03-09-2021, 06:43 AM
Understanding Compressors
Compressors are mechanical devices used to increase pressure in a variety of
compressible fluids, or gases, the most common of these being air. Compressors are used throughout industry to provide shop
or instrument air; to power air tools, paint sprayers, and abrasive blast equipment; to phase shift refrigerants for air
conditioning and refrigeration; to propel gas through pipelines; etc. As with pumps, compressors are divided into centrifugal
(or dynamic or kinetic) and positive-displacement types; but where pumps are predominately represented by centrifugal
varieties, compressors are more often of the positive- displacement type. They can range in size from the fits-in-a-glovebox
unit that inflates tires to the giant reciprocating or turbocompressor machines found in pipeline service. Positive-
displacement compressors can be further broken out into reciprocating types, where the piston style predominates, and rotary
types such as the helical screw and rotary vane.
In this guide, we will use both of the terms compressors and air compressors to refer mainly to air compressors, and in a
few specialized cases will speak to more specific gases for which compressors are used.
Types of Air Compressor
Compressors may be characterized in several different ways, but are commonly divided into types based on the functional
method used to generate the compressed air or gas. In the sections below, we outline and present the common compressor types.
The types covered include:
Piston
Diaphragm
Helical Screw
Sliding vane
Scroll
Rotary Lobe
Centrifugal
Axial
Due to the nature of the compressor designs, a market also exists for the rebuilding of air compressors, and
reconditioned air compressors may be available as an option
over a newly purchased compressor, including [url=http://www.gkaircompressor.com/air-compressors/special-process-gas-
compressor/]special process gas compressors.[/url]
Piston Compressors
Piston compressors, or reciprocating compressors, rely on the reciprocating action of one or more pistons to compress gas
within a cylinder (or cylinders) and discharge it through valving into high pressure receiving tanks. In many instances, the
tank and compressor are mounted in a common frame or skid as a so-called packaged unit. While the major application of piston
compressors is providing compressed air as an energy source, piston compressors are also used by pipeline operators for
natural gas transmission. Piston compressors are generally selected on the pressure required (psi) and the flow rate (scfm).
A typical plant-air system provides compressed air in the 90-110 psi range, with volumes anywhere from 30 to 2500 cfm; these
ranges are generally attainable through commercial, off-the-shelf units. Plant-air systems can be sized around a single unit
or can be based on multiple smaller units which are spaced throughout the plant.
To achieve higher air pressures than can be provided by a single stage compressor, two-stage units are available.
Compressed air entering the second stage normally passes through an intercooler beforehand to eliminate some of the heat
generated during the first-stage cycle.
Speaking of heat, many piston compressors are designed to operate within a duty cycle, rather than continuously. Such
cycles allow heat generated during the operation to dissipate, in many instances, through air-cooled fins.
Piston compressors are available as both oil-lubricated and oil-free designs. For some applications which require oil-
free air of the highest quality, other designs are better suited.
Diaphragm Compressors
A somewhat specialized reciprocating design, the diaphragm compressor uses a motor-mounted concentric that oscillates a
flexible disc which alternately expands and contracts the volume of the compression chamber. Much like a diaphragm pump, the
drive is sealed from the process fluid by the flexible disc, and thus there is no possibility of lubricant coming into
contact with any gas. Diaphragm air compressors with spare parts are
relatively low capacity machines that have applications where very clean air is required, as in many laboratory and medical
settings.
Helical Screw
Compressors
Helical-screw compressors are rotary compressor machines known for their capacity to operate on 100% duty cycle, making
them good choices for trailerable applications such as construction or road building. Using geared, meshing male and female
rotors, these units pull gas in at the drive end, compress it as the rotors form a cell and the gas travels their length
axially, and discharge the compressed gas through a discharge port on the non-drive end of the compressor casing. The rotary
screw compressor action makes it quieter than a reciprocating compressor owing to reduced vibration. Another advantage of the
screw compressor over piston types is the discharge air is free of pulsations. These units can be oil- or water- lubricated,
or they can be designed to make oil-free air. These designs can meet the demands of critical oil-free service.
Sliding Vane Compressors
A sliding-vane compressor relies on a series of vanes, mounted in a rotor, which sweep along the inside wall of an
eccentric cavity. The vanes, as they rotate from the suction side to the discharge side of the eccentric cavity, reduce the
volume of space they are sweeping past, compressing the gas trapped within the space. The vanes glide along on an oil film
which forms on the wall of the eccentric cavity, providing a seal. Sliding-vane compressors cannot be made to provide oil-
free air, but they are capable of providing compressed air that is free of pulsations. They are also forgiving of
contaminants in their environments owing to the use of bushings rather than bearings and their relatively slow-speed
operation compared to screw compressors. They are relatively quiet, reliable, and capable of operating at 100% duty cycles.
Some sources claim that rotary vane compressors have been largely overtaken by screw compressors in air-compressor
applications. They are used in many non-air applications in the oil and gas and other process industries.
Scroll Compressors
Scroll air compressors use stationary and orbiting spirals which decrease the volume of space between them as the
orbiting spirals trace the path of the fixed spirals. Intake of gas occurs at the outer edge of the scrolls and discharge of
the compressed gas takes place near the center. Because the scrolls do not contact, no lubricating oil is needed, making the
compressor intrinsically oil-free. However, because no oil is used in removing the heat of compression as it is with other
designs, capacities for scroll compressors are somewhat limited. They are often used in low-end air compressors and home
air-conditioning compressors.
Rotary Lobe Compressors
Rotary-lobe compressors are high-volume, low-pressure devices more appropriately classified as blowers. To learn more
about blowers, download the free Thomas Blowers Buying Guide.
Centrifugal Compressors
Centrifugal compressors rely on high-speed pump-like impellers to impart velocity to gases to produce an increase in
pressure. They are seen mainly in high-volume applications such as commercial refrigeration units in the 100+ hp ranges and
in large processing plants where they can get as large as 20,000 hp and deliver volumes in the 200,000 cfm range. Almost
identical in construction to centrifugal pumps, centrifugal compressors increase the velocity of gas by throwing it outward
by the action of a spinning impeller. The gas expands in a casing volute, where its velocity slows and its pressure rises.
Centrifugal compressors have lower compression ratios than displacement compressors, but they handle vast volumes of gas.
Many centrifugal compressors use multiple stages to improve the compression ratio. In these multi-stage compressors, the gas
usually passes through intercoolers between stages.
Axial Compressors
The axial Low-
Pressure Water Lubricating Oil-free Compressor achieves the highest volumes of delivered air, ranging from 8000 to 13
million cfm in industrial machines. Jet engines use compressors of this kind to produce volumes over an even wider range. To
a greater extent than centrifugal compressors, axial compressors tend toward multi-stage designs, owing to their relatively
low compression ratios. As with centrifugal units, axial compressors increase pressure by first increasing the velocity of
the gas. Axial compressors then slow the gas down by passing it through curved, fixed blades, which increases its pressure.
Power and Fuel Options
Air compressors may be powered electrically, with common options being 12 volt DC air compressors or 24 volt DC air
compressors. Compressors are also available that operate from standard AC voltage levels such as 120V, 220V, or 440V.
Alternative fuel options include air compressors that operate from an engine that is driven off of a combustible fuel
source such as gasoline or diesel fuel. Generally, electrically-powered compressors are desirable in cases where it is
important to eliminate exhaust fumes or to provide for operation in settings where the use or presence of combustible fuels
is not desired. Noise considerations also play a role in the choice of fuel option, as electrically driven air compressors
typical exhibit lower acoustical noise levels over their engine-driven counterparts.
Additionally, some air compressors may be powered hydraulically, which also avoids the use of combustible fuel sources
and the resulting exhaust gas issues.
Compressor Machine Selection in an Industrial Setting
In selecting air compressors for general shop use, the choice will generally come down to a piston compressor or a
helical-screw compressor. Piston compressors tend to be less expensive than screw compressors, require less sophisticated
maintenance, and hold up well under dirty operating conditions. They are much noisier than screw compressors, however, and
are more susceptible to passing oil into the compressed air supply, a phenomenon known as “carryover.” Because piston
compressors generate a great deal of heat in operation, they have to be sized according to a duty cycle—a rule of thumb
prescribes 25% rest and 75% run. Radial-screw [url=http://www.gkaircompressor.com/air-compressors/variable-frequency-water-
lubricating-oil-free/]Variable Frequency Water Lubricating Oil-Free Screw Compressor[/url] can run 100% of the time and
almost prefer it. A potential problem with screw compressors, though, is that oversizing one with the idea of growing into
its capacity can lead to trouble as they are not particularly suited to frequent starting and stopping. Close tolerance
between rotors means that compressor needs to remain at operating temperature to achieve effective compression. Sizing one
takes a little more attention to air usage; a piston compressor may be oversized without similar worries.
An autobody shop which uses air constantly for painting might find a radial-screw compressor with its lower carryover
rate and desire to run continuously an asset; a general auto-repair business with more infrequent air use and low concern for
the cleanliness of the supplied air might be better served with a piston compressor.
Regardless of the compressor type, compressed air is usually cooled, dried, and
filtered before it is distributed through pipes. Specifiers of plant-air systems will need to select these components based
on the size of the system they design. In addition, they will need to consider installing filter-regulator-lubricators at the
supply drops.
Larger job site compressors mounted on trailers are typically rotary-screw varieties with engine drives. They are
intended to run continuously whether the air is used or dumped.
Although dominant in lower-end refrigeration systems and air compressors, scroll compressors are beginning to make
inroads into other markets. They are particularly suited to manufacturing processes that demand very clean air (class 0) such
as pharmaceutical, food, electronics, etc. and to cleanroom, laboratory, and medical/dental settings. Manufactures offer
units up to 40 hp that deliver nearly 100 cfm at up 145 psi. The larger capacity units generally incorporate multiple scroll
compressors as the technology does not scale up well once beyond 3-5 hp.
If the application involves compressing hazardous gases, specifiers often consider diaphragm or sliding-vane compressors,
or, for very large volumes to compress, kinetic types.
Compressors are mechanical devices used to increase pressure in a variety of
compressible fluids, or gases, the most common of these being air. Compressors are used throughout industry to provide shop
or instrument air; to power air tools, paint sprayers, and abrasive blast equipment; to phase shift refrigerants for air
conditioning and refrigeration; to propel gas through pipelines; etc. As with pumps, compressors are divided into centrifugal
(or dynamic or kinetic) and positive-displacement types; but where pumps are predominately represented by centrifugal
varieties, compressors are more often of the positive- displacement type. They can range in size from the fits-in-a-glovebox
unit that inflates tires to the giant reciprocating or turbocompressor machines found in pipeline service. Positive-
displacement compressors can be further broken out into reciprocating types, where the piston style predominates, and rotary
types such as the helical screw and rotary vane.
In this guide, we will use both of the terms compressors and air compressors to refer mainly to air compressors, and in a
few specialized cases will speak to more specific gases for which compressors are used.
Types of Air Compressor
Compressors may be characterized in several different ways, but are commonly divided into types based on the functional
method used to generate the compressed air or gas. In the sections below, we outline and present the common compressor types.
The types covered include:
Piston
Diaphragm
Helical Screw
Sliding vane
Scroll
Rotary Lobe
Centrifugal
Axial
Due to the nature of the compressor designs, a market also exists for the rebuilding of air compressors, and
reconditioned air compressors may be available as an option
over a newly purchased compressor, including [url=http://www.gkaircompressor.com/air-compressors/special-process-gas-
compressor/]special process gas compressors.[/url]
Piston Compressors
Piston compressors, or reciprocating compressors, rely on the reciprocating action of one or more pistons to compress gas
within a cylinder (or cylinders) and discharge it through valving into high pressure receiving tanks. In many instances, the
tank and compressor are mounted in a common frame or skid as a so-called packaged unit. While the major application of piston
compressors is providing compressed air as an energy source, piston compressors are also used by pipeline operators for
natural gas transmission. Piston compressors are generally selected on the pressure required (psi) and the flow rate (scfm).
A typical plant-air system provides compressed air in the 90-110 psi range, with volumes anywhere from 30 to 2500 cfm; these
ranges are generally attainable through commercial, off-the-shelf units. Plant-air systems can be sized around a single unit
or can be based on multiple smaller units which are spaced throughout the plant.
To achieve higher air pressures than can be provided by a single stage compressor, two-stage units are available.
Compressed air entering the second stage normally passes through an intercooler beforehand to eliminate some of the heat
generated during the first-stage cycle.
Speaking of heat, many piston compressors are designed to operate within a duty cycle, rather than continuously. Such
cycles allow heat generated during the operation to dissipate, in many instances, through air-cooled fins.
Piston compressors are available as both oil-lubricated and oil-free designs. For some applications which require oil-
free air of the highest quality, other designs are better suited.
Diaphragm Compressors
A somewhat specialized reciprocating design, the diaphragm compressor uses a motor-mounted concentric that oscillates a
flexible disc which alternately expands and contracts the volume of the compression chamber. Much like a diaphragm pump, the
drive is sealed from the process fluid by the flexible disc, and thus there is no possibility of lubricant coming into
contact with any gas. Diaphragm air compressors with spare parts are
relatively low capacity machines that have applications where very clean air is required, as in many laboratory and medical
settings.
Helical Screw
Compressors
Helical-screw compressors are rotary compressor machines known for their capacity to operate on 100% duty cycle, making
them good choices for trailerable applications such as construction or road building. Using geared, meshing male and female
rotors, these units pull gas in at the drive end, compress it as the rotors form a cell and the gas travels their length
axially, and discharge the compressed gas through a discharge port on the non-drive end of the compressor casing. The rotary
screw compressor action makes it quieter than a reciprocating compressor owing to reduced vibration. Another advantage of the
screw compressor over piston types is the discharge air is free of pulsations. These units can be oil- or water- lubricated,
or they can be designed to make oil-free air. These designs can meet the demands of critical oil-free service.
Sliding Vane Compressors
A sliding-vane compressor relies on a series of vanes, mounted in a rotor, which sweep along the inside wall of an
eccentric cavity. The vanes, as they rotate from the suction side to the discharge side of the eccentric cavity, reduce the
volume of space they are sweeping past, compressing the gas trapped within the space. The vanes glide along on an oil film
which forms on the wall of the eccentric cavity, providing a seal. Sliding-vane compressors cannot be made to provide oil-
free air, but they are capable of providing compressed air that is free of pulsations. They are also forgiving of
contaminants in their environments owing to the use of bushings rather than bearings and their relatively slow-speed
operation compared to screw compressors. They are relatively quiet, reliable, and capable of operating at 100% duty cycles.
Some sources claim that rotary vane compressors have been largely overtaken by screw compressors in air-compressor
applications. They are used in many non-air applications in the oil and gas and other process industries.
Scroll Compressors
Scroll air compressors use stationary and orbiting spirals which decrease the volume of space between them as the
orbiting spirals trace the path of the fixed spirals. Intake of gas occurs at the outer edge of the scrolls and discharge of
the compressed gas takes place near the center. Because the scrolls do not contact, no lubricating oil is needed, making the
compressor intrinsically oil-free. However, because no oil is used in removing the heat of compression as it is with other
designs, capacities for scroll compressors are somewhat limited. They are often used in low-end air compressors and home
air-conditioning compressors.
Rotary Lobe Compressors
Rotary-lobe compressors are high-volume, low-pressure devices more appropriately classified as blowers. To learn more
about blowers, download the free Thomas Blowers Buying Guide.
Centrifugal Compressors
Centrifugal compressors rely on high-speed pump-like impellers to impart velocity to gases to produce an increase in
pressure. They are seen mainly in high-volume applications such as commercial refrigeration units in the 100+ hp ranges and
in large processing plants where they can get as large as 20,000 hp and deliver volumes in the 200,000 cfm range. Almost
identical in construction to centrifugal pumps, centrifugal compressors increase the velocity of gas by throwing it outward
by the action of a spinning impeller. The gas expands in a casing volute, where its velocity slows and its pressure rises.
Centrifugal compressors have lower compression ratios than displacement compressors, but they handle vast volumes of gas.
Many centrifugal compressors use multiple stages to improve the compression ratio. In these multi-stage compressors, the gas
usually passes through intercoolers between stages.
Axial Compressors
The axial Low-
Pressure Water Lubricating Oil-free Compressor achieves the highest volumes of delivered air, ranging from 8000 to 13
million cfm in industrial machines. Jet engines use compressors of this kind to produce volumes over an even wider range. To
a greater extent than centrifugal compressors, axial compressors tend toward multi-stage designs, owing to their relatively
low compression ratios. As with centrifugal units, axial compressors increase pressure by first increasing the velocity of
the gas. Axial compressors then slow the gas down by passing it through curved, fixed blades, which increases its pressure.
Power and Fuel Options
Air compressors may be powered electrically, with common options being 12 volt DC air compressors or 24 volt DC air
compressors. Compressors are also available that operate from standard AC voltage levels such as 120V, 220V, or 440V.
Alternative fuel options include air compressors that operate from an engine that is driven off of a combustible fuel
source such as gasoline or diesel fuel. Generally, electrically-powered compressors are desirable in cases where it is
important to eliminate exhaust fumes or to provide for operation in settings where the use or presence of combustible fuels
is not desired. Noise considerations also play a role in the choice of fuel option, as electrically driven air compressors
typical exhibit lower acoustical noise levels over their engine-driven counterparts.
Additionally, some air compressors may be powered hydraulically, which also avoids the use of combustible fuel sources
and the resulting exhaust gas issues.
Compressor Machine Selection in an Industrial Setting
In selecting air compressors for general shop use, the choice will generally come down to a piston compressor or a
helical-screw compressor. Piston compressors tend to be less expensive than screw compressors, require less sophisticated
maintenance, and hold up well under dirty operating conditions. They are much noisier than screw compressors, however, and
are more susceptible to passing oil into the compressed air supply, a phenomenon known as “carryover.” Because piston
compressors generate a great deal of heat in operation, they have to be sized according to a duty cycle—a rule of thumb
prescribes 25% rest and 75% run. Radial-screw [url=http://www.gkaircompressor.com/air-compressors/variable-frequency-water-
lubricating-oil-free/]Variable Frequency Water Lubricating Oil-Free Screw Compressor[/url] can run 100% of the time and
almost prefer it. A potential problem with screw compressors, though, is that oversizing one with the idea of growing into
its capacity can lead to trouble as they are not particularly suited to frequent starting and stopping. Close tolerance
between rotors means that compressor needs to remain at operating temperature to achieve effective compression. Sizing one
takes a little more attention to air usage; a piston compressor may be oversized without similar worries.
An autobody shop which uses air constantly for painting might find a radial-screw compressor with its lower carryover
rate and desire to run continuously an asset; a general auto-repair business with more infrequent air use and low concern for
the cleanliness of the supplied air might be better served with a piston compressor.
Regardless of the compressor type, compressed air is usually cooled, dried, and
filtered before it is distributed through pipes. Specifiers of plant-air systems will need to select these components based
on the size of the system they design. In addition, they will need to consider installing filter-regulator-lubricators at the
supply drops.
Larger job site compressors mounted on trailers are typically rotary-screw varieties with engine drives. They are
intended to run continuously whether the air is used or dumped.
Although dominant in lower-end refrigeration systems and air compressors, scroll compressors are beginning to make
inroads into other markets. They are particularly suited to manufacturing processes that demand very clean air (class 0) such
as pharmaceutical, food, electronics, etc. and to cleanroom, laboratory, and medical/dental settings. Manufactures offer
units up to 40 hp that deliver nearly 100 cfm at up 145 psi. The larger capacity units generally incorporate multiple scroll
compressors as the technology does not scale up well once beyond 3-5 hp.
If the application involves compressing hazardous gases, specifiers often consider diaphragm or sliding-vane compressors,
or, for very large volumes to compress, kinetic types.