Guidelines for Air to Air Heat Exchanger Design
Following are the typical steps for an air to air heat exchanger design (Please refer the above attached picture):
Select heat exchanger type (i.e. decide number of passages).
Find out the inlet and outlet temperatures (t1, t2, T1 and T2) of both the sides of the heat exchanger. These values are specified based on the application of the air to air heat exchanger.
Calculate heat load by using the following formulae:
H = m1 * c1 * (T2 – T1) = m2 * c2 * (t1 – t2) ……..Eqn.1
H - Total heat load of the heat exchanger.
m1 & m2 – Mass of air (flowing per unit time) for side 1 & 2 respectively.
c1 & c2 – Specific heat of air flowing in side 1 & 2 respectively.
Calculate LMTD (Log Mean Temperature Difference) by using the following formula:
LMTD = [ (T1 – t2) – (t1 – T2) ]/[ ln (T1 – t2) /(t1 – T2) ]…….Eqn.2
- Find out the value of overall heat transfer coefficient (U) using the following formula:
1/U = 1/h1 + w/K + 1/h2………..Eqn.3
U – Overall heat transfer coefficient of the air to air heat exchanger.
h1 & h2 – Convection heat transfer coefficient air of both the sides of the heat exchanger.
K – Thermal conductivity of the tube material.
- Apply the following equation to calculate required surface area:
H = U*A*LMTD……Eqn.3
A – Required surface area.
Once we got the total required surface area, we can size the air to air heat exchanger.