Making the Most of Heat Exchangers

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Different types of applications require different types of heat exchangers. How do you figure out which type of heat exchanger is best? How do you get the most out of the heat exchanger you already have?

First, you might want to consider the effect of fouling or deposition of dirt on heat-transfer surfaces. The "Process Heat Transfer and Multiphase Operations Group" page,

www.cpe.surrey.ac.uk/dptri/hms/fouling.htm, from the Chemical & Process Engineering Department of the University of Surrey, England, details ways to reduce or control fouling through proper heat exchanger design and selection. The page includes a table that compares the performance of six types of heat exchangers, including two of the most common types ," shell-and-tube and plate-and-frame. Four specialized types ," double pipe, cartridge, spiral, and scraped surface ," also are compared. A link to novel low-fouling heat-transfer surfaces is available. Methods of preparing low-fouling metal surfaces by using modern surface modification techniques are discussed.

Second, you might want to read the operating manual and/or start-up manual. Many such manuals are available online. For example, technical information manuals about SEC heat exchangers and energy transfer stations are available in portable document format (PDF) at www.heatexchangers.ca/manuals.html.

Shell-and-tube heat exchangers are probably the most commonly used heat exchangers in the process industries. Substantial information about shell-and-tube heat exchanger design and construction is available from the Wolverine Engineering Data Book II at www.wlv.com/products/databook/databook.pdf. The document is a complete table of contents with links to a 305-page online manual that covers everything from basic heat-transfer considerations through condensation to application examples of extended-surface heat exchanger tubes.

Plate-and-frame heat exchangers make close approach and temperature cross-applications economically feasible. Specific information about their operation is available in the SEC Heat Exchangers Commissioning Manual,

www.secplateandframe.com/manuals/SEC-PHE-Commissioning-Manual.pdf and the Polaris Plate Heat Exchangers Operation and Maintenance Manual, www.polarisphe.com/pdfiles/OM.pdf. Plate-and-frame heat exchangers have a high turbulence and a thin plate material and are easy to clean and maintain with minimum downtime. However, their use often is limited by operating pressure, pressure drop considerations and the presence of large amounts of suspended solids in the process fluids.

Information about the double-pipe or double-concentric-tube heat exchanger is provided at www.fda.gov/ora/inspect_ref/itg/itg34.html. This heat exchanger could be considered a simple shell-and-tube type with a single tube in the shell. It is most useful in applications in which a small ," less than 100 sq. ft. ," heat-transfer surface is sufficient and prevention of intermingling of process fluids is critical.

The spiral heat exchanger also resembles a shell-and-tube type in a number of respects. However, the spiral is more compact (has a smaller footprint for the same heat-transfer area) and is said to be easier to clean and maintain. It provides more efficient heat transfer in some difficult applications. Two examples to consider include Sentry spiral tube heat exchangers (www.sentry-equip.com/PDF%20files/Spiral.pdf) and Graham Heliflow heat exchangers (www.graham-mfg.com/heliflow.html).

Cartridge heat exchanger examples include the Hydrohelix (www.hydrothermal.com/chwork.htm) and the Firerod (www.watlow.com/literature/specsheets/files/heaters/1523_1100.pdf) models. Many are designed for special applications in which a common type of heat exchanger will not work.

Scraped-surface heat exchangers provide an alternative for heat transfer when viscous materials or heat-sensitive materials are processed. Rotating wiper blades keep the heat-transfer surface clean and minimize product overheating or baking. The scraped-surface heat exchanger also can be used for cooling a viscous material. The "Waukesha Cherry-Burrell Products: Scraped Surface Heat Exchangers" page (www.gowcb.com/products/heatex/ssheatx.htm) includes links to Votator II horizontal and vertical scraped-surface heat exchanger specifications and operating manuals. These pages thoroughly describe the design and operation of scraped-surface heat exchangers.

The chemical engineering (ChE) links collection from Dr. Bernhard Spang (www.cheresources.com/chelinks.shtml) now are available as part of "The Chemical Engineers' Resource Page," www.cheresources.com. One of Spang's articles about the mean overall heat-transfer coefficient is "U in Heat Exchangers," www.cheresources.com/uexchangers.shtml. Additional heat-transfer coefficients for refining applications,

www.processassociates.com/process/heat/uvalues1.htm, might also be of interest.

Although not suitable for design purposes, the "Overall Heat Transfer Coefficient" calculation page at www.processassociates.com/process/heat/u_calc.htm can be useful for estimating purposes, as can other heat-transfer resources found under Process Associates of America's "Process Tools," www.processassociates.com/process/tools.htm#heat.

Hodel is

Chemical Processing's Internet columnist. Contact him at aehodel@netscape.net.

 

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