Class 7 - Class 7 Heat Exchanger Design 1 Simplified View...

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1 Class 7 Heat Exchanger Design
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2 Simplified View of Heat Exchangers used in preliminary stages of design factors ignored: source/sink for energy transferred rate of energy transfer type and size of equipment only overall heat duty considered Example from HYSYS Vinyl Chloride simulation ( 29 out in Q m H H = - "one-sided" heat exchanger Example 18.1
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3 Heat Transfer Media (utilities) Utility exchangers use media such as cooling water, steam, flue gases, refrigerants, etc. Table 18.1 provides list of different media covering range -150ºF to 2000ºF Cooling water Cooling Tower Process Stream Heat Exchangers 90°F 120°F Process streams cooled to as low as 100°F Local abundant source of cool water in some areas: coastline, riverbanks Direct air cooling to 120°F If process streams are > 250°F, use waste heat boilers to create process steam. (Huerisitic 27, pg 168)
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4 Cooling below 100°F: refrigerants used, designated by R-number R-717: ammonia R-290: propane R-134a: tetrafluoroethane Many HFC's ("freons") phased out since 1980 because of ozone layer damage. Chilled water can be used for cooling to 45°F Chilled brines to 0°F Refrigeration cycle required: compressor, condensor, expansion valve, + utility exchanger Heat Transfer Media (utilities) Heat medium: commonly steam LP: 50 psig MP: 150 psig HP: 450 psig (298°F) (366°F) (459°F) Steam condenses in heat exchanger. Steam through-flow trapped. For heating above 450°F to 750°F: Dowtherm Molten salts (to 1100 o F), molten metals (to 1400 o F) for higher temperatures Fired furnaces (to 2000 o F) for high temperatures
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5 Temperature Driving Force When two heat transfer fluids considered, 2-sided heat exchanger model used. With negligible heat losses, 1 m 2 m 2 in T 2 out T 1 in T 1 out T ( 29 ( 29 1 1 1 1 2 2 2 2 P out in P in out m Q m C T T m C T T UA T = - = - = energy balances transport equation m T : mean temperature driving force determines required heat exchange area smallest ΔT at ends: minimum temperature approach
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Temperature Driving Force Common design strategy: specify inlet conditions of each stream pressure drops a minimum approach temperature Compute end for minimum, exit temperatures, heat duty Optimal minimum approach temperature depends on conditions: cryogenic: 1-to-2°F; 10 o F or less for temperatures < ambient ambient: 10°F; 20 o F for ambient to 300 o F high temperature: 100°F When one fluid is boiled: nucleate boiling: ΔT's 20°F to 45°F natural convection: ΔT's < 10°F film boiling: ΔT > 100°F transition region:
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This note was uploaded on 11/14/2011 for the course CHEN 4520 taught by Professor Wiemer during the Fall '11 term at Colorado.

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Class 7 - Class 7 Heat Exchanger Design 1 Simplified View...

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