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Reciprocating vs screw compressor selection
- Thread starter albertpet
- Start date
A screw compressor is most efficent running flat out a recip like part load better. A number of salesman will oversize the recip to insure its at part load. One of the best solutions is to put a screw compressor at base load and usr a small recip to trim out the load.
crjones,
That is an interesting observation. I've found that multi-stage recips really don't like changing suction conditions and I've found that a combination of unloader valve and changing rpm to allow a flooded screw to operate over a wide range of conditions.
The problem on multi-stage recips is that a reasonably small change in suction conditions will be magnified 3-5 times in the second stage and Ratio2 times on the suction side of the third stage--a clear opportunity for a high-temp trip and/or a serious rod-load problem.
For example let's assume you have a 3-stage recip set up to take air from 40 psig suction to 1,600 psig discharge (27 ratios, 3 per stage). If the suction drops to 20 psig then the frist stage discharge drops from 150 psig to 90 psig. This drops the second stage discharge from 486 to 300. Now instead of the third stage going from 486 to 1,600 it has to go from 300 to 1,600 (5.1 ratios). The result is that third stage discharge temperature goes from around 290F to 418F (assuuming 90F out of the interstage cooler). On most compressors that is the difference between running and being down on a high-temp trip. The actual dynamics are far from being this simple, but it is quite common for a properly set-up machine to take nearly all of a suction-change in either the second or third stage.
On the other hand if you are doing 27 ratios with a flooded screw (say -3 psig to 300 psig) and the suction drops to -6.6 psia (to get to the same 39 ratios in the earlier example) then the result will be to use 30% more horsepower, but the machine shouldn't see high-temp trips.
David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
That is an interesting observation. I've found that multi-stage recips really don't like changing suction conditions and I've found that a combination of unloader valve and changing rpm to allow a flooded screw to operate over a wide range of conditions.
The problem on multi-stage recips is that a reasonably small change in suction conditions will be magnified 3-5 times in the second stage and Ratio2 times on the suction side of the third stage--a clear opportunity for a high-temp trip and/or a serious rod-load problem.
For example let's assume you have a 3-stage recip set up to take air from 40 psig suction to 1,600 psig discharge (27 ratios, 3 per stage). If the suction drops to 20 psig then the frist stage discharge drops from 150 psig to 90 psig. This drops the second stage discharge from 486 to 300. Now instead of the third stage going from 486 to 1,600 it has to go from 300 to 1,600 (5.1 ratios). The result is that third stage discharge temperature goes from around 290F to 418F (assuuming 90F out of the interstage cooler). On most compressors that is the difference between running and being down on a high-temp trip. The actual dynamics are far from being this simple, but it is quite common for a properly set-up machine to take nearly all of a suction-change in either the second or third stage.
On the other hand if you are doing 27 ratios with a flooded screw (say -3 psig to 300 psig) and the suction drops to -6.6 psia (to get to the same 39 ratios in the earlier example) then the result will be to use 30% more horsepower, but the machine shouldn't see high-temp trips.
David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
cryotechnic
Chemical
@ David,
After reading your post one question came up to me.
Most of what your explaining is clear to me.
For the first and the second stage you are saying that: if the suction pressure drops, the discharge pressure drops. But at the 3rd stage you are saying, when the suction pressure drops, the ratio will change.
Why is that? I mean, the stroke of the recip's are not changing, so also for the 3rd stage.
I would expect that also the discharge pressure of the 3rd stage would be different.
I'm assuming that the 3 stages are all driven by one motor.
Greetings
Cryotechnic
"Math is the ruler of your potential succes...."
After reading your post one question came up to me.
Most of what your explaining is clear to me.
For the first and the second stage you are saying that: if the suction pressure drops, the discharge pressure drops. But at the 3rd stage you are saying, when the suction pressure drops, the ratio will change.
Why is that? I mean, the stroke of the recip's are not changing, so also for the 3rd stage.
I would expect that also the discharge pressure of the 3rd stage would be different.
I'm assuming that the 3 stages are all driven by one motor.
Greetings
Cryotechnic
"Math is the ruler of your potential succes...."
The whole compresor must still discharge into line pressure. We would wish that the additional ratios required would be spread evenly across the stages, but that never happens. What happens is that one of the three stages (rarely the first) takes the bulk of the load and the other stages stay about the same number of ratios. I've never been able to determine the parameters that will allow me to predict which particular stage will carry the extra load.
At the end of the process, if the final stage hasn't reached line pressure, then its discharge valve will not open and the gas in the cylinder will stay for another revolution of the crank. It is a very dynamic process.
When the conditions are such that the second stage takes the extra load, then the added temperature isn't quite as dramatic as when the load is taken in the third stage.
The "rule of thumb" that I like to use is that the inlet pressure to any stage sould always be within 5% (in absolute pressure units) of the design conditions. The design conditions set the valve stiffness and the clearance of each cylinder and a significant change in the suction pressure of any cylinder will result in bad things happening (one common problem is that valves that are too stiff for a lower pressure will partially fill the cylinder and the actual ratios will be higher than expected, resulting in higher temperatures and higher rod load).
David
At the end of the process, if the final stage hasn't reached line pressure, then its discharge valve will not open and the gas in the cylinder will stay for another revolution of the crank. It is a very dynamic process.
When the conditions are such that the second stage takes the extra load, then the added temperature isn't quite as dramatic as when the load is taken in the third stage.
The "rule of thumb" that I like to use is that the inlet pressure to any stage sould always be within 5% (in absolute pressure units) of the design conditions. The design conditions set the valve stiffness and the clearance of each cylinder and a significant change in the suction pressure of any cylinder will result in bad things happening (one common problem is that valves that are too stiff for a lower pressure will partially fill the cylinder and the actual ratios will be higher than expected, resulting in higher temperatures and higher rod load).
David
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