Chemical Engineering June 2014 - 52

Engineering Practice
jacketed cylinder, similar to that of
the jacketed section of a reciprocating
cylinder. The liquid jacket around
the cylinder is used for cooling. The
rotor is filled with blades, which are
free to move in and out of longitudinal
slots in a rotor. Blade configurations
range from 8 to 12 blades,
depending on the manufacturer
and required pressure differentials.
Blades are forced out against the
cylinder wall by a centrifugal force,
creating individual cells of gas that
are compressed as the rotor turns.
As the gas approaches the discharge
port, the area is reduced and the gas
discharged.
Sliding (rotary) vane compressors
are designed for use in harsh environments.
When it comes to dirty
process gases, sour gases, wet gases,
flare-recovery gases, vapor-recovery
services and other dirty or low-pressure-ratio
CPI applications, the sliding-vane
compressor is widely used.
Unlike other types of compressors
that are used for small compression
units (such as oil-flooded screw
compressors, where the lubrication
oil is mixed with the gas stream and
is then reused), the lubrication of a
sliding-vane compressor usually
relies on once-through lubrication,
thus not contaminating or breaking
down the oil viscosity.
The sliding (rotary) vane system
design does not have critical tolerances,
due to its design and working
principles. This is in contrast
to a reciprocating compressor or an
oil-flooded screw compressor, which
require tight tolerances for proper
operation. Since sliding (rotary)
vane compressors do not operate at
close tolerances, it could be considered
a rugged and reliable machine
for many applications.
Reciprocating compressors.
There are some disadvantages for
tiny reciprocating compressors.
The first disadvantage of reciprocating
compressors is low reliability
(and low availability). Another
critical issue is pulsation. Some
CPI operators of low-flow (small)
services still prefer small reciprocating
compressors to other compressor
types (such as oil-flooded
screw compressors or sliding-vane
compressors). These CPI operators
have reported acceptable reliability
and acceptable maintenance
expenses associated with these
units. The reality is that the reciprocating-compressor
technology
is a mature technology. There are
more manufacturers that supply
these machines compared to other
options (such as oil-flooded screw
compressors), and today, competition
among vendors has improved
reliability and reduced spare-part
and maintenance costs for specific
industries. These vendors have also
accumulated a wealth of knowledge
and experience in design, fabrication,
operation and troubleshooting
of reciprocating compressors.
For many applications, reciprocating
compressor technology may
be inherently less reliable and more
expensive than competing alternatives,
and requires a bigger and
heavier machine. However, reciprocating
compressors are often a good
option for some specific small or
tiny compressor units.
Oil-flooded screw compressors.
To provide lubrication, sealing and
cooling and to avoid any metal-tometal
contact, the lubrication oil is
injected into the compression chamber
of oil-flooded screw compressors.
As screw rotors mesh, the compression
chamber volume decreases,
compressing the gas/lubricant
mixture toward the discharge port,
where the stream exits as the chamber
passes over the port. Oil-flooded
screw compressors are often a good
option in medium- and low-volume
applications and those with low suction
pressure. The continuous flow
of cooling lubricant permits much
higher single-stage compression
ratios and suction-pressure variations.
In some cases, this can help
to eliminate the need for an aftercooler
for many small machines. An
oil-flooded screw compressor could
be a good option for many CPI tiny
or small compressors.
There is no general rule for the
selection of small/tiny (10-100
kW) compressor type. One of the
above-mentioned three compressor
types can usually be used for a
small-compressor unit application.
52 ChemiCal engineering www.Che.Com June 2014
The oil-flooded screw compressor is
technically the best option for many
CPI applications, however, the reciprocating
compressor or sliding-vane
compressor could also be used.
Generally, a rotary-vane compressor
offers the minimum cost. For
some applications, this option may
cost 10-25% less than a comparable
oil-flooded screw machine. Meanwhile,
sliding-vane compressors are
often selected if the oil-gas combination
can be anticipated to cause
problems in the oil-flooded screw
compressor. This could be the case
for some process-gas streams with
heavy gases and other elements.
Final notes on small compressors
The small compressor market (10-
100 kW) is an evolving and changing
market. When it comes to the
need for small or tiny compressors,
one of the following compressor
types should be used: the oil-flooded
screw compressor, the reciprocating
compressor and the sliding (rotary)
vane compressor. The oil-flooded
screw compressor could technically
be the best option for many
applications. The small compressor
package should usually be flexible
both for the suction pressure and
for the capacity with an acceptable
efficiency at a part-load operation.
The desired suction-pressure range
often is from a low pressure (even
0 barg or lower) to a higher suction
pressure (more than two-times the
rated suction pressure).
z
Author
Amin Almasi is a rotating
equipment consultant in Australia
(Email: amin.almasi@
ymail.com). He previously
worked at Worley Parsons
Services Pty Ltd. (Brisbane,
Australia), Technicas Reunidas
(Madrid, Spain) and
Fluor Corp. (various offices).
He holds a chartered professional
engineer license
from Engineers Australia
(MIEAust CPEng - Mechanical), a chartered
engineer certificate from IMechE (CEng MIMechE),
RPEQ (registered professional engineer
in Queensland) and he also holds M.S. and B.S.
degrees in mechanical engineering. He specializes
in rotating machines including centrifugal,
screw and reciprocating compressors, gas and
steam turbines, pumps, condition monitoring and
reliability. Almasi is an active member of Engineers
Australia, IMechE, ASME, Vibration Institute,
SPE, IEEE, and IDGTE. He has authored
more than 60 papers and articles dealing with
rotating machines.
Edited by Suzanne Shelley
http://www.Che.Com
Chemical Engineering June 2014

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