Transcript Chapter7
Condensation and
Boiling
Until now, we have been considering
convection heat transfer in homogeneous
single-phase (HSP) systems
Boiling and condensation, however,
provide much higher heat transfer rates
than those possible with the HSP systems
Condensation
Condensation occurs when the temperature of a vapor is reduced
below its saturation temperature
Condensation heat transfer
Film condensation
Drop wise condensation
Heat transfer rates in drop wise condensation may be as much as
10 times higher than in film condensation
Laminar Film condensation on a
vertical wall (VW)
y
y
y
x
x
Tsat
y
A
g
T
(x)
u
l y y
A
A
Condensate Film
( v )gAy
l
u
l y yy
A
Laminar Film condensation on a
vertical wall (cont..)
1/ 4
3 1/ 4
l
4xkl (Tsat Tw ) l
h fg g(l v )k
( x )
h(x)
4x (Tsat Tw ) l
h fg g(l v )
Averagecoeff.
h fg g(l v )k
h L 0.943
L(Tsat Tw ) l
3
l
where L is theplatelength.
T otalheat trans
fer rate:
q h L A(Tsat Tw )
Condensation rate:
h L A(Tsat Tw )
q
m
h fg
h fg
1/ 4
Example
Laminar film condensation of steam
Saturated steam condenses on the outside of a 5 cm-diameter
vertical tube, 50 cm high. If the saturation temperature of the
steam is 302 K, and cooling water maintains the wall temperature
at 299 K, determine: (i) the average heat transfer coefficient, (ii)
the total condensation rate, and (iii) the film thickness at the
bottom of the tube.
Given: Film condensation of saturated steam
Required: (i) Average heat transfer coefficient, (ii) total
condensation rate, (iii) and film thickness
1. Effect of tube curvature negligible
2. Effect of liquid sub cooling negligible
3. Laminar
Example (contd...)
y
x
The Average heat trasn sfer coefficent is given by :
Tsat
y
A
1/ 4
g
_
h
'
3
h g( v )k
l
l
0.943 fg
L(Tsat Tw )v
l
T
(x)
Condensate Film
Evaluate hfg at the saturation temperature of
302 K
From Table of water properties :
h
fg
2.432 106 J / kg
v 0.03kg / m3
Example (contd...)
Also, for water
k l 0.611W/mK
l 996 kg/m3
l 0.87 10-6 m 2 /s
h fh g l v k l3
h 0.943
L
T
T
sat
w
l
1/ 4
( 2.432 10 )(9.81)996 0.03(0.611)
0.943
6
(
0
.
5
)(
3
)(
0
.
87
10
)
6
3
1/ 4
7570 W/m2 K
(ii) T he totalcondensation rateis :
Q
h AT (7570)(3) (0.05)(0.5)
4
m
7
.
33
10
kg/s
6
h fg
h fg
( 2.432 10 )
Example (contd...)
(iii) T hefilm thicknessis
1/ 3
3 l
v l
l g
T hemass flow rate per unit widthof film is :
m
(7.33 104 )
4.67 103 kg/ms
D
( )(0.05)
3(0.87 10 )(4.67 10 )
Hence,
(996)(9.81)
-6
3
1/ 3
1.08 104 m
Boiling
Boiling occurs when the surface temperature Tw exceeds
the saturation temperature Tsat corresponding to the liquid
pressure
Heat transfer rate: qs h(Tw Tsat ) hTe
where Te Tw Tsat (excess temperatu
re)
Boiling process is characterized by formation of vapor
bubbles, which grow and subsequently detach from the
surface
Bubble growth and dynamics depend on several factors such
as excess temp., nature of surface, thermo physical
properties of fluid (e.g. surface tension, liquid density, vapor
density, etc.). Hence, heat transfer coefficient also depends
on those factors.
Pool Boiling Curve
qs
Te
Modes of Pool Boiling
Free convection boiling Te 5 C
Nucleate boiling
Transition boiling
Film boiling
5 C Te 30 C
30 C Te 120 C
Te 120 C