Do cold storage maintenance, you must understand these knowledge!

Date:Jun 11, 2020

[China Refrigeration Network] 

First, Refrigeration System "Three Prevents"

1, prevent evaporation temperature is too low

If the temperature difference between the evaporation temperature and the warehouse temperature increases, the evaporation temperature will be too low, which will lead to a decrease in the refrigeration coefficient and an increase in energy consumption.  It is estimated that under the condition that other conditions remain unchanged, when the evaporation temperature decreases by 1℃, it will consume 1 ~ 2% more electricity.  In addition, an increase in temperature difference will also increase the dehumidification capacity of the air cooler and reduce the humidity in the warehouse, thus causing an increase in dry food consumption and a decrease in food quality.

The difference between the evaporation temperature and the warehouse temperature is generally 10℃, the evaporation temperature installed in fruit and vegetable cold storage is generally designed to be around -10℃, the freezing temperature is -28℃, and the freezing room is -33℃, while in actual operation, the temperature difference in many fruit and vegetable storage warehouses is around 15℃.

Causes of Low Evaporation Temperature and Solutions;

If the evaporator is too small, increase the evaporation area of the evaporator or replace the evaporator.

If the cold capacity of the compressor is too large, the number of compressors to be started should be reduced according to the change of the warehouse load.

The evaporator did not defrost in time, so defrost in time.

If the opening of expansion valve is too small, the opening degree of expansion valve shall be increased.

2, prevent condensation pressure is too high

To prevent the condensation pressure from being too high and the condensation pressure from rising, the compression function will increase, the refrigerating capacity will decrease, the refrigerating coefficient will decrease, and the energy consumption will increase.  It is generally believed that the more economical and reasonable condensation temperature is 3 ~ 5℃ higher than the outlet temperature of cooling water.  Causes of Condenser Pressure Rise and Solutions;

The condenser is too small.  Replace or add condenser.

The number of condensers put into operation is small.  Increase the number of units in operation.

The cooling water flow is insufficient.  Increase the number of water pumps and increase the water flow.

3, prevent exhaust temperature is too high

If the exhaust temperature is too high, the lubrication condition of the compressor will deteriorate, increasing friction and energy consumption.  At the same time, too high exhaust temperature will also enhance the heat exchange between refrigerant gas and cylinder wall, resulting in a decrease in compressor efficiency.  Causes of Excessive Exhaust Temperature and Solutions;

Poor cooling of compressor cylinder.  Increase cooling water to improve cooling conditions.

Compressor suction superheat degree is too high, improve suction pipe insulation or increase the liquid supply volume of evaporator.

Second, the refrigeration system maintenance

The refrigerant circulating in the system needs to be clean and free of impurities, but it is impossible to be completely clean in actual operation, which is related to the working conditions and the insufficiently clean refrigerant added to the system.  Among the impurities controlled are: lubricating oil, air, water, etc. These impurities entering the refrigeration system are extremely unfavorable to the operation of the refrigeration device.

1, lubricating oil

Lubricating oil should have appropriate viscosity, viscosity is too large, flow resistance is large, friction work increases;  If the viscosity is too small, the friction surface cannot form an oil film, and if the friction force increases, the friction work will also increase.  The viscosity of lubricating oil decreases with the increase of temperature, which will lead to deterioration of lubrication and increase of friction work.  Therefore, when its temperature rises, it should be cooled in time or the cooling effect should be improved to reduce the oil temperature.  When lubricating oil and refrigerant dissolve each other, smooth oil return shall be ensured, otherwise the compressor will be damaged due to lack of oil.  At the same time, the evaporation pressure will also drop due to too much oil in the refrigerant, which will eventually lead to a drop in the refrigeration coefficient.

2, water

Ways for water to enter the system:

Air was not evacuated during installation or maintenance, or was not completely evacuated.

Evacuate the air during installation or maintenance and install the dryer.

Enter from an unsealed place.  Don't have leakage points and avoid negative pressure operation.

Water in oil or refrigerant.

3, air

Air entering the system will increase the condensed water pressure in the system and the total pressure. Air will also form a gas layer on the condenser surface, generating additional thermal resistance, which will reduce the heat transfer efficiency and lead to an increase in condensation pressure and condensation temperature. For every 1kg/cm2 increase in condensation pressure, the power consumption will increase by about 6.8%.  In addition, the adiabatic index of air (K=1.41) lies in the adiabatic index of refrigerant (ammonia k=1.28, freon 12k=1.13, freon 22k=1.18), causing the exhaust temperature to rise, so air should be discharged in time.

4, Remove dirt, frost, etc. from heat exchange equipment

Statistics show that 1.5mm scale on the water side surface of the condenser will increase the condensation temperature by 2.8℃ and increase the power consumption of the refrigeration device by about 9%.  In addition, scaling will also corrode equipment and shorten its service life.  Therefore, the condenser should be cleaned frequently, and the thickness of scale should not exceed 1mm at most.

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