Placing a secondary MDE filter stack after a portabe HP compressor presents three major problems.
First is the type and design of the BPR of the original primary filter.
Also I should allow Mike F to respond on his reccomendation of the MDE BPR for this application.
By contrast I couldnt recommend a MDE BPR off the primary stack prior to the secondary because of the method of design and the opening pressure blast caused at the BPR opening pressure setting and pressure surge jet.
Second the flow circuit for the MDE Series 40,50 models for portable compressors was designed for the older BS4001 air purity. Subsiquently for BS12021 you either have to up the BPR pressure setting and cause more pressure blast problems to the scondary filter chemical or you keep to the original BPR pressure setting and get less filter chemical life than you could.
Third the opening "throat" of the small MDE filter chemical cartridges (ie the hole diameter of the plastic cartridge) introduces a "pressure jet" effect on the filter chemical this can and will a times disrupt the chemical stack, expand the contact area of the chemical matrix thus reduce the dwell time and lower the cartrige chemical life if you use it as a secondary filter stack
Four, To answer the "back to front" aspect is that by following the engraved flow direction on the metal filter base this is for primary filtration where in order to extend the small chemical volume life expectancy you use the inside metal wall of the filter tower to act as a cooler to chill the gas and produce water condensate off the vapour dew point thus using the filter as both a separator and filter.
To achieve this the dirty wet gas flows up ouside the plastic chemical cartridge then down the filter chemical to the outlet port side of the filter base.
Now as to using it as a secondary filter stack your now "driving" the particulate dust out of the filter cartridge and out into you customers cylinders for one
One of the very reasons why Bauer and others flow in the opposite direction, up through the filter chemical stack then out at the top and down the inside of the metal tower.
Ive only just started lots more to come Iain