Appendix B Heat-Exchanger Design

Transcription

1 474 Appendix B Heat-Exchanger Design The information given below is useful in preliminary conceptual design and flowsheeting. More theory and data can be found by consulting specialized references [1 5]. B.1 Heat-transfer Fluids Water Critical pressure and temperature values of water are 220 bar and C. Steam is a valuable heating agent below 200 C, where the saturation pressure is about 24 bar. Superheated steam can be used to enlarge the temperature range. Liquid water is excellent for cooling, but also for heating at mild temperatures below 100 C. For higher temperatures thermal fluids are more suitable. Salt Brines Salt brines are water solutions of inorganic salts. Aqueous CaCl 2 solutions of maximum 25% are recommended down to 20 C. Salt brines are low cost but expensive in operation. Antifreezes described below are preferable. Glycol Solutions Ethylene glycol can be used in principle down to 35 C, but in practice is limited to 10 C because of high viscosity. Propylene glycol has the advantage of being nontoxic. Other antifreeze fluids, such as methanol and ethanol solutions raise safety and toxicity problems. Refrigerants Refrigerants remove heat from a body or process fluid by vaporization. Ammonia (R717) seems to be popular again after years of decline in favor of chlorinated hydrocarbons (CFCs). Because of damage to the ozone layer, the CFCs are being replaced by refrigerants based on hydrochlorofluorocarbons (HCFC), although these are not completely inoffensive. Thermodynamic properties of new HCFC can be found in the Perry s Handbook (1997). One of the most recommended is R134a for replacing R12. Thermal Fluids By using thermal fluids the heat - transfer operations can be carried out over a larger temperature interval but at reasonable operating Chemical Process Design: Computer-Aided Case Studies. Alexandre C. Dimian and Costin Sorin Bildea Copyright 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN:

2 B.2 Heat-transfer Coeffi cients 475 Table B.1 Properties of some thermal fluids. Fluid Composition Temperature range P sat max. temp. C p # r # h # 3 C bar kj/kgk kg/m cp Dowtherm A (C6H5) 2 O / (C6H5) 2 L V Dowtherm J Alkylated aromatics Dowtherm Q Alkylated aromatics L Syltherm 800 Siloxane Syltherm XLT Siloxane # = values at minimum and maximum temperatures. pressures. Table B.1 shows the properties of some thermal fluids produced by Dow Chemicals. The best known Dowtherm A, is a mixture of diphenyl oxide/ diphenyl capable of working as liquid or vapor up to 400 C at a maximum pressure of 10 bar. Other fluids are based on mixtures of heavy hydrocarbons. Silicones are excellent liquid heating/cooling media over a wide temperature range, as for example between 100 C and 400 C. More information can be found on the Internet sites of producers. Inorganic Salts Several formulations are known but the most widely used salt is a molten mixture of the eutectic NaNO 2 (40%)/NaNO 3 (7%) /KNO 3 (53%), for operation between 146 C (melting point) and 454 C. B.2 Heat-transfer Coefficients The overall heat -transfer coefficient between two fluids separated by a wall is the reciprocal of the sum of the individual resistances. Partial heat -transfer coefficients depend on the hydrodynamic regime and physical properties of fluids, particularly viscosity and thermal conductivity. Table B.2 shows typical values for partial heat - transfer coefficients that can be used in preliminary design. The assumed values have to be checked by rigorous calculation in final design. Particularly attention should be given to two -phase mixtures, hydrogen- rich gases and condensation of vapors with noncondensable gases. Table B.3 gives values of thermal resistance due to fouling. Thicker walls of stainless steel should also be included in the overall heat -transfer coefficient. The

3 476 Appendix B Heat-Exchanger Design Table B.2 Partial heat-transfer coeffi cients. Fluid h (W/m 2 K) Fluid h (W/m 2 K) Gases Gases, low pressure Gases, high pressure Hydrogen-rich gases Boiling liquids Boiling water Boiling organics Liquids Condensing vapor Water, turbulent regime Condensing steam Dilute aqueous Thermal fluids Solutions Light organic liquids Organics Viscous organic liquids Organics with NC Heavy-ends Refrigerants 1500 Brines Molten salts NC: noncondensables. Table B.3 Fouling as the equivalent heat-transfer coeffi cient. Fluid Fouling (W/m2 K) Cooling water (towers) Organic liquids & light hydrocarbon 5000 Refrigerated brine Steam condensate Steam vapor Condensing organic vapors 5000 Condensing thermal fluids 5000 Aqueous salt solutions Flue gases combination of different situations leads to the overall heat -transfer coefficients listed in Tables B.4 to B.7. Note that fouling is included. B.3 Shell-and-tubes Heat Exchangers Figure B.1 shows the American TEMA (Tubular Exchanger Manufacture Association) standards that are largely accepted. The codification makes use of three letters that indicate the type of stationary head, shell and rear head, respectively. One of the most common types is AES or AEL that designate removable channel and cover (A), one - pass shell (E) and fixed tube sheet (L) or floating-rear (S). A similar

4 Table B.4 Overall heat - transfer coeffi cients for shell - and - tubes heat exchangers. B.3 Shell-and-tubes Heat Exchangers 477 Hot fluid Cold fluid U (W/m 2 K) Heat exchangers Water Water Organic solvents Organic solvents Light oils Light oils Heavy oils Heavy oils Gases Gases Coolers Organic solvents Water Light oils Water Heavy oils Water Gases Water Water Brine Organic solvents Brine Gases Brine Water Natural gas mixture with hydrogen Water or brine Gases, moderate pressures Heaters Steam Organic solvents Steam Light oils Steam Heavy oils Steam Gases Dowtherm Heavy oils Dowtherm Gases Flue gases Steam or hydrocarbon vapors Condensers Steam Water Organic vapors Water Organics vapors, high NC, A Water Organics vapors, low NC, V Water Thermal fluid vapors Tall oil Tall oil, vegetable oil vapors Water Vaporizers Steam Aqueous diluted solutions Steam Light organics Steam Heavy organics Evaporators Steam Sea water (long tube falling film) Steam Sea water (long tube rising film) Steam Sugar solution (agitated film)

5 478 Appendix B Heat-Exchanger Design Table B.5 Overall heat - transfer coeffi cients for air - cooled heat (bare tube basis). Process fluid U (W/m2 K) Water cooling 500 Light-organics cooling Fuel-oil cooling 150 High viscous liquid cooling Hydrocarbon gases, 3 10 bar Hydrocarbon gases, bar Condensing hydrocarbons Table B.6 Overall heat-transfer coeffi cients for jacketed vessels. Jacket Vessel U (W/m 2 K) Steam Aqueous solutions (glass-lined CS) ( ) Steam Light organics (glass-lined CS) ( ) Steam Viscous solutions (glass-lined CS) (50 200) Water, brine, thermal fluid Aqueous solutions (glass-lined CS) ( ) Water, brine, thermal fluid Light organics (glass-lined CS) ( ) Water, brine, thermal fluid Viscous solutions (glass-lined CS) (50 150) Table B.7 Overall heat - transfer coeffi cients for immersed coils in agitated vessels. Coil Pool U (W/m 2 K) Steam Diluted aqueous solutions Steam Light oils Steam Heavy oils Water or brine Aqueous solutions Light oils Aqueous solutions NC: noncondensables. type is BEM but with bonnet - type cover. The type A is preferred when fouling in tubes is likely. The types NEN designates channel integral with tube sheets and removable cover. In preliminary design the problem is the selection of the right type of exchanger and its sizing that complies with design specifications. Conversely, the design

6 B.3 Shell-and-tubes Heat Exchangers 479 Figure B.1 Type of shell - and - tubes heat exchangers following TEMA standards [1]. should be developed so as to use standard heat exchangers as much as possible. The designer should decide which side, shell or tube, is appropriate for each fluid, and find a compromise between heat - transfer intensity and maximum pressure drop. For example, cooling water usually passes through tubes in low -pressure condensers. When the flow velocity cannot ensure high transfer then 2, 4, or 6 passes are recommended. Note that at higher pressures the tubes are more appropriate for condensing, while the cooling water is better fed in the shell side, where the fluid velocity can be manipulated by means of baffles.

7 480 Appendix B Heat-Exchanger Design Rules for fluid side selection are: 1. Corrosion: most corrosive fluid to the tube side. 2. Fouling: fouling fluids in tubes. 3. Fluid temperatures: high-temperature fluid in tubes. 4. Pressure: high-pressure fluids in tubes. 5. Pressure drop: lower pressure drop can be obtained in one or two passes. 6. Condensing steam and vapor at low pressures: shell side. 7. Condensing gas liquid mixtures: tube side with vertical position. 8. Stainless and special steels: corresponding fluid in tubes. Allowable pressure drop is the key design parameter. This is in general 0.5 to 0.7 bar for liquids, occasionally larger for tube -side flow, and of 0.1 bar for gases. The shell diameter depends on the number of tubes housed, as well as the limitations set by pressure and temperature. The diameter may vary between 0.3 and 3 m. High values are valid for fixed - tube sheet. If a removable bundle is necessary then the shell diameter is limited to 1.5 m. Tube size is designated by outside diameter CO.D.) thickness length. Diameters are normalized in inch or mm. Examples are 1/4, 3/8, 1/2, 5/8, 3/4, 1, 1 1 / 4, and 1 1 / 2 inches. Tubes of 3/4 in or schedule 40 (19/15 mm), as well as 1 in (25/21 mm) are the most widely used. Tube lengths may be at any value up to 12 m, the more common values being of 6, 9, and 10 m. Triangular layout of tubes is the most encountered. The tube pitch is 1.25 times the outside diameter. Exact tube counting can be obtained by means of specialized design programs. For preliminary calculations the number of tubes can be found by means of relations based on the factor C = ( D /d ) 36, where D and d are the bundle and outside tube diameters, respectively. The total number of tubes N t can be calculated by means of the following relations [1] : 1 tube pass: 2 Nt = C C C C 4 2 tube pass: 2 Nt = C C C C 4 4 tube pass: 2 Nt = C C C C 4 6 tubes pass: N t = C C C C 4 As an illustration, Table B.8 shows some layouts for 3 / 4 and 1 inch O.D. tubes. Double - pipe heat exchangers are widely used for smaller flow rates. When the heat-transfer coefficient outside is too low, a solution consists of using longitudinal finned tubes as an extended surface. B.4 Air-cooled Heat Exchangers Air-cooled heat exchangers are employed on a large scale as condensers of distillation columns or process coolers. The approach temperature the difference between process outlet temperature and dry - bulb air temperature is typically of 8 to 14 C above the temperature of the four consecutive warmest months. By air -

8 B.6 Plate Heat Exchangers 481 Table B.8 Shell and tubes layout. Shell I.D., in One pass Two pass Four pass 3 / 4 in O.D. 1 in O.D. 3 / 4 in O.D. 1 in O.D. 3 / 4 in O.D. 1 in O.D / / humidification this difference can be reduced to 5 C. Air - cooled heat exchangers are manufactured from finned tubes. A typical ratio of extended to bare tube area is 15 : 1 to 20 : 1. Finned tubes are efficient when the heat -transfer coefficient outside the tubes is much lower than inside the tubes. The only way to increase the heat transferred on the air side is to extend the exchange area available. In this way the extended surface offered by fins significantly increases the heat duty. For example, the outside heat -transfer coefficient increases from W/m2 K for smooth tubes to or more when finned tubes are used. Typical overall heat-transfer coefficients are given in Table B.5. The correction factor F T for LMTD is about 0.8. B.5 Compact Heat Exchangers Compact heat exchangers (CHEs) are characterized by high efficiency in reduced volume, but much higher cost. If area density of the shell -and-tubes heat exchangers can achieve 100 m 2 /m 3, the compact heat exchangers have significantly higher values, between 200 and 1500 m 2 /m 3. However, because of higher cost, the CHEs are employed only in special applications. Some common types are briefly presented. B.6 Plate Heat Exchangers Plate heat exchangers are intensively used in the food and pharmaceutical industries, but less so in chemical industries. Because of the small cross section, intensive heat transfer can be realized, as for example from 400 W/m 2 K with viscous fluids up to 6000 W/m 2 K for water. Gasket plate devices are the most common. The effective area per plate can be larger than 1 m 2. Up to 400 plates can be

9 PVC Manufacturing by Suspension Polymerization assembled in a frame. However, the operation is limited to 30 bars and 250 C. Welded plate heat exchangers are similar. The operation can rise to 80 bars and 500 C, but cleaning is problematic. Plate fin heat exchangers are manufactured by assembling plates separated by corrugated sheets, which form the fins. The plates are made from aluminum sealed by brazing. The operation of these devices requires clean fluids. The main applications can be found in cryogenic and natural - gas liquefaction for pressures and temperatures up to 60 bars and 150 C. B.7 Spiral Plate Exchangers Spiral plate exchangers are usually of two types: spiral -plate and spiral-tubes. Very intensive heat transfer can be achieved, with transfer area per unit up to 250 m 2. The operation is limited to 20 bars and 400 C. However, the cost of such devices is high. References 1 Perry s, Chemical Engineers Handbook, 7th edn, McGraw-Hill, New York, USA, Sinnot, R.K, Coulson & Richardson s Chemical Engineering, vol. 6, Butterworth-Heinemann, 2nd revision, Linnhoff, B., D. W. Townsend, D. Boland, G. F. Hewitt, B. Thomas, A. R. Guy, R. H. Marsland, User Guide on Process Integration for the Efficient Use of Energy, The Institution of Chemical Engineers, 2nd edn, McCabe, W. L., Smith, J. C., Harriott, P., Unit Operations of Chemical Engineering, 7th edn, McGraw - Hill, New York, USA, Ludwig, E. E., Applied Process Design, vol. 3, 3rd edn, Butterworth - Heinemann, 1999