Unit Conversion Constants; The Answer to Deriving the 500 in Q = 500 x gpm x delta t

This is just a quick post to share the answer to the challenge I posted about a week back.  Specifically, I suggested that you might want to try your hand at deriving the “500” in the common water side load equation we use just about every day out there in the HVAC field.


Here is my answer, (I actually came out at 501 and rounded it to 500).

500 gpm t 2

The constant involves multiplying the minutes per hour times the number of gallons per cubic foot, the density of water, and the specific heat of water.  To adjust the constant for different conditions or different fluids, you would simply adjust these factors to match the condition or fluid in question.

David Sellers
Senior Engineer – Facility Dynamics Engineering

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