Finding the Best Heat Exchanger
Why heat exchangers? The correct answer to this question is, of course, the need for thermal processing of various products, be they within the food, dairy, chemical or pharmaceutical industries.
In today's marketplace, a wide range heat exchangers styles are available, and on applications that are referred to as "standards," one of the first problems that occurs is that it is possible to find many different types of heat exchangers, all performing the same duties. Is this right or wrong? To come to a conclusion, one must analyze the reasons why one heat exchanger is chosen over another and if the choice is correct based upon available information.
Consider a hypothetical application where several types of heat exchangers could be utilized. Let's select a plate-, spiral- and tubular-type design, and let's forget everything regarding product specifics, concentrating on why choices were made:
- The first customer chose a plate heat exchanger because the price was lower.
- The second customer was prejdiced against designs differing from coventional tubular designs and felt comfortable with his or her selection.
- The third customer chose a spiral-type heat exchanger because he or she was afraid of possible gasket problems. Furthermore, he or she could not guarantee the maximum temperature limit of his or her steam, as could the first customer.
So with a single application, you can see three different approaches taken, where all seem to perform equally as well and all of the clients are pleased.
Distinguishing ExchangersTo match the best heat exchanger to the specific application, a processor must be able to choose from the many different types of heat exchanger equipment. To make the correct choice, the criteria that one must look for are both operating temperatures and pressures, the product's fluid properties, space, maintenance, multiproduct usage on the same equipment and, of course, price.
Most processors have several different styles of heat exchangers, which can include kettles, plates, tubulars, spirals and scraped surface heat exchangers. Within these processing plants, the use of this equipment is many and varied. They can be used to pasteurize, sterilize, preheat, cool, deep cool, crystallize, slush freeze, temper, gel or polymerize. Let's take a closer look at each.
Kettle. This is simply a tank with an outer jacket designed to contain heating or cooling media. Product is heated or cooled while being mixed, blended or agitated. However, kettles are neither thermally efficient nor continuous in operation.
Plate Heat Exchangers. These units 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 pass 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. These 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, it also can 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.
Shell and Tube Heat Exchanger. This consists of a bundle of parallel tubes with the ends expanded in tube sheets. The bundle is contained in a cylindrical shell. Connections are such that the tubes can contain either the product or the media, depending upon the application.
Scraped Surface Heat Exchangers. These consist of a tube or cylinder that has an inner tube (the heat exchange surface) and an outside tube. Between the two is the annular space, where the media flows counter-current to the product. Inside the inner cylinder, a rotating bladed shaft is positioned concentrically to continually agitate and remove the product from the heat exchange wall areas.
Reasons for SelectionThe No. 1 consideration when selecting a heat exchanger is price, but a processor should never let the dollars for the initial capital investment be the sole consideration -- that is, ignoring his or her long-range goals and requirements. The cost per square foot of heat exchange area can range from a low of $25 to as high as $2,500 (table 1).
It is hoped that a processor knows where he or she wishes to be a year from now. If he or she does, than the processor should know current and future type and/or phase of product (if it is a sensitive or complex product and materials of construction required. Both are critical to the overall operating efficiencies and success of his or her operation. Other important considerations are:
- Will the processor eventually need to process a product with acid-like pHs?
- Does the processor intend to use higher solid-type products?
- Will any of the intended product mix exhibit chemical changes during processing?
- What is the true product mix (liquid/liquid; particulates; or still pumpable liquid with high solids)?
Although the application guidelines below are a good place to start, don't consider any of the data shown as firm for two reasons: First, the various equipment manufacturers are always trying to improve upon their products. Second, depending upon how an individual product reacts under thermal processing conditions, it may be able to, or necessitate, being processed in an alternate type of heat exchanger than originally first thought.
Design limitations can be given, but they are only guidelines because the key here is the product. How does it react, what is its color or general condition, is it starch based, has it bloomed properly -- these can all be quickly ascertained once thermal processing is completed. Although one type of exchanger may be quite capable of handling the product, it may not be as good as if it had been done in another type.
The trade-off on current processing needs vs. future needs, many of which are unknown, can be confusing. The various heat transfer duties needed in the typical processing plant are too numerous to mention in this article, but one of the biggest considerations is the processors' ability to run trouble free for the longest possible time in full production state. This is the criteria that establishes the real bottom line that most processors must consider when contemplating various heat exchangers for their thermal processing requirements.
Therefore, this important decision of how to maximize their plant and their investment in filling equipment also is key to selecting heat exchanger equipment. So a processor must make sure that both types of equipment can work in conjunction with each other, operate for extended periods of time on his or her full range of products, consider that the type of equipment selected can be easily inspected and maintained and, in the latter, must be able to be accomplished with a minimum amount of downtime to the entire process.
Sometimes, many applications require combinations of heat exchangers to properly produce the type of end product needed by the processor. This might involve a kettle for preheating, a plate for doing the "carrier" liquid and a scraped surface heat exchanger doing the final heating or the final chilling/deep cooling.
It is hoped that from this short synopsis, you will remember, if nothing else, that the proper selection of a heat exchanger is difficult, and becomes dependent upon both the products and the applications to be handled, either currently or in the future. Once these are answered correctly, a processor will know which type of heat exchanger will be ultimately selected for his or her process requirements.
SIDEBAR: Kettle: Open tank for heating/cooling for a full range of products, but on a batch production basis.
Plate Heat Exchanger (liquid/liquid): Liquids with less than 5% of nonabrasive solids, generally micron size range. Viscosities less than 20,000 cPs.
Special Tubular Heat Exchanger (liquid/liquid; steam or gas/liquid): Liquids with high solids (30 to 40% with maximum size range of particulates at 0.125 to 0.5" in dia. if pressure conditions allow. High viscosities.
Spiral Heat Exchanger (liquid/liquid; steam or gas/liquid; gas/gas): Liquids with low solids (5 to 10%), in suspension, to 0.5" in dia. in special cases.
Shell and Tube Heat Exchanger (liquid/liquid; steam or gas liquid): Restricted to solutions with low solids contents and viscosities under 10,000 cPs.
Scraped Surface Heat Exchanger (liquid/liquid; liquid/steam or gas): For high viscosity (up to 100,000 cPs) and heavy fouling and crystallizing applications. Solids contents greater than 75% to normal and particulates up to 1" maximum. For complex or sensitive products that require gentile processing.
Temperature (oF): >500
Pressure (psig): 50 to 75
Flow Rate (Nominal/Maximum): N/A
Plate Heat Exchanger
Temperature (oF): 300 to 450*
Pressure (psig): 150 to 300
Flow Rate (Nominal/Maximum): >500 gal/min
Special Tubular Heat Exchanger
Temperature (oF): >1,000
Pressure (psig): >1,000
Flow Rate (Nominal/Maximum): No Limit
Shell and Tube Heat Exchanger
Temperature (oF): >1,000
Pressure (psig): >1,500
Flow Rate (Nominal/Maximum): No Limit
Scraped Surface Heat Exchanger
Temperature (oF): >450
Pressure (psig): 500
Flow Rate (Nominal/Maximum): 100 gal/min
*Depending upon gasket material
SIDEBAR 3:The material to be processed determines the best heat exchanger configuration.
Recommendations Based on Materials to be Processed
Nonviscous-to-Nonviscous Liquid (e.g., wine cooler)
Plate exchanger usually requires the smallest surface and is suitable for fabrication in various metallurgies with conventional gaskets. For high temperature liquids, a plate exchanger with special gaskets. A spiral can also be utilized but may not meet the sanitary requirements. Special tubular is appropriate but expensive. For high volumetric flow rates, pressures or temperatures, shell and tube type may be used, particularly if carbon steel is suitable as a material of construction.
Nonviscous Liquid to Steam (e.g., sugar solution)
Plate exchanger has high heat-transfer rate and is especially applicable with steam temperatures less than 270oF (132oC) with standard elastomers. Shell and tube 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 Liquid to Water or Steam (e.g., corn syrup heater)
Depending upon viscosity limit, a scraped surface heat exchanger, special tubular or plate heat exchangers are applicable. If the requirement is nonsanitary, a shell and tube or spiral may be used.
Viscous to Viscous Liquid (e.g., oil/oil cooler)
Plate exchanger is the most efficient due to turbulent flow on both sides. High heat-transfer coefficient and high turbulence due to even flow distribution are important. However, a plate heat exchanger regenerator is restricted to low viscosities. With high viscosities, a special tubular may be required. Spiral type can be used for liquids of not-too-high viscosity because of good flow distribution, when the flow is turbulent in the two single passages. Pressure drop must be sufficiently high to yield a velocity to create turbulence. Reynolds number should be greater than 1,000.
Heat-Sensitive Liquids (e.g., protein solution heater)
Temperature and holdup time are 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 hold-up times. Strict temperature control is essential. Wall temperature and fouling considerations may be very important with heat sensitive or corrosive liquids. A scraped surface heat exchanger with large diameter rotor shaft becomes the only solution when viscosities and/or solids restrict use of other types.
Vapor Condensation(e.g., steam condenser)
If stainless steel or high-alloy material must be used, a spiral type is frequently the best solution. If extensive and frequent manual cleaning is necessary, a plate exchanger, possibly a box condenser, may be utilized. Shell and tube condenser is applicable if carbon steel can be used throughout, or at least for the shell.
Cooling Water (e.g., using seawater as 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 (e.g., titanium, aluminum bronze).
High-Temperature Application (e.g., vegetable oil heater)
Usually custom-made due to the importance for high-thermal stresses A plate heat exchanger with special compressed gasketing material can be used for some limited applications. Also, shell and tube and spiral exchangers are suitable.
High Viscosity, Fouling or Crystallizing (e.g., peanut butter, sauces, gravy)
Scraped surface heat exchangers are the only solution due to scraping of the heat transfer wall by the scraping blades.