How to Calculate Cooling Capacity for a DOAS unit

This is an easy calculation if you know the cfm (airflow to be conditioned) and have a psychrometric chart handy. 

The formula for calculating total cooling is:

Qt = cfm x (Hf-Hi) x 4.5

Where:

  • Qt = Total capacity in btu/h
  • Hf = Enthalpy of the leaving air
  • Hi = Enthalpy of the entering air
  • 4.5 = constant 

What you need to know:

  1. Airflow (cfm) 

    The first piece of information you need to know is how much outside air you need to condition. This is usually based on how much air (cfm) you are exhausting, or how much fresh air the mechanical code (IMC) requires for the occupants. If you don’t know how to calculate this, give us a call!

  2. Design entering air temperature (EAT)

    Next you need to pick the design entering air temperature (dry bulb and wet bulb). If you are unsure, you can look use the ASHRAE fundamentals handbook as a guide. In Kansas City, for example, you might decide on 98/78 degF (dry bulb/wet bulb).

  3. Leaving air temperature (LAT)

    This is where the psychrometric chart will come in handy (you will need it again later though, so don’t get rid of it yet). Pick your desired space condition (75 degF 50% rh for example), and then move straight to the left on the chart until you reach the saturation curve (100% rh). For the purpose of this post, that will be your desired LAT. A typical LAT is 55/55 (dry bulb/wet bulb).

The final step before plugging everything into a calculator is to determine the Hf and Hi values.

Using your psychrometric chart, plot the conditions from steps #2 and #3 and then find each point’s enthalpy value. 

Now enter everything into the formula!

If we assume 9,000 cfm of fresh air, the values and calculation looks like this:

  • cfm = 9,000
  • Hf = 23.5 (based on 98/78 EAT)
  • Hi = 41.7 (based on 55/55 LAT)
  • Qt = cfm x (Hf-Hi) x 4.5
  • Qt = 9,000 x (41.7-23.5) x 4.5
  • Qt = 737,100 btuh 

The last step would be to convert btuh into tons by diving by 12,000.

Qt = 737,100 / 12,000 = 61.5 tons

If you need any help with this give us a call at 816-842-5400 and one of our sales engineers or territory managers can help!

cfm Distributors has DOAS equipment available up to 70 tons (in a single machine), and we have a few select models that directly fit competitor’s curbs. A DOAS replacement without a curb adapter? Count us in!

ASHRAE 0.4% Design Conditions for the major cities in cfm Distributors Territory

SummerSummerWinter
Dry BulbWet BulbDry Bulb
Kansas City, MO97.2°F76.0°F2.6°F
Omaha, NE93.4°F74.9°F-7.8°F
Des Moines, IA93.4°F76.2°F-6.9°F
Springfield, MO95.0°F74.5°F-2.1°F
Wichita, KS100.4°F73.8°F4.0°F
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Brad Telker

Vice President of Sales, Applied Systems Group at cfm Distributors, Inc.
Brad joined the cfm team in 2006, and now as Vice President of Sales, Applied Systems Group he focuses on business development, as well as helping contractors and engineers find creative and unique solutions to any size HVAC project. When he’s not at work, Brad enjoys reading, running and spending time with his family.
Brad Telker
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