William Lull’s comment that “the tale is told by your before/after weight measurements of the filters to compare the amount of particulate removed in each system” is a good one and I’m looking forward to seeing how much dirt we pick up over the two year trail window. There can be a really big difference in how much dust a filter is rated to retain as illustrated in the data for the filters we are using in our project. This first image is the data sheet for flexible bag filter.
This second image is for the rigid pocket filter we are comparing it to.
As you can see, the filter that the KPB folks have been using up until this point was rated to retain about 190 grams of dust when loaded to a pressure drop of 1.5 inches w.c. I’m not so good at understanding metric units in the context of what I see day to day so I launched my copy of VersaVerter (a free, shareware units conversion program) and found that 1 pound = 454 grams and 1 ounce = 28 grams.
So, the flexible bag filter we are testing would gain about 0.4 pounds or about 7 ounces if we ran it until the pressure drop was about 1.5 inches w.c. That doesn’t seem like much until you realize that the weight gain is dust that we generally can’t see but that is in the air we breath.
In contrast, the alternative we are exploring via our test is rated to retain an incredible 3,400 grams of dust at 1.5 inches w.c. That’s over 7 pounds of dust if we actually ran it long enough to reach a pressure drop of 1.5 inches.w.c. and the results of the independent lab test are accurate. Given the flat loading rate we are seeing, I’m thinking we won’t get there.
But, I do think I expect to see the weight gain of both filters to be in the same ball park given that:
- We are running them for the same amount of time with a similar flow profile (i.e. they will have had about the same amount of air pass through them),
- The intakes are at the same level, several hundred feet in the air on top of a high rise building,
- The filters have the same MERV rating, and
- The frames are in about the same condition in terms of construction and gasketing (i.e. the air that is bypassed due to leakage between the frame and filter is about the same and a very small fraction of the total).
The real problem is that we are trying to measure, in the field, a change in weight of ounces for in something that weights a couple of pounds with two years elapsing between the first and last measurement. So, we hope our postal scale will do the trick and that its calibration doesn’t shift much over the two year window. In hindsight, I’m wishing I had taken a chunk of plastic or something that would be immune to weigh changes due to humidity shifts and accumulated dirt to use as a reference standard, but I didn’t. I guess that’s something to add to my list for the next chance I get to run a test like this.
Meanwhile, if you’d like to see what a formal filter test report by an independent testing lab looks like, you can find one online on the independent, nonprofit research and development organization RTI‘s website. As you will see, one of the things they track is the amount of dust fed into the system over the course of the test.
Senior Engineer – Facility Dynamics Engineering