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Air conditioned air for compressor suction

Air conditioned air for compressor suction

(OP)
Dear all,

During last summer we have faced frequent issues of discharge Air temperature high in Rotary screw oil lubricated single stage air compressors(Rated Pressure 8 bar,500 CFM, 75 KW) because of summer heat.

Can we use air conditioned (cooling coil)dehumidified space to reduce suction air temperature which may also save energy use of compressor? Can we use dehumidified air for compressor suction?

Thanks in advance.

RE: Air conditioned air for compressor suction

Of course you can use air conditioned air at the suction of your compressor, but that is not likely to be the most cost effective way to solve your problem.

It is most likely that you can fix your problem by increasing oil flow much less expensively. Take a look at thread1036-183642: Wet Gas Screw compressor to see a pretty good discussion of temperature management in oil-flooded screws.

The arithmetic is a bit complex, but at least it is closed form and can be directly solved without iterations. You can find a version that works at thread391-294813: Heat of Compression with CO2 System (there is an attachment to the last post that has a corrected version of the calculations).

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Air conditioned air for compressor suction

This is a common problem with air compressors. Any form of additional cooling will solve the problem. Often simply opening the doors of the sound enclosure is sufficient but, of course, that defeats the purpose of the enclosure. Also make sure that the hot cooling air is not recirculating back to the intake. A breeze in the wrong direction can easily do that.

RE: Air conditioned air for compressor suction

I faced this problem some years ago with a rotary screw compressor with a refrigerated air dryer in the same enclosure, this was solved by adding an exhaust fan and a louvered intake grille to the enclosure, and we were a bit shocked to find that we required 20 changes per hour to keep the heat level down. This was due to our High ambient summer air temperatures of better than 100 degrees F.
B.E.

You are judged not by what you know, but by what you can do.

RE: Air conditioned air for compressor suction

As they say, compressors are much better at producing heat than compressing air. I saw one compressor where they had a hose just pouring water onto the unit. Not recommended.

You can keep the compressor room cooler by directly venting the hot exhaust air outside of the compressor room (depending on how cold you get in winter, it can be used for space heating during the winter season). Alternatively, if you have a process that requires hot water, many compressor manufacturers have the option to add a heat recovery system. If you have multiple units, you could even re-distribute the load.

zdas04's suggestion might be the most cost effective though.

RE: Air conditioned air for compressor suction

As you suggest, one way is to use colder air as suction air to the compressor. In addition to increasing lube oil flow, allowing colder air to pass through the lube oil fin fan cooler also helps.

RE: Air conditioned air for compressor suction

Can someone tell me whats wrong with an air cooled heat exchanger on the outlet air? Just one fan and no pumps or complex air conditioning system.

Everything said here is valid, but essentially it is just about where in the sequence you remove heat from the air you're compressing to the ambient air.

Sure, your min discharge air temp won't fall below 10C higher than ambient for a simple fin fan cooler, but given that the OP hasn't actually given us any temperature figures we're all just spouting hot air.....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Air conditioned air for compressor suction

The second thread I referenced above has the math to determine the discharge temperature of an oil-flooded screw compressor and it is pretty messy. There is a lot to it, but trying to fix a gas temperature problem by cooling the intake air or cooling the air on the outside of the fin-fan cooler will rarely allow you to meet gas temperature targets. The thermal mass of the oil is just too dominant.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Air conditioned air for compressor suction

"Can someone tell me whats wrong with an air cooled heat exchanger on the outlet air? Just one fan and no pumps or complex air conditioning system."
All air compressors already include this in the package. The issue is that as ambient heat and humidity get past a certain point, you cannot fit a large enough cooler and fan into a reasonably sized packaged unit. Technically, the problem is trivial. Most customers buy on price, so compressor manufacturers have to design to price points.

RE: Air conditioned air for compressor suction

If it comes with one then fine, but there was no mention of one in the OP or indeed any idea of actual data to work with.

A 500cfm compressor is a decent size compared to a space air conditioning unit so I think it will be difficult to get air temp down to anything worthwhile..

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Air conditioned air for compressor suction

(OP)
"The second thread I referenced above has the math to determine the discharge temperature of an oil-flooded screw compressor and it is pretty messy. There is a lot to it, but trying to fix a gas temperature problem by cooling the intake air or cooling the air on the outside of the fin-fan cooler will rarely allow you to meet gas temperature targets. The thermal mass of the oil is just too dominant."

You mean to say lowering suction air temperature doesn't help?

"Can someone tell me whats wrong with an air cooled heat exchanger on the outlet air? Just one fan and no pumps or complex air conditioning system."

The issue is not with discharge air temperature. The issue is compressor trip due to high oil temperature.
There will be heat exchanger(just like car's radiator with a blower) which cools the oil which gets heated in the compression process. Because of the high ambient temperature in the summer, the heat exchange is not effective and the hot oil is recirculated back to screw raising the oil temperature.

RE: Air conditioned air for compressor suction

santu,

Your OP stated "last summer we have faced frequent issues of discharge Air temperature high". Your latest post now says "The issue is not with discharge air temperature".

One is the opposite of the first and now you say the issue is actually the oil temperature (!)

So how about making the oil cooler a bit bigger?, or maybe cleaning the radiator? or adding a second one.

What were temperatures experienced. (Oil in, oil out, ambient air?) How big is your oil cooler?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Air conditioned air for compressor suction

What I was trying to say is that the ΔT of the oil absolutely dominates the process and it takes a lot of cooling for the air temp to make much difference.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Air conditioned air for compressor suction

In thermodynamic terms it is far less energy efficient to cool the inlet air below ambient than it is to cool the compressor oil to say 10 degrees above ambient. The former requires a refrigeration system while the latter only requires a heat exchanger.

je suis charlie

RE: Air conditioned air for compressor suction

Agreed, the last time I checked, almost all the compression heat on these machines is rejected at the lube oil fin fan cooler - in fact, the lube oil fin fan cooler cooling duty is very nearly equal to the compressor power.

RE: Air conditioned air for compressor suction

I don't doubt that the increase in pv energy of the gas after compression is only a small fraction of the work done but surely the intercooler/aftercooler cooling duties would be of similar magnitude to the oil cooler? I suppose some designs might use the oil as an inter/after cooling medium?

je suis charlie

RE: Air conditioned air for compressor suction

Gruntguru
Take a look at the attachment to the last post in the second thread I linked above. Remember that you are only lowering the suction temp from 560R to maybe 530R. Not a huge change.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Air conditioned air for compressor suction

santu398
Have you checked the air filters on the oil cooler radiators ? Partial plugging of these filters in high ambient temps can be enough to trip the high oil temp alarm.
Also have you talked to the manufacturer of the compressor , about adding another oil cooler ?
You do need to talk to the manufacturer, all we are doing here is guessing aided and abetted by the lack of information at your end. You do not even tell us who makes the compressor.
B.E.

You are judged not by what you know, but by what you can do.

RE: Air conditioned air for compressor suction

I did studies to use the propane refrigeration system to cool the inlet air to the gas turbine running the refrigeration compressor, a 5 stage centrifugal. Guess what, it wins, the output energy from the turbine went up faster than the energy used to cool the air intake. You ask what does that have to do with the op? A gas turbine is a huge air compressor!

RE: Air conditioned air for compressor suction

dcasto,
The reason that interesting anecdote is totally irrelevant is that the 5 stage centrifugal better not be oil flooded, or ugly things are going to happen.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

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