C. H. GILMOUR
Union Carbide Chemicals Co., South Charleston, W. Va.
Application of Heat Exchangers in Chemical Plants Because of the large investment and number of problems the heat exchanger represents, engineers should pay more attention to its design, specification, and performance in chemical plants
'TL
HEAT EXCHANGER is usually considered to be a n accessory piece of equipment in the modern chemical plant. In the reaction system, the large and costly reactor, though actually a heat exchanger, assumes the prime position of importance. Again, in the purification system, the distillation column-in reality, a direct contact heat exchanger-receives more engineering attention than the half dozen exchangers that are required to make the column function. The large systems, which provide the heat sources and sinkssteam refrigeration and cooling waterall appear to be more significant than the lowly heat exchangers which stabilize the operation of the plant by recovering heat or transferring it from source to sink. A look a t the total investment in equipment for a chemical plant, shows that heat exchangers are probably the most important pieces of equipment and therefore should require the most engineering attention. If a heat exchanger does not provide the desired temperature of feed to the reactor, then less heat is recovered in the form of process steam and more heat is rejected to the cooling water. If the feed preheater to a distillation column does not perform in accordance with its design, additional steam is required in the reboiler and additional water is required in the residue cooler. Because there are usually several interrelated heat exchangers in a system, there is a compounding influence of the malfunction of one or two exchangers so that it is often difficult to pinpoint the source of deficiency in attaining plant capacity. Sometimes heat exchangers do not perform in accordance with design. Too often, the blame is placed on the fabricator who has no knowledge of the process in which the heat exchanger is to be used and who has been given an inadequate set of design specifications. I t is normally impractical to supply the fabricator with process design information, but it is reasonable to expect the user to apply sound principles in developing. performance specifications from which an adequate heat exchanger may be rated and fabricated. No set of rules can be devised, for proper applica-
tion of heat exchangers, which will be relevant to all chemjcal plants and which will be devoid of personal preference. However, there seems to be an increasing interest in heat transfer on the part of engineering departments of chemical companies and a decided trend toward specifying heat exchangers on the basis of mechanical construction rather than on performance. Therefore some of the principles now being used in the chemical industry for specifying heat exchangers are discussed.
Classifications Two general types of heat exchangers are considered here: the shell-and-tube type and the special (proprietary) type. These are further classified in accordance with their mechanical configuration, installed position, and heat transfer process function. Shell-and-Tube Heat Exchangers. These exchangers are normally named in accordance with their mechanical configuration. Fixed tubesheet Return bend (U-tube) Floating tubesheet e Bayonet 0 Double-pipe 0
0 0
From the standpoint of process applications each of the above types has certain limitations. The fixed tubesheet unit is used when the shell side fluid is not likely to foul the surface. The shell side is not accessible for mechanical cleaning. Fixed tubesheet exchangers should not be used when the temperature differential between the shell and the tubes exceeds 100' F. Although chemical cleaning of the shell side may be possible and expansion joints in the shell may permit operation at differential temperatures above 100' F., this type of exchanger has limited usefulness in a chemical plant. The return bend heat exchanger is popular because it has a removable bundle, is not subject to stresses due to differential thermal expansion, and is low in cost. However, cleaning on the inside of the tubes is not easy to do, partial retubing is not practical, and vertical installation is not desirable.
Floating tubesheet exchangers, because of their versatility, are widely used in chemical plant service. T h e tubes are straight, both sides are accessible for cleaning, and there is nm problem of thermal expansion. When there is an extreme difference in temperature between the tube a n d shell side fluids, the bayonet type of heat exchanger is commonly used. The double-pipe exchanger is used when tube and shell side flow rates are low but about equal, when temperature range is relatively high, and when a close temperature approach is necessary. Sensible Heat Transfer Exchangers. This exchanger is one in which the heat transfer process consists of raising or lowering of the sensible heat of the process fluid. If there is a process fluid on both sides, the unit is called a heat exchanger. If the process fluid is being heated, regardless of the type of heating medium, the exchanger is called a heater. The term "cooler" is used to designate an exchanger in which the process fluid is being cooled. The closer the approach between temperatures on opposite sides of a heat exchanger the greater the heat recovery. More area and more investment are required. The choice of approach temperature must be made on the basis of several factors-e.g., the value of the recovered heat, the space available for installation of heat exchangers, the over-all plant cost, the materials of construction. These factors vary from plant to plant. The optimum approach temperature usually lies between 20' and 50' C. I n heaters and coolers, it is often economical to heat or cool to within 5' or 10' C. of the service fluids. Close approach in coolers reduces vent losses and product recycle. It is not good practice to allow cooling water to rise to too high a temperature even though apparent economy in water supply results. When temperatures of the water a t the hot end of water-cooled heat exchangers exceed 50' C., excessive fouling and corrosion are likely to occur, and, therefore, the maintenance and replacement costs will increase. Heat exchangers are normally installed VOL. 52, NO. 6
JUNE 1960
465
horizontally when: the unit consists of more than one body, there is more than one pass on the tube side, and there is a limitation on over-all length. The advantages of vertical installation are often overlooked. A long vertical unit is economical in first cost, a small amount of horizontal space is required, and a minimum number of bends, valves, and piping are utilized. The vertical exchanger is preferred for heating or cooling two-phase fluids. Short tube horizontal exchangers are considered to be more easily serviced than long tube vertical units, but major repairs are more effectively made in a maintenance shop in which case the entire unit rather than the bundle only, would be removed. Condensers. Condensers are used for recovery of the purified vapors from distillation columns or evaporators. They may be horizontal or vertical. The condensing vapor may be placed either outside or inside the tubes. There are inherent disadvantages associated with horizontal or vertical vapor-in-shell condensers. Carbon steel can rarely be used for construction of the shell, uniform vapor distribution is difficult to attain, pressure drop is abnormally high, and condensate subcooling, though possible, requires excessive surface area. Horizontal, vaporin-shell condensers should contain vertically cut baffles and may be multipass on both the shell and tube sides. Vertical, vapor-in-shell condensers should be designed for down-draft vapor flow and single-pass both sides. I n vapor-in-tube condensers, vapor distribution is good, the quantity of high cost materials is a minimum, pressure drop is reasonable except a t extremely low pressure levels, and condensate subcooling is readily accomplished, The horizontal vapor-intube condenser may be multipass on the tube side but vapor should be made to flow downward. The vertical vapor-intube condenser must be single pass on both sides and vapor flow should be downward. The fluids flow strictly countercurrent to one another. For service at normal pressure levels, the vertical vapor-in-tube condenser is the most economical design. I t may seem to be a contradiction to say that for service at high vacuum, the horizontal vapor-in-shell condenser is the preferred design. However, a t high vacuum, vapor lines are largeapproaching the dimensions of the condenser-vapor temperatures are usually high, and condensate subcooling is not too important. Thus, at high vacuum, because the dimensions of the vapor line and condenser shell are approximately equal, vapor distribution is good. A minimum number of rows are required for condensing because of the high temperature difference and, as a result, pressure drop is low. The
466
vapor pressure of the liquid is, of course, low so only a small amount of subcooling is required to prevent loss of material to the vacuum pump. These advantages are enhanced if low-finned tubes, rather than bare tubes, are used in the vapor-in-shell design. Although the discussion has been limited to condensers installed either horizontally or vertically, do not ruleout installation in an inclined position. The desirability of downdraft vapor flow has been stressed. There seems to be a strong desire on the part of process engineers to call for vertical updraft condensers (sometimes referred to as knockdown condensers). This type of condenser is highly impractical ; performance is unstable, pressure drop is high, design procedures are not available, and, hoped for economies are probably not really attained. Reboilers, Evaporators, Calandrias and Vaporizers. Various names are associated with heat exchangers in which the process consists of generation of vapor from a boiling liquid. I n chemical plants, the boiling process is normally conducted either on the shell side of horizontal kettle-type vessels or on the tube side of vertical shell-and-tube exchangers. Kettle-type reboilers are used for batch operations or continuous operations in which liquid hold-up is either desired or is not a problem. Tubes are subject to build-up of tar, carbon, or salts in this type of vaporizer but cleaning is possible because bundles are removable and tubes are spaced on a square pattern, often on wider than normal pitch. Vaporizing on the shell side of vertical exchangers is not commonly done because the surface above the liquid level is mostly ineffective. There are several reasons for the use of vertical, natural-circulation reboilers: liquid hold-up is low; material costs are low (fixed tube sheet and steel shells); fouling rates are low; process side cleaning is quick (bundles need not be removed, only heads); and it is difficult to design a low-volume, high surface-area, kettle-type unit. The tubes in vertical natural circulation reboilers are usually large in diameter (1 to 2 inches) and short in length (6 to 10 feet). This type of reboiler is most effective for use at or above atmospheric pressure. At high vacuum, the boiling point-rise reduces the thermal syphon effect. Circulation rates usually range from 10 to 20 pounds of liquid per pound of vapor generated. When boiling point-rise is low it is possible to use small diameter ("8 inch), long tubes (20 feet) to advantage. This exchanger is a long-tube vaporizer and usually is a once-through unit in which 100% of the liquid is vaporized. Forced circulation vaporizers are nothing more than heaters and are figured as sensible heat transfer ex-
INDUSTRIAL AND ENGINEERING CHEMISTRY
changers in which the liquid is superheated and subsequently flashed in the low pressure kettle section of the distillation column. Liquid velocities in this type of vaporizer are high (10 to 20 feet per second), pressure drop is high and back-pressure should be maintained so that boiling of the liquid within the exchanger does not occur. For clean liquids, an up-draft vaporizer may be used in which boiling takes place inside horizontal tubes. This type of vaporizer is similar to the dry-type chillers used in refrigeration units. Excess area needs to be supplied to assure that all of the inlet liquid is vaporized. Inclined natural circulation calandrias may be used provided inclination is greater than 1 5 O from horizontal and the boiling fluid is inside the tubes. Vertical evaporators in which the liquid to be vaporized is introduced into the top of a vertical tube and falls in film-form down the heated wall are attractive. Theoretical heat transfer rates are high and fabricating costs are low. However, operating data obtained on this type of evaporator indicate that design limitations are not fully understood. Because of the high vapor rates, concurrent flow of vapor and liquid is required. For satisfactory performance, a low temperature difference between wall and boiling liquid must be maintained ( l o o C.), excessive entrainment must be expected, and there is a minimum liquid rate below which the film breaks into a rivulet and heat transfer falls off. Fouling in exchangers contributes mostly to maintenance costs and is usually the scapegoat for disagreement between design and performance. To minimize difficulties due to fouling, clean exchangers before they need it. Special Types Special types of heat exchangers may be applied for solving special problems. This group, representing less than 15% of all units, contains equipment possessing unique features and costing less than conventional exchangers. Nonmetallic Construction. Evidently, the shell-and-tube heat exchanger has some shortcomings because a host of special types of heat exchangers, geared to special jobs, are now available. Most of these special exchangers are for use at rather small heat loads but others are competitive with the average size shelland-tube units. Heat exchangers may be obtained, small and large, which are constructed of impervious graphite. Thus, in some situations in which corrosion is extreme, a genuine saving may be realized by using graphite units. In fact, the price of these units, in standard sizes, is competitive with exchangers constructed of stainless steel. Often, the graphite exchanger may be the most economical
HEAT EXCHANGERS choice even when corrosion is not extreme. Graphite units are available in block form also. Single blocks are used for real small flows or modules may be joined together to increase surface area to several hundred square feet. Some all-graphite units consist of cylindrical modules which when joined together have the appearance of a shell-and-tube unit. Haveg, another nonmetallic material is also used in the construction of heat exchangers. The disadvantage of low conductivity is offset by the high resistance to corrosion in some media. Small, low cost, shell-and-tube heat exchangers from 2 to 125 square feet, containing nonferrous or stainless steel tubes--'/b and 3/8 inch in diameter and 1 to 6 feet in length-are available for use in chemical plant service. These are used to cool residue or side streams, heat catalyst feed streams, condense vent vapors, and vaporize liquids. They are low in cost and, because of the small diameter tubes and well-baffled shell side, they transfer a lot of heat per dollar of investment. Extended Surface. Thin-wall, standard diameter tubes are now available in which the ratio of outside-to-inside surface area is approximately 4 instead of the usual value of 1.2. In many cases, the use of these extended surface tubes results in an economic increase in performance per unit volume of exchanger. Many existing units are being retubed with these tubes and many new units are being designed and fabricated with this tubular product. As demands are made upon water supply systems, water costs go up, maintenance costs go u p because of accelerated corrosion due to temperature increases, and heat exchanger investments increase because of scarcity of water (lower AT). As a consequence, considerable attention is being given to the cooling of process streams by air. Transverse finned tubes (15 to 1 ratio of outside to inside area) are used for these coolers which consist of a bank of tubes, about four rows deep, over which an air stream is either blown or drawn-forced or induced draft. Coolers represent only part of the heat exchangers in chemical plants so even though air-cooled units are being used in increasing numbers there is no serious threat to the shell-and-tube heat exchanger business. Longitudinal finned-tube units are also used in chemical plant service. Both double-pipe designs (fins on outside of inner tube) and fin-tube bundles are used. They are particularly applicable for high temperature or high pressure service or where fin-side fluid heat transfer rates are low. Pipe Coils. Helical, spiral (pancake),
and serpentine coils are used quite extensively in chemical plants for various cooling services, especially at high temperature and pressure and for low flow, viscous materials and slurries. Coils are sometimes immersed in a tank of water and other times are sprayed with water to obtain higher rates of transfer. Although pressure drop is high, cost is low and this type of cooler can often be made by the user in his own shop. Spiral Types. The spiral plate exchanger is constructed by winding, spirally, two parallel plates which terminate at the periphery of a cylindrical shell. Two separate rectangular passages are thus created through which the two fluids flow countercurrently. Unique features include : perfect distribution, high turbulence, no possibility of inter-leakage, no bypassing, counterflow in a long path, compactness, no expansion problems, ease of cleaning, and resistance to fouling. Sizes are available from 15 to 1500 square feet. Multiple bodies are readily connected for either parallel or series flow. They are especially adaptable to solids-containing fluids because the single spiral path prevents settling. The dimensions of a 1000-square foot spiral heat exchanger would be 42 inches in diameter by 60 inches long, and the length of path would be 100 feet. An equal area shell-and-tube unit would consist of eight bodies, each 10 inches in diameter by 12 feet long containing 37 I-inch outside diameter tubes. These spiral exchangers are not limited to use for sensible heat transfer but are admirably suited for use as reboilers and vapor condensers. They are especially suited for condensing at high vacuum because of low pressure drop and adaptability for condenser and vent subcooling. Another spiral type of heat exchanger consists of a multiplicity of coils wound spirally ter'minating a t manifolds. These are available in sizes from 2 to 100 square feet in various tube diameters and coil lengths to 27 feet. A countercurrent surface area of 100 square feet is contained in a space 25 by 23 inches. These exchangers are inexpensive and are used for low-flow sensible heat transfer as well as for condensing. They are used effectively as intercondensers on steam jet vacuum pump installations. Plate Types. Multiple plate-in-frame heat exchangers were probably developed primarily for the food and drug industries because of the ease of cleaning and inspecting of surfaces. However, this type of cooler is beginning to be used in the chemical industry because of certain unique features not available in shelland-tube exchangers. The heat transfer surfaces consist of adjacent corrugated plates. Gaskets, cemented to the plates,
are shaped so that separate channels for the two fluids are formed. The corrugations add strength to the plates, provide high surface area, and cause considerable turbulence to the fluids. Area can be increased or decreased by, respectively, adding or subtracting plates. Plates are easily cleaned and replaced. Heat transfer rates are high, especially for viscous fluids. The plates are not welded and therefore not subject to welding stresses, pressure drop is low, inter-leakage of fluids i5 impossible, multipass flow is possible. These exchangers are available for pressures u p to 200 p.s.i.g. and in almost any material of construction. The chemical industry is using heat transfer surfaces made u p of single or double-embossed plates electric-welded together to form coillike channels for hot or cold fluids. The outer surfaces of the plates are in contact with process fluids inside tanks or shells. These plate units are inexpensive, light in weight, easy to clean, easy to remove and replace, can be shaped to fit special contours and can be made of all metals. They are fully annealed and can be provided with specially conditioned surfaces, and may be coated easily. The main application is for reactor cooling and tank heating or cooling. Tanks may be constructed from these double plates and thus increase heat transfer performance. Scraped Surface Exchangers. Scraped-surface heat exchangers are applied when it is necessary to transfer heat to or from slurries, crystallizing fluids, or highly viscous materials. These exchangers consist of large tubes, jacketed, and containing a slow speed rotating shaft to which are attached spring-loaded blades. The blades help propel the fluid through the exchanger. Several shells are usually connected in series. These are used for both heating and cooling of the process fluids. A higher speed, close-clearance, scraped-surface heat exchanger called a Votator processes highly viscous materials attaining heat transfer rates as high as 400 B.t.u. per hour per square foot per O F. More delicate scraped-surface units are available for evaporating viscous materials under high vacuum and effectively removing entrainment and residue. These consist of rotating blades within fairly large cylindrical or conical shells, vertical or horizontal, with ample space for disengagement of liquid and vapor and provision for returning liquid to the wall. RECEIVED for review February 17, 1960 ACCEPTEDFebruary 23, 1960 Division of Industrial and Engineering Chemistry, 137th Meeting, ACS, Cleveland, Ohio, April 1960.
-
VOL. 52, NO. 6
a *
*JUNE 1960
467
