Page 50 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

46 Chapter 2 Heat transfer processes in industrial scale

It is often convenient to correlate heat transfer data in terms of heat transfer factor j h defined as
0:14
m
0:67
j h ¼ StðPrÞ (2.22)
m w
The j h factor correlates with Reynolds number in both laminar and turbulent flow regime, similar to
the correlation of friction factor with Reynolds number for pressure drop. The correlation can be used
for circular and noncircular pipes of uniform cross section by using hydraulic diameter instead of tube
diameter for noncircular cases. Since pipes are rougher than heat exchanger tubes, a more accurate
result can be obtained for heat transfer coefficient in exchanger tubes by rearranging Eqn. (I) in
Table 2.6 as
0:14

h i D i 1 = 3 m
(2.23)
Nu ¼ ¼ j h ½RePrŠ
k f m w
Kern (see Further Reads) defines heat transfer factor as
0:14
1=3 m
(2.24)
j H ¼ NuðPrÞ
m w
where j H ¼ j h Re
In Table 2.7,
ðh con Þ ¼ mean condensation film coefficient for a single tube (W/m C)
2
1
k con ¼ condensate thermal conductivity (W/m C)

3
r con ¼ condensate density (kg/m )
3
r ¼ vapour density (kg/m )
v
m con ¼ condensate viscosity (Pa.s)
All condensate properties are evaluated at average temperature of condensing and tube wall
temperature
2
g ¼ acceleration due to gravity (m/s )
L ¼ tube length (m)
N T ¼ total number of tubes in tube bundle
N r ¼ average number of tubes in vertical tube row
(¼two-thirds of the number of tubes in the central row)
G ¼ condensate loading per unit tube length (kg/m s)
W
G ¼ 2=3 ¼ modified condenser loading
LN
T
W ¼ condensate rate (kg/s)
L ¼ tube length (m)
n T ¼ number of tubes
ðh nb Þ and ðh fb Þ¼ nucleate pool boiling and film boiling coefficient (W/m C)
2
3
k l ; C pl ; r m ¼ thermal conductivity (W/m C), specific heat, density of liquid (kg/m ),
2
l
l
viscosity of liquid (Pa.s)
l ¼ latent heat of condensation/vaporisation (J/kg)
3
r ¼ density of vapour (kg/m )
v