Screw-Type Water Cooled Unit Normal Operation Indicators and Failure Handling

2024-08-31

Refrigeration equipment is a complex system composed of refrigerators, condensers, evaporators, expansion valves and many equipment accessories that are interrelated and interact with each other. Therefore, once a fault occurs in the refrigeration equipment, attention should not be focused only on a certain part, but a comprehensive inspection and comprehensive analysis of the entire system should be carried out. This requires the accumulation of practical experience and the guidance of theory. Through the summary of long-term practice, operators have explored a lot of experience in checking faults and summarized it into a complete set of basic methods of "listening, touching, looking, and analyzing".
"Listening": Listen to whether the sounds of equipment such as compressors and expansion valves during operation are normal;
"Touching": Touch the changes in heat and cold at the connections of relevant components and pipelines in the system, and touch the heat and cold conditions and vibration conditions of the compressor;
"Looking": Look at the magnitudes of high and low pressure values, oil pressure values, and water pressure values during operation. The magnitudes of the set values of differential pressure relays, temperature relays, and pressure relays. Look at the height of the oil level and liquid level, and look at the frosting and condensation on the evaporator, return air pipe and infusion pipe;
"Analyzing": Use the relevant theories of the operation of refrigeration equipment to analyze and judge phenomena, find the cause of the failure, and eliminate it purposefully.
I. Signs of normal operation of the unit
When the refrigerator is started, one should first know whether the refrigeration system is operating normally. The following is a brief introduction to the content and signs of normal operation:

The cooling water of the condenser should be sufficient, and the water pressure should meet more than 0.12 MPa. The water temperature should not be too high. For screw refrigeration units, the reading of the oil pump pressure gauge should be 0.15-0.3 MPa higher than the exhaust pressure.
Under any circumstances, the oil temperature of fluorine refrigerators should not exceed 70 °C, and the minimum should not be lower than 30 °C. Under normal operation, the lubricating oil should not foam (except for fluorine refrigerators).
The exhaust temperature of the refrigerator R22 should not exceed 125 °C. If the excessive exhaust temperature rises further, it will be very close to the flash point (160 °C) of the refrigeration oil, which is unfavorable to the equipment. Therefore, from a practical point of view, the exhaust temperature cannot be too high. When it is too high, it should be shut down to find out the reason. Different refrigeration forms have different requirements for temperature. For example, refrigerators working under air conditioning conditions have higher exhaust temperatures than refrigerators working under standard conditions, which is normal.
The level of condensing pressure. It is mainly determined according to the water source situation, the structural form of the condenser and the refrigerant used. Under normal circumstances, for water-cooled condensers, R22 and ammonia should not exceed 1.8 MPa. In addition, when the unit is just started up, due to the large load on the condenser, it is normal for the pressure to be higher for a short period of time.
The liquid level of the liquid storage tank should not be lower than one-third of the liquid level indicator, and the oil level of the crankcase should not be lower than the horizontal center line of the indicator window (if there are two oil level indicator windows, the oil level should be between the center lines of the two glasses).
The automatic return oil pipe of the fluorine oil separator should be normal when it is cold and hot from time to time. The cold and hot cycle is about 1 hour. There should be no obvious temperature difference before and after the filter of the liquid pipeline. Frosting must not occur, otherwise it is blocked. Fluorine refrigerators should be cool on the flat side and hot on the dry side. The joints of the fluorine system should not leak oil. Oil leakage indicates fluorine leakage.
When touching the horizontal condenser by hand during operation, the upper part should be hot and the lower part should be cool.
The junction of hot and cold is the refrigerant liquid level. The oil separator is also hot on the upper part and not very hot on the lower part. The junction of hot and cold is the oil level or liquid level. The low-pressure end of the safety valve or bypass valve of the refrigerator should feel cool. If it is not cool, it means that there is gas leakage between high and low pressures.
During operation, the steam pressure and suction pressure should be similar, and the exhaust pressure at the high-pressure end should be similar to the condensing pressure and the pressure of the liquid storage tank. If they are not similar, it is abnormal.
Under a certain water flow rate, there should be a temperature difference between the inlet and outlet of cooling water. If there is no temperature difference or the temperature difference is extremely small, it indicates that the heat transfer surface of the heat exchange equipment is fouled and needs to be shut down for cleaning.
The refrigerator itself should be sealed and there should be no leakage of refrigerant and lubricating oil. For shaft seals, when the standard cooling capacity is 12.6×1000 kJ/h, the shaft seal is allowed to have a small amount of oil leakage. Refrigerators with a standard cooling capacity >12.6×1000 kJ/h are not allowed to have oil leakage of more than 10 drops per hour.
Fluorine refrigerators are not allowed to have oil dripping from the shaft seal.
The expansion valve body is evenly frosted or dewed, but there should be no thick frosting at the inlet. When flowing through the expansion valve, only a dull and faint sound can be heard.
The pointers of the pressure gauges in the system should be relatively stable and the thermometers should indicate correctly.
II. Fault analysis of the compressor
The reasons why the refrigerator cannot be started are nothing more than two aspects: motor faults and mechanical faults of the refrigerator. Now they are described separately as follows.
First check the main circuit. Is there electricity in the power supply? Is the fuse blown? Is the contact of the switch good? Is there single-phase operation? When one phase of the three-phase power supply is burned out, the motor can also rotate, but the sound is abnormal and the speed will also slow down. When this situation is found, it should be shut down immediately, otherwise the motor is easy to burn.
If the power supply voltage is too low, the motor sound will also be abnormal after startup. The voltage should not be lower than 90% of the rated voltage. Otherwise, the rated power of the motor will drop significantly and it will not be able to drag the compressor; when the allowable current of the input line is small and cannot meet the needs of the motor, the motor also cannot drag the compressor.
Check the differential pressure relay and high and low pressure relays. Because the differential pressure relay and high and low pressure relays are relay protections adopted for the safe operation of refrigerators. When the oil pressure (high pressure and low pressure) of the refrigerator is abnormal, the refrigerator can be stopped. Check whether the contacts of the pressure relay are disconnected, and check whether the relay is disconnected due to the high pressure setting value being too small or the low pressure setting value being too large. In addition, if there are valves in the system that are not opened, it will also cause the pressure relay to disconnect. Check whether the contacts of the differential pressure relay are disconnected. If the oil pressure cannot be established, the contacts will be disconnected. If the reset button is not pressed during startup, the contact is in the self-locking disconnected state. After the relay works once, it needs to wait 5 minutes to reset. If it is within 5 minutes, because the heating element still keeps the contact in the disconnected state, so it cannot be started.