Air and Gas Compressors (2 PDH) PDHengineer.com

PDHengineer.com
Course № M-2019
Air and Gas Compressors
This document is the course text. You may review this material at
your leisure before or after you purchase the course. If you have not
already purchased the course, you may do so now by returning to the
course overview page located at:
http://www.pdhengineer.com/pages/M‐2019.htm
(Please be sure to capitalize and use dash as shown above.)
Once the course has been purchased, you can easily return to the
course overview, course document and quiz from PDHengineer’s My
Account menu.
If you have any questions or concerns, remember you can contact us
by using the Live Support Chat link located on any of our web pages,
by email at administrator@PDHengineer.com or by telephone toll‐
free at 1‐877‐PDHengineer.
Thank you for choosing PDHengineer.com.
© PDHengineer.com, a service mark of Decatur Professional Development, LLC. M‐2019 C1

Introduction
Air and Gas Compressors (2 PDH)
PDHengineer.com
Course No. M-2019
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, Mark’s Standard handbook for Mechanical Engineers, sales
brochures from Ingersol- Rand and Gardener-Denver, and Process Engineer’s 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.
1

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.
2

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
Slip
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.
The internal leakage due to clearance.
3

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.
4

Adiabatic compression takes place when no heat is transferred into or out of the gas
during compression. For adiabatic compression,
pv k = 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:
pv n = 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.
5

Figure 1 – Illustration of various types of compressors.
Figure 2 – Approximate ranges of application for reciprocating, centrifugal, and axial flow
conditions.
6

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 preset
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.
7

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.
8

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
9

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
10

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 standalone
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.
11

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.
12

Figure 7 – Sketches of various moisture separator designs.
13

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 ¾, ½, ¼ , 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.
14