1、 Factors affecting evaporation temperature changes:

During the actual operation of a refrigeration device, the variation of evaporation temperature is very complex. In addition to being directly controlled by an expansion valve (throttle valve), it is also related to the thermal load of the cooled object, the heat transfer area of the evaporator, and the capacity of the compressor. When one of these three conditions changes, the evaporation pressure and temperature of the refrigeration system will inevitably change accordingly. Therefore, to ensure stable operation of the evaporation temperature within the specified range, operators need to timely understand the changes in evaporation temperature, and adjust the evaporation temperature in a timely and correct manner based on the changes in evaporation temperature.

1. The impact of changes in heat load on evaporation temperature:

The so-called heat load refers to the heat released by the cooled object. The change in heat load is the change in the amount of heat released by the cooled object. During the operation of a refrigeration device, changes in heat load often occur. When the heat load increases and other conditions remain unchanged, the evaporation temperature will increase, the low-pressure pressure will also increase, and the degree of overheating of the suction will also increase. In this case, the expansion valve can only be opened to increase the circulation of refrigerant, and the expansion valve cannot be closed to reduce the low-pressure pressure due to the increase in low-pressure pressure. This will increase the degree of superheat in the intake, increase the exhaust temperature, and worsen the operating conditions. When adjusting the expansion valve, the adjustment amount should not be too large each time. After adjustment, a certain period of operation must be passed to reflect whether the heat load and cooling capacity are balanced.

The impact of energy changes in refrigeration compressors on evaporation temperature. When the energy of the refrigeration compressor is increased, the suction capacity of the compressor correspondingly increases. Under other conditions that remain unchanged, there will be an increase in high pressure, a decrease in low pressure, and a decrease in evaporation temperature. In order to continue maintaining the evaporation temperature required by the production process, it is necessary to open the expansion valve to increase the low-pressure pressure to the specified range. After increasing the energy of the refrigeration compressor for a period of time, as the temperature of the cooled object decreases, the evaporation temperature and low-pressure pressure will gradually decrease (the expansion valve does not make any adjustments), which is because the temperature of the cooled object decreases and the heat load decreases. In this case, it should not be mistaken for a decrease in pressure. It is due to insufficient liquid supply to open the expansion valve and increase the liquid supply. Instead, the expansion valve should be closed to reduce the energy operation of the refrigeration compressor. Otherwise, excessive energy may occur, which may cause the refrigeration unit to run with liquid or run out of oil.

2. The impact of changes in heat transfer area on evaporation temperature:

The heat transfer area mainly refers to the evaporation area of the evaporator, and the change in 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 and unchanged, but in actual operation, due to insufficient liquid supply or oil accumulation in the evaporator, the evaporation area is constantly changing. The impact of increasing or decreasing evaporation area on evaporation temperature is basically similar to that of increasing or decreasing heat load on evaporation temperature. When the evaporation area increases, the evaporation temperature will increase; When the evaporation area decreases, the evaporation temperature will decrease. In order to maintain the required temperature, the energy and expansion valve should be adjusted to drain and clean the evaporator, in order to maintain a relative balance between the heat transfer area and the cooling capacity.

After the high-pressure steam of the refrigerant discharged by the compressor enters the condenser, it needs to be cooled by the cooling medium (otherwise it cannot be liquefied). If the cooling effect is not good, the heat of the refrigerant in the condenser cannot be taken away smoothly, so the condensation temperature will naturally increase, and the corresponding condensation pressure will also increase.

From the design of the refrigeration system, the determination of the condensation temperature should be based on the cooling environment, which means that the condensation temperature should be higher than the temperature of the cooling medium, otherwise the heat of the refrigerant in the condenser cannot be transferred to the cooling medium. Taking water-cooled units as an example, the condensation temperature of water-cooled units is affected by the cooling water temperature, and the majority of cooling water cooling methods currently use cooling towers. According to the principle of cooling towers, the cooling limit of cooling water is related to the wet bulb temperature of the environment (it can only be close to the wet bulb temperature, not lower than the wet bulb temperature). In this way, based on the statistical data of climatic conditions, the temperature that cooling water can maintain under normal conditions can be determined (the outlet temperature of cooling towers used in general air conditioning is 32 ℃ under rated conditions). Based on this condition, combined with the reasonable heat exchange temperature difference of the condenser, the reasonable condensation temperature of the refrigeration host can be determined during design.

The so-called reasonable heat transfer temperature difference is calculated based on the heat transfer coefficient of a new heat exchanger. After the heat exchanger is used to produce scaling, the heat transfer coefficient will decrease, and the heat transfer temperature difference will increase. However, if the temperature of the cooling medium is still limited by the environment, the condensation temperature will rise.

After the size and compression efficiency of the condenser and evaporator are determined, for example, the standard operating condition of the unit design is that when the unit is operating at 100% full load, the outlet water of the condenser is 40 ℃, and the outlet water of the evaporator is 2 ℃.

Their control logic: The standard operating condition of the evaporator is 2 ℃, so the control program will target 2 ℃. When the water outlet does not reach 2 ℃, the program will load. When the temperature reaches 2 ℃, the unit will load down, and the control program of the main engine is to maintain the water outlet of the unit at around 2 ℃ and perform corresponding loading/unloading. Of course, during this process, the expansion valve also needs to make corresponding actions. If the evaporator is full of liquid, the size of the expansion valve is generally determined based on the overheating size of the compressor. If it is a direct expansion type, the size of the expansion valve should be determined based on the suction superheat of the compressor.

The heat exchanger can be designed separately, but it must be combined with conventional heat exchange conditions (such as a conventional water condenser with water inlet of 37 ℃ and water outlet of 32 ℃, and a conventional evaporator with water inlet of 12 ℃ and water outlet of 7 ℃, which is suitable for the operating conditions of the general host).

The heat exchanger manufacturer can design the heat exchanger into various sizes (heat exchange capacity) for the convenience of the host manufacturer's matching selection. Of course, there are also many cases of designing heat exchangers separately for a certain type of host.

2、 Main operating parameters:

Refrigeration capacity=refrigerant circulation capacity * enthalpy difference on the evaporator side

For our refrigeration cycle, when the evaporation pressure rises, the enthalpy difference on the evaporator side changes relatively little, and can be assumed to be a fixed value when making such calculations.

The refrigeration capacity is proportional to the refrigerant circulation capacity (kg/s)

For compressors, the displacement (theoretical suction) of the compressor is a fixed value in m3/h

Assuming that the change in pressure has little effect on the volumetric efficiency of the compressor and can be ignored, then the actual displacement of the compressor=theoretical displacement * volumetric efficiency (fixed)

That is, the actual displacement of the compressor is also a fixed value.

The unit of displacement is m3/h and the unit of refrigerant circulation is kg/s.

Their difference is: refrigerant circulation rate=compressor discharge rate/suction side specific volume (m3/kg)

Then, the refrigeration ratio is directly proportional to the refrigerant circulation quantity and inversely proportional to the specific volume on the suction side.

The specific volume on the suction side, as shown on the pressure enthalpy diagram, is influenced by the pressure and temperature on the suction side.

Suction side pressure=evaporation pressure - suction side pipeline pressure loss

In general, due to the small change in superheat on the suction side with the evaporation temperature, the pressure loss value of the suction side pipeline is relatively small compared to the evaporation pressure, and its change can be ignored.

All the reasons are attributed to the evaporation temperature and its corresponding evaporation pressure.

The evaporation temperature increases, the evaporation pressure increases, the specific volume decreases, the refrigerant circulation volume increases, and the refrigeration capacity increases.

This is why the higher the evaporation temperature, the greater the cooling capacity.