Maintenance and Repair of Evaporators (1) Maintenance of Evaporators 1) Regularly conduct leak detection work on evaporators. Leakage is a common fault phenomenon of evaporators. During use, it is necessary to pay attention to frequent leakage detection. When the ammonia evaporator leaks, there is a pungent smell, but no frost forms at the leakage point.

Phenolphthalein test paper can be used to check the leakage point, as ammonia is alkaline and turns red when it meets phenolphthalein test paper. When visually inspecting, usually a certain area of the evaporator that does not frost is the leakage point. You can also use soapy water to find the leak at the leakage point. The inspection of leakage in Freon evaporators can be carried out using halogen lamps and halogen leak detectors, or soapy water can be used to find the leaks.

When conducting the inspection, you can first visually check if there are any oil stains on the evaporation tubes. This is because Freon and oil are miscible. When Freon leaks, oil will also seep out from the leakage point. Therefore, wherever there are oil stains, there will be a leak. When using a halogen lamp to check for leaks, if there is a Freon leak at a certain location, the flame of the halogen lamp burning changes from blue to light green, light green, grass green, purple green, purple and other colors, which can be used to determine the amount of Freon leakage. If the flame is dark green or purple, the phosgene in the flame is toxic and this method should not be used for a long time to check. In this case, soapy water can be used to check the leakage point. When there is a slight leakage, a halogen leak detector should be used for leak detection.

2) Regularly check the frosting condition of the evaporator. When the frost layer is too thick, defrosting should be done in time. When frosting is abnormal, it may be caused by blockage. The cause should be identified and eliminated in time. When the evaporator is not in use for a long time, it is advisable to transfer the refrigerant to the liquid storage tank or condenser to maintain the pressure of the evaporator at around 0.05MPa(gauge pressure). If it is an evaporator in a salt water tank, it also needs to be rinsed with tap water. After rinsing, fill the tank with tap water. 4) For the evaporator in the chiller, the quality of the chilled water should be guaranteed. The chilled water should be softened water that has been treated. When tap water forms scale, descaling treatment should be carried out.

(2) Repair of Evaporators 1) Maintenance of ammonia evaporators. The main faults of the ammonia evaporator are: oil and dirt accumulate inside the pipe fittings and clog the pipes together. Cracks or needle-shaped small holes appear on the exterior of the pipe fittings due to rust and other factors.

① Blockage. If oil and mechanical impurities are mixed in the evaporator and the viscosity is high, blocking the pipeline, hot ammonia defrosting should be carried out first. If defrosting cannot solve the problem, the ammonia in the evaporator should be drained completely, and then blown away with compressed air at 0.6MPa(gauge pressure). If blowing it away doesn't solve the problem. It indicates that there are obstructions in the pipe, such as wooden plugs, sandpaper used for cleaning pipe fittings, cotton yarn, etc., left inside the pipe fittings. In this situation, pipe cutting treatment is generally required. However, in order to reduce the number of pipe cuts. The blockage inside the pipe should be found. The blockage can be located based on the frosting conditions before and after the pipe, the sound of liquid flowing inside the pipe and other throttling phenomena. Locate the blockage, cut off the pipe fittings, remove the blockage, then weld the pipe fittings back together. Only after passing the pressure test and leak detection can the evaporator be put back into use.

② Plug the leak. If there is leakage in the evaporator tubes, such as needle-shaped small holes at the smooth wall tubes or top tubes, welding repair should be carried out. If the production task is busy and immediate welding repair cannot be carried out, it is advisable to block the leakage with rubber plates and secure it with pipe clamps. Welding repair can be carried out during the off-season of production. If there is leakage in the finned tubes of the cold air blower, the treatment method should be to close the liquid inlet valve and return valve of the cold air blower, open the hot ammonia defrosting and return liquid valve and the main defrosting and return liquid valve, and use the circulation bucket or drain bucket to evacuate. At the same time, open the defrosting water and increase its pressure. This way, it can evacuate quickly and thoroughly. Do not use the method of opening the return air valve to evacuate. This method takes a long time to evacuate and it is difficult to clean the ammonia solution thoroughly. After the equipment is evacuated, the relevant connection points can be disassembled and connected to the atmosphere, and then welded and repaired by electric welding or gas welding. Some equipment cannot be evacuated due to internal leakage of the valve, but can be evacuated to below atmospheric pressure. At this time, a sharp punch can be used to punch down the needle-shaped small hole. When no ammonia leaks out, electric welding can be used for welding repair. This treatment can only be carried out when the service life of the equipment is within s years and there is no severe rust on the pipe fittings. During the treatment, electric welding must be used and gas welding must not be used for repair.

2) Repair of Freon evaporators:

There is too much oil stain. Freon can dissolve with lubricating oil. Therefore, when the refrigerant circulates in the system, it is inevitable that there will be lubricating oil adhering to the inner surface of the pipe fittings. Especially when the design or installation does not meet the requirements, there will be a large amount of lubricating oil in the evaporator tubes, which will affect the heat transfer effect and ultimately affect the operation effect of the refrigeration system. When conducting maintenance, the refrigerant in the evaporator should be completely drained. The connection points of the supply and return gas pipelines should be disassembled and connected to a nitrogen cylinder or air compressor. Use nitrogen or compressed air to blow off oil stains and other impurities. If too much oil is blown away, it indicates that the oil return from the evaporator is not smooth. The possible reasons may be that no oil return bend is made at the end of each group of evaporation tubes, the diameter of the return gas pipe is too thick, the flow rate is insufficient, or the horizontal section of the return gas main pipe is not inclined towards the machine direction, etc. Repair it depending on the specific situation.

① Blockage. If the evaporator has oil, mechanical impurities, or even blockages caused by wooden plugs, cotton yarn, etc., it can be repaired according to the maintenance methods for ammonia evaporators. ② Leakage. When there is leakage at the union joint of the system connection pipe or the evaporator tube, it can be tightened with a wrench. If it is not loose, the nut should be removed to check whether the bell mouth is skewed or damaged and whether the thread of the bolt is damaged. If the bell mouth is damaged, it should be cut with a pipe cutter and re-expanded. It should be noted that the tube expander should be placed upright; otherwise, the expanded bell mouth will be skewed and leakage will occur when it is installed. The maintenance method for steel pipe leakage is the same as the welding repair method for ammonia evaporators.