Electric Heat Calorimetry Simplified



Electric Heat Calorimetry is a fairly accurate way to measure air flow volume and other quantities is HVAC systems. it is necessary to know airflow to properly achieve heating, cooling and dehumidification.

On a heat pump in the heating mode if we have too much air flow, efficiency will be high but the system will be noisy and drafts can be felt. If air flow is too low pressures will run high, efficiency will be down, heating output will be down and destruction to the system will result!

Basics: to measure the quantity (mass) of air that passes through a furnace or air handler if measured in CFM (Cubic Feet Per Minute) has a specific heat of .018 Btus/cubic foot (see definitions below). The amount of heat added to the air from electric heating elements is 3.4 Btus per hour for each Watt. 

Example: a 5Kw heating element will add 5 X 3,400 Btus per hour or exactly 17,000 Btus/hour to the moving air. We will use 17 degrees temperature rise in our example to make clean division.

Read on for the formula.

Density of air=0.0755 pounds per cubic foot at 70F, ignoring humidity at sea level.

Specific Heat of air =.24 Btus per pound
at 70F, ignoring humidity at sea level.

Specific Heat of air =.018 Btus per cubic foot at 70F, ignoring humidity at sea level.

Specific heat of 60 cubic feet of air = 1.08 at 70F, ignoring humidity at sea level.

Specific heat of 1CFM of air in 60 minutes = 1.08 at 70F, ignoring humidity at sea level.

CFM=  Btus/hr (electric heat input) divided by  1.08 X Temperature Rise. Works with other known Btu inputs.

Btus/hr =CFM X 1.08 X Temperature rise.

Method 1: If you have a 17 Degree temperature rise (difference between input and output) as a result of 17,000 Btus per hour input. Dividing 17,000 by 17 equals 1000. which is very close using 1 as a factor. If we multiply 17 degrees by 1.08 we get 18.36, when divided into 17,000 we get 925.9 CFM.

Method 2: Take Btus/hour input, divide by temperature rise to get a baseline, in our case 1000. Then multiply the results by 1/1.08 which is .9259 and the rest is history. So for a fairly accurate estimate divide the Btu/hr input by the temperature rise to get CFM. in our example if instead of using 5Kw we used 10Kw (34,000 Btus per hour)  Our temperature rise will be 34 degrees and the division will result in the same number.

For the most accurate reading one should measure the exact number of Btus/hour by measuring the input voltage to the heating elements and the current draw in amperes to get watts and then multiplying by 3.4 to get Btus/hour.



Another version at the link below.

http://www.bacharach-training.com/norm/cfm.htm


Copyright © 2007 Scott Meenen
All rights reserved