The maximum size of an Ex pz purge enclosure is determined by the pressure and flow that can be reached inside the enclosure. For this 'how to' post, I'll use our 5500 series purge system for the examples.
Depending on your application, there are three vent options that you can use with your system. The EPV-5500-…-01, EPV-5500-…-02, or EPV-5500-…-03. Each vent opens at a different back pressure and has different flow rates, which are listed below.
The EPV-5500-…-01 opens at the lowest back pressure and has the highest flow rate. This is the vent that I would recommend to use on a large enclosure.
Each of these vents has three different flow charts that you can find in the manual or on the datasheet. The graphs are based on enclosure size.
One graph is used when your enclosure is less than or equal to 5 cubic feet; one when your enclosure is between 5 and 15 cubic feet; and one when your enclosure is greater than 15 cubic feet. See, for example, the EPV-5500…-01 graphs below.
To demonstrate how to calculate the maximum enclosure size, we'll use the 15 cubic feet or greater flow chart. Based on the graph, in an ideal situation, we could have a flow rate of 30 SCFM if we can reach 4” wc of pressure inside the enclosure. The larger enclosure that you have, the lower the pressure will be inside of your enclosure.
The maximum purge time that the 5500 system will allow is 166 minutes. For North American applications, you must complete four volume exchanges. If it is an ATEX application, you must complete five volume exchanges.
We will use four volume exchanges for this example. The formula to calculate the maximum enclosure size in a perfectly sealed enclosure that allows us to reach 4” wc with a flow rate of 30 SCFM is:
(maximum purge time / number of volume exchanges) X flow rate (SCFM) = maximum enclosure size
(166/4) X 30 = 1245 cubic feet
Are you going to be able to reach 4” wc in a 1245 cubic foot enclosure? That is doubtful. Realistically, for enclosures over 15 cubic feet, you would fall somewhere between 1.5” and 2.0” wc. If we use 1.5” wc, the graph indicates the flow rate would be 7 SCFM. If we use the same formula above, the more realistic maximum enclosure size would be:
(166/4) X 7 = 290.5 cubic feet
As discussed, the maximum enclosure size is determined by the pressure that you can reach inside the enclosure and the corresponding flow rate. Be sure to base your calculation on realistic, achievable pressure and flow rate and not on a theoretical, ideal situation.
More information about the 5500 series