Heat Exchangers
DOE-HDBK-1018/1-93
TYPES OF HEAT EXCHANGERS
(2-1)
D Tlm
D T2 D T1
lnD T2
D T1
Heat transfer in a heat exchanger is by conduction and convection. The rate of heat
transfer, "Q", in a heat exchanger is calculated using the following equation.
(2-2)
Q
UoAoD Tlm
Where:
=
Heat transfer rate (BTU/hr)
Q
Uo = Overall heat transfer coefficient (BTU/hr-ft2-F)
Ao = Cross sectional heat transfer area (ft2)
DTlm =
Log mean temperature difference (F)
Consider the following example of a heat exchanger operated under identical conditions as a
counter flow and then a parallel flow heat exchanger.
T1
=
represents the hot fluid temperature
T1in
=
200F
T1out
=
145F
Uo = 70 BTU/hr-ft2-F
Ao = 75ft2
T2
=
represents the cold fluid temperature
T2in
=
80F
T2out
=
120F
Counter flow DTlm =
(200 120oF) (145 80oF)
ln
(200 120oF)
(145 80oF)
72oF
Rev. 0
ME-02
Page 7
