Page 105 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10B 5/30/2001 4:18 PM Page 73

Heat Transfer 73

T 1 210°F 190
For tube side: m 155,000 lb/hr T 1
Cp 0.55 Btu/lb (°F) 180
t 1 135°F; t 2 168°F
170
Now, estimate the exchanger performance if the hot fluid 160
(shell-side) is to be increased 35% greater than the original 150
2
design: A 187 ft . Note that the method does not specifi-
cally incorporate fouling, but it should be acknowledged. 140
Determine the outlet temperature when U is established. TEMPERATURE °F 130

1. On shell side: M 1.35 (65,000) 87,750 lb/hr; the Cp s and 120 DEWPOINT 120°F.
T 1 are kept the same. t
2. On tube side: m, Cp, t 1 , U, and A remain the same. 110 2
T (105°F)
2
T 1 T 2
3. Now calculate R m Cp >MCPs 100
105°F
t 2 t 1
90 (110 – 5) t (90°F)
1
1155,0002 10.552 > 187,7502 10.582
80
1.675 0 0.5 1.0 1.5 2.0
4. UA/(mc) (170)(187) / (155,000)(0.55) 0.37, see Figure HEAT LOAD, BTU/HR. (MM)
10-35.
5. Using Figure 10-35, at UA/mc 0.37 and R 1.675, read P Figure 10-37. Finding a counterflow weighted MTD. (Reprinted
0.25. with permission: Gulley, Dale E., Heat Exchanger Design Handbook,
6. Now using Figure 10-34A, at P 0.25 and R 1.675, read F © 1968 by Gulf Publishing Company, Houston, Texas. All rights
0.954. reserved.)
7. Using P and R to find exit temperatures:
t 2 P(T 1 t 1 ) t 1 0.25 (210 133) 133 152°F Subscripts:
T 2 T 1 R(t 2 t 1 ) 210 1.675 (152 133) 178°F
1 Inlet
2 Outlet
This same concept incorporating the TEMA charts can be
2
used to (1) determine the ft heat transfer area required for Weighted Mean Temperature Difference (MTD) applies to
an exchanger and (2) determine flow rate and outlet tem- the more complicated shell and tube heat exchangers. Gul-
59
135
perature of the fluids (shell or tube side) . ley discusses several important cases in which the conven-
tional LMTD for fluid temperature change and the
where corresponding MTD correction factors (Figure 10-34A—J) do
2
A Heat exchanger surface area, ft . not adequately represent the design requirements, see Figure
Cp Specific heat capacity (tube or cold side), 10-37. From the sample listing that follows, recognize that the
Btu/(lb)(°F) heat release for each section of an exchanger is necessary to
Cp s Specific heat capacity (shell or hot side), properly analyze the condition. It can be misleading if end
Btu/(lb)(°F) point conditions previously cited are used to describe some of
F MTD correction factor, Figure 10-34. the special cases. It is necessary to break the heat transfer cal-
F Heat exchanger efficiency, dimensionless.
m Mass flow rate (tube or cold side), lb/hr. culations into zones and calculate the weighted MTD. Typical
59, 70
M Mass flow rate (shell or hot side), lb/hr. services requiring the use of weighted MTS’s are
P (t 2 t 1 ) / (T 1 t 1 ), Figure 10-34.
q Rate of heat transfer, Btu/hr. 1. Overhead condensers with steam and hydrocarbon
R Ratio of the heat capacities of tube-side to condensing.
shell-side fluid, dimensionless 2. Exchangers with change of phase.
3. Amine overhead condensers.
1T 1 T 2 2
, Figure 10-34. 4. Pure component condensers with subcooling.
1t 2 t 1 2
5. Condensers with large desuperheating zones such as
t Temperature of tube-side fluid, °F.
for refrigerants, chemicals, and steam.
T Temperature of shell-side fluid, °F.
6. Pure component vaporizing with superheating.
T 1m Log mean temperature difference for counter-
7. Vertical reboilers in vacuum service.
current flow, °F.
8. Desuperheating-condensing-subcooling.
U Overall heat transfer coefficient in exchanger,
9. Condensing in presence of noncondensable gases.
2
Btu/(ft ) (hr)(°F).