CHARACTERISTICS AND APPLICATIONS OF PNEUMATICS

 CHARACTERISTICS  AND APPLICATIONS OF PNEUMATICS

PNEUMATICS – An over view

Fluid power system using air as a medium for developing, transmitting, controlling and utilizing power is commonly referred to as Pneumatics.

Pneuma is a Greek word means Breath or Wind or Air.

Pneumatics means the study of air movement and air phenomena. Today it is not possible to imagine the modern industry without using Pneumatics for the automation. It finds application in the diverse branch of the industry. The real practical industrial application of Pneumatics in production dates back to about 1950.

FUNCTIONS OF PNEUMATICS:

In Pneumatics in the majority of applications compressed air is used for one or more of the following functions.

• The use of sensor to determine status of process.
• Information processing.
• Switching of actuators by means of final control elements.
• Carrying out of work.

THE TYPES OF MOTION PERFORMED BY PNEUMATIC COMPONENTS:

• Linear
• Swivel
• Rotary.

APPLICATIONS OF PNEUMATIC SYSTEM:

General methods of Material handling

• Clamping
• Shifting
• Positioning
• Orienting

General applications

• Opening of system valves for Air, Water or Chemicals
• Packaging
• Door or Chute control
• Forming operations like bending, drawing etc.,
• Stamping and embossing of components
• Feeding and Transfer of materials.
• Turning and inverting of parts.
• Stacking of components.
• Sorting of parts.
• Spot welding
• Pick and place operations
• Work or tool feeding in Machine tools  
• Dental drills etc.,

CHARACTERISTICS OF COMPRESSED AIR:

Advantages and distinguishing characteristics of compressed air

Availability:    Air is available practically everywhere for compression, in unlimited quantities.

Transport:     Air can be easily transported in pipelines, even over larger distances. It is not necessary to return the compressed air to the storage.

Storage:     A compressor need not be in continuous operation. Compressed air can be stored in and removed from a reservoir.

Temperature:     Compressed air is insensitive to temperature fluctuations. This ensures reliable operation even under extreme conditions of temperature.

Explosion proof:     Compressed air offers no risk of explosion or of fire, hence no expensive protection against explosion is required.

Cleanliness:     Compressed air is clean since any air which escapes through leaking pipes or elements does not cause contamination.

Simple components: The operating components are simple in construction.

Speed:     Compressed air is a very fast working medium, cylinders have working speed up to 2m/sec.

Overload safe:     Pneumatic tools and operating components can be loaded to the point of stopping and they are therefore over load safe.

The negative characteristics:

LIMITA

Preparation:        Compressed air requires good preparation. Dirt and condensate

should not be present.

Compressible:    Due to compressibility it is not always possible to achieve uniform and

constant piston speeds with compressed air.

Force available:    Compressed air is economical only up to a certain force requirement. Under the normal working pressure of 6-7 bar and dependent on the travel and speed, the output limit is between 2000 – 3000 Kgs.

Noise level:    The exhaust air is loud. This problem has now, however, been largely solved due to silencers.

Cost:            Compressed air is relatively expensive means of conveying power.

The high energy costs are partially compensated by inexpensive components and higher performance.           

A comparison with other forms of energy is an essential part of the selection process when considering pneumatics as a control or working medium. This evaluation embraces the total system from the input signals (Sensors) through the control part (processor) or to the output devices (actuators). All factors must be considered such as :

• Work or output requirements
• Preferred control methods
• Resources and expertise available to support the project
• Systems currently installed which are to be integrated with the new project

CRITERIA FOR A WORKING MEDIUM:

Choice of working media

• Electrics
• Mechanical
• Pneumatics
• Hydraulics
• A combination of above.

Selection criteria for working section

• Force and Speed
• Type of motion
• Size and Service life
• Sensitivity
• Safety & reliability
• Energy controls.
• Controllability.
• Handling.
• Storage.

CRITERIA FOR A CONTROL MEDIUM:

Choice of control media

• Mechanical
• Electrical
• Electronics
• Pneumatics
• Hydraulics

Selection criteria for control section

• Reliability of components
• Sensitivity to environment influence
• Ease of maintenance & repair.
• Switching time of components
• Signal speed.
• Space requirements.
• Service life.
• Training requirement for operator & maintenance.
• Project modification of the control system.

PNEUMATICS AND CONTROL SYSTEM DEVELOPMENT:

The product development in pneumatics can be considered in a number of areas:

• Actuators
• Sensors and input devices
• Processors
• Control systems
• Accessories

Each of these product groups are important in the development of pneumatic solutions. The demands are for system / component reliability but with:

• Accessibility for repair and/or maintenance, or
• Low cost of replacement
• Ease of mounting and connection
• Low planned maintenance requirements
• Interchangeability and flexibility
• Compact design
• Cost commensurate with the above
• Readily available
• Documentation support
• Minimum training required to support the product.

BASIC PNEUMATIC SYSTEM

Pneumatic cylinder rotary actuators and air motors provide the force and movement of most pneumatic control systems, to hold, move, and form and process material.

To operate and control these actuators, other pneumatic components are required i.e. air service units to prepare the compressed sir and valves to control the pressure, flow and direction of movement of the actuators.

A basic pneumatic system consists of two main sections:

• The Air Production and Distribution system
• The Air Consuming System

Air Production System

The component parts and their main functions are:

1. Compressor  

Air taken in at atmospheric pressure is compressed and delivered at a higher pressure to the pneumatic system. It thus transforms mechanical energy into pneumatic energy.

2. Electric Motor

Supplies the mechanical power to the compressor. It transforms electrical energy into mechanical energy.

3. Pressure Switch

Controls the electric motor by sensing the pressure in the tank. It is set to a maximum pressure at which it stops the motor and a minimum pressure at which it restarts it.

4. Check Valve

Lets the compressed air from the compressor into the tank and prevents it leaking back when the compressor is stopped.

5. Tank

Stores the compressed air. Its size is defined by the capacity of the compressor. The larger the volume, the longer the intervals between compressor runs.

6. Pressure Gauge

Indicates the Tank pressure.

7. Auto Drain

Drains all the water considering in the tank without supervision.

8. Safety Valve

Blows compressed air off if the pressure in the tank should rise above the allowed pressure.

9. Refrigerated Air Dryer

Cools the compressed air to a few degrees above freezing point and condenses most of the air humidity. This avoids having water in the downstream system.

10. Line filter

Being in the main pipe, this filter must have a minimal pressure drop and the capability of oil mist removal. It helps to keep the line free from dust, water and oil.

The Air Consuming System

1.Air Take off

For consumers, air is taken off from the top of the main pipe to allow occasional condensate to stay in the main pipe, when it reaches a low point water off from beneath the pipe will flow into an Automatic drain and the condensate will be removed.

2. Auto Drain

Every descending tube should have a drain at its lowest point. The most     efficient method is an Auto Drain which prevents water from remaining in the tube.

3. Air Service Unit

Conditions the compressed air to provide clean air at optimum pressure and occasionally adds lubricant to extend the life of those pneumatic system components which need lubrication.

4.Directional Valve

Alternately pressurizes and exhaust the cylinder connections to control the direction of movement.

5.Actuator

Transforms the potential energy of the compressed air into mechanical work. Shown is a linear cylinder; it can also be a rotary actuator or an air tool etc.

   

6.Speed Controllers

Allow an easy and step less speed adjustment of the actuator movement.

PROPERTIES OF COMPRESSED AIR

Physical properties of air

Air contains Nitrogen 78%, Oxygen 21%, Others 01%(CO2, Argon, Hydrogen, Neon etc.)

Pressure

One Pascal(Pa) is equal to the constant pressure on a surface area of 1 m2 with the vertical force of one Newton(1 N) also Pressure (P) is defined as Force (F) per unit Area (A) and its unit in SI system is Pascal (Pa)

P = F/A in N/m²

Conversion units

1 Pascal    =    1N/m²            1 bar        =    1.013kg/cm²

100kPa    =    1 bar            1 bar        =    14.5 psi

1000kPa    =    1 mPa

Air pressure relationship

Since everything on earth is subjected to the absolute atmospheric pressure (pat), this pressure cannot be felt. The prevailing atmospheric pressure is therefore regarded as the base and any deviation is termed:

Gauge pressure    =    pg    or    Vacuum pressure    =    pv

The Atmospheric pressure does not have a constant value. It varies with the geographical locations and weather. The range from the absolute zero line to variable atmospheric pressure line is called vacuum range and above this, the pressure range.

The absolute pressure pab is composed of pressure pat and pressure pg. In practice, gauges are used which show only the excess pressure pg. Pressure pab is approximately one bar (100 kPa) greater than pg value.

Boyle-Mariotte’s law

At constant temperature, the volume of a given mass of gas is inversely proportional to the absolute pressure.

I.e., p1.V1 = p2.V2 = p3.V3 = Constant.

Air Humidity

Atmospheric air always contains a percentage of water vapour. The amount of moisture present will depend on the atmospheric humidity and temperature.

When atmospheric air cools it will reach a certain point at which it is saturated with moisture. This is known as the dew point. If the air cools further it can no longer retain all the moisture and the surplus is expelled as miniature droplets to form a condensate.

The actual quantity of water that can be retained depends entirely on temperature; 1m3 of compressed air is only capable of holding the same quantity of water vapour as 1m3 of atmospheric air. The table below shows the number of grams of water per cubic metre for a wide temperature range from - 40° C to + 40° C.

For the temperature range of pneumatic applications the table below gives the exact values. The upper half refers to temperature above zero, the lower to below zero. The upper rows show the content of a standard cubic metre, the lower ones the volume at the given temperature.

Temperature °C	0		5		10		15		20		25		30			35		40
g/ m3n (Standard)	4.98		6.99		9.86		13.76		18.99		25.94		35.12			47.19		63.03
g/m3 Atmospheric)	4.98		6.86		9.51		13.04		17.69		23.76		31.64			41.83		54.108
Temperature °C		0		-5		-10		-15		-20		-25		-30	-35		-40
g/ m3n* (Standard)		4.98		3.36		2.28		1.52		1		0.64		0.4	0.25		0.15
g/ m3 (Atmospheric)		4.98		3.42		2.37		1.61		1.08		0.7		0.45	0.29		0.18

Table: Water saturation of Air (Dew point)

    Relative humidity

With the exception of extreme weather conditions, such as a sudden temperature drop, atmospheric air is never saturated. The ratio of the actual water content and that of the dew point is called relative humidity, and is indicated as a percentage. 

Relative humidity (r.h.)       =  

Actual water content                   X   100% 

Saturated quantity (dew point)

Example 1:     Temperature 25°C, r.h. 65%. How much water is contained in 1 m3?

        Dew point 25°C = 24 g/ m3 X 0.65 = 15.6g/m3.

When air is compressed, its capacity for holding moisture in vapour form is only that of its reduced volume. Hence, unless the temperature rises substantially, water will condense out.

Example 2:     10m3 of atmosphere at 15° and 65 % r.h. is compresses to 6 bar gauge pressure. The temperature is allowed to rise to 25° C. How much water will condense out?

From table ; at 15°C, 10 m3 of air can hold a maximum of 13.04g/m3 X 10m3 = 130.4g

At 65% r.h. the air will contain 130.4 X 0.65 = 84.9g (a)

The reduced volume of compressed air at 6 bar pressure can be calculated:

p1v1 = p2v2   i.e. p1v1 = v2   

i.e.  1.013 bar X 10 m3  = 1.44 m3

        p2   6 + 1.013               

From table: 1.44 m3 of air at 25° C can hold a maximum of 23.76 g X 1.44 = 34.2 g (b)

The condensation equals the total amount of water in the air (a) minus the volume that the compressed air can absorb (b) , hence 84.9 – 43.2 = 50.6 g of water will condense out.

This condensate must be removed before the compressed air is distributed, to avoid harmful effects in the line and the pneumatic components.