compressor power calculation 2

[sah14002000]

Dear friends;
I am going to report the power for a compressor which shall transmit natural gas through pipeline.In this regard I have some questions .
First of all I want you to refer me to a good reference or guideline to calculate the power required for different type of compressors.I have found several methods but I am a little confused especially regarding the efficiency which shall be used in calculation(e.g. isentropic, polytropic,...).how can I calculate efficiency?
The other concern is about the difference of figures for duty that are calculated based on formula and output of some simulation software like hysys.would you please somebody make it clear for me?
thanks in advanced

 

[zdas04]

That was a lot of questions in one chunk.

First, positive displacement compressors (e.g., recips, dry screws, and flooded screws) are isentropic machines. Dynamic compressors (e.g., centrifugal, or axial compressors and dynamic blowers) are not constant entropy machines (i.e., they are polytropic).

Second, many manufacturers have free programs to evaluate their equipment. The best is from Ariel for their recips. This program is absolutely the most effective computer model I've ever used (if actual conditions differ from model output, then there is unequivocally a problem with the compressor, not the program). A couple of the screw compressor manufacturers have good programs (I like Gardner Denver's RotoSize, but Frick's Coolware has some nice features--GD's program is downloadable, but Coolware requires permission from Frick to start using). There are good programs with some of the dynamic machines, but I don't have any experience with them and can't make a recommendation.

Third, programs like HySys necessarily use generic factors in their compressor calculations. They tend to be pretty good for a general feel, but I'm not sure that you can always find the configuration they recommend. What I mean is that they will let you select a technology (e.g., let you decide between a recip and a centrifugal), but not make the selection between manufacturers. I like to get the manufacturers program and use their built-in efficiency factors for the options that they actually provide.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"It is always a poor idea to ask your Bridge Club for medical advice or a collection of geek engineers for legal advice"

 

[dcasto]

I always start with doing the HP requirements in my head to make sure where I am at.

for a recip compressor HP = 22 * ratios * MMSCFD * number stages

This is for ratios of from 2 to 4. If you have more than 4, you do 2 stages at 2 ratios each. Or if you need 12 ratios, you do a 3 and 4 ratio so you end up with 3 * 22 + 4*22 = 154 HP/MMSCFD

If you are using a screw compressor, increase the power.

Next You have to use the manufacturers results. They have built in all the efficencies for the standard valves, the slide unloaders, the areodydamics, whatever for their equipment.

In the simulators, they don't know what type of compressor so you have to give them some efficency. I use polytropic because I have a feel for what that efficency is for what type of compressor I'm using. For recips, its 78% to 82%. For centrifugals, 65% to 75%, for screws its 50% to 75%. The range is based mainly on ratios.