How to Size a Water to Air Heat Exchanger

How to Size a Water to Air Heat Exchanger

The water to air heat exchangers purpose is to transfer water heat into the forced air heating system of your home or commercial building. These units generally work by taking heated water though coils of copper tubing and running it back and forth against a surface of aluminum fins. This design uses the heat from the liquid enclosed in the tubing and transfers it to the air with the aid of the aluminum fins.

These finned coil heat exchangers are used for many applications in both residential and commercial buildings. They are available in many different sizes and BTU ratings. Some of the common applications are:

  • Outdoor wood furnace heat exchangers.
  • Forced air heating and cooling through water to air transfer.
  • Liquid cooling by air to water or liquid transfer applications.
  • Garage and out building heat.
  • Geothermal heating and cooling.
  • Many other applications utilizing liquid to air or air to liquid transfer.

Sizing of the unit required for the application if there is no space restriction, can be determined by the use of either the theoretical heat transfer formula or the practical heat transfer formula.

Theoretical Heat Transfer

This method is based on the fact that colder liquid draws heat from warmer liquid when passed over a conductive material.

Qt = m x Cp x ΔT, where:
Q = (Bth/h) = Total head load
m (lbs/h) = Mass flow rate of fluid
Cp (Btu/f, lbs) = Specific heat of fluid at constant pressure
ΔT (F) = Change in temperatures between inlet and outlet of the fluid

This provides a theoretical heat load from a fluid changing ΔT temperature at mass flow rate m with the fluids specific heat capacity Cp


Practical Heat Transfer

While also providing the heat transfer properties of a fluid this method in addition depends on the fluid properties, materials of contraction and geometric properties of the specific heat exchanger.

The maximum potential heat transfer is expressed through te practical heat transfer formula.

Qp = U x A x LMTD, where:
U = Overall heat transfer coefficient
A = Surface area
LMTD = Logarithmic mean temperature difference of the inlet and outlet temperatures

In turn, logarithmic mean temperature difference for counter flow heat exchanges is expressed below.

LMTD = [(Thi-Tco) – (Tho – Tci)] / (In[(Thi-Tci)]), where:
Thi = Inlet temperature of hotter fluid
Tho = Outlet temperature of hotter fluid
Tco = Outlet temperature of colder fluid
Tci = Outlet temperature of hotter fluid

Theoretical formula Qt values and practical formula Qp values should be compared to determine if the capacity of the heat exchanger is specifically adequate.

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