How do I estimate a pressure/flow curve for a filter of specified area, efficiency rating, MERV and MPR?(Duct work is negligible)

I am trying to pick an appropriately rated fan/fans to maintain a target cfm:

find flow @ pressure

I'm trying to add a filter to a wall-mounted AC unit. If I can't do this I will need to replace the whole system. I'm trying to calculate the resulting static pressure assuming I can boost flow to maintain cfm. I have chosen a filter and I know my target cfm; How do I find static pressure so I can choose the right fan to boost the unit.

constraints are as follows:

  • the unit is rated for approx. 400 c.f.m. on a 240V circuit
  • inbuilt blower motor: 8P DC280-340V 30W
  • filter area is ~$3ft^2$
  • filter efficiency is 93%
  • filter MERV rating 12
  • filter MPR rating 1900
  • I don't have any more information

How do I figure this out? I've looked at the Affinity laws:

$ \frac{Q_{1}}{Q_{2}} = \frac{N_{1}}{N_{2}} $

$ \frac{H_{1}}{H_{2}} = (\frac{N_{1}}{N_{2}})^2 = (\frac{Q_{1}}{Q_{2}})^2 $

$ \frac{P_{1}}{P_{2}} = (\frac{N_{1}}{N_{2}})^3 = (\frac{Q_{1}}{Q_{2}})^3 = (\frac{H_{1}}{H_{2}})^\frac{3}{2} $

Also perhaps important:

I don't really have any test equipment. I was hoping to estimate all necessary values based on product specs and labels. I'd like to know how to calculate my estimated pressure and flow, although perhaps I might up-vote a ballpark answer if it helps me pick a fan:

I have ~2.5" gap space behind my baffle to install my boost fan/fans. I was thinking of installing some 120mm, 110V, 100cfm muffin fans in this space. What total CFM should I aim for? Are muffin fans the right choice?

  • $\begingroup$ Seem to remember a similar question - have you checked? $\endgroup$ – Solar Mike Apr 26 '19 at 18:46
  • $\begingroup$ I think this might be related to this question: 2015-Engineering: CFM in relation to Static Pressure. Although I had some trouble understand some of the answers, or seeing how they derived some equations given the Fan Laws. $\endgroup$ – kipbits Apr 26 '19 at 18:49
  • $\begingroup$ It looks like the inbuilt fan for the unit takes 240V; is it realistic splice in the wiring for my boost fans directly to the wiring on the unit, or do I need to wire it from the wall. It would be nice to have a single on/off for the whole unit and boost fans. $\endgroup$ – kipbits Apr 26 '19 at 19:25
  • $\begingroup$ I'm looking at a variety of fans on ebay: reddit.com/r/kipplebits/comments/bhv1gm/fans it is my understanding that centrifugal fans generally provide more pressure. wish I could figure out the math to get a definite answer... $\endgroup$ – kipbits Apr 27 '19 at 3:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.