Choosing the Correct Prefilter Pore Size

When looking for a prefilter - whether as a replacement in an existing system or for a new application - it is of vital importance to determine the proper pore size rating and removal efficiency required. This involves gathering information on the fluid to be filtered, what filter is it protecting, flow, throughput expectations, etc.

For a replacement filter, it is necessary to keep in mind that not all vendors rate their prefilters the same way. One cannot necessarily call up a different vendor and say “I need a 0.45 micron prefilter to replace the one I’m using”. A conscientious vendor will take the time to get the detailed information about your application in order to spec in the proper filter. A less diligent vendor may send you whatever 0.45 micron filter they have available, often to the surprise and dismay of the customer.

Then the customer asks “Why is your 0.45 micron filter different than the one I’m already using?” The answer lies in how different vendors rate their prefilters, both in terms of pore size and removal efficiency.

Nominal vs. Absolute Prefiltration

Liquid filters are usually designated as absolute - meaning they will remove virtually all particles above the pore size rating, or nominal - meaning they will remove some or most of the particles above the pore size rating.

If the existing prefilter has an absolute rating, it is a straightforward exercise to identify a replacement. Absolute rated filters are typically membrane filters with a very high removal efficiency (99.999% or more). Select an absolute rated filter with proper materials of construction for the fluid to be processed and it should function pretty much the same. An example of this would be using a 0.45 micron or 0.65 micron absolute rated membrane as a prefilter to a 0.22 micron final sterilizing filter. The larger pore size will remove most of the particles that might prematurely clog the final filter and even some of the micro-organisms the sterilizing filter is designed to remove, extending the life of the final filter.

Prefilter Classification

It is important to note that some filter suppliers will reclassify the higher pore size at a tighter rating without calling it absolute. For example, a 0.65 micron membrane will remove some 0.22 micron particles, so it might be labeled as a 0.22 micron prefilter. In this case, replacement with an absolute 0.22 micron filter in the prefilter position would probably result in reduced flow, premature clogging (reduced throughput) of the prefilter, and a very unhappy customer.

If the existing prefilter has a nominal rating, this usually implies some type of depth filter (polypropylene, fiberglass, etc.) is being used. The key in this situation is to remember that not all suppliers use the same definition for nominal. One might specify their nominal filters at 99% removal efficiency, while another might specify 80% (some even lower). Getting them mixed up might result in one of two outcomes, both less than ideal.

High to Low Prefilter Efficiency, and Vice Versa

If an 80% efficient filter is replaced with a 99% efficient one, the flow would probably be slower (or higher pressure required to maintain flow) and throughput would be lower. This would lead to more frequent changeouts of the prefilter (although the final filter would still be well protected).

In the reverse case where an 80% efficient filter replaces a 99% efficient one, there might be an improvement in flow and required pressure drop, but the prefilter would allow a higher level of contaminants through which could result in premature clogging of the final filter.

In both cases there would be a noticeable difference in system performance and an unhappy customer.

Real life examples:

• A customer inquired about replacing their 0.1 micron polypropylene depth filter. Because we are aware of the different definitions of nominal being used, we requested and received a sample of the filter they were trying to replace. Upon testing the sample for flow rate and dirt holding capacity, it was discovered that it was actually closer to what CPF would classify as a 1.0 micron depth filter. Had the initial testing not been conducted, the customer would have been in for a surprise.
• A distributor was trying to replace a 0.22 micron nylon membrane prefilter. He naturally offered a general grade 0.22 micron nylon membrane filter as a trial. The customer reported back much lower flow rate and greatly reduced service life compared to their current filter. Upon further investigation and requesting a sample of the incumbent filter, it was discovered that the filter was not rated as absolute. Testing the membrane from the sample showed it had the flow rate and bubble point more typical of an 0.8 micron nylon membrane! When the customer tried what was essentially a general grade 0.8 micron nylon filter, the performance was virtually the same as what they were seeing before.

From these examples, it is clear that one must be careful to get all the required information when specifying a replacement prefilter. In the second example above, had the distributor done the proper homework, they could have specified the correct replacement from the start, saving the customer from wasting time (and money) testing the wrong filter.