Answer: Low evaporation temperature is reflected in the evaporation pressure being lower than the normal value. The reasons for low evaporation temperature are:
(1) The heat exchange area of the evaporator is too small during design and production;
(2) The compressor is too large or the system is not filled with enough refrigerant, etc.;
(3) The capillary tube or expansion valve is blocked or opened too small, resulting in insufficient refrigerant supply;
(4) The load of the dryer is too small;
(5) Frost on the surface of the evaporator copper tube affects heat transfer;
(6) The compressed air contains too much oil, and a layer of oil stain is formed on the surface of the evaporator copper tube, affecting heat transfer;
(7) There is a chronic leak in the refrigeration system pipeline.
If the evaporation temperature drops very low, so that the surface temperature of the heat exchange copper tube is below 0℃, water vapor will condense into frost on the surface of the copper tube, which will bring many adverse consequences to the refrigeration system, such as: "liquid compression", gas path blockage, resulting in a significant decrease in the compressor refrigeration coefficient, and increased energy consumption. Therefore, when compressed air with a large moisture content enters the cold dryer, the evaporation temperature of the refrigerant in the tube should at least ensure that the surface temperature of the evaporator copper tube is above 0°C.
In order to prevent the evaporation temperature from being too low, energy bypass protection is set in the cold dryer. When the refrigerant evaporation pressure drops to a certain value, the hot gas bypass valve for heat supplementation automatically opens (the opening increases), and the high-pressure and high-temperature refrigerant steam in the condenser is directly injected into the evaporator. The evaporation pressure is raised to a normal level.
