Many manufacturers offer metal clad or metal bowls as options only, with their main filter or lubricator lines having only a polycarbonate type filter bowl without any metal guarding or over-bowl-shroud.
The polycarbonate bowl offers an advantage in that you can see through the sides to the inside of the bowl. This is quite 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 the operator to manually drain the bowl as and when needed.
In the photo, the metal filter bowl has been removed from the filter housing, exposing the polycarbonate filter bowl underneath.
However, do check the specs on your filter unit. You have to be absolutely sure that the filter is rated for the pressure it will see in your application, particularly if your compressed air discharge pressure is over 100 PSI.
If you are running an air system where the pressure may exceed the level that a polycarbonate bowl is rated for, you will want to select an air filter brand with metal filter bowls, even if it means going to a different manufacturer than the one you presently use.
Often the metal filter bowl cover has slots in it, allowing an operator to see through to the clear bowl beneath, to check the debris level. The metal shroud provides a shield in the event the polycarbonate bowl shatters.
That same oil has a reputation for being detrimental to some polycarbonate filter bowls, hazing the interiors and ultimately degrading them to the point of cracking and creating the potential for a catastrophic bowl failure.
If you have oil carryover into your compressed air, and do not have a coalescent filter to remove the compressor oil, that oil may damage the polycarbonate of the filter bowl. At this point, an all-metal filter bowl would be best.
If it were up to me, and there were no over-riding reasons why I should not, I would always select a metal bowl for my air filters.
Unfortunately, some mini-air filters may not be available with a metal bowl option at all.
Here is information about compressed air filters if you wish to understand more about this essential compressed air piece of equipment.
In getting compressed air to your application, "it is better to use a longer and larger diameter air hose than an (electrical) extension cord" for your portable compressor!