09-11-3** Question—Essay Question—(Prize for participation)

When the heat exchanger starts operating and it is found that the temperature distribution across it is uneven, it is necessary to check whether all the air has been removed, whether the heat exchange plates have been installed incorrectly, and whether there are any blockages, and take appropriate corrective actions. When it is found that two media are mixing in the heat exchanger, especially flammable and explosive media, the operation should be stopped immediately, and the plates with perforations or cracks should be identified and replaced. The heat exchanger must have its temperature and pressure strictly controlled to stay within allowable limits; otherwise, it will accelerate the aging of the gaskets. During the operation of the heat exchanger, since the equipment is filled with fluid and under pressure, it is not allowed to tighten the bolts firmly. When tightening the clamping bolts and nuts of the heat exchange fins, it is necessary to strictly control the distance between the plate bundles at the two ends; otherwise, the heat exchange plates or gaskets may be damaged. The sliding rollers on the movable head of the heat exchanger should be lubricated regularly to prevent rusting, ensuring smooth and easy disassembly. Under normal conditions, there is no need to shut down the heat exchanger. It is only when the pressure drop exceeds the allowable value, backwashing proves ineffective, production capacity drops suddenly, or there is cross-contamination of fluids or significant leakage that cannot be controlled that the heat exchanger is shut down to investigate the cause. Clean or replace damaged heat exchanger components. The steam inlet valve is controlled based on the hot water outlet temperature of the heat exchanger.

When putting the heat exchanger into operation, introduce the cold material first, followed by the hot material. The heat exchange rate should be controlled using the outlet valve, with the inlet kept fully open.

It should be the inlet valve that needs to be controlled; the outlet valve is set to an open state, as per the studies*

In a heat exchanger, the temperature is controlled by the heat transfer medium passing through the inlet. If the temperature at the cold water outlet is set to 50 degrees, the temperature control valve activates and adjusts its opening degree automatically to maintain that temperature. When the temperature drops below the set value, the temperature control valve operates to raise the temperature again. That’s how heat exchangers work.

Typical heat exchangers use two control valves to regulate temperature: one is installed in the main flow path of the heat exchanger, and the other is placed in the bypass circuit. A controller is used to operate these two control valves, thereby controlling the temperature! This control scheme appears similar to split-range control in form, but the definition of split-range control is generally that \"the output of one regulator drives two or more control valves to operate, with each valve acting only within a certain range of the regulator’s output signal.\" There is therefore a difference: in the temperature control of a heat exchanger, the two control valves operate synchronously across the entire signal range, so it does not resemble split-range control!

Heat exchanger feed: 1: Heater: First introduce the cold material, then introduce the hot material； If steam heating is used, allow cold material to enter first, then gradually turn on the steam for preheating, and finally use steam under normal operating conditions ； The steam flow is controlled by the temperature at the outlet of the cold material; the steam control valve is installed in the steam inlet pipeline before the heater. 2: Cooler: Advanced cooling water, for the incoming hot material ； The flow rate of the cooling water is controlled by the temperature at the outlet of the hot material, and the cooling water control valve is installed on the cooling return pipeline.

1 Inspection and pressure testing before operation: Heat exchangers that have been manufactured or overhauled must undergo pressure testing in accordance with relevant regulations. Water is generally used as the testing medium; thus, a water pressure test is carried out. The pressure applied during this test is 1.25 to 1.5 times the design pressure. For heat exchangers that will be in use for a short period of time, this pressure level can be reduced. During the pressure test, it is necessary to check for leaks at the small floating heads, expansion joints, welds, pipe box gaskets, and connection valves; any leaks found must be addressed. 2 Commissioning of the heat exchanger: After pressure testing, the water remaining inside the heat exchanger should be drained completely, to prevent water hammer and vaporization that could damage the internal components when steam and hot oil enter, due to the presence of large amounts of water. When the unit is put into operation, the main body of the heat exchange equipment and its accessories are connected using flange bolts, with gaskets used for sealing. Due to the difference in materials between these components, during the heating process, especially when the temperature exceeds 200°C, uneven expansion occurs in various parts, which leads to loosening of the flange surfaces and resulting in leakage of the medium. Therefore, during the startup of the unit, steam purging, pressure testing, and heating are carried out to achieve thermal tightening. During operation, the heat exchanger should have the cold source activated first and then the hot source; the outlet should be opened before the inlet. This is because activating the hot source first will cause the various components to expand due to heat, while introducing the cold medium later will cause those same components to contract sharply. Such extreme expansion and contraction can easily lead to leaks in the seals. Also, since cold media are generally lighter in density and have lower boiling points, when the hot media enters first, the entire heat exchanger is in a high-temperature state. The small amount of cold media that enters later vaporizes suddenly in large quantities, causing overpressure in the heat exchanger and leading to leakage at its cover. Therefore, when putting cold exchange equipment into use, it is necessary to start with the cold side before moving to the hot side, opening the outlet first and then the inlet, so as to ensure a smooth exit path for the medium, prevent overpressure, and avoid seal leakage. Commissioning of the cooler. Coolers are generally cooled using circulating water. During this circulation process, the water evaporates continuously, resulting in its concentration; as a result, scale and dirt accumulate in the water, and certain microorganisms and algae develop, increasing the corrosiveness of the water. Therefore, before putting the cooler into use, a thin but dense protective layer must be formed on the metal surface to prevent corrosion of the equipment during operation – this is what is known as pre-coating for coolers. During the operation of the cooler, in addition to following the same procedures as those for operating heat exchangers, the outlet valve should be used for regulating the temperature of the oil. Using the inlet valve for regulation leads to short circuits in the flow within the cooler and a slow flow rate, resulting in hotter conditions at the top and cooler conditions at the bottom. Moreover, due to difficulties in controlling the water volume, it is not easy to cool the oil to the specified temperature range; therefore, the inlet valve should not be used for adjusting the cooler. 3 Routine Inspection and Maintenance Routine inspection is an important means of detecting and addressing sudden failures promptly. During operation, heat exchangers should be inspected as follows to ensure their safe operation. 4 Temperature: Temperature is a key control parameter during the operation of heat exchangers. By measuring and monitoring the inlet and outlet temperatures of the various fluids within the heat exchanger, as well as any changes in these temperatures, it is possible to determine the flow rate of the media and assess the efficiency of heat exchange. The quality of heat exchange efficiency depends mainly on the heat transfer coefficient. The most likely cause of the reduced heat transfer efficiency is scaling on the heat exchanger, which leads to a decrease in the heat transfer coefficient; therefore, the heat exchanger needs to be cleaned. During operation, sudden changes in the flow rate, temperature, and pressure of the medium should be prevented, as both overheating and overpressure can cause leaks in the heat exchanger. Therefore, during operation, it is necessary to strive for smooth operation, with steady changes in flow rate and temperature. Regularly check and adjust the water outlet temperature of the cooler to keep it below 50°C, as exceeding this temperature accelerates the growth of microorganisms and the decomposition of corrosion products, leading to tube corrosion and perforation; at the same time, scaling becomes more severe. 6 Pressure: Regularly check the pressure changes in the heat exchanger, especially the pressure difference between the inlet and outlet. This allows one to determine whether there is scaling, blockage, or leakage in the heat exchanger. The leakage of high-pressure liquid into low-pressure liquid causes the pressure of the low-pressure fluid to rise, increases the pressure drop of the high-pressure medium, and can lead to contamination and other consequences. If a sudden change in pressure is detected during operation, in addition to checking issues with the heat exchanger itself, other factors such as whether the flow path of the medium is unobstructed should also be considered. 7. Leaks: External leaks from heat exchangers are easy to see and detect. Such leaks can occur at various sealing surfaces, or they may be caused by sand holes and cracks in the equipment. In the case of leaks at sealing surfaces, it is necessary to check whether they are caused by loose bolts or damaged gaskets. Internal leakage in heat exchangers is not easy to detect, but it can be identified through the analysis of medium temperature, pressure, flow rate, abnormal noises, and vibrations. For example, in a heat exchanger where crude oil and second-line diesel are exchanged for heat, the second-line diesel flows through the tube side while crude oil flows through the shell side. Due to the high pressure of the crude oil, in the event of a leak, crude oil can mix into the second-line diesel. This can result in an increase in pressure on the tube side, a rise in pressure drop on the shell side, as well as issues such as poor quality of the second-line diesel and a change in its color to black. These phenomena can be used to determine that there is an internal leak in the heat exchanger. For coolers, a sampling pipe can be connected to the pipeline before the cooling water outlet valve, and samples can be taken regularly to check for any mixture of the cooled medium, thereby determining whether there is internal leakage in the cooler. 8 Vibration: The fluid inside the heat exchanger generally flows at high speeds, and both the pulsating flow and lateral movements of the fluid can cause vibration in the foundation supports. If an increase in foundation vibration is observed, in addition to checking for any abnormal operations, it is also necessary to examine whether the foundation of the heat exchanger is damaged and whether the anchor bolts are loose. 9 Heat retention: Damage to the heat retention (insulation) layer directly affects the thermal efficiency of the heat exchanger. Furthermore, due to the damage of the insulation layer, moisture accumulates on the outside of the casing, causing localized corrosion of the casing. Therefore, regular inspections should be carried out, and any damage to the insulation layer should be repaired as soon as possible. 10 Methods of shutting down and handling after shutdown: A heat exchanger that requires inspection and maintenance must be shut down and processed accordingly. The method for shutting down a heat exchanger is the opposite of that for starting it up: first turn off the heat source, then the cold source; first close the inlet, then the outlet. To avoid affecting other operations, the bypass valve should be opened before shutting down the heat exchanger. After deactivation, the heat exchanger is purged with steam; once purged, it is handed over for maintenance. When purging the oil in the cooler, the water inlet and outlet valves should be closed, and the vent valve should be opened, to prevent the heat exchanger from overpressuring and damaging its sealing surfaces and internal components due to water vaporization. Purging cannot be carried out in the presence of water hammer, as this could damage the anti-corrosion coating. For heat exchangers that are taken out of use in winter, the water and other fluids stored within them must be drained, the units should be purged with air, and then filled with N2 as a protective measure to prevent corrosion of the equipment ； If dismantling is required, the parts exposed to the atmosphere should be treated to prevent corrosion, such as by applying a protective coating.

The temperature control of the heat exchanger is achieved through the inlet valve; for example, to control the secondary temperature in a plate heat exchanger, it is necessary to control the primary inlet valve

This post was last edited by leiyu85639 on 2009-11-4 12:04. Heat exchanger operation: To shut down a heat exchanger, the heat source should be turned off first followed by the cold source; the inlet should be closed before the outlet. To avoid affecting other operations, the bypass valve should be opened before shutting it down. After deactivation, the heat exchanger is purged with steam; once purged, it is handed over for maintenance. When purging the oil in the cooler, the water inlet and outlet valves should be closed, and the vent valve should be opened, to prevent the heat exchanger from overpressuring and damaging its sealing surfaces and internal components due to water vaporization. Purging cannot be carried out in the presence of water hammer, as this could damage the anti-corrosion coating. For heat exchangers that are taken out of use in winter, the water and other fluids stored within them must be drained, the units should be purged with air, and then filled with N2 as a protective measure to prevent corrosion of the equipment ； If dismantling is required, the parts exposed to the atmosphere should be treated to prevent corrosion, such as by applying a protective coating. The operation method is the reverse. The temperature of a heat exchanger is generally controlled using the inlet valve, but the measurement point is usually taken at the outlet, or control is applied separately at both the inlet and outlet. Inlet and outlet valve status: Generally, the cold stream is F.O, and the hot stream is F.C. For special operating conditions, selection and use should prioritize the principle of meeting the process requirements. /

6# gxthomas uses proportional control: one controller regulates two control valves, one of which adjusts the flow rate within the 0–100% range, while the other does the same but in the 100–0% range, thereby maintaining a stable flow rate.