Compressor

      The compressor is used to increase the pressure of the air to that desired In the system. Numerous types with differing Pressure ratings are available. Today the market has a wide range of Compressors which has its own advantages, disadvantages which has to be seen from different viewpoints like initial cost, operating efficiency, maintenance and quality of air determined. Reciprocating Compressors are the one which we widely use.
These are built in a wide range of models from a few HP to very large sizes. Recipes themselves differ in their construction like vertical, Balanced oppose, Water cooled/air cooled, single stage/multi stage and so on. The new entrant to the Air compressor in the screw compressor. These use two helically fluted screws which mesh and compress air while they run. Other types of compressors are available like vane compressor, Centrifugal compressor of which centrifugal slowing was introduced.

Oil Free Compressor

      Since the lubricating oil used to lubricate the compressors contributes to the bulk of the oil carry over in the compressed air, compressor manufacturers have come out with Oil Free Compressor. Oil free Reciprocating compressors do not use oil in the crankcase, they have teflon rings which are self lubricating. These are quite popular and available in India. The latest trend is the so-called Dry Screw. Normally screw compressors are flood lubricated i.e., the lubricating oil is injected into the compressor chamber and air oil mixture comes out of the compressor block. The oil is separated from air by means of oil separator filter and air is sent to the outlet of the compressor.
      The dry screw compressor does not get in touch with oil at all and the compressed air is oil free. To obtain the advantages of Lubricated compressors like cost and maintenance over non-lubricated compressors, people have started using the concept of removing oil from standard lubricating compressors using high efficiency oil removing filters in the airline. This oil removing filter uses high efficiency micro fiber paper media made out of Borosilicate, another suitable material which uses the principle of coalescing technique. These filters which are available cán filter oil content as low as 0.01 mg/m3 also. These filters should be protected by providing a pre- filter for removing the dirt and dust particles. Depending upon the application the quality of Compressed air is recommended.Generally for textile machinery a 3-3-2- will be sufficient.

Problems Caused by Water in Compressed Air:

      Any one responsible for maintaining and operating plant compressed-air systems will be aware of problems caused by water in the air system.
The problems are very apparent to those who operate pneumatic tools, rock drills, automatic air-powered machines, sandblasting equipment, pneumatic logic devices and controls, air gaging, and paint spray equipment.

The principal problems might be summarized as follows:

1. Washing away required lubricants.
2. Causing rust and scale to form within pipelines.
3. Increased wear and maintenance of pneumatic devices.
4. Sluggish and inconsistent operation of air valves and cylinders.
5. Malfunction and high maintenance of control Instruments and air logic devices.
6. Product spoilage by spotting in paint and other types of spraying.
7. Rusting of parts that have been sandblasted.
8. Freezing in exposed lines during cold weather.
9. Further condensation and possible freezing of moisture at the exhaust whenever air devices are rapidly exhausted.

      In the last case, some rock drills exhibit a 70°F (21°C) drop in temperature from Inlet to exhaust. Most portable pneumatic tools have a considerably smaller temperature drop, but the problem can exist.
The increased use of control systems and automatic machinery has made these problems more serious and has caused increased awareness of the need for better-quality compressed air. Air-drying equipment is now a standard part of any well-designed compressed-air system.

Aftercooler

      Driers are sized according to the flow, pressure, temperature, and dew point required of the compressed air.
If these conditions are changed, the resulting efficiency of the dryer can be drastically affected. One may refer to previous sections for Information on selecting the proper dryer: Normally, the temperature of compressed air leaving a compressor varies from 180 to 350°F (82° to 177°C) depending on the type of compressor, method of cooling, and the number of stages in the compressor.

      Consequently, It is essential that an aftercooler be Installed upstream of 50% to 75% of the water vapor contained in the air.The exact amount of water vapor removed depends on the Intake conditions of the compressor and the type of aftercooler. As the air temperature is reduced,water vapor condenses and is withdrawn as a liquid.
After coolers are available in either. air or water-cooled units and in horizontal or vertical models. Smaller units are available that mount on tank-mounted compressors and are cooled by air flowing from the compressor flywheel across the heat-transfer surfaces. Larger coolers are available that use no electricity or water and only use outside air blowing over finned tubes.

Separators

      A number of different types of separators are available to remove the condensed water from the air stream. The most common type uses the centrifugal principle, which drops out most of the liquid to a drain or sump area. The moisture is then periodically discharged by a manually or automatically opened drain valve.

Drain Valves And Moisture Traps

      There are many different types of manual and mechanical traps and drains. Although not the same, traps and drains do enable accumulated liquids to be held for draining from receivers, coolers, driers, drop legs, and the like. The manual units range from a simple petcock to a ball, gate, or globe valve. The simplest type of mechanical trap consists of a ball-float arrangement that activates when rising water reaches a preset level. No power is required to open this type. The least complicated power-activated drain is solenoid type that is energized by a timer signal). The solenoid plunger is then raised and the fluid is dumped, Electric- motor-driven units are available with solid-state controls to program the time Intervals between opening and also the duration of the open cycle. Finally, there are pneumatically activated drains that have the interval between openings and the duration of the opening timed by air logic controls.

Filters

      There is a wide variety of coalescing and particulate filters that should be used with the various types of compressed-air dryers. The air dryer manufacturer should be consulted for recommendations on the type of filters to be installed, with the dryer being purchased. Following the manufacturer's specifications will improve the quality of the compressed air. The coalescing filter is designed to remove oil and water aerosols by means of coalescence (l.e.. / Intercepting and concentrating aerosols into larger droplets that can be collected and removed from the filter). The particulate filter is designed to remove solids, such as dirt, scale, rust, or other contaminants, that might get into the system stem downstream from the compressor or the dryer: Particulate filters are efficiency rated by the size of the smallest particle that the element will retain Filters for the removal of odors and vapors are also available. High performance compressed air filters are used to remove the oil aerosols in the compressed air. The filter Is of three dimensionally layered binder free microfiber media. This filter separates the oil particles based on the principle of coalescing, it will stop every thing bigger than 0.01 micron. Mainly three types of filters are used. They are General purpose Pre-filter, High Performance micro oil removal filter and High Performance submicro oil removing filter. The General Purpose Pre-filter has an Efficiency of 85%, with particle removal at the rate of 5 micron. Maximum operating temperature of 800C and is used as a coarse filter.

      The High Performance Micro Oil removal filter has an Efficiency of 99.99%, with particle removal at the rate of 1 micron, Maximum operating temperature of 800C, Maximum oil removal rate at 0.5 PPM andIs used to remove oil and water. The High Performance submicro Oil removal filter has an Efficiency of 99.9999%, with particle removal at the rate of 0.01 micron, Maximum oil removal rate at 0.01 PPM and this filter removes complete oil.

Receivers

      Receivers serve many purposes in a compressed air-system, the major purpose being a reservoir for the compressed air. In a compressed-air drying system, there are advantages to Installing the receiver before or after the dryer equipment, depending on specific conditions. When Installed before the dryer, the receiver acts as an abuser chamber to smooth the compressor pulsations. It also acts as an additional separator, permitting liquid water, oil, and solid particulate matter not removed by the aftercooler separator to settle out from the air stream. In some Installations, only a portion of the compressed-air supply needs to be dried; therefore, separate discharge lines can be taken from the receiver for each requirement.

      When the air dryer is installed downstream of the receiver, care should be taken to assure that the air flow through the dryer does not exceed the dryer design capacity. If Intermittent high air demand or the full. Volume of the receiver Is discharged through the drier at greater than the driers capacity. liquid entrainment, elevated dew points, desiccant damage, carry-over of desiccant or absorptive material, and, in some cases, damage to the dryer equipment could occur.
When the receiver is located after the dryer, the rated compressor capacity regulates the maximum. flow through the dryer. With this arrangement, the receiver stores dry air, which is readily available for peak demand loads. Also, for the most economical operation of some heat-reactivated driers, the newly reactivated dryer tower may not have time to cool completely before going back on drying service. Therefore, the compressed air will be at an elevated temperature for a few minutes until the flow of compressed air cools the desiccant. During these short periods, elevated temperatures are dissipated by the large volume of cool dry air in the receiver. When a heat-of-compression type of dryer is used, the receiver must be installed downstream from the dryer. Regardless of its location, an adequately sized receiver will Improve the overall operation of the compressed air system.