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The overall heat transfer coefficient is employed in calculating the rate of heat
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transfer Q from one fluid at an average bulk temperature T 1 through a solid surface to a

second fluid at an average bulk temperature T 2 (where T 1 > T 2). The defining equation
is generally only applicable to an incremental element of heat transfer surface dF for

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which the heat transfer rate is dQ , and the equation is strictly valid only at steady state
conditions and negligible lateral heat transfer in the solid surface, conditions generally

true enough in most practical applications. The defining equation is (2.1)

It is used in calculating the heat transfer, typically by convection or phase

transition between a fluid and a solid.
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The heat transfer coefficient has SI units in watts per squared meter kelvin: W/(m K).

There are numerous methods for calculating the heat transfer coefficient in different

heat transfer modes, different fluids, flow regimes, and under different thermo-

hydraulic conditions. Often it can be estimated by dividing the thermal conductivity of

the convection fluid by a length scale. The heat transfer coefficient is often calculated

from the Nusselt number (a dimensionless number). There are also online

calculators available specifically for heat transfer fluid applications.

It is to be noted that often the value for δ is referred to as the difference of two radii

where the inner and outer radii are used to define the thickness of a pipe carrying a

fluid, however, this figure may also be considered as a wall thickness in a flat plate

transfer mechanism or other common flat surfaces such as a wall in a building when the

area difference between each edge of the transmission surface approaches zero.

In the walls of buildings the above formula can be used to derive the formula

commonly used to calculate the heat through building components. Architects and

engineers call the resulting values either the k-value or the R-value of a construction

assembly like a wall. Each type of value (R or U) are related as the inverse of each other

such that R-Value = 1/k-Value and both are more fully understood through the concept

of an overall heat transfer coefficient.

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