When selecting a heat exchanger to solve heating or cooling process needs, there are many options and variables to consider. Determining the heat exchanger style and material that are optimal for an application should be the first and most important step. In the course of this process, the fluid, thermal performance, temperature and pressure limits, pressure drop, fluid flow capacity, maintenance issues and expansion plans must all be considered.
Style Selection.Before looking into the proper material to select for your heat exchanger, you must determine what heat exchanger style will best fit the processing needs. The two types of heat exchangers available are internal/immersion and external.
Common immersion-style heat exchangers include serpentine coils, grid coils, pipe coils, plate-style coils and electric heaters. If space is available in the process tank, these exchangers are the simplest and most economical choice. Most immersion heat exchanger manufacturers design the heat exchangers specifically for customer applications.
External heat exchangers usually are chosen when in-tank space is limited. These units require one pump to circulate the heating or cooling media and another pump to recirculate the process solution. Common external-style heat exchangers include shell-and-tube style, tube-plate, and plate-and-frame heat exchangers.
Material Choice.Once a suitable heat exchanger type is selected, the best material and required surface area must be determined. In order to select the appropriate heat exchanger for a process heating application, the following selection criteria must be evaluated:
- Process fluid characteristics.
- Desired thermal performance.
- Pressure and temperature limits.
- Pressure drops across heat exchanger.
- Fluid flow capacity.
- Maintenance and repair considerations.
- Ability and ease of future expansion.
Process Fluid.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 chromic, chromic acid with fluorides, and plating applications are known to cause titanium immersion coils to fall apart. However, polyvinylidene fluoride (PVDF) is an excellent material for applications such as these, and PVDF coils will perform well in them for years. PVDF also is a good material for hydrochloric acid and sulfuric acid pickling, sulfuric anodizing and nitric acid applications. Likewise, each material type is more or less suited to specific materials and applications. For instance, stainless steel is a good material for alkaline cleaning and caustic applications.
Thermal Performance.A heat exchanger must be designed to take advantage of the physical properties of the materials of construction, but at the same time, minimize any insulating effects of the materials. The thermal conductivity of the heat exchanger material plays a major role in determining the thermal performance of the heat exchanger.
For instance, the low thermal conductivity of plastic means that heat exchangers constructed of plastics are almost universally designed with a limited range of wall thicknesses. For practical purposes, to fabricate any plastic heat exchanger, it is necessary to use smaller diameter tubing to allow for an acceptable pressure rating and counteract the negative effect of the material, which has a low thermal conductivity. This requires a design approach that is different from traditional metal heat exchanger design where, due to the relatively high thermal conductivity of most metals, the material thickness is not as major of a factor in heat exchanger design. Table 2 displays the thermal conductivity values of materials commonly used for heat exchanger construction.
Pressure and Temperature Limits.Each heat exchanger, depending on material and construction, will have its own pressure and temperature limits. Should you favor a plastic heat exchanger in terms of materials of construction, remember that the general design criteria to consider when specifying a plastic heat exchanger are no different than that of a metallic unit. However, due to the specific limitations of pressure and temperature, it is imperative that when using a plastic heat exchanger, the proper thermoplastic material be chosen for a given application. The temperature/pressure range of thermoplastics is wide. For example, PVDF is rated for 230 psig at 68°F (20°C) and derates to 35 psig at 281°F (138°C). In applications where the temperatures and pressures required are greater than the above, metal heat exchangers would need to be considered.
Pressure Drops.The pressure drop through a heat exchanger is a function of the heating or cooling media flow rate, surface smoothness and fouling characteristics. Fouling reduces the cross-sectional area for heat to be transferred and causes an increase in the resistance to heat transfer across the heat exchanger. Heat exchangers with smooth tubes are less likely to have fouling issues. However, process solutions with particulates can abrade the tube surfaces, causing them to become rough and increasing pressure drop.
One method used to determine the abrasion resistance of materials is the Taber CS17 abrasion test. In this test, an abrasive wheel, weighing one kilogram, is cycled over the face of a solid plate of the material being tested, and the resultant weight loss of the plate is measured after 1,000 cycles. Table 3 displays the abrasion resistance of common heat exchanger materials.
Many thermoplastics are extremely abrasion resistant. For instance, PVDF is five to 10 times more abrasion resistant than stainless steel and 10 to 30 times more abrasion resistant than carbon steel. Polypropylene (PP) and polyethylene (PE) are approximately twice as abrasion resistant as stainless steel.
Fluid Flow Capacity.The fluid flow capacity of a heat exchanger is dependent on exchanger material type and thickness, operating pressure and cross-sectional area available for fluid flow. Heat exchangers with small fluid flow areas, used to increase surface area, will have a lower fluid flow capacity. Multiple heat exchangers operating in parallel may be required to meet the fluid flow demands.
Maintenance and Repair.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.
Ability and Ease for Future Expansion.The ability to expand the surface area of the heat exchanger may be important if it is anticipated that the heating or cooling needs of the process are going to change over time. Several types of heat exchangers lend themselves very well to expansion. Some grid-style immersion heat exchangers are made with modular panels; if more surface area is necessary, additional panels can be added to the existing unit. Likewise, additional plates can be added to most plate-and-frame style heat exchangers to increase the surface area.
Many manufacturers provide data sheets with the important information required to assist you in the heat exchanger selection and design process. At the same time, understanding the options available and required information upfront will lead to a smoother and more efficient heat exchanger design process.
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