Thermal conductivity is an important factor when determining the actual required surface area of a heat exchanger. For example, the thermal conductivity for titanium is 90 times that of PVDF. However, when comparing the actual required surface area for a titanium immersion heat exchanger, the PVDF unit will require only approximately three times the surface area.
The surface area of the heat exchanger is inversely related to the overall heat transfer coefficient (U), which is not only based on the thermal conductivity and wall thickness of the material from which the heat exchanger is fabricated, but also on surface effects at the inside and outside wall of the heat exchanger tubing. As U doubles, the required surface area is cut in half.
To optimize the design of the heat exchanger, it is necessary to maximize the heat transfer coefficient. Calculation of the overall heat transfer coefficient is actually a highly iterative process dependent on the material used and the physical and thermodynamic properties of the fluids on both the inside and outside of the tube wall. Equation 1 represents the generalized equation for calculation of the heat transfer coefficient for a cylinder.
H12is the surface transfer coefficient at the inside surface of the tube
H23is surface transfer coefficient at the outside surface of the tube
R2is the radius of the inside of the cylinder
R3is the radius of the outside of the cylinder
k23is the thermal conductivity of the cylinder material
It can be seen in Equation 1 that thermal conductivity is only one of the factors that control the overall heat transfer coefficient. Other major factors include the inside and outside radii of the tubes.
To combat the low thermal conductivity, plastic heat exchangers are designed with smaller tubes and are thus able to fit more surface area in the same footprint. For instance, a titanium heat exchanger with 0.75" dia. tubes would have a tube wall thickness in the range of 0.030 to 0.036". A comparable plastic heat exchanger would have a tube wall thickness of 0.024" with a wall diameter of 0.25". A titanium and plastic heat exchanger with the same footprint, 84 x 24", would have surface area of 30.0 ft2and 150 ft2, respectively.