Why would thermoplastics —rather poor conductors, or good insulators, depending on your perspective — even be considered a feasible material for heat exchangers? The answer becomes apparent when you realize that their design features minimize the insulating effect of the materials of construction. These two advantages make thermoplastic heat exchangers worth considering for many corrosive metal processing applications as well as high purity applications for pharmaceutical and other industries.
Thermoplastic heat exchangers commonly are manufactured from polyethylene, polyvinylidene fluoride (PVDF) and several PFA derivatives, all of which offer excellent chemical resistance. Because the internal tubing is smooth compared to most metals, plastic heat exchangers are advantageous in terms of low fouling characteristics during operation. They are suited for plating/metal processing, pharmaceutical, food and beverage, energy and power, and high-purity applications.
Serviceability Considerations for Thermoplastic Exchangers
For applications where fouling of the heat exchanger surface is an issue, the ability to clean and maintain the heat exchanger at optimal performance will be part of the selection process. Common methods to remove scale or solution buildup are to chemically clean or use a pressurized water jet. Because only the outside of the exchanger is exposed to the chemicals, immersion heat exchangers may be easier to maintain than external exchangers. External exchangers must be flushed with a chemical solution to remove buildup.
The chemical inertness of plastic heat exchangers makes them a suitable choice for use in process heating and cooling applications where system purity is an issue. Additionally, plastic heat exchangers are non-conductive and will not develop stray currents, which can be critical in specific applications such as chemical processes where combustibility is a significant concern. For applications where acids and solvents are used such as metal etch and pre-process cleaning/treating, these chemicals easily cause corrosion buildup in metal. They can eat through the pipe, causing leaks that can contaminate a chemical batch and, at some point, require metal coil replacement. Thermoplastics have excellent chemical and corrosion resistance and will not rust, pit, scale or corrode. Acids, bases, solvents and many other corrosive waste mixtures that are destructive to most metals have little to no effect on the materials used to produce thermoplastic heat exchangers.
Design Criteria for Thermoplastic Exchangers
The general design criteria when specifying a plastic heat exchanger are no different than those for a metallic unit. However, they must be designed to take advantage of the excellent physical properties of thermoplastics while at the same time minimize their insulating effect. Due to the specific limitation of pressure and temperature, it is imperative that the correct thermoplastic material be chosen. The temperature/pressure range of thermoplastics is wide. For example, PVDF is rated up to 230 psig at 68°F (20°C) and de-rates down to 35 psig at 280°F (138°C). This de-rating is a result of keeping the same tube wall diameter and thickness of the plastic heat exchangers to minimize the insulating effect. The key to any plastic heat exchanger design is to use smaller diameter tubes to allow for a relatively thin tube wall.
Depending on the process fluid to be used in the application, possible heat exchanger materials can be eliminated right from the beginning of the design process. For example, hard chrome, chromic acid with fluorides and acid-etch applications within the chemical plating industry are known to cause titanium immersion coils to fail in a relatively short amount of time. However, polyvinylidene fluoride is an excellent material for applications such as these, and PVDF coils will perform well for many years. PVDF is also a good material for hydrochloric acid and sulfuric acid pickling, sulfuric anodizing and nitric acid applications. While many plastics are limited to water or water/glycol for heating and cooling media, PVDF allows for the use (up to 30 psi of saturated steam) for heating. And, unlike metal heat exchangers, plastic heat exchangers are not etched or degraded by similar chemical applications.
Sizing Considerations for Thermoplastic Exchangers
The surface area of the heat exchanger is inversely related to the overall heat transfer coefficient, U, which is affected by the thermal conductivity and wall thickness of the material from which the heat exchanger is fabricated. 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 U. Other performance parameters like surface effects also influence the overall U, but they apply to either material selection.
For example, the thermal conductivity for titanium is 90 times that of PVDF, showing that the PVDF conducts heat rather poorly. However, when comparing the actual required surface area for a titanium immersion heat exchanger, the PVDF unit will only require approximately three times the surface area. Using the example of immersion heat exchangers, it is also important to note that even though plastic heat exchangers would require more surface area than titanium units, they both can occupy a comparable volume in the tank. Such space efficiency is due to the dense tube bundles utilized in plastic heat exchangers, compacting a large heat transfer surface area within a confined volume.
Process Heating and Cooling Applications
In the chemical industry, tube plate heat exchangers are used often and are installed externally for heat transfer between low-viscosity fluids. Typically, clean to moderately polluted media are run through the heat exchanger and can be used as a heater/cooler as well as a condenser for vapors.
Another important chemical application is the cooling of sulfuric acid. When sulfuric acid is diluted with water, the exothermic reaction produces heat. This heat must be removed to maintain the proper process temperature. The combination of a strong acid and excellent chemical resistance makes the use of a thermoplastic heat exchanger a practical solution. Additionally, the tube plate style heat exchanger makes it possible to cool the diluted sulfuric acid while at the same time recovering the resulting heat (as reflected by the higher outlet temperature of the cooling solution) for use in other processes.
In many applications in metal plating/coating and chemical production facilities, hazardous fumes are created as by-products of the manufacturing process. The dangerous chemicals that are contained within the fumes cannot be freely vented into the atmosphere but instead must be extracted from the drawn-off air prior to release into the atmosphere. For applications such as these, a thermoplastic gas/liquid heat exchanger is a suitable and environmentally friendly choice. Economically, thermoplastic gas/liquid heat exchangers, when compared to metal, can have less maintenance, which can result in a lower operating cost than a fiberglass unit of comparable volume. Additionally, similar to the tube and plate in liquid applications, the gas/liquid heat exchanger is well suited for extracting heat from the exhausted gases for use in other process heating applications.
New thermoplastic heat exchanger designs and welding techniques are constantly being developed that will further enhance the versatility of their use in process cooling and heating applications. This versatility provides a broad opportunity to replace metal heat exchangers with suitable plastic heat exchangers in current applications. Additionally, future design improvements will further widen the opportunity for users to benefit from the advantages that thermoplastics offer over exotic metals.