Heat Exchanger Basics
Heat Exchanger TypesHeat exchanger types include kettles, plates, tubulars, spirals and scraped surface units.
Kettle heat exchangers are simply tanks with an outer jacket designed to contain heating or cooling media. The product is heated or cooled while being mixed, blended or agitated. However, kettles are neither thermally efficient nor continuous in operation, and product moisture content is often lost during processing.
Gasketed plate heat exchangers consist of a number of corrugated metal sheets or heat transfer plates clamped together in a frame. The adjoining plates are spaced by gaskets, which form a narrow, uninterrupted space through which liquid flows. The fluids are separated by the gaskets and flow through alternate channels (passes). By arranging these channels in groups, and by further including intermediate separating/connecting plates, several fluid streams can be accommodated at once.
Special tubular heat exchangers can be of several different designs. Double- or triple-pipe units consist of two or three concentrically mounted tubes. The heating or cooling medium flows through the inner tube; on a triple-tube arrangement, the medium can also pass through the annular space between the intermediate and outer tubes. Product travels in the opposite direction through the annulus between the two inner tubes or through the inside tube.
The cost per square foot of heat exchange area can range from a low of $25 for basic plate types to as high as $1,400 for some scraped surface units (table 1). However, as with any equipment purchase, the initial capital investment is often far less important than the system's ability to meet the plant's goals and requirements.
Evaluating the ApplicationAlthough price is often the first criterion evaluated by purchasing managers, other important selection criteria include:
Table 2 outlines some basic application guidelines for the different heat exchanger types, and table 3 lists some design limitations. However, keep in mind that new equipment is continuously being developed to handle increasingly complex applications. Also, some products might require a specific type of heat exchanger to avoid adverse reactions during thermal processing.
Ideally, the product should be tested in the unit before the purchase is made. Questions to ask include: How does the product react and taste? Did its color change? What is the general condition of the product? If the product is starch-based, has it bloomed properly? These characteristics all can be quickly ascertained once thermal processing is completed. Although one type of exchanger might be quite capable of handling the product, it might not perform as well or produce the same level of quality as another type in the same application.
It is important to consider both near-term and longer-range goals when evaluating the different heat exchanger types. For example, will the plant eventually need to process a product with an acid-like pH? Will it ever handle higher-solids type products? Will any of the intended products exhibit chemical changes during processing?
In many cases, a plant's future needs are unknown. However, any heat exchanger selected should run trouble-free at full production levels for the longest possible time. A heat exchanger also should work well in conjunction with other equipment in the plant, and it should be capable of being inspected and maintained with a minimum amount of downtime to the entire process (table 4).
Making the Right SelectionMany applications require combinations of heat exchangers to properly produce the end product. This might involve a kettle for preheating, a plate for handling the “carrier” liquid and a scraped surface heat exchanger for the final chilling/deep cooling. By understanding the different types of heat exchangers and the requirements of the application, facilities at which process cooling is critical can select the right equipment for their application and optimize their investment.
Sidebar:Nonviscous-to-Nonviscous Liquids (e.g., wine coolers) -- For high-temperature liquids, a plate exchanger with special gaskets or a spiral exchanger can be used, but these types might not meet the sanitary requirements of the application. A special tubular heat exchanger is appropriate but expensive. For high volumetric flow rates, pressures or temperatures, a shell-and-tube type can be used, particularly if carbon steel is suitable as a material of construction.
Nonviscous Liquids to Steam (e.g., sugar solutions) -- A plate exchanger has a high heat transfer rate and is especially applicable with steam temperatures less than 270oF (132oC) with standard elastomers. A shell-and-tube type applies if it can be made in carbon steel or copper alloy. If high-pressure steam is used, a spiral or shell-and-tube heat exchanger is adequate.
Viscous Liquids to Water or Steam (e.g., a corn syrup heater) -- Depending on the viscosity limit, a scraped-surface heat exchanger, special tubular or plate heat exchanger is applicable. If the requirement is non-sanitary, a shell-and-tube or spiral can be used.
Viscous-to-Viscous Liquids (e.g., an oil/oil cooler) -- A high heat transfer coefficient and high turbulence due to even flow distribution are important criteria. A plate exchanger is the most efficient due to the turbulent flow it provides on both sides. However, plate heat exchanger regenerators are restricted to low viscosities. With high viscosities, a special tubular exchanger might be required. A spiral type can be used for liquids with low or medium viscosities because it provides good flow distribution for turbulent flows in the two single passages. However, the pressure drop must be sufficiently high to yield a velocity that creates turbulence. (The Reynolds number should be greater than 1,000.)
Heat-Sensitive Liquids (e.g., a protein solution heater) -- The temperature and holdup time are the deciding factors; thus, small channel volumes, high heat-transfer coefficients and even flow distribution are important. Plate exchangers fulfill these requirements best. Spiral type and special tubular styles have the longest holdup times; however, strict temperature control is essential. Wall temperature and fouling considerations might be important with heat-sensitive or corrosive liquids. A scraped-surface heat exchanger with large diameter rotor shaft becomes the only solution when viscosities or solids prohibit the use of other types.
Vapor Condensation (e.g., a steam condenser) -- If a stainless steel or high-alloy material must be used, a spiral type exchanger is often the best solution. If extensive and frequent manual cleaning is necessary, a plate exchanger (possibly a box condenser) can be used. A shell-and-tube exchanger is applicable if carbon steel can be used throughout, or at least for the shell.
Cooling Water (e.g., using seawater as a cooling medium) -- Cooling water or seawater circulating in a heat exchanger as the cooling medium can best be handled by either plate or shell-and-tube heat exchangers if they are fabricated out of specialized alloys such as titanium, aluminum or bronze.
High-Temperature Applications (e.g., a vegetable oil heater) -- High-temperature applications usually require custom-made heat exchangers because allowance for high thermal stresses is extremely important. A plate heat exchanger with a special compressed gasketing material can be used for some limited applications. Shell-and-tube and spiral exchangers also are suitable.
High-Viscosity, Fouling or Crystallizing Applications (e.g., peanut butter, sauces, starches, gravies) -- Scraped-surface heat exchangers are the only solution because the heat transfer wall must be continually scraped clean.