Chemical Engineering February 2012 - 37

Feature Report
Engineering Practice
Choosing
Oil-injected
Screw
Compressors
FIGURE 1. This medium-sized screw compressor is
Follow this guidance to improve the
design, performance and reliability
of these workhorse machines
Amin Almasi
Rotating Machine Consultant
S
crew compressors are widely
used throughout the chemical
process industries (CPI) and
represent a substantial percentage
of all positive-displacement
compressors that are now sold and
in operation in CPI facilities. In general,
screw compressors are efficient,
reliable, simple and compact compressors
that are competitive from both
an initial cost and an operating cost
standpoint compared to other types
of positive-displacement compressors
over a range of operating conditions.
Screw compressors have many inherent
advantages over other compressor
types, especially in the low- to
medium-flow range. These positivedisplacement
compressors draw a constant
inlet volume, meet varying differential
pressure requirements, and are
not significantly affected by changes
in gas conditions, such as changes in
density or gas composition.
And while screw compressors may
transmit relatively low-level pulsations,
they do not generate the types
of severe pulsations that are typical of
reciprocating compressors. Also, cylinder
valves and valve unloaders -
which can be problematic - are not
required.
driven by an electric motor and installed on a steel frame
skid. The top suction and side discharge nozzle arrangement
is quite common in mid-size and large machines
(typically >0.8 MW). Thermal and pulsation considerations
are important when designing and installing the
screw compressor's piping arrangement, particularly with
regard to the discharge line design. Oil system and auxiliaries
are typically included on the same skid
Two design options
Two types of twin screw compressors
are available:
* Oil-free (dry type), and
* Oil-injected (wet type)
Although both types use screw rotors,
they are significantly different,
and this article focuses on oil-injected
screw compressors.
Oil-injected (wet type) screw compressors.
These are the newest and
most advanced types of screw compressors.
Compared to their dry alternatives,
oil-injected screw compressors
enable much higher compression
ratios, simplified mechanical design
(mainly because they do not require
timing gears), more-efficient operation
and advanced reliability.
Oil-free (dry type) screw compressors.
In oil-free (dry-type) screw compressors,
the gas is compressed entirely
through the action of the screws,
without the assistance of the oil. Timing
gears ensure that the male and
female screw rotors maintain precise
alignment. These compressors usually
have lower maximum-dischargepressure
capability, and they need
relatively high rotational speeds. In
many cases, they require gear systems
to increase the driver speed. Because
they are more complex to manufacture
and cost more than oil-injected
screw compressors, dry-running compressors
tend to only be used where
entrained oil carry-over (even in very
small quantities) is not acceptable.
The benefits of oil injection
Today, oil-injected screw compressors
have largely replaced reciprocating
compressors and centrifugal compressors
for many CPI applications. The
general arrangement for a small, oilflooded
screw compressor package
(Figures 1 and 2) requires a compressor
and motor installed on a horizontal
oil separator. Medium and large
machines (those above 0.8 MW) are
typically installed on skids.
In an oil-flooded screw compressor,
lubricating oil is deliberately injected
into the gas-compression space and
is allowed to mix freely with the process
gas. The oil acts as the coolant,
enabling the gas-discharge temperature
to be controlled. During operation,
differential expansion between
the screws and the casing - or more
precisely, the contacting surfaces that
are separated by the oil film - that
results from thermal growth can reduce
the running clearances and create
interferences between the casing
and the screw rotors. By injecting a
calculated flow of oil, the discharge
temperature can be reliably controlled
(usually in the range of 75-110°C),
and the oil film can help to minimize
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Chemical Engineering February 2012

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