Chemical Engineering February 2012 - 39

FIGURE 3. The male and female screw rotors, and internal and external casing are
shown in this vertically split, oil-injected screw compressor. This machine uses an
asymmetric 4+6 screw rotor arrangement (4 male lobes and 6 female lutes), the most
common screw proile in various applications. The suction nozzle is located at the top
35 bar for a ratio of length-to-diameter
(L/D) of 1, to as low as 7 bar for L/D
= 2.3 for a screw configuration with a
4+6 profile. The reason for this limitation
is rotor deflection. The larger
the L/D value, the greater the bearing
span and thus the larger the rotor deflection.
The use of rotors with higher
rigidity (those with, for instance, a 6+8
profile combination and L/D = 1), can
provide a differential pressure capability
of as much as 50 or 60 bar.
The male and female rotors of a
screw compressor theoretically contact
each other along a sealing line.
There is a clearance distribution along
this sealing line, which influences the
efficiency and reliability of the screw
compressor. The amount of clearance
depends on a variety of factors, including
profile error, thermal deformation,
center displacement and the relative
rotational angle between rotors.
Medium and large-size screw compressors
(roughly above 0.7 MW)
typically incorporate sleeve bearings
(also called journal bearings or sliding
bearings) and self-adjusting, multisegment
thrust bearings. For difficult
services, specially designed bearings
may be used.
Since the highest loads in a screw
compressor are axial, the thrust bearing
is the component that is most
susceptible to premature failure. The
compressor should therefore be protected
by using embedded temperature
sensors mounted in the thrust bearings
(the axial bearings). The design
should allow bearings to be easily removed
without requiring the removal
of the screw rotors or the upper half on
the horizontally split machine. Oil-injected
screw compressors require only
one seal, where the driver shaft enters
the compressor casing.
Gas contamination (that is expected
during various modes of operation)
should be accurately defined by the
user. For example, handling sour gas
or other corrosive agents will dictate
material types, oil type, and other key
machine or operating considerations.
The ability to achieve the required
outlet pressure, regardless of the gas
molecular weight, gas composition
or operation changes, is another key
criteria for CPI applications. For instance,
a screw compressor that starts
on nitrogen and then gradually brings
in a hydrogen-rich gas mixture should
not change in performance. This operational
flexibility is a great advantage
compared to dynamic compressors,
such as centrifugal compressors.
The screw compressor module must
be fully integrated into the CPI plant
system. In other words, in the screw
compressor package - particularly
for the inlet system and discharge facilities
- the oil type and some other
auxiliaries should be customized to
meet the site-specific needs of each
process application.
Meanwhile, if particulate contamination
is present in the gas, solid or
liquid, especially in large quantities,
erosion and lubrication problems may
result. Thus, a properly sized knockout
drum, separator and filter should
be installed upstream of the screw
compressor inlet.
Managing lubricant oils
Oil-injected screw compressors are
typically fitted with a slide valve,
which permits the suction volume to
be reduced. This system gives operators
step-less control of capacity, while
maintaining high overall efficiency.
Oil-injected screw compressors are
usually constant-speed machines.
The use of a throttle valve to control
suction capacity may be used only for
small screw compressors, since this
approach can reduce efficiency and
also may cause some operational and
reliability problems, such as excessive
discharge temperature, internal leakage
and excessive rotor deflection.
The maximum temperature that
gases are allowed to reach is usually
determined by the lubricant type (CE,
January 2012, pp. 44-48). For mineral
lubricants, the normal maximum
temperature maximum is in the range
75-90°C (sometimes up to 95°C). Some
synthetic lubricants will permit this
discharge temperature to be extended
up to 120°C.
The quantity of oil injected into the
compressor is adjusted to make sure
that the combined gas and lubricant
discharge temperature does not exceed
the specified values. Mineral oil
viscosity drops rapidly with rising
temperature. Also mineral oils can be
affected by some gases (such as oxygen),
especially above 80°C. In general,
compared to mineral oils, synthetic
lubricants can operate at higher temperatures
because they have high viscosity
indices; thus, they can be used
(and are more stable) over a large
temperature range. Today, synthetics
are being increasingly used in screw
compressors.
All oil-injected screw compressors
require an oil-separation system,
which is usually combined with the lubrication
system. Depending on the selected
lubricant, oil pumps, cooler and
filters, the system must be designed
with an appropriate oil separator retention
time and proper oil-temperature
control loop. The oil separator
vessel is generally equipped with suitable
internals and wire mesh demister
devices (or alternative devices) to
separate the oil that is entrained in
the gas stream.
The lower part of the discharge vessel
(oil-separation vessel) serves as
the reservoir for the oil management
system and can usually be designed
CHEMICAL ENGINEERING WWW.CHE.COM FEBRUARY 2012 37
http://WWW.CHE.COM
Chemical Engineering February 2012

Table of Contents for the Digital Edition of Chemical Engineering February 2012
