VFD on Liquid Ring Compressor

[david6245]

Hello,

I am looking into potential energy savings on retrofitting an A/C motor with a VFD. The machine is a positive displacement, SIHI, liquid ring compressor. The performance curve I have is a bit confusing to me. Typically, I see centrifugal type machines as having the required power increase with increasing flow. However, the curve for my positive displacement compressor shows the opposite relationship. It shows that as the flow decreases, the power graph increases. So if a VFD is to slow the speed of the motor at times of low flow, will I actually be using more power at this low flow operating point? This seems counter-intuitive to me. I hope I am just misunderstanding the compressor curve, which I have attached.

Any help would be appreciated.

Thanks.

 

[ccjersey]

I don't read the graph like you did. Power vs discharge pressure is shown in the second graph and clearly rises as discharge pressure rises. Volume pumped varies inversely with the discharge pressure and may be what you were looking at instead of power.

The problem with the information is the machine is rated at a constant 390 rpm under all conditions of discharge pressure tested. Your application may be constant pressure, variable flow in which case, you would need to evaluate actual power use vs flow rate at a specified discharge pressure. I would also assume there is a minimum rpm that the compressor should operate at which the manufacturer could specify.

 

[ozmosis]

as ccjersey points out, the power curve is related to a fixed speed liquid ring compressor.
The application of a variable flow using a VFD can bring energy savings but you need to observe the following:
1) There is typically a min speed for LR Compressors. You need to check with the manf but it is about half speed.
2) Ensure the motor you wish to operate is suitable for VFD operation: a large number use insualtion class that would potentially be destroyed with VFD use. We typically advise sinewave filter on the output if in question.
3) Check the application the LR compressor is used on. A lot of them are on applications where explosive gases are used: not good for VFD use!
These are the parts I remember but there may be more. Check with SIHI before going ahead.