Why Does the Evaporation Temperature of the Refrigeration System Drop Too Low? And Why is the Condensing Pressure Too High?

2025-01-06

I. Reasons for Excessively Low Evaporation Temperature and Solutions

Reason 1: The evaporator (air cooler) is too small

There are design problems, or the actual stored variety is different from the designed one, resulting in an increased heat load. For example, a cold storage designed for storing apples is used to store garlic sprouts. Since the harvesting period of garlic sprouts in one production area is only a few days, it is impossible to achieve the daily intake of 5% - 15% of the storage capacity as with apples. Instead, the cold storage needs to be filled within 3 - 5 days. Therefore, to reduce the temperature to the appropriate storage temperature in a timely manner, it can only be achieved by lowering the evaporation temperature.
Solution: Increase the evaporation area of the evaporator or replace the evaporator.
Reason 2: The refrigeration capacity of the compressor is too large
After the load of the storage room decreases, the energy of the compressor is not reduced in a timely manner. The compressor of the cold storage is matched according to the maximum load of the refrigeration system. The maximum load of a fruit and vegetable cold storage occurs during the cargo - loading stage. At most other times, the load of the compressor is less than 50%. After the loading is completed and the temperature drops to the appropriate storage temperature, the system load is greatly reduced. If a large - capacity machine is still running, it is like using a big horse to pull a small cart, increasing the temperature difference and power consumption.
Solution: According to the change of the storage room load, reduce the number of compressors in operation or use an energy - regulating device to reduce the number of working cylinders.
Reason 3: The evaporator is not defrosted in a timely manner
Frost formation on the evaporator coils reduces the heat transfer coefficient, increases the thermal resistance, reduces the heat transfer effect, and decreases the evaporation of the refrigerant. With the energy of the compressor remaining unchanged, it will lead to a decrease in the evaporation pressure of the system and a corresponding decrease in the evaporation temperature. Therefore, defrosting should be carried out in a timely manner.
Solution: Defrost in a timely manner.
Reason 4: There is lubricating oil in the evaporator
The lubricating oil in the evaporator forms an oil film on the wall of the evaporation coil, which also reduces the heat transfer coefficient, increases the thermal resistance, reduces the heat transfer effect, and decreases the evaporation of the refrigerant, resulting in a decrease in the evaporation pressure of the system and a corresponding decrease in the evaporation temperature.
Solution: Drain the oil from the system in a timely manner and use hot ammonia defrosting to remove the lubricating oil in the evaporator.
Reason 5: The expansion valve is opened too small
When the expansion valve is opened too small, the liquid supply to the system is insufficient. With the energy of the compressor remaining unchanged, the evaporation pressure decreases, leading to a decrease in the evaporation temperature.
Solution: Increase the opening degree of the expansion valve.

II. Reasons for Excessively High Condensing Pressure and Solutions

An increase in condensing pressure will lead to an increase in compression work, a decrease in refrigeration capacity, a decrease in the coefficient of performance, and an increase in energy consumption. It is estimated that, under other unchanged conditions, for every 1°C increase in the condensing temperature corresponding to the condensing pressure, the power consumption will increase by about 3%. Generally, it is considered that a more economical and reasonable condensing temperature is 3 - 5°C higher than the outlet temperature of the cooling water. The reasons for the increase in the pressure of the condenser and the solutions are as follows:

The condenser selected is too small: Replace or increase the condenser.
The number of condensers in operation is insufficient: Increase the number of condensers in operation.
The cooling water flow is insufficient: Increase the number of water pumps in operation to increase the water flow.
The cooling water temperature is too high:
- (1) Supplement low - temperature water (tap water or well water).
- (2) Use defrosting water.
- (3) Ensure the cooling effect of the cooling tower.
Regularly clean the cooling tower to keep it clean: When the water temperature is close to the wet - bulb temperature of the air, turn off the cooling tower fan to reduce power consumption.
The heat - transfer area of the condenser is reduced: When adding refrigerant, strictly control the amount added to ensure sufficient heat - transfer area of the condenser.
Uneven water distribution in the condenser: When the water distribution is uneven, the water flow in some pipes is large, and the water flow in some pipes is small, which will reduce the heat - transfer efficiency and increase the condensing temperature.
There is scale in the condenser pipes: The scale in the condenser pipes increases the thermal resistance, reduces the heat - transfer coefficient, and decreases the heat - exchange effect, causing the condensing temperature to rise. Improve the water quality and remove the scale in a timely manner.
There is air in the condenser: The air in the condenser increases the partial pressure in the system and the total pressure. The air also forms a gas layer on the surface of the condenser, generating additional thermal resistance, reducing the heat - transfer efficiency, and leading to an increase in the condensing pressure and condensing temperature. Release the air in a timely manner.