Precautions for the maintenance and repair of evaporators

　　Maintenance and Repair of Evaporators (1) Maintenance of Evaporators 1) Regularly perform leak detection on the evaporator. Leakage is a common fault in evaporators, so frequent leak checks should be conducted during use. When an ammonia evaporator leaks, there is a pungent odor, and the leak point does not frost.

　　Phenolphthalein test paper can be used to check leak points because ammonia is alkaline and turns phenolphthalein paper red. Visually, areas on the evaporator where frost does not form are usually leak points. Soap water can also be used to find leaks. For Freon evaporators, halogen lamps and halogen leak detectors can be used, or soap water can be applied to find leaks.

　　During inspection, first visually check for oil stains on the evaporator tubes because Freon and oil are miscible; when Freon leaks, oil also seeps out at the leak point. Therefore, where there is oil, there is a leak. When using a halogen lamp for leak detection, if Freon leaks at a certain point, the flame color changes from blue to light green, grass green, purple-green, purple, etc., indicating the amount of Freon leakage. If the flame is dark green or purple, the phosgene in the flame is toxic, so this method should not be used for long periods. In such cases, soap water should be used to check for leaks. For very small leaks, a halogen leak detector should be used.

　　2) Regularly check the frosting condition of the evaporator. When the frost layer is too thick, defrosting should be done promptly. Abnormal frosting may be caused by blockages, which should be identified and eliminated in time. 3) When the evaporator is to be shut down for a long time, refrigerant should be pumped into the liquid receiver or condenser to keep the evaporator pressure around 0.05 MPa (gauge pressure). For evaporators in brine tanks, rinse with tap water and then fill the tank with tap water after rinsing. 4) For evaporators in chillers, ensure the quality of chilled water. Chilled water should be treated softened water. If scale forms when using tap water, descaling treatment should be performed.

　　(2) Repair of Evaporators 1) Maintenance of Ammonia Evaporators. The main faults of ammonia evaporators are: internal blockage of pipes due to oil and dirt accumulation; external cracks or pinholes caused by corrosion and other factors.

　　① Blockage. If oil and mechanical impurities mix inside the evaporator, causing high viscosity and blocking the pipes, first perform hot ammonia defrosting. If defrosting does not solve the problem, evacuate the ammonia from the evaporator, then blow out with compressed air at 0.6 MPa (gauge pressure). If blowing does not solve the problem, it indicates there are solid blockages inside the pipe, such as wooden plugs, sandpaper, cotton yarn left inside. Generally, pipe cutting is required. To reduce the number of cuts, locate the blockage by observing frosting before and after the pipe, or by listening for liquid flow sounds indicating throttling. After locating the blockage, cut the pipe, remove the blockage, weld the pipe, and after pressure testing and leak checking, the evaporator can be put back into use.

　　② Leak Repair. If there is a leak in the evaporator tubes, such as pinholes on smooth wall tubes or top tubes, welding repair should be done. If welding cannot be done immediately due to busy production, temporarily block the leak with a rubber sheet secured by pipe clamps, and perform welding during the off-season. If the finned tubes of the air cooler leak, close the air cooler's liquid inlet and suction valves, open the hot ammonia defrost liquid return valve and the defrost liquid return main valve, use a circulation or drainage bucket to evacuate, and open defrost water to increase pressure for faster and cleaner evacuation. Do not use the method of opening the suction valve to evacuate, as it takes longer and ammonia liquid is hard to remove. After evacuation, disconnect relevant connections to atmosphere and perform welding repair with electric or gas welding. If valve internal leakage prevents full evacuation but pressure below atmospheric can be reached, use a punch to enlarge the pinhole until no ammonia leaks, then weld with electric welding. This method is only suitable for equipment used less than 5 years without severe corrosion. Electric welding must be used, not gas welding.

　　2) Repair of Freon Evaporators:

　　① Excessive Oil Contamination. Freon can dissolve with lubricating oil, so during refrigerant circulation, lubricating oil inevitably adheres to the inner surface of pipes, especially if design or installation is improper, causing more oil accumulation inside evaporator tubes, which affects heat transfer and ultimately the refrigeration system's performance. During repair, evacuate the refrigerant from the evaporator, disconnect the supply and suction pipe joints, connect a nitrogen cylinder or air compressor, and blow out oil and other impurities with nitrogen or compressed air. If excessive oil is blown out, it indicates poor oil return in the evaporator, possibly due to lack of oil return bends at the end of each evaporator tube set, oversized suction pipe diameter, insufficient flow velocity, or horizontal suction main pipe not sloping toward the machine. Repair accordingly based on the situation.

　　① Blockage. If the evaporator is blocked by oil, mechanical impurities, or even wooden plugs and cotton yarn, repair can be done following the ammonia evaporator maintenance method. ② Leakage. If there is leakage at system connection pipes or evaporator tube unions, tighten with a wrench. If no looseness is found, remove the nut and check if the flare is skewed or damaged, or if the bolt threads are damaged. If the flare is damaged, cut off the flare with a pipe cutter and re-flare it. Ensure the flaring tool is properly aligned; otherwise, the flare will be skewed and leak after assembly. The repair method for steel pipe leaks is the same as welding repair for ammonia evaporators.