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74 Applied Process Design for Chemical and Petrochemical Plants

Example 10-5. Calculation of Weighted MTD 59 except in those cases when heat losses to the atmosphere or
other outside medium are either known or planned.
A gas is to be cooled from 190°F to 105°F with partial con- Plant 130 presented a technique for comparative heat
densation taking place. The dew point is 120°F. The cooling exchange performance evaluation that is based on his effi-
water enters at 90°F and leaves at 110°F. Figure 10-37 illus- ciency method and can include almost any style and appli-
trates the basic exchanger functions. cation of exchanger.
In condensers where heat loss is desired, insulation
1. The heat load is often is omitted from piping carrying hot fluids to take
advantage of the heat loss to the atmosphere. In any heat
Q (Desuperheat) = 420,000 Btu/hr exchange equipment the heat released or lost by one fluid
Q (Condensation) = 1,260,000 Btu/hr must be accounted for in an equivalent gain by a second
fluid, provided that heat losses are negligible or otherwise
1,680,000 Btu/hr
considered.
2. The cooling water temperature rise in each zone is
Heat Load or Duty
Desuperheating: T, °F = (110 – 90) (420,000)/1,680,000
The heat load on an exchanger is usually determined by
= 5°F
the process service conditions. For example, the load on a
Condensation: T, °F = (110 – 90) (1,260,000)/1,680,000
condenser for vapors from a distillation column is deter-
= 15°F
mined by the quantity and latent heat of vaporization at the
condensing conditions, or for gas coolers, by the flow of gas
3. For desuperheating, the LMTD is
and the temperature range required for the cooling.
190 → 120
For sensible heat changes:
110 ← 90
80 15 q W c p 1t 2 t 1 2 (10-18)
Reading Figure 10-33: For latent heat changes:
LMTD 38.8°F
(10-19)
q W l v
4. For condensation, the LMTD is
For cooling (or heating) and latent heat change (con-
120 → 105 dense or boil):
105 ← 90
(10-20)
Q q' Wc p 1t 2 t 1 2 Wl v
15 15
LMTD = 15°F An item of heat exchange equipment can be used for any
of these heat changes, or any combination of them, pro-
5. The weighted LMTD for calculations is vided the loads are established to correspond with the phys-
ical and thermal changes actually occurring or expected to
Q total
Wt. LMTD occur in the unit. Thus, the heat load must be known for the
Q des Q cond
design of an exchanger, although it may be determined on
existing equipment from operating data. This latter is
1LMTD2 des 1LMTD2 cond
termed performance evaluation.
1,680,000
17.7°F
420,000 1,260,000

38.8 15
Example 10-6. Heat Duty of a Condenser
For shells in series it is necessary to develop a weighted with Liquid Subcooling
MTD correction factor, and a graphical technique is pre-
sented by Gulley. 59 The overhead condenser on a distillation column is to
subcool the condensed vapors from the condensation tem-
Heat Balance perature of 46.4°F down to 35°F. The specific heat of the
liquid is 0.3 Btu/lb (°F), and the latent heat of vaporization
In heat exchanger design, the exchange of the heat at 46.4°F is 265 Btu/lb. The vapor rate to the condenser is
between fluids is considered to be complete (i.e., 100%) 740.3 lb/hr. What is the total heat load on the condenser?