A Practical Guide to Fluoropolymer Heat Exchangers
In addition to chemical resistance, fluoropolymer exchangers are versatile. A single heat exchanger design can be used with many different chemicals without fear of contamination. Since their debut as shell-and-tube models, fluoropolymer heat exchangers have found their way into other configurations, primarily reactor and immersion coils.
When choosing a heat exchanger, a general rule of thumb is if the chemical makeup of a given system permits the use of stainless steel or copper, those materials tend to be the most economical. However, if the system requires a more exotic metal material such as titanium, tantalum or Inconel, it is worthwhile to consider fluoropolymer units. Although the cost of a fluoropolymer exchanger is similar to those made of glass and graphite, the maintenance required for glass and graphite units usually is more than the original unit cost.
Service life for a fluoropolymer heat exchanger is long. Units usually require a smaller spare-parts inventory than metal exchangers, and damaged tubing can be repaired or removed without significantly affecting performance. Unlike exotic metal tubes, fluoropolymer components do not need to be sent to a special welding shop for repair, which can take several weeks. Depending on the type of exchanger, simple repairs can be handled by plant maintenance crews.
Some typical process applications for fluoropolymer exchangers include:
- Simple heating and cooling to satisfy process requirements or storage conditions.
- Condensing for material recovery or reactor refluxing.
- Vaporization for process separation or purification.
- Crystallizing for salt production or waste recovery.
- Absorbing in a cooler unit for recovery processes.
Because the unit's tubing is extruded fluoropolymer resin, there is a strong correlation between pressure capability and operating temperature (figure 1). Generally, a fluoropolymer unit is designed to operate at pressures up to 40 psig and temperatures up to 300oF (149oC); however, it can operate above those limits if it is manufactured of proprietary materials.
One obstacle to the use of fluoropolymer exchangers is the general assumption that fluoropolymers are inherently poor conductors of heat and thus unlikely candidates for use as heat exchangers. Depending on the size and design of the unit, however, the thermal performance of fluoropolymer exchangers can virtually match the performance of metals. In addition, the use of certain resins effectively can double the unit's heat transfer capabilities.
Shell-and-Tube ExchangersUnlike conventional shell-and-tube exchangers, which are gasketed or welded together, fluoropolymer models consist of many small-diameter tubes. The flexible fluoropolymer bundles are fused together into a single header or tube sheet that facilitates its connection to standard piping assemblies. The tube bundle is removable for maintenance. The most commonly used shells are made of carbon steel, but other materials are available.
Fluoropolymer shell-and-tube ex-changers range in size from large units with more than 1,000 ft2 heat transfer area to smaller units with 5 ft2 of area. They are single pass, usually with countercurrent flow. Typically, process fluid flows through the tubing with service media in the shell. Fluoropolymer shell-and-tube units have been used successfully as dilution coolers for sulfuric acid, bleach coolers and cooler absorbers for recovery of hydrochloric acid.
Reactor and Immersion CoilsReactor coils are straight bundles of tubing used as tank heaters or as coolers for crystallizer operations. They can be made to order for a given length. End fittings typically are either fluoropolymer or stainless steel, and special fittings are supplied for mounting the coils horizontally or vertically through tank nozzles. When used as coolers for crystallization processes, the coils are located vertically around the periphery of an agitated vessel. As the crystal forms, the movement of the tubing, combined with the nonstick quality of the fluoropolymer resin, prevents crystal from sticking to the tube.
Fluoropolymer immersion coils are used in metal finishing and chemical processing, where the resins' inherent characteristics are especially attractive. Those characteristics include corrosion resistance, long service life, minimum maintenance, high thermal efficiency and durability under heat and pressure.
Immersion coils are recommended for control of tank temperatures in chemical processes and pickling operations. Coils containing up to 500 tubes can be configured in a rigid U-shape or a straight cross-tank design to fit most tank sizes. Suspending the coil from the top of a sealed process tank simulates the effect of a shell-and-tube exchanger.
Another suitable application for immersion coils involves metal-finishing baths where heating and cooling can be accomplished using relatively small coils. Here, the coils usually are not confined in a rigid frame, providing greater flexibility for installation.
Shell-and-tube and immersion coil heat exchangers also are available in ultrahigh purity (UHP) fluoropolymer resins. These corrosion-resistant units have low extractables -- materials that could add contaminants or detract from the effectiveness of the fluids being heated or cooled. UHP fluoro-polymer heat exchangers primarily are used to heat or cool fluids used by the semiconductor and food industries. Additionally, shell-and-tube configurations with nonmetallic shells are available to eliminate any metal presence in proximity to fluid-handling processes.
Installation, Maintenance and TroubleshootingThe successful performance and length of service of a fluoropolymer exchanger depends on proper installation, operation and maintenance. Immersion heat exchangers typically should be located in a tank where they will not come in contact with the work being processed. In most cases, the coil should be mounted about 12" (30.5 cm) from the tank bottom to reduce the possibility of temperature stratification and to keep the coils clear of any sludge buildup.
Stainless steel end connections should not be immersed in the liquid or mounted in front of a fume exhaust as they are susceptible to chemical attack by liquid or vapor. Coils with fluoropolymer end hardware can be completely submerged in most chemicals.
Solids buildup around the coil will inhibit heat transfer and can cause mechanical damage to the unit. Although fluoropolymer resins have nonstick properties, solids still can buildup around the coil over time. A regular preventive maintenance program that includes a solids buildup inspection is recommended. For applications susceptible to solids buildup, a regularly scheduled light acid/water coil soak can remove the buildup.
Certain steps should be taken to ensure safe and proper operation of fluoropolymer heat exchangers. Before operating the unit, users should make certain the entire system is clean. Tubes and nozzle connections can be blinded or plugged by refuse left in the pipe during installation. All piping should be flushed with water before connecting the unit.
During startup, all discharge valves first should be opened to prevent excess pressure buildup on the coils. After the discharges are open, the supply-side valves should be opened slowly to ensure a safe startup. Excessive pressure buildup at the coil inlet may damage the tube sheet. Any hammer impact on the inlet may distort the tube sheet and cause irreparable damage to the coil.
If the coil does not provide the proper performance, check the following:
- Filters or strainers for debris. Clean as required.
- Steam pressure, temperature and flow rate at the exchanger inlet vs. design specifications.
- Cooling water flow and temperature vs. design conditions.
- Temperature controllers for proper functioning.
- Steam trap for proper size and operation.
- Back pressure in condensate or water return lines for obstructions.
- Nozzle for accumulated debris.Clean if necessary.
Properly installed and maintained, a fluoropolymer heat exchanger can provide years of corrosion-free service with minimum maintenance in most heating and cooling applications.