Page 228 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10F 5/30/2001 4:35 PM Page 191

Heat Transfer 191

fp process-side fouling k l 33,600D i G t 4 0.8 3c l l 4 0.4
F friction h 1 0.023 (10-175)
d i 3 l,b 43k l 4 b
g gas phase
h heating medium
7. Select mechanical features of the vertical reboiler
i inlet (feed leg) piping system
1
a. Tubes are preferably 1-in. minimum O.D., 1 / 4 -in.,
m mass
1
1 / 2 —2-in. maximum.
t tube
l L liquid base b. Vertical, with tube length preferable 6 ft to a maxi-
b boiling mum of 12 ft.
p process on boiling side 8. Determine average values
tp two phase a. Two-phase density, tp , at x E /3.
A point A in flow loop, Figure 10-110 b. Pressure drop factor,
, at 2x E /3;
is from Figure
B point B in flow loop, Figure 10-110 10-113.
C point C in flow loop, Figure 10-110
Obtain X tt to use with Figure 10-113.
E reboiler exit system
Due to slippage effects of the gas phase past the liq-
uid phase, the R L is not a simple function of the
At pressures greater than about 100 psig, the slope of
weight fraction of vapor. Using the parameter: 45
the vapor pressure curve, ( t/ p) s is low enough not
to influence the sensible heating zone equation, as
X tt = Correlation parameter, dimensionless (tt refers to the
most of the tube is in vaporization. However, at low turbulent-turbulent flow mechanism)
pressures and vacuum service, a large portion of the
tube is in sensible heat. W L 0.9 g 0.5 L 0.1
The pressure at the inlet to the tubesheet at point B, 45 X tt a b a b a b (10-176)
W g L g
Figure 10-110:
1 x 0.9 g 0.5 L 0.1
a b a b a b (10-177)
1Z A Z B 2 L g L 1 F in 2 x
p B p A (10-172) L
144g c 144
Often, approximately:
Static pressure loss in outlet leg where density, tp , varies with
vaporization: W L g 0.5
X tt a b (10-178)

P static g>g c tp dZ (10-173) W g L
where
where
X tt correlation for turbulent-turbulent flow,
F friction loss at inlet, ft of liquid
dimensionless
Z vertical height, ft W mass flow rate, lb/sec
2
p B boiling pressure, lb/in abs
g acceleration of gravity, ft/sec 2
c. Determine tp and
at exit conditions.
Subscripts
tp two phase
A point A in flow loop
(10-179)
tp g R g l R l
B point B in flow loop
and; R l (1 R g ), volumetric fraction of liquid at
5. The calculation of the term for Equation 10-170, Figure
any point along the vertical tube. Read R l using Fig-
10-110:
ure 10-114 and 10-115.
l g c F B C
p> L (10-174)
144g L
R l volume fraction of liquid phase, dimensionless
R g volume fraction of vapor phase, dimensionless
Fair states that the second term in the preceding equation
45
may be neglected. Fair reports that typical units show the 2
Obtain
from Figure 10-116, for Equation 10-181
sensible heating zone at 4—60% for T 20°F, and 4—49%
for both the average conditions and the exit condi-
for T 30°F for selected organics and also water. The val-
tions
ues vary with pressure, Table 10-30.
1 g 2 0.5 1 l 2 0.1
6. The convection heat transfer rate inside the tubes is 9. X tt (10-180)
expressed by the Dittus-Boelter equation: 45, 82 1 l 2 1 g 2 1W l >W g 2 0.9