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Fault analysis and handling of refrigeration units in cold storage

文字:[大][中][小]2022/12/14    浏览次数:     

  Many failures of refrigeration units in cold storage are caused by compressor problems.Based on the different problems of refrigeration unit failures,an analysis is conducted to determine how to solve them reasonably.Below is a detailed introduction to the content of refrigeration equipment failure problems.
  1.Fault Analysis and Handling of Cold Storage Temperature Failure
  The temperature in the cold storage is too high.Upon inspection,it was found that the temperature in both warehouses is only between 14℃and O℃,and the supply solenoid valves in both warehouses are in an open state.The compressor starts frequently,and the situation does not improve when switching to another compressor,while there is thick frost on the return pipe.Upon entering these two warehouses for inspection,it was found that there was thick frost on both evaporator coils.After defrosting,the situation improved.At this time,the number of compressor starts decreased and the temperature of the warehouse also decreased,but it was not ideal.Later,it was found that the upper and lower limits of the low-pressure controller were mistakenly set to 0.11-0.15NPa(gauge pressure,the same below),meaning that the compressor was stopped at a pressure of 0.11MPa and started at a pressure of 0.15 lPa.The corresponding evaporation temperature range was about-2O℃to-18℃.Obviously,this setting was too high and the amplitude difference was too small.Therefore,the upper and lower limits of the low-voltage controller are readjusted,and the adjusted value is:O O5-0.12MPa,corresponding to an evaporation temperature range of approximately-2O℃-l8℃.After that,the system was restarted and normal operation was restored.
  2.Several reasons for frequent start-up of refrigeration compressors
  The running compressor is controlled by high and low pressure relays to start and stop,but most high pressure relays must be manually reset before restarting the compressor.Therefore,frequent compressor starts and stops are generally not caused by high pressure relays,but mainly by low pressure relays:
  (1)The amplitude difference of the low-voltage relay is too small,or in a joint control system of temperature relay and low-voltage relay,the amplitude difference of the temperature relay is too small;
  (2)The suction and discharge valve or safety valve of the compressor leaks,so after shutdown,high-pressure gas will leak into the low-pressure system.The pressure will quickly rise and the compressor will be started.After startup,the pressure of the low-pressure system will be rapidly reduced,and the low-pressure relay will act to shut down;
  (3)The automatic return valve of the oil separator is leaking;
  (4)Expansion valve ice blockage.
  3.The running time of the compressor is too long
  The fundamental reason for the long running time of the compressor is insufficient refrigeration capacity of the device or excessive heat load of the cold storage,mainly including:
  (1)The evaporator frost is too thick or there is too much oil stored;
  (2)Insufficient circulation of refrigerant in the system,or insufficient patency of liquid refrigerant pipelines;
  (3)Due to leakage of the suction and exhaust valve plate,severe leakage of the piston ring,or inability of the compressor to increase load,the actual gas transmission capacity of the compressor is significantly reduced;
  (4)Due to thermal insulation damage,inadequate door closure,or the placement of a large amount of hot goods,the thermal load of the cold storage is too high;
  (5)Control components such as temperature relays,low-voltage relays,or supply solenoid valves have malfunctioned,resulting in the lower limit of the warehouse temperature.But the compressor cannot stop in time.
  4.Rapid balance of high and low pressure after compressor shutdown
  This is mainly due to severe leakage or breakage of the suction and exhaust valve plates,as well as gasket breakdown between the high and low pressure of the cylinder,causing high-pressure gas to quickly enter the suction chamber after shutdown.
  5.The compressor cannot be loaded or unloaded normally
  The main reason for the energy regulation system controlled by oil pressure is that the lubricating oil pressure is too low.(Generally caused by excessive bearing clearance and pump clearance),it can be solved by tightening the oil pressure regulating valve;The piston of the unloading oil cylinder is severely leaking oil,and the oil circuit is blocked;The oil cylinder is stuck with the piston or other mechanism;The solenoid valve is not functioning properly or there is residual magnetism in the iron core.
  Refrigeration system faults and analysis
  Firstly,the frost on the evaporator coil should not exceed 3mm per strand.If the frost is too thick,it will increase the thermal resistance,resulting in a certain heat transfer temperature difference between the evaporator and the cold storage.The refrigerant in the evaporator cannot absorb sufficient heat to vaporize,and a large amount of refrigerant will absorb heat and vaporize on the return air pipeline,causing an increase in frost formation on the return air pipeline;In addition,due to the small or even zero overheating felt by the expansion valve,it turned down or closed,causing the compressor to quickly shut down at low pressure.But the solenoid valve was not closed,and there was still a certain heat load in the cold storage.After the evaporator pressure rose,the compressor started again,causing frequent starts.The thicker the frost on the evaporator,the more severe this situation becomes.In fact,the frost on the evaporator coils of the two low-temperature cold storage units in this system is too thick,reaching 1-2cm,which seriously affects heat transfer and makes it impossible for the temperature of the storage to drop.After defrosting,the temperature of the two low temperature warehouses can be reduced to?6~C 5qC.
  Secondly,the setting values of the high and low voltage controllers are incorrect.The refrigerant used in this refrigeration equipment is R22,and the high-pressure cut-off pressure(upper limit)is often selected as gauge pressure of 1.7-1.9 MPa.The low pressure relay pressure(lower limit)can be taken as the refrigerant saturation pressure corresponding to the design evaporation temperature minus 5℃(heat transfer temperature difference),but generally not less than the gauge pressure O.01 MPa.The adjustment amplitude difference of low-voltage switches is generally 0.1~0.2MPa.Sometimes the scale of the pressure control setting value is not very accurate,and the actual action value should be based on the measured value during debugging.When testing the low-pressure controller,slowly close the suction shut-off valve of the compressor and pay attention to the indicated value on the suction pressure gauge.The indicated values when the compressor stops and restarts are the upper and lower limits of the low-pressure controller.The high-pressure controller can slowly close the discharge shut-off valve of the compressor and read the reading of the discharge pressure gauge when the compressor stops,which is the high-pressure cut-off pressure.Confirm the reliability of the pressure gauge before the experiment;The discharge valve should not be fully closed to ensure safety.
  Once again,there is insufficient refrigerant in the system.In devices with a liquid reservoir,due to the regulating function of the reservoir,unless there is a severe shortage of refrigerant,the liquid supplied from the reservoir to the liquid pipe cannot be continuous,which affects the normal operation of the device.In general,"insufficient refrigerant",that is,a low liquid level,will not have a significant impact on the operation of the system.However,in a device without a liquid reservoir,the amount of refrigerant in the system directly determines the level of refrigerant in the condenser,and thus affects the operation of the condenser and the subcooling of the liquid refrigerant.Therefore,when the refrigeration dose in the system is insufficient,it will inevitably cause the following changes in the device's operating conditions:
  (1)The compressor runs non-stop,but the temperature of the storage cannot decrease;
  (2)The discharge pressure of the compressor decreases;
  (3)The suction pressure of the compressor is lower,the suction superheat increases,the frost at the back of the evaporator melts,and the compressor cylinder head heats up;
  (4)A large number of bubbles can be seen in the center of the liquid flow in the liquid supply indicator;
  (5)The liquid level in the condenser is significantly low.
  It should be noted that when the opening of the thermal expansion valve is too small,there may also be a decrease in suction pressure,partial frost and melting of the evaporator,and frost and melting of the suction pipe.Therefore,in situations where the refrigerant level cannot be accurately observed.To determine whether the refrigeration dose in the system is insufficient,the following method can be used:stop using the thermal expansion valve,open and adjust the manual expansion valve appropriately,observe the operation of the system,and see if it can return to normal.If it can return to normal,it is due to improper adjustment of the thermal expansion valve,otherwise it is due to insufficient refrigerant in the system.Insufficient refrigerant in the system,if not charged enough,is always caused by leakage.Therefore,after determining the insufficient refrigerant in the system,leakage should be detected first,and refrigerant should be added after eliminating the leakage.

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