Basic Knowledge Of Refrigeration Maintenance And Commissioning

1. Condensation temperature
The condensation temperature of the compressor system refers to the temperature at which the refrigerant condenses in the condenser, and the corresponding refrigerant vapor pressure is the condensation pressure.
The condensation temperature is one of the main operating parameters in the refrigeration cycle. For the actual refrigeration device, due to the small range of other design parameters, the condensation temperature can be said to be the most important operating parameter. It is directly related to the cooling effect of the refrigeration device, safety and reliability. and energy consumption levels.
2. Evaporation temperature
The evaporation temperature refers to the temperature when the refrigerant evaporates and boils in the evaporator, which corresponds to the corresponding evaporation pressure. The evaporation temperature is also an important parameter in the refrigeration system.
The evaporation temperature is ideally the refrigeration temperature, but the evaporation temperature of the refrigerant in actual operation is slightly lower than the refrigeration temperature by 3 to 5 degrees.
3. Suction temperature
The suction temperature refers to the temperature when the refrigerant enters the compressor, which is generally higher than the evaporation temperature. Because the evaporation temperature is the saturation temperature of the refrigerant, and the suction temperature is the temperature of the superheated gas, at this time the refrigerant becomes a superheated gas. At this time, the difference between the suction temperature and the evaporation temperature is the suction superheat.
4. Superheat
Definition of superheat: refers to the temperature difference between the low-pressure side and the steam in the temperature-sensitive bulb.
The method of measuring superheat: measure the evaporation pressure at a position as close as possible to the temperature sensing bulb, convert the reading into temperature, and then subtract the temperature from the actual temperature measured at the temperature sensing bulb. The superheat should be between 5-8°C.
5. Supercooling
Definition of subcooling degree: the difference between the saturated liquid temperature corresponding to the condensing pressure of the condenser and the actual temperature of the liquid at the outlet of the condenser.
In engineering, the exhaust pressure is generally regarded as approximately the condensing pressure, and the difference between the temperature of the saturated liquid corresponding to the exhaust pressure and the temperature of the liquid at the outlet of the condenser is regarded as the subcooling degree. The reason for this approximation is that the pressure drop in the condenser is small compared to the evaporator. For air-cooled condensers, a subcooling degree of 3 to 5 degrees is more appropriate. When the refrigeration system circulates normally, the outlet of the condenser generally has a certain degree of subcooling.
6. Effect of suction superheat
If there is no superheat in the suction, it may cause back air to carry liquid, and even cause wet stroke liquid shock to damage the compressor. In order to avoid this phenomenon, a certain degree of suction superheat is required to ensure that only dry steam enters the compressor (determined by the nature of the refrigerant, the existence of superheat means that the liquid refrigerant evaporates).
However, too high a degree of superheat also has disadvantages. A high degree of superheat will cause an increase in the discharge temperature of the compressor (exhaust superheat), and the deterioration of the operating condition of the compressor will reduce the service life. Therefore, the suction superheat should be controlled within a certain range.
The expansion valve senses the temperature difference between the return air temperature and the actual evaporating pressure (corresponding to the saturation temperature) through the temperature-sensing part placed on the return air pipe of the compressor or the outlet of the evaporator (the temperature difference is the superheat of the suction air), and set Adjusting the opening of the expansion valve based on the fixed superheat is equivalent to adjusting the liquid supply of the evaporator, and finally controlling the suction superheat.
Now some models (such as frequency conversion multi-line) also have expansion valves that specifically control the degree of condensation subcooling. When the degree of subcooling is insufficient, increase the opening of the expansion valve of the subcooling circuit to increase the amount of liquid sprayed to cool the refrigerant in the main circuit and improve the condensation effect.
The temperature of the refrigerant when it evaporates in the evaporator has a great influence on the cooling efficiency. For every 1 degree it decreases, the power needs to be increased by 4% to produce the same cooling capacity. Therefore, if conditions permit, appropriately increase the evaporation temperature. It would be beneficial to increase the efficiency of the refrigeration system.
7. Evaporation temperature adjustment
Evaporation temperature adjustment is to control the evaporation pressure in actual operation, that is, to adjust the pressure value of the low-pressure pressure gauge. During operation, the opening of the thermal expansion valve (or throttle valve) is adjusted to adjust the low-pressure pressure. If the opening degree of the expansion valve is large, the evaporation temperature rises, the low pressure also rises, and the cooling capacity will increase; if the opening degree of the expansion valve is small, the evaporation temperature decreases, the low pressure also decreases, and the cooling capacity decreases.
8. Factors affecting the evaporation temperature
In the actual operation of the refrigeration device, the change of the evaporation temperature is very complicated. In addition to being directly controlled by the expansion valve (throttle valve), it is also related to the heat load of the cooled object, the heat transfer area of the evaporator and the capacity of the compressor. related. When one of these three conditions changes, the evaporation pressure and temperature of the refrigeration system will inevitably change accordingly. Therefore, to ensure the stable operation of the evaporation temperature within the specified range, the operator needs to know the change of the evaporation temperature in time. According to the evaporation temperature According to the changing law of the system, the evaporation temperature can be adjusted in a timely and correct manner.
9. Effect of heat load on evaporation temperature
Heat load refers to the heat release of the object to be cooled. When the heat load increases and other conditions remain unchanged, the evaporation temperature will increase, the low-pressure pressure will also increase, and the superheat of the suction gas will also increase. In this case, the expansion valve can only be opened to increase the refrigerant circulation, but the expansion valve cannot be closed to reduce the low pressure due to the increase of the low pressure. Doing so will result in greater suction superheat, increased exhaust temperature, and worsened operating conditions. When adjusting the expansion valve, the adjustment amount should not be too large each time, and it must be operated for a certain period of time after adjustment to reflect whether the heat load and cooling capacity are balanced.
The impact of the energy change of the refrigeration compressor on the evaporation temperature. When the energy of the refrigeration compressor is increased, the suction capacity of the compressor will increase accordingly. When other conditions remain unchanged, the high pressure will increase and the low pressure will decrease. The evaporation temperature will also drop accordingly. In order to continue to maintain the evaporation temperature required by the production process, it is necessary to open a large expansion valve to raise the low pressure to the specified range. After the refrigeration compressor increases the energy to run for a period of time, as the temperature of the object to be cooled drops, the evaporation temperature and low pressure will gradually decrease (the expansion valve does not make any adjustments). This is because the temperature of the object to be cooled decreases and the heat load decreases. . In this case, it should not be mistaken for the pressure drop, which means that the liquid supply is insufficient to open the expansion valve to increase the liquid supply. Instead, the expansion valve should be closed to reduce the energy operation of the refrigeration compressor.
10. Effect of heat transfer area change on evaporation temperature
The heat transfer area mainly refers to the evaporation area of the evaporator, and the change of the heat transfer area mainly refers to the change in the size of the evaporation area. In a complete refrigeration device, the evaporation area is usually fixed, but in actual operation, due to insufficient liquid supply or oil accumulation in the evaporator, the evaporation area is constantly changing. The influence of the increase and decrease of the evaporation area on the evaporation temperature is basically similar to that of the increase and decrease of the heat load on the evaporation temperature. When the evaporation area increases, the evaporation temperature increases; when the evaporation area decreases, the evaporation temperature decreases. In order to maintain the required temperature, the energy and expansion valve should be adjusted, and the evaporator should be drained and cleaned to maintain the relative balance between the heat transfer area and the cooling capacity.
11. The relationship between evaporation pressure and evaporation temperature
The lower the evaporating pressure (low pressure), the lower the evaporating temperature.
The relationship between evaporation temperature and cooling capacity is: when the refrigerant flow rate is constant, the lower the evaporation temperature, the greater the temperature difference with the heat load (hot air), and the greater the cooling capacity. In other words, the lower the evaporation pressure, the greater the cooling capacity , and the same refrigerant with the same mass evaporates at different temperatures, and its latent heat of evaporation is different. The lower the evaporation temperature, the greater the latent heat of evaporation and the stronger the heat absorption capacity.
Condensing temperature: 40°C, degree of superheat: 10°C, degree of subcooling: 5°C, and other conditions unchanged, the influence of the change of evaporation temperature on the cooling capacity, power and COP of the compressor.