Heat
xchangers tor
Chemical Process Applications KENNETH B. MILLETT The Griscom-RussellC o m p a n y , New York, N. Y.
The commercial availability of corrosion-resistant materials enables the maker of chemicals to reduce manufacturing costs by saving heat through the use of efficient and durable heat interchangers. The data required by the manufacturer for the most appropriate selection of apparatus, together with the salient factors to be considered in preparing specifications of duty to be performed are outlined. Design features, such as efficient and practical structures for cross flow and leak-proof longitudinal baffles, are dis-
cussed, as well as the resistance method of determining the surface needed. Special designs for high-pressure operations and for extended or cccompensated”surface are featured: Tubular construction without shell enclosures but with a series of metallic heat-transferring yokes between tubes for use with dirty or corrosive fluids is also described. A table of heat units transferred per dollar of purchase price for a range between 50 and 5500 square feet of surface is included for various duties.
T
HE utilization of efficient and durable heat conservation equipment by the chemical process industries is becoming necessary to an increasing degree. Owing to a better understanding of the science of heat transfer and a rapidly increasing source of construction materials to withstand the unusual corrosion conditions inherent to the production of chemicals, the manufacturer is now able to design heat exchange apparatus with a guarantee of performance to meet specific operating requirements and to construct it of proper materials for minimum depreciation. Assuming that the chemical plant has adequate information on the local costs of power for pumping, either purchased or produced in the owner’s plant, the cost of water or refrigerant for cooling, and the cost of steam for heating, the exchanger manufacturer can proceed with his design to meet the plant’s specifications as to duty. Necessarily, pumping cost applied to the number of hours of operation per year is the proper gage of the most economical pressure loss to allow the designer in working out the resistance to flow of one or both media through the exchanger. The economics of the particular installation, including estimated initial cost plus all installation charges, interest on the investment, depreciation and obsolescence, pumping charges and any additional labor required, must be studied before the capital expenditure can be approved. A knowledge of these figures will enable the purchaser to specify the optimum values for allowable pressure drop on both hot and cold fluids handled by the exchanger. Cooperation between user and manufacturer will ensure the best selection of construction materials to withstand the particular conditions of aorrosion existing a t the plant. However, in many instances the experience of the fabricator may be sufficiently broad to govern the choice of materials unaided
by a user to whom the contemplated application of heat exchangers introduces metallurgical problems with which he has had no experience. The subject of exchangers is quite broad; it includes apparatus in which one fluid is being heated while another is being cooled, with or without gain or loss of latent heat, and with or without change in the physical state of one or both media. Condensers, heaters, evaporators, distillers, and reboilers are properly classified as heat exchangers. However, this discussion will be confined to apparatus in which a fluid, either liquid or noncondensable gas, is being cooled or heated in juxtaposition to the same or another fluid which is being heated or cooled. Such equipment is commonly termed a heater, a cooler, a heat exchanger, or an interchanger. The exact designation is governed by the primary object for which the apparatus is installed.
Data Required by Manufacturer The manufacturer should receive from the purchaser a concise specification of the duty which is to be performed by the exchanger, including the following items: A description of the material to be heated or cooled should include its specific gravity a t a certain temperature, its specific heat a t the average operating temperature, and its thermal conductivity and viscosity a t not less than two temperatures in the operating range. If a change of state is to occur in the exchanger, the latent heat or heat of vaporization should be specified as well as the boiling or condensing temperature under the prevailing pressure. The degree of cleanliness and scaling tendency of both fluids under the specified operating temperatures should also be stated, as well as any tendency to crystallization. 367
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Method of assembly
1. Parts of longitudinal baffle assembled before pulling ball cage bar t o engage parts
3.
h
2.
I
A”
Ball cage bar pulled t o engage all parts ready for placing in shell
Ball cage bar shown pulled into final position with all parts locked and center baffle sealed in shell
FIGURE1. BALL-BARBAFFLEASSEMBLED IN SHELL
The quantity of each fluid passing through the exchanger in an hour, or other time period, should be given, as well as the inlet and outlet temperatures and the allowable resistance to %owor pressure drop through the unit. Maximum operating pressures for both fluids are essential, as well as the maximum hydrostatic or other test pressure which is to be applied before shipment. The user could advantageously specify whether his preference is for vertical, horiaontal, or inclined installation; whether removable tube bundles or spare units are required for operation while the original installation is shut down for inspection or cleaning; and what materials of construction his experience would indicate as the best to resist the corrosion conditions prevailing.
cannot be used for cooling a liquid to a temperature below the exit temperature of the coolant. This duty requires strictly counterflow construction with the same number of passes for both liquids. The relative position of the fluids must be determined to decide whether the coolant is to pass through the tubes or the shell. If the fluid to be cooled is corrosive to common metals, it may be preferable to pass it through the tubes and the connected heads which are fabricated of expensive Spacer Ring one piece
. , Jsplit
Ring
two halves
Duty Specifications and Type of Unit With this information a t hand, or as much of it as the purchaser is prepared to furnish, the designer is in a position to work out the duty specifications and to select the type of exchanger best suited to them. The quantity of heat to be exchanged per hour, neglecting radiation losses, is commonly expressed in British thermal units, although some operators prefer the pound centigrade unit (p. c. u.) which has a value nine-fifths that of the B. t. u. If the heat to be absorbed hourly, the initial and final temperatures, and the specific heat of the cooling medium are known, the quantity of heat to be exchanged is readily calculated. In the case of water, quantities should be investigated for both winter and summer temperatures and load conditions. The mean temperature difference may be based on counterflow or parallel flow of the two fluids, or a combination of them. The type of flow relation required largely determines the selection of passes in the exchanger design. Obviously a cooler with two passes in the tubes and one pass in the shell
Floating Head Tube Sheet FloHting Head Cover FIGURE2. ARRANGEMENT OF PERFECTED SPLIT-RINGJOINT
noncorrosive material, rather than through the shell with its greater weight of costly metal. However, if one of the fluids is dirty or scale forming, it should be run through straight tubes in a unit arranged with easily removable heads and covers to facilitate cleaning the bore of the tubes. If no adverse considerations exist, it is preferable to run the material which is being cooled through the shell in order to take advantage of radiation from an uninsulated surface.
INDUSTRIAL AND ENGINEERING CHEMISTRY
APRIL, 1938
Temperature conditions for both fluids and the materials selected for shell and for tubes will determine whether a fked tube sheet design can be used safely. This involves an investigation of tube wall temperature and shell temperature under the most difficult operating conditions and of the tensile stresses imposed upon either tube or shell metal under these conditions. If means must be provided in the design to accommodate expansion strains, either floating-head, flexed-tube, helicalcoil, or U-tube bundle construction can be used. Frequently an expansion member is built into the shell. The coil or Utube alternates are to be recommended only for use with clean nonscaling fluids flowing through them. If a floating head is employed, the design can be further perfected by having the tube bundle completely removable from the containing shell for cleaning or inspection. This is usual practice although with clean fluid in the shell the construction can be cheapened by combining a floating head with a nonrernovable bundle. The number of tube passes or length of tube travel is governed by the pressure drop allowed by the customer, which in turn is determined by annual pumping costs. I n general, a long unit with one- or two-tube passes is cheaper to construct than a short multiple-tube pass design. Space limitations frequently control this selection, however. The size and gage, or wall thickness, of the tubes depend largely upon the cleanliness and viscosity of the fluid being handled; the latter property governs the mass velocity which can be used within the specified pressure-drop allowance. For this reason turbulence strips or ribbons are occasionally inserted in the tubes to impart spiral flow to a clean viscous liquid, thereby decreasing the fluid film resistance a t the expense of an increase in pressure drop. The spacing of the tubes and their geometrical pattern are very important. For clean fluids a t low pressure or for conditions where chemical cleaning is adequate, close tube pitch with a triangular arrangement is used. For dirty liquids where
FIGURE 3.
369
frequent mechanical cleaning between tubes is necessary, a liberal spacing and a square layout are employed. For high vacuum in the shell a wide tube pitch with tubes staggered to prevent channeling or by-passing of vapors and condensate is utilized. The tube spacing can be decreased advantageously towards the outlet of the shell where the vapor density is greatest.
Baffle Structures The baffle structure in the shell space is a most important consideration in design. The aim should be to secure the maximum turbulence consistent with the allowable pressure loss. Eddy losses formed by dead pockets in the baffle system are to be avoided, as well as large layers of fluid between the outermost row of tubes and the wall of the shell. Flow of liquid a t right angles to the tubes is desirable. In some cases relieving areas in the baffles are necessary to reduce pressure losses, as well as an enlargement of the annular space between the tube and the transverse baffle. Obviously the baffle pitch, or spacing, must be carefully worked out to supplement the cross flow velocity most advantageously. Where the duty requires two passes of a fluid through the shell of an exchanger employing a removable tube nest or bundle, the designer is confronted with the problem of building a rigid longitudinal baffle with a length almost equal to the tubes and with both edges so constructed as to form a bottle-tight joint with the bore of the shell. The assembly of this baffle with its transverse members mush be capable of easy extraction from the shell after many weeks or months of service and of maintaining tightness between the two shell passes after replacement in the exchanger. The Ball-Bar design meeting all these requirements satisfactorily is shown in Figure 1. Turbulent flow is to be preferred to streamline flow because the fluid film resistance is less and consequently the exchanger is smaller and less expensive. The baffle design should there-
COMPRESSION-RING EXTRACTION STEAMHEATER WITH HEADCOVER,RING GASKET, AND PARTITION SEALINGPLATEREMOVED
I
1
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VOL. 30, NO. 4
FIGURE 4. ROTATABLE BREECH-BLOCK FORUING FOR HIGH-PRESSURE T h e water-head cover is readily removed f r o m t h e head or replaced with a single tool by which it is rotated through a small angle equivalent t o t h e width of t h e engaling projections.
tube sheet. This feature is accomplished by means of a twopart split bolting ring backed by a one-piece ring, both of which are detached from the floating tube sheet before the tube nest is withdrawn through the bore of the sheII. This detail is illustrated by Figure 2.
High-pressure Designs
I I
turbulence where the allowable pressure drop this class of flow. flowing through a baffled shell it is advantaof heat transfer as well as price considerations to enclose the tube nest as closely as possible. A minimunI clearance between the outer tube row and the shell !vent extraction of the bundle unless some means ided to disengage the floating head cover from its
For pressures of the higher order in the tube space-that is, 600 pounds per square inch and upward-certain refinements in design of the liquid channels or boxes are necessary. Conventional low-pressure designs when subjected to heavy pressures are not suitable because of the extravagant size and number of bolts required for strength and the maintenance of tightness. This need has been met by the use of a special closure, either in the form of an oval mild-steel ring or hoop gasket in compression (Figure 3) or a rotatable breech-block forging similar to ordnance practice (Figure 4). The bolting to ensure liquid tightness with only a narrow metallic gasket can be light since all heavy pressure is transmitted directly to forged lugs integral with the heads.
STEAM INLET-,
WATER
aurLEr-
1
W A T E R INLET/
:SHELL D R A I N /
FIGURE 5. STRAIGHT-TUBE EXTR.4CTION
HEATER FOR LOW-PRESSURE SERVICE, HEADS,STEAM IMPACT BAFFLE,AND AIR
STEAM
SHOWING PARTITION SEALING PLATE I N W A T E R
BAFFLE
I
APRIL, 1938
INDUSTRIAL AND ENGl:NEERING CHEMISTRY
371
I n such operations it is essential that there be no leakage between passes in the stationary and floating heads; otherwise by-passing will occur and predicted performance will not be obtained. Advantage is taken of the unbalanced pressure between passes due to flow resistance through the tubes to bring the pressure differential to bear upon a relatively thin sealing plate which is supported by the pass partitions within the external head cover; a tight joint is thus secured. This feature appears in Figure 5. The method of connecting tubes to sheets or plates is important. Customary steam condenser practice using packed or unpacked ferrules is not suitable for exchangers in chemical service. There must be a positive, leak-proof bond between the tube and both sheets. This is best accomplished by using a series of shallow parallel grooves in the drilled tube hole and roller expanding the tube into it with sufficient pressure to force the metal into the serrations. A tube can be replaced, when necessary, by drilling out the ends without damaging the grooves in the tube hole.
economical production, and with these as a basis the number of tubes and the shell size are selected. Then the effective tube length measured on the outside of the tube between the sheets can be calculated. A suitable safety factor is introduced to offset any unknown variables present in the operations, and the nearest commercial over-all tube length is selected. The exchanger chosen is then checked for resistance to flow or pressure drop on both hot and cold fluids to ensure compliance with specified requirements. This figure is a function of mass velocity, average viscosity, size of tube bore, length of travel, specific gravity, and character of flow, whether turbulent or streamline. Entrance and exit pressure drop is also investigated where it has a bearing on over-all resistance to flow; the figure is a function of mass velocity and number of passes. Suitable safety factors are introduced to neutralize any uncertainties in the operating conditions.
Calculation of Surface
The introduction of extended surface to exchanger construction has solved numerous problems which have been troubling the designer of conventional shell and bare tube transfer apparatus. By “extended surface” is meant equipment in which the usual bare tube wall between media is replaced by elements consisting of tubes or pipes to which are mechanically bonded, without brazing, solder, or other extraneous fluxed metal, fins of corrosion-resistant material extending either radially to the axis of the tube or forming a helical winding around it; the most effective fin height is determined by numerous test runs. These elements are illustrated by Figure 6. I n many applications of exchangers it is necessary to extract heat from a viscous liquid of low conductivity and low specific heat, such as petroleum oil, water with low viscosity and high specific heat and conductivity being used as the coolant. Or it may be specified that the oil must be heated with steam, a medium which parts with its latent heat content rapidly. Theoretically, these duties indicate ample surface in contact with the oil because of its reluctance to have its heat content disturbed, either by subtraction or addition, and a minimum of area against the water or steam. Bare tube equipment is not the answer to these requirements since the metal area is substantially the same against both media. On the other hand, “compensated surface” offered by these finned elements is very suitable because the fins can be presented to the oil with many times the area inside the tube, and the water or steam can flow through the latter over the minimum of transfer surface. Where the flow is parallel to the tube axis, radial fins are used. With flow a t right angles
Determination of the amount of surface required to meet the customer’s conditions of operation requires a survey of the resistance or, conversely, the conductance of the fluid and dirt films and the metal wall separating the two media, as well as the mean temperature difference. Various theoretical methods are in use for evaluating fluid film resistances inside and outside of tubes; these methods are functions of mass velocity, film viscosity, thermal conductivity, length of travel (whether streamline or turbulent flow), and baffle spacing. Dirt or, reciprocally, cleanliness factors are important items, particularly in duties where liberal transfer rates are to be expected arid a carelessly selected dirt factor will have a large effect upon the total conductance and therefore the surface. The correct choice of factors depends upon a true knowledge of the actual operating conditions and long experience with numerous installations of similar nature. Metal resistance caused by the tube wall varies with the thickness and conductivity of the material. The inclusion in an alloy of a small percentage of an element having a low conductivity value has a marked effect on the over-all conductance of the metal. An example is 18 per cent chrome-8 per cent nickel stainless steel having a value of 10.7 contrasted with low-carbon steel for which the figure is 25. With mean temperature difference and over-all conductance determined by calculation, it is possible to establish the amount of surface theoretically required in the type of exchanger best suited to the particular problem. A wide variety of tube layouts has been developed by the manufacturer for
Special Designs
FIGURE 6. HEATTRANSFER ELEMENT, WITH HELICAL FINS(Above) AND LONGITUDINAL FINS(Below)
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FIGURE7. (Above) STACK OF TUBEFLO SECTIONS SHOWING COMPACT GROUPINGOF FIVE UNITSCONNECTED IN SERIES, AND (Below)TUBING AND FINSFOR TUBEFLO SEC-
assembled exchanger which is about 20 feet in length. The exchange of heat occurs through the closely spaced fins linking the four tubes into a single assembly. These sections are illustrated by Figure 7. m e r e low-carbon or special alloy steel tubes cannot be used because of corrosion, and cast iron is necessary, the construction is similar except that four cored passages in a single casting replace the tubes. Several body pieces of this description, as well as cast heads and return bends, are bolted together to obtain the requisite transfer surface in one or more stacks. It is difficult, if not impossible, to give typical selling prices covering heat exchangers either on a square footage basis or on a duty basis. There are so many operating condition variables, designs, and materials of construction that generalities are meaningless. The following tabular data are illustrative of the broadness of this field:
Heat per Hr. per of Selling Dollar Duty
Surface Price Sq. ft. B . t . u. High-pressure water heater 8590 648
to:the tubes the helical fin is more suitable. For downfalling condensate on a vertical tube with radial fins, the cooling is most satisfactory and the exit temperature approaches closely to that of the cooling medium a t the inlet. This is valuable in solvent recovery applications. For exchanging heat between liquids, one or both of which may be too dirty to handle in shell and bare tube units without excessive maintenance charges for frequent cleaning, a design is used which utilizes tubes of medium bore only and requires no shell. Four steel tubes, 2.5 inches in outside diameter, are yoked together by means of narrow steel fins closely spaced along the length of the tubes. The tubes are roller-expanded simultaneously against the fins, which are assembled in spaced relation; the spacing depends upon the operating pressure which may be as high as 5000 pounds per square inch. Suitable forged stationary heads, with caststeel return bends and connectors, are used to complete the
Vapor condensers Alkali exchangers Turpentine condensers Ammonia liquor coolers Naphthalene condensers Vapor condensers Wash oil coolers Water-to-water exchangei Air heaters Wash oil coolers Gas coolers Gas coolers Alkali exchangers Gas coolers Oil heaters Alkali coolers Oil coolers Special acid coolers
162 1675 768 1892 270 139 600 148 1360 1212 671 1258 151 2165 506 214 54 157
5830 5540 5100 3210 3030 2900 < 2560 2430 1728 1400 1145 1072 960 916 890
836 782 645
Notes
1000 lb./sq. in. tube operating pressure
...
...
... ... ...
...
Helical coil 'type Copper-extended surface
... ... All nickel construction ...
Partly nickei construction \
G r a p h i t e 'id ns t r u c t i o n against acid A l l stainless-steel c o n struction
Organic liquor exchangers
55
473
Tar heat exchangers Preheaters for organic vapor using oil Solvent recovery coolers
308
374
1885 5470
366 355
Aluminum ' kitended sur-
285
326
Ca&iron tubeflo eeotions
1899 576
176 121
380
107 98
Catalyst-filled tubes Copper and Everdur oonEtrn"ti"" I". -_".--. Same Same
Tar heat exchangers Preheaters f o r organic vapor using oil Vapor condensers Vapor condensers Vapor condensers
R E C E I ~December D 1, 1937.
640
...
fnrr
