The issue of internal and external pressure ratios in screw compressors

This post was last edited by lovestory on 2011-12-17 22:15. When the exhaust pressure is lower than the pressure at the end of internal compression, at the moment when the two are connected, gas flows rapidly toward the exhaust port under the effect of the pressure difference (Pi-Pd), causing the gas pressure in the elementary volume to drop sharply to Pd. Only then, as the elementary volume continues to shrink, is the gas expelled. This is known as over-compression phenomenon. When the exhaust pressure is greater than the pressure at the end of internal compression, that is, when the gas pressure Pi in the elementary volume has not yet reached the desired value Pd, it is already in communication with the exhaust port. In this way, at the moment of connection, the gas in the exhaust port will rapidly flow back into the elemental volume, causing the pressure there to rise suddenly from Pi to Pd; thereafter, as the elemental volume continues to shrink, the gas is expelled. This is known as undercompression. The internal pressure ratio is the ratio of the pressure at the end of internal compression to the intake pressure; therefore, adjusting the internal pressure ratio does not affect the exhaust pressure. 1# kb0227

The internal pressure ratio refers to the degree of pressure increase exerted by the rotor of the screw compressor itself, while the external pressure ratio is determined by the combination of the condenser and evaporator. When the external pressure ratio is greater than the internal pressure ratio, it is referred to as under compression; when the external pressure ratio is less than the internal pressure ratio, it is called over compression. Both conditions lead to a decrease in the efficiency of the unit (not the compressor’s efficiency – it’s easy to confuse the efficiency of the entire unit with that of the compressor as well).

3# fld8882005 If the compressed gas is not cooled (in which case there is no condensation pressure), then how is the exhaust pressure determined? Is it related to the pressure at the end of compression?

For screw compressors, the compression process within them is independent of the back pressure.

It’s not easy to represent the internal pressure ratio with pictures; it’s much easier to understand by looking at the structure of a screw compressor. The forum has many actual photos of screw compressor maintenance.

This post was last edited by yapbest on 2011-2-15 at 11:39. I also want to learn some knowledge in this area. On our WuLeng screw compressor, the vibration level increases suddenly when the load is raised above 90%. The inlet pressure is 0.08 (gauge pressure), the outlet pressure is 1.1 (gauge pressure), giving an internal pressure ratio of 6.1; I’m not sure what’s causing this

The external pressure ratio = condensation pressure/evaporation pressure = (exhaust pressure + 0.1)/(suction pressure + 0.1). The internal pressure ratio is related to the rotor structure; when the internal pressure ratio differs from the external pressure ratio, the efficiency of the compressor decreases.

Reply to 2# magnolialjp1986 – it’s quite interesting. Under-compression is equivalent to an increase in the duration of a single complete unit of work time ； What about over-compression? The unit compression time remains unchanged; it’s just that the compressor’s power consumption increases. It’s like going around in a big circle, or using a heavy engine to drive a small vehicle. Is that what you mean?

Could someone explain why the internal pressure ratio and the external pressure ratio are not equal?

Reply to 10# yapbest: Hehe, Wu Leng’s machines tend to develop this problem after a while.