Air Flow and Energy in Coils

In the United States HVAC industry, airside flows are almost always reported in cubic feet per minute (cfm) for design and calculation purposes.  As you are probably well aware, a standard formula to calculate the thermal transfer in a heating or cooling coil is:

Btuh = cfm * 1.08 * (To-Ti)

Hmm.

We know that the specific heat of air will relate energy flow to temperature change in the air.  We also know that specific heat refers to a unit mass of air.  We also note that we are looking at Btu-per-hour, while the flow rate is in cubic-feet-per-minute.   So the factor of 1.08 must accommodate these items, among others.  Let’s investigate a little.  We will be using our old English units for this one.

The density (mass) of air is approximately 0.00242 slugs per cubic foot at 50 degF.   Or, multiplying by 32.2, about 0.07786 Lbm per cubic foot.

The specific heat of air is roughly 0.24 Btu/Lbm-DegR at 40 degF (the Rankine scale has the same incremental values as the Fahrenheit scale fortunately!)

So let’s see where that 1.08 comes from.  We need to convert cfm to lbs-per-hour.  And this gets multiplied times specific heat:

CubicFeet/minute * (.07786 Lbs/CubicFoot * 60 minutes/1 hour) = Lbs-per-hour

(.07786 Lbs/CubicFoot * 60 minutes/1 hour) * 0.24 Btu/lbm-DegR = conversion factor.

Which works out to 1.12!  Close enough for HVAC work!  And indeed, if you tweak the temperatures at which you select the specific heat and density of the air, you can hit the 1.08 on the nose.

Note, by the way, that we are looking at the coil power, not the energy.  The energy is equal to the coil power multiplied by time!

Lets do an example.  Imaging a coil with an airflow flow of 50,000 cfm, an entering air temperature of 75 degrees and a leaving air temperature of 55 degrees.

The cooling rate is:  50,000 cfm * 1.08 * (75-55) = 1,080,000 Btuh.

You may recall that cooling power is often reported as tons for large HVAC systems, one ton equivalent to 12,000 Btuh.  If you wish to solve directly for tons, simply divide the formula by 12,000:

The cooling rate is ( 50,000 cfm * (75-55) * 1.08 ) / 12,000 = 90 tons

Note that if you wanted to carry the units on the 1.08 conversion factor, it would be (Btu-min/CubicFoot-hr-DegR)  Which kind of explains why you don’t see it much.

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