exhaust fan sizing for cooling

[swoosh172]

I have a 1,000 SF electrical room, with some transformers, panelboards, etc that is a free standing building. All four sides and the roof are exterior. I have close to 40,000 btuh of heat load due to internal equipment but my question is if I want to keep the room below 104F with an ambient of 94F do I take into account thermal loads through the envelope? I don't think I would because the indoor temp will be higher than the outdoor temp. Any help would be appreciated.

 

[scha0786]

just to cool the heat from the equip you will need roughly 3700 cfm. If I were you I would look at supplying 3700 cfm and not exhausting. Supplying air and distributing via ductwork will help mix the air better in the room. I would do a quick cooling and heating load on the room. If you have trane trace this can literally take 20 minutes.

If you supply lets say 4000 cfm with a inline fan and have a mixing box with return and outside air. This way you can modulate the mixing dampers via space or discharge air temp. If this is in a cold weather environment I would throw some electric heat in the room (duct coil or a unit heater). Then I would supply low and relief high. You can go simple and just have a barometric damper weighted. Or you can install a motorized damper to modulate open and close to maintain a certain space pressure.

hope this gives you some ideas, although maybe my ideas a dumb

 

[jkauwale]

Hi swoosh172. Your approach of installing an exhaust fan, in order to maintain space temperatures suitable for the equipment in an electrical room is a common and sound design approach. With regards to the sizing of the exhaust fan, you will not need to account for the heat load due to conduction, but you will need to account for the solar radiation load, especially if you have windows. As scha0786 stated, running a Trane Trace cooling load calculation will be able to help you size the exhaust fan and get the total heat load on your building.

Some additional points to consider, that I forgot in one of my designs.
- How will you control the exhaust fan? Typically provide a Hand-OFf-Auto switch with a thermostat.
- When in Auto mode the thermostat will control the exhaust fan to maintain the set point temperature.
- When in Hand mode the exhaust fan will run continuously. (for use when personnel come into the room to work on the equipment)



Justin K, P.E.

http://www.engproguides.com

 

[swoosh172]

I have Trane Trace but I have never used it to figure out exhaust fan cooling only airflows. The interior temperature setpoint is always set to 72F for cooling which always gives some conduction load. If I set the interior setpoint to 104F with supply air set to 94F, then I get some crazy cooling numbers. The building has no windows, so I would have no solar gain, therefore I would just ignore all conduction through the walls and roof correct and would only consider the equipment heat generation? Thanks again!

 

[jkauwale]

Yes, if you have no windows, then I believe you are correct. The only thing I could see is if you have a thin metal building, that had a surface temperature greater than 104 F due to solar radiation. Is that possible? What kind of construction do you have?

Justin K, P.E.

http://www.engproguides.com