Page 196 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10F 5/30/2001 4:35 PM Page 159

Heat Transfer 159

1. Heat Balance
h o
t w t a a b1T v t a 2 (10-132)
h io h o
A. Oil
where
Q mc p t 19,0002 10.42 1600 5002 360,000 Btu>hr
t w wall temperature, °F
t a average temperature of cold fluid, °F
B. DOWTHERM A T v temperature of vapor, °F
t a (500 600)/2 550°F
Q 1W2 1 2 or W Q> T v 620°F
Q DOWTHERM Q Oil 360,000 Btu>hr
h io . . . Basing the coefficient on the outside area, h i may be con-
620°F 111.3 Btu>lb 1physical property data2
verted from inside to outside as follows:
D¿ i
h io h i (10-133)
W 360,000>111 .3 3235 lb>hr D¿ o
g condensate flow 8.3 gal.>min D¿ i 0.620
0.827 (tube properties, see Table 10-3)
D¿ o 0.750
2. Log Mean Temperature Difference (LMTD) h i 360 Btu/(hr) (ft ) (°F) (given)
2
2
h io 360 0.827 298 Btu/(hr) (ft ) (°F)
620°F = T T = 620°F
1 2 To solve the preceding equation for t w , assume
t = 600°F
2
2 h o 150 Btu/(hr) (ft ) (°F).
500°F = t
1 Thus,
11502
t w 550 1620 5502
150 298
t 1 t 2
LMTD t w 550 10.3352 1702 573°F
t 1
2.3 log
Now determine h o from Figure 10-67B:
t 2
t 1 620 500 120°F
when
t 2 620 600 20°F
t T v t w 620 573 47°F
120 20 D 0.750 in.
LMTD
2.3 log 10 1120>202 (D ) ( t) (0.750) (47) 35.3
2
From the graph, h o 270 Btu/(hr) (ft ) (°F)
100
55.9°F
12.3210.7782 Because the assumed value does not agree with the
LMTD 56 F calculated value, assume h o 290, and repeat the
calculations.
290
t w 550 1620 5502
3. Clean Overall Heat Transfer Coefficient (U c )
290 298
t w 550 10.492 1702 584°F
1>U 1>h i 1>h o L>k r i ro
and t T v t w 620 584 36°F
To determine a clean overall heat transfer coefficient, 1D'2 1 t2 136.02 10.7502 27.0 and
2
let r i , r o , and L/k equal zero. Thus, h o 290 Btu>1hr2 1ft 2 1°F2
1 1 1
(10-131) This is the design value for h o as the assumed value
U c h i h o
equals the calculated value.
A. Film Coefficient on Outside (h o )
To calculate the outside film coefficient, you need B. Clean Overall Heat Transfer Coefficient (U c )
to know the difference in temperature of the con-
densing vapor (T v ) and the pipe wall temperature 1>U c 1>h io 1>h o
(t w ). The pipe wall temperature is determined by 1>U c 1>298 1>290
trial-and-error calculations using the following 1>U c 0.0036 0.00345 0.00681
equation: 70 U c 147 Btu>1hr2 1ft 2 1°F2
2