Page 139 - Mechanical Engineer's Data Handbook

128 MECHANICAL ENGINEER'S DATA HANDBOOK

Refrigeration effect RE = cp( TI - T3) + cpq,( T3 - T,)

Work in W=cp-- (T2 - cpqt( T3 - T,)
VC
RE
Coefficient of performance COP = -
W
T, ( )~
T
where: A= ( I, qt = turbine isentropic effi- 4 I I
-=
m
m
12 13 \Pl/
ciency, qc =compressor isentropic efficiency.

3.14 Heat transfer

Heat may be transmitted by conduction, convection or
radiation.

3.14. I Conduction

Heat transfer by conduction is the transfer of heat from
one part of a substance to another without appreciable
displacement of the molecules of the substance, e.g.
heat flow along a bar heated at one end. This section
deals with conduction of heat through a flat wall, a
composite wall, a cylindrical wall and a composite
cylindrical wall. A table of thermal-conductivity coeffi-
cients is given.
x
1
Thermal resistance R = - -
=
kA UA
3.14.2 Conduction through wall
Conduction from JIuid to Jluid through wall
Let:
k=conductivity of wall, Wm-lK-' In this case the surface coefficients are taken into
A=area of wall, m2 account.
x = thickness of wall, m kA
t = temperature ("C) q=Aha(ta-tl)=-(tl -t2)= Ahb(t2-tb)
q = heat flow rate, W X
h=heat transfer coefficient, WrnW2K-'
U =overall heat transfer coefficient, Wm-2K-1
R = thermal resistance KW-
kA
Heat flow q=-(t,-t,)
X
k
Overall heat transfer coefficient U =-
X
Therefore, q= UA(t, -t2)