Compare the performance of three commonly used refrigeration compressors, and clearly understand the outdoor unit II

The outdoor unit is an important part of the whole central air conditioner, and the compressor is the heart of the outdoor unit. The length of air conditioner and the cooling and heating capacity of air conditioner are all important working parts of the compressor. Compressors are generally used in rooftop machines, air-cooled screws, water-cooled screws, etc. In the last article, we learned the basic knowledge of these three compressors. Today, let’s continue to understand the performance characteristics of these three compressors and refrigerators. What are their specific characteristics?

In the refrigeration system, because the working principle of reciprocating refrigeration compressor and screw refrigeration compressor is different, the reasons affecting their pressure loss and leakage loss are also different.

For reciprocating refrigeration compressor, the main reasons that affect its pressure loss and leakage loss are the quality of air valve and its sealing when closed. This is because the spring resistance (compression spring) should be overcome when the suction valve is opened, and when the gas flows through the air valve, because of the small cross section and high flow speed, a certain flow resistance will be produced, so the pressure of the gas in the cylinder of the reciprocating refrigeration compressor is always lower than that in the suction pipe during the suction process. In the same way, the pressure of gas in the cylinder of reciprocating refrigeration compressor is always higher than that in the suction pipe. If the passage section of the air valve is smaller, the greater the resistance loss will be. If the weight of the valve plate is large and the spring force of the valve is also large, the resistance loss will also increase, so the pressure coefficient value will decrease.

For screw refrigeration compressor, the main reason that affects its pressure loss and leakage loss is the gas flow rate.

In screw refrigeration compressor, the performance of screw is the key. If the tooth shape of the screw is symmetrical circular arc, its manufacture is simple. If the tooth shape of screw is asymmetric linear, it has a large gas transmission capacity and high efficiency. If the length-diameter ratio of screw is reduced, it can make the screw have good strength, increase the operation reliability of screw refrigeration compressor, and help screw refrigeration compressor develop towards high pressure ratio.

In the screw refrigeration compressor, the two pairs of screws with the same diameter and length, and the pair with large rotor area utilization coefficient have large displacement. On the surface, the greater the rotor area utilization coefficient, the better the performance of screw refrigeration compressor. However, if the rotor area utilization coefficient is too large, the strength and stiffness of the screw will be reduced.

In screw refrigeration compressor, reducing the number of teeth of screw can increase the area between the teeth of screw and increase the displacement of screw refrigeration compressor. On the surface, the smaller the number of teeth of screw, the better the performance of screw refrigeration compressor. However, if the number of teeth of the screw is too small, the bending strength and stiffness of the screw will be reduced.

In the screw refrigeration compressor, by increasing the peripheral speed of the screw, the size and mass of the screw refrigeration compressor can be reduced, and the relative leakage of gas through the gap in the screw refrigeration compressor can be reduced, which is beneficial to improve the volumetric efficiency and thermal efficiency of the screw refrigeration compressor. On the surface, the faster the circumferential speed of screw, the better the performance of screw refrigeration compressor. However, if the circumferential speed of the screw is too fast, the flow loss of gas in the suction and exhaust ports and the circumferential speed between teeth will increase accordingly.

Among the three common refrigeration compressors (reciprocating, screw and centrifugal), the inertia caused by reciprocating motion is the main disadvantage of reciprocating refrigeration compressors. Because of the inertia caused by reciprocating motion, the air valve and crank-connecting rod mechanism in reciprocating refrigeration compressor are most vulnerable to breakage.

Among the three common refrigeration compressors (reciprocating, screw and centrifugal), the huge noise generated during operation is the main disadvantage of screw refrigeration compressor. Because it is often affected by refrigerant gas periodically passing through suction and exhaust ports at high speed and leaking through gaps, it is necessary to select reasonable screw running speed in screw refrigeration compressor.

Among three common refrigeration compressors (reciprocating, screw and centrifugal), surge is the main disadvantage of centrifugal refrigeration compressor.

The surge of centrifugal refrigeration compressor is caused by the fact that when the cooling water inflow of condenser is reduced to a certain extent, the flow of centrifugal refrigeration compressor is reduced to a very small extent, and serious gas outflow occurs in its channel, and its outlet pressure drops suddenly. Although the centrifugal refrigeration compressor and condenser work together, the pressure of gas in the condenser does not decrease at the same time, so the pressure of gas in the condenser is greater than the outlet pressure of the centrifugal refrigeration compressor, causing the gas in the condenser to flow back to the centrifugal refrigeration compressor until the gas pressure in the condenser drops to equal to the outlet pressure of the centrifugal refrigeration compressor.

That is, the centrifugal refrigeration compressor starts to send air to the condenser again, the flow rate increases, and the centrifugal refrigeration compressor resumes normal operation. However, when the gas pressure in the condenser returns to the original gas pressure, the flow rate of the centrifugal refrigeration compressor decreases again, the outlet pressure of the centrifugal refrigeration compressor starts to drop, and the gas flows backwards again. In this way, periodic oscillation of air flow occurs. Therefore, in the centrifugal refrigeration compressor, the cooling water quantity of the condenser should not be too small, otherwise, it will cause strong vibration when the centrifugal refrigeration compressor is running, and even damage the centrifugal refrigeration compressor in severe cases.

In the refrigeration system, these three kinds of common refrigeration compressors have their own advantages and disadvantages, so when choosing compressors, we should choose their greatest advantages.

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