Page 211 - Mechanical Engineers' Handbook (Volume 4)

200 Heat-Transfer Fundamentals

The critical heat flux (point C of Fig. 30) is given by 28
h gg ( ) 0.25 1 0.5

q fg v c l v v
c
24 2 v l
2
For a water–steel combination, q 1290 kW/m and T 30 C. For water–chrome-
c e,c
plated copper, q 940–1260 KW/m and T 23–28 C.
2
c e,c
Film Pool Boiling
The heat transfer from the surface to the liquid is due to both convection and radiation. A
total heat-transfer coefficient is defined by the combination of convection and radiation heat-
transfer coefficients of the following form for the outside surfaces of horizontal tubes:
29
h 4/3 h 4 /3 hh 1/3
c
r
where
h 0.62 k ( )g(h 0.4c T ) 1/4
3
v
vv
p,v
e
l
fg
c
D T e
v
and
r
4
5.73 10 (T T )
8
h s sat
r
T T sat
s
The vapor properties are evaluated at the film temperature T (T T )/2. The
f
s
sat
temperatures T and T are in kelvins for the evaluation of h . The emissivity of the metallic
sat
r
s
solids can be found from Table 17. Note that q hA (T T ).
s
sat
Nucleate Boiling in Forced Convection
The total heat-transfer rate can be obtained by simply superimposing the heat transfer due
to nucleate boiling and forced convection:
q q boiling q forced convection
For forced convection, it is recommended that the coefficient 0.023 be replaced by 0.014 in
the Dittus-Boelter equation (Section 2.1). The above equation is generally applicable to
forced convection where the bulk liquid temperature is subcooled (local forced convection
boiling).
Simplified Relations for Boiling in Water
For nucleate boiling, 30
h C( T ) 0.4
p
n
e
p a
where p and p are, respectively, the system pressure and standard atmospheric pressure. The
a
constants C and n are listed in Table 24.
For local forced convection boiling inside vertical tubes, valid over a pressure range of
5–170 atm, 31
3 p /1.551
h 2.54( T ) e
e
3
6
where h has the unit W/m C, T is in C, and p is the pressure in 10 N/m .
2
e