ME 150 – Heat and Mass Transfer
Chap. 17: Heat Exchangers
Heat Exchangers A heat exchanger is a thermo-mechanical device that transfers heat from one working medium to another. Both working media are typically fluids (liquid or gas).
Concentric tubular HX Prof. Nico Hotz
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ME 150 – Heat and Mass Transfer
Chap. 17: Heat Exchangers
Household radiator
Car coolant radiator
Prof. Nico Hotz
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ME 150 – Heat and Mass Transfer
Chap. 17: Heat Exchangers
Double-pipe HX
Shell-and-tube HX
Prof. Nico Hotz
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ME 150 – Heat and Mass Transfer
Chap. 17: Heat Exchangers
Types of Heat Exchangers: Based on the flow arrangement:
1) Parallel Flow
2) Counter flow
Prof. Nico Hotz
3) Cross flow
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ME 150 – Heat and Mass Transfer
Chap. 17: Heat Exchangers
Types of Heat Exchangers: Based on the construction: Concentric tubes (double-pipe):
Finned and unfinned constructions:
Prof. Nico Hotz
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ME 150 – Heat and Mass Transfer
Chap. 17: Heat Exchangers
Shell-and-tubes construction:
Many more types of heat exchangers available: heat exchangers with fluids in direct contact, regenerators, etc.
Prof. Nico Hotz
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ME 150 – Heat and Mass Transfer
Chap. 17: Heat Exchangers
Overall Heat Transfer: Tcold flow Cold fluid:
Rtot = Rcold , conv + Rwall , cond + Rhot , conv
Rcold,conv Tcold wall
Wall:
Rwall,cond Thot wall
Hot fluid:
Rtot =
Rhot,conv
Thot flow
1 t 1 + + hcold ⋅ Acold k wall ⋅ Ac hhot ⋅ Ahot
U ⋅ Awet =
1 Rtot
→ qtot = U ⋅ Awet ⋅ (Thot , flow − Tcold , flow )
Prof. Nico Hotz
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ME 150 – Heat and Mass Transfer
Chap. 17: Heat Exchangers
Varying Fluid Temperatures along the Heat Exchanger: qtot =
1 ⋅ ΔTlm = U ⋅ Awet ⋅ ΔTlm Rtot
ΔT1 − ΔT2 with the log-mean temperature: ΔTlm = ln(ΔT1 / ΔT2 ) (where ΔT1/2 depends on configuration)
Energy Balances: qtot = m hot ⋅ c p ,hot ⋅ (Thot ,inlet − Thot ,outlet ) qtot = m cold ⋅ c p ,cold ⋅ (Tcold ,outlet − Tcold ,inlet )
Prof. Nico Hotz
Enthalpy change on hot and cold side are identical for perfectly insulated heat exchanger.
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ME 150 – Heat and Mass Transfer
Prof. Nico Hotz
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