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Refrigeration system blowdown and air tightness inspection method
The cold storage refrigeration system should be a clean, dry and tight closed circulation system. What if there are mechanical impurities or other contaminants in the system?
First, the impact on the device
1. Accelerate the wear of the friction surface, and if it is serious, it may cause the cylinder to damage the machine;
2. Affecting the sealing performance of the valve seat of the suction and exhaust valve, so that the exhaust pressure of the compressor is lowered and the suction pressure is increased;
3, may cause some of the narrower pipe fittings such as thermal expansion valves, capillaries and strainers, etc., affecting the normal flow of refrigerant, and thus affect the refrigeration capacity of the refrigerator. Therefore, mechanical impurities or other contaminants are not allowed in the system, and the system must be cleaned after installation.
The mechanical impurities and other contaminants in the refrigeration system are derived from iron filings, welding slag, scale, sand, rust and other impurities caused by equipment and piping during manufacturing, construction, installation and maintenance.
For small refrigeration systems, the basic method of cleaning up is to first disconnect the compressor from the other parts of the refrigeration system, then depressurize the high pressure nitrogen through the pressure reducing valve to 0.5 ~ 0.6MP and then connect to the condenser inlet for purging . Under the action of air pressure, the nitrogen gas is swept through the entire refrigeration system except the compressor in the direction of the original refrigerant circulation. The length of the purge time should be determined on a case-by-case basis.
For larger systems, in order to reduce gas flow resistance and gas flow, the cleaning operation can be carried out in stages. The sewage outlet should be set at the lowest point of each cleaning section, and the sewage discharge can be repeated several times until the gas discharged from the sewage outlet is blown on the white paper or the white cloth without obvious stain, the system can be considered to have been blown clean. If a solenoid valve is installed in the system, try to open the solenoid valve when blowing. If the compressed air of the compressor itself is used for blowing, the exhaust temperature of the compressor should not exceed 130 °C. Otherwise, the blowdown must be intermittent. Before the blown, the trichloroethylene solution may be added to the relevant parts to dissolve and discharge the oil, and then the oil is dissolved before being blown.
Second, how to check the air tightness of the refrigeration system
The refrigerants currently used (especially Freon-type refrigerants) have a strong permeability, and therefore, the refrigeration system is required to have good air-tightness. After the completion of the blowdown work, the refrigeration system must be carefully and meticulously tested for air tightness.
The air tightness test is generally divided into three methods: pressure test, vacuum test and working test:
1 stress test
The magnitude of the test pressure is usually determined by the difference in the type of refrigerant used in the refrigeration system and the test site.
(1) The nitrogen bottle is connected to the exhaust multi-channel of the refrigeration compressor through a pressure reducing valve.
(2) Close all valves of the refrigeration system to the atmosphere except the exhaust bypass, and open all the valves in the system.
(3) Open the nitrogen bottle valve, let the system first increase the nitrogen pressure to 0.3 ~ 0.5MP, do the initial test leak, and consider that after the basic no leakage, the specific pressure test is carried out on the high pressure part and the low pressure part respectively.
(4) Pressurize the test pressure value to the low pressure system. After the pressure is balanced, record the specific reading of the pressure gauge and the ambient temperature. Keep 6L, allow the pressure drop to be below 0.02MP, and continue to maintain 18~24L. If the ambient temperature does not change much, if the gauge pressure does not change, it is considered that the low pressure pressure test leak is qualified.
(5) Close the shut-off valve or manual bypass valve before the thermal expansion valve, and only pressurize the high pressure part of the system to the test pressure value of the high pressure system. After the pressure is balanced, record the specific reading of the pressure gauge and the ambient temperature. Maintain 6L, allow pressure drop in 4 small refrigeration systems. Pressure tests must be performed with dry nitrogen.
2 vacuum test
Vacuum leak test is to let the refrigeration system under a proper vacuum for a certain period of time, whether the reading of the vacuum gauge changes to reflect whether the air penetrates into the system to verify the sealing performance of the system. This test must be performed for refrigeration systems where the evaporation pressure is near or below atmospheric pressure.
3 working fluid test
Due to the high permeability of refrigerants such as Freon, when there are only small leaks in the system, it is often difficult to determine whether the system has a leak hazard by using only the above two methods. Therefore, such a system also requires a test leak. The basic method of working fluid leakage test is that the system is filled with vacuum and charged with the same refrigerant as the refrigerant used in the system. The charge is equal to about 0.1 MP lower than the ambient pressure at ambient temperature.
If the system can maintain 16 to 24L without pressure drop, it is considered that the working fluid test is qualified. The test fluid leakage can also be carried out by the following method: first, the working medium is filled with a working pressure of 0.1 to 0.2 MP, and then filled with dry nitrogen to make the total atmospheric pressure reach 0.8 to 1 MP. If the system can maintain 16 to 24L without pressure drop, it is considered that the working fluid test is qualified. This method has a higher test success rate.
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