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.

 

[KGisME]

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.

Ahhh, this will work, thank you!

To work through this and make sure I have my units properly accounted for, if I solve for volume flow rate (Q), I will get

Q = Power (W) / delta-P (Pa)

using the motor nameplate info (115V, 1.3A) to get power, let's try this out (I'll still go measure actual motor current and voltage with a couple DMMs before I act on these results)
1.3" WC = 323.5 Pa [N/m^2]

Q = 149.5 W / 323.5 Pa
Q = 149.5 (J/s) / 323.5 [N/m^2]
Q = 149.5 ([kg*m²]/s³) / 323.5 (kg/[m*s²])
Q = 0.462 m³/s = 979 cfm

Hmmm that seems way too high. I think I need to use the actual shaft output power instead of electrical input power? If I apply a rough guesstimate for the static efficiency of a small shaded pole motor of 0.20, we get

Q = 149.5* 0.20 / 323.5 = 195.8 CFM

Still seems pretty high. Help?

Edit: oh wait, I'm not taking into account the static efficiency of the fan itself. No idea what to use to estimate that. I can get measurements on the blower wheel diameter and width if that will help?

 

[KGisME]

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.

I would definitely use this equation if I could, but unfortunately I don't have the heat dissipated by the computer either. We've tried to get that info in the past without success, but I could try again.

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........ 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.

Yeah, I was thinking the same thing, so I did test it a second time to make sure. Agreed, I need to find a fan that will be run in its ideal region of the fan curve for this application.

 

[KGisME]

Testing with a balometer (if they can work when oriented horizontally?) is probably the best way. They're so expensive though, I'm not sure I can get them to buy one, and if I did it would take awhile.

Hmm would I be able to make a reasonable estimate of the flow rate through a very small (~2"D x 8"L) duct with a hot wire anemometer? I'm not so sure

 

[LittleInch]

At these sort of powers and very small differential pressures yu cannot calcualte this with even an order of magnitude as there are too many things tyu don't know. Your only way is to measure it using some sort of air velocity meter and then figure ut what area that air is flowing at to get your required CFM and your measured differential pressure.

You fan could be anywhere on your fan curve of CFM vs DP. Unless you can find that fan curve you won't get any where close and if its a flatish curve maybe not even then.

 

[KGisME]

AH-HA! Found some small, relatively lower cost vane anemometer/ balometer kits for low flows

Wohler USA FA 410 KIT (4155) Vane Anemometer Kit w/Funnel Set | Test Equipment Depot

Buy the Wohler USA FA 410 KIT (4155) Large Vane Anemometer Kit with FA 410 Measuring Funnel Set for only $654.3 USD at the Test Equipment Depot. FREE Shipping on orders over $75! Shop our Vane Anemometers catalog today!

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Testo 417-KIT-2 (0563 2417) Vane Anemometer with Measurement Funnels and Flow Straightener | Test Equipment Depot

Buy the Testo 417-KIT-2 (0563 2417) Vane Anemometer with Measurement Funnels and Flow Straightener for only $594.15 USD at the Test Equipment Depot. FREE Shipping on orders over $75! Shop our Vane Anemometers catalog today!

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Testo 417-KIT-1 (0563 1417) Vane Anemometer with Measurement Funnels | Test Equipment Depot

Buy the Testo 417-KIT-1 (0563 1417) Vane Anemometer with Measurement Funnels for only $492.15 USD at the Test Equipment Depot. FREE Shipping on orders over $75! Shop our Vane Anemometers catalog today!

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or
https://www.tequipment.net/TSI/RVA801/ plus
https://www.tequipment.net/TSI/801750/HVAC-Accessories/

Anyone have a recommended brand between TSI/Alnor, Testo, or Wohler?

 

[Snickster]

I calculate the ideal flowrate as follows:

Power = work per time = F(d)/t = P(A)(L/t) = PQ as previously stated

P is in pounds per square foot and Q is in cubic ft./sec
Power is in ft-lb/sec = (550)HP

1.3" wg = 0.1083 ft wg = 6.76 PSF

Therefore:

6.76(CFM/60) = (1/30)(550)

CFM ideal = 162.7 CFM

Assuming a 30% efficiency since you are operating high up on the curve towards minimum flow then actual CFM:

Actual CFM = 0.3(162.7) = 49 CFM

 

[KGisME]

Yeah, that seems more like the right ballpark. If I can get approved to buy one of these low-flow balometer kits, I'll report back with my actual test data when I get it.

 

[pierreick]

Hi,
Probably this paper could support your work.
Pierre

 

[goutam_freelance]

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

For fan power calculation you need to consider total head drop, i.e including velocity head also. So either estimate of velocity at duct or duct diameter is required.

 

[MintJulep]

The manufacturers web page offers a bunch of data for the price of your contact information.

Maybe try that before you start buying test equipment.

 

[KGisME]

The manufacturers web page offers a bunch of data for the price of your contact information.

Maybe try that before you start buying test equipment.

Yes, I linked to the manufacturer's page for this model in my original post, so I am familiar with their website. The data they provide is lacking compared to most fan manufacturers I've worked with in the past. I tried requesting more data through their form several times, with the first time being almost 2 months ago, and they never so much as acknowledge receiving my request, let alone get back to me with any additional information.

 

[Snickster]

I found this searching the web. It is an expanded Fasco catalogue. It don't list performance for the furnace blowers I assume because they are bought a certain size for a certain furnace without the need to know performance except by the furnace designer. They do list a line of general centrifugal fans with performance parameters. There is one with model A080 which looks similar to the A088 you have.