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Mechanical Design of Heat Exchangers The many configurations and types of heat exchangers necessary for the variety of fluids and wide range of temperature and pressure encountered in the chemical industry make choice of design a complex problem in economics32493
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Tm WIDE RANGE of applications of heat exchangers in the chemical industry has led to a variety of constructions. Many types have been designed to accommodate the simple fluids, solutions, or slurries which must be cooled, condensed, or boiled. The extremes of temperatures and the pressures involved in these processes have also been considered.
These standards, developed by fabricators,are primarily described by the“Standards of Tubular Exchanger Manufacturing Association.” The fourth edition of this booklet has just been published. A further effort at construction standardization is presently being undertaken by the American
Standards Association, through the efforts of Sectional Committee B-78,“Standardization of Heat Exchangers for Chemical Industry Use.”Standardization reduces first costs,speeds delivery, and permits interchange of parts. In the chemical industry,because of its dynamic technology,
write-off time for process equipment is relatively rapid. First cost is of prime importance, but the first cost of a heat exchanger must be considered,together with many other factors, for the choice of a heat exchanger design is a very complex problem in economics.Important variables in this problem are the cost of outage time or the cost of operating at reduced efficiencies because of fouling, corrosion, leakage, or structural failure. The cost of some modifications can be justified, such as those that permit chemical cleaning or facilitate plugging tubes or replacing surface. Additional costs can be justified when they are necessary to accommodate severe cyclic conditions, or when the fluids involved are lethal.
Special Heat Exchangers
Sometimes these factors lead to exchanger requirements not covered by the standard tubular exchanger, and nonstandard shell-and-tube exchangers come into the picture, as well as miscellaneous special types, involving coiled tubes, plates, extended surface, and unusual construction materials, such as graphite or glass.Basically, heat exchangers must be designed to be structurally sound for their intended service. Usually, pressure part thicknesses will be satisfactory
if they meet the requirements of the ASME Code for Unfired Pressure Vessels.Beyond that, the TEMA Standards are used as a guide. The codes generally call for the use of materials conforming to the specifications of the ASTM. It is also the practice of many heat exchanger users to write their own more or less rigid specifications.Generally speaking, for the low pressure heat exchangers, fabrication requirements and corrosion allowances will be the governing factor in determining tube and shell thicknesses. Accommodating Mechanical and
Chemical Cleaning
An important consideration in most chemical plant applications is the fouling of the heat exchanger surface. Heat exchangers reflect this problem in a variety of construction details. Most
head, channel, and cover plate designs permit ready access to tube ends so that the inside of tubes can be rodded clean. Where cleaning externally is important, the construction may permit removal of the entire tube bundle from the heat exchanger so that access to the outside of the tubes may be gained. While a tube pattern of triangular pitch is the most economical of space and materials, an in-line or square pitch tube pattern results in lanes for mechanical cleaning devices. Where chemical cleaning is possible, vents and drains are used as connections for circulating the solutions in and out.
Reducing Stresses Due to Differential Expansion
In the first heat exchanger drawing (p. 468), a fixed-tube sheet arrangement is shown. This is the simplest and least expensive type of construction; but if the fluids on the tube side and shell side are of significantly different temperatures,so that the tubes and shell are at different temperatures, there will be a differential expansion between tubes and shell that might cause excessive stresses. The result: of these stresses may be fatigue failure, leaking tubes at the tube seats, or perhaps an acceleration of corrosion. To avoid these types of failures, construction modifications are introduced. One example is an expansion joint in the heat exchanger shell. Another common device is the floating tube sheet. While one end of the tube bundle is secured to the shell, the other end is permitted to float with packed seals to prevent leakage. The U-tube arrangement will accommodate differential expansion between tubes and shell and also different rates of expansion between adjacent tubes. ‘Note the flanged construcdon that permits disassembly and removal of the tube bundle. The use of bent tubes between fixed tube sheets is another accepted means of accommodating differential expansions. An additional advantage claimed for this arrangement is the natural shedding of tube scale which accompanies tube flexing during heating and cooling.Directing Tube-Side and Shell-Side Flow Tube-side flow is channeled readily within the tubes, any number of passes using pider plates within the heat exchanger heads. On the shell side, a variety of devices are employed. The most common is the half-moon baffling. These drilled plates, while directing the flow back and forth across the tubes, also act as tube supports or spacers. Another familiar baffle is the disk and donut. Longitudinal flow piders may also be used and, if necessary, tubes may be supported with lattice arrangements which minimize flow obstruction.Preventing Erosion Erosion of heat exchangers in service is generally avoided by designing for low fluid velocities if the fluid is of an erosive nature. Two common mechanical devices are also employed in shell-andtube exchangers to overcome this problem:the shell-side impingement baffle,and the tube-side bell-mouthed tube insert. Both are employed at fluid inlets.