Back pressure valve in Centrifugal Compressor

[MacMcMacmac]

Hello. Forgive me, I first posted this in the Chemical Engineering forum by mistake.

I am an operator/maintenance person at a compressor plant which provides 300psi dry air to a supersonic blowdown windtunnel. The unit in question is a 10 stage centrifugal GEC compressor, built in roughly 1959/1960. Design capacity is 40 lbs/sec delivery at 300 psi. In this installation, we have a hydraulically operated back-pressure valve which allows us to adjust the discharge pressure of the compressor independent of the actual tank pressure. I am not sure what the exact purpose of this valve is. It would seem to me that anywhere below the choke point, it would be beneficial to run as low of a back pressure as possible, to maximize air delivery and minimize power consumption. Currently, it is set to 280psi, i.e., the compressor discharge pressure is 280psi regardless of what the downstream pressure is. Obviously, once the tank pressure exceeds the back-pressure valve setting, that is then the pressure the compressor is acting against. There are a few possible conditions that I can see that would require this back pressure valve, the most important of which would be to prevent overwhelming the desiccant air dryer after filters when the tunnel blows down while the compressor is pumping. When this occurs, there is a rapid drop in the back pressure until the valve catches up and returns it to its set point. Also, there is a takeoff for the air before the non-return valve at the tanks, which allows another facility to access air at the back pressure set point regardless of what pressure is in the tank. This is useful for some simulations where a constant pressure is required, but the capacity of the tanks is not required (50,000 cubic ft) so they will not have to wait the 40 minutes necessary to fully charge the tanks.

Other than the two scenarios I have described, is there any performance issues with running a very low back pressure? It is adjustable on the control panel from 100 to 400 psi.

The original manufacturer is long out of business, otherwise I would be asking them.

 

[Warpspeed]

My thoughts on this are that the back pressure valve sets the operating point for the compressor for both flow and pressure ratio. That also sets (limits) the shaft drive torque requirement of the compressor.

So basically you run your compressor at a fixed, and fairly efficient design point on the flow map, at a constant drive horsepower that your prime mover can sustain, and be entirely happy with.

 

[dcasto]

the compressor has to put up some 560,000 feet of head and move 32,000 ACFM, if it doesn't have that, it will create it by internal recycleing of air. When that happens, we call it surgeing. It will destroy the machine.

You could put a huge intake choke and cut the inlet pressure to a deep vacuum, say 3 psia and only set the dischrage to about 50 psig, that would give the required ACFM and head. But the controls would be a nightmare and huge. That compressor looks like it use over 15,000HP.

 

[MacMcMacmac]

Thanks for your ideas on this subject. The compressor in question is 11,250hp, and I previously calculated the output at 29,000cfm, using a pump up time calculation spreadsheet from Kaeser compressors.