## Tuesday, March 16, 2010

### CFM to BTU formula

A BTU is a British thermal unit, which is the measure of energy to raise one CC of water one degree Celsius.

But you probably want to know about airflow in CFM (not water), and the amount of cooling (or heating) available in 1 cfm or airflow.

In Houston, we tend to cool things more than heat. We also try to drive moisture out of the air (dehumidify), but I won't discuss that now. The general equation to answer your question is like this: Qsens = cfm * 1.08 * (Tin - Tout) <-- "delta T"

You need to determine what your "delta T" is. For cooling a space to 75degF (23.89 degC), your dT is usually 20degF (-6.67 degC) with your supply air temperature (SA=55degF 12.78degC) and your return air temperature (RA=75degF 23.89 degC). This has been idealized and simplified. For cooling, 1cfm will do about 21.6btu/h of cooling. For heating, (SA=90/32.2, RA=68, 1cfm => 23.76btu/h)

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CFM * 1.08 * tRise = Btu/hr
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General information follows.

That factor is based on standard conditions which are at sea level and A standard humidity level which i do not remember off hand. As altitude increases the factor decreases. At 2100 feet it is roughly 1.0.
The humidity also affects this number. Generally the greater the humidity the greater the factor.

1.08 Will be good enough for most applications though.

I'll give an example of the use:

Suppose the room temperature is 65.
We have a 1200 CFM blower. We measure an output temperature of 100.
100 - 65 = 35 tRise.

Now we have all the numbers to get our BUT/hr.

1200 * 1.08 * 35 = 45,360 btu/hr

This particular problem often compilments another, And that is how do we verify the CFM is as cited or measured with an anemometer.

Assuming we have electric heat and the temperature probe is not in line of site of the heating elements (to be sure we are not measuring the IR radiation) All you need to know is the power consumption. Either measure it with an amp meter. Or go with cited figure(not as accurate).

CFM = btu / (1.08 * tRise)

For reference: 1 kw/hr = 3412.14 btu

assume we have a 15kw heater and a tRise of 22

15kw * 3412.14 = 51182.12 btu

now we can solve for CFM

51182.12 / (1.08 * 22) = 2154.

In that example our result is 2,154 CFM.

If that conflicts with a suspected figure An investigation is required.

Keep in mind the factor is based on standard conditions. As a rule of thumb the result should be within 10% of the calculation. Anything out of this range is a trouble spot. Most likely to be a dirty air filter, or incorrect power factoring.