Editing Heat transfer coefficient (section)

== Composition ==
{{See also| Lumped-element model}}
A simple method for determining an overall heat transfer coefficient that is useful to find the heat transfer between simple elements such as walls in buildings or across heat exchangers is shown below. This method most readily accounts for conduction and convection.  Effects of radiation can be similarly estimated, but introduce non-linear temperature dependence. The method is as follows:

:<math> \frac{1}{U \cdot A} = \frac{1}{h_1 \cdot A_1} + \frac{dx_w}{k \cdot A} + \frac{1}{h_2 \cdot A_2} </math>

Where:

: <math> U </math> =  the overall heat transfer coefficient (W/(m<sup>2</sup>·K))
: <math> A </math> = the contact area for each fluid side (m<sup>2</sup>) (with <math> A_{1} </math> and  <math> A_{2} </math> expressing either surface)
: <math> k </math> = the [[thermal conductivity]] of the material (W/(m·K))
: <math> h </math> = the individual convection heat transfer coefficient for each fluid (W/(m<sup>2</sup>·K))
: <math> dx_w </math> = the wall thickness (m).

As the areas for each surface approach being equal the equation can be written as the transfer coefficient per unit area as shown below:

:<math>  \frac{1}{U} = \frac{1}{h_1} + \frac{dx_w}{k} + \frac{1}{h_2} </math>

or

:<math>  U = \frac{1}{\frac{1}{h_1} + \frac{dx_w}{k} + \frac{1}{h_2}} </math>

Often the value for <math>dx_w</math> 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 [[R-value (insulation)#U-factor/U-value|U-Value]] or the [[R-value (insulation)|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/U-Value and both are more fully understood through the concept of an [[#Overall heat transfer coefficient|overall heat transfer coefficient]] described in lower section of this document.