Chemical Engineering February 2012 - 38

Engineering Practice
differential expansion between the
contact surfaces. This permits relatively
large pressure rises across each
stage of the compressor and obviates
the need for jacket or rotor cooling.
For optimum efficiency, internal
clearances within the compressor
must be kept as small as possible. The
injected oil also serves as a sealant,
partially filling the clearance between
the screw rotors and casing.
The injected oil also acts as a lubricant
enabling the main rotor (the
drive rotor, which is generally the
male rotor) to drive the female rotor
through a film of oil. With this design,
oil-injected screw compressors do not
require a timing gear between the rotors
to ensure that there is no metalto-metal
contact. This results in a simpler
and less expensive compressor
design, compared to dry screw compressors
that require a timing gear.
This lubricant effect also permits
the use of a slide valve, which acts as
the capacity-control system. The valve
is capable of controlling volume flow
between typically 20-100%, providing
one of the most flexible compression
systems available.
The lubricating oil also acts as a corrosion
inhibitor, protecting the compressor
from the constituents in the
compressed gas. This feature enables
oil-injected compressors made from
conventional materials (such as carbon
steel or grey cast iron) to be used
for difficult gases (those containing
sulfides, chlorides and so on).
The oil acts as a noise dampener.
Combined with the fact that screw
compressors have relatively low
rotor-lobe passing frequencies, this enables
most installations to meet permitted
noise levels without extensive
sound attenuation.
Additional design aspects
Generally, grey cast iron is suitable for
the compressor casing, for operating
pressures to 15 barg. Casings made
from spheroidal graphite (SG) iron
can be used for pressures up to around
35 barg. Proper steel grades should be
used for pressures above 35 barg.
Figure 3 shows a vertically split,
oil-injected screw compressor in the
workshop for repair. In the start of the
suction stroke, gas enters the rotor
flutes. The suction port is
sealed from the gas-filled
flutes by the meshing of the
next lobe. The trapped gas is
then briefly moved axially
along, until the flutes reach
the end wall and the compression
stroke commences.
The volume trapped in
the flutes is reduced until
trapped gas reaches the
discharge port. The volume
ratio is an inherent physical
feature of a given screw
compressor design. The discharge
pressure of screw
compressors is limited by various
factors, including the gas discharge
temperature, the rotor length-to-dia.
ratio (L/D ratio), the material and casing
design (the common limit is below
70 barg).
Rotor speeds are such that dynamic
balancing is required for proper vibration
control. Screw rotors usually rotate
around 10-25% below their critical
(resonance) speeds.
Rotors made from forged carbon
FIGURE 2. In this medium-sized screw compressor,
the effects of injected oil result in relatively
lower speed (compared to comparably sized, drytype
screw compressors), which enables direct connection
between the electric-motor driver and the
screw compressor. This eliminates speed-increasing
gears, which are usually required in dry-type
screw compressors
as those above 0.3 MW). The screw
configuration with a 4+6 asymmetric
profile that is designated 'D' is predominantly
used for small and medium
oil-injected screw compressors
(such as those around and below 0.5
MW). Figure 4 shows the rotors of a
screw compressor.
Some vendors provide screw rotors
steel are commonly used for all noncorrosive
applications. High-alloy
steels (such as 13% Cr and 4% Ni)
tend to be used only for corrosive or
other special applications. Nearly all
stainless steels are prone to galling.
Particular attention is required for
screw compressors that have relatively
low operating clearances (around or
below 80% of commonly used clearances)
when using stainless-steel or
alloy-steel materials. Alloy steels and
stainless steels are also rarely used
because of high price, operational
problems and lack of proper references.
Rather, for most screw compressors,
a suitable carbon-steel grade is
typically specified.
Screw profile options
Various screw profiles are available.
Selection depends on the process conditions,
and the anticipated flowrate
and pressure conditions. The most
common profile is called the asymmetric
4+6 screw (which has 4 male
lobes and 6 female flutes). The screw
configuration with a 4+6 asymmetric
profile that is designated 'A' is predominantly
used for relatively large
oil-injected screw compressors (such
36 CHEMICAL ENGINEERING WWW.CHE.COM FEBRUARY 2012
with a range of coatings to improve
wear resistance and maintain the
desired seal clearance. But, there are
diverse opinions on the value of such
coatings because of costs, operational
problems, relatively low life and some
reported reliability issues.
In general, rotor coating is not generally
recommended - except for the
purpose of renewing clearances during
machine repair or screw rotor rehabilitation.
Because the metal surfaces
in modern screw-type compressors are
configured in a three-dimensional spiral
slope, machining them to precise
dimensions during repair is very difficult.
Thus, some modern polymeric
coatings that match rotor clearances
have been used successfully in recent
years for screw rotor repair. Compared
to other repair or replacement options,
rotor coating can be carried out at relatively
low cost and low risk.
In a recent case study, the application
of a screw rotor coating using a
special polymeric coating on the contacting
surfaces during machine renovation
provided 4% improvement in
machine efficiency.
In general, differential pressure on
any stage should not exceed the maximum
allowable values, depending on
rotor design. As a rough indication,
this limit could vary from as much as
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Chemical Engineering February 2012

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