Editing Heat transfer coefficient (section)

==Overall heat transfer coefficient==
The '''overall heat transfer coefficient''' <math>U</math> is a measure of the overall ability of a series of conductive and convective barriers to transfer heat. It is commonly applied to the calculation of heat transfer in [[heat exchanger]]s, but can be applied equally well to other problems.

For the case of a heat exchanger, <math>U</math> can be used to determine the total heat transfer between the two streams in the heat exchanger by the following relationship:

:<math>q = UA \Delta T_{LM}</math>

where:
:<math>q</math> = heat transfer rate (W)
:<math>U</math> = overall heat transfer coefficient (W/(m<sup>2</sup>·K))
:<math>A</math> = heat transfer surface area (m<sup>2</sup>)
:<math>\Delta T_{LM}</math> = [[logarithmic mean temperature difference]] (K).

The overall heat transfer coefficient takes into account the individual heat transfer coefficients of each stream and the resistance of the pipe material.  It can be calculated as the reciprocal of the sum of a series of thermal resistances (but more complex relationships exist, for example when heat transfer takes place by different routes in parallel):

:<math>\frac {1} {UA} = \sum \frac{1} {hA} + \sum R </math>

where:
:''R'' = Resistance(s) to heat flow in pipe wall (K/W)
:Other parameters are as above.<ref>Coulson and Richardson, "Chemical Engineering", Volume 1, Elsevier, 2000</ref>

The heat transfer coefficient is the heat transferred per unit area per kelvin.  Thus ''area'' is included in the equation as it represents the area over which the transfer of heat takes place.  The areas for each flow will be different as they represent the contact area for each fluid side.

The ''[[thermal resistance]]'' due to the pipe wall (for thin walls) is calculated by the following relationship:

:<math>R = \frac{x}{kA}</math>

where
:<math>x</math> = the wall thickness (m)
:<math>k</math> = the thermal conductivity of the material (W/(m·K))

This represents the heat transfer by conduction in the pipe.

The ''[[thermal conductivity]]'' is a characteristic of the particular material.  Values of thermal conductivities for various materials are listed in the [[list of thermal conductivities]].

As mentioned earlier in the article the ''convection heat transfer coefficient'' for each stream depends on the type of fluid, flow properties and temperature properties.

Some typical heat transfer coefficients include {{citation needed|date=April 2025}}:

* Air - ''h'' = 10 to 100 W/(m<sup>2</sup>K)
* Water - ''h'' = 500 to 10,000 W/(m<sup>2</sup>K).