1. The suction temperature is too high-mainly caused by the increase in the degree of superheat of the suction. Note that the high suction temperature does not mean the suction pressure is high, because the suction is superheated steam.
Under normal circumstances, the compressor cylinder head should be half cold and half hot. If the suction temperature is too high, all cylinder heads will heat up. If the suction temperature is higher than the normal value, the exhaust temperature will rise accordingly. The main reasons for the excessively high suction temperature of the exhaust gas thermometer are:
(1) The refrigerant charge in the system is insufficient. Even if the expansion valve is opened to the maximum, the liquid supply will not change. In this way, the refrigerant vapor will overheat in the evaporator and the suction temperature will increase.
(2) The opening of the expansion valve is too small, resulting in insufficient refrigerant circulation in the system, less refrigerant entering the evaporator, high superheat, and high suction temperature.
(3) The filter screen of the expansion valve port is blocked, the liquid supply in the evaporator is insufficient, the amount of refrigerant liquid is reduced, and a part of the evaporator is occupied by superheated steam, so the suction temperature increases.
(4) The suction temperature is too high due to other reasons, such as poor heat insulation of the return air pipe or too long pipe, which can cause the suction temperature to be too high.
2. The suction temperature is too low-mainly caused by the large liquid supply of the evaporator and the low suction superheat.
(1) The refrigerant charge is too much, which occupies part of the volume in the condenser, which increases the condensation pressure, and the liquid entering the evaporator increases accordingly. The liquid in the evaporator cannot be completely vaporized, so that the gas sucked by the compressor contains liquid droplets. In this way, the temperature of the return air duct drops, but the evaporation temperature does not change because the pressure does not drop, and the degree of superheat decreases. Even if the expansion valve is closed, there is no significant improvement.
(2) The opening of the expansion valve is too large. Because the temperature sensing element is loosely tied, the contact area with the return pipe is small, or the temperature sensing element is not wrapped with insulating material and its wrapping position is wrong, etc., the temperature measured by the temperature sensing element is inaccurate and close to the ambient temperature, causing the expansion valve to operate. The opening degree increases, resulting in excessive liquid supply.
The compressor discharge temperature can be read from the discharge thermometer on the discharge pipe. It is related to the refrigerant's adiabatic index, compression ratio (condensing pressure/evaporating pressure) and suction temperature. The higher the suction temperature and the higher the compression ratio, the higher the exhaust temperature, and vice versa.
When the suction pressure does not change, when the exhaust pressure increases, the exhaust temperature rises; if the exhaust pressure does not change, when the suction pressure drops, the exhaust temperature also rises. Both of these cases are caused by the increase in compression ratio. Too high condensing temperature and exhaust temperature are detrimental to the operation of the compressor and should be prevented. Excessive exhaust temperature will cause the lubricating oil to become thinner or even carbonize and coke, thereby worsening the compressor lubrication conditions. The exhaust temperature on the exhaust gas thermometer is directly proportional to the compression ratio (condensing pressure/evaporating pressure) and the suction temperature. If the superheating temperature of the suction is high and the compression ratio is large, the exhaust temperature is also high. If the suction pressure and temperature do not change, when the exhaust pressure increases, the exhaust temperature also increases.