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compressor room exhaust

compressor room exhaust

compressor room exhaust

(OP)
air compressor in a 8 x 14 room.  the manufacturer says the unit produces about 32000 btu of heat.  the owner is looking to exhaust some of the heat from the room to keep it around 85* was thinking of installing an exhaust fan ducted to outdoors and a make-up air duct.  how much cfm do i need to exhaust.  
thanks

RE: compressor room exhaust

You need to do a heat/cooling budget.  Not only do you need to account for the equipnment load, but also for any other additional loads, ie. solar load on the roof, surrounding temperature of adjacent spaces, and acount for any insulation.  The next question is where is the make up air going to come from and what is the temperature of the makeup air.  This will impact the quanty of air required.  Also watch the temperature and the humidity of the air that you dump into the space or you could have new issues to deal with. Two possible reference sources are:

Society of Naval Architects and Marine Engineers (SNAME) Technical and Research Bulletin No. 4-16, Calculations for Merchant Ship Heating, Ventilation and Air Conditioning Design.  

Society of Naval Architects and Marine Engineers (SNAME) Technical and Research Bulletin No.  4-7, Thermal Insulation Report.

RE: compressor room exhaust

emalsyd,

You would have had more help on the HVAC/R forum.  At any rate, allow me to shed some light.  cgwilson is right in that in addition to the compressor itself, normally a cooling load calcualtion is performed where you also account for other contributors to heat such as the adjacent spaces, the outside environment, and also other interanl heat generators such as lights, etc.

Anyway, regarding the compressor itself, 32,000 Btu's equates to about 3-tons of cooling (12,000 btu's/ton).  Assuming the space is small and there isn't a whole lot of additional heat contribution, let's assume 5-tons of cooling.  You can accomplish this with mechancial cooling of course (like an AC unit).  If outside air is used to ventilate the space, the design temperature will be 10 to 15 deg F above outside summer design conditions.  Therefore, if in your area it's 95 deg F on a hot summer day, you'll only be able to keep the space at 105 to 110 deg F.

If you use conditioned air from adjacent spaces to ventilate the room, it could be done as you suggest, but again, the compressor room will be 10 to 20 deg F higher than the adjacent space temps.  So let's assume 72 deg F from adjacent spaces and 85 deg F in the compressor room. Also, let's assume 60,000 Btu/hr (5-tons) as I mentioned earlier (although the actual number should be calculated as discussed via laod calculations).  Now the appropriate equation is:

q (sensible heat gain) = 1.10 x CFM x delta T (temp change.  60,000 = 1.1 x CFM x (85 - 72).  Therefore, CFM = 4,196.  This is a pretty hefty airflow from an adjacent space and would need to be "made-up" somewhere for those spaces.  With mechanical cooling, the dischareg air temperatures are much lower (55 deg F vs 72), but the airflow is a lot less too, plus you can recirculate the air rather than just exhaust it.

So, unless you can increase the 85 degree interior design temperature (frequently electrical equipment will have a 40 deg C or 104 F max space temp) or unless your outside temperatures are really low, you might have to consider mechanical cooling.    

RE: compressor room exhaust

Depending on your wetbulb you could also consider evaporative cooling to help drop your required airflow in the space.

We've successfully used evap cooling in areas like this where the cfm requirement of the space without any cooling would make the space a wind tunnel.

But, I'm in a dry climate...

RE: compressor room exhaust

Another option is to duct the compressor cooling fan discharge air directly to the outdoors and install an extra large makeup air louver in the mechanical room wall (assuming it is on an outside wall).  Your mechanical room will then stay much closer to the outside ambient temperature.  If the compressor has a centrifugal fan, the backpressure due to the ductwork should not be a problem.  Even propellor fans can sometimes be ducted if you oversize the ductwork and minimize elbows & transitions.  Regardless of the fan type, you need to get the compressor manufacturer to tell you what external static pressure their fan will provide.  I've done this twice - once on a 200 HP Atlas Copco and once on a 40 HP Ingersol Rand - both with factory concurrence.  You'll need to carefully calculate the static pressure losses through every elbow, transition, louver, screen, and filter.  However, CountOlaf was right - keeping the room at 85 Deg F will be hard in most parts of the US.

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