Page 222 - Chemical Process Equipment - Selection and Design

192 HEAT TRANSFER AND HEAT EXCHANGERS

TABLE 8.10-(confinued)
On a bank of N horizontal tubes: r= condensation rate per unit length from the bottom tube

C. Boiling
Single immersed tube: 4 heat flux (W/mZ), p, =critical pressure, bars, p, = p/p,
a = 0.1000q0.7p0.69[l .8p?.17 + 4p:.’+ lOp:O], W/m’ K (37)
Kettle and horizontal thermosiphon reboilers
+
a= 0.27 e~p(-O.O27BR)q~.~p~~~p:.’~ (38)
an=
BR = difference between dew and bubblepoints (“K); if more than 85, use 85
&“C = [ 250 W/m2 K, for hydrocarbons
1000 W/rn K, for water

Critical heat flux in kettle and horizontal therrnosiphon reboilers
qmax = 80,700pcp~.35(1 - pr)0.9~b, W/mZ
qb = (external peripheral surface of tube bundle)/
(total tube area); if B0.45, use 0.45

Boiling in vertical tubes : th er mosi p h on re boi I ers
Critical heat flux: p, critical pressure, bars; Di tube ID, m; L tube length, rn
-
p,),
4 = 393,000(D~/L)0~35p~1p~~z5(1 W/m’
Heat transfer coefficient with Eqs. (42)-(48) and following procedure
ac=0.023(y7 O.* ( ), 0.4A iF
m(1-x)

F= 1 for 1/X, 50.1
F=2.35(1/Xff + 0.213)0.736 for I/%, >0.1
S= 1/(1 + 2.53 x
x, = [(l - x~/xlo~g~P,/P,~o~5~S//S9~o~1
Procedure for finding the heat transfer coefficient and required temperature difference when
the heat flux 0, mass rate of flow m and fraction vapor x are specified
1. Find &, Eq. (48)
2. Evaluate Ffrom Eqs. (45), (46)
3. Calculate a=, Eq. (43)
4. Calculate Re, = mF’.25(l - x)D/q,
5. Evaluate S from Eq. (47)
6. Calculate anb for a range of values of AT,,,
7. Calculate atp from Eq. (42) for this range of AT,,, values
8. On a plot of calculated q = afpATs,, against atp, find the values of atp and ATsat
corresponding to the specified q