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Many manufacturers offer metal cladded or metal bowls as options only, as their main filter or lubricator lines have polycarbonate type filter bowls without any metal guarding or over-bowl.
The polycarbonate bowl offers an advantage in that you can see through the sides to the inside of the bowl. This is particularly important if your filter bowl has a manual drain. Your maintenance operator needs to make sure that the level of contaminant in the bowl, the water and sludge that the filter will remove from the air stream, stays below the bowl separator and in the "quiet zone" at the bottom of the bowl. Being able to see the level allows you to manually drain the bowl as and when needed.
If you are interested in details about compressed air filter bowl components, please click here.
Pressure is a big consideration in filter or lubricator bowl material selection. Most polycarbonate bowls are "good" up to 150 PSI, else they wouldn’t be released for industrial use.
However, do check the specs on the unit you have to be absolutely sure, particularly if your compressed air discharge pressure is over 100 PSI.
If you are running an air system where the pressure may exceed that level, you will want to select a metal bowl, even if it means going to a different manufacturer than the one you presently use.
If you select a metal bowl, be sure to get one with a sight-glass so that you can tell if water is building up in the bowl.
Metal is opaque and you won't be able to tell what the water / debris level is in your metal filter bowl is, otherwise.
Even with an auto-drain on the bowl, having the site glass means you’ll be able to diagnose a drain blockage quickly.
If the air pressure in your system is close to the upper limit of the polycarbonate filter bowl capacity, you can also consider a bowl with a metal shroud. The metal shroud has "slots" in it allowing you to see through to the clear bowl beneath to check the debris level, as well as providing a shield in the event the polycarbonate bowl shatters.
You may have read elsewhere on this site that compressor oil carryover into the air lines creates problems for Buna seals in valves and cylinders. That same oil has a reputation for being very detrimental to most polycarbonate type bowls, hazing the interiors and ultimately degrading them to the point of cracking and then catastrophic failure.
So if you've got oil carryover in your air, and no coalescent filter to remove oil, that oil may damage the polycarbonate, and you'd be better off selecting a metal or metal shrouded bowl.
If it were us, and there were no over-riding commercial reasons why not, we'd always select a metal bowl for our air filters. The commercial reasons are price, of course, but also that some "mini-filters" may not be available with a metal bowl option.
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Source: http://www.rolair.net
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