Air or Water-Cooled Air Compressor

[threeS]

I am now selecting the Air Compressor for a new power station. The expected capacity of Air Compressor is around 7 bar and 50m3/min.

The Air Compressor (two-stage) will be screw type with intercooler and aftercooler. We require the water-cooled Air Compressor but the Bidder counterproposed the air-cooled Air Compressor.

In power station, cooling water supply is not a problem so I can't understand why the Bidder had such proposal. Is the air-cooled unit cheaper than water-cooled, or are there any other technical consideration ?

Can anyone help ?

 

[quark]

You should go with water cooled air compressor in my opinion. Air cooled compressors occupy more space and specific power consumption will be higher when compared to water cooled compressors.

The optimum power consumption of a two stage compressor depends upon the intermediate air pressure and temperature. The more you deviate from adiabatic temperatures the more you loose.

Regards,

 

[zdas04]

You didn't say if this was a flooded screw or a dry screw. With a flooded screw the temperature change across the stages is so small that it is unusual to use water cooling (it is also unusual to have interstage cooling in a flooded screw). For a dry screw the liquid cooling would be a better choice.

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.

 

[rmw]

Check carefully his design temperature for his offering of the air cooled system.

You don't want to find out on some hot summer day, when the compressor is suffering from insufficient cooling, that its design point is 16C and the ambient is exceeding 38C.

rmw

 

[crjones]

Having designed these packages, the air cooled is cheaper to build. As rmw pointed out an air cooled unit performace is subject to the ambient conditions. As an intercooler an air cooled unit can only economically achieve 15 degrees imperfect.

 

[jay165]

The air cooled exchanger is probably part of his standard package and it is thus cheaper to build as crjones pointed out. Unlike your case I've found most suppliers and operating folks have a bias toward ariel coolers even if cooling water is readily available and the operating costs are actually lower for water cooling.

 

[Montemayor]

Maybe I'm interpreting wrong, but I believe Eric said he requires a "water-cooled Air Compressor but the Bidder counterproposed the air-cooled Air Compressor".

There's a big difference in terminology when describing compressors. Perhaps Eric means something else, but what he wrote is that he wants a screw compressor with built-in (actually, probably cast) water jacket -- much like what has been traditionally manufactured in the reciprocating compressor field. His bidder quoted on a finned, air-cooled screw compressor. The intercooler and the aftercooler are something(s) completely apart. These can be either air-cooled or water cooled. In this case, I would suspect that Eric also wants water cooling in both coolers, taking advantage of the closer discharge air temperature approach to the cooler water. I think David Simpson is also working with this concept in his reply. I also suspect, like David, that this involves an oil-flooded screw - which is more practical and much more reliable and efficient than a dry one. Therefore, any intercooler is not employed. An aftercooler, however, will be needed with the customary bulky oil separation equipment.

I've not seen or heard of any jacketted oil-flooded screw compressor and I doubt if one could be justified for the simple reason(s) that David has pointed out. Perhaps two (2) screw compressors are planned in order to obtain what Eric describes as "two-stage", but I strongly doubt this also. I've also never seen a jacketted dry screw; the casting is very complicated and expensive to make - and in the majority of cases, not justified.

What I really suspect here is that the compressor specifications were prepared by someone who is applying what they believe to be analogous specifications between a reciprocating machine and a screw - when, in reality, there are few (if any) similarities. Eric should explain the lack of detailed basic data that the above posts have revealed and, as a consequence, we will probably find out the rest of the story - which will clear things up.

Art Montemayor
Spring, TX

 

[zdas04]

Two-stage flooded screws are starting to be available. Most packagers are putting an air-derivative machine (max pressure on the casing of around 250 psig) in front of a process screw (rated at 600 psig) without an interstage cooler between them. The critical temperature on a flooded screw is the final discharge temp. If you are going from atmospheric to 500 psig then you'll take about 7 ratios in the first stage (up to 85 psig) and then about 5 ratios in the second stage. The first stage will add about 25F to the stream and the second will add about 20F, so if your oil inlet temp is around 160F then the outlet temp is perfect. Finally, separate the oil from the gas and cool the gas in the after cooler. Since this is a 7 ratio application, two stages of flooded screw wouldn't be needed.

The after cooler is the only place in a flooded configuration that it makes any sense to be discussing air-cooled vs. water cooled. Trying to remove the heat of compression using a case jacket is just not feasible or very smart.

I had a water-cooled after cooler many years ago (on an lp dry screw) in a space that the ambient temperature was over 110F and a 20F approach to ambient was just too hot. Water cooling worked well. I've had hundreds of air-cooled after coolers that have all worked pretty well too. You just need to look at your environment and make a decision.

If this is a dry screw, then two stages and both interstage and after cooling are required. Look at your environment and decide what cooling you really need. If the packager won't provide it, several of us can give you a list of suppliers that will (although most of the ones I know don't speak m^3/min).


David

 

[threeS]

quark, David, rmw, crjones, jay165, Montemayor, Thanks.

Based on your posts, it can be concluded that the key factors for deciding the type of compressos are price and ambient temperature. Since the air compressors will be installed in the tropical region, water-cooled is therefore better than air-cooled. Comparing with cost of a power plant, price difference between water-cooled and air-cooled is very minimal. So we will insist the water-cooled type.

Just to respond the questions in your posts:-
a) Air compressor, intercooler and aftercooler will be packed inside one house. Only water cooling piping and power cable connection is required at site.
b) Air compressor is dry type and Air dryer will be installed downstream of air compressor to ensure the air quality to instrumental equipment inside power plant.
c) Air compressor is screw type.