Heating area covered by the boiler
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What is the heating area for each, a 1-ton hot water boiler and a 1-ton steam boiler? Which one is taller? Why?
This post was last edited by zgj2405 on 2011-2-13 08:56. Basics of heating: What are the basic properties of water and water vapor? Answer: The basic physical properties of water and water vapor include specific gravity, specific volume, latent heat of vaporization, specific heat, viscosity, temperature, pressure, enthalpy, entropy, etc. The specific gravity of water is approximately 1 (t/m3, kg/dm3, g/cm3). The specific volume of steam is the reciprocal of its specific gravity, and it is determined by pressure and temperature. The latent heat of vaporization of water refers to the amount of heat absorbed when water turns into steam under saturated conditions at a certain pressure or temperature, or the amount of heat released when steam turns back into water; its unit is KJ/Kg. The specific heat of water refers to the amount of heat absorbed per unit mass of water for every 1°C increase in temperature, and its unit is KJ/Kg·°C; it is typically taken as 4.18 KJ. The concept of specific heat for water vapor is the same as that for water, but it is not a constant; it depends on temperature and pressure. 2. How is the output of a hot water boiler expressed? Answer: There are three ways to express the output of hot water boilers: kilocalories per hour (Kcal/h), tons per hour (t/h), and megawatts (MW). (1) Kilocalories per hour is a unit in the metric system; it indicates the amount of heat generated by a hot water boiler per hour. - h) j, F4 |8 q (2) “Ton” or “steam ton” is a colloquial term borrowed from steam boilers. It refers to the amount of heat that a hot water boiler can supply per hour; this amount is equivalent to the heat required to heat a certain mass of water (usually expressed in tons) from 20°C and completely convert it into steam. (3) Megawatt (MW) is a unit of power in the International System of Units, with the base unit being W (1 MW = 106 W). This expression should be used in official documents. The conversion relationships among the three units of measurement are as follows: 600,000 kilocalories/hour (60×10⁴ Kcal/h) ≈ 1 steam ton/hour [1 t/h] ≈ 0.7 MW. 3. What is the heat consumption index? How should it be stipulated? Answer: The heat consumption per unit area of building space is generally referred to as the heat consumption index, or simply the heat index; it is expressed in units of W/m2 and is usually denoted by qn. It represents the amount of heat required to heat each square meter of floor area. The heating load indicators for various types of buildings in the Yellow River Basin can be referred to in Table 2-1.
| Building Type | Non-energy-efficient Buildings | Energy-efficient Buildings |
|---------------|-------------------------------|----------------------------|
| Residential buildings | 56–64 | 38–48 |
| Residential complexes | 56–64 | 38–48 |
| Schools or other educational institutions | 60–80 | 50–70 |
| Office buildings | 60–80 | 55–70 |
| Hotels | 60–70 | 50–60 |
| Canteens and restaurants | 115–140 | 100–130 |
The figures in the table are merely approximate values. They vary depending on factors such as building structure, materials used, orientation, air leakage rate, and geographical location. In regions with higher latitudes, the heating load indicators tend to be higher. 4. How to determine the circulating water volume? How to determine the relationship between steam volume, heat, and area? Answer: For hot water heating systems, the circulation water flow rate is calculated using the following formula: G = ×3600 = 0.86Q/(tg-th). In this formula, G represents the calculated water flow rate in kg/h; Q is the designed heat load of the heat user; c is the specific heat capacity of water, namely 4187 J/kg·°C; tg and th are the designed supply and return water temperatures in °C. As a general rule, an estimated value of 2–2.5 kg/h per square meter of floor area is used. For steam-driven heat exchange units, since the temperature difference between the supply and return water is designed to be 20°C, the water flow rate is typically set at 2.5 kg/h. The steam consumption of a heating system can be calculated using the following formula:
G = 3.6Q/r + Δh
Where:
G – Designed steam flow rate, kg/h
Q – Thermal load of the heating system
r – Latent heat of vaporization of steam, kJ/kg
Δh – Enthalpy difference between saturated condensate and condensate at the discharge state, kJ/kg
When making heating estimates for the Qingdao area, it is generally assumed that each ton of superheated steam can be used to heat 12,000 square meters of buildings. How is the flow rate of the system determined? How is the pipe diameter determined? Answer: The maximum flow velocity of steam in pipes can be selected according to the following table: Unit: (m/s). Steam properties: Superheated steam, Saturated steam. For nominal diameters >200, the value is 80; for nominal diameters ≤200, it is 35. The diameter of steam pipes should be determined by referring to tables and calculations based on factors such as flow rate, allowable flow velocity, pressure, temperature, and allowable pressure drop. 6. How is the flow rate in the water system determined? How is the pipe diameter determined? Answer: Generally, the flow velocity of circulating water is between 0.5 and 3. The finer the pipe diameter and the longer the pipe length, the greater the resistance, which requires a lower flow velocity. To avoid hydraulic imbalances, the flow velocity is generally kept low, or in other words, the pipe diameter is chosen to be larger; refer to the table below: Pipe Diameter DN20 DN25 DN32 DN50 DN80 Flow Velocity (m/s) 0.3 0.5 0.6 0.82 0.9 When selecting the diameter of the main pipeline, it is necessary to take into account future load growth plans. 7. How is air removed from the water system? What are the hazards? Answer: The air in water systems is generally allowed to flow away through a certain slope provided in the piping, with an exhaust valve installed at the highest point to release it. Exhaust valves come in manual and automatic types. The slope of the pipes is 0.003 for upward flow and 0.005 for downward flow. If the air in the pipes is not removed, air locks will form, hindering circulation and affecting heating. It can also cause corrosion to the pipelines. Air entering the steam heater can disrupt its normal operation; in severe cases, it may cause accidents. 8. How are the system’s water loss rate and water replenishment rate determined? What are the common causes of water loss? Answer: According to the \"Code for Design of Urban Heating Networks\", the flow rate of the make-up water device in a closed heating network should be 2% of the circulation flow rate of the heating system, while the flow rate of water supplied in case of emergencies should be 4% of the heating circulation flow rate. Reason for water loss: Poor sealing of pipes and heating systems, resulting in leaks in the system ； Draining water for system maintenance ； Accidental water leakage ； Users steal water ； System pressure relief, etc. 9. How many types of constant pressure methods are there for water systems? How is constant pressure achieved in each case? What is the typical constant pressure for a system? Answer: Common methods for maintaining pressure in hot water heating systems include pressure control using an expansion tank, ordinary make-up water pumps, gas pressure tanks, steam for pressure regulation, variable frequency control of make-up water pumps for pressure maintenance, as well as using stable tap water pressure for such purposes – there are various ways to achieve pressure regulation through make-up water. Hot water systems utilizing hybrid heaters should adopt an overflow pressure stabilization method. (1) Pressure maintenance using an expansion tank: A tank is installed at a height of 2-3 meters above the highest point in the heating system; maintaining a constant pressure at this point is referred to as pressure maintenance using an expansion tank. Its advantages are stable pressure and resistance to power outages ； The disadvantage is that the height of the water tank is limited; when the highest building has many floors and is located far from heat sources, or when heating high-temperature water is required, it becomes difficult to achieve the necessary installation height for the expansion tank. (2) Constant pressure control by the regular make-up water pump: The method of using the make-up water pump in the heating system to continuously supply water in order to maintain a constant pressure at a specific level is known as constant pressure control by the make-up water pump. The advantage of this method is its simple equipment, low investment, and ease of operation. The disadvantages are fear of power outages and electricity waste. (3) Pressure maintenance in gas pressure tanks: Gas pressure maintenance includes nitrogen pressure maintenance and air pressure maintenance; both methods rely on the combined action of a low-pressure tank and a make-up water pump to maintain a constant pressure in the heating system. Nitrogen pressurization involves filling a pressure tank with nitrogen. Air pressure regulation involves filling with air; to prevent the air from dissolving in water and causing corrosion of the pipes, bladders are often installed in the air pressure regulation tank to separate the air from the water. The advantages of a gas constant-pressure heating system are: safe and reliable operation, as well as the ability to effectively prevent vaporization and water hammer in the system ； Its disadvantages are: the equipment is complex, relatively large in size, and quite expensive; it is mostly used in high-temperature water systems. (4) Steam constant pressure: Steam constant pressure is maintained by the pressure in the steam space of the boiler’s drum. For two or more boilers, a steam constant-pressure system with an external expansion tank can also be used. Additionally, the water spray heater and the steam-driven heater produced by our company can also be considered a form of steam constant pressure control. The advantages of steam constant-pressure operation are: a simple system, low investment, and economical operation. Its disadvantage is that the steam pressure used for pressure control depends on the combustion conditions of the boiler, resulting in large pressure fluctuations; if not managed properly, steam entering the water side can cause water hammer. (5) Variable-frequency speed control and constant-pressure operation for make-up water pumps: The basic principle is to adjust the power supply frequency in response to changes in pressure within the heating system, thereby adjusting the speed of the make-up water pumps in a smooth, stepless manner and ensuring timely regulation of the amount of make-up water supplied, so as to maintain a constant pressure at the constant-pressure point of the system. The advantages of this method are: energy savings and easy adjustment of control pressure. The disadvantages are: high investment costs and fear of power outages. (6) Constant pressure of tap water: During heating, the pressure of tap water meets the required constant pressure value for the heating system and remains stable. Tap water can be directly connected to the return pipe of the heating system to replenish water and maintain pressure. The advantages of this method are obvious: it’s simple, and requires minimal investment and operating costs ； Its disadvantages are: a narrow range of applicability, and direct heating without water treatment can cause scaling in the heating system. (7) The pressure control methods for water overflow include pressure control via a pressure control valve, pressure control through an overhead water tank, and pressure control using an inverted U-tube. ; e, [$ P# K3 a4 P During operation, the highest point of the system must be filled with water and have a certain head reserve, usually around 4 meters. Since most systems operate with flow from top to bottom, and the resistance during the forward flow is much smaller than that during the return flow, the head pressure at the highest point is higher during operation than when the system is at rest. Therefore, the static pressure value can be set a bit lower; generally, 1 to 4 m is appropriate. Minimizing the constant-pressure value is intended to make full use of the work potential of steam. 10. How to control the supply and return water temperatures during operation? What is the typical temperature difference between supply and return water in heating systems in our country? Answer: The regulations followed in China’s heating design are: a supply water temperature of 95°C, a return water temperature of 70°C, with a temperature difference of 25°C. However, in recent years, drawing on advanced experiences in heating systems both domestically and internationally, there has been a trend toward lower supply and return water temperatures as well as smaller temperature differences. The designed supply and return water temperatures are now 80/60°C, with a temperature difference of 20°C. 11. What is specific friction? What value is the skin friction coefficient usually chosen as? What is the general range of the total resistance in water systems? Of these, how many are inside and outside the station respectively? Answer: The resistance along a unit length is called the specific friction resistance. Under normal circumstances, the value for main lines is 30–70 Pa/m; for branch lines, it should be selected based on the allowable pressure drop, typically ranging from 60–120 Pa/m, and should not exceed 300 Pa/m. Generally, in a heating system with a heating area of 50,000 m², the total resistance of the heating system is 20–25 meters of water column. Of this, the resistance of the user-side system is 2–4 meters, that of the external network system is 4–8 meters of water column, and that of the piping system at the heat exchange station is 8–15 meters of water column. 12. What are the different forms of heat exchange? What is the heat transfer coefficient? What is the main heat exchange mechanism of a surface-type heat exchanger? Answer: There are three forms of heat exchange (or heat transfer): conduction, convection, and radiation. For counter-flow heat exchangers, the main forms of heat transfer are convection and conduction. The formula for calculating heat transfer by convection is: Q=αA(t2-t1), while the formula for heat transfer by conduction is: Q=（λ/δ）A(t2-t1). Convective heat transfer occurs on both sides of the heat transfer elements in plate heat exchangers, while conductive heat transfer takes place within the elements themselves. 13. What are the types of surface heat exchangers? What are its principles, advantages, and disadvantages? Answer: The main types of surface heat exchangers include: shell-and-tube heat exchangers, plate heat exchangers, heat pipe heat exchangers, etc. It can be divided into many forms, all of which share the same drawbacks: large size, high space requirement, high investment costs, low heat exchange efficiency (compared to hybrid types), and short lifespan ； Their advantage is that the quality of the condensate water remains less polluted, and it is easy to recover. 14. What are the disadvantages of ordinary mixed-type heat exchangers? Answer: A conventional mixed-type heat exchanger has steam entering from its side, and the water circulation is driven entirely by electricity. Although it offers advantages such as small size and high thermal efficiency, it has the following disadvantages: 1. It does not save electricity; the circulation pump must always be in operation under any circumstances ； / U. p0 G3 t# J$ z 2. Unstable: severe vibration and noise occur when the steam inlet pressure is low or the water inlet pressure is high ； 3. Similarly, there is also the problem of difficulty in recovering condensate water. 15. What types of valves are commonly used in heating systems, and what are their respective properties? Answer: The valves commonly used in heating systems include: globe valves, gate valves (or plug valves), butterfly valves, ball valves, check valves, safety valves, pressure reducing valves, pressure stabilizing valves, balance valves, control valves, as well as various self-acting control valves and electric control valves. Among them:
Gate valve: Used to stop the flow of the medium; it has a certain degree of regulation capability, but experiences high pressure loss. It is commonly used in heating systems to stop the flow of steam. In valve specifications, “J” is used to denote a gate valve.
Globe valve: Also used to stop the flow of the medium; when the valve is fully open, the medium can pass through just like in a regular pipe, without any need to change the direction of flow, which results in lower pressure loss. Gate valves have poor regulating performance, and the letter “Z” is used in valve codes to indicate gate valves. Check valve: Also known as a non-return valve or one-way valve, it allows fluid to flow in only one direction; if the pressure behind the valve is higher than that in front of it, the check valve will close automatically. There are various types of check valves, mainly including lift-type and swing-type ones. The lift-type valve body resembles a globe valve; it has high pressure loss, which is why it is less commonly used in modern heat exchange station systems. It is indicated by “H” in the valve model. Butterfly valve: It achieves regulation and opening/closing by changing the angle of the valve disc; since the valve disc is always situated in the flowing medium, significant resistance is generated, which is why it is less commonly used. It is indicated by “D” in the valve model. Safety valve: Primarily used to release pressure when the medium is under overpressure, thereby protecting equipment and systems. In some cases, modified slightly-open type pressure relief valves can be used as system pressure control valves. There are many structural types of safety valves, which are indicated by the letter “Y” in the valve model. 16. What is the function of a dirt remover? Where is it usually installed in the system? Answer: The function of a strainer is to remove debris from the water system. The station internal dirt removers are generally large in size, and are installed before the steam-driven heaters or on the return water pipes to prevent debris from entering the heaters. The strainers located at the entrance wells outside the building are generally small in size; they are installed on the water supply pipes. Some systems have them, while others do not. Their purpose is to prevent debris from entering the users’ radiators. The new generation of gas-fired heaters are equipped with a dirt remover 17. Sometimes it is observed that some users’ radiators get hot while others do not – why is that? How to solve it? Answer: This is known as hydraulic imbalance in the system. The causes are quite complex; generally, they include the following: (1) Unreasonable pipe diameter design, with certain sections having pipes that are too narrow ； (2) Some components present excessive resistance, such as valves that cannot be fully opened ； (3) There are debris blocking the system. (4) The air in the system cannot be completely removed due to reasons such as an incorrect slope direction of the pipes ； (5) Significant water loss from the system ； (6) The system pressure is set too low, resulting in operation without sufficient water ； (7) The flow rate and head of the circulating water pump are insufficient ； To resolve system imbalances, it’s necessary to first identify the causes and then take appropriate measures. What are the advantages and disadvantages of steam heating and hydronic heating? Answer: Although steam heating systems have the advantage of lower initial investment, they result in excessive energy waste. According to estimates by authoritative agencies, steam heating consumes about 30% more energy than water heating; as a result, this heating method is gradually being phased out in recent years. The waste of energy in steam heating is mainly manifested in: (1) The quality of domestic steam traps is substandard; they have a short service life and poor performance, resulting in the simultaneous discharge of steam and water ； (2) The piping system has high heat dissipation; in addition to high operating temperatures, damaged insulation and lack of timely maintenance are also contributing factors ； (3) The system has severe leakage; for the same leakage area, the heat carried away by steam is much greater than that carried away by water. In addition to being uneconomical, steam heating is also unsafe, as it can lead to burns and pipe bursts caused by water hammer. Many systems generate vibration and water hammer noises during operation, which affect people’s work and rest. Additionally, the air in rooms heated by steam is dry, which is uncomfortable. Although the investment in the plumbing system increased appropriately, it overcame the aforementioned drawbacks.
| Building Type | Non-energy-efficient Buildings | Energy-efficient Buildings |
|---------------|-------------------------------|----------------------------|
| Residential buildings | 56–64 | 38–48 |
| Residential complexes | 56–64 | 38–48 |
| Schools or other educational institutions | 60–80 | 50–70 |
| Office buildings | 60–80 | 55–70 |
| Hotels | 60–70 | 50–60 |
| Canteens and restaurants | 115–140 | 100–130 |
The figures in the table are merely approximate values. They vary depending on factors such as building structure, materials used, orientation, air leakage rate, and geographical location. In regions with higher latitudes, the heating load indicators tend to be higher. 4. How to determine the circulating water volume? How to determine the relationship between steam volume, heat, and area? Answer: For hot water heating systems, the circulation water flow rate is calculated using the following formula: G = ×3600 = 0.86Q/(tg-th). In this formula, G represents the calculated water flow rate in kg/h; Q is the designed heat load of the heat user; c is the specific heat capacity of water, namely 4187 J/kg·°C; tg and th are the designed supply and return water temperatures in °C. As a general rule, an estimated value of 2–2.5 kg/h per square meter of floor area is used. For steam-driven heat exchange units, since the temperature difference between the supply and return water is designed to be 20°C, the water flow rate is typically set at 2.5 kg/h. The steam consumption of a heating system can be calculated using the following formula:
G = 3.6Q/r + Δh
Where:
G – Designed steam flow rate, kg/h
Q – Thermal load of the heating system
r – Latent heat of vaporization of steam, kJ/kg
Δh – Enthalpy difference between saturated condensate and condensate at the discharge state, kJ/kg
When making heating estimates for the Qingdao area, it is generally assumed that each ton of superheated steam can be used to heat 12,000 square meters of buildings. How is the flow rate of the system determined? How is the pipe diameter determined? Answer: The maximum flow velocity of steam in pipes can be selected according to the following table: Unit: (m/s). Steam properties: Superheated steam, Saturated steam. For nominal diameters >200, the value is 80; for nominal diameters ≤200, it is 35. The diameter of steam pipes should be determined by referring to tables and calculations based on factors such as flow rate, allowable flow velocity, pressure, temperature, and allowable pressure drop. 6. How is the flow rate in the water system determined? How is the pipe diameter determined? Answer: Generally, the flow velocity of circulating water is between 0.5 and 3. The finer the pipe diameter and the longer the pipe length, the greater the resistance, which requires a lower flow velocity. To avoid hydraulic imbalances, the flow velocity is generally kept low, or in other words, the pipe diameter is chosen to be larger; refer to the table below: Pipe Diameter DN20 DN25 DN32 DN50 DN80 Flow Velocity (m/s) 0.3 0.5 0.6 0.82 0.9 When selecting the diameter of the main pipeline, it is necessary to take into account future load growth plans. 7. How is air removed from the water system? What are the hazards? Answer: The air in water systems is generally allowed to flow away through a certain slope provided in the piping, with an exhaust valve installed at the highest point to release it. Exhaust valves come in manual and automatic types. The slope of the pipes is 0.003 for upward flow and 0.005 for downward flow. If the air in the pipes is not removed, air locks will form, hindering circulation and affecting heating. It can also cause corrosion to the pipelines. Air entering the steam heater can disrupt its normal operation; in severe cases, it may cause accidents. 8. How are the system’s water loss rate and water replenishment rate determined? What are the common causes of water loss? Answer: According to the \"Code for Design of Urban Heating Networks\", the flow rate of the make-up water device in a closed heating network should be 2% of the circulation flow rate of the heating system, while the flow rate of water supplied in case of emergencies should be 4% of the heating circulation flow rate. Reason for water loss: Poor sealing of pipes and heating systems, resulting in leaks in the system ； Draining water for system maintenance ； Accidental water leakage ； Users steal water ； System pressure relief, etc. 9. How many types of constant pressure methods are there for water systems? How is constant pressure achieved in each case? What is the typical constant pressure for a system? Answer: Common methods for maintaining pressure in hot water heating systems include pressure control using an expansion tank, ordinary make-up water pumps, gas pressure tanks, steam for pressure regulation, variable frequency control of make-up water pumps for pressure maintenance, as well as using stable tap water pressure for such purposes – there are various ways to achieve pressure regulation through make-up water. Hot water systems utilizing hybrid heaters should adopt an overflow pressure stabilization method. (1) Pressure maintenance using an expansion tank: A tank is installed at a height of 2-3 meters above the highest point in the heating system; maintaining a constant pressure at this point is referred to as pressure maintenance using an expansion tank. Its advantages are stable pressure and resistance to power outages ； The disadvantage is that the height of the water tank is limited; when the highest building has many floors and is located far from heat sources, or when heating high-temperature water is required, it becomes difficult to achieve the necessary installation height for the expansion tank. (2) Constant pressure control by the regular make-up water pump: The method of using the make-up water pump in the heating system to continuously supply water in order to maintain a constant pressure at a specific level is known as constant pressure control by the make-up water pump. The advantage of this method is its simple equipment, low investment, and ease of operation. The disadvantages are fear of power outages and electricity waste. (3) Pressure maintenance in gas pressure tanks: Gas pressure maintenance includes nitrogen pressure maintenance and air pressure maintenance; both methods rely on the combined action of a low-pressure tank and a make-up water pump to maintain a constant pressure in the heating system. Nitrogen pressurization involves filling a pressure tank with nitrogen. Air pressure regulation involves filling with air; to prevent the air from dissolving in water and causing corrosion of the pipes, bladders are often installed in the air pressure regulation tank to separate the air from the water. The advantages of a gas constant-pressure heating system are: safe and reliable operation, as well as the ability to effectively prevent vaporization and water hammer in the system ； Its disadvantages are: the equipment is complex, relatively large in size, and quite expensive; it is mostly used in high-temperature water systems. (4) Steam constant pressure: Steam constant pressure is maintained by the pressure in the steam space of the boiler’s drum. For two or more boilers, a steam constant-pressure system with an external expansion tank can also be used. Additionally, the water spray heater and the steam-driven heater produced by our company can also be considered a form of steam constant pressure control. The advantages of steam constant-pressure operation are: a simple system, low investment, and economical operation. Its disadvantage is that the steam pressure used for pressure control depends on the combustion conditions of the boiler, resulting in large pressure fluctuations; if not managed properly, steam entering the water side can cause water hammer. (5) Variable-frequency speed control and constant-pressure operation for make-up water pumps: The basic principle is to adjust the power supply frequency in response to changes in pressure within the heating system, thereby adjusting the speed of the make-up water pumps in a smooth, stepless manner and ensuring timely regulation of the amount of make-up water supplied, so as to maintain a constant pressure at the constant-pressure point of the system. The advantages of this method are: energy savings and easy adjustment of control pressure. The disadvantages are: high investment costs and fear of power outages. (6) Constant pressure of tap water: During heating, the pressure of tap water meets the required constant pressure value for the heating system and remains stable. Tap water can be directly connected to the return pipe of the heating system to replenish water and maintain pressure. The advantages of this method are obvious: it’s simple, and requires minimal investment and operating costs ； Its disadvantages are: a narrow range of applicability, and direct heating without water treatment can cause scaling in the heating system. (7) The pressure control methods for water overflow include pressure control via a pressure control valve, pressure control through an overhead water tank, and pressure control using an inverted U-tube. ; e, [$ P# K3 a4 P During operation, the highest point of the system must be filled with water and have a certain head reserve, usually around 4 meters. Since most systems operate with flow from top to bottom, and the resistance during the forward flow is much smaller than that during the return flow, the head pressure at the highest point is higher during operation than when the system is at rest. Therefore, the static pressure value can be set a bit lower; generally, 1 to 4 m is appropriate. Minimizing the constant-pressure value is intended to make full use of the work potential of steam. 10. How to control the supply and return water temperatures during operation? What is the typical temperature difference between supply and return water in heating systems in our country? Answer: The regulations followed in China’s heating design are: a supply water temperature of 95°C, a return water temperature of 70°C, with a temperature difference of 25°C. However, in recent years, drawing on advanced experiences in heating systems both domestically and internationally, there has been a trend toward lower supply and return water temperatures as well as smaller temperature differences. The designed supply and return water temperatures are now 80/60°C, with a temperature difference of 20°C. 11. What is specific friction? What value is the skin friction coefficient usually chosen as? What is the general range of the total resistance in water systems? Of these, how many are inside and outside the station respectively? Answer: The resistance along a unit length is called the specific friction resistance. Under normal circumstances, the value for main lines is 30–70 Pa/m; for branch lines, it should be selected based on the allowable pressure drop, typically ranging from 60–120 Pa/m, and should not exceed 300 Pa/m. Generally, in a heating system with a heating area of 50,000 m², the total resistance of the heating system is 20–25 meters of water column. Of this, the resistance of the user-side system is 2–4 meters, that of the external network system is 4–8 meters of water column, and that of the piping system at the heat exchange station is 8–15 meters of water column. 12. What are the different forms of heat exchange? What is the heat transfer coefficient? What is the main heat exchange mechanism of a surface-type heat exchanger? Answer: There are three forms of heat exchange (or heat transfer): conduction, convection, and radiation. For counter-flow heat exchangers, the main forms of heat transfer are convection and conduction. The formula for calculating heat transfer by convection is: Q=αA(t2-t1), while the formula for heat transfer by conduction is: Q=（λ/δ）A(t2-t1). Convective heat transfer occurs on both sides of the heat transfer elements in plate heat exchangers, while conductive heat transfer takes place within the elements themselves. 13. What are the types of surface heat exchangers? What are its principles, advantages, and disadvantages? Answer: The main types of surface heat exchangers include: shell-and-tube heat exchangers, plate heat exchangers, heat pipe heat exchangers, etc. It can be divided into many forms, all of which share the same drawbacks: large size, high space requirement, high investment costs, low heat exchange efficiency (compared to hybrid types), and short lifespan ； Their advantage is that the quality of the condensate water remains less polluted, and it is easy to recover. 14. What are the disadvantages of ordinary mixed-type heat exchangers? Answer: A conventional mixed-type heat exchanger has steam entering from its side, and the water circulation is driven entirely by electricity. Although it offers advantages such as small size and high thermal efficiency, it has the following disadvantages: 1. It does not save electricity; the circulation pump must always be in operation under any circumstances ； / U. p0 G3 t# J$ z 2. Unstable: severe vibration and noise occur when the steam inlet pressure is low or the water inlet pressure is high ； 3. Similarly, there is also the problem of difficulty in recovering condensate water. 15. What types of valves are commonly used in heating systems, and what are their respective properties? Answer: The valves commonly used in heating systems include: globe valves, gate valves (or plug valves), butterfly valves, ball valves, check valves, safety valves, pressure reducing valves, pressure stabilizing valves, balance valves, control valves, as well as various self-acting control valves and electric control valves. Among them:
Gate valve: Used to stop the flow of the medium; it has a certain degree of regulation capability, but experiences high pressure loss. It is commonly used in heating systems to stop the flow of steam. In valve specifications, “J” is used to denote a gate valve.
Globe valve: Also used to stop the flow of the medium; when the valve is fully open, the medium can pass through just like in a regular pipe, without any need to change the direction of flow, which results in lower pressure loss. Gate valves have poor regulating performance, and the letter “Z” is used in valve codes to indicate gate valves. Check valve: Also known as a non-return valve or one-way valve, it allows fluid to flow in only one direction; if the pressure behind the valve is higher than that in front of it, the check valve will close automatically. There are various types of check valves, mainly including lift-type and swing-type ones. The lift-type valve body resembles a globe valve; it has high pressure loss, which is why it is less commonly used in modern heat exchange station systems. It is indicated by “H” in the valve model. Butterfly valve: It achieves regulation and opening/closing by changing the angle of the valve disc; since the valve disc is always situated in the flowing medium, significant resistance is generated, which is why it is less commonly used. It is indicated by “D” in the valve model. Safety valve: Primarily used to release pressure when the medium is under overpressure, thereby protecting equipment and systems. In some cases, modified slightly-open type pressure relief valves can be used as system pressure control valves. There are many structural types of safety valves, which are indicated by the letter “Y” in the valve model. 16. What is the function of a dirt remover? Where is it usually installed in the system? Answer: The function of a strainer is to remove debris from the water system. The station internal dirt removers are generally large in size, and are installed before the steam-driven heaters or on the return water pipes to prevent debris from entering the heaters. The strainers located at the entrance wells outside the building are generally small in size; they are installed on the water supply pipes. Some systems have them, while others do not. Their purpose is to prevent debris from entering the users’ radiators. The new generation of gas-fired heaters are equipped with a dirt remover 17. Sometimes it is observed that some users’ radiators get hot while others do not – why is that? How to solve it? Answer: This is known as hydraulic imbalance in the system. The causes are quite complex; generally, they include the following: (1) Unreasonable pipe diameter design, with certain sections having pipes that are too narrow ； (2) Some components present excessive resistance, such as valves that cannot be fully opened ； (3) There are debris blocking the system. (4) The air in the system cannot be completely removed due to reasons such as an incorrect slope direction of the pipes ； (5) Significant water loss from the system ； (6) The system pressure is set too low, resulting in operation without sufficient water ； (7) The flow rate and head of the circulating water pump are insufficient ； To resolve system imbalances, it’s necessary to first identify the causes and then take appropriate measures. What are the advantages and disadvantages of steam heating and hydronic heating? Answer: Although steam heating systems have the advantage of lower initial investment, they result in excessive energy waste. According to estimates by authoritative agencies, steam heating consumes about 30% more energy than water heating; as a result, this heating method is gradually being phased out in recent years. The waste of energy in steam heating is mainly manifested in: (1) The quality of domestic steam traps is substandard; they have a short service life and poor performance, resulting in the simultaneous discharge of steam and water ； (2) The piping system has high heat dissipation; in addition to high operating temperatures, damaged insulation and lack of timely maintenance are also contributing factors ； (3) The system has severe leakage; for the same leakage area, the heat carried away by steam is much greater than that carried away by water. In addition to being uneconomical, steam heating is also unsafe, as it can lead to burns and pipe bursts caused by water hammer. Many systems generate vibration and water hammer noises during operation, which affect people’s work and rest. Additionally, the air in rooms heated by steam is dry, which is uncomfortable. Although the investment in the plumbing system increased appropriately, it overcame the aforementioned drawbacks.
A salute to the sofa! ! ! ! ! ! ! ! !
Compared to steam boilers, hot water boilers differ in terms of heat content due to the difference in steam enthalpy and the deficit enthalpy of water. Generally, direct supply is used for hot water heating, while a secondary network is employed for high-temperature water ； At present, direct steam heating is rarely used; instead, steam-water heat exchangers are employed for heat exchange in the secondary network, making the operation costs and management more complex compared to hot water heating. Relatively speaking, there are more systems that use direct hot water supply at present compared to those with steam heating
When using steam with a pressure of 0.6 MPa for heating, how much steam is consumed per square meter per hour? Assuming 0°C outside and 20°C inside, does anyone have more accurate data?