# Air flow requirement for cooling an enclosure?

Discussion in 'The Projects Forum' started by Jameye09, Aug 8, 2012.

1. ### Jameye09 Thread Starter New Member

Aug 8, 2012
1
0
I've been asked to determine how much compressed air a cabinet cooler will use based on the BTU/hr load in the panels and average temperature days.

Unfortunately, I haven't taken any thermo or had an electronics class yet, so this has been rather difficult for me. Any help is appreciated. Please bear with me!

All that I know is that there are two power sources in the enclosure (I don't know what other devices are in it, and can't go look). One is a ControlLogix 1756 that has a max output of 100 W. The other I was simply told is a 250 W DC power source. My supervisor told me that this information would be sufficient to determine BTU load.

The enclosure measures 36" X 48" X 10". It is at an elevation of 783', and is exposed to the sun. I believe that the enclosure is made of unpainted stainless steel. It is sealed, because it is located in a Class I hazardous area.

The average high temperature of the area is 85 °F. The cabinet cooler is designed to maintain a temperature of 90 °F within the enclosure. These are the coolers we are considering: http://www.airtx.com/stainless-steel-nema-4-12.html

Questions:

Do I have sufficient information to make an approximation about the required air flow?

Did my supervisor only give me information about the power sources because theoretically all of the power provided to the panel will be consumed by devices within the panel, and therefore be dissipated as heat?

I think I need to calculate temperature rise. Without considering solar energy, I believe that would be ΔT=70 °F. That seems really high. I used an input power of 350 W divided by a surface area of 35.7 sq. ft., which gives me 9.81 W/sq. ft. I then used a graph to find my ΔT. I got this equation and graph from: http://www.hoffmanonline.com/stream_document.aspx?rRID=16169&pRID=16168

Can anyone provide me with the equation(s) I need, or point me to a source I might find helpful?

I may be totally wrong about everything, but I thought it was best to share my thought process up to this point. I am at a loss.

Thanks so much!

2. ### wayneh Expert

Sep 9, 2010
12,368
3,224
You need to estimate an energy balance for the enclosure, including conduction (heat flow from the enclosure to the air), convection (moving air), radiation (sunlight), and internal heat generation. You'll need to use worst case scenarios, for instance not the average high but the all-time high, in the sun, all components stretched to the max, cooling vent full of dust, etc.

The power and energy calcs for electrical components will usually be in watts and watt-hours. You'll need to convert to BTUs or whatever unit you want, just be consistent.

Engineering handbooks and online sources may help you estimate these effects. For instance the enclosure manufacturer may have a wealth of info they will share.