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 December 2005
library  >  Application Notes  >  General Articles

## Calculating flow rate in an enclosure

a first in the field of thermoelectric cooling, teca introduces the ahp-6263 model with a half ton of cooling capacity. this is the largest commercially available thermoelectric air conditioner anywhere. the ahp-6263 is a 3 phase, 240 vac air conditioner. suitable for nema-12, nema-4 and nema-4x.

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if temperatures and conditions are at levels where the ambient air outside the enclosure can be used to cool the electrical equipment, then filter fans are an ideal choice. filter fans are simply a fan (typically a tubeaxial fan) connected to a housing with an integral air filter, which mount as a system to an enclosure wall or door. in order to select the proper filter fan for your application, rittal recommends that you use a formula to determine the air flow required to cool an enclosure, as measured in m3/h, since filter fans have varying performance ratings.

first, let's define some of the variables of the formula (v = f x qv ÷ t)

maximum ambient temperature (tu) = highest temperature reached outside the enclosure, measured in degrees c.

maximum internal temperature (ti) = highest temperature allowed by the equipment found in the enclosure., measured in degrees c.

heat loss (qv) = amount of heat radiated in the enclosure.

measured in watts. difference in temperature (t) = ti minus tu. altitude factor (f) = compensation for the altitude above sea level at which the enclosure is installed. measured in m3k/wh.

• 0 - 100 m is factor of 3.1
• 100 - 250 m is factor of 3.2
• 250 - 500 m is factor of 3.3
• 500 - 750 m is factor of 3.4
• 750 - 1000 m is factor of 3.5

air flow (v) = air flow required to cool the enclosure as measured in m3/h.

based on these variables, here's a sample.

tu = 25°c

ti = 40°c

qv= 800 w

difference in temperature = 15° c

(f) = 3.3

apply that to the formula, and it would be 3.3 x 800 ÷ 15 for a required air flow of 176 m3/h. based on this calculation you can then go to suppliers' literature and determine which fans have the required air displacement performance. it is critical to derate the performance of the fan for flow obstructions such as finger guards, filters, and louvers.

some filter fan suppliers provide ratings which take into account the airflow loss of these obstructions. often a performance diagram showing airflow vs. static pressure is also shown. in systems where a filter fan is used in conjunction with an outlet louver, such as in the diagram above the rating for the filter fan and exhaust louver together must be used.

to accurately size the fan, the static pressure of the fully loaded enclosure needs to be measured in order to determine actual air throughput of the filter fan in a given situation. since such static pressure measurement is only possible in an loaded enclosure with test equipment, this value is often estimated. thermal modeling using cfd type software can also provide an estimate of actual fan performance in a loaded enclosure.

if you need more assistance or would like more information on rittal's filter fans, call rittal at 1-800-477-4000 or log on to www.enclosureinfo.com.

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