Page 215 - Chemical Process Equipment - Selection and Design

8.4. DATA OF HEAT TRANSFER COEFFICIENTS 185

TABLE 8.5. Ranges of Overall Heat Transfer Coefficients in Various Types of
Exchangers [ U Btu / (hr)(sqft)("F)] a
Equipment Process Lp
Shell-and-tube exchanger [Fig. gas (1 am)-gas (1 atm) 1-6
8.Wl gas (250 atm)-gas (250 atm) 25-50
liquid-gas (1 atm) 2-42
liquid-gas (250 atm) 35-70
liquid-liquid 25-200
liquid-condensing vapor 50-200
Double-pipe exchanger [Fig. 8.4(a)l gas (1 atm)-gas (1 atm) 2-6
gas (250 atm)-gas (250 atm) 25-90
liquid-gas (250 atm) 35-100
liquid-liquid 50-250
Irrigated tube bank water-gas (1 atm) 3-1 0
water-gas (250 atm) 25-60
water-liquid 50-1 60
water-condensing vapor 50-200
Plate exchanger [Fig. 8.8(all water-gas (1 atml 3-10
water-liquid 60-200
Spiral exchanger [Fig. 8.8(c)l liquid-liquid 120-440
liquid-condensing steam 160-600
Compact [Fig. 8.6(h)l gas (1 am)-gas (1 atm) 2-6
gas (1 atm)-liquid 3-10
Stirred tank, jacketed liquid-condensing steam 90-260
boiling liquid-condensing steam 120-300
water-liquid 25-60
Stirred tank, coil inside liquid-condensing steam 120-440
water-liquid 90-210
a 1 Btu/(hr)(sqft)("F) = 5.6745 W/m' K
Data from (HEDH, 1983).

borne in mind that very few proposed correlations are more difference is 234.5 and Ng = 4.48. The height of a contact zone then
accurate than &20% or so. is obtained as the product of the number of transfer units and the
Along with, rate of heat transfer, the economics of practical height Hg of a transfer unit. Several correlations have been made of
exchanger desi,gn requires that pumping costs for overcoming the latter quantity, for example, by Cornell, Knapp, and Fair (1960)
friction be taken into account. and modified in the Chemical Engineers Handbook (1973, pp.
18.33, 18.37). A table by McAdams (1954, p. 361) shows that in
DIRECT CONTACT OF HOT AND COLD STREAMS spray towers the range of Hg may be 2.5-10 ft and in various kinds
of packed towers, 0.4-4 ft or so.
Transfer of heat by direct contact is accomplished in spray towers, Heat transfer coefficients also have been measured on a
in bowers with a multipnicity of segmented baffles or plates (called volumetric or cross section basis. In heavy hydrocarbon fraction-
shower decks), and in a variety of packed towers. In some processes ators, Neeld and O'Bara (1970) found overall coefficients of 1360-
heat and mass transfer occur simultaneously between phases; for 3480 Btu/(hr)("F)(sqft of tower cross section). Much higher values
example, in water cooling towers, in gas quenching with water, and have been found in less viscous systems.
in spray or rotary dryers. Quenching of pyrolysis gases in transfer Data on small packed columns were correlated by Fair (1972)
lines or towers and contacting on some trays in fractionators may in the form
involve primarily heat transfer. One or the other, heat or mass
transfer, may be the dominant process in particular cases. Ua = CG"L", Btu/(hr)(cuft)("F), (8.34)
Data of direct contact heat transfer are not abundant. The
literature has been reviewed by Fair (1972) from whom specific data where the constants depend on the kind of packing and the natures
wilE be cited. of the fluids. For example, with air-oil, lin. Raschig rings, in an
One rational measure of a heat exchange process is the number 8 in. column
of transfer units. In term of gas temperatures this is defined by
Ua = 0.083G0.94L0.25. (8.35)

When G and L are both 5000 lb/(hr)(sqft), for instance, this formula
gives Uu = 2093 Btu/(hr)(cuft)("F).
The logarithmic mean temperature difference usually is applicable. In spray towers, one correlation by Fair (1972) is
For example, if the gas goes from 1200 to 150°F and the liquid
.
Btu/
countercurrently from 120 to 400"F, the mean temperature hga = 0.043 G 0.8L0.4/Z0-5 (hr) (cuft) ("F) (8.36)