Help me calculate the CFM of this fan in our application

[KGisME]

Hi, we have this draft inducer fan cooling a piece of avionics in a simulator as described here https://www.eng-tips.com/threads/reliable-alternative-to-this-fan.526827/. It’s this one https://www.regalrexnord.com/produc...draft-inducer-blower-115-volts-flange-no-a088 . The motor nameplate on this particular one I measured says 1/30 HP, although the newest copies of this same model use a slightly different motor that’s 1/25 HP. (Is that mechanical output/ shaft HP?) Both are shaded pole motors.

We want to replace this fan with something more reliable, as we’ve had a couple bearing failures. We’d also like one a little quieter. I’m trying to figure out how to calculate the CFM this fan is delivering so I know what CFM I need in a potential new fan. I might need to look at something that's intended more for electronics cooling, like a EBM-Papst or Delta Electronics, as opposed to one meant for HVAC applications.

I measured the SP in the short duct between the fan and the computer at 1.3” W.C.

How can I calculate / estimate the airflow volume based on the data I have? The fan calculators I’ve found online all require you to input airflow volume.

 

[3DDave]

Power = volume rate * delta-P when all the dimensions/units are compensated for. If you measure the actual power consumption of the fan and neglect temperature rise of the air, then you can solve for volume rate.

Cooling is more a function of mass flow rate, but if the density of the air isn't changing too much then volumetric rate is a sufficient measure and easier to do.

You could get a big plastic bag and fit it into a drum - I am thinking of a cardboard drum used to deliver either packing material or some small item suspended in packing material. Anyway, fill the bag so it fits the volume of the drum, which you can calculate by measuring it, and run a hose from the bag to the inlet and time how long it takes to displace all the air from the bag.

Example drum: https://www.opentip.com/BASCO-55-Gallon-Fiber-Drum-Open-Head-Fiber-Cover-p-8975377.html?ats=gs&gQT=1

There are CFM measuring devices available; I would expect most commercial HVAC companies would have them; some residential would also. The ones that pop up on search are anemometers to measure air speed and require knowing the duct size. Probably they assume some turbulent distribution to even the flow out, though any measurement near the outlet of a fan is going to be highly variable and need a pretty lengthy settling distance.

I see a device called a balometer for capturing airflow and dealing with errors that using an anemometer could introduce. For example:

 

[Snickster]

The CFM can be calculated using the following formula:

Q = 1.1 CFM (T2-T1)

Where Q is the heat dissipated by the computer in BTU/hr= power rating of computer in watts time 3.415
CFM is the actual CFM of your blower in Standard Cubic Feet Per Minute
(T2-T1) = temperature difference between fan inlet an air exhausted from the computer in deg. F. Typically computers or sensitive electronics are designed for 104F maximum temperature.

For instance for a 400 watt rated computer with 70F inlet air measured at fan inlet, and 104F exhaust air measured at exit port of the computer, the CFM is calculated as follows:

400(3.415) = 1,256 btu/hr = 1.1 (CFM) (104-70)

CFM = 33.6 CFM

This will be the flowrate of your fan. If you say your are measuring 1.3 inches wg. then this would be the static pressure at calculated flowrate required to size the fan. Also note that regardless of what your present flowrate, the actual required flowrate can be determined by setting a computer exhaust temperature to whatever you want if you want it to be less than 104F, and subtract your actual fan inlet temperature to get your temperature difference, and then solve for CFM by using the actual power rating of your computer in the above equation. However if you do increase the flowrate you will need to increase the static pressure output of the fan in accordance with the square of the higher flow divided by the present flow since pressure loss through your computed is proportional the ratio of the flowrates squared. So if your present flow is really 33.6 CFM and you want to increase your flow to 50 CFM the new pressure output of your fan required will be (1.3)(50/33.6)^2 = 2.2 inches w.g. in order to really get the 50 CFM.

I would double check the static pressure measured as this appears a little high for such small flowrate. This makes sense though because it appears you have a blower that is probably rated a higher flow at lower pressure say about 100 CFM at 0.5 static pressure normal operating point, but operating in at a point high up on the fan curve due to the pressure loss through the computer being high due to high flow resistance causes the flow to reduce as the operating point moves towards the left on the fan curve where likely you are at a point at or below the fan minimum recommended flow. That is why it is noisy and ruining the bearings. Your fan is an induced blower flue gas fan for a furnace and you are using it to cool a computer, and using it as a force draft fan instead an induced draft fan. You need to select a new fan such that the operating point of static pressure and flow required is closer to the midpoint on the fan curve at the best efficiency point.