Compressed Air UsageCompressed air is a major prime mover in many industries. People say air is free but compressed air is not free and is even costly. The compressed air provided for use in plants is designated ‘Instrument air’, ‘Plant air’ or ‘Process air’. Instrument air is cleaned and dried compressed air, used to prevent corrosion in some instruments. Plant air is compressed air but is usually neither cleaned nor dried, although most of the condensate and oil, are removed by a separator near the compressor, especially if adequate cooling can take place. Plant air is used for cleaning, power tools, blowing out vessels, etc: if used for air-powered tools exclusively, some suspended oil is advantageous for lubrication, although filter/lube units are usually installed in the air line to the tool.
Costing of Compressed Air (Example)(1) Single stage compression at 100 psig
- 25 BHP is consumed to produce 100 cfm.
- It consumes 150 units of power in every shift = 150 x 5.0 = 750 Rs / shift.
(2) Two stage up to 100 psig
- 19 BHP for 100 cfm
- = 114 units of power in a shift
- = 114 x 5 = 570 Rs /shift.
(3) For two stage compression upto 215 psig (15 kg/cm^2g)
- 28 BHP / 100 cfm of air
- 168 units / shift
The above does not include maintenance and spares, water cost for compressor headers, intercoolers and aftercoolers and operating costs for cooling water pumps.
Units of Air FlowAir flow specified for any compressor is based on the air flow at suction conditions and not in compressed conditions.
CompressorsCompressors are the heart of the system. This is the equipment used to compress the atmospheric air to a higher pressure which in turn is used as a source of power in pneumatic tools or to operate various machines, or used as process/instrument air.
Two Main Types of Air Compressors are
(1) Positive Displacement Type
(2) Dynamic Compressors
1) The Positive Displacement Type Compressor is one in which successive volume of air/gas are sourced into a closed space and elevated to higher pressure. Hence the discharge is in batches. e.g. Reciprocating, Screw, Sliding Vane, etc.
2) Dynamic Compressors are fast moving rotary compressors in which high speed rotating parts accelarate the gas as it passes through the element. The velocity head is converted to the pressure head by the diffuser/casing. e.g. Centrifugal compressors/Axial flow.
Following are the Three Main Types of the Compressors Used in Indian Industry:
(1) Reciprocating Compressor:
• Most common
• Positive displacement type
• Lubricated and Non Lubricated versions are available.
• No economical substitutes in lower ranges.
• Available in 50 cfm to 20000 cfm
• Discharge pressure high as 60000 psig (4250 kg/cm2g)
(2) Screw Compressors :
Basically twin helical lobe in which both lobes rotate simultaneously to compress the air/gas between
• Available upto 20000 cfm
• Not for vey high discharge pressure
• In case of lube type, carry over of oil is very high.
• Screw compressors are best suited for relatively high flow and low pressures
• Advantages are economy with regard to
(a) lesser space requirement
(b) less expensive foundation
(c) Low power cost
(d) low maintenance cost.
(3) Centrifugal Compressors :
• Specially meant for large air flow.
• Maintencance is very low.
• Heat generated is very less.
• Air flows upto 650000 cfm and pressure up to 550 psig
• These are sensitive to dust/dirt.
• Care to be taken to provide dirt free air at suction.
• These are oil free.
• Impellers are rotated at very high speed 20000 to 60000 rpm.
• Consumes less space compared to equivalent reciprocating compressors.
• Power consumption is also less
Contaminants in Compressed Air Lines(1) Water
• Corrodes internls surfaces of air piping, valves, fittings, pneumatic tools etc.
• May damage end products like food/pharama/paint etc.
• Air dryers must be installed at the outlet to remove the water.
• Act as tiny missiles which attack the internal parts of an system such as valves/pipes/pneumatic cylinders, etc.
• Choke small nozzles
• Contaminates end products
• Use Filters
• The Maximum particle size acceptable in instrument air line is 3 micron.
(3) Oil :
• Coagulation of dirt
• Causes pressure drops and flow restrictions
• Contaminate end products
• Use Filters
• Instrument air lines oil content max 1 ppm acceptable limitations excluding non condensables.
Accessories in Compressed Air System
Intercoolers and Aftercoolers• Cool the air between stages.
• Generally attached with dampner and Moisture separator.
• Perform as
(1) Dampening pulsations thus allowing smooth flow to the next stage avoiding uneven loads.
(2) Cooling the air thus ensuring the second stage receives cooled air. This results in low power consumption and cheaper grade material can be used.
(3) Cooling causes moisture to be separated. If this is allowed in next stage, it causes damage to piston ring/cylinders. Hence ADT is used.
• Available in air/water cooled versions.
• Water-cooled are preferred as efficiency is very high.
• Pressure drop of air side to be low.
Pulsation Vessels/Dampners• Pressure vessels designed to provide a buffer in the compressed air line so as to reduce the air pulsation by the reciprocating action of compressors.
• Generally installed after every stage of compressor and after final discharge.
Moisture Separator and Oil SeparatorsMoisture separator: -
(a) Impingement Baffle type
(b) Centrifugal type
(c) Demister Pad type
Moisture separator should be located after the:
(i) outlet of Intercooler and aftercooler
(ii) bypass line of dryers
(iii) after bends in pipelines
(iv) at regular intervals.
Installation ADT is preferred afterwards.
Air DryerThe air dryer shall be used to remove the moisture from the air. Air dryers are of following types:
- Purge Type
- No loss split flow (Electrical heater type)
- Heat of compression type
- Blower reactivated type