Page 183 - 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

26 Cryogenic Process Engineering

design principles applicable at ambient temperatures are 2. Centrifugal Compressors
also valid for low-temperature design. However, underly-
Technological advances achieved in centrifugal compres-
ing each aspect of such a design must be a thorough under-
sor design have resulted in improved high-speed compres-
standing of the effect of temperature on the properties of 3
sion equipment with capacities exceeding 280 m /sec in
the fluids being handled and the materials of construction
a single unit. As a consequence of their high efficiency,
being selected.
better reliability, and design upgrading, centrifugal com-
pressors have become accepted for low-pressure cryo-
A. Compression Systems genic processes such as air-separation and base-load LNG
plants.
Compression power accounts for more than 80% of the
Separately driven centrifugal compressors are adapt-
total energy required in the production of industrial gases
able to low-pressure cryogenic systems because they can
andtheliquefactionofnaturalgas.Inordertominimizethe
be coupled directly to steam turbine drives, are less critical
cost and maintenance of cryogenic facilities, special care
from the standpoint of foundation design criteria, and lend
must be exercised to select the appropriate compression
themselves to gas turbine or combined cycle applications.
system. The three major types of compressors widely used
Isentropic efficiencies of 80 to 85% are usually obtained.
today are reciprocating, centrifugal, and screw. Currently,
there is no particular type of compressor that is generally
preferred for all applications. The final selection will ul- 3. Screw Compressors
timately depend on the specific application as well as the
effect of plant site and existing facilities. Most screw compressors are of the oil-lubricated type.
There are two types—the semihermetic and the open-drive
type. In the former, the motor is located in the same hous-
1. Reciprocating Compressors
ing as the compressor, while in the latter the motor is lo-
The key feature of reciprocating compressors is their cated outside of the compressor housing and thus requires
adaptability to a wide range of volumes and pressures with a shaft seal. The only moving parts in screw compressors
high efficency. Some of the largest units for cryogenic gas are two intermeshing helical rotors. The rotors consist of
4
production range up to 15,000 bhp (1.12 × 10 kW) and one male lobe, which functions as a rolling piston, and a
use the balanced–opposed machine concept in multistage female flute, which acts as a cylinder. Since rotary screw
designs with synchronous motor drive. When designed for compression is a continuous positive-displacement pro-
multistage, multiservice operation, these units incorporate cess, no surges are created in the system.
manual or automatic, fixed or variable, volume clearance Screw compressors require very little maintenance be-
pockets and externally actuated unloading devices where cause the rotors turn at conservative speeds and are well
required. Balanced–opposed units not only minimize vi- lubricated with coolant oil. Fortunately, most of the oil
brations, resulting in smaller foundations, but also allow can easily be separated from the gas in screw compres-
compact installation of coolers and piping, further increas- sors. Typically, only small levels of impurities of between
ing the savings. 1.0 and 2.0 ppm by weight remain in the gas after compres-
Operating speeds of larger units are as high as 277 rpm sion. Charcoal filters can be used to reduce the impurities
with piston speeds for air service up to 4.3 m/sec. The below this level.
larger compressors with provision for multiple services A major advantage of screw compressors is that they
reduce the number of motors or drivers and minimize the permit the attainment of high-pressure ratios in a single
accessory equipment, resulting in lower maintenance cost. mode. To handle these same large volumes with a recip-
Compressors for oxygen service are characteristically rocating compressor would require a double-stage unit.
operated at lower piston speeds, of the order of 3.3 m/sec. Because of this and other advantages, screw compressors
Maintenance of these machines requires rigid control of are now preferred over reciprocating compressors for he-
cleaning procedures and inspection of parts to ensure lium refrigeration and liquefaction applications. They are
the absence of oil in the working cylinder and valve competitive with centrifugal compressors in other appli-
assemblies. cations as well.
Engine drivers of the variable-speed type can generally
operate over a 100 to 50% variation in the design speed B. Expansion Devices
with little loss in operating efficiency since compressor
fluid friction losses decrease at the lower revolutions per The primary function of a cryogenic expansion device is
minute. to reduce the temperature of the gas to provide useful