How many tons of air conditioning is this DX or Chilled Water coil?

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

For example-

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!

Brad Telker
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Brad Telker

Brad has been in the HVAC industry and at cfm Distributors since 2006, and is actively involved in HARDI as well as the local Kansas City chapter of ASHRAE. Brad has been a semi-professional blogger since late 2015 and is a huge fan of Apple's new 3D Touch technology.
Brad Telker
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