3 Stage Bag Filters

Did you buy a 3 or more stage bag filter system? Are you thinking of buying one? Keep reading.

It is common to see #2 bag filters connected in line as part of the winterization process. The first bag will have a higher porosity than the subsequent bag. Typically we see three bags in series, but I have seen up to five.

Though bag filters are useful in certain applications, especially as a prefilter, winterization is not one of those applications. Bag filters are Nominal filters, not Absolute. This means that if a bag is rated as 10 microns, it is like saying it will retain most 10 micron particles. It will let through bigger particles and retain smaller particles.

Even if you filter down using a 1-micron bag, you will find problems and contaminants downstream using this system. It is always necessary to use a lenticular or cartridge filter to ensure you get the absolute filtration required.

As your fats warm up when in contact with steel over an extended system, fats may melt. This type of filtration only removes solids, unlike lenticular or OSN membrane filters. As fats become liquid, they pass through any bag.

As fats accumulate in your bags, care must be taken as to not exceed differential pressure so the bags don't rupture. Bag filters must be changed at about 10 PSI dP to avoid rupture and contamination of downstream processes.

Some bag filter systems get out of hand by having up to five housings. This is because these systems are not absolute rated and excessive filtration is necessary. The convenience of bags is lost in the increased operating costs and cleanup of so many housings. It is a flawed application of these filters and shows the negligence of the vendors.

If you purchased one of these, shame on the vendor. If you are thinking of buying and you do so after reading this, shame on you.