Page 306 - 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

252 Heat Exchangers

Pressure drop Decrease in static pressure of a stream
between the entrance and the exit of a heat exchanger.
Sensible heat transfer Transfer of heat required to cause
a change of temperature in a fluid. (Compare to latent
heat transfer.)
Thermal duty Total amount of heat transferred from one
stream to the other in a heat exchanger.
Vaporization Conversion of a liquid to a vapor by adding
the latent heat of vaporization to the liquid. “Boiling”
is a commonly used synonym but is not as precise.
Vaporizer Heat exchanger with the primary function of
vaporizing a liquid by transferring heat from a hot
stream. Also termed “reboiler” in some applications.

HEAT EXCHANGERS play an essential role in chemi-
cal processing. In the typical process plant, heat exchang-
ers bring the feed streams to the proper temperature for
the reactors, provide vapor and liquid reflux streams for
the separation and purification steps, and finally cool the
products for storage and shipping. But the same types
of heat exchangers are used in a wide variety of auxil-
iary services in process plants and many other places as FIGURE 1 Typical temperature profiles for several process heat
well; examples include lubricating-oil coolers for all kinds exchanger applications: (a) product cooler; (b) feed heater with
of machinery, compressor intercoolers and aftercoolers condensing stream; (c) multicomponent feed heater with vapor-
ization and superheating; (d) pure-component product condenser;
for gas pipeline systems, chillers in refrigeration and air-
(e) multicomponent product condenser; (f) typical feed-effluent
conditioning installations, and vapor generators and con- heat exchanger.
densers in conventional, nuclear, geothermal, and solar
thermal power plants. Heat exchangers come in many dif-
ferent configurations and with surface areas ranging from
2
0.1 to 100,000 m . The selection of type or configura- flow rates of the two streams. Since specific heats usually
tion of heat exchanger is governed by the nature of the vary little with temperature, the local stream temperatures
streams flowing in the exchanger (e.g., liquid or gas, high are linear functions of the heat exchanged between the
or low pressure, high or low temperature) and the service streams, as shown in Fig. 1a.
(e.g., heating or cooling, condensing, vaporizing) to be If the process fluid needs to be heated (e.g., for feed to a
performed. The size of the heat exchanger is governed by chemical reactor), the hot fluid supplying the heat is likely
the amount of heat to be transferred and the rate of heat to be saturated steam at a high enough pressure that the
transfer, which can vary by several orders of magnitude. condensing temperature is greater than the final temper-
ature of the process fluid. The heat exchanger is usually
designed so that the pressure drop in the condensing steam
is negligible compared to the static pressure, so the steam
I. APPLICATIONS IN CHEMICAL condenses at essentially constant temperature, as shown
ENGINEERING in Fig. 1b. The heat given up by the steam is the latent heat
of condensation and is equal to the sensible heat gained
A simple but common heat exchanger application in a by the process fluid.
chemical process plant is cooling a hot liquid or gas prod- A somewhat more complicated situation occurs if a liq-
uct from the process (called the “process fluid”) to a tem- uid process fluid made up of several components (e.g.,
perature low enough that it can be safely stored. The crude oil) is to be partially vaporized, possibly as a feed
coolant is likely to be air or water, which would be heated to a distillation column. The liquid heats up sensibly until
in the heat exchanger. If none of the fluids involved reach it reaches the temperature at which the first bubble of va-
their boiling or condensing temperatures, no phase change por is formed; this temperature is the “bubble point” (see
occurs, and the process fluid is “sensibly cooled” and the Fig. 1c). The bubble is richer than the liquid in the more
coolant “sensibly heated.” A heat balance relates the inlet volatile components of the mixture. As heating continues,
and outlet temperatures, the specific heats, and the mass more vapor is formed and the temperature continues to