Page 199 - Chemical process engineering design and economics

Process Heat Transfer 181

5
5
Q = 261.4 kcal/kg (650 kg/hr) = l.VxlO kcal/h (6.75xl0 Btu/h)
To calculate the logarithmic-mean temperature difference, the terminal tem-
peratures of the condenser must be fixed. Because the condensation is essentially
isobaric, the inlet and outlet temperatures of the ammonia stream are 41.4°C
(106.5 °F). From Table 4.1, the inlet cooling-water temperature is 30°C (86.0 °F)
if cooling-tower water is used. Also, for thermodynamic considerations the exit
water temperature must be less than 41.4°C, and it is calculated from Equation
4.7.6. If the lower value of the approach temperature difference of 5 °C (9.0 °F) is
selected from Table 4.4, a low cooling-water flow rate will be needed. Thus, exit
water temperature is 36.4°C. Therefore, from Equation 4.7.5, the logarithmic-
mean temperature difference,

41.4-36.4-(41.4-30.0)
(At) LM =——————————————— = 7.765 °C (14.0 °F)
5.0
hi ——
11.4

For isothermal condensation, the logarithmic-mean temperature difference
correction factor, F, equals one. Therefore, from Equation 4.7.3 for the existing
heat exchanger, the available overall heat-transfer coefficient,

Q
u = ————

AF(At) LM
1.7xl0 5 kcal 1 1 4.183xl0 3 J 1 h
U 0 =
h 46m 2 7.765 °C 1 kcal 3600s
2
0 = 97.5Btu/h-fV -°F.
From Equation 4.7.11, the clean overall heat-transfer coefficient,

hj ho 5000 (8000)
3
2 0
2
U oC = ——— = ——————— = 3.077xl0 W/m -K (542 Btu/h-ft - ?)
hj + ho 5000 + 8000
Then, from Equation 4.7.10, the available fouling resistance,

U oC -U 0 3077-553.5
2
2
3
ROA = ————— =——————— =1.482xlO~ m -°C/W (2.60x10" h-ft -°F/Btu)

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