Calculating purge times for type X, Y and Z systems

10/21/2005

In order to calculate the amount of time it takes to complete a full purge cycle, you need to know:

%%POINT%%Enclosure volume

%%POINT%%Flow rate

%%POINT%%Purge media

Compressed instrument air

If you are using compressed instrument air to purge your enclosure, you can refer to predetermined purge times. These times are represented as a ratio of 1 minute per “X” cubic feet of enclosure volume. The “X” is a variable that changes with different size systems. This ratio is based on the NEC/NFPA-496 standards that require four volume exchanges. For IEC and EN standards, five volume exchanges must be performed. If there is a motor inside the protected enclosure, 10 volume exchanges must be performed to satisfy both NEC and IEC standards.

Inert gas and other purge media

If your purge media is something other than compressed instrument air, such as an inert gas, a density correction factor must be used to determine proper purge time. The density correction factor is determined by the expression:

Density Correction Factor = (molecular weight of air / molecular weight of protective gas)

The molecular weight of air at sea level is 29 AMUs (Atomic Mass Units). If you divide this number by the molecular weight of the protective gas used, you get a constant that is called the Density Correction Factor. Then multiply the exchange time you determined as if you were using instrument air by the correction factor.

For example, if you were using a system that required 1 minute per cubic foot of exchange time using instrument air, and your enclosure is 10 cubic feet, your standard exchange time would be 10 minutes. If you were using Nitrogen as your purge media, the correction factor would be:

(29 AMU / 28 AMU) = 1.04

The new exchange time would be:

10 minutes * 1.04 = 10.4 minute

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