Page 185 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN004D-156 June 8, 2001 15:28

28 Cryogenic Process Engineering

system, although not as critical a component as the others ments, mechanical design limitations, and economic con-
mentioned in this section. In simplest terms the expansion siderations. The principal industrial exchangers finding
valve resembles a normal valve that has been modified to use in cryogenic applications are coiled-tube, plate–fin,
handle the flow of cryogenic fluids. These modifications reversing, and regenerator types.
include exposing the high-pressure stream to the lower
part of the valve seat to reduce sealing problems, a valve 1. Coiled-Tube Exchangers
stem that has been lengthened and constructed of a thin-
Construction of these widely used heat exchangers in-
walled tube to reduce heat transfer, and a stem seal that is
volves winding a large number of tubes in helix fash-
accomplished at ambient temperatures.
ion around a central core mandrel with each exchanger
containing many layers of tubes, both along the princi-
E. Heat Exchangers and Regenerators pal and radial axes. Pressure drops in the coiled tubes are
equalized for each specific stream by using tubes of equal
One of the more critical components of any low-
length and carefully varying the spacing of these tubes in
temperature liquefaction and refrigeration system is the
the different layers. A shell is fitted over the outermost
heat exchanger. This point is readily demonstrated by con-
tube layer, and this shell together with the outside sur-
sidering the effect of the heat exchanger effectiveness on
face of the core mandrel form the annular space in which
the liquid yield of nitrogen in a simple Linde-cycle liq-
the tubes are nested. Coiled-tube heat exchangers offer
uefaction process operating between a lower and upper
unique advantages, especially for those low-temperature
pressure of 0.1 and 10 MPa, respectively. The liquid yield
design conditions where simultaneous heat transfer be-
under these conditions will be zero whenever the effec-
tween more than two streams is desired, a large number
tiveness of the heat exchanger falls below 90%. (Heat ex-
of heat transfer units is required, and high operating pres-
changer effectiveness is defined as the ratio of the actual
sures in various streams are encountered. The geometry of
heat transfer to the maximum possible heat transfer.)
these exchangers can be varied widely to obtain optimum
Fortunately, most cryogens, with the exception of he-
flow conditions for all streams and still meet heat transfer
lium II, behave as “classical”fluids. As a result, it has been
and pressure drop requirements.
possibletopredicttheirbehaviorbyusingwell-established
Optimization of the coiled-tube heat exchanger is quite
principles of mechanics and thermodynamics applicable
complex. There are numerous variables, such as tube and
to many room-temperature fluids. In addition, this has per-
shell flow velocities, tube diameter, tube pitch, and layer
mitted the formulation of convective heat transfer correla-
spacing. Other considerations include single-phase and
tions for low-temperature designs of simple heat exchang-
two-phase flow, condensation on either the tube or shell
ers that are similar to those used at ambient conditions and
side, and boiling or evaporation on either the tube or shell
utilize such well-known dimensionless quantities as the
side. Additional complications come into play when mul-
Nusselt, Reynolds, Prandtl, and Grashof numbers.
ticomponent streams are present, as in natural gas lique-
However, the requirements imposed by the need to op-
faction, since mass transfer accompanies the heat transfer
erate more efficiently at low temperatures has made the use
in the two-phase region.
of simple exchangers impractical in many cryogenic ap-
Many empirical relationships have been developed to
plications. In fact, some of the important advances in cryo-
aid the optimization of coiled-tube exchangers under
genic technology are directly related to the development
ambient conditions. Many of the same relationships are
of rather complex but very efficient types of heat exchang-
currently being used in low-temperature applications as
ers. Some of the criteria that have guided the development
well. A number of these relationships are tabulated in
of these units for low-temperature service are (1) a small
readily available cryogenic texts. However, no claim is
temperature difference at the cold end of the exchanger
made that these relationships will be more suitable than
to enhance efficiency, (2) a large ratio of heat-exchange
others for a specific design. This can be verified only by
surface area to heat-exchanger volume to minimize heat
experimental measurements on the heat exchanger.
leak, (3) a high heat-transfer rate to reduce surface area,
(4) a low mass to minimize start-up time, (5) multichan-
2. Plate–Fin Exchangers
nel capability to minimize the number of exchangers, (6)
high-pressure capability to provide design flexibility, (7) a These types of heat exchangers normally consist of heat-
low or reasonable pressure drop to minimize compression exchange surfaces obtained by stacking alternate layers of
requirements, and (8) minimum maintenance to minimize corrugated, high-uniformity, die-formed aluminum sheets
shutdowns. (fins) between flat aluminum separator plates to form in-
The selection of an exchanger for low-temperature op- dividual flow passages. Each layer is closed at the edge
eration is normally determined by process design require- with solid aluminum bars of appropriate shape and size.