Will Centrifugal Compressor Pulls Vacuum at Suction

[DoraeS]

Hi,

I am a process engineer and I would like to know when a centrifugal compressor suction is blocked, will the compressor pulls vacuum at its suction?
I have heard that a reciprocating compressor will create vacuum at its suction, is this true?

Please help.


Thanks.

 

[BigInch]

I think not, if it is below its surge limit.
Like an airplane wing, a certain gas density and speed combination is required to begin to develop useful lift, or impart work to the gas being compressed.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[zdas04]

Any dynamic compressor such as a centrifugal or an ejector will have a pretty narrow suction range where it will work. Slightly outside that range will significantly impact mass flow rate. Considerably outside that range and mass flow rate drops to zero. I was recently awarded a patent for a very specific case of ejector, and in our testing we found that stopping suction flow actually increased the indicated suction pressure. I would expect something like that to happen with a centrifugal compressor as it went into surge.

For a PD compressor, it is going to try to move the same volume every cycle. With a recip, you will quickly reach the point that you don't have enough dP to open the suction valves and the piston will just slide up and down until the cylinder melts from windage losses. It won't pull much of a vacuum in the shut-suction scenario. A flooded screw doesn't have valves, and the oil flood keeps considerable mass flowing through the process so it will just keep dropping suction pressure into a deep vacuum. When it runs out of anything to compress it will just recycle oil at a decreasing discharge pressure (machines without an oil pump will lose the driving force for oil and will eat themselves, machines with an oil pump will probably keep running).

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[MortenA]

David - isnt it the (mass) flow rate thats the limting factor (more than the suction pressure) for a centrifugal? OK i realise that its a dynamic relation, as the pressure drop so does the desity and that changes the aerodynamics?

Best regards

Morten

 

[zdas04]

I think that is right, but the exact cause and effect get murky as mass flow rate goes toward zero. When we ran into it on the ejector it was so surprising that we added a day of testing to evaluate it. We found that for a very short time, lowering mass flow rate lowered suction pressure, but at some (fairly high) mass flow rate the suction pressure began increasing. We approached it from 20 different directions and never got a good theoretical explanation. This ejector was super-sonic so all the math was compressible flow and really messy. I would think that in a centrifugal compressor velocities would be a smaller fraction of sonic velocity and the breakover point might not be as sharp (but it will still be pretty sharp, and pretty early). The actual surge point on a centrifugal compressor seems to be a pretty narrow range of mass flow rates.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.