Heat Exchangers

• Types of heat exchangers

• Constraints on temperature differences

• Materials

• Costing

Shell-and-Tube Heat Exchanger

Double-Pipe Exchanger

Spiral Heat Exchanger

Plate Heat Exchanger

Shell-and-Tube Heat Exchanger

Shell and TubeExchanger TermsFanaritis, J.P., Bevevino, J.W. "Designing shell and tube heat exchangers". Chemical Engineering, 83(13), 62-71, 1976.

Bayonet type - floating inner tubesand floating heads for large temperature differences

Heat Exchanger SelectionBrown, 1985

Shell and Tube Plate Spiral

General purposegas/liquid service

Liquids, low Psteam

Gases, boiling orcondensing

No temperaturecross

High temperaturecross OK

No to moderatetemperature cross

Low fouling Medium fouling High fouling

Good for alloys Limited space

Air-Cooled Exchangers

• Design for worst case - summer air temperatures of 30-35 C

• Typical approach temperature is 50 C• Maximum process fluid temperature ca. 260

C• Typical inlet process fluid temperature <

150 C

Ganapathy, 1978; Ulrich, 1982

Air-Cooled or Fin-Fan Exchanger

Air Cooling vs Water Cooling

Air cooling (+)• corrosive• water pollution• water consumption• maintenance

Water cooling (tower) (+)• temperature control• space• freeze up• noise• achievable T• power

Heat Integration

• Temperature profiles– crossovers– phase changes– correct delta T

• Keeping heat in the process saves utility costs– integrated processes save money– heat exchangers (always) leak

T cross

Design EquationQ U A MTD

Q=amount of heat transferred (from process design)A=heat exchanger areaMTD=mean temperature difference=fTLMf=correction factor for exchanger geometryf=1 for true counter-current; f<1 when cross-flow

TT T

TT

LMh c

h

c

ln