Chemical engineering design

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Chemical Engineering Design I: Procedures for Chemical Engineering Design 1 II: Principles of Chemical Engineering Design 3 III: Selection of Pumps for Chemical Applications 6 3.1 Centrifugal Pumps 6 3.2 Positive Displacement Pumps 7 3.3 Determining the Suitable Pump Type 7 3.4 Transporting Special Media 7 3.5 Influence of the Suction Tank’s Design 8 3.6 Properties of the Fluid to Be Transported 8 3.7 Selection of Vacuum Pumps 10 IV: Classification and Selection of Valves 12 4.1 Classification of Valves 12 4.2 Advantages and Disadvantages of Various Valves 12 V: Chemical Materials 16 5.1 Metal Materials 16 5.2 Non-Metallic Materials 16 VI: Freezing and Cooling Equipment 17 6.1 Freezers 17 6.2 Refrigerants 17 6.3 Coolants 19 6.4 Comparison of Screw Compressors with Reciprocating Piston Compressors, Absorption Compressors, and Centrifugal Compressors 20 6.5 Cooling Towers 21 VII: Chemical Instruments and Automation 21 7.1 Thermometers 21 7.2 Flow Meters 23 7.3 Pressure Measurement Instruments 25 7.4 Level Gauges 26 7.5 Control Valves 27 7.6 Development of Automation Solutions 27 VIII: Common Concepts in Chemical Engineering Design 28 8.1 Re (Reynolds Number) 28 8.2 Viscosity 29 8.3 Calculation of Flow Resistance in Fluid Transport 29 8.4 Saturated Vapor Pressure 30 I: Procedures for Chemical Engineering Design 1. Selection of Site (A feasibility study and market analysis must be conducted first, along with economic and financial assessments). 2. Conduct geological and hydrological surveys, determine the zero level, and prepare a geological survey report for the entire plant area. 3. Draw a process flow diagram of the entire product with control points. 4. Draw detailed equipment drawings (in cases where conditions do not permit, equipment sketches can be drawn, but the size, self-weight, and total weight of the equipment must be specified). 5. Draw a layout plan, indicating the size, height, weight of the equipment as well as their placement locations. Specify the location and size of the control room, as well as the location and size of the tank area. It is important to note that, while adhering to the prerequisites of the manufacturing process, it is necessary to reduce the production cycle time. b. Reduce costs and consumption. c. When formulating the lifting plan, full consideration should be given to ease of lifting and safety. d. Consider ease of operation. e. Minimize the floor area, building height, and beam span as much as possible. f. Take into account the overall layout and aesthetics. g. Make every effort to take into account the mandatory requirements of the design and comply with chemical industry standards. 6. The main plant shall be designed by the civil engineering department or structural engineer (and, if conditions permit, the foundations for the auxiliary buildings, sewage tanks, and other large-scale equipment shall also be designed). 7. Start civil engineering work, then prepare detailed equipment drawings (including the location and size of pipe connections), and discuss automated control solutions. 8. Begin manufacturing of the equipment; for other large-scale equipment (or those that cannot be manufactured by the company), start the bidding process for instruments and automation systems. 9. Create detailed piping diagrams, and based on these diagrams, compile and organize a list of pipe and valve components (including pipes, valves, flanges, gaskets, screws, nuts, filters), which is then submitted to the equipment department for procurement. 10. Start installation, making adjustments as needed according to the conditions on site. 11. After installation is complete, conduct flaw detection, hydraulic testing, and airtightness tests. 12. Tune the entire system (including the operation of the equipment, setting the zero and full-scale values for automated instruments, and performing static and dynamic tests on the automation system). 13. Carry out trial operations to check that everything is functioning properly. II. Principles of Chemical Engineering Design Throughout the chemical engineering design process, we must be familiar with its principles and spirit in order to apply them. Flexibly applying the details in chemical engineering design ensures compliance with standards without compromising production. Achieve maximum utilization of resources. Now let’s gain a further understanding of the requirements for chemical engineering design and related topics. Since safety is of primary importance in chemical production, we must analyze the safety aspects involved in the production process. Familiarize yourself with the properties of our raw materials and products so that we can match them accordingly. Chemical industry standards classify the degree of hazard of substances into five levels (for details, see GBJ16-87). This classification is determined primarily by taking into account the physicochemical properties, quantities of raw materials, intermediates, and finished products used, generated, and stored during the production process, as well as their fire and explosion hazards, as well as the characteristics of the production process itself. Only by determining the hazard level can we establish the fire protection distance and explosion protection rating. It plays a decisive role in our selection of equipment, instruments, operating methods, and fire-fighting gear. Among the five grades of A, B, C, D, and E, the vast majority of our company’s raw materials are classified as Grade A hazardous materials. It is defined as follows: 1. Liquids with a flash point of less than 28°C; 2. Liquids with an explosion limit of less than 10%; 3. Substances that can decompose on their own at room temperature or oxidize in air, thereby causing rapid spontaneous combustion or explosion; 4. Substances that, when exposed to water or water vapor in the air at room temperature, produce flammable gases that can lead to combustion or explosion; 5. Strong oxidizers that are highly prone to combustion or explosion when exposed to acids, heat, impact, friction, catalysts, or flammable inorganic substances such as organic compounds or sulfur; 6. Substances that can cause combustion or explosion upon impact, friction, or contact with oxidizers or organic compounds; 7. Substances used in closed systems where the operating temperature is equal to or higher than the substance’s own auto-ignition point. As long as one of these conditions is met, the substance is classified as a Class A explosive hazard; substances such as benzene, toluene, and propylene all fall into this category of highly dangerous materials. For Class A hazardous materials, the relevant regulations are as follows: 1. Fire separation distance for fixed-roof vertical storage tanks of Class A on the ground: when the volume of a single tank is greater than 1,000 cubic meters, the distance between tanks should be 0.6D (where D is the diameter of the tank); when the volume is 1,000 cubic meters or less, the distance between tanks should be 0.75D. However, it is also stipulated that for above-ground fixed storage tanks holding Class A and B liquids with a single-tank capacity of no more than 1,000 cubic meters, the fire separation distance may be no less than 0.6D when a fixed cooling fire protection system is used. It is also stipulated that when equipped with submersible foam fire extinguishing systems, fixed cooling water systems, and foam fire extinguishing systems for extinguishing liquid fires within fire dikes, the spacing between storage tanks can be reduced appropriately; however, the distance between above-ground storage tanks should not be less than 0.4D. 2. The fire separation distance between above-ground Class A fixed-roof vertical storage tanks and process units must be greater than 25 meters ； The fire separation distance from Class A warehouse areas is between 25 and 35 meters (depending on the size of the storage tanks). 3. The fire separation distance between Class A process units and power distribution rooms as well as pump rooms should not be less than 20 meters, with a minimum of 15 meters ； The fire protection distance from open flames and emission sources should not be less than 30 meters ； The fire separation distance from sewage treatment plants should not be less than 30 meters. Fire separation distance from roads outside the plant (roadside) is 15 meters ； A fire protection distance of 10 meters from the main roads within the plant (road edges) ； A fire protection distance of 5 meters from secondary roads within the plant (along the road edges) ； The fire separation distance from civil buildings shall be no less than 25 meters ； The fire separation distance from important public buildings shall be no less than 50 meters. 4. Class A explosion-proof workshops with explosion hazards should be located separately, and should be of open or semi-open design. They should utilize frame or shed structures supported by reinforced concrete columns or steel columns, with the steel columns being equipped with fire-resistant protective layers ； There should be no fewer than two safety exits in the factory building. 5. The distance between the control room and the production equipment should be at least 15 meters, and it should also be located away from electrical explosion-proof areas (hazardous areas). If it is necessary to be in an explosion-proof area, no doors or windows should be installed on the walls facing the hazardous area, and explosion-proof positive pressure measures must be taken to maintain a positive pressure of 5–10 mm of water column inside, thereby preventing hazardous gases from entering the control room. In addition, other factors must also be taken into consideration: a: Placement upwind of equipment that handles toxic gases; b: It is necessary to stay away from equipment that generates high temperatures and pressures, as well as equipment containing flammable or toxic substances; c: At least one side of the control room should not face the production facilities. 6. Regarding the classification of explosion-proof ratings in explosive gas environments: (for example, dⅡBT4) The letter “d” denotes an explosion-proof design, and this designation refers to the selection of electrical equipment for use in explosive gas environments, that is, it indicates the different safety levels required for various electrical devices in explosive gas zones (Zone 0, Zone 1, Zone 2). For the selection of rotary motors, they are classified into explosion-proof type (code d), positive pressure type (p), enhanced safety type (e), and spark-free type (n) ； The selection of lighting fixtures is divided into flameproof type (code d) and increased safety type (e) ； The selection of signals and alarm devices is divided into intrinsically safe types (ia, ib), flameproof types (code d), and enhanced safety types (e). IIB represents the classification of the explosivity level of gas or vapor mixtures, and is divided into three categories: IIA, IIB, and IIC. Among these, IIA has the lowest level (such as common alkanes like methane and ethane) ； Aromatic hydrocarbons such as benzene and toluene); Category IIB mainly includes propyne, ethylene, cyclopropane, 1,2-epoxypropane, coke oven gas, etc ； IIC is the highest (mainly hydrogen, acetylene, carbon disulfide, ethyl nitrate, water gas, etc.). These grades are primarily distinguished based on the maximum test safety gap (MESG) or the minimum ignition current (MICR). T4 represents 135＜T≤200℃; it is defined as a temperature range within which a certain medium can be ignited. It is mainly divided into T1 – 450℃ < T, T2 – 300 < T ≤ 450℃, T3 – 200 < T ≤ 300℃, T4 – 135 < T ≤ 200℃, T5 – 100 < T ≤ 135, and T6 – 85 < T ≤ 100℃. 7. Term explanation: Flash point: Also known as the ignition point. One of the indicators for the properties of flammable liquids. It is the temperature at which the mixture of vapor and air on the surface of a liquid produces a flash of blue flame when it comes into contact with fire. When measured using standard instruments, there are two types: open-cup and closed-cup. Generally, the former is used to measure liquids with a high flash point, while the latter is used to measure liquids with a low flash point. The temperature is lower than the ignition point. The flash point and ignition point of flammable liquids indicate the likelihood of an explosion or fire, and are of great significance for the safety of their transportation, storage, and use. Design of plant (facility) pipe racks and roads: When a pipe rack crosses above a road, its clear height should be: 4.5 meters or more for secondary roads (the roads within the facility are generally secondary roads); 6 meters or more for primary roads (main roads); and 7 meters or more for railways. When there are supports in the pipe gallery, the calculation should be based on the height at the bottom of the supports. The clear height of the maintenance passage under the pipe gallery shall be not less than 3.1 m. The clear height when the pipeline passes above platforms and walkways shall be not less than 2.1 meters. When the combined units are arranged side by side, a common maintenance (also for firefighting) road is provided, with a width of 5.2–6 m, and secondary roads have a width of 4.5 m. There are specific requirements for the installation and layout of pumps, which must meet the needs of safe operation and maintenance; in addition, the spacing between the operating surfaces also needs to be taken into consideration. The clear distance in the operation passage between pumps should be no less than 1 m, the distance in the operation passage in front of the pumps should be no less than 1.25 m, and the maintenance passage for the pumps should be no less than 3.5 m to allow cranes and trucks to access them; the width of the maintenance passage for small pumps can be reduced to 2.5 m. When arranging pumps of different sizes, there are generally three methods: 1. Align the centerlines of the pump outlets; the advantage is that it simplifies uniform operation. 2. The pump foundation surface should be leveled to facilitate the installation of sewage pipes or ditches, as well as to ease foundation construction. 3. Align the foundation surface on the power side. The advantages are that cable wiring is easy and cost-effective; the pump switch and ammeter are located on the same line, and the motor is easy to operate. Of course, if the sizes of the pumps differ too much, it will result in an excessively long suction pipe. Another thing to note is that the distance between the valve handwheel and any protruding parts of the pump or columns should be at least 750 mm, while the distance between electric motors should be 1.5–1.8 meters. When installing the pump, the foundation surface is generally 200–600 mm higher than the floor level, and the installation height must be uniform. Of course, when considering the installation height, the pump’s NPSH factor must be taken into account. (Cavitation refers to the phenomenon in which, when liquid enters the first stage impeller of a pump, the static pressure is lower than or equal to the saturated vapor pressure at that temperature; as a result, the liquid vaporizes and bubbles are formed. These bubbles move with the liquid to areas of higher pressure, where they suddenly condense. The surrounding liquid then rushes in quickly, creating a hydraulic shock. The forces generated by this vaporization and condensation can cause erosion, vibration, and a decline in the pump’s performance – this phenomenon is known as cavitation.) When installing a filter in front of the pump inlet, its base height should be chosen to facilitate the cleaning and removal of the filter. In addition, the drainage funnel and buried pipes also need to be considered. If the pump is used to draw liquid from a tank, a bottom valve should be installed at the bottom of the suction pipe, and there must be a column of liquid equal to the suction height when the pump is started. The pump outlet shut-off valve should preferably be a valve with low resistance, such as a gate valve; globe valves should be avoided as much as possible, in order to reduce pressure drops and prevent damage to the pump. The pump outlet pressure gauge should be installed between the pump outlet and the first shut-off valve. The bearings of pumps generally require cooling water for cooling; cooling water pipes should be equipped with flow detectors or funnels to monitor the water flow and prevent interruptions. In winter, measures must be taken to prevent freezing. Centrifugal pumps (such as water pumps) should be equipped with a vent port at the upper part of the pump body, and a drain port at the bottom. 3. Selection of chemical pumps: When selecting pumps, it is necessary to take into account the characteristics of different types of pumps. Pumps are classified into two main categories based on the principle by which they act on liquids: vane pumps and positive displacement pumps. Vane pumps draw in or discharge liquid through the centrifugal force generated by the rotating vanes inside the pump. Positive displacement pumps draw in or force out liquid by means of the compressive action generated by the reciprocating or rotating motion of the pump’s piston or rotor. Vane pumps are classified into centrifugal pumps, axial flow pumps, and vortex pumps depending on the structure of the vanes inside the pump. Positive displacement pumps are further divided into piston (plunger) pumps and rotary pumps. 3.1 Centrifugal pumps: The viscosity of the liquid at the delivery temperature should not exceed 650 mm2/S; otherwise, the pump’s efficiency decreases significantly. (When the viscosity is greater than 650 mm2/S, the performance of centrifugal pumps declines significantly, and such pumps are generally not used. However, since centrifugal pumps deliver fluid without pulsations, do not require safety valves, and allow for easy flow regulation, they are often employed in chemical production to transport liquids with a viscosity of up to 1000 mm2/S.) The flow rate is high while the head is relatively low. The amount of gas dissolved or entrained in the liquid should not exceed 5% (by volume). When the liquid contains solid particles, special centrifugal pumps (such as slurry pumps) are advisable. When a large variation in flow rate with minimal change in head is required, centrifugal pumps with a flat flow-rate–head curve are suitable; whereas when a small variation in flow rate with a large change in head is needed, centrifugal pumps with a steep flow-rate–head curve are appropriate. 3.2 Positive-displacement pumps: When the viscosity of the liquid at the delivery temperature is greater than 650 mm2/S, and the flow rate is low while the head is relatively high, reciprocating pumps are suitable. Gases dissolved in or entrained in the liquid allow a concentration of slightly more than 5% (by volume). When precise measurement of the liquid is required, a plunger metering pump can be used; when absolute leakage prevention is necessary, a diaphragm metering pump is suitable. Gear pumps and triple-screw pumps should not be used for liquids with poor lubricity properties, as reciprocating pumps are a better choice in such cases. For applications with low flow rates, low temperatures, and stable pressure requirements, rotary pumps or twin-screw pumps are suitable choices. 3.3 Determining the pump selection: Based on the flow rate and head required by the system, the type of pump to be used is initially determined according to the classification and applicable range of pumps. Since centrifugal pumps have a simple structure, do not cause pulsations in the fluid flow, and allow for easy flow rate adjustment, they should be preferred whenever possible, except in those situations where they are not suitable. Once the pump model has been selected, the pump series and material can be chosen based on the parameters of the process unit and the properties of the medium. Then, the specific model (specifications) of the pump is determined based on the samples and relevant technical documents provided by the pump manufacturer. 3.4 For the transportation of special media: Applicable viscosity range in mm2/s – Blade pumps: Centrifugal pumps, <150; Vortex pumps, <37.5. Positive displacement pumps: Reciprocating pumps, <850; Metering pumps, <800. Rotary piston pumps: 200–10,000. Single-screw pumps: 10–560,000. Twin-screw pumps: 0.6–100,000. Tri-screw pumps: 21–600. Gear pumps: <2,200. When a pump transports liquid containing gas, its flow rate, head, and efficiency all decrease. The higher the gas content, the faster the efficiency declines. As the gas content increases, the pump generates additional noise and vibration; in severe cases, this can lead to increased corrosion or even flow interruption and shaft failure. To ensure reliable pump operation, measures can be taken to reduce the gas content in the liquid: 3.5 Influence of the suction tank’s design The design of the suction tank and the arrangement of the pump’s suction pipes should ensure that each pump in parallel can draw in an equal amount of liquid. The pump’s suction inlet should have a certain depth of submersion within the suction tank, as well as a certain height above the bottom of the tank. The inlet pipe, return pipe, and waste liquid collection pipe of the liquid absorption tank should be kept away from the pump’s suction port, to prevent bubbles from being drawn into the pump before they have dissipated. At the same time, the suction pipe should not be placed in the center of the tank, nor too close to the tank walls; it should generally be at a distance of more than 1.5D from the walls to avoid the formation of vortices or vacuum conditions. Ensure that the pipe joints are properly sealed to prevent air from leaking in. When arranging the inhalation pipeline, areas where air pockets may form should be avoided. 3.6 Properties of the fluid being transported When transporting liquids containing solid particles, these particles suspended in the liquid cannot absorb, store, or transfer energy in the same way as the liquid, nor can they transfer kinetic energy to the liquid. The presence of solid particles results in lower pump head and efficiency compared to pumping clean water. When transporting volatile liquids, the impact of the properties of these liquids on the pump must be taken into account: 1) High pressure at the pump inlet; 2) The vaporization pressure changes significantly with temperature. 3) Strict requirements for the pump’s shaft seal, as well as the impact of the pump’s suction pressure on the vaporization of the material. For transporting liquids that must not leak, sealless pumps (magnetic drive pumps and shielded pumps) or mechanically sealed pumps equipped with leakage collection and alarm devices should be used. Comparison of magnetically driven pumps and shielded pumps:
Item | Magnetically Driven Pump | Shielded Pump
--- | --- | ---
Thickness of isolation sleeve (or shield) | 3 times that of the shielded pump’s isolation sleeve |
Consequences of failure of the isolation sleeve (or shield) | Fluid leaks into the atmosphere | A second leak prevention layer (motor casing) prevents fluid leakage, but it can damage the motor stator
Efficiency | Slightly lower | Slightly higher
Remote operation | Not available currently | Possible
Requirements for manufacturing technology and equipment | Lower | Higher
Driver | Standard motor or turbine | Specialized motor
Noise | Slightly higher (motor with fan) | Slightly lower (motor without fan)
Axial length | Longer | Shorter
Coupling | Coupling is present; alignment is required | No coupling
Bearing wear monitor | Still in the trial stage | Available
Price | Basically similar | Basically similar
Ease of routine maintenance | Easy | Difficult
Application range (power in KW/temperature in °C/pressure in MPA) | Good performance | Good performance
Suitable for: Normal pressure and temperature conditions, clean fluids that do not easily vaporize | Generally not suitable | There are specialized models available
Suitable for fluids containing solid particles | Generally not suitable; especially not suitable for fluids containing iron particles | There are models designed for fluids with solid particles (external cleaning fluid required)
Suitable for fluids with high melting points and tendency to crystallize | Generally not suitable | There are specialized models available
Suitable for highly corrosive fluids | Good performance | Not suitable (due to limitations of the shield)
For transporting corrosive fluids, corrosion-resistant pumps should be used. These pumps have corrosion-resistant materials in their flow-through parts, while other non-corrosion-resistant components, such as brackets, need to be protected from corrosion. The clearance of the sealing ring (flange ring) is larger than that of the water pump, to prevent operation at low flow rates and thus avoid increased corrosion due to rising liquid temperature. When stopping the pump, the suction valve should be closed promptly, or a shutdown seal should be used to prevent the medium from leaking out of the pump. For commonly used metal pumps, the materials used for their flow-through components include ordinary cast iron, high-silicon cast iron, stainless steel, high-alloy steel, titanium and its alloys, etc. Different materials can be selected depending on the properties of the medium and the temperature range. High-alloy steels, titanium, and their alloys are expensive, and their use should generally be avoided unless it is absolutely necessary. Metal pumps generally have better temperature resistance, pressure resistance, and operational stability than non-metallic pumps. Materials for the flow-through components of non-metallic pumps include: polyvinyl chloride, fiberglass-reinforced plastic, polypropylene, F46, fluorocarbon alloys, PVDF, ultra-high molecular weight polyethylene, graphite, ceramics, and glass-lined materials. The material should also be selected based on the properties of the medium and the temperature range. Generally, non-metals have lower temperature and pressure resistance compared to metals. Therefore, it is commonly used in applications with low flow rates, as well as those with low temperatures and operating pressures. 3.7 Selection of Vacuum Pumps Vacuum pumps are used to achieve a vacuum level below atmospheric pressure; they are typically classified based on the degree of vacuum or the pumping speed. They can generally be divided into vane type, positive displacement type (piston type, screw type), and water ring type. There are several indicators that can describe the performance of a vacuum pump: a) Vacuum level: expressed in terms of absolute pressure P, with units of Kpa, Torr, and mmHg (1 atmosphere = 101.325 Kpa = 1 Torr = 760 mmHg = 1.01325 bar). b) Pumping speed: refers to the volume of gas drawn into the vacuum pump’s inlet per unit of time (i.e., the volumetric flow rate under normal temperature and pressure conditions). Per cubic unit: cubic meters per hour, liters per second. The pumping speed of a vacuum pump is related to the suction pressure; the higher the suction pressure, the greater the pumping speed. c. Ultimate vacuum: refers to the stable lowest pressure value that can be achieved by the vacuum pump during pumping, also known as the maximum degree of vacuum. d. Selection of various pumps: Mechanical pumps and piston pumps are suitable only for applications with low vacuum levels and little water vapor present; for example, W-type reciprocating vacuum pumps. Vane pumps are used in situations where a large volume of gas needs to be pumped, and an auxiliary pump is necessary in such cases. Rotary vane vacuum pumps have a relatively low pumping speed. Molecular pumps are suitable for applications requiring high precision. Water ring pumps are appropriate for low-vacuum applications, such as those involving the removal of water vapor. Jet pumps, including water jet pumps, use water under pressure of 0.2–0.3 MPa as a driving force; they are suitable for removing water vapor and condensable gases. They function by utilizing the principle of conversion between static pressure energy and dynamic pressure energy during fluid flow. Steam jet pumps can handle various gases, and different numbers of stages can be selected according to specific requirements. Oil diffusion pumps are suitable for high vacuum levels, as are molecular centrifugal pumps. 4. Valve classification and selection: Valves are components used in fluid flow or pressure systems; they serve to regulate the flow rate or pressure of the fluid. Its functions include cutting off or connecting the medium, controlling flow rate, changing the flow rate, altering the direction of the medium flow, preventing backflow of the medium, and controlling or releasing pressure. 4.1 Classification of valves Based on their purpose and function, valves can be classified into: shut-off valves (whose role is to open and close the flow of medium in a pipeline, such as ball valves, gate valves, globe valves, butterfly valves, and diaphragm valves) ； Control valves (which are used to regulate parameters such as the flow rate and pressure of a medium, including control valves, throttle valves, and pressure reducing valves, etc.) ； Check valves (which serve to prevent the backflow of fluid in pipelines, such as check valves and foot valves) ； Shunt valves (which are used to distribute, separate, or mix fluids in pipelines, such as distribution valves and steam traps) ； Safety valves can be classified according to their actuation method into: manual valves ； Power-driven valves (such as electric valves, pneumatic valves) ； Automatic valves (valves that do not require external force to operate, but use the energy of the medium itself to move the valve, such as check valves, safety valves, self-acting pressure relief valves, and steam traps) can be classified by nominal pressure into: vacuum valves (operating at pressures below standard atmospheric pressure) ； Low-pressure valves (nominal pressure less than or equal to 1.6 Mp); Medium-pressure valves (nominal pressure of 2.5 MPA, 4.0 MPA, 6.4 MPA) ； High-pressure valves (nominal pressure 10MPA—80MPA) ； Ultra-high pressure valves (greater than 100 MPa) can be classified by temperature range as: ultra-low temperature valves (operating at temperatures below –80°C) ； Low-temperature valves (operating temperature between –40°C and –80°C) ； Normal temperature valves (operating temperature higher than –40°C and lower than or equal to 120°C) ； Medium-temperature valves (operating temperature above 120°C and below 450°C) ； High-temperature valves (operating at temperatures above 450°C) are generally classified by taking into account both their operating principle and function as well as their structural design; this is the common classification method used in China. They can be divided into: gate valves ； Butterfly valve ； Stop valve ； check valve ； Plug valve ； Ball valve ； Clamp valve ； Diaphragm valve ； Plunger valves, etc. 4.2 Advantages and disadvantages of various valves Gate valves: Gate valves are valves in which the closing element (gate) moves vertically along the axis of the passage; they are primarily used in pipelines to shut off the flow of medium, that is, to either fully open or fully close it. Generally, gate valves cannot be used to regulate flow. It can be used in low-temperature and low-pressure conditions as well as in high-temperature and high-pressure conditions, and it can be adapted to different valve materials. However, gate valves are generally not used in pipelines for transporting media such as slurry. Advantages: ① Low fluid resistance ； ② The torque required to open and close it is low ； ③ It can be used in ring piping systems where the medium flows in both directions; in other words, the flow direction of the medium is not restricted ； ④ When fully open, the sealing surface is subject to less erosion by the working medium compared to a globe valve ； ⑤ Its structural design is relatively simple, and it is easy to manufacture ； ⑥ The structural length is relatively short. Disadvantages: ① It has large external dimensions and opening height, requiring a lot of space for installation ； ② During the opening and closing process, there is relative friction between the sealing surfaces, resulting in significant wear; in addition, high temperatures can easily cause scuffing ； ③ Generally, gate valves have two sealing surfaces, which adds some difficulties to processing, grinding, and maintenance. ④ The opening and closing time is long. Butterfly valve: A butterfly valve is a type of valve that uses a disc-shaped closing element to rotate back and forth by about 90° in order to open, close, and regulate the flow of fluid. Advantages: ① Simple structure, small size, light weight, and low material consumption; not suitable for large-diameter valves ； ② Rapid opening and closing, low flow resistance ； ③ It can be used in media containing suspended solid particles, and depending on the strength of the sealing surface, it can also be used for powdered and granular media. It can be used for two-way opening, closing, and regulation in ventilation and dust removal pipelines, and is widely applied to gas pipelines and water conduits in metallurgy, light industry, power, and petrochemical systems. Disadvantages: ① The flow regulation range is limited; when it is set to 30%, the flow rate exceeds 95%. ② Due to the structure of butterfly valves and the limitations of their sealing materials, they are not suitable for use in high-temperature and high-pressure pipeline systems. The normal operating temperature is below 300°C, and the pressure rating is below PN40. ③ Its sealing performance is inferior to that of ball valves and globe valves, so it is used in applications where high sealing requirements are not necessary. Ball valve: It evolved from the plug valve; its closing element is a sphere, and opening and closing are achieved by rotating this sphere 90° around the axis of the valve stem. Ball valves are primarily used on pipelines to shut off, distribute, and change the direction of fluid flow; those designed with a V-shaped opening also possess excellent flow regulation capabilities. Advantages: ① It has the lowest flow resistance (actually 0) ； ②It does not get stuck during operation (without lubricant), allowing it to be reliably used in corrosive media and low-boiling-point liquids ； ③Complete sealing can be achieved over a wide range of pressures and temperatures ； ④Fast opening and closing can be achieved; the opening and closing time for certain structures is as low as 0.05–0.1 seconds, ensuring their suitability for use in automated systems on test benches. When the valve is opened and closed quickly, the operation is smooth with no shocks. ⑤The spherical closure element can automatically position itself at the boundary position ； ⑥The working medium is reliably sealed on both sides ； ⑦When in the fully open and fully closed positions, the sealing surfaces of the ball and the valve seat are separated from the medium, so the medium flowing through the valve at high speeds does not cause erosion of these sealing surfaces ； ⑧With its compact design and low weight, it can be considered the most suitable valve structure for low-temperature medium systems ； ⑨The valve body is symmetrical; especially in the case of welded valve bodies, it can withstand stresses from the pipes effectively ； ⑩The closing element can withstand the high pressure difference during closure. ⑾Ball valves with fully welded valve bodies can be buried directly underground, preventing the internal components of the valve from being corroded; their maximum service life can reach 30 years, making them the ideal valves for oil and gas pipelines. Disadvantages: ① Since the main material used for the seat seal ring in ball valves is polytetrafluoroethylene, it is inert to almost all chemicals. It also possesses a number of advantages, including a low friction coefficient, stable performance, resistance to aging, a wide operating temperature range, and excellent sealing properties. However, the physical properties of polytetrafluoroethylene, including its high coefficient of expansion, sensitivity to cold flow, and poor thermal conductivity, require that the design of the valve seat seal take these properties into account. Therefore, when the sealing material hardens, the reliability of the seal is compromised. Moreover, polytetrafluoroethylene has a low temperature resistance and can only be used at temperatures below 180°C. Above this temperature, the sealing material will age. When considering long-term use, it is generally not used at 120°C. ②Its regulating performance is somewhat inferior to that of globe valves, especially pneumatic valves (or electric valves). Globe valve: It is a valve in which the closing element (valve disc) moves along the center line of the valve seat. Based on this manner of movement of the valve disc, the change in the valve seat opening is directly proportional to the stroke of the valve disc. Due to the relatively short opening and closing stroke of the valve stem in this type of valve, as well as its highly reliable shut-off function, and because the change in the valve seat orifice is proportional to the stroke of the valve disc, it is highly suitable for regulating flow rate. Therefore, this type of valve is very suitable for use in cutting off or regulating flow, as well as for throttling. Advantages: ① During opening and closing, the frictional force between the valve disc and the sealing surface of the valve body is lower than that of gate valves, thus it is wear-resistant. ②The opening height is generally only 1/4 of the valve seat passage, so it is much smaller than gate valves ； ③Usually, there is only one sealing surface on the valve body and the valve disc, which makes the manufacturing process simpler and facilitates maintenance. ④Due to its filler, which is generally a mixture of asbestos and graphite, it has a high temperature resistance rating. Gauge valves are generally used for steam valves. Disadvantages: ① Due to the change in the flow direction of the medium as it passes through the valve, the minimum flow resistance of globe valves is also higher than that of most other types of valves ； ②Due to the long travel distance, its opening speed is slower than that of a ball valve. Plug valve: It is a rotary valve whose closing element is in the form of a plunger; by rotating it 90°, the passage opening on the plunger is connected to or separated from the passage opening on the valve body, thereby enabling the valve to be opened or closed. The shape of the valve plug can be cylindrical or conical. Its principle is basically similar to that of a ball valve, which was developed from a plug valve. It is primarily used in oil field extraction, as well as in the petrochemical industry. Diaphragm valve: It refers to a valve in which a flexible diaphragm or composite diaphragm is installed within the valve body and cover, with the closing element being a compression device connected to the diaphragm. The valve seat can be wavy or the wall of a straight flow channel. Advantages: ① The control mechanism is separated from the medium flow path, which not only ensures the purity of the working medium but also prevents the medium in the pipeline from impacting the working components of the control mechanism. No separate sealing is required at the valve stem, unless such a measure is needed as a safety feature when dealing with harmful media ； ②Since the working medium comes into contact only with the diaphragm and the valve body, both of which can be made from a variety of different materials, this valve is capable of effectively controlling a wide range of working media, especially those that are chemically corrosive or contain suspended particles. ③It has a simple structure, consisting of only three components: the valve body, the diaphragm, and the valve cover assembly. This valve is easy to disassemble and repair quickly, and the diaphragm can be replaced on-site in a short time. Disadvantages: ① Due to the limitations imposed by the valve body lining process and diaphragm manufacturing process, it is difficult to produce large valve body linings and large diaphragms; therefore, diaphragms are not suitable for larger pipe diameters and are generally used in pipelines with a diameter of DN≤200mm. ②Due to limitations imposed by the diaphragm material, diaphragm valves are suitable for low-pressure and low-temperature applications. Generally not exceeding 180℃ ； ③The adjustment performance is relatively poor, allowing adjustments only within a narrow range (it can be used for flow regulation when the valve is closed to 2/3 of its open position). Safety valve: It refers to a device used as an overpressure protection mechanism in pressurized containers, equipment, or pipelines. When the pressure inside a device, container, or pipeline rises above the allowable level, the valve opens automatically, allowing for complete discharge in order to prevent further increases in pressure within that device, container, or pipeline ； When the pressure drops to the specified value, the valve should close automatically and promptly, thereby ensuring the safe operation of the equipment, containers, or pipelines. Steam traps: When conveying media such as steam and compressed air, some condensate water is formed. To ensure the efficiency and safe operation of the equipment, it is necessary to remove this unnecessary and harmful fluid promptly, so as to maintain proper functioning of the equipment. It has the following functions: ① It can quickly remove the condensed water that is generated ； ②Prevent steam leakage ； ③Remove air and other non-condensable gases. Pressure relief valve: A valve that reduces the inlet pressure to a desired outlet pressure through regulation, and relies on the energy of the medium itself to maintain the outlet pressure stable automatically. Check valve: also known as a backflow valve, non-return valve, backpressure valve, and one-way valve. These valves open and close automatically due to the force generated by the flow of the medium itself within the pipeline; they are a type of automatic valve. Check valves are used in piping systems; their main function is to prevent the backflow of fluid, to stop pumps and driving motors from rotating in reverse, and to prevent the leakage of fluid from containers. Check valves can also be used in pipelines that supply auxiliary systems whose pressure may rise above the system pressure. They can mainly be divided into swing-type (rotating around the center of gravity) and lift-type (moving along the axis). Definition of flow coefficient: 1. C: The volume of water at a temperature of 5–40°C that flows through the valve per hour (expressed in m3), when the valve is fully open and the pressure difference across it is 1 kgf/cm2. C is the general symbol for the flow coefficient, which has been widely used in China for a long time. 2. Kv: The volume of water at a temperature of 5–40°C that flows through the valve per hour, in cubic meters, when the valve is fully open and the pressure difference across it is 100 KPA. Kv=1.01C, which is currently recommended for use in our country. 3. Cv: The volume of water flowing through the valve per hour (in m3), at a temperature of 15°C (60°F) and with a pressure difference of 1 lb/in2 across the valve when it is fully open. Cv=1.167C V. Chemical Materials Materials: are divided into metallic materials and non-metallic materials. 5.1 Metallic Materials These can generally be classified into ferrous metals (cast iron, steel, etc.) and non-ferrous metals (copper, aluminum). Metal materials possess mechanical properties such as strength, hardness, and impact toughness. Metallic materials are composed of various elements. We mainly use ferrous metals such as iron, steel, and alloy steel. Iron-carbon alloys with a carbon content in the range of 0.1% to 2% are called steel (commonly known as wrought iron), while those with a carbon content greater than 2% are called cast iron (commonly known as pig iron). In addition to iron and carbon, steel also contains elements such as silicon (Si), manganese (Mn), phosphorus (P), and sulfur (S). Steel can be divided into carbon steel and alloy steel based on its chemical composition ； Based on their uses, they can be divided into structural steel, tool steel, and special-purpose steel ； Based on quality, including the amount of phosphorus and sulfur impurities, it can be divided into ordinary steel, high-quality steel, and ultra-high-quality steel ； Stainless steel refers to an iron-carbon alloy with a chromium content of over 12%. Based on their microstructure, they can be classified into martensitic stainless steels, ferritic stainless steels, and austenitic stainless steels. The steel grade indicates the properties of the steel (chemical composition, quality, and type); for example, the commonly used A3 denotes first-class steel of category A among ordinary carbon steels ； Another example is 1Cr18Ni9 stainless steel, whose chemical composition consists of 0.10% carbon, 18% chromium, and 9% nickel; it exhibits excellent corrosion resistance in cold phosphoric acid and dilute nitric acid. By adding a small amount of titanium (less than 1%) to such steels, 1Cr18Ni9Ti is obtained, which further enhances corrosion resistance. If an additional small amount of molybdenum (2%~3%) is added, 1Cr18Ni12Mo2Ti and 1Cr18Ni12Mo3Ti are formed, thereby significantly improving the chemical stability of stainless steel in media such as urea and dilute sulfuric acid. 5.2 Non-metallic materials are divided into inorganic non-metallic materials and organic non-metallic materials. Generally, we use plastics and rubbers, which are organic non-metallic materials. Common plastics are polyvinyl chloride, polyethylene, polypropylene, and polytetrafluoroethylene. ①Polyvinyl chloride has decent corrosion resistance, but it is not resistant to organic solvents. Its typical operating temperature does not exceed 50°C, but its high strength and good weldability make it the most widely used and largest-volume material in chemical corrosion protection. ②Polyethylene has better corrosion resistance compared to polyvinyl chloride; this is evident in its fact that, in addition to resisting other types of corrosion, it is also virtually insoluble in any organic solvents. Its corrosion resistance in formic acid and hydrofluoric acid environments is also superior to that of polyvinyl chloride. However, polyethylene is prone to environmental stress cracking; it can experience sudden cracking when exposed to low stresses or strains in certain media. In addition, aliphatic hydrocarbons, halogenated hydrocarbons, and aromatic hydrocarbons at room temperature can cause it to swell. ③Polypropylene is the lightest among commercial plastics, typically at 0. 9. Compared with polyvinyl chloride, its advantage is a higher operating temperature; its disadvantages are a large linear expansion coefficient, a low elastic modulus, a large shrinkage rate during molding, and high brittleness at low temperatures. It has excellent corrosion resistance, but it cannot be used with strong oxidizing agents such as concentrated nitric acid, fuming sulfuric acid, chlorosulfonic acid, etc., at room temperature. At room temperature, almost all organic solvents cannot dissolve polypropylene, but certain chlorinated hydrocarbons, aromatic hydrocarbons, and high-boiling-point aliphatic hydrocarbons can cause polypropylene to swell; moreover, as the temperature rises, its degree of swelling and solubility increase as well. ④Polytetrafluoroethylene is a type of fluoroplastic; its corrosion resistance exceeds that of all existing engineering plastics, which is why it is known as the \"king of plastics\". Strong acids, strong bases, strong oxidizing agents, and solvents of almost any concentration have no effect on it, not even at high temperatures. Only molten alkali metals or their ammonia solutions, chlorine trifluoride, and elemental fluorine have an effect on it, but this effect is only evident at high temperatures. The long-term operating temperature for polytetrafluoroethylene is around 180°C. Its main drawback is the difficulty in molding. 6 Freezing and Cooling Equipment 6.1 Freezers The purpose of cooling is to achieve a temperature lower than that of the surrounding environment, in order to provide the cooling capacity required for various manufacturing processes. Based on the different structures and working principles of refrigerators, they can be classified into piston-type and screw-type, centrifugal type, and absorption type. Since centrifugal compressors are mostly used in large-scale refrigeration systems to compress refrigerant vapors with high molecular weights, they are quite unrelated to our industry. We mainly introduce positive-displacement types (piston and screw). Its working principle is as follows: the compressor compresses the refrigerant, causing its volume to decrease rapidly, its pressure to rise, and its potential energy to drop; this process releases a large amount of heat, which is then cooled down using cooling water. Then, through the throttle valve, it passes from the high-pressure area to the low-pressure area, causing its volume to increase rapidly and thus its potential energy to rise, while its temperature drops sharply. Thus, a large amount of heat is absorbed, achieving heat exchange through the coolant. The coolant then enters the process area to exchange heat with the process medium. 6.2 Refrigerant refers to the working fluid in a refrigeration system. At low temperatures, heat is absorbed due to evaporation ； At high temperatures, heat is released through condensation. Due to this effect, it can transfer heat from a lower temperature to a higher temperature, thereby creating a lower temperature. Common refrigerants include ammonia and Freon. The advantages of ammonia (R717) are its low standard boiling temperature, moderate pressure in condensers and evaporators, high cooling capacity per unit volume, large specific heat of fusion, high latent heat of vaporization, low throttling losses, easy detection in the event of leaks, and low cost. The disadvantages are that it is toxic, has an irritating odor, is corrosive to copper and its alloys in the presence of water, and poses an explosion risk when mixed with air in certain proportions. The advantages of Freon (commonly R12 – dichlorodifluoromethane CF2Cl2, R22 – chlorodifluoromethane) are that it is non-toxic, does not corrode metals, is difficult to ignite and explode, has a relatively high molecular weight, a large specific heat capacity, and a low freezing point. The disadvantages are high cost, low surface heat transfer coefficient, low cooling capacity per unit volume, damage to the ozone layer, and a tendency to cause the greenhouse effect. Among them, R11, R12, R113, etc. will be banned by 2030. Chemical name Symbol Molecular formula Molecular weight Boiling point at atmospheric pressure °C Critical temperature °C Critical pressure (absolute) MPa Critical specific volume (L/KG) Freezing point °C Vaporization heat at atmospheric pressure (KJ/KG) Trichlorotrifluoroethane R113 C2F3CL3 187 48 214 3.4 1.7 -36.6 144.6 Dichlorotetrafluoroethane R114 C2F4CL2 171 3.5 146 3.3 1.7 -94 137.3 Chlorodifluoromethane R22 CHF2CL 87 -41 96 5.9 1.9 -160 235 Dichlorodifluoromethane R12 CF2CL2 121 -30 112 4.1 1.8 -155 167.2 Chlorotrifluoromethane R13 CF3CL 104 -81 29 3.9 1.7 -180 150 Dichlorofluoromethane R21 CHFCL2 103 8.9 178.5 5.2 1.9 -135 Trichlorofluoromethane R11 CFCL3 137 23.7 197.8 4.4 1.8 -111 182.3 Ammonia R717 NH3 17 -33 4 132.4 11.3 4.1 -77. 7 1368.2 6.3 The coolant is an intermediate medium used in indirect heat transfer refrigeration systems to transfer cold energy. An ideal coolant should possess the following properties: ① it should not solidify or vaporize within the operating temperature range; ② it should have a high specific heat capacity; ③ it should have a low density and low viscosity; ④ it should have a high thermal conductivity; ⑤ it should not corrode equipment, be non-toxic, and exhibit good chemical stability; ⑥ it should be inexpensive. Common cryogens include water, inorganic salt solutions, and organic cryogens. Their characteristics are as follows: A. Water: It can only be used as a coolant at temperatures above 0°C. B. Inorganic salt water solutions: In medium and low temperature applications, salt water solutions are generally used as coolants; common ones include solutions of calcium chloride and sodium chloride. C. In applications where the use of corrosive refrigerants is not allowed, aqueous solutions such as ethylene glycol and dichloromethane can be used. 6.4 Comparison of screw compressors with reciprocating piston compressors, absorption compressors, and centrifugal compressors: Piston compressors have a long history of use and are currently the most widely used type of compressor in China. Due to its wide pressure range, it can adapt to a broad energy range, and boasts advantages such as high speed, multiple cylinders, adjustable energy output, high thermal efficiency, and suitability for various cooling applications ； Its disadvantages are a complex structure, many vulnerable components, a short maintenance cycle, sensitivity to wet strokes, pulse vibrations, and poor operational stability. China has decades of experience in manufacturing this type of product; it is easy to process, relatively inexpensive, and thus widely used. With energy shortages and severe environmental pollution, there are higher demands on refrigerators. Under such circumstances, the application scope of piston-type refrigerators is showing a trend of gradual reduction. The screw compressor is a new type of compression device; compared to reciprocating compressors, its advantages include ① a compact machine structure, small size, low floor space requirement, and light weight. ②It has high thermal efficiency, requires fewer components, and the total number of parts in the compressor is only 1/10 that of a piston-type compressor. It has few vulnerable components, operates safely and reliably, and is easy to operate and maintain. ③There is no pulsation in the gas, and the operation is smooth; the unit does not require a special foundation as it does not sit high above the base. Oil is injected into the rotor chamber during operation, which results in a low exhaust temperature – the ammonia refrigerant generally does not exceed 90°C. ⑤It is not sensitive to wet conditions; the entry of wet steam or a small amount of liquid into the machine poses no risk of liquid hammer. ⑥It can operate at higher pressure ratios; with single-stage compression, the temperature of the ammonia vapor can reach -40°C. ⑦The effective compression stroke can be adjusted using a slide valve, enabling stepless cooling capacity control from 10% to 100%. The disadvantages are: it requires complex oil treatment equipment, such as oil separators and oil coolers with excellent separation capabilities; it generates significant noise, usually above 85 decibels, thus sound insulation measures are necessary. There is also a type of absorption refrigeration that can be driven directly by a heat source, especially by the large amount of waste heat generated in petrochemical production processes. Compared to compression refrigeration, both methods utilize the fact that a refrigerant absorbs heat when it evaporates at low pressure and releases heat when it condenses at high pressure. The difference lies in the way pressure is increased: compression refrigeration uses mechanical compression to raise the vapor pressure, whereas absorption refrigeration relies on absorption at low pressure and desorption (thermal compression) at high pressure to increase the vapor pressure. Therefore, in an absorption refrigeration cycle, in addition to an evaporator and a condenser, there are also a regenerator, an absorber, and other components. Such as lithium bromide refrigerators. The advantages include: simple structure, stable operation, low vibration, low noise, and safety. The disadvantage is: lithium bromide has a strong corrosive effect on steel ； The cooling water consumption is very high, about twice that of compression-type refrigerators; therefore, unless a cheap heat source is available, using this type of refrigerator is less economical than using a compression-type one. Compared to piston-type compressors, centrifugal compressors feature higher rotational speeds, greater cooling capacity, less mechanical wear, fewer vulnerable components, simpler maintenance, longer continuous operation times, lower vibration levels, smoother operation, and lower requirements for the foundation. When high cooling capacity is needed, units with a given power level are lighter in weight, smaller in size, and require less space. The cooling capacity can be adjusted continuously within a range of 30% to 100%. Multi-stage compression and throttling are easy to implement, allowing various temperatures to be achieved in each stage to meet the requirements of certain chemical processes. Automation is also straightforward. For large refrigeration systems, industrial steam turbines with high efficiency can be used for drive, which offers economic advantages for enterprises that have waste heat in the form of steam. The disadvantages are: lower efficiency compared to piston refrigerators, higher noise frequency, high consumption of cooling water, and surge can occur if not operated properly. 6.5 Cooling Towers A cooling tower is a device or structure in which hot water is sprayed inside the tower in the form of droplets or a water film and flows from top to bottom, while air flows inside the tower from bottom to top or horizontally; it utilizes the evaporation of water as well as the heat transfer between air and water to remove heat from the water. The circulating water systems in chemical plants typically use mechanical ventilation counterflow cooling towers and mechanical ventilation crossflow cooling towers. The size classification of cooling towers into large, medium, and small categories is as follows: those with a cooling water flow rate per unit exceeding 1500 m3/h are considered large-scale ； When the cooling water volume load per cell is greater than 500 m3/h but less than or equal to 1500 m3/h, it is classified as medium-sized ； A single-cell cooling water flow rate of less than or equal to 500 m3/h is considered small ； A cooling tower consists of a water spray device, a water distribution system, an air distribution device, air ducts, a water remover, a fan, and the tower structure. When selecting a cooling tower, the following points should be considered: A. Tower structure materials—structural stability, durability, resistance to atmospheric and water corrosion, and precise assembly fit. B. Even water distribution, less wall flow, and a properly selected spraying device that makes clogging less likely. C. The type of wetting filler meets the requirements regarding water quality and temperature, and it has been scientifically evaluated. D. The performance of the dehydrator meets **the specified standards. E. The fan is properly matched, enabling long-term normal operation without vibration or abnormal noise. The leaves have good resistance to water erosion and sufficient strength. E. Low power consumption and easy for regular maintenance. F. Low cost; small and medium-sized steel-frame glass cooling towers also require light weight. During the heat exchange process, 80% of the heat exchange occurs through evaporation, relying on the difference in saturated vapor pressures. Only 20% of heat exchange is achieved through contact heat transfer due to differences in temperature and pressure. 7. Chemical Instrumentation and Chemical Automation: In chemical production, it is not sufficient to rely on sight, senses, and experience alone to carry out proper operations and identify abnormalities in the production process; a large number of automated instruments are also necessary to control the production process. Depending on their functions and measurement purposes, they can be roughly classified into thermometers, flow meters, pressure gauges, level gauges, etc. 7.1 Thermometers Thermometers are the devices we use to measure temperature, and they play a very important role in chemical production. Based on their different measurement principles, they can be divided into direct-type and indirect-type types; the ones that are commonly used are mostly of the direct-type, including glass tube thermometers, pressure thermometers, bimetallic thermometers, thermal resistance thermometers, and thermocouple thermometers. Indirect types include optical thermometers, radiation thermometers, etc. Compared to indirect methods, the direct method has the advantages of being simple, reliable, inexpensive, and having high accuracy; it generally enables the measurement of the true temperature. The disadvantages are: long latency time and susceptibility to corrosion. Very high temperatures cannot be measured. A glass tube thermometer uses the principle of thermal expansion and contraction to measure temperature. Due to the differences in the expansion coefficients of temperature-measuring media as well as their boiling and freezing points, the common glass tube thermometers include kerosene thermometers, mercury thermometers, and red ink thermometers. Its advantages are a simple structure, easy use, relatively high measurement accuracy, and low cost. The disadvantage is that the measurement limits and accuracy are constrained by the quality of the glass and the properties of the temperature-measuring medium. It also cannot be transmitted over long distances and is fragile. A pressure thermometer uses the volume expansion or pressure change that occurs in a liquid, gas, or saturated vapor within a sealed container when heated as the signal for measurement. Its basic structure consists of three parts: a temperature bulb, capillaries, and an indicator gauge. It is one of the earliest methods applied to temperature control in production processes. The pressure-type temperature measurement system remains a widely used measurement method for on-site temperature indication and control. The advantages of pressure-type thermometers are: simple structure, high mechanical strength, and resistance to vibration. It has a low price and does not require external energy. The disadvantage is that the temperature measurement range is limited, generally ranging from -80 to 400℃ ； High heat loss results in a slower response time ； The instrument sealing system (thermocouple bulb, capillary, bimetallic strip) is damaged and difficult to repair; it must be replaced ； The measurement accuracy is greatly affected by the ambient temperature and the installation location of the thermocouple, resulting in relatively low precision ； There is a limit to the capillary transfer distance ； Bimetallic temperature sensors are a type of solid expansion thermometer that utilize the geometric displacement that occurs when two metals with different coefficients of expansion, yet firmly bonded together, are heated, as a signal for measuring temperature. Advantages: Simple structure, low price ； Easy to maintain ； More durable, shock-resistant, and impact-resistant than glass thermometers ； It has a large field of view. The disadvantages are: low measurement accuracy, limited range and application scope, and no capability for remote transmission. A thermistor thermometer measures temperature by utilizing the property that the resistance value of a metal conductor changes as temperature changes. As a resistance material for temperature-sensitive elements, it is required that the resistance have a certain functional relationship with temperature, possess a high temperature coefficient, a high resistivity, and a low heat capacity. It should maintain stable physicochemical properties over the entire temperature measurement range, and the relationship between resistance and temperature must exhibit good reproducibility. Common materials for thermal resistors include platinum, copper, and nickel. The forming instrument is an armored thermoresistor. An armored thermoresistor combines the temperature sensing element, insulating material, and wires together within a metal tube; as a result, it can have a small outer diameter, possesses good mechanical properties, and is resistant to vibration. It also has the advantages of fast response time and a small time constant. Except for the temperature-sensing element, all other parts of the armored thermoresistor can be designed in a cable-like structure, allowing it to bend freely and enabling temperature measurement in various complex structural environments. Thermistors should be the most widely used in chemical production. Its advantages: high measurement accuracy ； Good reproducibility, with long-term stability and accuracy on the right side ； Fast response time ； It does not require cold junction compensation compared to thermocouples. The disadvantage is: it is more expensive than thermocouples in Guiyang ； External power supply required ； High thermal inertia ； Avoid using in areas with mechanical vibration. A thermocouple thermometer is made by welding two conductors of different materials, A and B (thermoelectrodes) together. The hot end is inserted into the medium to be tested, while the other end is connected to a wire, thus forming a circuit. If the temperatures at the two ends are different, a thermoelectromotive force is generated in the circuit, and the difference in the function values at the two ends of this thermoelectromotive force reflects the temperature. Thermocouples play a significant role in industrial temperature measurement, and they are used extensively for remote temperature sensing in production processes. Its advantages are: small size and easy installation ； The signal can be transmitted over long distances for indication and control purposes ； Fewer response times compared to pressure-type thermometers ； It has a wide temperature measurement range, especially when it comes to measuring high temperatures ； Low price, good reproducibility, and high precision. The disadvantage is that there is a nonlinear relationship between the thermoelectromotive force and temperature ； Accuracy: Lower than that of heat capacity resistors ； Under the same conditions, thermocouple joints tend to age easily ； The cold end needs to be compensated. 7.2 Flow Meters In chemical production, it is often necessary to measure the quantity of a fluid or to determine its flow rate, and for this purpose flow meters are required. Based on the different structures of flow meters, they can be roughly classified into: rotameter flow meters ； Volumetric flow meter ； Electromagnetic flowmeter ； Differential pressure flow meter ； Turbine flow meter ； Vortex flow meter ； Mass flow meter. We will briefly describe their structure, suitable environments, and requirements for selection. A rotameter, also known as a float flow meter, is a relatively simple type of flow meter that is mainly used for measuring flow rates in medium and small diameter pipes in applications where relatively low precision requirements apply. It is generally used for diameters smaller than ф50, with a wide range; typically 10:1, with a minimum of 5:1. Low pressure loss. Its sensing element consists of a vertical conical tube that widens from bottom to top, and a float that moves up and down along the conical tube as the fluid flow rate changes. As the fluid flows upward past the float, forces such as differential pressure, fluid dynamic pressure, and friction act on the float; these forces balance the downward weight of the float. Different positions of the float in the cone tube correspond to different flow rates. The common types of rotameters include glass tube rotameters and metal tube float rotameters. The glass tube rotameter features a simple structure, low cost, and easy installation and use, making it a type of flowmeter that is widely used in production and scientific research experiments. However, due to the limitations of the glass tube material, it cannot be used in fragile or high-temperature and high-pressure environments. And it cannot be transmitted over long distances. Compared to glass tube flowmeters, metal tube flowmeters also feature resistance to high pressure and high temperatures, a robust structure, and the ability to avoid breaking. It can adapt to harsh working conditions. It can also transmit over long distances, with a standard signal output. The downside is that it’s relatively expensive. Positive displacement flowmeters are the most accurate type of flowmeters among all flowmeters. The main types include helical gear type, worm gear type, screw type (double rotor type), scraper type, piston type and vane type, drum type, and membrane type (for gas measurement). The working principle of positive displacement flow meters relies on two specially shaped measuring elements that rotate while in contact (or not in contact) with each other; taking elliptical gears as an example, these are two interlocking gears, one being the driving gear and the other the driven gear. When material enters, the driving wheel, under the effect of pressure, causes the driven wheel to operate; with each rotation of the rotor, four volumes of fluid from the crescent-shaped cavities formed by the rotor and the shell wall are discharged. By its relationship with time, the instantaneous flow rate and cumulative flow rate can be calculated. The main advantage of positive displacement flowmeters is their high measurement accuracy ； The installation of straight pipe sections has little impact on measurement accuracy ； Suitable for high-viscosity applications ； The range is relatively wide, generally between 5:1 and 10:1, with special cases reaching 30:1 or higher ； It can utilize medium dynamics, requires no external energy source, offers direct reading, and is simple and convenient. The disadvantage is that it is generally only suitable for small to medium diameters ； The price is relatively high ； Due to part deformation, it is generally not suitable for use in high and low temperatures, within the range of -30 to 160℃ ； In the measurement using positive-displacement flowmeters, it is necessary to prevent the presence of particulate impurities; therefore, a filter is usually installed before the flowmeter, although this increases the resistance in the pipeline ； And gas-liquid mixing must be prevented ； Due to the small gap, the detection element is prone to getting stuck, resulting in poor safety. An electromagnetic flowmeter uses the conductivity of fluids to measure fluid flow rates. Its feature is that when measuring fluid flow, this instrument is not affected by temperature, pressure, density, viscosity, or the composition of the fluid. Since there are no protruding parts or moving components inside the tube, it is suitable for measuring wastewater containing suspended solid particles as well as coal slurry. It is particularly suitable for measuring corrosive media. It has a wide measurement range and can measure the flow rate of fluids in both forward and reverse directions. With current technology, it is required that the conductivity of the medium under test must reach or exceed 0.01 u S/cm. Differential pressure dosimeters have a wide range of applications, among which the throttling differential pressure flow meter is the most widely used. Its working principle is as follows: the fluid filling the pipe, as it flows past the throttling element within the pipe, causes the flow stream to narrow locally at that element. At this point, the flow velocity increases while the static pressure decreases, resulting in a pressure difference before and after the throttling element. The greater the flow rate, the larger the pressure difference; thus, the flow rate can be determined based on this pressure difference. Classified by structural form, they can be divided into standard orifice plates, standard nozzle types, classic venturi tubes, venturi nozzles, 1/4 circle orifice plates, conical inlet orifice plates, annular orifice plates, eccentric orifice plates, etc. Its advantages and disadvantages are: it has a relatively simple structure, stable performance, a long service life, and is inexpensive ； A wide variety of options available, with many different types ； Repeatability and precision are average, with a narrow range, typically 3:1 to 4:1 ； The head loss is relatively high ； It has high installation requirements, such as the need for long straight pipe sections. Turbine flowmeters are the main type among vane flowmeters; they also include anemometers and water meters. Its principle is as follows: when the fluid to be measured flows through the sensor, the fluid exerts a force on the impeller, causing it to rotate; the rotation speed is proportional to the average flow velocity in the pipeline. The rotation of the impeller changes the magnetic resistance value of the magnetoelectric converter, resulting in periodic changes in the magnetic flux within the detection coil. This generates a periodic induced electromotive force, namely a pulse signal, which, after being amplified by an amplifier, is sent to a secondary instrument for display. Its advantages are: high accuracy, ranging from ±0.25% to ±0.5%R, and precision of up to ±0.15%R (for liquids) ； Good repeatability ； No zero-point drift, strong anti-interference capability ； Compact and lightweight, easy to install and maintain, with high flow capacity ； Good safety performance (even if the impeller fails and gets stuck, flow will not be interrupted) ； The disadvantage is that changes in the properties of the fluid (density, viscosity) have a significant impact on the dosimeter, requiring compensatory measures to be taken ； The instrument is greatly affected by the flow velocity distribution and swirling flow; therefore, a sufficient straight pipe section must be maintained upstream and downstream of the sensor ； High requirements are placed on the cleanliness of the medium ； Flowmeters with a diameter of DN50mm or less are highly affected by the properties of the fluid, making it difficult for them to maintain excellent performance. The working principle of the vortex flow meter involves the use of a vortex generator in the fluid, which causes regular vortices to be generated alternately on both sides of the generator. These vortices are arranged asymmetrically downstream of the vortex generator, resulting in a certain frequency. This frequency can be calculated using the formula f = St * v / (1 – 1.27d/D) * d, where St is the Strouhal number, a dimensionless value that is related to the vortex generator and the Reynolds number ； v is the flow velocity ； d is the frontal width of the occurring body ； The flow velocity can be obtained using D (the nominal diameter). Generally speaking, the output signal (frequency) of a vortex flow meter is not affected by changes in the fluid’s properties or composition; this means that the instrument coefficient depends only on the shape and size of the vortex generator as well as the Reynolds number. Its advantages are: simple and robust structure, as well as easy installation and maintenance ； Suitable for a variety of fluids, including liquids, gases, vapors, and some mixed phases ； It has high accuracy, generally around ±1%R ； Wide flow range, up to 10:1 or 20:1 or higher ； Low head loss ； No zero drift ； The price is relatively low ； The disadvantage is that it is not suitable for low Reynolds number conditions (Re < 20,000), and its use is limited in cases of high viscosity, low flow velocity, and small diameters ； High requirements are placed on the environment; areas with vibrations should be avoided as much as possible, and there needs to be a long straight pipe section on the upstream side ； The instrument coefficient is low, and it becomes lower as the diameter increases. The signal resolution decreases, so the diameter should not be too large; it is generally used for DN25~DN300mm. A mass flow meter is a new type of flow measurement instrument that can be used directly to measure the mass flow and density of a medium. It features high measurement accuracy, a wide range, good stability, and low maintenance requirements, and is widely used in the petrochemical industry. They are divided into Coriolis mass flow meters and thermal mass flow meters. The Coriolis mass flow meter is more commonly used. It is a flow meter based on the Coriolis force principle discovered by the Greeks. Inside the sensor’s housing, there is a pair of parallel measuring tubes that, under the action of electromagnetic drive coils mounted at the ends of the tubes, vibrate in a manner similar to that of a tuning fork. When a fluid flows through two parallel measuring tubes, an acceleration perpendicular to the direction of flow is generated, along with a corresponding Coriolis force. This force causes the measuring tubes to oscillate and twist, and this twisting phenomenon is known as the Coriolis effect. The degree of twist of the measuring tube is directly proportional to the mass flow rate passing through it. Mass flow meters are characterized by their high precision ； The measured mass flow rate is independent of the fluid’s temperature, pressure, viscosity, conductivity, and flow state, but the fluid must be fully filled ； It is necessary to ensure that the measuring tube is free from corrosion, wear, and scaling, as all of these can affect the accuracy of measurements ； The zero-point drift is large, and the head loss is high ； Avoid mixing liquid and gas ； Sufficiently strong brackets must be installed in front of and behind the sensor flange to prevent pipeline vibrations from causing measurement errors. There are also ultrasonic flowmeters, among others. 7.3 Pressure measuring instruments In chemical production, pressure is often one of the important operational parameters. Measurements of pressure and vacuum are encountered frequently. Instruments for measuring pressure can generally be divided into four categories: liquid-column pressure gauges, elastic pressure gauges, pressure (differential pressure) transmitters, and piston pressure gauges. A liquid-column pressure gauge converts the pressure to be measured into a liquid column height for measurement, based on the principles of statics. Based on their structural forms, there are U-tube manometers, single-tube manometers, and inclined-tube manometers, etc. It is characterized by its simple structure, ease of use, and low cost. The disadvantages are: large size, inconvenient reading, fragile glass tubes, and low precision. It is only suitable for measuring low or micro pressures, where the pressure difference and negative pressure are not significant, the requirements are modest, and the environment is not complex. The working fluid for liquid-column pressure gauges is usually mercury, water, or alcohol. Elastic pressure gauge: It converts the pressure to be measured into a displacement resulting from the deformation of an elastic element for measurement. Based on the different elastic elements, they can be classified into diaphragm tube type, bellows type, diaphragm type, corrugated tube type, and leaf spring type. It is characterized by its simple structure, ease of use, and low cost. By adding additional components such as recording mechanisms, control elements, and electrical conversion devices, it can be used to create pressure recorders, electric contact pressure gauges, pressure control alarms, and more. It is mainly used to measure vacuum level and pressure, and it can provide on-site indication, enable remote centralized control, record data, issue alarms, or send signals. Crystalline and corrosive media can also be measured using diaphragm or membrane structures. Pressure sensors convert the pressure to be measured into various electrical signals for measurement. Based on their working principle, they can be classified into resistive, capacitive, piezoresistive, inductive, piezoelectric, Hall effect, etc. The output signal can be resistance, current, voltage, or frequency, depending on the form. Due to its fast response time, strong resistance to environmental interference, corrosion resistance, high precision, and wide range of pressure and vacuum levels, it has become an important type of pressure measurement instrument. It is mainly used in remote transmission rooms for pressure signals, as well as for sending signals or centralized control. When used in combination with display, regulation, and recording instruments, it can form a self-regulating system. It is widely used in industrial production automation and the aviation industry. The most commonly used type is the capacitive sensor: it measures pressure by detecting changes in capacitance. When the pressure or pressure difference being measured moves the plates, the distance between them changes, which in turn alters the capacitance. An amplification circuit then converts this change into current, voltage, or frequency signals for output. A piston-type pressure gauge measures the pressure by converting it into the mass of the counterweight applied to the piston. Based on their design, they can be divided into single-piston and double-piston types. It is characterized by high measurement accuracy, but this accuracy is affected by temperature and gravitational acceleration; calibration is required during use, and its structure is relatively complex. It is generally used as a standard pressure measuring instrument to calibrate other types of pressure gauges. 7.4 Level Instruments In chemical production, it is often necessary to measure the height of various interfaces, a process known as level measurement. Generally, based on their working principle, they can be classified into direct-reading type, differential pressure type, buoyancy type, electrical type, acoustic type, nuclear radiation type, etc. 　　　　　A direct-reading glass level gauge is the simplest and most economical type of level gauge for measuring liquid levels, based on the principle of a communicating vessel. Glass level gauges require no external power source and are safe and explosion-proof. There are various types of glass level gauges, with the most typical ones being glass tube level gauges and glass plate level gauges. Float level gauge: It consists of a float made of metal or another material (usually copper or stainless steel); as the liquid level rises or falls, the float moves up and down. Through a pivot point and based on the principle of levers, the pointer at the other end also rotates, thereby indicating the corresponding value. It can also be converted into pneumatic or electrical signals using certain devices for remote transmission. It is used to directly indicate the level of various open or pressurized liquids. Compared with glass tube level gauges, it has the advantages of being resistant to breakage and providing clear readings, and is especially suitable for liquid media that are sticky and can adhere to the walls of glass tubes. The disadvantage is that the accuracy grade is not high, usually at level 2.5 ； Moreover, its range is limited, which imposes constraints on measuring heights and liquid levels. The main types include float-type (including buoy-type and float-level gauges), barrel-type level gauges, magnetic flap-type level gauges, etc. A differential pressure level gauge measures liquid level by utilizing differential pressure; it operates on the principle that the static pressure of a liquid column is proportional to the height of that column. The main types include pressure-type level gauges, differential pressure type level (or) interface gauges, and blow-out pressure level gauges. Differential pressure level transmitters are a widely used method in the petrochemical industry. For corrosive or viscous media, flanged differential pressure transmitters (which can be equipped with capillaries) can be used for measurement. Capacitive level gauges are used to measure the level of non-conductive fluids. It consists of two coaxial metal sleeves that are insulated from each other and fixed in place, forming a coaxial cylindrical capacitor. When the liquid level is zero, the two electrodes of the sensor form a capacitor, and the capacitance at this point is 0. As the liquid level rises, the space between some of the electrodes is filled with the medium being measured, so the change in capacitance is proportional to the liquid level. By detecting this change in capacitance, it is possible to determine the liquid level. Others include ultrasonic level gauges, fork-type level gauges, floating magnetic level gauges, magnetostrictive level gauges, radio frequency admittance level gauges, radar level gauges, and radioactive level gauges, among others. 7.5 Control Valves: Regulating valves receive signals from regulators in order to adjust the flow rate of the medium being controlled, thereby keeping the parameter in question within the desired range. A control valve consists of an actuator and a control mechanism. In the production process, the most commonly used actuators are pneumatic actuators and electric actuators. Under normal circumstances, to meet explosion-proof requirements, it is advisable to use pneumatic actuators. A pneumatic actuator is a driving device powered by compressed air, with a linear displacement of its output rod; if a lever mechanism such as a crank is used, it can be converted into angular displacement. Pneumatic actuators can be classified into diaphragm type, piston type, and long-stroke type based on their structure. A regulating mechanism is a device that directly changes the flow rate of the regulating medium, and it consists of a valve body, valve core, valve seat, valve stem, and packing. Based on the different structural designs of the valve body, they can be classified into single-seat valves, double-seat valves, angle valves, three-way valves, eccentric rotary valves, butterfly valves, ball valves, quick-cut valves, diaphragm valves, valve-body-separated valves, low-noise valves, bellows-sealed valves, low-temperature control valves, and plug valves. Valve positioner: Its function is to receive the output signal from the regulator and use this signal to control the pneumatic actuator. Once the pneumatic actuator moves, the displacement of the valve stem is fed back to the valve positioner through a mechanical mechanism; thus, the positioner and the actuator form a closed loop. 7.6 Development of automation solutions – Open-loop control systems: A system is considered an open-loop control system whenever the system’s output signal has no influence on the control process. Closed-loop control system: Any system in which the system’s output signal has a direct impact on the control action is called a closed-loop control system. In chemical production, determining what the most appropriate approach is is of utmost importance. First, it is necessary to understand the contents of the process, become familiar with its parameters, and determine the relationships between these parameters. It is also important to take into account other factors as well as the current practical conditions, such as the climate and hydrological conditions in the area, as well as the level of automation that should be achieved. Automation replaces only those tasks that are too demanding or require too high a level of precision and complexity for humans to handle, but it cannot accomplish tasks that are simply impossible to complete. For example, in terms of temperature increase, if the saturated steam pressure resulting from heating is only 2 kgf/cm2, then even if the heating valve is opened to 100%, the temperature cannot rise to 150°C. Since steam at a pressure of 2 kilograms has a temperature of only 120°C, and at least 4.85 kgf/cm2 is required to reach 150°C, this is an impossible task for valves. Therefore, automation is merely a tool for chemical production; it cannot change the chemical reactions themselves. Its role is merely to reduce labor intensity, improve product precision, and simplify operational steps, thereby increasing labor productivity and reducing production costs. In the production process, various factors, different situations, and various details related to production must be taken into account. This requires a thorough understanding of the production process, as well as a good knowledge of chemical parameters and solid fundamental skills. One cannot rely solely on knowledge of chemistry or merely on an understanding of automation; without integrating the two, efforts are often wasted and counterproductive. When determining a scheme, it is necessary to know the properties of the medium, such as density, state, boiling point, specific heat capacity, viscosity, saturated vapor pressure, etc ； It is also necessary to know the properties of the entire chemical reaction process, such as the reaction temperature, reaction time, and the amount of heat absorbed or released ； It is also necessary to know the pipe diameter, material, pressure rating, and temperature rating, as well as the temperature and pressure of the water (or chilled water) available in the utility systems, the temperature and pressure of steam, and the direction of flow and the temperature and pressure conditions of the fluid within the pipes ； It is also necessary to be aware of the performance of certain instruments, such as head loss and the impact of the environment on them. 8 Common Concepts in Chemical Engineering Design 8.1 Re (Reynolds number) is a dimensionless parameter used to compare the flow conditions of fluids. In 1883, the British scientist Reynolds proposed that when a viscous fluid flows around objects with similar geometric shapes, as long as vlρ/η is the same, the flow patterns of the fluids will be similar. V represents the flow velocity of the fluid, l represents the linear velocity of the object (such as the diameter of a circular pipe), ρ represents the density of the fluid, and η represents the viscosity of the fluid. The value of the Reynolds number serves as a basis for indicating the flow state of a fluid. It can generally be divided into laminar flow, transitional flow, and turbulent flow. Laminar flow is also known as stagnant flow. It refers to the orderly flow of a particle’s path of motion; in a circular tube, flow is laminar when the Reynolds number of the fluid is less than 2300. Turbulence, also known as turbulent flow, refers to a flow in which the paths of moving particles are extremely chaotic and the streamlines change very easily. In a circular pipe, the fluid is in a turbulent state when its Reynolds number is greater than 10,000. Between turbulence and laminar flow, it is transitional flow. 8.2 Viscosity Viscosity is an umbrella term for dynamic viscosity, kinematic viscosity, and relative viscosity. We often refer to dynamic viscosity as viscosity. It is defined as η=τ/D. where τ is the shear stress ； D is the velocity gradient in the direction perpendicular to the stratosphere. That is, the shear stress of fluid flow divided by the velocity gradient in the flow direction. Among them, the SI unit is pascal·second, while millipascal·second is commonly used in chemical engineering. The centimeter-gram-second system units used in the past were pascal and centipascal. Kinematic viscosity is defined as υ=η/ρ, where η is dynamic viscosity ； ρ is its density. That is, dynamic viscosity divided by the fluid’s density. The SI unit is meters squared per second. Chemical plants commonly use millimeters per second. The definition of relative viscosity is: the ratio of the dynamic viscosity of a fluid to that of water at the same temperature. It is a column of dimensionless quantities. Sometimes it also refers to the ratio of the dynamic viscosity of a polymer solution to that of the pure solvent at the same temperature. 1 Pa•s (pascal-second) = 10 P (dyne-second/cm2 = poise) = 1000 cP (centipoise). 1 Newton = 105 dynes. 1 centipoise = centistokes × density (specific gravity). 1 stoke = 100 centistokes = 10-4 m2/s. 1 stoke = 1 cm2/second = 100 mm2/second. 8.3 Calculation of flow resistance in fluid transport: Based on the law of conservation of mechanical energy for incompressible fluids. The head delivered by the pump should be the same as the head loss in the piping. We mainly calculate the resistance of the fluid. Generally, the resistance is divided into two parts: one is the resistance within straight circular pipes, and the other is the local resistance caused by fittings, valves, etc. ΣR=ΣRr+ΣR1={λ(l+Σle)+Σż}u²/2d. ΣR represents the local resistance; le is the equivalent length of the local resistance; l is the length of the pipeline; Σż is the sum of the resistance coefficients for various local resistances; λ is the friction coefficient; d is the diameter of the pipe. To calculate the resistance of a circular pipe, it is first necessary to determine the friction coefficient λ, for which the Reynolds number must be calculated. Once the Reynolds number is known, the friction coefficient can be found by referring to a table that relates the friction coefficient, the wall roughness ε, and the Reynolds number. To calculate the local resistance, the equivalent length method can be used; that is, the local resistance is equivalent to the resistance of a straight pipe of a certain length having the same diameter as the pipe in question, and this length of straight pipe constitutes the equivalent length. In this way, the formula for calculating the resistance of straight pipes can be used to determine the local resistance; that is, when calculating the resistance of a pipeline, the length of the straight sections in the pipeline along with the equivalent lengths of fittings, valves, etc., are combined to form the total length of the pipeline. Local resistance can also be expressed as the product of the drag coefficient and kinetic energy. 8.4 Saturation vapor pressure refers to the pressure of the vapor that is in equilibrium with a liquid or solid at a certain temperature. If the liquid is water, it is called vapor pressure. For example, at 20°C, the vapor pressure of water is 2.4 KPA (17.7 mmHg) ； The vapor pressure of ethanol is 5.9 KPA (43.9 mmHg). The saturated vapor pressure at a certain temperature also represents the boiling point at that temperature. When two or more liquids or solids are mixed, and the proportions of the mixture vary, its saturated vapor pressure also changes. When a liquid contains non-volatile solutes, its vapor pressure is lower at the same temperature than that of the pure solvent; therefore, at the same pressure, the boiling point of the solution is higher than that of the pure solvent. When the temperature of the heating vapor is constant, the heat transfer temperature difference during the evaporation of a solution is higher than the boiling point of the pure solvent.