TYPES OF AIR COMPRESSORS

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Reciprocating Air Compressors

Reciprocating air compressors are positive displacement compressors. This means they are taking in successive volumes of air, which is confined within a closed space, and elevating this air to a higher pressure. The reciprocating air compressor accomplishes this by using a piston within a cylinder as the compressing and displacing element.

The reciprocating air compressor is considered single acting when the air compression is accomplished using only one side of the piston. A compressor using both sides of the piston is considered double acting.

The reciprocating air compressor uses a number of automatic spring loaded valves in each cylinder that open only when the proper differential pressure exists across the valve.

Inlet valves open when the pressure in the cylinder is slightly below the intake pressure. Discharge valves open when the pressure in the cylinder is slightly above the discharge pressure.

A compressor is considered to be single stage when the entire compression is accomplished with a single cylinder or a group of cylinders in parallel. Many applications involve conditions beyond the practical capability of a single compression stage. Too great a compression ratio (absolute discharge pressure/absolute intake pressure) may cause excessive discharge temperature or other design problems.

For practical purposes most plant air reciprocating air compressors over 100 horsepower are built as multi-stage units in which two or more steps of compression are grouped in series. The air is normally cooled between the stages to reduce the temperature and volume entering the following stage.

Reciprocating air compressors are available either as air-cooled or water-cooled in lubricated and non-lubricated configurations, may be packaged, and provide a wide range of pressure and capacity selections.

Rotary Air Compressors

Rotary air compressors are positive displacement compressors. The most common rotary air compressor is the single stage helical or spiral lobe oil flooded screw air compressor. These compressors consist of two rotors within a casing where the rotors compress the air internally. There are no valves. These units are basically oil cooled (with air cooled or water cooled oil coolers) where the oil seals the internal clearances.

Since the cooling takes place right inside the compressor, the working parts never experience extreme operating temperatures. The rotary compressor, therefore, is a continuous duty, air-cooled or water cooled compressor package.

Because of the simple design and few wearing parts, rotary screw air compressors are easy to maintain, operate and provide great installation flexibility. Rotary air compressors can be installed on any surface that will support the static weight.

The two-stage oil flooded rotary screw air compressor uses pairs of rotors in a combined air-end assembly. Compression is shared between the first and second stages flowing in series. This increases the overall compression efficiency up to fifteen percent of the total full load kilowatt consumption. The two-stage rotary air compressor combines the simplicity and flexibility of a rotary screw compressor with the energy efficiency of a two stage double acting reciprocating air compressor. Two stage rotary screw air compressors are available air-cooled and water-cooled and fully packages.

The oil free rotary screw air compressor utilizes specially designed air ends to compress air without oil in the compression chamber yielding true oil free air. Oil free rotary screw air compressors are available air-cooled and water-cooled and provide the same flexibility as oil flooded rotaries when oil free air is required.

As you can see, rotary screw air compressors are available air cooled and water cooled, oil flooded and oil free, single stage and two stage. There is a wide range of availability in configuration and in pressure and capacity.

Centrifugal Air Compressors

The centrifugal air compressor is a dynamic compressor, which depends on transfer of energy from a rotating impeller to the air. The rotor accomplishes this by changing the momentum and pressure of the air. This momentum is converted to useful pressure by slowing the air down in a stationary diffuser.

The centrifugal air compressor is an oil free compressor by design. The oil-lubricated running gear is separated from the air by shaft seals and atmospheric vents.

The centrifugal is a continuous duty compressor, with few moving parts, that is particularly suited to high volume applications--especially where oil free air is required.

Centrifugal air compressors are water-cooled and may be packaged; typically the package includes the after cooler and all controls.

Tank Size

Never consider an oversized compressor tank size for an undersize compressor cfm rating. Consider the following example, 5 hp compressor equipped with a 60-gallon receiver tank. The compressor performance rating is 12 cfm @ 100 psi or about a C+ on a Bell Curve. It is assumed that the large compressor tank will make up for any performance shortage of the compressor during operation with a 1/4" siphon nozzle. The 1/4" siphon nozzle will consume 21 cfm @ 80 psi. The simple fact is the receiver tank has an inside area of 8 cubic feet. The tank will hold 30 cfm @ 50 psi.and 62 cfm @ 100 psi. Consider the amount of time the blast machine could operate above 80 psi if the compressor wasn't turned on and the tank started with 100 psi inside pressure. Rough math tells you that every 10-psi of tank pressure equals about 6 cfm of compressed air with the 60 gallon tank. If the gun requires 21 cubic feet of compressed air per minute to maintain 80 psi the tank pressure would drop to roughly 70 psi after one minute of operation. Two minutes 40 psi, etc. If the compressor pump were operational supplying 12 cfm of the required 21cfm, then a tank size of 60 gallons (6.2 cfm/10psi tank pressure) would permit blasting above 80 psi for roughly two minutes! A 120-gallon tank would permit 4 minutes and a 120-gallon tank is 24 inches by 68 inches long.

COMPRESSED AIR SYSTEMS

Compressed air contains concentrated oil, water vapor, dirt and other contaminants that can damage airlines and pneumatic components on the blast machine. Prior to using the compressed air to operate the blast cabinet, it must be dried, filtered and regulated to the desired operating pressure. Properly prepared compressed air prolongs equipment life, increases efficiency and reduces blast equipment maintenance costs. This is a typically recommended set-up for a blast operation. Some components may not be required (always consult with compressed air experts for your specific application):

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