Six Questions About Air Compressors

1. Intake temperature is too high

（1）An excessively high intake temperature is primarily caused by increased superheat in the intake steam. It is important to note that a high intake temperature does not necessarily indicate high intake pressure, as the intake medium is superheated vapor. Under normal operating conditions, the compressor cylinder head should exhibit a temperature gradient—cool on one side and warm on the other. However, when the intake temperature is too high, the entire cylinder head becomes overheated. As a result, an elevated intake temperature will also lead to a corresponding rise in exhaust temperature.

(2) The main reasons for the high suction temperature:

① Insufficient refrigerant filling in the system. Even if the expansion valve is opened to the maximum, the liquid supply will not change. In this way, the refrigerant vapor will be overheated in the evaporator, causing the suction temperature to rise.

② The expansion valve is opened too small. This causes insufficient circulation of the system refrigerant, a small amount of refrigerant entering the evaporator, a large degree of superheat, and thus a high suction temperature.

③ The expansion valve port filter is clogged. The liquid supply in the evaporator is insufficient, the amount of refrigerant liquid is reduced, and part of the evaporator is occupied by superheated steam, so the suction temperature rises.

④ Other reasons cause the suction temperature to be too high. For example, if the return air duct is poorly insulated or the duct is too long, it can cause the suction temperature to be too high.

2. The suction temperature is too low

(1) Too much refrigerant is filled. It occupies part of the volume in the condenser and increases the condensing pressure, which increases the amount of liquid entering the evaporator. The liquid in the evaporator cannot be completely vaporized, so the gas sucked into the compressor contains liquid droplets. In this way, the temperature of the return air pipe drops, but the evaporation temperature does not change because the pressure does not drop, and the superheat decreases. Even if the expansion valve is closed, there is no significant improvement.

(2) An overly large opening of the expansion valve is caused by inaccurate temperature sensing. This inaccuracy may result from the temperature sensor being loosely attached, resulting in minimal contact with the return air pipe, or from improper installation—such as lacking insulation or being positioned incorrectly. As a result, the sensor reads a temperature closer to the ambient environment rather than the actual return air temperature, leading the expansion valve to open more than necessary and causing an excessive supply of refrigerant.

(3) Reason: Insufficient refrigerant filling will condense from the evaporator to the compressor. In addition, due to external reasons, the refrigerant evaporates insufficiently or even does not evaporate in the evaporator, which will cause severe frost and even wet compression.

(If the central air conditioner does not return enough air or the air conditioner box filter is seriously clogged, the return air pipe of the chiller compressor will frost and the exhaust temperature will be very low)

3. Abnormal exhaust temperature

Abnormal exhaust temperature – influencing factors: adiabatic index, compression ratio, suction temperature.

(1) The compressor’s exhaust temperature can be monitored via the thermometer installed on the exhaust pipe. This temperature is influenced by several factors, including the refrigerant’s adiabatic index, the compression ratio (defined as condensing pressure divided by evaporating pressure), and the suction temperature. An increase in suction temperature or compression ratio will lead to a rise in exhaust temperature, while a decrease in these parameters will result in a lower exhaust temperature.

(2) When the suction pressure remains unchanged and the exhaust pressure increases, the exhaust temperature rises; if the exhaust pressure remains unchanged and the suction pressure decreases, the exhaust temperature also rises. Both situations are caused by the increase in compression ratio. Excessive condensation temperature and exhaust temperature are not good for the operation of the compressor and should be prevented. Excessive exhaust temperature will make the lubricating oil thinner or even carbonize and coke, thereby deteriorating the lubrication conditions of the compressor.

(3) The exhaust temperature is proportional to the compression ratio (condensing pressure/evaporating pressure) and the suction temperature. If the suction superheat temperature is high and the compression ratio is large, the exhaust temperature will also be high. If the suction pressure and temperature remain unchanged, when the exhaust pressure increases, the exhaust temperature also increases.

4. The main reasons for the increase in exhaust temperature are:

(1) When the suction temperature is elevated, the temperature of the refrigerant vapor after compression also becomes higher.

(2) A high condensation temperature leads to increased condensation pressure, which in turn raises the exhaust temperature.

(3) If the exhaust valve plate is damaged, high-pressure vapor undergoes repeated compression, resulting in a temperature rise. This causes both the cylinder and cylinder head to overheat, and the reading on the exhaust pipe thermometer also increases.

5. High exhaust pressure

(1) Exhaust pressure generally corresponds to the condensing temperature. Under normal circumstances, the exhaust pressure of the compressor is very close to the condensing pressure. A high exhaust pressure will increase the compression work, reduce the gas transmission coefficient, and thus reduce the refrigeration efficiency. When the condensing pressure increases, the compressor exhaust temperature also increases.

(2) If the exhaust temperature is too high, it will increase the consumption of the compressor lubricating oil, making the oil thinner and affecting lubrication; when the exhaust temperature is close to the flash point of the compressor oil, it will also cause part of the lubricating oil to carbonize and accumulate at the suction and exhaust valve ports, affecting the sealing of the valve.

(3) Lowering the temperature of the cooling medium can reduce the condensing temperature and the condensing pressure, but this is subject to environmental conditions and is difficult to select artificially. Increasing the flow rate of the cooling medium can reduce the condensing temperature a little (this method is often used). However, the flow rate of cooling water or air cannot be increased unilaterally, because this will increase the power of the cooling water pump or fan and motor, and should be considered comprehensively.

6. Insufficient exhaust volume

Insufficient exhaust volume – mainly compared with the designed air volume of the compressor, is one of the most common faults of the compressor. It is mainly caused by the following reasons:

(1) The air intake filter is blocked by dirt or the compressor suction pipe is too long and the pipe diameter is too small, which increases the suction resistance, affects the suction volume and reduces the exhaust volume.

(2) The compressor speed is reduced, which reduces the exhaust volume. If the compressor is used in an improper environment, the exhaust volume will also be affected.

(3) The packing is not tightly sealed and leaks, which reduces the exhaust volume. The first reason is that the packing itself does not meet the requirements during manufacturing; the second reason may be due to wear and tear during installation, which causes leakage; generally, lubricating oil is added to the packing to play a role in lubrication, sealing and cooling.