8.2 Pneumatic Systems

The word pneumatics is a derivative of the Greek word pneuma, which means air, wind, or breath. Pneumatics can be defined as that branch of engineering science that pertains to gaseous pressure and flow. As used in this manual, pneumatics is the portion of fluid power in which compressed air, or other gas, is used to transmit and control power to actuating mechanisms.

This section discusses the basic principles of pneumatics, characteristics of gases, heavy-duty air compressors, and air compressor maintenance. It also discusses the hazards of pneumatics, methods of controlling contamination, and safety precautions associated with compressed gases.

Basic Principles of Pneumatics

Gases differ from liquids in that they have no definite volume, that is, regardless of the size or shape of a vessel, a gas will completely fill it. Gases are highly compressible, while liquids are only slightly so. Also, gases are lighter than equal volumes of liquids, making gases less dense than liquids.

Boyle’s Law

When the automobile tire is initially inflated, air that normally occupies a specific volume is compressed into a smaller volume inside the tire. This increases the pressure on the inside of the tire.

Charles Boyle, an English scientist, was among the first to experiment with the pressure-volume relationship of gas. During an experiment when he compressed a volume of air, he found that the volume decreased as pressure increased, and by doubling the force exerted on the air, he could decrease the volume of the air by half (Figure 8-35).


Figure 8-35 — Gas compressed to half its original size by a doubled force.

Temperature is a dominant factor affecting the physical properties of gases. It is of particular concern in calculating changes in the state of gases. Therefore, the experiment must be performed at a constant temperature. The relationship between pressure and volume is known as Boyle's law.

Boyle's law states when the temperature of a gas is constant, the volume of an enclosed gas varies inversely with pressure. Boyle's law assumes conditions of constant temperature. In actual situations this is rarely the case. Temperature changes continually and affects the volume of a given mass of gas.

Charles’ Law

Jacques Charles, a French physicist, provided much of the foundation for modem kinetic theory of gases. Through experiments, he found that all gases expand and contract proportionally to the change in absolute temperature, providing the pressure remains constant. The relationship between volume and temperature is known as Charles's law.

Charles's law states that the volume of a gas is proportional to its absolute temperature if constant pressure is maintained.

Kinetic Theory of Gases

In an attempt to explain the compressibility of gases, consider the container shown in Figure 8-36 as containing a gas. At any given time, some molecules are moving in one direction, some are travelling in other directions, and some may be in a state of rest. The average effect of the molecules bombarding each container wall corresponds to the pressure of the gas. As more gas is pumped into the container, more molecules are available to bombard the walls, thus the pressure in the container increases.


Figure 8-36 — Molecular bombardment that creates pressure.

Increasing the speed with which the molecules hit the walls can also increase the gas pressure in a container. If the temperature of the gas is raised, the molecules move faster, causing an increase in pressure. This can be shown by considering the automobile tire. When you take a long drive on a hot day, the pressure in the tires increases, and a tire that appeared to be soft in cool morning temperature may appear normal at a higher midday temperature.

Compressibility and Expansion of Gases

Gases can be readily compressed and are assumed to be perfectly elastic. This combination of properties gives gas the ability to yield to a force and return promptly to its original condition when the force is removed. These are the properties of air that are used in pneumatic tires, tennis balls, and other deformable objects whose shapes are maintained by compressed air.

Pneumatic Gases

Gases serve the same purpose in pneumatic systems as liquids serve in hydraulic systems. Therefore, many of the same qualities that are considered when selecting a liquid for a hydraulic system must be considered when selecting a gas for a pneumatic system.

Qualities

The ideal fluid medium for a pneumatic system must be a readily available gas that is nonpoisonous, chemically stable, nonflammable, and free from any acids that can cause corrosion of system components. It should be a gas that will not support combustion of other elements.

Gases that have these desired qualities may not have the required lubricating power. Therefore, lubrication of the components must be arranged by other means. For example, some air compressors are provided with a lubricating system, some components are lubricated upon installation, or in some cases lubrication is introduced into the air supply line (inline oilers).

Two gases that meet these qualities and are most commonly used in pneumatic systems are compressed air and nitrogen. Since nitrogen is used very little except in gas-charged accumulators, we will discuss only compressed air.

Compressed Air

Compressed air is a mixture of all gases contained in the atmosphere. However, in this manual it is referred to as one of the gases used as a fluid medium for pneumatic systems.

The unlimited supply of air and the ease of compression make compressed air the most widely used fluid for pneumatic systems. Although moisture and solid particles must be removed from the air, a pneumatic system does not require the extensive distillation or separation process required in the production of other gases.

Compressed air has most of the desired characteristics of a gas for pneumatic systems. It is nonpoisonous and nonflammable but does contain oxygen, which supports combustion. The most undesirable quality of compressed air as a fluid medium for a pneumatic system is moisture content. The atmosphere contains varying amounts of moisture in vapor form. Changes in the temperature of compressed air will cause condensation of moisture in the system. This condensed moisture can be very harmful to the system and may freeze the line and components during cold weather. Moisture separators and air dryers are installed in the lines to minimize or eliminate moisture in systems where moisture would deteriorate system performance.

An air compressor provides the supply of compressed air at the required volume and pressure. In most systems the compressor is part of the system with distribution lines leading from the compressor to the devices to be operated.

Compressed air systems are categorized by their operating pressure as follows:

Heavy-Duty Air Compressors

Compressors are used in pneumatic systems to provide requirements similar to those required by pumps in hydraulic systems. They furnish compressed air as required to operate the units of the pneumatic systems.

Even though manufactured by different companies, most compressors are quite similar. They are governed by a pressure control system that can be adjusted to compress air to the maximum pressure.

Compressor Types

Rotary. The rotary compressor has a number of vanes held captive in slots in the rotor. These vanes slide in and out of the slots, as the rotor rotates. Figure 8-37 shows an end view of the vanes in the slots.


Figure 8-37 — Rotary compressor operation.

The rotor revolves about the center of the shaft that is offset from the center of the pumping casing. Centrifugal force acting on the rotating vanes maintains contact between the edge of the vanes and the pump casing. This feature causes the vanes to slide in and out of the slots as the rotor turns.

Notice in the variation in the clearance between the vanes and the bottom of the slots, as the rotor revolves. The vanes divide the crescent-shaped space between the offset rotor and the pump casing into compartments that increase in size and then decrease in size as the rotor rotates. Free air enters each compartment as successive vanes pass across the air intake. This air is carried around in each compartment and is discharged at a higher pressure due to the decreasing compartment size (volume) of the moving compartments as they progress from one end to the other of the crescent-shaped space.

The compressor is lubricated by oil circulating throughout the unit. All oil is removed from the air by an oil separator before the compressed air leaves the service valves.

Screw. Screw compressors are most often direct drive, two-stage machines with two precisely matched spiral-grooved rotors (Figure 8-38). The rotors provide positive- displacement internal compression smoothly and without surging. Oil is injected into the compressor unit and mixes directly with the air as the rotors turn, compressing the air.


Figure 8-38 — Screw compressor.

The oil has three primary functions:

All large volume compressors have protection devices that shut them down automatically when any of the following conditions develop:

Other features that may be observed in the operation of the air compressors is a governor system whereby the engine speed is reduced when less than full air delivery is used. An engine- and compression-control system prevents excessive buildup in the receiver.

Intercoolers

When air is compressed, heat is generated. This heat causes the air to expand, thus requiring an increase in power for further compression. If this heat is successfully removed between stages of compression, the total power required for additional compression may be reduced by as much as 15 percent. In multistage reciprocating compressors, this heat is removed by means of intercoolers that are heat exchangers placed between each compression stage. Rotary air compressors are cooled by oil and do not use intercoolers.

Aftercoolers

It is obvious that the presence of water or moisture in an air line is not desirable. The water is carried along through the line into the tool where the water washes away the lubricating oil, causing the tool to run sluggishly and increases maintenance. The effect is particularly pronounced in the case of high-speed tools where the wearing surfaces are limited in size, and excessive wear reduces efficiency by creating internal air leakage.

Further problems may result from the decrease in temperature caused by the sudden expansion of air at the tool. This low temperature creates condensation that freezes around the valves, ports, and outlets. This condition obviously impairs the operational efficiency of the tool and cannot be allowed.

The most satisfactory means of minimizing these conditions is the removal of the moisture from the air immediately after compression and before the air enters the distribution system. This may be accomplished in reciprocating compressors through the use of an aftercooler that is an air radiator that transfers heat from the compressed air to the atmosphere. The aftercooler reduces the temperature of the compressed air to the condensation point where most of the moisture is removed. Cooling the air not only eliminates the difficulties which moisture causes at points where air is used but also ensures better distribution.

Receiver Tank

The receiver tank is of welded steel construction and is installed on the discharge side of the compressor. It acts as a surge tank as well as a condensation chamber for the removal of oil and water vapors. It stores enough air during operation to actuate the pressure control system and is fitted with at least one service valve, a drain or blow-by valve, and a safety valve.

Pressure-Control System

All portable air compressors are governed by a pressure-control system. The control system is designed to balance the compressor's air delivery and engine speed with varied demands for compressed air.

The rotary compressor output is governed by varying the engine speed. The engine will operate at the speed required to compress enough air to supply the demand at a fairly constant pressure. When the engine has slowed to idling speed as a result of low demand, a valve controls the amount of free air that may enter the compressor.

A screw compressor output is governed by automatic control that provides smooth, stepless capacity regulation from full load to no load in response to the demand for air. From a full load down to no load is accomplished by a floating-speed engine control in combination with the variable-inlet compressor.

Air Compressor Maintenance

A number of built-in features make portable compressors easy to maintain:

Remember: a good maintenance program is the key to a long machine life. So it is up to both the operator and the mechanic to ensure that the maintenance is performed on time, every time.

Air Cleaner Servicing

The air cleaner contains a primary and secondary dry filter element (Figure 8-39). An air filter restriction indicator is located at the rear of the air filter housing to alert the operator of the need to service the filters. When a red band appears in the air filter restriction indicator, secure the compressor and service the filters.


Figure 8-39 — Air filter.

Use compressed air to clean the primary element; however, never let the air pressure exceed 30 psi. The secondary filter is not cleanable and should be replaced when necessary. Reverse flush the primary element by directing compressed air up from the inside out. Continue reverse flushing until all dust is removed. Should any oil or greasy dirt remain on the filter surface, replace the element. When the element is satisfactorily cleaned, inspect it thoroughly before installation. Inspection procedures are as follows:

After the element has been installed, inspect and tighten all air inlet connections before resuming operation.

 

CAUTION

Do not strike the element against any hard surface to dislodge dust. This will damage the sealing surfaces and possibly rupture the element.

Compressor Oil Change

Many articulated-piston compressors are oil lubricated, that is, they have an oil bath that splash-lubricates the bearings and cylinder walls as the crank rotates. The pistons have rings that help keep the compressed air on top of the piston and keep the lubricating oil away from the air. Rings, though, are not completely effective, so some oil will enter the compressed air in aerosol form. Air compressors that use oil as a lubricant require regular oil checks and periodic oil changes, and they must be operated on a level surface. Check the manufacturer’s specifications for oil change increments.

Main Oil Filter Servicing

The main oil filter is a replaceable cartridge. The servicing of the filter is required as indicated by the maintenance indicator on the filter, or each time the compressor oil is changed. Under normal operating conditions, the oil is changed at approximately 500 operating hours. Under severe conditions, more frequent servicing is required.

Demister or Separator Element

The demister, or separator element, is located inside the receiver tank (Figure 8-40). Replacement of the demister is indicated by the maintenance indicator (usually mounted on the receiver tank but also can be remote-mounted) or any sign of oil in the air at the service valves. You can reach the demister after removing the plate on the end of the receiver tank.


Figure 8-40 — Demister.

Contamination Control

As in hydraulic systems, fluid contamination is the leading cause of malfunctions in pneumatic systems. In addition to the solid particles of foreign matter that find their way to enter the system, there is also the problem of moisture. Most systems are equipped with one or more devices to remove contamination. These include filters, water separators, air dehydrators, and chemical dryers. Most systems contain drain valves at critical low points in the system. These valves are opened periodically to allow the escaping gas to purge a large percentage of the contaminants, both solids and moisture, from the system. In some systems these valves are automatic, while in others they must be operated manually.

Removing lines from various components throughout the system and then attempting to pressurize the system, causing a high rate of air flow through the system, does complete purging. The air flow will cause the foreign matter to be dislodged and blown from the system.

 

CAUTION

If an excessive amount of foreign matter, particularly oil, is blown from any one system, the lines and components should be removed and cleaned or, in some cases, replaced.

In addition to monitoring the devices installed to remove contamination, it is your responsibility as a mechanic to control the contamination. You can do this by using the following maintenance practices:

Potential Hazards

All compressed gases are hazardous. Compressed air and nitrogen are neither poisonous nor flammable, but should be handled with care. Some pneumatic systems operate at pressures exceeding 3,000 psi. Lines and fittings have exploded, injuring personnel and property. Literally thousands of careless workers have blown dust or other harmful particles into their eyes by careless handling of compressed air outlets.

If you ever have to handle nitrogen gas, remember that it will not support life, and when released in a confined space, it will cause asphyxia (the loss of consciousness as a result of too little oxygen and too much carbon dioxide in the blood). Although compressed air and nitrogen seem safe in comparison with other gases, do not let overconfidence lead to personal injury.

Safety Precautions

To minimize personal injury and equipment damage when using compressed gases, observe all practical operating safety precautions, including the following:

Test your Knowledge

5. A pneumatic system with an operating pressure of 500 psi is known as what type of system?

A. High-pressure
B. Medium-pressure
C. Medium high-pressure
D. Low-pressure

Summary

In this chapter, you were introduced to hydraulic and pneumatic systems. You learned how the hydraulic system and all its components, including a reservoir, pump, control valves, and cylinders, generate great power to perform material-handling operations. In addition you learned about the pneumatic system, including the laws that control compressed gases and the safety needed to properly operate both systems. This information will enable you to master the knowledge of these systems and to be a better construction mechanic.