Demonstrating HVAC Filtration Savings with Data Loggers

Earlier this year, I wrote a string of posts that looked at 4-20 milliamp current loops and how you could interface them with a data logger for monitoring just about anything out in the field. I also did a set of posts that looked at how to build the DC power supply panel you would need to power the current loops if your particular brand of data logger could not do that.  Then, earlier this week I mentioned that I would provide you with an example of something that I think represents “low hanging fruit” in terms of reducing the consumption of energy and resources;  an opportunity that likely exists in just about any facility with an air handling system of any size.

Believe it or not, the photos below along with the content of this post ties all of those topics together.

What you are looking at is a field trial comparing:

  • Top Photo – A fairly conventional approach to commercial office building filtration that uses a MERV 4 (ASHRAE Dust Spot Efficiency 20%) roll media type prefilter with a flexible 8 pocket MERV11 (ASHRAE dust spot efficiency 60-65%) bag fitler (MERV stands for Minimum Efficiency Reporting Value).
  • Bottom Photo – Extended surface area MERV11 rigid pocket extended surface area filters using no prefilter.

The gray boxes are a couple of power supply panels that contain the power supplies and data loggers …

… that are driving and monitoring the velocity and static pressure 4-20 ma transmitters …

… that I am using to document flow and filter pressure drop.

The air handling systems are two virtually identical nominal 160,000 cfm double duct VAV units serving commercial office space in the Kaiser Permanente Building in Portland, Oregon.

The project evolved because Jeff Shapiro, a planned maintenance project coordinator for Kaiser, wanted to move away from their conventional approach to filtration (the roll media and bag filters I mentioned previously) which are changed on the basis of time (roll media every 3-6 months and the final filters every 2 years). Instead, he wanted to use extended surface area technology and change the filters based on best life cycle cost.  I became  involved because I have been very interested in this for a long time and happen to know Doug Potter of Air Filters Northwest, who was the vendor Jeff was working with to purchase his filters.

Jeff was having a hard time making the financial case for the alternative approach; the extended surface area filters are significantly more expensive than the conventional ones and Jeff was having a hard time convincing his purchasing department that they should spend a lot more money for something:

  • That they would throw away in two years based on the current approach to filter changes.
  • That had the same rating as the less costly alternative.

I was able to work with Jeff to model the two options using a spreadsheet tool I have been developing and look at the bigger picture, i.e. the life cycle cost perspective (one of my favorite topics as you probably know from previous posts).  The model said that even with the $0.03/kWh electricity that the KBP enjoys (yes, you read that right; the KBP has a very good contract with the local utility, but that price won’t last forever), the alternative approach would:

  • Save electrical energy
  • Last at least 3 years if not longer
  • Reduce the labor associated with maintaining the filters, and
  • Reduce the facility’s waste stream.

    From a financial standpoint, if you allowed the filters to run to the same change out pressure, you would start making money with the alternative strategy after about 24-30 months; sooner if the price of electricity went up.

    Towards the end of the consulting effort, on a phone call with Jeff, our collective “light bulbs” came on and we realized that we had a perfect opportunity to demonstrate the validity of what our numbers said. We could keep doing what we were doing in one of the air handling systems and try the new approach in the other system. Both systems were virtually identical and saw very similar load profiles which should mean the results would be about as “apples to apples” as  you could get in a field setting.

    Thus began a joint effort between Jeff, a facility owner/engineer, Ken Montaque and Jesse Miller, the KBP operating team, Doug, a filter salesman, and myself, a commissioning provider and HVAC consultant, to see what reality had to say about what we all had come to believe.   To put our plan into action, we coordinated a filter change for both air handling systems that included:

    • Inspection and cleaning of the filter plenums, frames and gasket systems with gasket repairs/replacement as necessary,
    • Calibration of the existing filter gauges,
    • Weighing a few filter modules selected at random so we could see how much dust they ended up capturing, and
    • Data loggers installed to measure average filter face velocity and filter pressure drop once every 15 minutes.

      We are a little over a year into our field trial at this point and I thought I would share the trends that are emerging. We have been logging average face velocity (as a proxy for flow) and filter pressure drop every 15 minutes starting right after the new filters were installed. Here is what a typical data set for two days of operation looks like for the AHU with the conventional filters.

      To get a feel for what the trends are, I made a table for each air handling unit, then took the pressure drop through the filters on the first Monday of the month at 4 am (the time the systems tend to be at minimum flow) and at 4 pm (the time the systems tend to be  at maximum flow) and averaged the pressures. I then plotted the results and here is what I got.

      Bear in mind that my methodology for making the table to generate the graph above is not very precise;  rather, it was just a technique I tried to give me a general picture of what is going on.  For instance, the filter pressure drop for the extended surface area technology is not really decreasing as the trend line suggests (I think the technology is good, but not that good!).  The trend line is currently tipped down a bit because of the flow rate that happened to exist at my arbitrary “first Monday of the month at 4 am and 4 pm” data point selection.  It’s low relative to the preceding months and if you throw it out, the trend shows a slight rise, which is what is actually going on.

      Similarly, the larger data scatter associated with the conventional filter data is related to similar phenomenon coupled with the loading and then changing of the prefilter, which occurs about every 3 – 6 months.  So, if you look at the data in detail, the conventional filter bank’s pressure drop tends to rise, drops a bit with a prefilter change, and then continues to rise.  What I was trying to capture with my somewhat crude analysis technique was the general trend, which is up in both cases, and the general difference in the trend, which is significant.

      Specifically, the difference in pressure drop represents about 7-10 kW at the peak flow condition;  over time, that will translate to a lot of energy saved at the kWh meter.

      For those who prefer a good old fashioned gauge instead of this electronic mumbo jumbo from data loggers that I keep talking about, here are pictures of the indications on the filter pressure drop gauges from one of my recent visits for the conventional filters …

      … and the extended surface area filters …

      … both with about the same flow rate through them.

      No matter how you look at it, the alternative technology seems to be performing as advertised; i.e. there is significantly less pressure drop through the extended surface area filter bank and the loading rate is very flat relative to the conventional approach. In fact, if you project the current trend out for the KPB standard filter change out interval of 2 years, you see a pretty amazing result.

      What the projection above is saying is that if the current trends persist, the conventional filters appear to be headed towards a pressure drop that is typical of what the KBP staff would see at the end of a 2 year cycle.  In contrast, the extended surface area technology, after two years of operation, will not have reached the clean pressure drop of the conventional technology.

      As you may guess, Jeff, Doug, and I are pretty excited about the results.

      • For me, it confirms what we have been seeing in other systems where we are trying the extended surface area technology.  They are in other locations with different operating characteristics and patterns, but they all are showing similar results.
      • For Jeff, it affirms his instinct that told him there had to be a better approach to filtration than the one they were using and that moving in that direction would save his company money in addition to benefiting the environment.
      • For Doug, it affirms his business model, which in general terms, is to sell not just filters; rather Doug wants to sell clean air in the form of installed filters operated for best life cycle cost.  The trick is in getting his customers to understand the value of that approach, and some hard data from a field trial should help make the case.

      In the bigger picture, we all think the implications of our data could be pretty significant in terms of reducing:

      • Electrical energy consumption
      • Green house gasses
      • Filter maintenance labor costs
      • Filter consumption
      • Facility waste streams

      when they are extrapolated to all of the air handling systems that exist out there. Periodically, I’ll update our results to keep you in touch with the progress of our experiment. Meanwhile, I’m planning a few posts that look at the theory and implications of some of the things I mention in this post in more detail.

      David Sellers
      Senior Engineer – Facility Dynamics Engineering

      Click here for an index to previous posts

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      5 Responses to Demonstrating HVAC Filtration Savings with Data Loggers

      1. Mark Hydeman says:

        Dave:
        I don’t understand the inconsistancies in your trend lines on the 4am/4pm readings. In the first one your trend line indicates that the extended media filter actulally discreases in pressure with loading (a questionable result).
        In the lower figure where you are projecting DP at 2 years of operation the trend line for the extended media filter (the green line) has a positive slope but the data points appear to be the same.
        Please explain the differences.
        – Mark Hydeman

      2. Mark Hydeman says:

        On a different note, you should take some measurements to isolate the prefilter on the “as found” case. Conventional wisdom is that prefilters should only be used during construction then removed during operation.
        – Mark

      3. Hi Mark,
        What you are talking about is what I was trying to explain when I said that I think the technology is good, but not that good! I think the actual flow at my arbitrary 4am and 4pm points varies. On the last data point, it’s low relative to the preceding months and if you throw it out, the trend shows a slight rise, which is what is actually going on and what I based the two year projection on. All that said I think the gauges in the pictures that follow tell the story; virtually identical units at identical flow rates with different types of filters that have run for the same amount of time.
        With regard to your second point, I actually have pressure drop data on just the prefilter fore the “conventional” approach. Its the green line in the graph showing the daily pattern. The red line is the final filter. Conventional wisdom aside, we see more buildings than not where they are running prefilters with final filters. Sometimes that’s a good idea, at least part of the time (for instance, if there’s a cottonwood grove nearby) Other times, there may note be much added benefit That’s a topic I plan to further discuss in a subsequent post.
        Thanks for the feedback,
        David.

      4. William Lull says:

        Dave:
        I would offer the simple observation that you could get those same measured results by removing one of the conventional filter cells. Do you have a particle counter? Did you make any comparative downstream readings?
        The tale is told by your before/after weight measurements of the filters to compare the amount of particulate removed in each system. Hopefully you will add that data, since, after all, the purpose of the filters is to remove particulates.
        Hopefully you will add this data.
        (PS – Tell Tim S and Tom M that I said hi. Working with Dave R right now on the U MD project.)

      5. David Mason says:

        Hey Dave, good work. Thanks for sharing. I attended a class you taught at SMUD a while back.

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