heat flow in pipe wall (K/W) Other parameters are as above. 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: where x = the wall thickness (m) k = 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: Air - h = 10 to 100 W/(m2K) Water - h = 500 to 10,000 W/(m2K). Thermal resistance due to fouling deposits Often during their use, heat exchangers collect a layer of fouling on the surface which, in addition to potentially contaminating a stream, reduces the effectiveness of heat exchangers. In a fouled heat exchanger the buildup on the walls creates an additional layer of materials that heat must flow through. Due to this new layer, there is additional resistance within the heat exchanger and thus the overall heat transfer coefficient of the exchanger is reduced. The following relationship is used to solve for the heat transfer resistance with the additional fouling resistance: = where = overall heat transfer coefficient for a fouled heat exchanger, = perimeter of the heat exchanger, may be either the hot or cold side perimeter however, it must be the same perimeter on both sides of the equation, = overall heat transfer coefficient for an unfouled heat exchanger, = fouling resistance on the cold side of the heat exchanger, = fouling resistance on the hot side of the heat exchanger, = perimeter of the cold side of the heat exchanger, = perimeter of the hot side of the heat exchanger, This equation uses the overall heat transfer coefficient of an unfouled heat exchanger and the fouling resistance to calculate the overall heat transfer coefficient of a fouled heat exchanger. The equation takes into account