Compressor capacity Control Method

[Sabergg1981]

HI All

Which methods of capacity control for a centrifugal compressor is better? WHY?

 

[RotaryG]

Your question can be a good title for a book!

Could you please be more specific as to the nature of the application and brief process specification for the compressor so that a more meaningful response can be given?

It also helps if you can advise the forum the extent of your familiarity with centrifugal compressors so that the rseponse can be to the point rather than a tutorial.

 

[Sabergg1981]

You are right. I am wondering between advantage and disadvantage of suction throttling rather than speed variation. if you have any specified question dont hesitate. thnx

 

[zdas04]

In Ariel's Senior Maintenance Tech school a few years ago I got a definitive answer to that question. "If you've never compressed a unit of gas then the efficiency of that unit is 100%". This works for every kind of compressor there is. Every capacity control scheme other than variable speed takes more energy than variable speed. Energy efficiency is rarely the controlling factor in compressor performance. For example, a recip wants to be within +/- 10% of design suction pressure and you can maintain this range with a suction control/recirc control scheme that is very energy inefficient, but can be the most process-effective technique to use.

You really have to state your process goals before you can decide on a method to achieve them.

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.

 

[rakuday]

Sabergg,

If the driver is an electric motor, suction throttling will be more advantageous than speed variation, as the VSD's for larger motors will be an expensive affair.

As other members have noted, you will have to give more details about your process requirements to get satisfactory replies.

Furthermore, after observing from your other threads, I get a feel that you are new to process equipments and are trying to understand the machineries thru the forum. Am not being rude, though this will help you get replies for specific questions, you will not be able to understand the compressors / pumps in totality unless you undergo a professional training / course on turbomachineries and related equipments.

 

[zdas04]

I spend an hour in my 5-day class talking about capacity control. That section includes the arithmetic to allow someone to assess the life cycle costs of various control methods. The only method with worse life-cycle economics than suction throttling is recirculation. I have evaluated compressors that had a higher pressure upstream of the suction control valve than the pressure downstream of the backpressure control valve. They were using $600/day in fuel to lower the pressure of the gas. It doesn't take many days to pay for a pretty expensive VFD in that case. I know that is an extreme case, but I've evaluated several hundred compressors and found that condition a dozen times (if you include the case where first stage discharge pressure is higher than second or third stage discharge pressure the number spikes to nearly 100). There are legitimate reasons for installing a suction controller and/or a backpressure controller, but efficiency is none of them.

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.