A reciprocating compressor or piston compressor is a positive-displacement

compressor that uses pistons driven by a crankshaft to deliver gases at high

pressure.[1][2] Pressures of up to 5,000 psig are commonly produced by multistage
reciprocating compressors.
The intake gas enters the suction manifold, then flows into the compression cylinder
where it gets compressed by a piston driven in a reciprocating motion via a
crankshaft, and is then discharged. Applications include railway and road vehicle air
brake systems oil refineries, gas pipelines, oil and gas production drilling and well
services, air and nitrogen injection, offshore platforms, chemical plants, natural gas
processing plants, air conditioning, and refrigeration plants. One specialty
application is the blowing of plastic bottles made of polyethylene
terephthalate (PET).
In the ionic liquid piston compressor many seals and bearings were removed in the
design as the ionic liquid does not mix with the gas. Service life is about 10 times
longer than a regular diaphragm compressor with reduced maintenance during use,
energy costs are reduced by as much as 20%. The heat exchangers that are used in
a normal piston compressor are removed as the heat is removed in the cylinder
itself where it is generated. Almost 100% of the energy going into the process is
being used with little energy wasted as reject heat.
https://en.wikipedia.org/wiki/Reciprocating_compressor

Centrifugal compressors, sometimes called impeller compressors or radial

compressors, are a sub-class of dynamic axisymmetric work-absorbing
turbomachinery.[1]
They achieve pressure rise by adding energy to the continuous flow of fluid through
the rotor/impeller. The equation in the next section shows this specific energy input.
A substantial portion of this energy is kinetic which is converted to increased
potential energy/static pressure by slowing the flow through a diffuser. The static
pressure rise in the impeller may roughly equal the rise in the diffuser.
Components of a simple centrifugal compressor
[edit]

Figure-1.1 - 2-Stage turboshaft, 1st-stage

flowpath, annular inlet, guide vanes, open impeller, vaned diffuser, vaneless return-
bend

A simple centrifugal compressor stage has four components (listed in order of

throughflow): inlet, impeller/rotor, diffuser, and collector.[1] Figure 1.1 shows each
of the components of the flow path, with the flow (working gas) entering the
centrifugal impeller axially from left to right. This turboshaft (or turboprop) impeller
is rotating counter-clockwise when looking downstream into the compressor. The
flow will pass through the compressors from left to right.

Inlet
[edit]
The simplest inlet to a centrifugal compressor is typically a simple pipe. Depending
upon its use/application inlets can be very complex. They may include other
components such as an inlet throttle valve, a shrouded port, an annular duct (see
Figure 1.1), a bifurcated duct, stationary guide vanes/airfoils used to straight or swirl
flow (see Figure 1.1), movable guide vanes (used to vary pre-swirl adjustably).
Compressor inlets often include instrumentation to measure pressure and
temperature in order to control compressor performance.
Bernoulli's fluid dynamic principle plays an important role in understanding vaneless
stationary components like an inlet. In engineering situations assuming adiabatic
flow, this equation can be written in the form:
Equation-1.1

where:

 0 is the inlet of the compressor, station 0

 1 is the inlet of the impeller, station 1
  p is the pressure

 ρ is the density and indicates that it is a function of pressure

 is the flow speed

 γ is the ratio of the specific heats of the fluid

Centrifugal impeller
[edit]

Figure 1.2.1 - Graphic modeling of the

impeller, similar to turbocharger impeller

The identifying component of a centrifugal compressor stage is the centrifugal

impeller rotor. Impellers are designed in many configurations including "open"
(visible blades), "covered or shrouded", "with splitters" (every other inducer
removed), and "w/o splitters" (all full blades). Figures 0.1, 1.2.1, and 1.3 show
three different open full inducer rotors with alternating full blades/vanes and
shorter length splitter blades/vanes. Generally, the accepted mathematical
nomenclature refers to the leading edge of the impeller with subscript 1.
Correspondingly, the trailing edge of the impeller is referred to as subscript 2.
As working-gas/flow passes through the impeller from stations 1 to 2, the kinetic
and potential energy increase. This is identical to an axial compressor with the
exception that the gases can reach higher energy levels through the impeller's
increasing radius. In many modern high-efficiency centrifugal compressors the
gas exiting the impeller is traveling near the speed of sound.
Most modern high-efficiency impellers use "backsweep" in the blade shape.[2]
[3][4]
A derivation of the general Euler equations (fluid dynamics) is Euler's pump and
turbine equation, which plays an important role in understanding impeller
performance. This equation can be written in the form:
Equation-1.2 (see Figures 1.2.2 and 1.2.3 illustrating impeller velocity triangles)

where:

 1 subscript 1 is the impeller leading edge (inlet), station 1

 2 subscript 2 is the impeller trailing edge (discharge), station 2
 E is the energy added to the fluid
 g is the acceleration due to gravity
 u is the impeller's circumferential velocity, units velocity
 w is the velocity of flow relative to the impeller, units velocity
 c is the absolute velocity of flow relative to stationary, units
velocity

Figuer1.2.2 -Inlet velocity triangles for centrifugal compressor

impeller

Figuer1.2.3 - Exit velocity triangles for centrifugal compressor impeller

Diffuser
[edit]
 Figure 1.3 - NASA CC3 impeller and
wedge diffuser

The next component, downstream of the impeller within a simple centrifugal

compressor may the diffuser.[5][4] The diffuser converts the flow's kinetic
energy (high velocity) into increased potential energy (static pressure) by
gradually slowing (diffusing) the gas velocity. Diffusers can be vaneless,
vaned, or an alternating combination. High-efficiency vaned diffusers are also
designed over a wide range of solidities from less than 1 to over 4. Hybrid
versions of vaned diffusers include wedge (see Figure 1.3), channel, and pipe
diffusers. Some turbochargers have no diffuser. Generally accepted
nomenclature might refer to the diffuser's lead edge as station 3 and the
trailing edge as station 4.
Bernoulli's fluid dynamic principle plays an important role in understanding
diffuser performance. In engineering situations assuming adiabatic flow, this
equation can be written in the form:
Equation-1.3

where:

2 is the inlet of the diffuser, station 2


4 is the discharge of the diffuser, station 4

(see inlet above.)

Collector
[edit]
 Figure 1.4 - Centrifugal
compressor model illustrating the main components

The collector of a centrifugal compressor can take many shapes and

forms.[5][4] When the diffuser discharges into a large empty
circumferentially (constant area) chamber, the collector may be termed a
Plenum. When the diffuser discharges into a device that looks somewhat
like a snail shell, bull's horn, or a French horn, the collector is likely to be
termed a volute or scroll.
When the diffuser discharges into an annular bend the collector may be
referred to as a combustor inlet (as used in jet engines or gas turbines) or
a return-channel (as used in an online multi-stage compressor). As the
name implies, a collector's purpose is to gather the flow from the diffuser
discharge annulus and deliver this flow downstream into whatever
component the application requires. The collector or discharge pipe may
also contain valves and instrumentation to control the compressor. In
some applications, collectors will diffuse flow (converting kinetic energy
to static pressure) far less efficiently than a diffuser.[6]
Bernoulli's fluid dynamic principle plays an important role in
understanding diffuser performance. In engineering situations assuming
adiatice flow, this equation can be written in the form:
Equation-1.4

where:

 4 is the inlet of the diffuser, station 4

 5 is the discharge of the diffuser, station 5
 (see inlet above.)
Historical contributions, the pioneers
[edit]
Over the past 100 years, applied scientists including Stodola (1903,
1927–1945),[7] Pfleiderer (1952),[8] Hawthorne (1964),[9] Shepherd
(1956),[1] Lakshminarayana (1996),[10] and Japikse (many texts
including citations),[2][11][citation needed][12] have educated young
engineers in the fundamentals of turbomachinery. These
understandings apply to all dynamic, continuous-flow, axisymmetric
pumps, fans, blowers, and compressors in axial, mixed-flow and
radial/centrifugal configurations.
This relationship is the reason advances in turbines and axial
compressors often find their way into other turbomachinery including
centrifugal compressors. Figures 2.1 and 2.2 illustrate the domain of
turbomachinery with labels showing centrifugal compressors.[13]
[14] Improvements in centrifugal compressors have not been
achieved through large discoveries. Rather, improvements have been
achieved through understanding and applying incremental pieces of
knowledge discovered by many individuals.
Aerodynamic-thermodynamic domain
[edit]

Figure 2.1 – Aero-thermo

domain of turbomachinery

Figure 2.1 (shown right) represents the aero-thermo domain of

turbomachinery. The horizontal axis represents the energy equation
derivable from The first law of thermodynamics.[1][14] The vertical
axis, which can be characterized by Mach Number, represents the
range of fluid compressibility (or elasticity).[1][14] The Z-axis, which
can be characterized by Reynolds number, represents the range of
fluid viscosities (or stickiness).[14] Mathematicians and physicists who
established the foundations of this aero-thermo domain include:[15]
[16] Isaac Newton, Daniel Bernoulli, Leonhard Euler, Claude-Louis
Navier, George Stokes, Ernst Mach, Nikolay Yegorovich Zhukovsky,
Martin Kutta, Ludwig Prandtl, Theodore von Kármán, Paul Richard
Heinrich Blasius, and Henri Coandă.
Physical-mechanical domain
[edit]
 Figure 2.2 – Physical domain
of turbomachinery

Figure 2.2 (shown right) represents the physical or mechanical domain

of turbomachinery. Again, the horizontal axis represents the energy
equation with turbines generating power to the left and compressors
absorbing power to the right.[1][14] Within the physical domain the
vertical axis differentiates between high speeds and low speeds
depending upon the turbomachinery application.[1][14] The Z-axis
differentiates between axial-flow geometry and radial-flow geometry
within the physical domain of turbomachinery.[1][14] It is implied that
mixed-flow turbomachinery lie between axial and radial.[1][14] Key
contributors of technical achievements that pushed the practical
application of turbomachinery forward include:[15][16] Denis Papin,
[17] Kernelien Le Demour, Daniel Gabriel Fahrenheit, John Smeaton,
Dr. A. C. E. Rateau,[18] John Barber, Alexander Sablukov, Sir Charles
Algernon Parsons, Ægidius Elling, Sanford Alexander Moss, Willis
Carrier, Adolf Busemann, Hermann Schlichting, Frank Whittle and
Hans von Ohain.
Partial timeline of historical contributions
[edit]

<168
Early turbomachines Pumps, blowers, fans
9

Origin of the centrifugal

1689 Denis Papin
compressor

1754 Leonhard Euler Euler's "Pump & Turbine"

 equation

1791 John Barber First gas turbine patent

First practical centrifugal

1899 A. C. E. Rateau
compressor

1927 Aurel Boleslav Stodola Formalized "slip factor"

1928 Adolf Busemann Derived "slip factor"

Frank Whittle and Hans First gas turbine using a

1937
von Ohain, independently centrifugal compressor

3D-CFD, rocket turbo-pumps,

>197
Modern turbomachines heart assist pumps,
0
turbocharged fuel cells

Turbomachinery similarities
[edit]
Centrifugal compressors are similar in many ways to other
turbomachinery and are compared and contrasted as follows:
Similarities to axial compressor
[edit]

Cutaway showing an axi-

centrifugal compressor gas turbine

Centrifugal compressors are similar to axial compressors in that they

are rotating airfoil-based compressors. Both are shown in the adjacent
photograph of an engine with 5 stages of axial compressors and one
stage of a centrifugal compressor.[10] The first part of the centrifugal
impeller looks very similar to an axial compressor. This first part of the
centrifugal impeller is also termed an inducer. Centrifugal
compressors differ from axials as they use a significant change in
radius from inlet to exit of the impeller to produce a much greater
pressure rise in a single stage (e.g. 8[19] in the Pratt & Whitney
Canada PW200 series of helicopter engines) than does an axial stage.
The 1940s-era German Heinkel HeS 011 experimental engine was the
first aviation turbojet to have a compressor stage with radial flow-
turning part-way between none for an axial and 90 degrees for a
centrifugal. It is known as a mixed/diagonal-flow compressor. A
diagonal stage is used in the Pratt & Whitney Canada PW600 series of
small turbofans.
Centrifugal fan
[edit]

A low speed, low-pressure centrifugal

compressor or centrifugal fan, with upward discharging cone used to
diffuse the air velocity

Centrifugal compressors are also similar to centrifugal fans of the

style shown in the neighboring figure as they both increase the energy
of the flow through the increasing radius.[1] In contrast to centrifugal
fans, compressors operate at higher speeds to generate greater
pressure rises. In many cases, the engineering methods used to
design a centrifugal fan are the same as those to design a centrifugal
compressor, so they can look very similar.
For purposes of generalization and definition, it can be said that
centrifugal compressors often have density increases greater than 5
percent. Also, they often experience relative fluid velocities above
Mach number 0.3[20] when the working fluid is air or nitrogen. In
contrast, fans or blowers are often considered to have density
increases of less than five percent and peak relative fluid velocities
below Mach 0.3.
Squirrel-cage fan
[edit]
 A low-speed, low-pressure
blower used for HVAC ventilation

Squirrel-cage fans are primarily used for ventilation. The flow field
within this type of fan has internal recirculations. In comparison, a
centrifugal fan is uniform circumferentially.
Centrifugal pump
[edit]

A 3D-solids model of a type of centrifugal pump

Cut-away of a centrifugal pump

Centrifugal compressors are also similar to centrifugal pumps[1] of
the style shown in the adjacent figures. The key difference between
such compressors and pumps is that the compressor working fluid is a
gas (compressible) and the pump working fluid is liquid
(incompressible). Again, the engineering methods used to design a
centrifugal pump are the same as those to design a centrifugal
compressor. Yet, there is one important difference: the need to deal
with cavitation in pumps.
Radial turbine
[edit]
Centrifugal compressors also look very similar to their turbomachinery
counterpart the radial turbine as shown in the figure. While a
compressor transfers energy into a flow to raise its pressure, a turbine
operates in reverse, by extracting energy from a flow, thus reducing
its pressure.[citation needed] In other words, power is input to
compressors and output from turbines.
Turbomachinery using centrifugal compressors
[edit]
Standards
[edit]
As turbomachinery became more common, standards have been
created to guide manufacturers to assure end-users that their
products meet minimum safety and performance requirements.
Associations formed to codify these standards rely on manufacturers,
end-users, and related technical specialists. A partial list of these
associations and their standards are listed below:

American Society of Mechanical Engineers :BPVC, PTC.[21]


[22]
 American Petroleum Institute: API STD 617 8TH ED (E1), API
STD 672 5TH ED (2019).[23][24]
 American Society of Heating, Refrigeration, and
Airconditioning Engineers: Handbook Fundamentals.[25]
 Society of Automotive Engineers[26]
 Compressed Air and Gas Institute[27]
 International Organization for Standardization ISO 10439,
ISO 10442, ISO 18740, ISO 6368, ISO 5389[28]
Applications
[edit]
Below, is a partial list of centrifugal compressor applications each with
a brief description of some of the general characteristics possessed by
those compressors. To start this list two of the most well-known
centrifugal compressor applications are listed; gas turbines and
turbochargers.[10]

Figure 4.1 – Jet engine cutaway

showing the centrifugal compressor and other parts

Figure 4.2 – Jet engine cross

section showing the centrifugal compressor and other parts

 In gas turbines and auxiliary power units.[29] Ref. Figures

4.1–4.2In their simple form, modern gas turbines operate on
the Brayton cycle. (ref Figure 5.1) Either or both axial and
centrifugal compressors are used to provide compression.
The types of gas turbines that most often include centrifugal
compressors include small aircraft engines (i.e. turboshafts,
turboprops, and turbofans), auxiliary power units, and
micro-turbines. The industry standards applied to all
centrifugal compressors used in aircraft applications are set
by the relevant civilian and military certification authorities
to achieve the safety and durability required in service.
Centrifugal impellers used in gas turbines are commonly
made from titanium alloy forgings. Their flow-path blades
are commonly flank milled or point milled on 5-axis milling
machines. When running clearances have to be as small as
possible without the impeller rubbing its shroud the impeller
is first drawn with its high-temperature, high-speed
deflected shape and then drawn in its equivalent cold static
shape for manufacturing. This is necessary because the
 impeller deflections at the most severe running condition
can be 100 times larger than the required hot running
clearance between the impeller and its shroud.
 In automotive engine and diesel engine turbochargers and
superchargers.[30] Ref. Figure 1.1Centrifugal compressors
used in conjunction with reciprocating internal combustion
engines are known as turbochargers if driven by the
engine's exhaust gas and turbo-superchargers if
mechanically driven by the engine. Standards set by the
industry for turbochargers may have been established by
SAE.[26] Ideal gas properties often work well for the design,
test and analysis of turbocharger centrifugal compressor
performance.
 In pipeline compressors of natural gas to move the gas from
the production site to the consumer.[31]Centrifugal
compressors for such uses may be one- or multi-stage and
driven by large gas turbines. Standards set by the industry
(ANSI/API, ASME) result in thick casings to achieve a
required level of safety. The impellers are often if not always
of the covered style which makes them look much like pump
impellers. This type of compressor is also often termed an
API-style. The power needed to drive these compressors is
most often in the thousands of horsepower (HP). The use of
real gas properties is needed to properly design, test, and
analyze the performance of natural gas pipeline centrifugal
compressors.
 In oil refineries, natural-gas processing, petrochemical and
chemical plants.[31]Centrifugal compressors for such uses
are often one-shaft multi-stage and driven by large steam or
gas turbines. Their casings are termed horizontally split if
the rotor is lowered into the bottom half during assembly or
barrel if it has no lengthwise split-line with the rotor being
slid in. Standards set by the industry (ANSI/API, ASME) for
these compressors result in thick casings to achieve a
required level of safety. The impellers are often of the
covered style which makes them look much like pump
impellers. This type of compressor is also often termed API-
style. The power needed to drive these compressors is
usually in the thousands of HP. Use of real gas properties is
needed to properly design, test and analyze their
performance.
 Air-conditioning and refrigeration and HVAC: Centrifugal
compressors quite often supply the compression in water
chillers cycles.[32]Because of the wide variety of vapor
compression cycles (thermodynamic cycle,
thermodynamics) and the wide variety of working fluids
(refrigerants), centrifugal compressors are used in a variety
of sizes and configurations. Use of real gas properties is
needed to properly design, test and analyze the
performance of these machines. Standards set by the
 industry for these compressors include ASHRAE, ASME &
API.
 In industry and manufacturing to supply compressed air for
all types of pneumatic tools.[33]Centrifugal compressors for
such uses are often multistage and driven by electric
motors. Inter-cooling is often needed between stages to
control air temperature. Road-repair crews and automobile
repair garages find screw compressors better adapt to their
needs. Standards set by the industry for these compressors
include ASME and government regulations that emphasize
safety. Ideal gas relationships are often used to properly
design, test, and analyze the performance of these
machines. Carrier's equation is often used to deal with
humidity.
 In air separation plants to manufacture purified end product
gases.[33]Centrifugal compressors for such uses are often
multistage using inter-cooling to control air temperature.
Standards set by the industry for these compressors include
ASME and government regulations that emphasize safety.
Ideal gas relationships are often used to properly design,
test, and analyze the performance of these machines when
the working gas is air or nitrogen. Other gases require real
gas properties.
 In oil field re-injection of high-pressure natural gas to
improve oil recovery.[31]Centrifugal compressors for such
uses are often one-shaft multi-stage and driven by gas
turbines. With discharge pressures approaching 700 bar,
casings are of the barrel style. Standards set by the industry
(API, ASME) for these compressors result in large thick
casings to maximize safety. The impellers are often if not
always of the covered style which makes them look much
like pump impellers. This type of compressor is also often
termed API-style. The use of real gas properties is needed to
properly design, test, and analyze their performance.
Theory of operation
[edit]
In the case where flow passes through a straight pipe to enter a
centrifugal compressor, the flow is axial, uniform, and has no vorticity,
i.e. swirling motion. As the flow passes through the centrifugal
impeller, the impeller forces the flow to spin faster as it gets further
from the rotational axis. According to a form of Euler's fluid dynamics
equation, known as the pump and turbine equation, the energy input
to the fluid is proportional to the flow's local spinning velocity
multiplied by the local impeller tangential velocity.
In many cases, the flow leaving the centrifugal impeller is traveling
near the speed of sound. It then flows through a stationary
compressor causing it to decelerate. The stationary compressor is
ducting with increasing flow-area where energy transformation takes
place. If the flow has to be turned in a rearward direction to enter the
next part of the machine, e.g. another impeller or a combustor, flow
losses can be reduced by directing the flow with stationary turning
vanes or individual turning pipes (pipe diffusers). As described in
Bernoulli's principle, the reduction in velocity causes the pressure to
rise.[1]
Performance
[edit]

Figure 5.1 – Illustration of

the Brayton cycle as applied to a gas turbine

Figure 5.2 – Example

centrifugal compressor performance map

While illustrating a gas turbine's Brayton cycle,[15] Figure 5.1 includes

example plots of pressure-specific volume and temperature-entropy.
These types of plots are fundamental to understanding centrifugal
compressor performance at one operating point. The two plots show
that the pressure rises between the compressor inlet (station 1) and
compressor exit (station 2). At the same time, the specific volume
decreases while the density increases. The temperature-entropy plot
shows that the temperature increases with increasing entropy (loss).
Assuming dry air, and the ideal gas equation of state and an
isentropic process, there is enough information to define the pressure
ratio and efficiency for this one point. The compressor map is required
to understand the compressor performance over its complete
operating range.
Figure 5.2, a centrifugal compressor performance map (either test or
estimated), shows the flow, pressure ratio for each of 4 speed-lines
(total of 23 data points). Also included are constant efficiency
contours. Centrifugal compressor performance presented in this form
provides enough information to match the hardware represented by
the map to a simple set of end-user requirements.
Compared to estimating performance which is very cost effective
(thus useful in design), testing, while costly, is still the most precise
method.[12] Further, testing centrifugal compressor performance is
very complex. Professional societies such as ASME (i.e. PTC–10, Fluid
Meters Handbook, PTC-19.x),[34] ASHRAE (ASHRAE Handbook) and
API (ANSI/API 617–2002, 672–2007)[31][33] have established
standards for detailed experimental methods and analysis of test
results. Despite this complexity, a few basic concepts in performance
can be presented by examining an example test performance map.
Performance maps
[edit]
Pressure ratio and flow are the main parameters[15][31][33]
[34] needed to match the Figure 5.2 performance map to a simple
compressor application. In this case, it can be assumed that the inlet
temperature is sea-level standard. This assumption is not acceptable
in practice as inlet temperature variations cause significant variations
in compressor performance. Figure 5.2 shows:

Corrected mass flow: 0.04 – 0.34 kg/s

Total pressure ratio, inlet to discharge (PRt-t =
Pt,discharge/Pt,inlet): 1.0 – 2.6
As is standard practice, Figure 5.2 has a horizontal axis labeled with a
flow parameter. While flow measurements use a variety of units, all fit
one of 2 categories:
Mass flow per unit time
[edit]
Mass flow units, such as kg/s, are the easiest to use in practice as
there is little room for confusion. Questions remaining would involve
inlet or outlet (which might involve leakage from the compressor or
moisture condensation). For atmospheric air, the mass flow may be
wet or dry (including or excluding humidity). Often, the mass flow
specification will be presented on an equivalent Mach number basis,

.[35] It is standard in these cases that the equivalent

temperature, equivalent pressure, and gas is specified explicitly or
implied at a standard condition.
Volume flow per unit time
[edit]
In contrast, all volume flow specifications require the additional
specification of density. Bernoulli's fluid dynamic principle is of great
value in understanding this problem. Confusion arises through either
inaccuracies or misuse of pressure, temperature, and gas constants.
Also as is standard practice, Figure 5.2 has a vertical axis labeled with
a pressure parameter. There is a variety of pressure measurement
units. They all fit one of two categories:

A △pressure, ie increase from inlet to exit (measured with a


manometer)
 A discharge pressure
The pressure rise may alternatively be specified as a ratio that has no
units:

A pressure ratio (exit/inlet)

Other features common to performance maps are:
Constant speed-lines
[edit]
The two most common methods for producing a map for a centrifugal
compressor are at constant shaft speed or with a constant throttle
setting. If the speed is held constant, test points are taken along a
constant speed line by changing throttle positions. In contrast, if a
throttle valve is held constant, test points are established by changing
speed and repeated with different throttle positions (common gas
turbine practice). The map shown in Figure 5.2 illustrates the most
common method; lines of constant speed. In this case, we see data
points connected via straight lines at speeds of 50%, 71%, 87%, and
100% RPM. The first three speed-lines have 6 points each while the
highest speed line has five.
Constant efficiency islands
[edit]
The next feature to be discussed is the oval-shaped curves
representing islands of constant efficiency. In this figure we see 11
contours ranging from 56% efficiency (decimal 0.56) to 76% efficiency
(decimal 0.76). General standard practice is to interpret these
efficiencies as isentropic rather than polytropic. The inclusion of
efficiency islands effectively generates a 3-dimensional topology to
this 2-dimensional map. With inlet density specified, it provides a
further ability to calculate aerodynamic power. Lines of constant
power could just as easily be substituted.
Design or guarantee point(s)
[edit]
Regarding gas turbine operation and performance, there may be a
series of guaranteed points established for the gas turbine's
centrifugal compressor. These requirements are of secondary
importance to the overall gas turbine performance as a whole. For this
reason, it is only necessary to summarize that in the ideal case, the
lowest specific fuel consumption would occur when the centrifugal
compressor's peak efficiency curve coincides with the gas turbine's
required operation line.
In contrast to gas turbines, most other applications (including
industrial) need to meet a less stringent set of performance
requirements. Historically, centrifugal compressors applied to
industrial applications were needed to achieve performance at a
specific flow and pressure. Modern industrial compressors are often
needed to achieve specific performance goals across a range of flows
and pressures; thus taking a significant step toward the sophistication
seen in gas turbine applications.
If the compressor represented in Figure 5.2 is used in a simple
application, any point (pressure and flow) within the 76% efficiency
would provide very acceptable performance. An "End User" would be
very happy with the performance requirements of 2.0 pressure ratio at
0.21 kg/s.
Surge
[edit]
Surge - is a low flow phenomenon where the impeller cannot add
enough energy to overcome the system resistance or backpressure.
[36] At low flow rate operation, the pressure ratio over the impeller is
high, as is back system backpressure. Under critical conditions, the
flow will reverse back over the tips of the rotor blades towards the
impeller eye (inlet).[37] This stalling flow reversal may go unnoticed
as the fraction of mass flow or energy is too low. When large enough,
rapid flow reversal occurs(i.e., surge). The reversed flow exiting the
impeller inlet exhibits a strong rotational component, which affects
lower radius flow angles (closer to the impeller hub) at the leading
edge of the blades. The deterioration of the flow angles causes the
impeller to be inefficient. A full flow reversal can occur. (Therefore,
surge is sometimes referred to as axisymmetric stall.) When reversed
flow reduces to a low enough level, the impeller recovers and regains
stability for a short moment at which point the stage may surge again.
These cyclic events cause large vibrations, increase temperature and
change rapidly the axial thrust. These occurrences can damage the
rotor seals, rotor bearings, the compressor driver, and cycle operation.
Most turbomachines are designed to easily withstand occasional
surging. However, if the machine is forced to surge repeatedly for a
long period of time, or if it is poorly designed, repeated surges can
result in a catastrophic failure. Of particular interest, is that while
turbomachines may be very durable, their physical system can be far
less robust.
Surge line
[edit]

Figure-6.2.1 Stall formation

The surge-line shown in Figure 5.2 is the curve that passes through
the lowest flow points of each of the four speed-lines. As a test map,
these points would be the lowest flow points possible to record a
stable reading within the test facility/rig. In many industrial
applications, it may be necessary to increase the stall line due to the
system backpressure. For example, at 100% RPM stalling flow might
increase from approximately 0.170 kg/s to 0.215 kg/s because of the
positive slope of the pressure ratio curve.
As stated earlier, the reason for this is that the high-speed line in
Figure 5.2 exhibits a stalling characteristic or positive slope within that
range of flows. When placed in a different system those lower flows
might not be achievable because of interaction with that system.
System resistance or adverse pressure is proven mathematically to be
the critical contributor to compressor surge.
Maximum flow line versus choke
[edit]
Choke occurs under one of 2 conditions. Typically for high speed
equipment, as flow increases the velocity of the flow can approach
sonic speed somewhere within the compressor stage. This location
 may occur at the impeller inlet "throat" or at the vaned diffuser inlet
"throat". In contrast, for lower speed equipment, as flows increase,
losses increase such that the pressure ratio eventually drops to 1:1. In
this case, the occurrence of choke is unlikely.
The speed-lines of gas turbine centrifugal compressors typically
exhibit choke. This is a situation where the pressure ratio of a speed
line drops rapidly (vertically) with little or no change in flow. In most
cases the reason for this is that close to Mach 1 velocities have been
reached somewhere within the impeller and/or diffuser generating a
rapid increase in losses. Higher pressure ratio turbocharger centrifugal
compressors exhibit this same phenomenon. Real choke phenomena
is a function of compressibility as measured by the local Mach number
within an area restriction within the centrifugal pressure stage.
The maximum flow line, shown in Figure 5.2, is the curve that passes
through the highest flow points of each speed line. Upon inspection it
may be noticed that each of these points has been taken near 56%
efficiency. Selecting a low efficiency (<60%) is the most common
practice used to terminate compressor performance maps at high
flows. Another factor that is used to establish the maximum flow line
is a pressure ratio near or equal to 1. The 50% speed line may be
considered an example of this.
The shape of Figure 5.2's speed-lines provides a good example of why
it is inappropriate to use the term choke in association with a
maximum flow of all centrifugal compressor speed-lines. In summary;
most industrial and commercial centrifugal compressors are selected
or designed to operate at or near their highest efficiencies and to
avoid operation at low efficiencies. For this reason there is seldom a
reason to illustrate centrifugal compressor performance below 60%
efficiency.
Many industrial and commercial multistage compressor performance
maps exhibits this same vertical characteristic for a different reason
related to what is known as stage stacking.
Other operating limits
[edit]
Minimum operating speed
The minimum speed for acceptable operation, below this value the
compressor may be controlled to stop or go into an "idle" condition.
Maximum allowable speed
The maximum operating speed for the compressor. Beyond this value
stresses may rise above prescribed limits and rotor vibrations may increase
rapidly. At speeds above this level the equipment will likely become very
dangerous and be controlled to lower speeds.
Dimensional analysis
[edit]
To weigh the advantages between centrifugal compressors it is
important to compare 8 parameters classic to turbomachinery.
Specifically, pressure rise (p), flow (Q), angular speed (N),
power (P), density (ρ), diameter (D), viscosity (μ) and elasticity
(e). This creates a practical problem when trying to
experimentally determine the effect of any one parameter. This
is because it is nearly impossible to change one of these
parameters independently.
The method of procedure known as the Buckingham π theorem
can help solve this problem by generating 5 dimensionless
forms of these parameters.[1][citation needed][16] These Pi
parameters provide the foundation for "similitude" and the
"affinity-laws" in turbomachinery. They provide for the creation
of additional relationships (being dimensionless) found valuable
in the characterization of performance.
For the example below Head will be substituted for pressure
and sonic velocity will be substituted for elasticity.
Buckingham Π theorem
[edit]
Main article: Buckingham π theorem
The three independent dimensions used in this procedure for
turbomachinery are:

 mass (force is an alternative)

 length

 time
According to the theorem each of the eight main parameters
are equated to its independent dimensions as follows:

Flow ex. = m3/s

ex. =
Head
kg·m/s2
Speed ex. = m/s

ex. =
Power
kg·m2/s3

Density ex. = kg/m3

Viscosity ex. = kg/m·s

Diameter ex. = m

Speed of
ex. = m/s
sound

Classic turbomachinery similitude

[edit]
Completing the task of following the formal procedure results in
generating this classic set of five dimensionless parameters for
turbomachinery.[1] Full-similitude is achieved when each one of
the 5 Pi-parameters is equivalent when comparing two different
cases. This of course would mean the two turbomachines being
compared are similar, both geometrically and in terms of
performance.

1 Flow-coefficient

Head-coefficient

3 Speed-coefficient

4 Power-coefficient

Reynolds-
5
coefficient

Turbomachinery analysts gain tremendous insight into

performance by comparisons of the 5 parameters shown in the
above table. Particularly, performance parameters such as
efficiencies and loss-coefficients, which are also dimensionless.
In general application, the Flow-coefficient and Head-coefficient
are considered of primary importance. Generally, for
centrifugal compressors, the Speed-coefficient is of secondary
importance while the Reynolds-coefficient is of tertiary
importance. In contrast, as expected for pumps, the Reynolds-
coefficient becomes of secondary importance and the Speed-
coefficient of tertiary importance. It may be found interesting
that the Speed-coefficient may be chosen to define the y-axis
of Figure 1.1, while at the same time the Reynolds coefficient
may be chosen to define the z-axis.
Other dimensionless combinations
[edit]
Demonstrated in the table below is another value of
dimensional analysis. Any number of new dimensionless
parameters can be calculated through exponents and
multiplication. For example, a variation of the first parameter
shown below is popularly used in aircraft engine system
analysis. The third parameter is a simplified dimensional
variation of the first and second. This third definition is
applicable with strict limitations. The fourth parameter, specific
speed, is very well known and useful in that it removes
diameter. The fifth parameter, specific diameter, is a less often
discussed dimensionless parameter found useful by Balje.[38]

1 Corrected mass flow coefficient

2 Alternate#1 equivalent Mach form

Alternate#2 simplified dimensional

3
form

4 Specific speed coefficient

5 Specific diameter coefficient

It may be found interesting that the specific speed coefficient

may be used in place of speed to define the y-axis of Figure
1.2, while at the same time, the specific diameter coefficient
may be in place of diameter to define the z-axis.
Affinity laws
[edit]
The following affinity laws are derived from the five Π-
parameters shown above. They provide a simple basis for
scaling turbomachinery from one application to the next.

From flow coefficient

From head coefficient

From power
coefficient

Aero-thermodynamic fundamentals
[edit]
The following equations outline a fully three-dimensional
mathematical problem that is very difficult to solve even with
simplifying assumptions.[10][39] Until recently, limitations in
computational power, forced these equations to be simplified
to an inviscid two-dimensional problem with pseudo losses.
Before the advent of computers, these equations were almost
always simplified to a one-dimensional problem.
Solving this one-dimensional problem is still valuable today and
is often termed mean-line analysis. Even with all of this
simplification it still requires large textbooks to outline and
large computer programs to solve practically.
Conservation of mass
[edit]
Also termed continuity, this fundamental equation written in
general form is as follows:

Conservation of momentum
[edit]
Also termed the Navier–Stokes equations, this fundamental
is derivable from Newton's second law when applied to fluid
motion. Written in compressible form for a Newtonian fluid,
this equation may be written as follows:
Conservation of energy
[edit]
The first law of thermodynamics is the statement of the
conservation of energy. Under specific conditions, the
operation of a Centrifugal compressor is considered a
reversible process. For a reversible process, the total
amount of heat added to a system can be expressed as

where is temperature and is entropy.

Therefore, for a reversible process:

Since U, S and V are thermodynamic functions of

state, the above relation holds also for non-
reversible changes. The above equation is known as
the fundamental thermodynamic relation.
Equation of state
[edit]
The classical ideal gas law may be written:

The ideal gas law may also be expressed as

follows

where is the density, is the

adiabatic index (ratio of specific heats),

is the internal energy per unit mass (the

"specific internal energy"), is the specific

heat at constant volume, and is the

specific heat at constant pressure.
With regard to the equation of state, it is
important to remember that while air and
nitrogen properties (near standard
atmospheric conditions) are easily and
accurately estimated by this simple
relationship, there are many centrifugal
compressor applications where the ideal
relationship is inadequate. For example,
centrifugal compressors used for large air
conditioning systems (water chillers) use a
refrigerant as a working gas that cannot be
modeled as an ideal gas. Another example
are centrifugal compressors design and built
for the petroleum industry. Most of the
hydrocarbon gases such as methane and
ethylene are best modeled as a real
gas equation of state rather than ideal gases.
The Wikipedia entry for equations of state is
very thorough.
Pros and cons
[edit]
Pros

 Centrifugal compressors offer the

advantages of simplicity of
manufacturing and relatively low
cost. This is due to requiring fewer
stages to achieve the same
pressure rise.
 Centrifugal compressors are used
throughout industry because they
have fewer rubbing parts, are
relatively energy efficient, and give
higher and non-oscillating constant
airflow than a similarly sized
reciprocating compressor or any
other positive displacement pump.
 Centrifugal compressors are mostly
used as turbochargers and in small
gas turbine engines like in an APU
(auxiliary power unit) and as main
engine for smaller aircraft like
helicopters. A significant reason for
this is that with current technology,
the equivalent airflow axial
compressor will be less efficient due
primarily to a combination of rotor
and variable stator tip-clearance
losses.
Cons
 Their main drawback is that they
cannot achieve the high
compression ratio of reciprocating
compressors without multiple
stages. There are few one-stage
centrifugal compressors capable of
pressure ratios over 10:1, due to
stress considerations which
severely limit the compressor's
safety, durability and life
expectancy.
 Centrifugal compressors are
impractical, compared to axial
compressors, for use in large gas
turbines and turbojet engines
propelling large aircraft, due to the
resulting weight and stress, and to
the frontal area presented by the
large diameter of the radial diffuser.
Structural mechanics, manufacture and
design compromise

[edit]
Ideally, centrifugal compressor impellers have
thin air-foil blades that are strong, each
mounted on a light rotor. This material would
be easy to machine or cast and inexpensive.
Additionally, it would generate no operating
noise, and have a long life while operating in
any environment.[clarification needed]
From the very start of the aero-
thermodynamic design process, the
aerodynamic considerations and
optimizations [29,30] are critical to have a
successful design. during the design, the
centrifugal impeller's material and
manufacturing method must be accounted for
within the design, whether it be plastic for a
vacuum cleaner blower, aluminum alloy for a
turbocharger, steel alloy for an air compressor
or titanium alloy for a gas turbine. It is a
combination of the centrifugal compressor
impeller shape, its operating environment, its
material and its manufacturing method that
determines the impeller's structural integrity
https://en.wikipedia.org/wiki/
Centrifugal_compressor

A compressor is a mechanical device that

increases the pressure of a gas by reducing
its volume. An air compressor is a specific
type of gas compressor.
Many compressors can be staged, that is, the
gas is compressed several times in steps or
stages, to increase discharge pressure. Often,
the second stage is physically smaller than
the primary stage, to accommodate the
already compressed gas without reducing its
pressure. Each stage further compresses the
gas and increases its pressure and also
temperature (if inter cooling between stages
is not used).
Types
[edit]
Compressors are similar to pumps: both
increase the pressure on a fluid (such as a
gas) and both can transport the fluid through
a pipe. The main distinction is that the focus
of a compressor is to change the density or
volume of the fluid, which is mostly only
achievable on gases. Gases are compressible,
while liquids are relatively incompressible, so
compressors are rarely used for liquids. The
main action of a pump is to pressurize and
transport liquids.
The main and important types of gas
compressors are illustrated and discussed
below:
Positive displacement
[edit]
A positive displacement compressor is a
system that compresses the air by the
displacement of a mechanical linkage
reducing the volume (since the reduction in
volume due to a piston in thermodynamics is
considered as positive displacement of the
piston).[vague]
Put another way, a positive displacement
compressor is one that operates by drawing in
a discrete volume of gas from its inlet then
forcing that gas to exit via the compressor's
outlet. The increase in the pressure of the gas
is due, at least in part, to the compressor
pumping it at a mass flow rate which cannot
pass through the outlet at the lower pressure
and density of the inlet.
Reciprocating compressors
[edit]
Main article: Reciprocating compressor
 A motor-driven
six-cylinder reciprocating compressor that can
operate with two, four or six cylinders.

Reciprocating compressors use

pistons driven by a crankshaft. They can be
either stationary or portable, can be single or
multi-staged, and can be driven by electric
motors or internal combustion engines.[1][2]
[3] Small reciprocating compressors from 5 to
30 horsepower (hp) are commonly seen in
automotive applications and are typically for
intermittent duty. Larger reciprocating
compressors well over 1,000 hp (750 kW) are
commonly found in large industrial and
petroleum applications. Discharge
pressures can range from low pressure to very
high pressure (>18000 psi or 124 MPa). In
certain applications, such as air compression,
multi-stage double-acting compressors are
said to be the most efficient compressors
available, and are typically larger, and more
costly than comparable rotary units.
[4] Another type of reciprocating compressor,
usually employed in automotive cabin air
conditioning systems,[citation needed] is the
swash plate or wobble plate compressor,
which uses pistons moved by a swash plate
mounted on a shaft (see axial piston pump).
Household, home workshop, and smaller job
site compressors are typically reciprocating
compressors 1.5 hp (1.1 kW) or less with an
attached receiver tank.
A linear compressor is a reciprocating
compressor with the piston being the rotor of
a linear motor.
This type of compressor can compress a wide
range of gases, including refrigerant,
hydrogen, and natural gas. Because of this, it
finds use in a wide range of applications in
many different industries and can be
designed to a wide range of capacities, by
varying size, number of cylinders, and
cylinder unloading. However, it suffers from
higher losses due to clearance volumes,
resistance due to discharge and suction
valves, weighs more, is difficult to maintain
due to having a large number of moving
parts, and it has inherent vibration.[5]
Ionic liquid piston compressor
[edit]
Main article: Ionic liquid piston compressor
An ionic liquid piston compressor, ionic
compressor or ionic liquid piston pump is a
hydrogen compressor based on an ionic
liquid piston instead of a metal piston as in a
piston-metal diaphragm compressor.
Rotary screw compressors
[edit]

aDiagram of a
rotary screw compressor

Main article: Rotary screw compressor

Rotary screw compressors use two
meshed rotating positive-displacement helical
screws to force the gas into a smaller space.
[1][6][7] These are usually used for
continuous operation in commercial and
industrial applications and may be either
stationary or portable. Their application can
be from 3 horsepower (2.2 kW) to over 1,200
horsepower (890 kW) and from low pressure
to moderately high pressure (>1,200 psi or
8.3 MPa).
The classifications of rotary screw
compressors vary based on stages, cooling
methods, and drive types among others.
[8] Rotary screw compressors are
commercially produced in Oil Flooded, Water
Flooded and Dry type. The efficiency of rotary
compressors depends on the air drier,
[clarification needed] and the selection of air
drier is always 1.5 times volumetric delivery
of the compressor.[9]
Designs with a single screw[10] or three
screws[11] instead of two exist.
Screw compressors have fewer moving
components, larger capacity, less vibration
and surging, can operate at variable speeds,
and typically have higher efficiency. Small
sizes or low rotor speeds are not practical due
to inherent leaks caused by clearance
between the compression cavities or screws
and compressor housing.[5] They depend on
fine machining tolerances to avoid high
leakage losses and are prone to damage if
operated incorrectly or poorly serviced.
Rotary vane compressors
[edit]
 Ecce
ntric rotary-vane pump

See also: Rotary vane pump

Rotary vane compressors consist of a rotor
with a number of blades inserted in radial
slots in the rotor. The rotor is mounted offset
in a larger housing that is either circular or a
more complex shape. As the rotor turns,
blades slide in and out of the slots keeping
contact with the outer wall of the housing.
[1] Thus, a series of increasing and
decreasing volumes is created by the rotating
blades. Rotary vane compressors are, with
piston compressors one of the oldest of
compressor technologies.
With suitable port connections, the devices
may be either a compressor or a vacuum
pump. They can be either stationary or
portable, can be single or multi-staged, and
can be driven by electric motors or internal
combustion engines. Dry vane machines are
used at relatively low pressures (e.g., 2 bar or
200 kPa or 29 psi) for bulk material
movement while oil-injected machines have
the necessary volumetric efficiency to achieve
pressures up to about 13 bar (1,300 kPa;
190 psi) in a single stage. A rotary vane
compressor is well suited to electric motor
drive and is significantly quieter in operation
than the equivalent piston compressor.
Rotary vane compressors can have
mechanical efficiencies of about 90%.[12]
Rolling piston
[edit]

Rolling piston
compressor

The Rolling piston in a rolling piston style

compressor plays the part of a partition
between the vane and the rotor.[13] Rolling
piston forces gas against a stationary vane.
2 of these compressors can be mounted on
the same shaft to increase capacity and
reduce vibration and noise.[14] A design
without a spring is known as a swing
compressor.[15]
In refrigeration and air conditioning, this type
of compressor is also known as a rotary
compressor, with rotary screw compressors
being also known simply as screw
compressors.
It offers higher efficiency than reciprocating
compressors due to less losses from the
clearance volume between the piston and the
compressor casing, it's 40% to 50% smaller
and lighter for a given capacity (which can
impact material and shipping costs when
used in a product), causes less vibration, has
fewer components and is more reliable than a
reciprocating compressor. But its structure
does not allow capacities beyond 5
refrigeration tons, is less reliable than other
compressor types, and is less efficient than
other compressor types due to losses from
the clearance volume.[5]
Scroll compressors
[edit]

Mechanism of a scroll
pump

Main article: Scroll compressor

A scroll compressor, also known as scroll
pump and scroll vacuum pump, uses two
interleaved spiral-like vanes to pump or
compress fluids such as liquids and gases.
The vane geometry may be involute,
archimedean spiral, or hybrid curves.[16][17]
[18] They operate more smoothly, quietly,
and reliably than other types of compressors
in the lower volume range.
Often, one of the scrolls is fixed, while the
other orbits eccentrically without rotating,
thereby trapping and pumping or
compressing pockets of fluid between the
scrolls.
Due to minimum clearance volume between
the fixed scroll and the orbiting scroll, these
compressors have a very high volumetric
efficiency.
These compressors are extensively used in air
conditioning and refrigeration because they
are lighter, smaller and have fewer moving
parts than reciprocating compressors and
they are also more reliable. They are more
expensive though, so peltier coolers or rotary
and reciprocating compressors may be used
in applications where cost is the most
important or one of the most important
factors to consider when designing a
refrigeration or air conditioning system.
This type of compressor was used as the
supercharger on Volkswagen G60 and G40
engines in the early 1990s.
When compared with reciprocating and rolling
piston compressors, scroll compressors are
more reliable since they have fewer
components and have a simpler structure, are
more efficient since they have no clearance
volume nor valves, and possess the
advantages both of surging less and not
vibrating so much. But, when compared with
screw and centrifugal compressors, scroll
compressors have lower efficiencies and
smaller capacities.[5]
Diaphragm compressors
[edit]
Main article: Diaphragm compressor
A diaphragm compressor (also known as a
membrane compressor) is a variant of the
conventional reciprocating compressor. The
compression of gas occurs by the movement
of a flexible membrane, instead of an intake
element. The back-and-forth movement of the
membrane is driven by a rod and a crankshaft
mechanism. Only the membrane and the
compressor box come in contact with the gas
being compressed.[1]
The degree of flexing and the material
constituting the diaphragm affects the
maintenance life of the equipment. Generally
stiff metal diaphragms may only displace a
few cubic centimeters of volume because the
metal cannot endure large degrees of flexing
without cracking, but the stiffness of a metal
diaphragm allows it to pump at high
pressures. Rubber or silicone diaphragms are
capable of enduring deep pumping strokes of
very high flexion, but their low strength limits
their use to low-pressure applications, and
they need to be replaced as plastic
embrittlement occurs.
Diaphragm compressors are used for
hydrogen and compressed natural gas (CNG)
as well as in a number of other applications.

A three-stage
diaphragm compressor

The photograph on the right depicts a three-

stage diaphragm compressor used to
compress hydrogen gas to 6,000 psi (41 MPa)
for use in a prototype compressed
hydrogen and compressed natural gas (CNG)
fueling station built in downtown Phoenix,
Arizona by the Arizona Public
Service company (an electric utilities
company). Reciprocating compressors were
used to compress the natural gas. The
reciprocating natural gas compressor was
developed by Sertco.[19]
The prototype alternative fueling station was
built in compliance with all of the prevailing
safety, environmental and building codes in
Phoenix to demonstrate that such fueling
stations could be built in urban areas.
Dynamic
[edit]
Air bubble compressor
[edit]
Also known as a trompe. A mixture of air and
water generated through turbulence is
allowed to fall into a subterranean chamber
where the air separates from the water. The
weight of falling water compresses the air in
the top of the chamber. A submerged outlet
from the chamber allows water to flow to the
surface at a lower height than the intake. An
outlet in the roof of the chamber supplies the
compressed air to the surface. A facility on
this principle was built on the Montreal
River at Ragged Shutes near Cobalt,
Ontario in 1910 and supplied 5,000
horsepower to nearby mines.[20]
Centrifugal compressors
[edit]

A single stage
centrifugal compressor

A single stage
centrifugal compressor, early 1900s, G.
Schiele & Co., Frankfurt am Main

Main article: Centrifugal compressor

Centrifugal compressors use a rotating
disk or impeller in a shaped housing to force
the gas to the rim of the impeller, increasing
the velocity of the gas. A diffuser (divergent
duct) section converts the velocity energy to
pressure energy. They are primarily used for
continuous, stationary service in industries
such as oil refineries, chemical and
petrochemical plants and natural gas
processing plants.[1][21][22] Their application
can be from 100 horsepower (75 kW) to
thousands of horsepower. With multiple
staging, they can achieve high output
pressures greater than 1,000 psi (6.9 MPa).
This type of compressor, along with screw
compressors, are extensively used in large
refrigeration and air conditioning systems.
Magnetic bearing (magnetically levitated) and
air bearing centrifugal compressors exist.
Many large snowmaking operations (like ski
resorts) use this type of compressor. They are
also used in internal combustion engines as
superchargers and turbochargers. Centrifugal
compressors are used in small gas
turbine engines or as the final compression
stage of medium-sized gas turbines.
Centrifugal compressors are the largest
available compressors, offer higher
efficiencies under partial loads, may be oil-
free when using air or magnetic bearings
which increases the heat transfer coefficient
in evaporators and condensers, weigh up to
90% less and occupy 50% less space than
reciprocating compressors, are reliable and
cost less to maintain since less components
are exposed to wear, and only generate
minimal vibration. But, their initial cost is
higher, require highly precise CNC machining,
the impeller needs to rotate at high speeds
making small compressors impractical, and
surging becomes more likely.[5] Surging is gas
flow reversal, meaning that the gas goes from
the discharge to the suction side, which can
cause serious damage, specially in the
compressor bearings and its drive shaft. It is
caused by a pressure on the discharge side
that is higher than the output pressure of the
compressor. This can cause gases to flow
back and forth between the compressor and
whatever is connected to its discharge line,
causing oscillations.[5]
Diagonal or mixed-flow compressors
[edit]
Diagonal or mixed-flow compressors are
similar to centrifugal compressors, but have a
radial and axial velocity component at the
exit from the rotor. The diffuser is often used
to turn diagonal flow to an axial rather than
radial direction.[23] Comparative to the
conventional centrifugal compressor (of the
same stage pressure ratio), the value of the
speed of the mixed flow compressor is 1.5
times larger.[24]
Axial compressors
[edit]

An animation of
an axial compressor.

Main article: Axial compressor

Axial compressors are dynamic rotating
compressors that use arrays of fan-like
airfoils to progressively compress a fluid. They
are used where high flow rates or a compact
design are required.
The arrays of airfoils are set in rows, usually
as pairs: one rotating and one stationary. The
rotating airfoils, also known as blades or
rotors, accelerate the fluid. The stationary
airfoils, also known as stators or vanes,
decelerate and redirect the flow direction of
the fluid, preparing it for the rotor blades of
the next stage.[1] Axial compressors are
almost always multi-staged, with the cross-
sectional area of the gas passage diminishing
along the compressor to maintain an optimum
axial Mach number. Beyond about 5 stages or
a 4:1 design pressure ratio a compressor will
not function unless fitted with features such
as stationary vanes with variable angles
(known as variable inlet guide vanes and
variable stators), the ability to allow some air
to escape part-way along the compressor
(known as interstage bleed) and being split
into more than one rotating assembly (known
as twin spools, for example).
Axial compressors can have high efficiencies;
around 90% polytropic at their design
conditions. However, they are relatively
expensive, requiring a large number of
components, tight tolerances and high quality
materials. Axial compressors are used in
medium to large gas turbine engines, natural
gas pumping stations, and some chemical
plants.
Hermetically sealed, open, or semi-
hermetic

[edit]

A small
hermetically sealed compressor in a common
consumer refrigerator or freezer typically has
a rounded steel outer shell permanently
welded shut, which seals operating gases
inside the system, in this case
an R600a refrigerant. There is no route for
gases to leak, such as around motor shaft
seals. On this model, the plastic top section is
part of an auto-defrost system that uses
motor heat to evaporate the water.

Compressors used in refrigeration systems

must exhibit near-zero leakage to avoid the
loss of the refrigerant if they are to function
for years without service. This necessitates
the use of very effective seals, or even the
elimination of all seals and openings to form a
hermetic system. These compressors are
often described as being either hermetic,
open, or semi-hermetic, to describe how
the compressor is enclosed and how the
motor drive is situated in relation to the gas
or vapor being compressed. Some
compressors outside of refrigeration service
may also be hermetically sealed to some
extent, typically when handling toxic,
polluting, or expensive gasses, with most non-
refrigeration applications being in the
petrochemical industry.
In hermetic and most semi-hermetic
compressors, the compressor and motor
driving the compressor are integrated, and
operate within the pressurized gas envelope
of the system. The motor is designed to
operate in, and be cooled by, the refrigerant
gas being compressed. Open compressors
have an external motor driving a shaft that
passes through the body of the compressor
and rely on rotary seals around the shaft to
retain the internal pressure.
The difference between the hermetic and
semi-hermetic, is that the hermetic uses a
one-piece welded steel casing that cannot be
opened for repair; if the hermetic fails it is
simply replaced with an entire new unit. A
semi-hermetic uses a large cast metal shell
with gasketed covers with screws that can be
opened to replace motor and compressor
components. The primary advantage of a
hermetic and semi-hermetic is that there is no
route for the gas to leak out of the system.
The main advantages of open compressors is
that they can be driven by any motive power
source, allowing the most appropriate motor
to be selected for the application, or even
non-electric power sources such as an internal
combustion engine or steam turbine, and
secondly the motor of an open compressor
can be serviced without opening any part of
the refrigerant system.
An open pressurized system such as an
automobile air conditioner can be more
susceptible to leak its operating gases. Open
systems rely on lubricant in the system to
splash on pump components and seals. If it is
not operated frequently enough, the lubricant
on the seals slowly evaporates, and then the
seals begin to leak until the system is no
longer functional and must be recharged. By
comparison, a hermetic or semi-hermetic
system can sit unused for years, and can
usually be started up again at any time
without requiring maintenance or
experiencing any loss of system pressure.
Even well lubricated seals will leak a small
amount of gas over time, particularly if the
refrigeration gasses are soluble in the
lubricating oil, but if the seals are well
manufactured and maintained this loss is very
low.
The disadvantage of hermetic compressors is
that the motor drive cannot be repaired or
maintained, and the entire compressor must
be replaced if a motor fails. A further
disadvantage is that burnt-out windings can
contaminate the whole systems, thereby
requiring the system to be entirely pumped
down and the gas replaced (This can also
happen in semi hermetic compressors where
the motor operates in the refrigerant).
Typically, hermetic compressors are used in
low-cost factory-assembled consumer goods
where the cost of repair and labor is high
compared to the value of the device, and it
would be more economical to just purchase a
new device or compressor. Semi-hermetic
compressors are used in mid-sized to large
refrigeration and air conditioning systems,
where it is cheaper to repair and/or refurbish
the compressor compared to the price of a
new one. A hermetic compressor is simpler
and cheaper to build than a semi-hermetic or
open compressor.
Thermodynamics of gas compression
[edit]
Isentropic compressor
[edit]
 A compressor can be idealized as internally
reversible and adiabatic, thus an
isentropic steady state device, meaning the
change in entropy is 0.[25]
The enthalpy change for a flow process can
be calculated.[26]
dH = VdP +TdS
Isentropic dS is zero.
dH = VdP
Non flow isentropic processes like some
positive displacement compressors may use a
different equation.[27]
dH = PdV
By defining the compression cycle as
isentropic, an ideal efficiency for the process
can be attained, and the ideal compressor
performance can be compared to the actual
performance of the machine. Isotropic
Compression as used in ASME PTC 10 Code
refers to a reversible, adiabatic compression
process[28]
Isentropic efficiency of Compressors:

is the enthalpy at the initial state

is the enthalpy at the final state for the actual process

is the enthalpy at the final state for the isentropic process

Minimizing work required by
a compressor

[edit]
Comparing reversible to
irreversible compressors

[edit]
Comparison of the differential
form of the energy balance for
each device.
Let be heat, be work,

be kinetic energy, and

be potential energy.
Actual Compressor:

Furthermore, and T is

[absolute temperature] (
) which produces:

or

Therefore, work-consuming
devices such as pumps and
compressors (work is
negative) require less work
when they operate
reversibly.[25]
Effect of cooling during
the compression process

[edit]

P-v (Specific volume vs.

Pressure) diagram
comparing isentropic,
 polytropic, and isothermal
processes between the
same pressure limits.

isentropic process: involves

no cooling,
polytropic process: involves
some cooling
isothermal process: involves
maximum cooling
By making the following
assumptions the required
work for the compressor to

compress a gas from to

is the following for each

process:

and
Flow processes VdP
All processes are internally reversible
The gas behaves like an ideal gas with constant specific heats
Isentropic (

, where

):

Polytropic (

):
Isother
mal (

or

):

By
com
pari
ng
the
thre
e
inter
nally
reve
rsibl
e
proc
esse
s
com
pres
sing
an
idea
l gas
from

to

, the
resu
lts
sho
w
that
isen
tropi
c
com
pres
sion
(

)
requ
ires
the
mos
t
wor
k in
and
the
isot
her
mal
com
pres
sion
(

or

)
requ
ires
the
leas
t
amo
unt
of
wor
k in.
For
the
poly
tropi
c
proc
ess (

)
wor
k
decr
ease
s as
the
exp
one
nt,
n,
decr
ease
s, by
incr
easi
ng
the
heat
reje
ctio
n
duri
ng
the
com
pres
sion
proc
ess.
One
com
mon
way
of
cooli
ng
the
gas
duri
ng
com
pres
sion
is to
use
cooli
ng
jack
ets
arou
nd
the
casi
ng
of
the
com
pres
sor.
[25]
Co
mpr
ess
ors
in
ide
al
ther
mo
dyn
ami
c
cycl
es

[edit
]
Idea
l
Ran
kine
Cycl
e 1-
>2
Isen
tropi
c co
mpr
essi
on
in a
pum
p
Idea
l
Carn
ot
Cycl
e 4-
>1
Isen
tropi
c co
mpr
essi
on
Idea
l
Otto
Cycl
e 1-
>2
Isen
tropi
c co
mpr
essi
on
Idea
l
Dies
el
Cycl
e 1-
>2
Isen
tropi
c co
mpr
essi
on
Idea
l
Bray
ton
Cycl
e 1-
>2
Isen
tropi
c co
mpr
essi
on
in a
com
pres
sor
Idea
l
Vap
or-
com
pres
sion
refri
gera
tion
Cycl
e 1-
>2
Isen
tropi
c co
mpr
essi
on
in a
com
pres
sor
NOT
E:
The
isen
tropi
c
assu
mpti
ons
are
only
appl
icabl
e
with
idea
l
cycl
es.
Real
worl
d
cycl
es
hav
e
inhe
rent
loss
es
due
to
ine
ffici
ent
com
pres
sors
and
turbi
nes.
The
real
worl
d
syst
em
are
not
truly
isen
tropi
c
but
are
rath
er
idea
lized
as
isen
tropi
c for
calc
ulati
on
purp
oses
.
Tem
pera
ture

[edit
]
Mai
n
artic
le:
Gas
laws
Com
pres
sion
of a
gas
incr
ease
s its
tem
pera
ture.
For
a
poly
tropi
c
tran
sfor
mati
on o
fa
gas:

T
h
e
w
o
r
k
d
o
n
e
f
o
r
p
o
l
y
t
r
o
p
i
c
c
o
m
p
r
e
s
s
i
o
n
(
o
r
e
x
p
a
n
s
i
o
n
)
o
f
a
g
a
s
i
n
t
o
a
c
l
o
s
e
d
c
y
li
n
d
e
r.

so

in which
p is
pressure,
V is
volume,
n takes
different
values
for
different
compres
sion
processe
s (see
below),
and 1 &
2 refer to
initial
and final
states.

 Adiabatic –
This mode
assumes
that no
energy
(heat) is
transferred
to or from
the gas
during the
compressi
, and all
supplied
work is
added to t
internal
energy of
the gas,
resulting in
increases
temperatu
and
pressure.
Theoretica
temperatu
rise is:[29]

with T1 and
T2 in
degrees
Rankine or
kelvins,
p2 and
p1 being
absolute
pressures

and
ratio of
specific
heats (appro
ximately 1.4
for air). The
rise in air
and
temperature
ratio means
compression
does not
follow a
simple
pressure to
volume
ratio. This is
less
efficient, but
quick.
Adiabatic
compression
or expansion
more closely
model real
life when a
compressor
has good
insulation, a
large gas
volume, or a
short time
scale (i.e., a
high power
level). In
practice
there will
always be a
certain
amount of
heat flow
out of the
compressed
gas. Thus,
making a
perfect
adiabatic
compressor
would
require
perfect heat
insulation of
all parts of
the
machine. For
example,
even a
bicycle tire
pump's
metal tube
becomes hot
as you
compress
the air to fill
a tire. The
relation
between
temperature
and
compression
ratio
described
above
means that
the value of

for an
adiabatic
process is

(the
ratio of
specific
heats).

 Isotherma
This mode
assumes t
the compr
gas remain
a constant
temperatu
throughou
compressi
expansion
process. In
cycle, inte
energy is
removed f
the system
heat at the
same rate
it is added
the mecha
work of
compressi
Isotherma
compressi
expansion
closely mo
real life wh
the compr
has a large
exchangin
surface, a
gas volum
a long tim
scale (i.e.,
small pow
level).
Compresso
that utilize
inter-stage
cooling be
compressi
stages com
closest to
achieving
perfect
isotherma
compressi
However,
practical
devices pe
isotherma
compressi
not attaina
For examp
unless you
an infinite
number of
compressi
stages wit
correspond
intercooler
you will ne
achieve pe
isotherma
compressi
For an
isothermal
process,

is 1, so
the value of
the work
integral for
an
isothermal
process is:

When
evaluated, the
isothermal wo
is found to be
lower than th
adiabatic wor
 Polytropic
model take
account bo
in tempera
the gas as
some loss
energy (he
the compr
componen
assumes t
may enter
the system
that input
work can a
both incre
pressure (
useful wor
increased
temperatu
adiabatic (
losses due
efficiency)
Compressi
efficiency
the ratio o
temperatu
theoretica
percent (a
 vs. actual
(polytropic
Polytropic
compressi

use a valu
between 0
constant-p
process) a
infinity (a
volume pr
For the typ
where an e
made to c
gas compr
an approxi
adiabatic p

the value o
will be bet

and .
Staged
compression

[edit]
In the case of
centrifugal
compressors,
commercial
designs
currently do n
exceed a
compression
ratio of more
than 3.5 to 1
any one stage
(for a typical
gas). Since
compression
raises the
temperature,
the compress
gas is to be
cooled betwe
stages makin
the
compression
less adiabatic
and more
isothermal. Th
inter-stage
coolers
(intercoolers)
typically resu
in some partia
condensation
that is remove
in vapor–liqui
separators.
In the case of
small
reciprocating
compressors,
the compress
flywheel may
drive a coolin
fan that direc
ambient air
across the
intercooler of
two or more
stage
compressor.
Because rotar
screw
compressors
can make use
cooling
lubricant to
reduce the
temperature
rise from
compression,
they very ofte
exceed a 9 to
compression
ratio. For
instance, in a
typical diving
compressor th
air is
compressed i
three stages.
each stage ha
a compression
ratio of 7 to 1
the compress
can output 34
times
atmospheric
pressure (7 ×
× 7 = 343
atmospheres)
(343 atm or
34.8 MPa or
5.04 ksi)
Drive motors
[edit]
There are ma
options for th
motor that
powers the
compressor:

 Gas turbin
the axial a
centrifuga
compresso
are part of
engines.
 Steam turb
water turb
possible fo
compresso
 Electric mo
cheap and
static com
Small mot
suitable fo
domestic e
supplies u
phase alte
current. La
motors can
used wher
industrial e
three
phase alte
 current su
available.
 Diesel eng
petrol eng
suitable fo
portable
compresso
support
compresso
 In automo
other type
vehicles (i
piston-pow
airplanes,
trucks, etc
or gasoline
engine's p
output can
increased
compressi
intake air,
more fuel
burned pe
These eng
power com
using their
crankshaft
(this setup
as a super
or, use the
exhaust ga
drive a tur
connected
compresso
setup know
turbocharg
Lubrication
[edit]
Compressors
that are drive
by an electric
motor can be
controlled usi
a VFD or pow
inverter,
however man
hermetic and
semi-hermeti
compressors
can only work
a range of or
fixed speeds,
since they ma
include built-i
oil pumps. Th
built-in oil pum
is connected
the same sha
that drives th
compressor,
and forces oil
into the
compressor a
motor bearing
At low speeds
insufficient
quantities of o
reach the
bearings,
eventually
leading to
bearing failur
while at high
speeds,
excessive
amounts of oi
may be lost
from the
bearings and
compressor a
potentially int
the discharge
line due to
splashing.
Eventually the
oil runs out an
the bearings a
left
unlubricated,
leading to
failure, and th
oil may
contaminate t
refrigerant, ai
or other
working gas.
[30]
Applications
[edit]
Gas
compressors
are used in
various
applications
where either
higher
pressures or
lower volume
of gas are
needed:

 In pipeline
transport o
purified na
from the p
site to the
consumer,
compresso
driven by
fueled by g
from the p
Thus, no e
power sou
necessary.
 In maritim
transport a
operations
carriers.
 Petroleum
refineries,
gas proces
plants,
petrochem
chemical p
and simila
industrial p
require
compressi
intermedia
end-produ
 Refrigerati
air
 conditione
ent use
compresso
move heat
refrigerant
(see vapor
compressi
refrigerati
 Gas turbin
systems co
the intake
combustio
 Small-volu
purified or
manufactu
gases requ
compressi
high press
cylinders f
medical, w
and other
 Various ind
manufactu
building pr
require co
air to pow
pneumatic
 In the
manufactu
blow moul
PET plastic
bottles an
containers
 Some aircr
require
compresso
maintain c
pressuriza
altitude.
 Some type
engines—s
turbojets a
turbofans—
compress
required fo
combustio
engine's
turbines p
combustio
compresso
 In underwa
diving, sel
contained
breathing
apparatus
hyperbaric
therapy, a
life suppor
equipmen
compresso
provide pr
breathing
gas either
or via high
gas storag
containers
diving cyli
[31][32] In
supplied d
air compre
generally u
supply low
air (10 to 2
for breathi
 Submarine
compresso
store air fo
use in disp
water from
buoyancy
chambers
buoyancy.
 Turbocharg
supercharg
compresso
increase in
combustio
engine per
e by increa
mass flow
inside the
so the eng
burn more
hence prod
more powe
 Rail and he
transport v
use compr
air to oper
vehicle or
 vehicle bra
and variou
systems (d
windscree
engine,
gearbox co
etc.).
 Service sta
and auto r
shops use
compresse
fill pneuma
tires and p
pneumatic
 Fire piston
heat pump
heat air or
gasses, an
compressi
gas is only
to that end
 Rotary lob
compresso
often used
provide air
pneumatic
conveying
powder or
Pressure re
can range
to 2 bar g.

Diving air com

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