How to measure BTU/hr. ?

gregteMarch 29, 2008

I have made a simple air to air heat exchanger that flows 3 CFM of air and lowers the air's temperature 10 degrees F.

As far as I can tell this means that the heat exchanger has a capacity of 135 BTU/hour. I derived this by using the following formula:

BTU/hr. = CFM * delta-T * 4.5

Is this correct?


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The basic equation is

q = m*Cp*delta-T

where q is the heat transfer rate, m is the mass flow rate, Cp is the specific heat of air and delta-T is the temperature difference.

This equation applies to both sides of the heat exchanger, and the q's must be equal. Your equation is using CFM, which is a measure of volumetric flow rate rather than mass flow rate. To convert, use

m = Q*rho

where Q is the volumetric flow rate and rho is the density of air.

You need to keep track of the units of measurement in all of these equations. It's possible that the 4.5 factor in your equation takes care of the gas properties (specific heat and density) and the units, but I couldn't say for certain without access to my reference books (don't keep them at home!).

    Bookmark   March 29, 2008 at 8:45PM
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BTU per hour = CFM x 1.08 x Delta T

The 1.08 factor gets the units straight.

    Bookmark   March 31, 2008 at 7:37AM
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Technically, since 1997 the Standard air calculation has been revised to accommodate for humidy. The original factor of 1.08 is for dry air.

BTUH = 60 x 0.075 (0.24 + 0.45W) x CFM x Delta T

BTUH = 4.5 x .2445 x 3 x 10

BTUH = 33

60 = min/hr
0.075 = lb(dry air)/ft3
0.24 = specific heat of dry air, btu/(lb* degrees f )
W = Humidity ratio, lb(water)/lb(dry air)
0.45 = specific heat of water vapor, btu/(lb* degrees f )

When W = 0.01 the value = 1.10
When W = 0.02 the value = 1.12
When W = 0.03 the value = 1.14

Thus, because a value of W = 0.01 approximates conditions found in many air-conditioning problems, the Sensible heat change (in btu/h) can be found as;

BTUH = 1.10 x CFM x Delta T

The preceeding was extracted from 1997 ASHRAE Handbook "Fundamentals", page 28.15.

Dan Martyn

    Bookmark   March 31, 2008 at 10:51AM
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We've been working on computing some BTUH in different locations with different humidity ratios. When I checked the 1997 handbook which you referenced I noticed they used slightly different figures for the values. For example, you mentioned that "when W = 0.01 the value = 1.10, when W = 0.02 the value = 1.12, when W = 0.03 the value = 1.14", whereas in the handbook, it shows these values as: 1.23, 1.25 and 1.27 respectively. I wasn't sure what that difference might be and I wanted to make sure before we started in on the calculations that we were doing them correctly.



    Bookmark   April 6, 2009 at 1:45PM
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