《化学工程设计技术》课程教学资源（参考资料，英文版）3 Heat Exchanger Design

HEAT EXCHANGER DESIGN GOALS: o Role ofheat exchangers in chemical processing o Basic concepts and terminology Types ofheat exchangers Design methodology 今S E Design → Rating /2499 Heat Exchanger desigr

8/24/99 Heat Exchanger Design 1 HEAT EXCHANGER DESIGN GOALS: u Role of heat exchangers in chemical processing u Basic concepts and terminology u Types of heat exchangers u Design methodology ð Sizing ð Design ð Rating

PURPOSE OF A HEAT EXHANGER o To Heat or Cool a stream flowing from item of equipment to another. The steam may be a ey A liquid 中Ag E A multiphase mixture o To Vaporize a liquid stream To Condense a vapor stream /2499 Heat Exchanger desigr

8/24/99 Heat Exchanger Design 2 PURPOSE OF A HEAT EXHANGER u To Heat or Cool a stream flowing from item of equipment to another. The steam may be a: ð A liquid ð A gas ð A multiphase mixture u To Vaporize a liquid stream u To Condense a vapor stream

SINGLE TUBE PASS. SINGLE SHELL PASS COUNTERCURRENT HEAT EXCHANGER Shell side Shell side Outlet Tube side Tube side Outlet Inlet nozzle nozzle 8/24/99 Heat Exchanger desi

8/24/99 Heat Exchanger Design 3 Shell Side Inlet Nozzle Shell Side Outlet Nozzle Tube Side Inlet Nozzle Tube Side Outlet Nozzle SINGLE TUBE PASS, SINGLE SHELL PASS COUNTERCURRENT HEAT EXCHANGER

TYPES OF HEAT EXCHANGER SERVICE o Fluid heated by external utility E Steam Ey Hot oil or molten salt E Combustion gas (furnace) E Electricity(resistive, inductive, microwave ◆ Fluid cooled by external utility E Cooling water E Refrigeration o Fluid heated or cooled by other process stream /2499 Heat Exchanger desigr

8/24/99 Heat Exchanger Design 4 TYPES OF HEAT EXCHANGER SERVICE u Fluid heated by external utility ð Steam ð Hot oil or molten salt ð Combustion gas (furnace) ð Electricity (resistive, inductive, microwave) u Fluid cooled by external utility ð Cooling water ð Refrigeration u Fluid heated or cooled by other process stream

TYPES OF HEATING AND COOLING CURVES Sensible heat- heating Sensible heat-Cooling T(K) Pure Component Condensation or vaporization Multicomponent Cooling/Condensation DUTY Q(KJ/s) /2499 Heat Exchanger Desigr

8/24/99 Heat Exchanger Design 5 DUTY Q (kJ/s) T (K) Sensible Heat - Heating Sensible Heat - Cooling Pure Component Condensation or Vaporization Multicomponent Cooling/Condensation TYPES OF HEATING AND COOLING CURVES

CALCULATION OF COOLING CURVES o Sensible Heat: 0=W Cp Tin-Tout For constant Cp:Q=w(Hin-Hout otherwise ◆ latent heat:Q=Wλ o Multicomponent Cooling Curves: Requires point-by- point flash calculations /2499 Heat Exchanger desigr

8/24/99 Heat Exchanger Design 6 CALCULATION OF COOLING CURVES u Sensible Heat: Q = W Cp (Tin - Tout) (For constant Cp: Q = W (Hin - Hout) otherwise) u Latent Heat: Q = W l u Multicomponent Cooling Curves: Requires point-by￾point flash calculations

FEASIBLE COOLING CURVE PAIRINGS Corollary of the Second Law of Thermodynamics Heat can only be transferred from a higher temperature to a lower one For heat exchangers this means that the higher temperature cooling curve and the lower temperature heating curve cannot intersect. o When this condition is satisfied, the pairing of a heating and cooling curve is said to be feasible /2499 Heat Exchanger desigr

8/24/99 Heat Exchanger Design 7 FEASIBLE COOLING CURVE PAIRINGS u Corollary of the Second Law of Thermodynamics: Heat can only be transferred from a higher temperature to a lower one. u For heat exchangers this means that the higher temperature cooling curve and the lower temperature heating curve cannot intersect. u When this condition is satisfied, the pairing of a heating and cooling curve is said to be feasible

FEASIBLE COOLING CURVE PAIRINGS TIin Tli Countercurrent Flow Cocurrent flow Tout Tout Tout Tout T2in T2in Q 0 INFEASIBLE COOLING CURVE PAIRINGS T Q Q /2499 Heat Exchanger desigr

8/24/99 Heat Exchanger Design 8 T T1in Q T1out T2in T2out Countercurrent Flow T2in T1in T1out T2out Q T Cocurrent Flow FEASIBLE COOLING CURVE PAIRINGS T Q T Q INFEASIBLE COOLING CURVE PAIRINGS

MAXIMUM HEAT EXCHANGER DUTY Q 2max, the maximum amount of heat that can be transferred in a heat exchanger, no matter how large it lS, occurs when the heating ana cooling curves eltner E Intersect at one end of the exchanger or the other or E Become tangent within the exchanger o For sensible heating with constant fluid heat capacities, the curves are straigh ght lines. They will intersect at that end of the exchanger whose entering fluid has the largest wCp, call it Wimax. /2499 Heat Exchanger desigr

8/24/99 Heat Exchanger Design 9 MAXIMUM HEAT EXCHANGER DUTY u Qmax, the maximum amount of heat that can be transferred in a heat exchanger, no matter how large it is, occurs when the heating and cooling curves either ð Intersect at one end of the exchanger or the other or ð Become tangent within the exchanger u For sensible heating with constant fluid heat capacities, the curves are straight lines. They will intersect at that end of the exchanger whose entering fluid has the largest WCp, call it Wcpmax

MAXIMUM HEAT EXCHANGER DUTY Tlin Tout Tout= T2in Q Q max For this example, wCp2>wCpl /2499 Heat Exchanger desigr

8/24/99 Heat Exchanger Design 10 MAXIMUM HEAT EXCHANGER DUTY Q T T1in T2out T1out = T2in Qmax 0 For this example, WCp2 > WCp1