Page 307 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology En007c-310 June 30, 2001 17:30

Heat Exchangers 253

rise, though not as rapidly as before; that is, both sensi- and then superheated, while the hot effluent stream enters
ble and latent heat transfer are occurring to the process the exchanger as a superheated vapor and is then cooled
fluid. Thermodynamic phase equilibrium calculations are and partially condensed. This case is diagrammed in
required to find the amount and composition of the vapor Fig. 1f.
phase, the temperature of the fluid, and the amounts of It is evident that a wide variety of heat transfer processes
sensible and latent heat transfer. These calculations are an occurs in heat exchangers in chemical process plants, and,
essential part of the design of any heat exchanger involv- like snowflakes, no two cases are identical. The task of the
ing phase changes of multicomponent mixtures. If heating engineer is to select and properly size a heat exchanger,
is continued and the liquid and vapor phases are kept in in- or a system of heat exchangers, to accomplish the desired
timate contact, the last liquid (rich in the less volatile com- thermal changes in the process streams.
ponents) vaporizes at the “dewpoint” temperature. Further
heating results in superheating the vapor.
Another common problem is the condensation of vapor II. CRITERIA FOR SELECTION
from a distillation column, possibly using water or air as
the coolant. The vapor may be either a nearly pure chem- Given the large variety of process heat transfer problems
ical species or a multicomponent mixture. If nearly pure, and the heat exchanger configurations available, the en-
the vapor will condense almost isothermally at its satura- gineer must select a type and design that satisfy several
tion temperature corresponding to the vapor pressure, as criteria. These are listed approximately in the order of
shown in Fig. 1d. If multicomponent, the vapor will begin their importance, though in any individual case one crite-
to condense at its dew point and continue through the two- rion or another may move up or down in the list of relative
phase region until it reaches the bubble point and is totally importance.
condensed, as in Fig. 1e. Through the two-phase region,
both condensation (latent heat transfer) and cooling of the 1. The heat exchanger must satisfy process specifica-
mixture (sensible heat transfer) occur simultaneously. If tions; that is, it must perform the required thermal change
the condensate is further cooled below the bubble point, on the process stream within the pressure drop limita-
the liquid is said to be subcooled. tions imposed. The basic thermal design equations are
The above examples have the common feature of the discussed in a later section, and these determine the size
thermal condition of the process fluid being altered by the of the heat exchanger. Equally important to a successful
use of steam for heating or air or water for cooling. Steam design is the proper utilization of the allowed pressure
(usually available at several pressures), water, and air are drops for each stream. As a general rule, the greater the
often termed “service” or “utility” streams, and have the allowable pressure drop, the higher the fluid velocity and
common feature of being generally supplied throughout heat transfer coefficient, resulting in a smaller and less ex-
the plant as required. Other service streams include special pensive heat exchanger. However, pressure drop increases
high-temperature heat transfer liquids such as Dowtherm, with fluid velocity more rapidly than does heat transfer,
hot oil, and occasionally liquid metals; sea water and var- and pumping costs soon become prohibitive. Also, exces-
ious refrigerants may also be available as coolants. sive velocities can cause damage by cavitation, erosion,
Use of service streams to thermally modify process and vibration. Therefore, the allowable pressure drop in
streams is simple, convenient, and operationally flexible, each stream should be carefully chosen (70 kPa is a typical
but it is inefficient in terms of energy conservation. Steam value for low-viscosity liquids, and 5–10% of the absolute
has to be made by burning a fuel; cooling water has to pressure is typical for low-pressure gases and vapors), and
be cooled in a cooling tower. In the typical process plant, as fully utilized as other considerations permit.
there are many hot streams that need to be cooled and 2. The heat exchanger must withstand service condi-
many cold streams that need to be heated. If the tem- tions. The most obvious condition is that the exchanger
peratures, flow rates, and locations within the plant are construction must be strong enough to contain the fluid
satisfactory, a hot process stream can be used to heat a pressures inside the exchanger, and design standards for
cold process stream in a heat exchanger (which in this safe construction are set by the various pressure-vessel
case is often called a feed-effluent exchanger), resulting codes. There are also thermally induced stresses due to
in a more energy-efficient plant. As an example, the hot the differential expansion of the various exchanger com-
vapor effluent (product) stream from an exothermic chem- ponents. There are mechanical stresses imposed by the
ical reactor may be used to heat the cold feed stream to exchanger weight and externally by piping stresses, wind
that reactor. While each stream may pass through all of loading, and mechanical handling during shipping, in-
the processes described above, a more typical situation stallation, and maintenance. The heat exchanger must
is one in which the cold feed steam starts out as a two- withstand corrosive attack, primarily achieved by suitable
phase gas/vapor–liquid mixture and is totally vaporized selection of the materials of construction. To minimize