COMPRESSED AIR GENERATION, PREPARATION AND DISTRIBUTION

 COMPRESSED AIR GENERATION, PREPARATION AND DISTRIBUTION

AIR COMPRESSORS

A compressor converts the mechanical energy of an electric motor or any prime mover into the potential energy of compressed air.

Air compressor fall into two main categories: Reciprocating and Rotary.

The principal types of compressor within these categories are shown below:

Reciprocating Compressors:-

Single stage piston compressor

                            

Air taken in at atmospheric pressure is compressed to the required pressure in a single stroke. Downward movement of the piston increase volume to create a lower pressure than that of the atmosphere, causing air to enter the cylinder through the inlet valve.

At the end of the stroke, the piston moves upwards, the inlet valve closes as air is compressed, forcing the outlet valve to open discharging air into a receiver tank. This type of compressor is generally used in systems requiring air in the 3-7 bar range.

Two stage piston compressor.

                            

In a single stage compressor, when air is compressed above 6 bar, the excessive heat created greatly reduces the efficiency. Because of this, piston compressors used in industrial compressed air systems are usually two stages. Air is taken in at atmospheric pressure is compressed in two stages to final pressure. If the final pressure is 7 bar, the first stage normally compresses the air to approximately 3 bar, after which it is cooled. It is then fed into the second stage cylinder which compresses it to 7 bars. The compressed air enters the second stage at a greatly reduced temperature after passing through the inter cooler, thus improving efficiency compared to that of a single stage unit. The final delivery temperature may be in the region of 120°C.

Diaphragm Compressor

Diaphragm compressor provides compressed air in the 3-5 bar range totally free of oil and is therefore widely used by food, pharmaceutical and similar industries.

The diaphragm provides a change in chamber volume. This allows air intake in the down stroke and compression in the up stroke.

Smaller types, with an electric motor of ≤ 1kW power, make portable compressors ideal for spray painting.

Rotary Compressors (Rotary Sliding Vane Compressor )

                                  

This has an eccentrically mounted rotor having a series of vanes sliding in radial slots. As the rotor rotates, centrifugal force holds the vanes in contact with the stator wall and the space between the adjacent blades decreases from the air inlet to outlet, so compressing the air. Lubrication and sealing is achieved by injecting oil into the air stream near the inlet. The oil also acts as a coolant to limit the delivery temperature.

Screw Compressor

  

Two meshing helical rotors rotate in opposite directions. The free space between them decreases axially in volume and this compresses the air trapped between the rotors. Oil flooding provides lubrication and sealing between the two rotating screws. Oil separates remove this oil from the outlet air. Continuous high flow rates in excess of 400 m3/min are obtainable from these compressors at pressures up to 10 bar. More so than the vane compressor, this type of compressor offers a continuous pulse free delivery.

The most common industrial type air compressor is still the reciprocating machine, although screw and vane types are finding increasing favour.

Compressor Rating

A compressor capacity or output is stated as Std. Volume Flow(Q), given in m3n/s or /min, or liters/min. The capacity may also be described as displaced volume, or Theoretical Intake Volume. For a piston compressor it is based on:

Q (l /min) = piston area in dm2 X stroke length in dm X number of first stage cylinder X rpm.

In the case of a two stage compressor, only the first stage cylinder should be considered.

The effective delivery is always less due to volumetric and thermal losses. Thermal loss occurs due to the fact that during when cooling down to ambient temperature.

Volumetric Efficiency

The ratio between free air delivered and displacement expressed as a percentage, is                     

Known as the volumetric efficiency, and will vary with the size, type and make of the machine, number of stages and the final pressure. The volumetric efficiency of a two stage compressor is less than that of a single stage type as both the first and second stage cylinder have dead volumes.

AIR RECEIVER   

An air receiver is a pressure vessel of welded steel plate construction, installed horizontally or vertically directly downstream from the after cooler to receiver the compresses air, thereby damping the initial pulsations in the air flow.

 

Its main functions are to store sufficient air to meet temporary heavy demands in excess of compressor capacity, and minimize frequent loading and unloading of the compressor but it also provides additional cooling to precipitate oil and moisture carried over from the after cooler, before the air is distributed further. To this end it is an advantage to place the air receiver in a cool location.

The vessel should be fitted with a safety valve, pressure gauge, drain, and inspection covers for checking or cleaning inside.

Sizing a receiver

Air receivers are sized according to the compressor output, size of the system and whether the demand is relatively constant or variable.

AIR DEHYDRATION

After coolers

After final compression, the air will be hot and, when cooling, will deposit water in considerable quantities in the airline system which should be avoided. The most effective way to remove the major part of this condensate, it to subject the air to after-cooling, immediately after compression.  After coolers are heat exchangers, being either air cooled or water cooled units.

Air Cooled

Consisting of nest of tube through which the compressed sir flows and over which a forced draught of cold air is passes by means of a fan assembly.

The outlet temperature of the cooled compressed air should be approximately 15°C above the ambient cooling air temperature.

Water cooled

Essentially, a steel shell housing tubes with water circulating on one side and air on the other, usually arranged so that the flow is in opposite direction through the cooler. A water cooled after cooler should ensure that the air discharged will be 10°C above the temperature of the cooling water. An automatic drain attached to or integral with, the after cooler, removes the accumulated condensation. After coolers should be equipped with a safety gauge, and it is recommended that thermometer pockets for air and water are included.

Air dryers

After coolers cool the air to within 10 - 15°C of the cooling medium. The control and operating elements of the pneumatic system will normally be at ambient temperature (approx 20°C). This may suggest that no further condensate will be precipitated, and that the remaining moisture passes out with the exhaust air released to atmosphere. However, the temperature of the air leaving the after coolers may be higher than the surrounding temperature through which the pipeline passes, for example during night time. This situation cools the compressed air further, thus condensing more of the vapour into water

The measure employed in the drying of air is lowering the dew point, which is the temperature at which the air is fully saturated (that is 100% humidity). The lower the dew point, the less moisture remains in the compressed air.

There are three main types of air dryer available which operate on an absorption, adsorption or refrigeration process.

Absorption (deliquescent) Dryer

The compressed air is forced through a drying agent such as dehydrated chalk or magnesium chloride which remains in a solid form, lithium chloride or calcium chloride which reacts with the moisture to form a solution which is drained from the bottom of the vessel. The drying agent must be replenished at regular intervals as the dew point increases as a function of consumption of the salt during operation, but a pressure dew pint of 5°C at 7 bar is possible.

The main advantage of this method are that is of low initial  and operating cost, but the inlet temperature must not exceed 30°C, the chemicals involved are highly corrosive necessitating carefully monitored filtering to ensure that a fine corrosive mist is not varied to the pneumatic system.

Adsorption (desiccant) Drying

A chemical such as silica gel or activated alumina in granular form is contained in a vertical chamber to physically absorb moisture from the compressed air passing through it. When the drying agent becomes saturated it is regenerated by drying, heating, heatless by a flow of previously dried air.

Wet compressed air is supplied through a directional control valve and passes through desiccant cloumn1. The dried air flows to the outlet port. Between 10 – 20 % of the dry air passes through orifice O2 and column 2 in reverse direction to reabsorb moisture from the desiccant to regenerate it. The regenerating air flow then goes to exhaust. The directional control valve is switched periodically by timer to alternately allow the supply air to one column and regenerating the other, to provide with continuous dry air. Extremely low dew points are possible with method, for example   –40°C.

A colour indicator may be incorporated in the desiccant to monitor the degree of saturation. Micro filtering is essential on the dryer outlet to prevent carry – over of adsorbent mist. Initial and operating costs are comparatively high, but maintenance cost tends to be low.

Refrigerant drying

This is a mechanical unit incorporating a refrigeration circuit, and two exchangers.

Humid high temperature air is pre-cooled in the first exchanger 1 by transferring part of its heat to the cooled output air.

It is then cooled by the refrigerator principle of heat extraction as a result of evaporating Freon gas in the refrigerator circuit, in heat exchanger 2. At this time, moisture and oil mist condenses and are automatically drained. The cold dry air return pipe passes through air heat exchanger 1   and gains heat from the incoming high temperature air. This prevents dew forming on the discharge outlet, increases volume and lowers relative humidity.

An output temperature of 2°C is possible by modern methods, although an output air temperature of 5°C is sufficient for most common applications of compressed air. Inlet temperature may be up to 60°C but it is more economical to pre cool to run at lower inlet temperatures.

As a general rule, the cost of drying compressed air may be 10-20% of the cost of compressing air.

Main line filter

A large capacity filter should be installed after the air receiver to remove contamination, oil vapours from the compressor and water from the air.

This filter must have a minimum pressure drop and the capability to remove oil vapour from the compressor in order to avoid emulsification with condensation in the line. It has no deflector, which requires a certain minimum pressure drop to function properly as the Standard Filter discussed .A built in or an attached auto drain will ensure a regular discharge of accumulated water.

The filter is generally a quick change cartridge type.

AIR DISTRIBUTION

The air main is a permanently installed distribution system carrying the air to the various consumers. There are two main layout configurations are Dead end line and Ring Main.

Dead End Line

To assist drainage, the pipe work should have a slope of about 1 in 100 in the direction of flow and it should be adequately drained. At suitable intervals the main can be brought back to its original height by using two long sweep right angle bends and arranging a drain leg at the low point.

Ring main

In a ring main system air can be fed from two sides to a point of high consumption. This will reduce pressure drop. However, this derives condensate in any direction and sufficient water take-offs with Auto Drains should be provided. Isolating valves can be installed to divide the air main into sections. This limits the area that will be shut down during periods of maintenance or repair.

Secondary lines

Unless an efficient after cooler and air dryer are installed the compressed air distribution pipe work acts as a cooling surface and water and oil will accumulate through out its length.

    Branch lines are taken off the top of the main to prevent water in the main pipe from running in to them, instead of into drainage tubes which are taken from the bottom of the main pipe at each low point of it. These should be frequently drained or fitted with an automatic drain.

Auto drains are more expensive to install initially, but this is off-set by the man—hours saved in the operation of the manual type. With manual draining, neglect leads to compound problems due to contamination of the main.

AIR SERVICE EQUIPMENT

Atmospheric air carries both dust and moisture. After compression, moisture condenses out in the after cooler and receiver, but there will always be some which will be carried over. To remove these contaminants, the air should be further cleaned as near as possible to the point of use.

Air Filter

The standard filter is combined water separator and filter. The water separation occurs mainly by a rapid rotation of the air, caused by the deflector at the inlet. The heavier particles of dirt, water and oil are thrown outward to impact on the wall of the filter bowl before running down to collect at the bottom. The liquid then be drained off through a manual drain cock or an automatic drain.

                             

Air quality  

The figure illustrates different levels of air quality for various applications.

  

Branches 1- 2 provide air direct from the air receiver. Branches 3-6 use air conditioned by a refrigerated type of drier. Branch 7 incorporates an additional dryer of the adsorption type.                   

Pressure Regulator

   

Regulation of pressure is necessary because at pressures above optimum, rapid wear will take place with little or no increase in out put. Air pressure which is too low is uneconomical because it results in poor efficiency.

Pressure regulators have a piston or diaphragm to balance the output pressure against an adjustable spring force.

The secondary pressure is set by the adjusting screw loading the setting spring to hold the main valve open, allowing flow from the primary pressure P1 inlet port to the secondary pressure P2 out let port. Then the pressure in the circuit connected to the outlet rises and acts on the diaphragm, creating a lifting force against the spring load.

When consumption starts P2 will initially drop and the spring, momentarily stronger than the lifting force from P2 on the diaphragm, opens the valve. If the consumption rate drops, P2 will slightly increase, this increase the force on the diaphragm against the spring force, diaphragm and valve will then lift until the spring force is equaled again. Te air flow through the valve will be reduced until it matches the consumption rate and the output pressure is maintained.

Air Lubricator

Lubricator is no longer necessity for the majority of modern pneumatic components. They are available pre-lubricated for life.

Certain equipments still requires lubrication. To ensure they are continually lubricated, a certain quantity of oil is added to the compressed air by means of a lubricator.

   

In a lubricator a pressure drop between inlet and outlet, directly proportional to the flow rate, is created and lifts the oil from the bowl into the sight feed dome as shown in the figure.

With a fixed size of restriction, a greatly increased flow rate would create an excessive pressure drop and produce an oil/air mixture. To over come the excessive and decreased flow, lubricators must have self adjusting cross sections to produce a constant mixture.