Page 229 - Chemical Process Equipment - Selection and Design

8.7. SHELL-AND-TUBE HEAT EXCHANGERS 199

EMPEE 8.9 When W=16ft, L=50ft.
Estimation of the surface Requirements of an Air Cooler Two fans will make the ratio of section length to width,
An oil is to be cooled from 300 to 150°F with ambient air at 90"F, 25/16=1.56 which is less than the max allowable of 1.8. At
with a total duty of 20 MBtu/hr. The tubes have 5/8 in. fins on 1 in. 7.5 HP/100 sqft,
OD and 2-5/16 in. triangular spacing. The tube surface is given by
A = 1.33AWL, sqft of bare tube surface, Power = __ 16(50) 7.5 = 60 HP.
100
N= number of rows of tubes, from 3 to 6,
W = width of tube bank, fb, From Figure 8.10(c), 6 rows,
L = length of tubes, ft.
According to thle data of Table 8.12, the overall coefficient may be A = 185 sqft/(MBtu/hr)
taken as U = 60 Btu/(hr)("F)(sqft of bare tube surface). Exchangers + 185(20) = 3700 sqft.
with 3 rows and with 6 rows will be examined. = 1.33(6)WL.
Approach =: 150 - 90 = 60"F,
Cooling range = 300 - 150 = 150"F, WhenW=16ft,L=29ft.
Since L/W = 1.81, one fan is marginal and two should be used:
From Figure S.9(f), 3 rows,
Power = [16(29)/100]7.5 = 34.8 HP.
A = 160 sqft/MBtu/hr)
-+ 160(201) = 3200 sqft The 6-row construction has more tube surface but takes less
= 1.33(3)iWL. power and less space.

tubes. Figure $.4(c) shows a typical construction and flow paths. 0 Thermal stresses can be accommodated inexpensively.
The versatility and widespread use of this equipment has given rise 0 A great variety of materials of construction can be used and may
to the development of industrywide standards of which the most be different for the shell and tubes.
widely observed are the TEMA standards. Classifications of
equipment and terminology of these standards are summarized on 0 Extended surfaces for improved heat transfer can be used on
Figure 8.11. either side.
Baffle pitch, or distance between baffles, normally is 0.2-1.0 A great range of thermal capacities is obtainable.
dimes the inside diameter of the shell. Both the heat transfer 0 The equipment is readily dismantled for cleaning or repair.
coefficient and the pressure drop depend on the baffle pitch, so that
rts selection is part of the optimization of the heat exchanger. The TUBE SIDE OR SHELL SIDE
window of segmental baffles commonly is about 25%, but it also is a
parameter in the thermal-hydraulic design of the equipment. Several considerations may influence which fluid goes on the tube
In order to simplify external piping, exchangers mostly are built side or the shell side.
with even numbers of tube passes. Figure 8.12(c) shows some The tube side is preferable for the fluid that has the higher
possible arrangtements, where the full lines represent partitions in pressure, or the higher temperature or is more corrosive. The tube
one head of the exchanger and the dashed lines partitions in the side is less likely to leak expensive or hazardous fluids and is more
opposite head. Partitiomng reduces the number of tubes that can be easily cleaned. Both pressure drop and laminar heat transfer can be
accommodated in a shell of a given size. Table 8.12 is of such data. predicted more accurately for the tube side. Accordingly, when
Square tube pitch in comparison with triangular pitch accommo- these factors are critical, the tube side should be selected for that
dates fewer tubes but is preferable when the shell side must be fluid.
cleaned by brushing. Turbulent flow is obtained at lower Reynolds numbers on the
Two shell passes are obtained with a longitudinal baffle, type F shell side, so that the fluid with the lower mass flow preferably goes
in Figures 8.1P(a) or 8.3(c). More than two shell passes normally on that side. High Reynolds numbers are obtained by rnultipassing
are not provided in a single shell, but a 4-8 arrangement is the tube side, but at a price.
thermally equivalent to two 2-4 shells in series, and higher
combinations are obtained with more shells in series. DESIGN OF A HEAT EXCHANGER
A substantial number of parameters is involved in the design of a
shell-and-tube heat exchanger for specified thermal and hydraulic
A wide range of design alternates and operating conditions is conditions and desired economics, including: tube diameter,
thickness, length, number of passes, pitch, square or triangular; size
obtainable with shell-and-tube exchangers, in particular:
of shell, number of shell baffles, baffle type, baffle windows, baffle
spacing, and so on. For even a modest sized design program, Bell
Single phases, condensation or boiling can be accommodated in (in HEDH, 1983, 3.1.3) estimates that 40 separate logical designs
either the tubes or the shell, in vertical or horizontal positions. may need to be made which lead to 240 = 1.10 x 10'' different paths
0 Pressure ran,ge and pressure drop are virtually unlimited, and can through the logic. Since such a number is entirely too large for
be adjusted independently for the two fluids. normal computer processing, the problem must be simplified with