Heat Integration Schematic

Transcription

1 PERANCANGAN SISTEM HEAT EXCHANGER PERANCANGAN PROSES KIMIA (CHEMICAL PROCESS DESIGN) Section 2 oleh: Dr. Istadi, ST, MT Kode Mata Kuliah : TKK 345 Beban : 3 SKS 1 Heat Integration Schematic 1

2 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. 3 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 l 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. 2

3 Thermal Fluids: By using thermal fluids the heat - transfer operations can be carried out over a larger temperature interval but at reasonable operating 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. 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. Properties of Some Thermal Fluid 3

4 Heat transfer Coefficients The overall heat - transfer coefficient between two fluids separated by a wall is the reciprocal of the sum of the individual dua resistances. s 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. Thicker walls of stainless steel should also be included in the overall heat - transfer coefficient. 7 Partial Heat Transfer Coefficient 8 4

5 Fouling as the equivalent heat transfer coeffi cient Shell and Tubes Heat Exchangers 10 5

6 Overall heat transfer coefficients for shell and tubes heat exchangers 6

7 Overall heat transfer coefficients for shell and tubes heat exchangers 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 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 fl uid, 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 baffl es. 7

8 Rules for fluid side selection are: Corrosion: most corrosive fluid to the tube side. Fouling: fouling fluids in tubes. Fluid temperatures: t high h - temperature t fluid in tubes. Pressure: high - pressure fluids in tubes. Pressure drop: lower pressure drop can be obtained in one or two passes. Condensing steam and vapor at low pressures: shell side. Condensing gas liquid mixtures: tube side with vertical position. Stainless and special steels: corresponding fl uid in tubes. Allowable pressure drop is the key design parameter 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. 8

9 Tube size is designated by outside diameter CO.D.) thickness length. Diameters are normalized in inch or mm 1/4, 3/8, 1/2, 5/8, 3/4, 1, 1 1/4, and 11/2 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. 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. Please be continued to: MODUL LATIHAN 4 (Heat MODUL LATIHAN 4 (Heat Exchanger Design with CHEMCAD CC THERM) 9

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