Page 198 - Chemical process engineering design and economics

180 Chapter 4

where the overall heat-transfer coefficient for the existing heat exchanger is calcu-
lated from Equation 4.20.

(4.20)
Q = U 0 A 0 F(At) LM
Thus, ROA can be calculated from Equation 4.19 after calculating U from
oC
Equation 4.16 and U 0 from Equation 4.20.
Next, add the fouling resistances caused by the inside and outside scale,

RoR = (Rfi)R + (Rfo)R (4-21)

where R^R is the required combined fouling resistance. Ro is calculated using
R
individual fouling resistances obtained from Table 4.3, assuming that one to one-
and-a-half years of service before cleaning is optimum.
For an existing heat exchanger to be adequate for new process conditions,

ROA ^ ROR (4-22)

A value of RoA larger than ROR means that the heat exchanger will last longer than
the optimal time before cleaning. For any value of Ro A smaller than ROR, the heat
exchanger will operate at less than the optimum time.
Tabele 4.7 lists the equations for rating heat exchangers and Table 4.8
outlines thecalculating procedure.

Example 4.3 Rating an Ammonia Condenser__________________

It is required to condense 650 kg/h (1430 Ib/h) of ammonia vapor at 14.8 bar
(14.6 atm) using water. The available heat exchanger is a 1-2 heat exchanger
2
with 46 m 2 (495 ft ) of surface area. The enthalpy of vaporization is 261.4
kcal/kg. Is this heat exchanger adequate for this service? Show why or why not.
Follow the solution procedure.
From Table 4.3, the following conservative values of the heat-transfer coef-
ficients and fouling resistances are selected. Because water is dirtier than ammo-
nia, locate the water on the tube side. Also, a condensing vapor is usually located
on the shell side.
2
hj = 5000 W/m -K
2
^ = 8000 W/m -K
2
4
R fi = 2.5 x KT m -k/W
4
2
R f o =lxlO~ m -kAV
Assuming that there is no subcooling of the condensed ammonia, from
Equations 4.7.1 and 4.7.2,

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