Basic Concepts of Heat Exchange
Heat exchange is a natural phenomenon occurring between fluids of different temperatures. Heat exchangers allow to control heat transfer and are used for various applications, such as radiant heating, solar heating, ice and snow melt, oil cooling and pre-heating, domestic water heating, and more.
Theoretical Heat Transfer
The basic principal of heat exchangers is based on the premise that colder fluid gains heat from a relatively hotter fluid while flowing over a conductive material in a heat exchanger. This is expressed by the following formula:
Qt = m x Cp x ∆T, where:
Q (Bth/h) = Total heat load
m (lbs/h) = Mass flow rate of fluid
Cp (Btu/F, lbs) = Specific heat of fluid at constant pressure
∆T (F) = Change in temperatures between inlet and outlet of the fluid
This formula gives us theoretical heat load from a given fluid undergoing ∆T temperature change at a mass flow rate m, with the fluid’s specific heat capacity Cp. It is a theoretical heat yield that can be transferred to or from a fluid.
Practical Heat Transfer
While theoretical heat load gives us an idea of the amount of heat that can be transferred to or from a fluid; the practical heat transfer also depends on the fluid properties, materials of contraction of the heat exchanger and physical geometry of the heat exchanger itself.
Practical heat transfer can be defined as the maximum potential heat transfer through a heat exchanger and is expressed by the following 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 exchangers is expressed by the following formula:
LMTD=[(Thi-Tco)- (Tho-Tci) ]/(ln[(Thi-Tco)- (Tho-Tci)]) , where:
Thi = Inlet temperature of hotter fluid
Tho = Outlet temperature of hotter fluid
Tco = Outlet temperature of colder fluid
Tci = Inlet temperature of colder fluid
Theoretical Qt and practical Qp values should be compared to determine if the heat exchanger has enough capacity for specific application.
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