# Can a compressor give the same discharge pressure even if suction pressure varies?

The task is to maintain compressor discharge pressure at 200 bars. The inlet pressure varies with time from 200 bars to 10 bars (emptying a compressed air tank and compressing to transfer it to another tank under 200 bars). Which means : At t=0 Pin=200, Pout=200, dP=0 bars At t=t1 Pin = 150, Pout=200, dP=50 bars ... At t=tf Pin = 10, Pout = 200, dP=190 bars

Is this possible for a compressor? I thought about installing a pressure regulator valve at the inlet to always maintain the pressure Pin at 10 bars. This way, dP will always be = 190 bars for all the above cases.

• Yes, there are types that can do that. It would help if you indicated what you are actually doing. If you are trying to fill a large LNG tanker, or a portable oxygen bottle, the smart solution will be different. Commented Mar 19 at 21:37

It is possible to maintain a constant discharge pressure for a compressor even if the suction pressure varies, but this requires a system that can adjust the compressor's output to match the changes in the suction pressure.

In your case, installing a pressure regulator valve at the inlet to maintain the inlet pressure at 10 bars could be a viable solution, as you suggested. By doing so, the difference between the inlet pressure and the discharge pressure (dP) will always be 190 bars, regardless of the fluctuations in the inlet pressure.

It's worth noting that this solution will not be effective if the suction pressure varies significantly beyond the range of 10 to 200 bars. In that case, the compressor's output would not be able to match the changes in the suction pressure, and the discharge pressure would not be maintained at 200 bars.

In summary, maintaining a constant discharge pressure for a compressor with a variable suction pressure is possible, but it requires a system that can adjust the compressor's output to match the changes in the suction pressure. A pressure regulator valve is one option that can be used to achieve this.

You could use a pressure regulator on the inlet side, but you wouldn't want to. This would mean that your pump would always be working hard. When the inlet pressure is 200 bar, theoretically your compressor should have to do 0 work.

A better solution would be to add some kind of control system to your compressor. Different compressor types would require different control systems.

You'll probably have a three stage compressor with intercooling. Would it be possible to introduce air from the tank at intermediate stages? For example introduce air at about 74bar first to the suction side of the high stage, then 27bar at the suction side of the second stage and finally 10bar at the low end, using 3 PRVs. This would save some energy, but adds complication.

In general the discharge pressure of a compressor is determined by the system into which it is discharging. In this case the pressure in the receiving tank determines the discharge pressure, (unless you are discharging into an extremely long pipe of insufficient diameter.) This pressure will rise as the tank fills.