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Air and Gas Compressors (2 PDH) PDHengineer.com Course No.
M-2019
Introduction Compressors are widely used in construction, power
plants, process industry, assembly plant, refineries, air
conditioning, and refrigeration, to mention some of the
applications. Compressors are power conversion machines, like pumps
and electric motors. Compressed air systems are alternatives to
hydraulic systems and electric operators in many applications. For
these reasons, engineers, operations, and maintenance personnel
should be aware of the applications and limitations of various
types of compressors. The same basic principles apply to all gas
and vapor compressors, as well as air compressors. Diagrams and
illustrations are taken from the following sources: Various issues
of Power magazine, Marks Standard handbook for Mechanical
Engineers, sales brochures from Ingersol- Rand and Gardener-Denver,
and Process Engineers guide to centrifugal compressors, by Igor
Karassik.
AIR COMPRESSORS Compressed air is free air that has been forced
into a smaller volume and is at a pressure higher than atmospheric.
Some of the terms and definitions used when discussing air
compressors are as follows:
Absolute Pressure The existing gage pressure plus the
atmospheric pressure measured from absolute zero Aftercooler Device
that dissipates heat caused by compression. This also effectively
removes moisture down to the saturation temperature Air Receiver
Tank into which compressed air is delivered and stored Atmospheric
pressure Pressure at a specific altitude. At sea level this is 14.7
psia.
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Brake Horsepower Total power input required to compress and
deliver a given quantity of air, including losses due to friction
and other mechanical losses. Capacity
SCFM - Standard cubic feet per minute. Delivered capacity in
cubic feet of air measured at 68 deg F and 14.7 psia. (per ASME
Power Test Code, but this standard may vary).
ICFM - Inlet cubic feet per minute. The capacity entering the
inlet filter in CFM at actual
inlet conditions. ACFM - Actual cubic feet per minute. Delivered
CFM as measured at actual conditions at
the compressor suction downstream of the inlet filter. ACFM
differs from ICFM primary by seal losses and to a much lesser
extent by the lower pressure condition at the compressor suction
due to pressure drop through the inlet filter. Since ACFM most
realistically expresses the user's intent, it is recommended that
compressors be specified in that unit.
Compression The reduction of a specified volume, resulting in an
increase in pressure Compression Efficiency Ratio of the
theoretical to the actual required to compress air. Compression
Ratio The ratio of the absolute discharge pressure to the absolute
inlet pressure. Compressor A machine designed for compressing a gas
or vapor from an initial pressure to a higher discharge pressure.
Design Pressure Maximum continuous operating pressure. Also
referred to as maximum working pressure. Design Speed Maximum
continuous operating speed of a compressor.
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Discharge Pressure Total pressure at the discharge flange of the
aftercooler. Free Air Air at atmospheric conditions. This may vary
with altitude, barometric pressure and temperature. Inlet Pressure
Total pressure at the inlet flange of the compressor or inlet
filter Inlet Temperature Temperature at the inlet flange of the
compressor or the inlet filter. Load Factor The ratio of the
average actual compressor output to the maximum rated output for a
defined period of time. I Moisture Separator A devise designed to
collect and remove moisture from the air during the cooling process
Pressure - Force per unit area
PSIG - Pressure above local atmospheric pressure PSIA - Equal to
gage pressure plus atmospheric
pressure. Pressure Drop - Loss of pressure commonly due to
friction. Rated Discharge Pressure - The highest continuous
operating pressure to
meet the specified conditions. It is lower than design pressure
by 10% or 15 psig.
Slip The internal leakage due to clearance.
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Speed The number of revolutions per minute Unloaded Horsepower
The power that is consumed to overcome frictional losses when
operated in an unloaded condition. Vacuum Pressure below
atmospheric Volumetric Efficiency The ratio of the actual quantity
of air delivered to the displacement of the compressor. (For
reciprocating compressors). Types of Compression A brief review of
the thermodynamics of gas compression is in order at this point.
The perfect gas law expresses the equation of state for gases:
144pv = RT,
where p is the absolute pressure in psia, v is the specific
volume in cubic feet per pound, R is a constant which depends on
the nature of the gas, and T is the absolute temperature in deg
F.
Specific heat is the amount of heat required to raise the
temperature of one pound of gas 1 deg F. The specific heat of a gas
has two distinct values, depending on whether the volume or the
pressure remain constant during the addition of heat:
Cp = specific heat at constant pressure Cv = specific heat at
constant volume
The factor or exponent k is the ratio between the specific heat
at constant pressure to the specific heat at constant volume.
k=Cp/cv The value of k for air is commonly taken as 1.4.
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Adiabatic compression takes place when no heat is transferred
into or out of the gas during compression. For adiabatic
compression,
pvk = constant Adiabatic compression is further characterized by
an increase in temperature during compression. Isothermal
compression occurs when the heat of compression is removed during
compression, so that the temperature of the gas remains constant.
The equation for isothermal compression is:
pv = constant Polytropic compression is characterized by the
equation:
pvn = constant When n=1, the polytropic compression is
isothermal. When n=k, it is adiabatic. The slope of a
pressure-volume curve is dependent on the value of n. If a
compressor is not cooled, and the compression takes place with 100%
efficiency, it would be Adiabatic. However, the inefficiency of the
compressor results in the addition of heat during compression. As a
result, the actual compression of an uncooled compressor is
polytropic, with a value of n greater than k. Design
Classifications There are two broad classifications of compressors;
positive displacement and dynamic. Positive displacement
compressors confine successive volumes of gas in an enclosed space
where pressure increases as the volume of the enclosed space
decreases. They can be thought of as constant volume-variable
pressure machines; that is, they move a certain volume of gas with
each stroke, and the pressure is that of the system into which they
discharge. With dynamic compressors, the mechanical action of
rotating impellers impart pressure and velocity to the gas. They
are constant pressure-variable volume machines. Categories of
Compressors Compressors are generally categorized as:
Reciprocating, Rotary, Centrifugal, and Axial. These are
illustrated below in Figure 1. Figure 2 shows approximate ranges of
capacity and pressure for each compressor category.
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Figure 1 Illustration of various types of compressors.
Figure 2 Approximate ranges of application for reciprocating,
centrifugal, and axial flow conditions.
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Reciprocating Compressors Reciprocating compressors (abbreviated
recips) may be considered for applications up to approximately 3000
ACFM. They are favored for low flow, high pressure services. They
can be arranged in configurations of two or more stages, with
maximum compression ratios per stage usually about 3:1 or 4:1.
Intercoolers can be used between stages to remove moisture and to
improve compressor efficiency. More on intercooling later. Single
stage compressors may also be double acting. That is discharging
through both ends of the cylinder, thus doubling the capacity per
stroke. Discharge pressure is generally limited to 3000 psia for
large machines, but can go up to 60,000 psia for small machines.
Pressure rise per stage is usually limited by the valve design, and
is generally about 1000 psia per stage for large compressors. Since
reciprocating compressors are constant volume machines, the inlet
ACFM remains essentially unchanged if the compressor operates at a
constant speed, regardless of the discharge pressure. A
reciprocating compressor will operate at any discharge pressure
within the power limit of the driver. If there is insufficient air
capacity in the compressor for the load to be handled, the air
demand sets the pressure, which may be less than the rated
pressure. If there is sufficient capacity, then a step control of
the compressor can maintain the receiver pressure within a pre-set
range. A step control unloads the compressor sequentially in
several steps in 1 to 2 psi increments as the load demand
decreases. If two or more recips operate in parallel, the
compressors should be controlled to unload sequentially, one after
the other. The cylinders of conventional reciprocating compressors
require oil lubrication. Carry-over of lubricating oil can be a
problem in some applications, such as instrument air. There are
"oil free" recips that can be used for these applications. Oil free
recips use teflon piston rings and valves, and so the air side is
truly oil free except for the small amount that carry over on the
piston rods. The trouble is that reciprocating compressors tend to
be maintenance intensive in any case, and oil free recips with
teflon rings and valves are particularly demanding. Rotary
Compressors Rotary compressors are positive displacement machines
like recips, and although they are very different configurations
from recips, they do share the basic defining characteristics They
compress gas by confining a specific volume of gas in a closed
space and increase the pressure by decreasing the volume of the
space. They are constant volume and variable pressure machines.
They are configured as screw, sliding vane, lobe, and liquid piston
compressors, as illustrated below.
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Figure 3 Illustration of a lobe compressor and a liquid piston
compressor.
Sliding vane rotary compressors trap gas between vanes as the
rotor passes an inlet opening. As the rotor continues toward the
discharge port, the volume of a cell between any two vanes
decreases, causing the gas pressure to rise. The vanes slide in and
out of slots as the rotor rotates, and are held against the casing
by centrifugal force. Capacities of sliding vane units range to
5000 CFM, and single stage compression to about 50 psi. Multiple
stages can by configured for higher pressures. Lobe compressors
come in two and three lobe design, with capacities from 5 to 50,000
CFM. Pressures above 15 psi can be reached by connecting two or
three lobe compressors in series. Lobe compressors have identical
impellers, or lobes, which trap and compress gas between the outer
surfaces of the lobes and the outer casing. Some screw and lobe
compressors require large quantities of lubricant to be sprayed
onto the rotating parts, and so are not suitable for applications
requiring oil free air or gas. Other screw and lobe compressors
have the rotating elements held in place with gears that prevent
actual contact of the screws or lobes, so the air chamber can
remain free of lubricating oil. Centrifugal Compressors In
centrifugal compressors, the gas travels essentially radially
through one or more stages of rotating impellers. The centrifugal
action of the impeller produces some pressure rise and a large
increase in air velocity. In the diffuser, velocity energy is
converted to static pressure. Velocity decreases, and pressure
increases. Pressure / volume curves are represented in Figure 4.
Each curve is for a different compressor speed.
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A compressor running at medium speed delivers a certain volume
V1 at pressure P1. Increasing to high speed increases volume to V2
, or old volume V1 can be delivered at a higher pressure P2.
Figure 4 Characteristic curves for a centrifugal pump. The
popularity of centrifugal compressors grew as the need for higher
capacities reached and exceeded the practical limits of
reciprocating compressors. Multistage
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centrifugal compressors can handle 150,000 CFM or more, and the
lower capacity has extended down to 500 CFM.
Figure 5 Sectional view of a centrifugal compressor. A
characteristic of centrifugal compressors is surge, or pumping.
Surge occurs at reduced loads. At reduced capacities, impellers do
not fully load, density is reduced, and full discharge pressure is
not developed. Since the pressure in the discharge line will then
be momentarily higher,a reverse surge will occur. The impellers
will then fill and again deliver full pressure. This cyclic effect
willcause pressure fluctuations within the compressor. These
fluctuations can be destructive over a period of time. Surge can
occur at about 50% of rated capacity in single stage units designed
for low compression ratios. Multistage units will surge at a higher
capacity, perhaps 75 to 80%. A steeply rising pressure curve at
reduced capacity will give an increased stable operating range at
rated pressure. Such a curve can be obtained with a backward
sloping impeller design. Axial Flow Compressors Axial flow
compressors are dynamic compressors in which the air flow is
parallel to the axis of rotation. The most common usage is in
combustion turbines and in ventilation systems. They can handle
very large volumes of air, and have a low pressure increase per
stage. Multistage axials can compress air to 150 psi. They deliver
a relatively fixed amount of air over a range of pressures. To
prevent surging at low
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loads, large axials can be fitted with a blowoff system to
ensure that enough air passes through the stages to remain stable.
Accessories The compressor itself is the heart of the air or gas
compressor system, but there are accessories that are generally
supplied as part of the system, either integral with the
compressor, or as stand-alone devices. Inlet Filters and Silencers
Filter-silencers can come as an integral package for each
compressor. They are essential to prevent particulate matter from
being ingested and damaging the compressor. Inlet filters usually
come in variations of oil bath or dry cartridge types. Intercoolers
and Aftercoolers Intercoolers are designed to remove the heat of
compression between the stages of multi-stage compressors.
Aftercoolers serve the same function following the final stage of
compression. They can be either water cooled or air cooled.
Atmospheric air contains moisture, and furthermore, the air may
pick up oil vapor as it passes through some compressors. Cooling
the air down to or below its initial temperature will remove
moisture down to the dew point, improving the quality of the air.
Another purpose of intercooling is to improve the efficiency of
compression. Refer to Figure 6 below, which is a pressure-volume
diagram of compression. The work input to the compressor in the
case of isothermal compression (perfect cooling) is represented by
the area ABCD, and is the least possible work for a given
compression. Perfect cooling means that all the heat of compression
is removed, with no pressure drop in the intercoolers. The other
extreme is adiabatic compression (no cooling). All of the heat of
compression is retained within the air. The work for adiabatic
compression is represented by the area ABCE. The dotted curve is
the approximate actual compression line, somewhere between
isothermal and adiabatic, and represents a certain degree of
intercooling. The work input to the compressor with some
intercooling is between the two extreme theoretical cases. The work
saved by intercooling is represented by the area between the actual
curve and the adiabatic curve.
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Figure 6 Pressure/volume diagram of a compressor. Separators
Intercoolers and aftercoolers can only remove water in proportion
to their ability to lower the temperature of the compressed air.
Moisture separators are generally used between the aftercooler and
the receiver, particularly in cases where moisture and oil cannot
be tolerated in the compressed air. Figure 7 below shows some of
the separator designs in use. Some use centrifugal force or rapid
changes in direction of flow to throw out moisture particles. The
air receivers also contribute to the removal of moisture simply by
providing a chamber of low air velocity to allow the moisture to
settle out and be removed by drain traps. In applications where
removal of entrained oil droplets is essential, more sophisticated
separators, such as coalescing filters, may be installed after the
receiver. Coalescing filters use a combination of baffles and a
coalescing medium to remove the maximum amount of impurities. Some
of these filter types are illustrated in Figure 7.
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Figure 7 Sketches of various moisture separator designs.
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Drives Compressor drives are commonly electric motors, steam
turbine or internal combustion engines. Recently, combustion
turbines have also become popular. As previously mentioned,
accessories such as these often are purchased as part of the
compressor package. Control Compressor controls are used to control
output and load. Steam-driven compressors usually have combination
speed and pressure governors that vary air capacity by changing
speed. There are two basic types of compressor controls: Throttling
governors vary steam pressure, reducing it as air discharge
pressure rises. Automatic cutoff governors change the cutoff point
of valves within the steam cylinder. Throttling governors are used
with a manually adjustable cutoff in plants that have a steady
supply of exhaust steam, or where conditions are so nearly constant
that automatic cutoff valves would have little chance to function.
Motor-driven and other types of constant speed compressors usually
have one of three types of controls: (1) Constant speed control
decreases compressor capacity in one or more steps by means of an
unloader. (2) Automatic start and stop control uses a starter and
pressure switch. This type of control works well where air demand
is intermittent with long periods of no demand, and where precise
pressure regulation is not necessary. (3) Dual control is a
combination of (1) and (2) . It allows continuous operation when
demand is nearly continuous, and automatic start and stop when
demand is low. Constant speed 5 step control unloads the compressor
in five steps, reducing capacity from full load to , , , and no
load as demand decreases. There also are three step and one step
unloading controls for smaller compressors. There are several other
considerations in designing a compressed air or gas system, such as
compressor cooling, reliability requirements, and environmental
concerns.
Conclusions Compressors may be considered to be minor pieces of
industrial equipment, but like electric motors and pumps, they are
essential to the reliable functioning of many complex functions and
mechanisms. As such, the design engineer and the operating engineer
must be aware of their applications and limitations, and to be
familiar with the definitions and terminology.