# Industrial Compressed Air System Description

May 2006

Compressed air systems may be noisy, but they are necessary. While they
may look intimidating, most modern compressed air system are composed of
similar major sub-systems and many sub-components. This document is
intended to provide you with the basic information about modern
compressed air systems.

Major compressed air sub-systems include the compressor, prime mover,
controls, treatment equipment and accessories, and the distribution
system. The compressor is the mechanical device that takes in ambient
air and increases its pressure. The prime mover powers the compressor
(typically the electric motor). Controls serve to regulate the amount of
compressed air being produced. The treatment equipment removes
contaminants and water from the compressed air, and accessories keep the
system operating properly. A distribution system consists of the piping
that is analogous to wiring in the electrical world - they transport
compressed air to where it is needed.

Compressed air storage can also serve to improve system performance and
efficiency. Figure 1.1 shows a representative industrial compressed air
system and its components.

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## Compressor Types

Many modern industrial air compressors are sold "packaged" with the
compressor, drive motor, and many of the accessories mounted on a skid
for ease of installation. This skid allows for movement by forklift.
Larger packages may require the use of an overhead crane. Some systems
also have an enclosure for sound attenuation, cleanliness, and
aesthetics.

As shown in Figure 1.2, there are two basic compressor types:
positive-displacement and dynamic. In the positive-displacement type, a
given quantity of air or gas is trapped in a compression chamber and the
volume which it occupies is mechanically reduced, causing a
corresponding rise in pressure prior to discharge. At constant speed,
the air flow remains essentially constant with variations in discharge
pressure. Dynamic compressors impart velocity energy to continuously
flowing air or gas by means of impellers rotating at very high speeds.
The velocity energy is changed into pressure energy both by the
impellers and the discharge volutes or diffusers. In the
centrifugal-type dynamic compressors, the shape of air flow and the
pressure (or head) generated.

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## 

## Positive-Displacement Compressors

These compressors are available in two types: reciprocating and rotary.
Reciprocating compressors work like bicycle pumps. A piston, driven
through a crankshaft and connecting rod by an electric motor, reduces
the volume in the cylinder occupied by the air or gas, compressing it to
a higher pressure. Single acting compressors have a compression stroke
in only one direction, while double-acting units provide a compression
stroke as the piston moves in each direction. Large, industrial
reciprocating air compressors are double-acting and water-cooled.
Multi-stage, double acting compressors are the most efficient
compressors available, and are typically larger, noisier, and more
costly than comparable rotary units. Reciprocating compressors are
available in sizes from less than 1 horse power (HP) to more than 600
HP.

Rotary compressors have gained popularity and are now the "workhorse" of
American industry. They are most commonly used in sizes from about 30 to
200 HP. The most common type of rotary compressor is the helical-twin,
screw-type (also known as rotary screw or helical-lobe). Male and female
screw-rotors mesh, trapping air, and reducing the volume of the air
along the rotors to the air discharge point. Rotary screw compressors
have low initial cost, compact size, low weight, and are easy to
maintain. Rotary screw compressors may be air or water-cooled. Less
common rotary compressors include sliding-vane, liquid-ring, and
scroll-type.

## Single-Acting, Reciprocating Air Compressors

This type of compressor is characterized by its "automotive" type piston
driven through a connecting rod from the crankshaft. Compression takes
place on the top side of the piston on each revolution of the
crankshaft. Single-acting, reciprocating air compressors may be
air-cooled or liquid-cooled. These may be single stage, usually rated at
discharge pressures from 25 to 125 pounds per square inch gauge (psig),
or two-stage, usually rated at discharge pressures from 125 psig to 175
psig or higher.

The most common air compressor in the fractional and single-digit HP
sizes is the air-cooled, reciprocating air compressor. In larger sizes,
single-acting reciprocating compressors are available up to 150 HP, but
above 25 HP are much less common. Two-stage and multi-stage designs
include inter-stage cooling to reduce discharge air temperatures for
improved efficiency and durability.

Pistons used in single-acting compressors are of the "automotive" or
"full skirt" design, the underside of the piston being exposed to the
crankcase. Lubricated versions have a combination of compression and
lubricant-control piston rings, which seal the compression chamber,
control the lubricant to the compression chamber, and act (in some
designs) as support for piston movement on the cylinder walls.

Lubricant-free, or non-lube designs, do not allow lubricant in the
compression chamber and use pistons of self-lubricating materials or use
heat resistant, non-metallic guides and piston rings which, are
self-lubricating. Some designs incorporate a distance piece or crosshead
to isolate the crankcase from the compression chamber.

Lubricant-less designs have piston arrangements similar to
lubricant-free versions but do not have lubricant in the crankcase.
Generally these have a grease pre-packed crankshaft and connecting rod
bearings.

## 

## Double-Acting, Reciprocating Air Compressors

Double-acting reciprocating compressors use both sides of the piston for
air compression, doubling the capacity for a given cylinder size. A
piston rod is attached to the piston at one end and to a crosshead at
the other end. The crosshead ensures that the piston travels
concentrically within the cylinder. These compressors may be single- or
multi-stage, depending on discharge pressure and HP size. These can
range upwards from 10 HP and with pressures upwards from 50 psig.

## 

## Lubricant-Injected Rotary Screw Compressors

The lubricant-injected rotary screw compressor powered by an electric
motor has become a dominant type of industrial compressor for a wide
variety of applications.

**Compression Principle.** The lubricant-injected, rotary screw
compressor consists of two intermeshing rotors in a stator housing
having an inlet port at one end and a discharge port at the other. The
male rotor has lobes formed helically along its length while the female
rotor has corresponding helical grooves or flutes. The number of helical
lobes and grooves may vary in otherwise similar designs.

Air flowing in through the inlet port fills the spaces between the lobes
on each rotor. Rotation then causes the air to be trapped between the
lobes and the stator as the inter-lobe spaces pass beyond the inlet
port. As the rotation continues, a lobe on one rotor rolls into a groove
on the other rotor and the point of intermeshing moves progressively
along the axial length of the rotors, reducing the space occupied by the
air, resulting in increased pressure. Compression continues until the
inter-lobe spaces are exposed to the discharge port when the compressed
air is discharged.

Lubricant is injected into the compression chamber during compression
and serves three basic functions: 1) it lubricates the intermeshing
rotors and associated bearings; 2) it takes away most of the heat caused
by compression; and 3) it acts as a seal in the clearances between the
meshing rotors and between rotors and stator.

## 

## Lubricant-Free Rotary Screw Compressors

The principle of compression in lubricant-free rotary screw compressors
is similar to that of the lubricant-injected rotary screw compressors
but, without lubricant being introduced into the compression chamber.
Two distinct types are available: the dry-type and the water-injected
type.

In the dry-type, the intermeshing rotors are not allowed to touch and
their relative positions are maintained by means of lubricated timing
gears external to the compression chamber. Since there is no injected
fluid to remove the heat of compression, most designs use two stages of
compression with an intercooler between the stages and an aftercooler
after the second stage. The lack of a sealing fluid also requires higher
rotation speeds than for the lubricant-injected type. Dry-type,
lubricant-free rotary screw compressors have a range from 25 to 4,000 HP
or 90 to 20,000 cfm. Single-stage units operate up to 50 psig, while
two-stage can achieve up to 150 psig.

In the water-injected type, similar timing gear construction is used,
but water is injected into the compression chamber to act as a seal in
internal clearances and to remove the heat of compression. This allows
pressures in the 100 to 150 psig range to be accomplished with only one
stage. The injected water, together with condensed moisture from the
atmosphere, is removed from the discharged compressed air by a
conventional moisture separation device. Similar to the
lubricant-injected type, lubricant-free rotary screw compressors
generally are packaged with all necessary accessories.

## 

## Dynamic Compressors

These compressors raise the pressure of air or gas by imparting velocity
energy and converting it to pressure energy. Dynamic compressors include
centrifugal and axial types. The centrifugal-type is the most common and
is widely used for industrial compressed air. Each impeller, rotating at
high speed, imparts primarily radial flow to the air or gas which then
passes through a volute or diffuser to convert the residual velocity
energy to pressure energy. Some large manufacturing plants use
centrifugal compressors for general plant air, and in some cases, plants
use other compressor types to accommodate demand load swings while the
centrifugal compressors handle the base load.

Axial compressors consist of a rotor with multiple rows of blades and a
matching stator with rows of stationary vanes. The rotating blades
impart velocity energy, primarily in an axial plane. The stationary
vanes then act as a diffuser to convert the residual velocity energy
into pressure energy. This type of compressor is restricted to very high
flow capacities and generally possess relatively high compression
efficiency. Mixed flow compressors have impellers and rotors which
combine the characteristics of both axial and centrifugal compressors.

## 

## Centrifugal Air Compressors

A centrifugal air compressor has a continuously flowing air stream which
has velocity energy, or kinetic energy, imparted to it by an impeller,
or impellers, which rotate at speeds that can exceed 50,000 revolutions
per minute (rpm). Approximately one half of the pressure energy is
developed in the impeller with the other half achieved by converting the
velocity energy to pressure energy as the air speed is reduced in a
diffuser and volute. The most common centrifugal air compressor is one
with two to four stages for pressures in the 100 to 150 psig range. A
water-cooled intercooler and separator between each stage returns the
air temperature to approximately ambient temperature and removes
condensed moisture before entering the next stage. An aftercooler cools
the air from the final stage and a moisture separator removes the
moisture prior to air delivery to distribution.

The inherent characteristic of centrifugal air compressors is that as
system pressure decreases, the compressor's flow capacity increases. The
steepness of the pressure head/capacity curve is dependent upon the
impeller design. The more the impeller blades lean backwards from the
true radial position, the steeper the curve.

Most standard centrifugal air compressor packages are designed for an
ambient temperature of 95° F and near sea level barometer pressure. The
dynamic nature of the centrifugal compressor results in the pressure
head generated by each impeller increasing as the air density increases.
The compressor mass flow and actual cubic feet per minute (acfm)
capacity at a given discharge pressure increases as the ambient
temperature decreases. Typically, a capacity control system is provided
with the compressor to maintain the desired capacity and to operate
within the motor horsepower limits. The control system regulates the air
flow by means of an inlet throttle valve or inlet guide vanes. The
amount of reduction in the flow rate is limited by a minimum point flow
reversal phenomenon known as surge. Control systems either unload the
compressor or blow off the excess air to atmosphere to avoid this
occurrence, which could result in excessive vibration and potential
damage to the compressor. Given adequate storage, some manufacturers
will operate the compressor controls in a load/unload mode at lower flow
conditions.

Centrifugal air compressors range from around 300 to more than 100,000
cfm but the more common air compressors are from 1,200 to 5,000 cfm and
with discharge pressures up to 125 psig. These may have several
impellers in line on a single shaft or with separate impellers
integrally geared.

Centrifugal air compressors provide lubricant-free air delivery as there
is no lubricant in the compression chambers. Lubrication for speed
increasing gears and the special high-speed shaft bearings is kept away
from the compression chambers by means of shaft seals, which may also
have air purge and vent connections.

Centrifugal air compressors are high-speed rotating machines and as
such, shaft vibration monitoring is mandated to record operational
trends and protect the equipment. Automatic control of the compressors
is typical and has been greatly improved by the use of microprocessors,
which monitor the pressure/capacity/ temperature characteristics as well
as main-drive motor current draw. It is important that the
manufacturer's recommended maintenance procedures be followed and that
certain maintenance procedures be carried out by qualified staff. This
is particularly true of attempts to remove an impeller from its shaft,
since special procedures and tools may be involved.

## Compressor Prime Movers

The prime mover is the main power source providing energy to drive the
compressor. The prime mover must provide enough power to start the
compressor, accelerate it to full speed, and keep the unit operating
under various design conditions. This power can be provided by any one
of the following sources: electric motors, diesel or natural gas
engines, steam turbines and combustion turbines. Electric motors are by
far the most common type of prime mover.

Electric motors are a widely available and economical means of providing
reliable and efficient power to compressors. Most compressors use
standard, polyphase induction motors. In many cases, either a standard
or a premium-efficient motor can be specified when purchasing a
compressor or replacement motor. The incremental cost of the premium
efficient motor is typically recovered in a very short time from the
resulting energy savings. When replacing a standard motor with a
premium-efficient version, careful attention should be paid to
performance parameters, such as full-load speed and torque. A
replacement motor with performance as close as possible to the original
motor should be used. When replacing a drive motor in a compressor that
uses a variable frequency drive as part of the control system, use an
inverter-duty motor.

Diesel or natural gas engines are common compressor power sources in the
oil and natural gas industries. Considerations such as convenience,
cost, and the availability of liquid fuel and natural gas play a role in
selecting an engine to power a compressor. Although the majority of
industrial compressed air systems use electric motors for prime movers,
in recent years there has been renewed interest in using non-electric
drives, such as reciprocating engines or turbines powered by natural
gas, particularly in regions with high electricity rates. Standby or
emergency compressors may also be engine-driven to allow operation in
the event of a loss of electrical power. Maintenance costs for
engine-driven systems are significantly higher than those that use
electric motors.

## 

## Compressed Air System Controls

Compressed air system controls serve to match compressor supply with
system demand. Proper compressor control is essential to efficient
operation and high performance. Because compressor systems are typically
sized to meet a system's maximum demand, a control system is almost
always needed to reduce the output of the compressor during times of
lower demand. Compressor controls are typically included in the
compressor package, and many manufacturers offer more than one type of
control technology. Systems with multiple compressors use more
sophisticated controls (network or system master controls) to
orchestrate compressor operation and air delivery to the system.

Network controls use the on-board compressor controls' microprocessors
linked together to form a chain of communication that makes decisions to
stop/start, load/unload, modulate, vary displacement, and vary speed.
Usually, one compressor assumes the lead with the others being
subordinate to the commands from this compressor.

System master controls coordinate all of the functions necessary to
optimize compressed air as a utility. System master controls have many
functional capabilities, including the ability to monitor and control
all components in the system, as well as trending data, to enhance
maintenance functions and minimize costs of operation. Other system
controllers, such as pressure/flow controllers, can also improve the
performance of some systems.

The type of control system specified for a given system is largely
determined by the type of compressor being used and the facility's
demand profile. If a system has a single compressor with a very steady
demand, a simple control system may be appropriate. On the other hand, a
complex system with multiple compressors, varying demand, and many types
of end uses will require a more sophisticated control strategy. In any
case, careful consideration should be given to compressor system control
selection because it can be the most important single factor affecting
system performance and efficiency. For information about efficiency and
compressor controls, see the fact sheet titled *Compressed Air System
Controls* in Section 2.

## Accessories

Accessories are the various types of equipment used to treat compressed
air by removing contaminants such as dirt, lubricant, and water; to keep
compressed air systems running smoothly; and to deliver the proper
pressure and quantity of air throughout the system. Accessories include
compressor aftercoolers, filters, separators, dryers, heat recovery
equipment, lubricators, pressure regulators, air receivers, traps, and
automatic drains.

**Air Inlet Filters.** An air inlet filter protects the compressor from
atmospheric airborne particles. Further filtration is typically needed
to protect equipment downstream of the compressor.

**Compressor Cooling.** Air or gas compression generates heat. As a
result, industrial air compressors that operate continuously generate
substantial amounts of heat. Compressor units are cooled with air,
water, and/or lubricant. Single-acting reciprocating compressors are
typically air-cooled using a fan, which is an integral part of the
belt-drive flywheel. Cooling air blows across finned surfaces on the
outside of the compressor cylinder's cooler tubes. Larger, water-cooled,
double-acting reciprocating air compressors have built-in cooling water
jackets around the cylinders and in the cylinder heads. The temperature
of the inlet water and the design and cleanliness of the cooler can
affect overall system performance and efficiency. Centrifugal
compressors are generally water-cooled.

Lubricant-injected rotary compressors use the injected lubricant to
remove most of the heat of compression. In air-cooled compressors, a
radiator-type lubricant cooler is used to cool the lubricant before it
is re-injected. The cooling fan may be driven from the main motor-drive
shaft or by a small auxiliary electric motor. In plants where good
quality water is available, shell and tube heat exchangers generally are
used.

**Intercooling.** Most multi-stage compressors use intercoolers, which
are heat exchangers that remove the heat of compression between the
stages of compression. Intercooling affects the overall efficiency of
the machine.

**Aftercoolers.** As mechanical energy is applied to a gas for
compression, the temperature of the gas increases. Aftercoolers are
installed after the final stage of compression to reduce the air
temperature. As the air temperature is reduced, water vapor in the air
is condensed, separated, collected, and drained from the system. Most of
the condensate from a compressor with intercooling is removed in the
intercooler(s), and the remainder in the aftercooler. Almost all
industrial systems, except those that supply process air to
heat-indifferent operations require aftercooling. In some systems,
aftercoolers are an integral part of the compressor package, while in
other systems the aftercooler is a separate piece of equipment. Some
systems have both.

**Separators.** Separators are devices that separate liquids entrained
in the air or gas. A separator generally is installed following each
intercooler or aftercooler to remove the condensed moisture. This
involves changes in direction and velocity and may include impingement
baffles. Lubricant-injected rotary compressors have an air/lubricant
coalescing separator immediately after the compressor discharge to
separate the injected lubricant before it is cooled and recirculated to
the compressor. This separation must take place before cooling to
prevent condensed moisture from being entrained in the lubricant.

**Dryers.** When air leaves an aftercooler and moisture separator, it is
typically saturated. Any further radiant cooling as it passes through
the distribution piping, which may be exposed to colder temperatures,
will cause further condensation of moisture with detrimental effects,
such as corrosion and contamination of point-of-use processes. This
problem can be avoided by the proper use of compressed air dryers.

Atmospheric air contains moisture. The higher the air temperature, the
more moisture the air is capable of holding. The term "relative
humidity" is commonly used to describe the moisture content although
technically, the correct term is "relative vapor pressure," the air and
the water vapor being considered as gases. When the air contains all the
moisture possible under the prevailing conditions, it is called
"saturated." Air at 80 percent relative humidity would contain 80
percent of the maximum possible.

When air is cooled, it will reach a temperature at which the amount of
moisture present can no longer be contained and some of the moisture
will condense and drop out. The temperature at which the moisture
condenses is called the dew point. In general, reducing the temperature
of saturated compressed air by 20° F will reduce the moisture content by
approximately 50 percent.

When air is compressed and occupies a smaller volume, it can no longer
contain all of the moisture possible at atmospheric conditions. Again,
some of the moisture will drop out as liquid condensate. The result of
both of these situations is a difference between the dew point at
atmospheric conditions and the dew point at higher pressures. Drying
compressed air beyond the required pressure dew point will result in
unnecessary energy and costs.

Different types of compressed air dryers have different operating
characteristics and degrees of dew point suppression. The most common
types of dryers are discussed below.

-   The refrigerant dryer is the most commonly used dryer in the
    industry, having relatively low initial and operating costs.
    Refrigerant-type air dryers (cycling and non-cycling) are not
    recommended for operation in sub-freezing ambient temperatures. The
    moisture in the compressed air can freeze and damage the dryer. Most
    refrigerated dryers are equipped with a precooler/reheater that
    reheats the dried compressed air with an air-to-air heat exchanger
    using the hot incoming air. This lowers the temperature of the
    incoming air before it passes through the refrigerant/thermal
    mass-to-air heat exchanger, reducing the heat load on the
    refrigerant system. Reheating the dried air prevents condensation on
    the outside of the compressed air piping in warm humid environments.
    The refrigerated dryer lowers the dew point of the air to the
    approximate temperature of the air exiting the refrigerant
    evaporator.

    Cycling dryers cool compressed air indirectly through a thermal
    storage medium (heat sink, thermal mass, chilled media, etc.) while
    non-cycling dryers directly cool compressed air in a refrigerant to
    air heat exchanger. Refrigerant-type cycling dryers are controlled
    with one or two thermostats to shut off the refrigerant compressor
    when it is not needed, and a thermal storage medium (sometimes
    referred to as heat sink, chilled media or thermal mass) prevents
    rapid cycling of the refrigerant compressor(s).

    Refrigerant-type, non-cycling dryers cool the air in a
    refrigerant-to-air heat exchanger. The cooling effect is from the
    evaporation of a liquid refrigerant causing moisture in the air to
    condense. The moisture then is removed and drained by a separator
    and drain. The temperature of the air leaving the refrigerant
    evaporator is controlled by a hot gas bypass valve.

-   Regenerative-desiccant-type dryers use a porous desiccant that
    adsorbs the moisture by collecting it in its myriad pores, allowing
    large quantities of water to be retained by a relatively small
    quantity of desiccant. Desiccant types include silica gel, activated
    alumina, and molecular sieves. Normally, the desiccant is contained
    in two separate towers. Compressed air to be dried flows through one
    tower, while the desiccant in the other is being regenerated.
    Regeneration is accomplished by reducing the pressure in the tower
    and passing previously dried purge air through the desiccant bed.
    Dryers of this type normally have a built-in regeneration cycle,
    which can be based upon time, dew point, or a combination of the
    two.

-   Deliquescent-type dryers use a drying medium that absorbs, rather
    than adsorbs, the moisture in the compressed air. This means that
    the desiccant medium is used up as it changes from solid to liquid
    and cannot be regenerated.

-   Heat-of-compression dryers are regenerative desiccant dryers that
    use the heat generated during compression to accomplish desiccant
    regeneration, so they can be considered as heat reactivated. There
    are two types: the single-vessel and the twin-tower.\
    \
    The single-vessel, heat-of-compression dryer provides continuous
    drying with no cycling or switching of towers. The twin-tower,
    heat-of-compression dryer operation is similar to other twin-tower,
    heat-activated, regenerative-desiccant dryers. The difference is
    that the desiccant in the saturated tower is regenerated by means of
    the heat of compression in all of the hot air leaving the discharge
    of the air compressor. The heat-of-compression dryers require air
    from the compressor at a sufficiently high temperature to accomplish
    regeneration. For this reason, it is used almost exclusively with
    centrifugal or lubricant-free rotary screw compressors.

-   Membrane technology dryers have advanced considerably in recent
    years. Membranes commonly are used for gas separation, such as in
    nitrogen production for food storage and other applications. The
    structure of the membrane allows molecules of certain gases (such as
    oxygen) to pass through (permeate) a semi-permeable membrane faster
    than others (such as nitrogen), leaving a concentration of the
    desired gas (nitrogen) at the outlet of the generator. When used as
    a dryer in a compressed air system, specially designed membranes
    allow water vapor (a gas) to pass through the membrane pores faster
    than the other gases (air) reducing the amount of water vapor in the
    air stream at the outlet of the membrane dryer, suppressing the dew
    point

**Compressed Air Filters.** Depending on the level of air purity
required, different levels of filtration and types of filters are used.
These include particulate filters to remove solid particles, coalescing
filters to remove lubricant and moisture, and adsorbent filters for
tastes and odors. A particulate filter is recommended after a
desiccant-type dryer to remove desiccant "fines." A coalescing-type
filter is recommended before a desiccant type dryer to prevent fouling
of the desiccant bed. Additional filtration may also be needed to meet
requirements for specific end uses.

Compressed air filters downstream of the air compressor are generally
required for the removal of contaminants, such as particulates,
condensate, and lubricant. Filtration only to the level required by each
compressed air application will minimize pressure drop and resultant
energy consumption. Elements should also be replaced as indicated by
pressure differential to minimize pressure drop and energy consumption,
and should be checked at least annually.

**Heat Recovery.** As noted earlier, compressing air generates heat. In
fact, industrial-sized air compressors generate a substantial amount of
heat that can be recovered and put to useful work. More than 80 percent
of the electrical energy going to a compressor becomes available heat.
Heat can be recovered and used for producing hot water or hot air.

**Lubrication.** In lubricant-injected rotary screw compressors,
lubricants are designed to cool, seal, and lubricate moving parts for
enhanced performance and longer wear. Important considerations for
compressor lubricants include proper application and compatibility with
downstream equipment, including piping, hoses, and seals. A lubricator
may be installed near a point-of-use to lubricate items such as
pneumatic tools. The lubricator may be combined with a filter and a
pressure regulator to make up what is commonly called a FRL
(filter-regulator-lubricator). The lubricant should be that specified by
the point-of-use equipment manufacturer.

**Pressure/Flow Controllers.** Pressure/flow controllers are optional
system pressure controls used in conjunction with the individual
compressor or system controls described previously. Their primary
function is to stabilize system pressure separate from and more
precisely than compressor controls. A pressure/flow controller does not
directly control a compressor and is generally not included as part of a
compressor package. A pressure/flow controller is a device that serves
to separate the supply side of a compressor system from the demand side.

**Air Receivers.** Receivers are used to provide compressed air storage
capacity to meet peak demand events and help control system pressure by
controlling the rate of pressure change in a system. If the receiver is
located before the air dryer, it is called a 'wet air' receiver. If it
is located after the dryer, it is called a 'dry air' receiver. Receivers
are especially effective for systems with widely varying compressed air
flow requirements. Where peaks are intermittent, a large air receiver
may allow a smaller air compressor to be used and can allow the capacity
control system to operate more effectively and improve system
efficiency. An air receiver after a reciprocating air compressor can
provide dampening of pressure pulsations, radiant cooling, and
collection of condensate. Demand-side control will optimize the benefit
of the air receiver storage volume by stabilizing system header pressure
and "flattening" the load peaks. Air receivers also play a crucial role
in orchestrating system controls, providing the time needed to start or
avoid starting standby air compressors.

**Traps and Drains.** Traps (sometimes called drains) allow the removal
of condensate from the compressed air system. Automatic condensate traps
are used to conserve energy by preventing the loss of air through open
petcocks and valves. Poorly maintained traps can waste a lot of
compressed air.

There are four methods to drain condensate from the receiver.

1.  **Manual.** Operators will manually open valves to discharge
    condensate. However, this is not automatic, and unfortunately, too
    often, manual valves are left open to drain condensate from moisture
    separators, intercoolers, refrigerated dryers, and filters, allowing
    compressed air to continually escape into the atmosphere.

2.  **Level-operated mechanical traps.** Float-type traps do not waste
    air when operating properly, but they often require a great deal of
    maintenance and are prone to blockage from sediment in the
    condensate. Inverted bucket traps may require less maintenance but
    will waste compressed air if the condensate rate is inadequate to
    maintain the liquid level (or prime) in the trap.

3.  **Electrically operated solenoid valves.** The solenoid-operated
    drain valve has a timing device that can be set to open for a
    specified time and at preset adjustable intervals. There are two
    issues with using these valves:

-   The period during which the valve is open may not be long enough for
    adequate drainage of accumulated condensate.

-   The valve will operate even if little or no condensate is present,
    resulting in the loss of valuable compressed air. Level-operated and
    electrically operated solenoid valves should have strainers
    installed to reduce contaminants, which block the inlet and
    discharge ports of these automatic devices. Motorized ball valves
    are also used with programmable timers. However, while fairly
    reliable, these valves can be even more wasteful as the duration of
    the valve opening is dependent on the valve actuator and is not
    adjustable.

4.  **Zero air-loss traps with reservoirs.** There are various types of
    zero air-loss traps.

-   A float or level sensor operates an electric solenoid or ball valve
    and maintains the condensate level in the reservoir below the
    high-level point.

-   A float activates a pneumatic signal to an air cylinder to open a
    ball valve through a linkage to expel the condensate in the
    reservoir to the low-level point.

-   Be sure to drain the reservoir often to prevent the accumulation of
    contaminants, which could foul the mechanisms of these traps.

The potential for freezing must be considered and provision made for
heated drains where necessary. The relatively common practice of leaving
a manual drain valve cracked open should not be tolerated because it
wastes costly compressed air. Contaminated condensate requires removal
of lubricant before the condensate is discharged to a sewer system. It
is recommended that the local sewage authority be consulted for
allowable contamination levels.

**Air Distribution Systems.** The air distribution system links the
various components of the compressed air system to deliver air to the
points-of-use with minimal pressure loss. The specific configuration of
a distribution system depends on the needs of the individual plant, but
frequently consists of an extended network of main lines, branch lines,
valves, and air hoses. The length of the network should be kept to a
minimum to reduce pressure drop. Air distribution piping should be large
enough in diameter to minimize pressure drop. A loop system is generally
recommended, with all piping sloped to accessible drop legs and drain
points.

When designing an air distribution system layout, it is best to place
the air compressor and its related accessories where temperature inside
the plant is the lowest (but not below freezing). A projection of future
demands and tie-ins to the existing distribution system should also be
considered. Air leaks are an important issue with distribution system
and are addressed in another fact sheet*.* It is important to note that
the majority of system leakage will be at the point of use and not in
the distribution piping.

Good design would have headers with a slight slope to allow drainage of
condensate and drop legs from the bottom side of the header should be
provided to allow collection and drainage of the condensate. The
direction of the slope should be away from the compressor.

Piping from the header to points-of-use should connect to the top or
side of the header to avoid being filled with condensate. Drainage-drop
legs from the bottom of the header should be installed to collect the
condensate.

*NOTE: This information was adapted from the "BestPractices for
Compressed Air Systems," 2003*

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The Compressed Air Challenge (CAC) is a nationwide government/industry
collabora­tive formed in 1997. The goals of the CAC are to raise
awareness in industry to the true costs and efficiency improvement
opportunities in compressed air systems oper­ations and to bring
compressed air best practices to the plant floor level for American
manufacturing. Contact us toll free at 866-442-4247

or
[[www.compressedairchallenge.org]{.underline}](http://www.compressedairchallenge.org/)
for more information