If you are measuring a Direct Expansion (DX) or Chilled Water coil in the field, or just quoting a replacement coil, you can quickly estimate its nominal tonnage based on the square footage of the FIN area of the coil.
This is a useful “rule of thumb”, which will help ensure that you have all the information you need before leaving the site or getting off the phone with your customer.
It’s also a good double check to make sure you aren’t missing something major.
Since most evaporator coils are designed for a maximum face velocity of 500 fpm (to eliminate condensate blow-off), and nominal airflow is typically 400 cfm/ton for comfort cooling applications- here is the formula for determining nominal tonnage based on square footage of the fin area of the coil.
(Ft^2 of fin area) x (500 fpm) / (400 cfm/ton) = Nominal Tonnage
If: Fin area of the coil is 58” x 20”
Then: Fin area = 58 x 20 / 144 = 8.0 ft^2
8.0 ft^2 x 500 / 400 = 10 Nominal Tons
To simplify the equation: (square feet of fin area x 1.25) = Nominal Tonnage
This can be done in reverse as well:
(Nominal tonnage) x (400 cfm/ton) / (500 fpm) = approx. fin area in square feet
So, if you are about to go measure a 30 ton evaporator coil, you should expect to find a coil that has approximately 24 square feet of
fin area (30 / 1.25 = 24 ft^2).
Note: All of this ASSUMES 500 feet per minute and 400 cfm/ton airflow.
Make up air units typically run at HALF of this airflow (~200 cfm/ton). And plenty of air units are designed for 300 fpm across the evaporator coil.
The point is, these equations should only be used as a “rule of thumb” to double check or to estimate the nominal tonnage of the system.
If the square footage is far off, then you might be dealing with a 100% outside air system, or a low discharge temperature application.
Let us know if you would like us to meet you in the field to measure a replacement coil. We’d love to help!
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