finding out CFM of a unit

[mohad_i]

hi.
i have seen a YouTube video about how to find out the actual CFM of a unit installed and running. at the beginning it was showing nice a great calculation formulas. but then i get confused when he used the electrical power input and assumed it as the cooling power output..

he used the formula , sensible heat capacity = WATTS X 3.413 BTU/h, which its right (conversion between KW and BTU/h).

But also he considered the WATTS in this Thermal units conversion formula is the same as the electrical power input KW.

I get confused here, what i know is the conversion formula can be used for the output power , from kw output power to BTU/h, not from the input electrical power to BTU/h.

what you think guy's?

where is my mistake? or am i correct.!

https://youtu.be/aRJH-wJZ1Gs

https://youtu.be/aRJH-wJZ1Gs

[Artrose]

hi.
i have seen a YouTube video about how to find out the actual CFM of a unit installed and running. at the beginning it was showing nice a great calculation formulas. but then i get confused when he used the electrical power input and assumed it as the cooling power output..

he used the formula , sensible heat capacity = WATTS X 3.413 BTU/h, which its right (conversion between KW and BTU/h).

But also he considered the WATTS in this Thermal units conversion formula is the same as the electrical power input KW.

I get confused here, what i know is the conversion formula can be used for the output power , from kw output power to BTU/h, not from the input electrical power to BTU/h.

what you think guy's?

where is my mistake? or am i correct.!

https://youtu.be/aRJH-wJZ1Gs

https://youtu.be/aRJH-wJZ1Gs

I'm not sure I totally understand the question, but here's something that might help?

That's the method we occasionally use to determine airflow across, (for instance) electric duct heaters in variable volume systems. It might be used if there's a particular reason why we can't do a duct traverse, or use a flow hood, or aren't particularly interested in being as accurate as possible for our airflow readings. This method has good potential for error in the field.

It's simple. With the system stabilized, measure the volts and amps of the electric duct heater. Ohms law states that volts times amps equals watts.
One watt is equivalent to 3.41 btu's. Multiply your watts times 3.41 which gives you your sensible capacity.

Take the temperature of the air before it passes through the electric heating element, and take the temperature of the air downstream of the heater.
Subtract the air temp entering from the air temp leaving for your delta T.

Plug your readings into the formula and the result is cubic feet of air per minute. We normally don't worry about the density of the air, or the power factor of the power supply.

This method normally works well enough for resistive loads like electric duct heaters, but there are much better methods for determining airflows. Basically, in my world, it's an extra trick in the trick bag.

[mohad_i]

I'm not sure I totally understand the question, but here's something that might help?

That's the method we occasionally use to determine airflow across, (for instance) electric duct heaters in variable volume systems. It might be used if there's a particular reason why we can't do a duct traverse, or use a flow hood, or aren't particularly interested in being as accurate as possible for our airflow readings. This method has good potential for error in the field.

It's simple. With the system stabilized, measure the volts and amps of the electric duct heater. Ohms law states that volts times amps equals watts.
One watt is equivalent to 3.41 btu's. Multiply your watts times 3.41 which gives you your sensible capacity.

Take the temperature of the air before it passes through the electric heating element, and take the temperature of the air downstream of the heater.
Subtract the air temp entering from the air temp leaving for your delta T.

Plug your readings into the formula and the result is cubic feet of air per minute. We normally don't worry about the density of the air, or the power factor of the power supply.

This method normally works well enough for resistive loads like electric duct heaters, but there are much better methods for determining airflows. Basically, in my world, it's an extra trick in the trick bag.

yah, thanks... there was my mistake.
i didn't notice he is using this method for resistive load only (electric resistive heater).

thanks

[mgenius33]

Good video, but I disagree with the free area.

[mgenius33]

Using the opening he measured, you’re only discounting 1/2” on each dimension. That’s roughly 90-95% free area of the actual listed grille size.

You will have very high readings using the grille area explained in this video.
On a standard RA grille I use 65-75% free air depending upon whether the louvers are tight gap or wide gap respectively.

[mgenius33]

I will say, you can use the basic grille dimensions if you maintain a 1” distance from the grille. However, this video shows him holding the vane flat to the grille surface. The distance from the grille does make a huge difference. The velocity does change as it approaches the grille louvers.

[mgenius33]

What distance you hold from the grille is predicted by the expected velocity. What I mean by that is, 450fpm at 1” may result in proper reading, while 150fpm may need 1/2” distance.

[mgenius33]

I get confused here, what i know is the conversion formula can be used for the output power , from kw output power to BTU/h, not from the input electrical power to BTU/h.

Remember energy is always conserved. If energy is not performing work, it will be conserved in the form of heat. So, if your input energy is not performing work it must be converted to heat.
Look up “conservation of energy” law.

[hvacker]

I might expect holding the instrument away from the grill might entrain air next to it and effect the reading. This might be how the meter is supposed to be used by the maker. I used mine like that.

[mgenius33]

Random assumptions are made with regard to methods of anemometer data collection.

I don’t believe there is a clear set of instructions provided by any vane anemometer mfg in order to provide confident results on a return grille traverse.

I do believe that a decent quality vane as simple as a 410 could provide excellent results a technician could feel confident with. Holding a vane free handed at various depths and speeds, using random free areas, is not going to provide those results.

[hvacker]

I never liked to NEED to use an anemometer because of other methods weren't possible. I didn't find them all that reliable. Following the inst. didn't always give the expected result.
I could expect surprises. System effect etc. and the instrument itself.

[Ed Janowiak]

Use a TrueFlow grid from the Energy Conservatory to get the most accurate / easiest to use results for measuring airflow in a residential application.

[mgenius33]

Use a TrueFlow grid from the Energy Conservatory to get the most accurate / easiest to use results for measuring airflow in a residential application.

Yes, but very pricey. How well do they work with insertion losses? Ideally filter back grilles are 300fpm, but we all know that’s a rarity especially in residential applications.

Never mind, I see that you use NSOP and TFSOP, in order to calculate a correction factor.

[hvacker]

I removed the grid from a Shortridge flow hood I wasn't using to use as a collector grid hooked up to my Alnor velometer.
It worked but I had no way to certify the readings so mainly used to troubleshoot.
But I can see the advantages of a grid on returns.