28-09-2012, 05:47 PM
Introduction to Pneumatics and Pneumatic Circuit Problems for FPEF Trainer
Intro_to_Pneumatics.pdf (Size: 721.72 KB / Downloads: 17)
Pneumatic Transmission of Energy
The reason for using pneumatics, or any other type of energy transmission on a
machine, is to perform work. The accomplishment of work requires the application
of kinetic energy to a resisting object resulting in the object moving through a
distance. In a pneumatic system, energy is stored in a potential state under the form
of compressed air. Working energy (kinetic energy and pressure) results in a
pneumatic system when the compressed air is allowed to expand. For example, a
tank is charged to 100 PSIA with compressed air. When the valve at the tank outlet
is opened, the air inside the tank expands until the pressure inside the tank equals the
atmospheric pressure. Air expansion takes the form of airflow.
To perform any applicable amount of work then, a device is needed which can
supply an air tank with a sufficient amount of air at a desired pressure. This device
is positive displacement compressor.
What a Positive Displacement Compressor Consists of
A positive displacement compressor basically consists of a movable member inside
a housing. The compressor has a piston for a movable member. The piston is
connected to a crankshaft, which is in turn connected to a prime mover (electric
motor, internal combustion engine). At inlet and outlet ports, valves allow air to
enter and exit the chamber.
How a Positive Displacement Compressor Works
As the crankshaft pulls the piston down, an increasing volume is formed within the
housing. This action causes the trapped air in the piston bore to expand, reducing its
pressure. When pressure differential becomes high enough, the inlet valve opens,
allowing atmospheric air to flow in. With the piston at the bottom of its stroke, inlet
valve closes. The piston starts its upward movement to reduce the air volume which
consequently increases its pressure and temperature. When pressure differential
between the compressor chamber and discharge line is high enough, the discharge
valve opens, allowing air to pass into an air receiver tank for storage.
Control of Pneumatic Energy
Working energy transmitted pneumatically must be directed and under complete
control at all times. If not under control, useful work will not be done and machinery
or machine operators might be harmed. One of the advantages of transmitting
energy pneumatically is that energy can be controlled relatively easily by using
valves.
Control of Pressure
Pressure in a pneumatic system must be controlled at two points - after the
compressor and after the air receiver tank. Control of pressure is required after the
compressor as a safety for the system. Control of pressure after an air receiver tank
is necessary so that an actuator receives a steady pressure source without wasting
energy.
Control of Pressure after A Compressor
In a pneumatic system, energy delivered by a compressor is not generally used
immediately, but is stored as potential energy in air receiver tank in the form of
compressed air.
In most instances, a compressor is designed into a system so that it operates
intermittently. A compressor usually delivers compressed air to a receiver tank until
high pressure is reached, then it is shut down. When air pressure in the tank
decreases, the compressor cuts in and recharges the tank.
Intermittent compressor
operation in this manner is a power saving benefit for the system.
A common way of sensing tank pressure and controlling actuation and de-actuation
of relatively small (2-15 HP) compressors, is with a pressure switch.
Pressure Switch
System pressure is sensed with a spring-loaded piston within the switch housing.
When pressure in the system is at its low level, the spring pushes the piston down. In
this position a contact is made causing an electrical signal to turn on the compressor.
As pressure in the receiver tank rises, it forces the piston upward. With system
pressure at its high level, the piston breaks the electrical contact shutting down the
compressor.