Although most engineers do not think plastics are practical materials for constructing heat exchangers, the high chemical inertness of thermoplastics -- along with their flexibility in fabrication -- offers advantages vs. exotic metal heat exchangers for highly corrosive applications. As long as care is taken in the design, bearing in mind the limitations of pressure and temperature, plastic heat exchangers operate reliably. Immersion and shell-and-tube style plastic heat exchangers can be used in many process heating applications.
The primary reason to use any plastic heat exchanger is corrosion resistance. One misconception about plastic exchangers is that they are less durable than metallic units made from exotic metals. This is not necessarily the case. The tubes used to manufacture exotic metal heat exchangers can be very thin due to the high cost per pound of the metal. The thinner tubes are less robust and must be handled with care. Additionally, corrosion of the tube wall is a big factor when the tube wall is already thin. The application area of exotic metal units requires specific knowledge of all components in the process stream. The addition of just a few parts per million of a salt ion, for example, can cause fast corrosion in an application where a specific metal normally would be appropriate for use.
Thermoplastic heat exchangers are made from many types of plastic, including polyethylene (PE), polypropylene (PP), polyvinylidene fluoride (PVDF) and several Teflon derivatives. These materials offer good chemical resistance for many applications. When compared to many metals, thermoplastic tubing is smooth; therefore, plastic heat exchangers are useful in applications where the fluid tends to adhere to the surface of a metallic unit. Plastic heat exchangers have been used for applications where system purity is an issue such as manufacturing in the semiconductor and pharmaceutical industries. The high purity of materials such as PVDF and perfluoroalkoxy resin (PFA) have been widely accepted where any contamination from the heat exchanger tubing is unacceptable. Plastic heat exchangers can be used at temperatures up to 300oF (149oC), which is well suited for steam heating of highly corrosive inorganic acids.
It is desirable to fabricate heat exchangers from the most cost-effective material with a relatively high heat transfer coefficient to minimize size and cost. At first glance, this might appear troublesome with regard to plastics. Compared to metals, thermoplastics are thermally nonconductive and always will require more surface area than a metallic unit. This fact necessitates special design considerations to produce a heat exchanger that is not so bulky and costly as to be impractical.
The thermal conductivity for titanium is 90 times that of PVDF, for example. 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 related to the overall heat transfer coefficient (U), which is based on the thermal conductivity and wall thickness of the material from which the heat exchanger is fabricated as well as on surface effects at the inside and outside wall of the heat exchanger tubing.
To calculate the surface area, use this equation:
Q is heat transfer rate
U is overall heat transfer coefficient and
ΔTlm is log mean temperature difference.
The required surface area is inversely related to the overall heat transfer rate (U). 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. Calculating the overall heat transfer coefficient is 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.
To calculate the heat transfer coefficient for a cylinder, use this generalized equation:
H12 is the surface transfer coefficient at the inside surface of the tube
H23 is the surface transfer coefficient at the outside surface of the tube
R2 is the radius of the inside of the cylinder
R3 is the radius of the outside of the cylinder and
k23 is the thermal conductivity of the cylinder material.
Each of the three units in the denominator of the equation represent resistances to heat transfer. In addition, other factors such as fouling and insulation also should be taken into account. The surface heat transfer coefficients are a function of the velocity and physical and thermodynamic properties of the fluids traveling inside and outside of the tube. For example, if the fluid inside the tube is traveling quickly, the inside heat transfer coefficient is increased. Conversely, if the fluid outside of the tube wall has minimal velocity, the heat transfer coefficient of the outside surface is low.
Plastic heat exchangers become more appropriate in applications where there is a poor condition for heat transfer at the inside or outside surface of the heat exchanger tubing. This occurs particularly in immersion-bath-type heat exchangers where the fluid outside the tube is quiescent and in applications where the fluid is flowing at a low velocity or is highly viscous. Immersion heat exchangers have virtually no velocity at the outside tube wall. Therefore, the outside surface heat transfer coefficient is low, which lowers the overall heat transfer rate. This is why a plastic immersion heat exchanger requires only three times the surface of a metallic unit even though the thermal conductivity of the material is 100 times lower. As either condition occurs, the negative effect of the relatively nonthermally conductive plastic material is minimized.
Plastic-Specific Design Considerations
Using plastic heat exchangers only as immersion heat exchangers would severely limit their use in the process industry. To design an effective plastic heat exchanger, it also is necessary to minimize the detrimental effect of the thermally nonconducive tube wall. As the diameter of a tube is increased, the thickness of the tube wall also must be increased to keep the same pressure rating. The key in any plastic heat exchanger design is to use smaller diameter tubes to allow for a relatively thin tube wall. Use of smaller diameter tubing allows for pressure ratings up to 230 psig while still maintaining sufficient heat transfer to minimize heat exchanger size. This is why nearly all plastic heat exchangers are constructed from smaller diameter tubing -- generally not more than 0.5". If an engineer designed a traditional shell-and-tube heat exchanger using standard plastic piping and pipe diameters, the heat transfer would be too poor to make a practical design.The breadth of thermoplastic materials available allows great flexibility when manufacturing plastic heat exchangers. Likewise, many different techniques can be used to fabricate plastic heat exchangers. Plastics can be heat fused and injection molded into heat exchanger components in ways that are not possible with metals. For plastic heat exchanger manufacturing, the critical point is welding the small tubing to a larger mass such as a tubesheet or header, to direct the fluid flow into the multiple tubes. In welding the small diameter tubing to the tubesheet, the tube wall can collapse before an acceptable weld can be made. Many plastic heat exchanger designs utilize a compression-type of mechanical joint at the end of the tubing. This is laborious and may be subject to the thermoplastic "creeping" over time. One plastic heat exchanger design utilizes an injection overmolding. In this manufacturing process, the header of the heat exchanger is injection molded around the tubes so that the tubing is actually part of the header or tubesheet.
New heat exchanger designs and plastic welding techniques are constantly being developed that will further enhance the versatility of plastic heat exchangers in processing applications.
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