Air Handling System Prefilters

There have been quite a few comments on my recent post where I share my data from a field  trial comparing filter technologies, both on the web site and in the form of e-mail.  So, I thought I would spend a post or two discussing some of the thought I had related to the comments. Generally, they are topics I was going to mention in relation to the filter discussion anyway.

Mark comment about the conventional wisdom in his area being that “prefilters should only be used during construction then removed during operation” got me to wondering what the formal definition of a prefilter was. I started by looking in my copy of the NAFA Guide to Air Filtration, something I have found to be a very useful resource as I have tried to understand the ins and outs of air filtration. (NAFA is the National Air Filtration Association.) In Chapter 1, the guide simply states that:

A prefilter is intended to extend the life of the second or final filter.

Later on, in Chapter 13 – Owning and Operating Cost, there is an entire paragraph dedicated to the topic of using prefilters to extend filter life. In checking out the NAFA web site as I wrote this to get the links I needed, I discovered that they now also publish a guideline on operating and maintaining filters, which I ordered and will fill you in on when it arrives.

Next, I went ot my copy of the ASHRAE Systems and Equipment Handbook (2008 version) and found a similar definition in on page 28.6 where it said that prefilters are:

… used upstream of a high-efficiency filter to extend the life of the better and more costly final filter.

Later in the chapter it went on to say:

Using coarse prefilters upstream of extended-surface filters is sometimes justified economically by the longer life of the main filters, and

Generally, prefilters should be considered only if they can substantially reduce the part of the dust that may plug the protected filter. A prefilter usually has an arrestance of at least 70% (MERV 3) but is commonly rated up to 92% (MERV 6), and

Temporary prefilters protecting higher-efficiency filters are worthwhile during building construction to capture heavy loads of coarse dust, and

Filters of 95% DOP efficiency and greater should always be protected by prefilters of 80 to 85% or greater ASHRAE average atmospheric dust-spot efficiency (MERV 13).

So, the common ground seems to be that prefitlers are generally used to protect and extend the life of final filters, which may include running them as a matter of standard practice, which is what I have tended to observe as I work around the country. In contrast, Mark’s observation about using them during construction and then removing them would seem to be a special case of the more general practice.

That leads to the question How do I decide if I should or should not use prefilters during normal operation?

In my research and experience thus far, that comes down life cycle cost, at topic I will go into in more detail in a subsequent post. But in general terms, owning and operating the prefilters should be cost effective relative to what would happen if you didn’t install them.

One of the more interesting experiences with regard to what would happen if you didn’t install them occurred during one of my first experiences where two nearly identical systems were operated with different filtration strategies to contrast them. This occurred six or seven years ago now when Ken Westlake and Dave Rabon, engineer’s for Melvin Mark’s Crown Plaza Building in downtown Portland were trying to figure out if they should use prefilters or not after converting to extended surface area filters.

Their decision to try the extended surface area technology came about as the result of a recommendation I had made during a utility funded retrocommissioning project. My recommendation came out of past experience and knowledge gained when Michael Chimack of the Energy Resources Center at the University of Illinois at Chicago and I co-authored an ACEEE paper titled Using Extended Surface Air Filters in Heating Ventilation and Air Conditioning Systems: Reducing Utility and Maintenance Costs while Benefiting the Environment.

That paper evolved when Mike and I submitted similar abstracts to ACEEE. What was exciting about Mikes work was that he had actual logged field data vs. my more empirical experience. To the best of my knowledge, this is one of the first times that data was methodically collected to compare extended surface area filters with conventional filters and the results affirmed the desirability of the technology from the standpoint of saving energy and other resources.

Anyway, my experience and Mike’s data had convinced Ken and Dave of the validity of the approach. What they were trying to decide was if they should leverage the extra dust holding capacity of the extended surface area filters and simply not use prefilters, which would totally eliminate the prefilter costs from the equation.

Or, should they maximize the filter life of his more expensive extended surface area filters (which my past experience indicated could approach or exceed 4 years) by protecting them with prefilters. So Ken and Dave, being practical folks, decided that since they had two identical systems, they would try running one with and one with-out prefilters and observe the results.  Initially, we had hoped to log data, just like in the KPB project I discuss in the previous post.  But tight budgets and a short program time line stalled out the high tech effort and we resorted to the original data logger, that being Ken and his observations and operating records.

The bottom line was that the final filters for both systems loaded up at about the same rate, which seemed mysterious until Ken did some research and figured out that their primary contaminate was rubber dust from the tires of cars zipping by the building. Specifically, the building is located right next to the Natio Parkway, a main drag through downtown Portland and the intakes for the systems are at street level. It turned out that the rubber dust and other contaminates generated by the high levels of traffic on the parkway was fine enough that it tended to pass right through the prefilters.

So, there’s a different angle on the need for prefilters; the contaminate has to be something they can filter out if they are going to protect the downstream filters. For Crown Plaza, the right answer was to simply not use prefilters. But, that same building located in Beaverton (a bit west of Portland) next to a cottonwood grove, the prefilters might have been much more desirable, at least for part of the year because the seeds from a cottonwood can quickly obstruct the mesh of an intake screen let alone a filter.

That brings me to another piece in the filter operarting puzzle, that being how do the filters load over time. It turns out its very site and system specific as Ken and Dave’s experience revealed. I’ll talk about that more in a subsequent post. Until then, thanks for your interest and feedback on my filter discussion.

David Sellers
Senior Engineer – Facility Dynamics Engineering

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