Do you understand the compression ratio, internal volume ratio, and internal pressure ratio of the compressor?

Just now, someone was discussing the issue of compression ratio and internal volume ratio. We have sorted out some relevant knowledge for your peers to learn.

1. The concept of compression ratio (external pressure ratio), internal volume ratio, and internal pressure ratio

Compression ratio: (external pressure ratio) is the ratio of the condensing pressure and the evaporating pressure of the refrigeration system (that is, the pressure corresponding to the designed condensing temperature and the evaporating temperature) (both use absolute pressure). Compression ratio refers to the ratio of absolute exhaust pressure to absolute suction pressure. For example, if the exhaust absolute pressure is 20 bar and the suction absolute pressure is 5 bar, then the compression ratio is 4.

Internal volume ratio: It is an important parameter of continuous compressors such as positive displacement compressors. The exhaust volume is the volume value when the inter-tooth volume is connected to the exhaust orifice, that is, the volume value at the end of the compression process; the suction volume is the volume value when the inter-tooth volume is disconnected from the suction orifice, that is, the volume value at the end of the suction process. The internal volume ratio is a parameter determined by the mechanical structure, that is, the ratio of the suction volume to the exhaust volume.

Internal pressure ratio: It is the ratio of the pressure in the exhaust chamber of the refrigeration compressor to the pressure in the suction chamber (that is, the pressure in the cylinder at the end of suction and exhaust) (both use absolute pressure).

2. The concept of over-compression and under-compression

For compressors, the compression ratio is a key concept to measure the size of the compressor compression stroke, and it is also a reference value for the optimization point of compressor efficiency. (The article comes from Refrigeration Baijia.com. For more information, please log in to Refrigeration Baijia.com to download and learn)

In layman's terms, taking a manual compressor such as a bicycle pump as an example, the length of the pump means different compression ratios, but there is definitely an optimal length that is most suitable for bicycle tires. The suction pressure of the pump is basically equal to the atmospheric pressure, and the exhaust pressure is the tire pressure required by the tire. It is a relatively stable operating condition, and it is easier to determine the compression ratio.

If you use the pump to inflate a balloon, the required pressure is relatively small compared to a bicycle tire, and there will be a problem of over-compression. This means that the gas pressure at the moment before the compressor completes the compression and exhaust is higher than the required pressure, and the compressor wastes power in vain.

On the other hand, if you use a bicycle pump to inflate a car tire, the same principle will cause under-compression, and the pressure after compression is not enough to meet the demand. In this way, the required pressure can only be achieved by repeated compression and one-way back pressure valves and other devices.

Also because of the design of the compressor structure, for scroll compressors and screw compressors, it is a continuous suction and exhaust principle, and over-compression and under-compression may occur; while for compressors that use back pressure control (limited pressure device) to achieve compression function in a single volume chamber, such as piston compressors, rolling rotor compressors, etc., there will be no over-compression, but under-compression will occur.

When the screw machine is under-compressed, at the end of compression, the exhaust pressure is lower than the condensing pressure, which will form a momentary reflux. When the exhaust pressure of the rotor is balanced with the condensing pressure, the gas will be discharged. This reflux will increase the power consumption of the compressor.

Whether it is over-compression or under-compression, the performance of the compressor will be lower than the optimal point to a certain extent, and the reliability will also be reduced.

3. The significance of compression ratio

When the exhaust pressure increases and the suction pressure decreases, the compression ratio increases, the power consumption of the compressor increases, the performance of the compressor decreases, and the performance of the system decreases. Conversely, when the exhaust pressure decreases and the suction pressure increases, the compression ratio decreases. Although the performance of the compressor decreases, the refrigeration capacity of the entire system increases. In general, the efficiency of the system increases.

The concept of compression ratio and the relationship between the system are of the greatest significance in helping us to determine which compressor is the most suitable compressor for the current application when selecting the compressor.

The following figure is the operating range diagram of several scroll compressors of the same type. The definition of the operating compression ratio is that the slope of the oblique line connecting any point in the figure with the origin is the compression ratio of the compressor working at this point. This can help us quickly and directional compare the compression ratio size at different working points.

When selecting a compressor, it is necessary to consider the compression ratio range, high pressure range and low pressure range of the actual application system in all operating conditions throughout the year and seasons, and then correspond to this operation diagram. The basis for judgment is that the operating range of the compressor can cover the range of the actual operation of the system. Then this compressor is the appropriate choice, otherwise it is not.

If there are many compressors that can meet the application requirements at the same time, there is room to consider other performance indicators, such as COP, noise, size, interface, etc. The first prerequisite is to determine whether the compressor can operate normally throughout the year.