Heat Added to Chilled Water Systems Due to Pump Efficiency

Norm Hall
/
August 9, 2010
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 We received an interesting question this week from a consultant engineer: “How much temperature increase will I get in the chilled water from the pump? My secondary chilled water pump has a capacity of 1400 GPM at 100 feet” Let’s look at the answer. ANSWER: Centrifugal pumps in hydronic systems do the work of moving gallons per minute of fluid against a differential head in feet. This work is defined as Brake Horsepower (BHP) also expressed in equation 1. The maximum amount of horsepower available to heat the water is 1 minus the pump efficiency multiplied by the motor horsepower. Bell and Gossett refers to this as the Heating Horsepower (HHP) expressed in equation 2. 1 HP equals 42.44 BTU/Minute (equation 3) and BTU/minute is GPM times ∆T times 8.33 times specific gravity times specific heat (equation 4). The later two items are equal to 1 for water. Using all of this we arrive at a formula for ∆T shown as equation 5.
 BHP= (GPM X Ft hd)/ (3960 X Eff)     – Equation 1 HHP= BHP X (1-Eff)                            – Equation 2 1 HP= 42.44 BTU/M                           – Equation 3 BTU/M = GPM X ∆T X 8.33 X 1 X 1   – Equation 4 ∆T = (42.44 X HHP)/ (8.33 X GPM)  – Equation 5 ∆T = (5.09 X HHP)/ GPM                    – Equation 6 Let’s look at the example requested by the engineer. We selected a 60 HP pump using the ITT Bell and Gossett ESP-PLUS program. The BHP, using equation 1, is (1400 X 100) / (3960 X .84) = 42.09 HP. At the design point we have an efficiency of 84% so HHP is 42.09 X .16 or 6.73 HP and ∆T is (5.09 X 6.73)/1400 or .02 degrees F. We can answer the question with the response, “Not very much”!

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