Chapter 17 - Pneumatic Systems
Bernoulli's Principles
As the conductor's cross sectional area changes, so does the pressure in the line (with constant flow rate)
Charles Law
At a constant pressure, the volume (V) of a gas varies proportionately to its absolute temperature (T)
Boyle's Law
At a constant temperate, absolute pressure (P) varies inversely to the volume (V)
Barometers
At sea level the tube must be more than 760mm or 29.92"
Pneumatic line installations
A water leg must be installed at the end of the main line to accumulate and drain the excess moisture.
Psia
Absolute pressure in pounds per square inch, where 0 PSIA corresponds to 29.9 inches of mercury vacuum, and 14.7 PSIA corresponds to 0 PSIG (pounds per square inch gauge).
Air reservoirs
Acts as a reservoir to accommodate any fluctuations in the system's supply. This dampens pulsation from the compressors and provides steady pressure to the system.
Properties of compressed air
Air flows from high to low pressure areas.
Axial flow compressor
Air is forced axially along the rotor by a series of fins and directed by the stationary vanes. The impeller cavity decreases from one stage to another. This compresses the air as it moves towards the discharge port.
Air lubricators
Air must be clean and dry before it passes through this.
Air slip
Air returning to the inlet side past the sealing surfaces in a screw compressor. This can be reduced by increasing the rpm of the rotors, however, a higher rpm also increases the noise to the point where the compressor must be isolated from the work area or in a sound reducing enclosure.
Properties of compressed air
Air will expand to fill any container
Charles' Law
Aka Gay-Lussac's law
Loop system
Allows air to move continuously around the system in either direction to supply the work stations.
Decentralized system
Allows for shorter supply lines which results in a more uniform air supply and pressure.
Unloading device
Allows the drive to run with the least hp demand when it reaches the maximum air pressure.
Membrane dryers
Because the membrane vents gas (water vapor) not condensate, there is no need for regeneration or fear of freezing.
3-position, 5-port, 4-way DCVs
Blocked centre
Dual air exhaust control circuit
Both exhaust ports have variable output, flow control valves to control the speed at which the air escapes from the actuator. This controls the speed at which the actuator moves.
Quick exhaust and shuttle valves
Both the quick-exhaust and shuttle valves are usually used with 3- or 4-way DCVs. Many quick-exhaust valves may also be used as shuttle valves by changing the line to their port connections.
Muffler
Breaks up the sound waves and smooths them out reducing their energy (db)
Water trap
Can be placed in many locations and are a vital component. Common placements are after the intercooler, after coolers and,0 air dryers.
Properties of compressed air
Can be reduced in volume
Air dryer
Can lower the dew point to as low as -400°C (-400°F)
Unloading fingers
Commonly used, consisting of a set of fingers controlled by a pilot line from the system's air pressure. These fingers hold the intake valves open when the maximum pressure is reached.
Vane compressors
Composed of a rotor with sliding vanes mounted in an eccentric housing. The vanes move in and out under centrifugal force.
Refrigeration dryers
Condense moisture by cooling the air in heat exchangers chilled by refrigerants. The air is reheated prior to entering the system preventing condensation from forming on the exterior of the air line downstream of the dryer.
Screw compressors
Consist of two screws, lobes or helix. Air is trapped between the meshing units and reduced in volume as it moves axially to the discharge port.
Grid system
Consists of a main line from the compressor which begins large and becomes progressively smaller as it reaches the end.
Decentralized system
Consists of two or more grids within the main system each with its own compressor.
Deliquescent dryers
Contain a desiccant, which absorbs moisture and they are consumed in the process.
Intake filters
Controls the amount of airborne contaminants allowed to enter the compressor. Most are felt or cotton material held in shape by a mesh or screen.
Aftercoolers
Cool the air after it has been compressed, and facilitates the removal of moisture and other entrained liquids.
Water control
Cooling temperature is kept above the dew point.
Quick-exhaust circuit
Coupled between the cylinder's head end and the DCV. Because the piston must extend very rapidly, the exhaust air is allowed to escape near the cylinder instead of returning through the DCV. The retraction stroke is not required to move as quickly, so the path through the DCV is adequate.
Compressor overheating
Damage to cooling system
Low oil pressure in a compressor
Damage to oil pick up
Grid system
Dead end
Low oil pressure in a compressor
Defective pump
Dynamic compressors
Deliver large volumes of air at relatively low pressure.
Positive displacement action compressors
Deliver moderate volumes of air at high pressure
Coalescing filters
Designed to remove all solid particles as small as 0.3 microns as well as nearly 100% of the oil and water vapors.
Compressor overheating
Dirty intake filter
Pneumatic circuits
Do not usually show the compressor or the maximum pressure control, but start off instead with a pneumatic pressure source symbol.
Barbs
Hose is pushed onto the fitting and they tighten their grip as the hose tries to pull off. Use clamps wherever possible.
Vane compressors
Lubrication is required where vanes make contact with the housing. Done by injecting oil from the air downstream of the compressor. Oil absorbs some of the heat of compression and must be cooled after it is removed from the air.
Lobe compressors
Mate marks on timing gears is required for disassembly. Often put on from the factory.
3-position, 5-port, 4-way DCVs
May be 2 or 3 position
Rotary compressors
May have a positive displacement or a dynamic action.
3-position, 5-port, 4-way DCVs
May have their pressure and exhaust ports reversed. This is used to control an actuator at a different pressure in each direction. Used on gate valves because the slight taper takes a little force to wedge the making surface to make a tight seal. Requires 3-5 × as much force to open.
Decentralized system
More versatile and adapt easily to changing requirements.
Reusing compressor heat
Most of the heat is wasted. Plant policy has usually been to waste this heat into the atmosphere but more companies are now using it to heat buildings and water.
Instrument air
Must be clean dry and free of oil.
Air lubricators
Must be located downstream from any device that might be contaminated by lubricating oil.
3-position, 5-port, 4-way DCVs
Normally the pressure port is the center port (1) and the outer two ports (3 & 5) are exhaust ports.
3-position, 5-port, 4-way DCVs
Not all 4 ways can be reversed. Their packing seal may be designed to only seal in one direction.
Pneumatic hose
Nylon, polyethylene, vinyl, Teflon and more.
Packed bore design
O-rings are fixed in grooves in the body through which the spool slides.
Air reservoirs
Receivers
Ways to increase pressure
Reduce the volume to create more impact on a smaller wall area
Pressure regulators
Reduces the line pressure.
Air reservoirs
Reduces the velocity of the air which allows any moisture carried over from the aftercooler to settle out.
Safety valve
Releases any excess pressure, possibly from a failed unloading valve of a pressure surge transmitted back to the system.
Solenoid-controlled DCVs
Remotely controlled by pilot pressure, direct solenoids, or solenoid and pilot valves.
Flow control valves
Restrict the rate of flow in a leg of the system.
Shuttle valve
Self activated
Fog lubricator
Serve best when the flow to the component is straight and short.
Mist lubrication
Serve best when the oil must be carried a long way, so that it doesn't settle in the line.
Safety valve
Set to 5-10% above the systems maximum pressure. The usual maximum pressure is about 100 psi (690kPa)
Plant air
Should be clean and have low moisture and oil content.
Air reservoirs
Should be located as close to the compressors as possible.
Muffler
Silencer
Air-cooled aftercoolers
Similar to automotive radiators.
Quick Release (Exhaust)
This valve is a special-purpose flow control valve used in pneumatic circuits . It is commonly used with a three-way or four-way DC valves to increase the flow and therefore, the speed of a cylinder
Two-hand or safety circuit
To advance or retract the piston both valves must be shifted allowing air to enter the cap and exit the head.
Variable speed drive
To control maximum air pressure, such as a gas or diesel engine power unit. At the maximum pressure setting, the engine rpm is reduced and the centrifugal clutch disengages allowing the engine to run at a low rpm with no load.
Throttling the air intake
To control maximum air pressure, thus allowing less free air to enter the compressor.
Pressure switch
To control maximum air pressure, to start and stop an electric drive motor.
Convert °C to °K
°K = °C + 273
Convert °F to °R
°R = °F + 459
Lobe compressors
Two closely drive lobes are driven by timing gears. The pressure range on these compressors is low, but the volume is high. Air is carried around without a mechanical reduction in volume. Pressure is build up from restriction at the discharge port and from the systems resistance to flow.
Loop system
Two or more compressors around ta continuous loop.
Membrane dryers
Typically made of thousands of fiber tubes through which the water vapors pass.
Air reservoirs
Unfired pressure vessel
Centrifugal compressors
Use a dynamic action to build up pressure. They do not have a passage from the inlet to the exhaust port sealed. When the compressor it not operating air can flow from one port to another in either direction. When in operation the dynamic action resists the reversal of flow.
Air-cooled intercoolers
Use a fan driven by the compressor to force air over finned tubes where the compressed air passes.
Regenerative desiccant dryers
Use a solid desiccant which absorbs, usually silica gel, activated alumina or a molecular sieve.
Pneumatic actuators
Use corrosion resistant materials such as aluminum, brass and stainless steel. Due to their lower pressure, they may use seals with lower capacities.
Pneumatic Systems
Use gas as a fluid medium instead of oil.
Flow control valves
Use the same action and symbol as hydraulic flow control valves with metered flow in one direction and free flow in the other.
Barometers
Use to measure vacuum and is measured in Hg or "Hg. A simple tube which is sealed at one end and filled completely with mercury and stands vertically with the open end placed in a pool of mercury. As the pressure of the pool changes the height in the tube changes.
Dry screw compressors
Use two timed gears to prevent contact. Gear backlash and bearing wear must be held to a minimum due to very small clearances.
Manually controlled DCV
Used for cylinders that must operate at speeds from creep to wide open.
FLR
Used to be joined by a short pipe nipples. When replacing these older units had to be wholly disassembled. New modular systems individual components can be removed.
Shuttle valve
Used when it is necessary to have two DCVs operating a single actuator.
Quick-exhaust circuit
Used where very rapid actuator speed is required
Spool sealing
Uses a dynamic seal, such as O-rings or U-sections in contact with the bore and spool. They are held in position by spacers and the spool has rounded corners to allow easy engagement of the seal.
Sticky valves in a compressor
Valve packing too tight
Water control
Water trap built into the intercooler
Quick exhaust and shuttle valves
When a cylinder is required to return rapidly these installed in the retract line instead of returning through the DCV
Circuits with multiple remote control positions
When a single piece of equipment needs to be operated from various
Regenerative desiccant dryers
When heat is used 78% of the time is used for heating and 25% for cooling.
Vacuum
When the pressure in the container is less than the pressure surrounding it (typically atmospheric) Perfect vacuum is 0 bar. When there is a perfect vacuum in a system the maximum force in the system cause by the vacuum is 1 bar.
Flareless joints
When these are required, ensure an internal tube support is used to prevent the hose from collapsing.
Fusible plug
Will melt and release all the pressure in the receiver if the air temperature becomes dangerously high.
Excessive noise in a compressor is caused by
Worn bearings
Low oil pressure in a compressor
Worn out bearings
Water trap
.
one micron
0.000039 of an inch
6.9 kPa
1 Psi
Pressure at sea level
1 bar/14.7 psi
1 Psi
2.04" Hg
Filtering action
4. A baffle below the filtering element creates a quiet zone which allows the particles and moisture to collect and prevents them from becoming entrained in the air flow again. 5. The air is then forced through the filtering element which removes the smaller particles. 6. Clean air then exits the filter and enters the system.
Water-cooled aftercoolers
A coil of tubing within a chamber that water passes through in the other direction. As the air cools it condenses and moisture collects within the chamber, the water drains into a water trap.
liquid-cooled intercoolers
A nest of tubes immersed in recirculating liquid. The liquid cools the air as it flows through the tubes.
Vane compressors
Dry air can be obtained from a vane compressor by using vanes with special wearing surface.
Air filters
Even though filters are resent there may still be foreign material present in the system.
Sticky valves in a compressor
Excessive carbon deposits on the valve components
3-position, 5-port, 4-way DCVs
Exhaust center
Excessive air pressure in the receiver
Faulty safety valve
Compressor overheating
Faulty thermal valve
Excessive air pressure in the receiver
Faulty unloading valve
FLR
Filter, regulator, lubricator
Charles' Law
For a constant volume of gas, the pressure (P) varies proportionately to its absolute temperature (T)
Ideal gas law
Gas does not compress without a change in temperature, Boyle's law and Charle's law are combined to create this.
Membrane dryers
Gas separation devices that consist of a permeable membrane surface that blocks nitrogen and oxygen molecules but allow water vapor to pass through. .
Two-hand or safety circuit
Has air blocked at both supply and exhaust port, keeping the actuator locked in position.
Reciprocating compressors
Have a positive displacement action.
Quick-disconnect couplers
Have a shut off recessed half and an open passage end.
Membrane dryers
Have no moving parts, are non electric and suitable for most hazardous locations.
Wet screw compressors
Have one rotor driving the other, this allows the timing gears to be eliminated. They require oil to reduce wear. Oil must be removed from the airstream after it leaves the compressor. This oil also reduces the amount of air slip and removes some of the heat.
Regenerative desiccant dryers
Heat may be used to speed the drying process but the chamber must be cooled before it is ready to use again.
Ways to increase pressure
Heating the air so that the molecules travel faster and increase the intensity and the amount of impact on the same wall area.
Cold weather handling
Heating the air supply is done in a steam mill by running steam lines next to or through air lines.
Air reservoirs
If there is no aftercooler this acts as one.
Compressor overheating
Improper oil level
Barometers
In areas where the atmospheric pressure is less than 1 bar, the achievable vacuum is also less. For every 1000 Ft. above sea level the pressure drops 1"Hg.
Compressor overheating
Inadequate circulation of cooling water or air flow
Cold weather handling
Infrared lamps or a heat source on the valve.
Drain valve
Installed at the lowest point of the receiver so that water and oil emulsion can settle out and be drained though this.
Air drop lines
Installed with a tee junction that goes up and then down to prevent moisture from flowing into the line.
Excessive noise in a compressor is caused by
Insufficient head clearance
Sticky valves in a compressor
Insufficient lubraication
Cold weather handling
Introducing a small amount of permanent anti freeze into the lubricator
Ways to increase pressure
Introducing more air into the confined space
Vacuum power
Is a form of fluid power.
Air drop lines
Lines can be placed anywhere along the main feed line to power working components.
Safety valve
Located in or on the compressor no the air receiver. This valve is only in case of failure in the system.
Excessive noise in a compressor is caused by
Loose flywheel
Excessive noise in a compressor is caused by
Loose or worn piston
Low oil pressure in a compressor
Low oil level
Centrifugal compressors
Operate at high speed and deliver large volumes of air. Multiple stage units are used to increase the working pressure delivery.
Ideal gas law
Other considerations such as humidity, heat, friction and efficiency come into effect when calculating for pneumatic systems. Even so, this law is still used for design calculations.
Boyle's Law
P1 × V1 = P2 × V2 Based on absolute pressure
Pneumatic line installations
Permanent lines are installed so that moisture flows towards the drain.
Solenoid-controlled DCVs
Permit very little control as they are wide open or shut and air is fast acting.
Low oil pressure in a compressor
Plugged suction strainer
Air reservoirs
Potentially explosive, built to specific standards and safe operation requires that the receiver is equipped with a relief or safety valve, a drain valve, a fusible plug and an air pressure gauge.
Lapped spool
Precision tolerance lapped parts, this design relies on lubrication between the mating surfaces to do the sealing.
3-position, 5-port, 4-way DCVs
Pressure centre
Pascals Law
Pressure in a confined, static fluid acts the same and equally in every direction, it always acts at right angles to the containing surface.
Spring guards
Protect the hose from kinking.
Solenoid-controlled DCVs
Some may be manually overridden to permit the valve to operate without electricity.
DCV Valves3-position, 5-port, 4-way DCVs
Some of the 4 ways valves are equipped with 5 ports.
Water trap
Some use a moisture separator within the unit to enhance the water removal process
Muffler
Sounds sintered bronze, felt, styrofoam and others.
Pneumatic hose
Straight lengths are used to connect components which are permanently in place and coils used with remote equipment.
Using fomulas
Temperatures and pressures must be in absolute for the purpose of the calculations of the gas laws. Therefore, 14.7 is added to 100 to give a psia reading of 114.7, and 459° is added to 200°F to obtain an absolute reading of 659°R.
Cold weather handling
Thaw gradually allowing the seals to soften slowly and the valve body and components to expand at an even rate and prevent cracks.
Packed spool
The O-rings are fixed in the spool
Cooling
The air between stages must be cooled because the hotter the air the less efficient the compression.
Refrigeration dryers
The air is reheated prior to entering the system preventing condensation from forming on the exterior of the air line downstream of the dryer.
Filtering action
The compressed air enters a polycarbonate (high-strength plastic) or an all metal filter bowl. The metal bowl mayor may not have a sight glass. It I. A deflector plate swirls the air around the filter bowl. 2. A shroud ensures that the swirling action occurs around the filter bowl and not around the filtering element. 3. This causes the larger particles and excess moisture to be thrown out to the side of the filter bowl.
Mist lubricator
The discharge valves, ports and cylinders of pneumatic systems are subject to extreme heat and are cooled by lubrication.
Moisture separator
The float rises with the increase of water until the valve unseats. The water is then expelled out of the water trap until the water levels allows the valve to reseat.
Pneumatic DCV differences from hydraulic
The method that the spools are sealed
Air filters
The minimum size of the particles removed depend on the rating of the filter measured in microns.
Loop system
The preferred system
Pneumatic DCV differences from hydraulic
The release of exhaust air into atmosphere at the valve or the actuator, or in a remote area.
Air dryer
The required dew point is based on the operating conditions of the system.
Membrane dryers
The shell is constructed from plastic and aluminum to prevent corrosion.
Dew point
The temperature at which condensation begins
Directional Control Valves
The valve controls the direction of airflow. They do this by shifting poppets or spools to open or close a passage between various ports
Flow Control
The valve is used in pneumatic circuits is to vary the speed of actuators in a specific part of the circuit
Check Valve
The valve is used in pneumatic systems to allow airflow in one direction while blocking flow in reverse.
Pressure Regulator
The valve is used to reduce and maintain the line pressure to the set point required by the actuators.
Pressure regulators
The working element is a main piston or diaphragm which controls a poppet or valve by means of a connecting pin.
Grid system
The workstation at the end may have less air when there is heavy demand.
Lobe compressors
There is no metal to metal contact therefore no surface lubrication or air stream lube is required.
Properties of compressed air
There must be pressure change to create air flow.
Regenerative desiccant dryers
They have 2 identical chambers where the air is dried. As the first chamber becomes saturated with moisture the flow is directed to the second chamber while it dries.
Coalescing filters
They use a similar desiccant to the air dryers.