Air Cooled Centrifugal vs. Air Rotary Screw compressors

[nickpa]

I am evaluating replacing an Air-Cooled Rotary Screw Air Compressor with an air cooled centrifugal air compressor or an air cooled rotary screw air compressor in the southeast USA. I am told that an air cooled centrifugal air compressors can not operate during the hot summer months; however, the manufacturer is telling me that maximum ambient conditions can be up to 140F. The rotary screw compressor manufacturer only list a maximum of 122F. Does anybody have any experience with an air cooled centrifugal compressor operating in climates similar to the southeast US?

 

[zdas04]

The southeast U.S. is a pretty benign place to run a compressor. I've seen both centrifugal compressors and flooded screw compressors running in Trinidad, Indonesia, Egypt which are all far more difficult environments (for heat and/or humidity) than you'll find anywhere north of Miami. If your economics and operating envelope point you to either a centrifugal or to a flooded screw then you will likely have a good result with either. If by "Air Cooled Rotary Screw" you are talking about
[ul]
[li]replacing a dry screw with a dry screw then it should work much like the machine you are replacing[/li]
[li]replacing a dry screw with a flooded screw then it should work much better[/li]
[li]replacing a flooded screw with a dry screw than you need to be prepared for more noise, more maintenance costs, and lower availability[/li]
[/ul]

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.
The plural of anecdote is not "data"

 

[nickpa]

By air cooled I mean the cooling media is air and not water to cool the high temperatures exhibited by the compression. Both the current and the replacement are dry screw. Where those compressors that you listed in those environments air or water cooled?

 

[zdas04]

Doesn't matter. The cooler and the cooler medium are external to the compression process.

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.
The plural of anecdote is not "data"

 

[nickpa]

I am not to worried about the compression process as I know that will work in the environmental conditions. I am more worried about using air to cool the compressor when the air temperature will change throughout the year.

 

[zdas04]

Welcome to the industry. For every water cooled compressor there are several dozen air cooled compressors. Maybe several hundred. Temperature changes day to night and winter to summer nearly everywhere on the planet, which is why you get an "approach to ambient" rating on the cooler instead of a final temperature rating. 20F approach to ambient is pretty normal (i.e., on a 100F day gas out of the cooler will be 120F, on a 10F day it will be 30F), I have specified 15F and got a really big cooler. I've heard of people specifying 10F approach to ambient, but I've never seen the results of that.

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.
The plural of anecdote is not "data"

 

[rakuday]

Nickpa,

Your compressor will work, but what is the performance you are achieving during those ambient conditions matter a lot. Centrifugal compressors performances depend a lot on the operating conditions. To add to zdas reply, if the "operating envelope" is well defined during the compressor selection, then it shouldn’t be a problem.

Getting back to your main question, air cooled compressors will work, provided the coolers are properly sized. This again points to how well you define your operating conditions throughout the year (max & min).

Furthermore, what is the intent of replacing your existing compressors, if it is related to capacity expansion, then note that for higher flows (beyond 2000+ cfm) air cooled centrifugals are not feasible and not economical too.

 

[nickpa]

The new compressor will be a 1000 scfm centrifugal and will replace a 1500 scfm rotary screw. The system demand is around 700 scfm so the rotary screw is over sized and has a ton of maintenance issues. The main reason for the centrifugal is for its high reliability. I hoping that the hot summer months will not cause multiple spurious trips since it will be installed outside.

 

[rakuday]

A centrifugal for 1000 cfm? Are you referring to the integrally geared compressors?

The centrifugals for low capacity ranges will not be very econimical. Check the motor rating and energy costs, before proceeding further. As an alternate you can check for direct driven centrifugal compressor offered by few manufacturers. Again, it might be costlier than the screw compressors.

 

[nickpa]

Since its for a power plant, we are not concerned about energy costs, more concerned about reliability.

 

[jbattershell]

the air cooled centrifugal compressor that I am familiar with will operate reliably in ambient temperatures of 105-110F. the after cooler is in most cases, a 10degree approach cooler. this compressor is integrally geared. it will have longer life cycle than dry screw, higher efficiency, and since it will not need to be rebuilt in 4 or 5 years like a dry screw will need, higher up time. the fan for the cooler is VFD controlled and will regulate it's speed based on reqirements meaning slower when ambients cool down and it will speed up when ambient temps increase.

I am an advocate for centrifugals in this size range, where the centrifugals can compete better against screw compressors.