http://en.wikipedia.org/wiki/Heat_transfer_coefficient

Overall heat transfer coefficient
The overall heat transfer coefficient U 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 exchangers, but can be applied equally well to other problems.

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

Q = UAΔTLM
where

Q = heat transfer rate (W)
U = overall heat transfer coefficient (W/(m²·K))
A = heat transfer surface area (m2)
ΔTLM = log 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):


where

R = Resistance(s) to heat flow in pipe wall (K/W)
Other parameters are as above.[5]
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 is calculated by the following relationship:


where

x = the wall thickness (m)
k = the thermal conductivity of the material (W/(m·K))
A = the total area of the heat exchanger (m2)
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:

Air - h = 10 to 100 W/(m2K)
Water - h = 500 to 10,000 W/(m2K)