INDUSTRIAL COMPRESSED AIR SYSTEMS

INTRODUCTION

This section intended for readers who want to gain an understanding of the basics of industrial compressed air systems.

Compressed air is used widely throughout industry and is often considered the "fourth utility" at many facilities. Almost every industrial plant, from a small machine shop to an immense pulp and paper mill, has some type of compressed air system. In many cases, the compressed air system is so vital that the facility cannot operate without it. Plant air compressor systems can vary in size from a small unit of 5 horsepower (hp) to huge systems with over 50,000 hp.

In many industrial facilities, air compressors use more electricity than any other type of equipment. Inefficiencies in compressed air systems can therefore be significant. Energy savings from systems improvements can range from 20-50% or more of electricity consumption. For many facilities this is equivalent to thousands, or even hundreds of thousands of dollars of potential annual savings, depending on use. A properly managed compressed air system can save energy, reduce maintenance, decrease downtime, increase production throughput, and improve product quality.

Compressed air systems consist of a supply side, which includes compressors and air treatment, and a demand side, which includes distribution and storage systems and end-use equipment. A properly managed supply side will result in clean, dry, stable air being delivered at the appropriate pressure in a dependable, cost-effective manner. A properly managed demand side minimizes wasted air and uses compressed air for appropriate applications. Improving and maintaining peak compressed air system performance requires addressing both the supply and demand sides of the system and how the two interact.

COMPONENTS OF AN INDUSTRIAL COMPRESSED AIR SYSTEM

A compressor is a machine that is used to increase the pressure of a gas. The earliest compressors were bellows, used by blacksmiths to intensify the heat in their furnaces. The first industrial compressors were simple reciprocating piston-driven machines powered by a water wheel.

A typical modern industrial compressed air system is composed of several major subsystems and many sub-components. Major subsystems 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. Controls serve to regulate the amount of compressed air being produced. The treatment equipment removes contaminants from the compressed air and accessories keep the system operating properly. Distribution systems are 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 shows a representative industrial compressed air system and its components.

Compressor Types

Many modern industrial air compressors are sold "packaged" with the compressor, drive motor, and many of the accessories mounted on a frame for ease of installation. Provision for movement by forklift is common. Larger packages may require the use of an overhead crane. An enclosure may be included for sound attenuation and aesthetics.

As shown in Figure , 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 the impeller blades determines the relationship between air flow and the pressure (or head) generated.

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 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 30-200 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 are available in sizes from 3-600 hp and may be air- or water-cooled. Less common rotary compressors include sliding-vane, liquid-ring, and scroll-type.

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 has a relatively high compression efficiency.

Mixed flow compressors have impellers and rotors which combine the characteristics of both axial and centrifugal compressors.

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, or steam engines or 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 energy-efficient motor can be specified when purchasing a compressor or replacement motor. The incremental cost of the energy-efficient motor is typically recovered in a very short time from the resulting energy savings. When replacing a standard motor with an energy-efficient one, careful attention needs to 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.

Diesel or natural gas engines are a common compressor power source in the oil and 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 powered by natural gas, especially 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.

The oldest method of driving compressors is through the use of a steam engine or turbine. In general, however, it is not economical to use a steam engine or turbine unless the steam is readily available within the plant for use as a power source.

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. Since 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. For systems with multiple compressors, sequencing controllers can be used to bring individual compressors on and off line as needed. Other system controllers, such as network controllers and demand controllers, can substantially improve performance for many 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 .

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 needed, however, 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. Reciprocating compressors of less than 100 hp 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 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.
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 such as forges and foundries, 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 .   Compressor filters and separators remove contamination (e.g., dirt, water, and lubricant) from the air before it enters and as it exits, the compressor. Depending on the level of air purity required, different levels of filtration and types of filters are used. Separators are devices which 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. The most common types are:

Refrigerant-type dryers cool the air to 35 to 40F and then remove the condensed moisture before the air is reheated and discharged.
Deliquescent-type dryers use a hygroscopic desiccant material with a high affinity for water. The desiccant absorbs water vapor and is dissolved in the liquid formed. Dew point suppression of 15 to 50F degrees can be expected when the proper bed level is maintained.
Twin tower regenerative-type dryers use a desiccant which adsorbs water vapor in the air stream. Adsorb means that the moisture adheres to the desiccant, collecting in the thousands of small pores within each desiccant bead. The composition of the desiccant is not changed and the moisture can be driven off in a regeneration process by applying dry purge air, by the application of heat, or a combination of both. Regenerative desiccant-type dryers typically are of twin tower construction. One tower dries the air from the compressor while the desiccant in the other tower is being regenerated, after the pressure in the tower being regenerated has been reduced to atmospheric pressure. The purge air requirement can range from 10 to 18% of the total air flow, depending on the type of dryer. The typical regenerative desiccant dryer at 100 psig has a pressure dew point rating of -20F to -40F, but a dew point as low as -100F can be obtained.

Compressed Air Filters. These include particulate filters to remove solid particles, coalescing filters to remove lubricant and moisture, and adsorbent filters for very fine contaminants. 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, and at least annually, to minimize pressure drop and energy consumption.

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% of the electrical energy going to a compressor becomes heat. Much of this heat can be recovered and used for producing hot water or hot air. See the Fact Sheet titled Heat Recovery with Compressed Air Systems for more information on this energy-saving opportunity.

Lubrication. Compressor 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. The lubricant should be that specified by the point-of-use equipment manufacturer.

Flow Controllers.  Besides regulating pressure, these devices also deliver varying volumes of air in response to changing demand. Various types of regulators are available for a wide range of applications. A regulator should be selected for a specific application, based upon type of equipment, supply and downstream pressure requirements, flow rate, and required pressure accuracy.

Air Receiver. Receivers are used to provide compressed air storage capacity to meet peak demand events and help control system pressure. 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.

Traps and Drains. Automatic condensate drains or traps are used to prevent the loss of air through open petcocks and valves. Drain valves should allow removal of condensate but not compressed air. Two types of traps are common: mechanical and electrical. Mechanical traps link float devices to open valves when condensate rises to a preset level. Electric solenoid drain valves operate on a preset time cycle, but may open even when condensate is not present. Other electrical devices sense liquid level and open to drain only when condensate is present. Improperly operating or maintained traps can create excessive air usage and waste energy.

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. 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 the Fact Sheet titled Compressed Air System Leaks.

USES OF COMPRESSED AIR

Industrial facilities use compressed air for a multitude of operations. Almost every industrial facility has at least two compressors, and in a medium-sized plant there may be hundreds of different uses of compressed air.
Uses include powering pneumatic tools, packaging and automation equipment, and conveyors. Pneumatic tools tend to be smaller, lighter, and more maneuverable than electric motor-driven tools. They also deliver smooth power and are not damaged by overloading. Air-powered tools have the capability for infinitely variable speed and torque control, and can reach a desired speed and torque very quickly. In addition, they are often selected for safety reasons because they do not produce sparks and have low heat build-up. Although they have many advantages, pneumatic tools are generally much less energy-efficient than electric tools. Many manufacturing industries also use compressed air and gas for combustion and process operations such as oxidation, fractionation, cryogenics, refrigeration, filtration, dehydration, and aeration. Table 1-1 lists some major manufacturing industries and the tools, conveying, and process operations requiring compressed air. For some of these applications, however, other sources of power may be more cost effective (see the Fact Sheet titled Inappropriate Uses of Compressed Air).

Table. Industrial Sector Uses of Compressed Air

Industry Example Compressed Air Uses
Food Dehydration, bottling, controls and actuators, conveying, spraying coatings, cleaning, vacuum packing
Textiles Agitating liquids, clamping, conveying, automated equipment, controls and actuators, loom jet weaving, spinning, texturizing
Apparel Conveying, clamping, tool powering, controls and actuators, automated equipment 
Lumber and Wood Sawing, hoisting, clamping, pressure treatment, controls and actuators
Furniture Air piston powering, tool powering, clamping, spraying, controls and actuators
Pulp and Paper Conveying, controls and actuators
Chemicals Conveying, controls and actuators
Petroleum Process gas compressing, controls and actuators
Rubber and Plastics Tool powering, clamping, controls and actuators, forming, mold press powering, injection molding
Stone, Clay, and Glass Conveying, blending, mixing, controls and actuators, glass blowing and molding, cooling
Primary Metals Vacuum melting, controls and actuators, hoisting
Metals Fabrication  Assembly station powering, tool powering, controls and actuators, injection molding, spraying

Compressed air also plays a vital role in many non-manufacturing sectors, including the transportation, construction, mining, agriculture, recreation, and service industries. Examples of some of these applications are shown in Table 1-2 below.

Table. Non-Manufacturing Sector Use of Compressed Air

Sector Example Compressed Air Uses
Transportation Pneumatic tools, hoists, air brake systems
Mining Pneumatic tools, hoists, pumps, controls and actuators
Agriculture Farm equipment, materials handling, spraying of crops, dairy machines
Power Generation Starting gas turbines, automatic control, emissions controls
Wastewater Treatment Vacuum filters, conveying
Recreation Ski resorts - snow making
Hotels - elevators, sewage disposal
Golf courses - seeding, fertilizing, sprinkler systems
Theaters - projector cleaning
Amusement parks - air brakes
Underwater exploration - air tanks 
Service Industries Pneumatic tools, hoists, air brake systems, garment pressing machines, hospital respiration systems, climate control

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