The concept of compression ratio, over-compression, and under-compression
The compression ratio of a compressor is theoretically the ratio of the absolute discharge pressure to the absolute suction pressure.
For example, if the absolute exhaust pressure is 20 bar and the absolute suction pressure is 5 bar, then the compression ratio is 4.
There are also volume ratios, such as the ratio of the discharge volume per revolution to the volume of the suction chamber.
The compression ratio mentioned here is the ratio of pressure and the ratio between the maximum and minimum volume of the compressor when compressing, and this data will never change.
For compressors, the compression ratio is a key concept to measure the size of the compressor’s compression stroke and a reference value for the point of optimization of the compressor’s efficiency.
In the case of a manual compressor like a bicycle pump, for example, the length of the pump means different compression ratios.
But there is definitely an optimal length that is best suited for bicycle tires.
The suction pressure of a bicycle pump is basically equal to the atmospheric pressure.
The exhaust pressure is the tire pressure needed for a bicycle tire, a relatively stable operating condition, and it is relatively easy to determine the compression ratio.
If you use a bicycle pump to inflate a balloon, the required pressure is small compared to a bicycle tire, and there is a problem of over-compression.
Meaning that the gas pressure before the compressor completes compression and exhaust is beyond the required pressure, and the compressor wastes power.
On the other hand, if you take a bicycle pump and inflate a car tire.
By the same token, under-compression will occur and the pressure after compression is completed will not meet the demand.
This leaves it up to the device of repeated compression and one-way backpressure control to achieve the required pressure.
For scroll compressors, over-and under-compression can occur.
With either over-or under-compression, the performance of the compressor will be somewhat below the optimum point and the reliability will be reduced.
This just can also explain the current refrigeration industry, many users of the air conditioning scroll compressor are used for high and medium-temperature cold storage.
The end result is low efficiency, the compressor overheats easily burned, which is the same reason.
The Meaning of Compressor Compression Ratio
When the discharge pressure increases and the suction pressure decreases.
The compression ratio increases, the compressor power consumption increases.
The performance of the compressor decreases, and the performance of the system decreases.
Conversely, when the discharge pressure decreases and the suction pressure increases, the compression ratio decreases.
Although the performance of the compressor decreases, the cooling capacity of the whole system increases.
Collectively, the efficiency of the system is increased.
This concept of compression ratio and the relationship between the system.
The greatest significance is to help us in the compressor selection, to determine which compressor is the most suitable for you.
When we do compressor selection, we need to consider the compression ratio range of the HVAC&R system in all operating conditions throughout the year.
That is the high-pressure range and low-pressure range.
Then corresponding to this operating chart, the basis of judgment is that the compressor operating range can cover the actual operation of the refrigeration system will occur in the range.
Then this compressor is the right choice, otherwise, it is not.
If many compressors are available at the same time to meet the needs of the application.
Only then do we have room to consider other performance indicators, such as COP, noise, size, interface, etc.
However, the primary prerequisite is to determine if the compressor can operate properly all year round.
Read more: how to choose the right refrigeration compressor.
The Impact of Over-compression, Under-compression
The compression ratio is too small, the compressor volume efficiency is seriously reduced (limit state: compression ratio of 1, the compressor stopped working), thus losing the refrigeration capacity.
The compressor compression ratio is too large, the compressor motor in a long-term high-load operation state will reduce the service life of the compressor, serious more will burn the compressor.
Not only that, the compressor scroll disk design strength is limited, the compression ratio is too large will directly lead to damage to the scroll disk, resulting in serious consequences.
Read more: Types of compressor