Understanding Blower Ratings


Thread Starter



I have a blower rated to deliver 2063 CFM @ 1.5" SP @ 1719 RPM, but it comes with blower curves that relate RPM/CFM/SP. I would like to measure my actual static pressure to see where I am at on the curve, but I am not sure where I should measure it. At the blower inlet? Outlet? Somewhere else? If anyone could give me any advice, I would greatly appreciate it.

Thank you,
The spec refers to the difference between the inlet and outlet pressure, the rise that the fan is creating. Use a differential Magnetic or transmitter across the fan.


Thank you Steve and Smart S,

After thinking about this some more, I realize that even if I am able to measure the differential pressure between my blower inlet and outlet, I still have some confusion interpreting the blower performance curves I get from my supplier.

The curves I get actually include three curves. One curve is for static pressure, one is for brake horsepower, and one is for CFM. From what I can tell (and please correct me if I am wrong), all three curves (sp, bhp, and CFM) make the assumption that the blower speed is a fixed RPM (in the case of the blower I am working with now, this is 1719 rpm). Making this assumption, the intersection of the SP curve and the CFM curve shows me an operating point at which my blower should move 2063 CFM when SP = 1.5 in wg and blower speed is 1719 rpm.

We control our blowers with variable speed drives, so we can set our blower speeds to whatever we want (we always end up running them slower than the speed stamped on the blower nameplate). I was wondering if you knew of a way that I could calculate my expected air flow at different blower RPMs. (If it is necessary, I do have a manometer that would allow me to take differential pressure readings between the blower inlet and outlet, or to measure static pressure anywhere I would need to).

If you can tell me if/how I can do this type of calculation or can provide any insight (or refer me to any reading material) that might help my understanding of blower curves, I would greatly appreciate it.

Thanks again,
Generally speaking, your CFM will be proportional to the rpm, for a given system curve (don't know if you're always pushing through the same system, or if it changes dynamically -- even filter loading will change your system curve).

If you know your rpm, that identifies a specific fan curve, which then knowing the dp, you can calculate cfm. Alternatively, you could generate equations, so that at a given dp, the cfm varies directly as the rpm, and relate the proportionality constants to the measured dp.

Alternately, if all you want to know is cfm, you could probably pick some portion of your system that never changes in terms of restriction. Then place a dp sensor across that fixed restriction, and cfm will be proportional to the square root of the dp. This will wash out the source of any variation, whether it be drift in your system curve or change in rpm, but it's the most direct path to cfm measurement. Or, look at the difference between (velocity + static) pressure, as measured by a pitot tube, and the static pressure alone, as measured by a static (perpendicular) tap. You might need a probe that averages across your duct.


Hi Steve,

Thanks for your thorough response. As you point out, it seems like the air flow calculations I am hoping to do would be easier (and have more chance of being accurate) if I can assume my system (or at least some portion of my system) doesn't change dynamically.

Unfortunately, I am not sure if I can make this assumption. I didn't mention this in my previous emails, but my system consists of a supply blower that pushes air through electric heaters followed by filters and then through a duct into a drying chamber. I also have an exhaust blower that pulls air out of the drying chamber and through a duct that goes outside of the building (possibly first passing through a thermal oxidizer). I use variable speed drives for both the supply blower and the exhaust blower. The dryer is not air tight as it has openings that allow the product that is being dried to be conveyed in and out of the dryer. I try to set the blower speeds so that the exhaust blower removes slightly more air than the supply blower supplies (so I have a slight negative draft from the room into the dryer). Otherwise, solvent vapors from the product I am drying can get into the room where the operators are.

With this in mind, I would say that the interaction of my two blowers affects my system curve (and possibly even my fan curves). This makes me think that simply measuring the dp between the inlet and outlet of my supply blower wouldn't allow me to use the fan curve supplied by my blower manufacturer to calculate cfm because I would guess the fact that my exhaust blower is pulling air away from the supply blower would affect the dp I measure between the supply blower inlet and outlet.

Your suggestion that I might be able to determine air velocity by looking at the difference between velocity and static pressure using a pitot tube seems promising to me, but when I investigated this possibility with Dwyer, they thought their products would not be suitable for my low air flows (which I estimated could be as low as 200 FPM). If you know of a company that offers pitot tube solutions that work with very low air flows, I would greatly appreciate it. For that matter, if you could recommend any device that would allow me to take accurate measurements at low air flows, I think it would be very useful to me.

Your suggestion that if I do have a portion of my system that never changes in terms of restriction makes me wonder if my filters would qualify. I can certainly measure the dp across my filters (although this probably changes as the filters get clogged). It would be useful if I knew that the CFM was proportional to the square root of this dp, but I think (and please correct me if I am wrong), unless I had some sort of instrument to measure CFM at a few different blower speed settings, I wouldn't know what the correct scale factor would be between CFM and square root of dp.

Any advice or information you provide (or tell me where I can find) will be greatly appreciated.

thanks again,
I'm lucky, I get to work with round pipes and commercial primary flow elements for liquids and steam.

But I notice that the AMCA organization seems to have written the book (the standards) on fan/blower air flow. A call to them might point out the white paper that best addresses your concerns.

They have a ton of downloadable standards on this site