Pressure And Temperature Of Evaporation And Condensation

Pressure And Temperature Of Evaporation And Condensation

1. Evaporating temperature and evaporating pressure

Evaporation temperature and evaporation pressure are two important parameters determined based on user needs. The evaporation temperature during equipment operation should be set according to the temperature requirements of the cooled medium and its working characteristics. For example, in a direct evaporation cold storage, if the air is natural convection, the evaporation temperature is usually 10~15℃ lower than the required storage temperature; if it is forced circulation, this difference will be reduced to 5~10℃. For liquid cooling applications, the evaporation temperature should be 4~6℃ lower than the average temperature of the cooled liquid.

From the perspective of energy saving, it is beneficial to appropriately increase the evaporation temperature. Studies have shown that when the cold storage temperature is increased from -30℃ to -25℃, the power consumption can be reduced by 9.8% due to the increase in evaporation temperature. Therefore, without affecting the product quality, it is possible to consider moderately increasing the evaporation temperature to achieve energy saving.

At this time, by reducing the heat transfer temperature difference of the evaporator, the evaporation temperature can be further increased without sacrificing the cooling effect. During the adjustment process, observe the change in evaporation pressure to determine whether the expansion valve opening is appropriate. If the expansion valve is opened too small, the liquid supply will be insufficient, which will cause the evaporation pressure and temperature to drop, and the compressor suction will be overheated and the exhaust temperature will rise; on the contrary, if the opening is too large, it may cause the evaporation pressure and temperature to be too high, and even cause a liquid hammer accident. Therefore, it is very important to correctly control the opening degree of the throttle valve.

2.Condensing temperature and condensing pressure

The condensing temperature refers to the saturation temperature of the refrigerant when it changes from gas to liquid inside the condenser, and the condensing pressure refers to the corresponding pressure value at this time. Under normal circumstances, the condensation temperature of a water-cooled condenser is 3-5℃ higher than the cooling water inlet temperature; for air-cooled units, the condensation temperature is approximately equal to the ambient temperature plus 10~15℃.

When the evaporation temperature remains unchanged, the increase in condensation temperature will cause the condensation pressure to rise, thereby increasing the compression ratio of the compressor, reducing the gas transmission coefficient, reducing the cooling capacity and increasing energy consumption. More importantly, a higher exhaust temperature will make the lubricating oil thinner and affect the lubrication effect. Especially when it is close to or exceeds the flash point of the lubricating oil, it may cause the lubricating oil to carbonize and deposit on the exhaust valve, damaging the valve sealing. In order to maintain a low condensing temperature, on the one hand, this can be achieved by lowering the inlet temperature of the cooling water; on the other hand, this can also be achieved by increasing the amount of cooling water.