Page 447 - Mechanical Engineers' Handbook (Volume 4)

436 Refrigeration

Vapor density (or molecular weight) is an important characteristic when the compressor
is centrifugal because the lighter gases require more impellers for a given pressure rise, that
is, head, or temperature lift. On the other hand, centrifugal compressors have a limitation
connected with the acoustic velocity in the gas, and this velocity decreases with the increas-
ing molecular weight. Low vapor densities are desirable to minimize pressure drop in long
suction and discharge lines.
Liquid density should be taken into account. Liquid velocities are comparatively low,
so that pressure drop is usually no problem. However, static head may affect evaporator
temperatures, and should be considered when liquid must be fed to elevated parts of the
system.
Latent heat should be high because it reduces the quantity of refrigerant that needs to
be circulated. However, large flow quantities are more easily controlled because they allow
use of larger, less sensitive throttling devices and apertures.
Refrigerant cost depends on the size of the installation and must be considered both
from the standpoint of initial charge, and of composition owing to losses during service.
Although a domestic refrigerator contains only a few dollars worth of refrigerant, the refrig-
erant in a cooling system for a typical chemical plant may cost thousands of dollars.
Other desirable properties. Refrigerants should be stable and noncorrosive. For heat-
transfer considerations, a refrigerant should have low viscosity, high thermal conductivity,
and high specific heat. For safety to life or property, a refrigerant should be nontoxic and
nonflammable, should not contaminate products in case of a leak, and should have a low-
leakage tendency through normal materials of construction.
With a flammable refrigerant, extra precautions have to be taken in the engineering
design if it is required to meet the explosion-proof classification. It may be more economical
to use a higher cost, but nonflammable, refrigerant.

4.3 Refrigerant Selection for the Open Cycle
Process gases used in the open cycle include chlorine, ammonia, and mixed hydrocarbons.
These gases create a wide variety of operating conditions and corrosion problems. Gas char-
acteristics affect both heat exchangers and compressors, but their impact is far more critical
on compressor operation. All gas properties and conditions should be clearly specified to
obtain the most economical and reliable compressor design. If the installation is greatly
overspecified, design features result that not only add significant cost but also complicate
the operation of the system and are difficult to maintain. Specifications should consider the
following:

Composition. Molecular weight, enthalpy–entropy relationship, compressibility factor, and
operating pressures and temperatures influence the selection and performance of compressors.
If process streams are subject to periodic or gradual changes in composition, the range of
variations must be indicated.

Corrosion. Special materials of construction and types of shaft seals may be necessary for
some gases. Gases that are not compatible with lubricating oils or that must remain oil-free
may necessitate reciprocating compressors designed with carbon rings or otherwise made
oilless, or the use of centrifugal compressors designed with isolation seals. However, these
features are unnecessary on most installations. Standard designs usually can be used to
provide savings in initial cost, simpler operation, and reduced maintenance.