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Compressed Air Basics

Compressor Selection

GUIDE TO AIR COMPRESSOR SELECTION

Customer satisfaction with a blast cabinet goes hand in hand with the air compressor and support equipment being used. It is imperative that the air compressor produces enough volume of compressed air (CFM - cubic feet per minute) to operate the blast cabinet per the compressor manufacturers specifications. For the customer, the most critical factor when choosing a compressor should be the volume of air (CFM) that the air compressor generates. The volume of air will be a large factor in determining the productivity of the blast cabinet as it relates to the corresponding blast nozzle.

The following factors should be considered when sizing a compressor:

Duty Cycle: The duty cycle is the percentage of time in ten minutes increments that the air compressor pump should be allowed to run. For instance, if the air compressor has a duty cycle of 50/50, and the air compressor will be running for 10 minutes, then it should run for a combined maximum of 5 minutes ON and 5 minutes OFF. As the duty cycle increases the pump can run for longer periods of time without a cooling break. Typically, rotary screw air compressors have longer duty cycles than reciprocating air compressors. Most piston air compressors are available with 100% duty cycles. This high duty cycle is normally a result in a slower compressor speed allowing cooler compressed air production. For more information, refer to our air consumption chart.

Air Volume (CFM): Blast cabinet users commonly size air compressors based on the compressor's horsepower (HP) rating. The historical rule of thumb states that each compressor horsepower would produce four CFM. Therefore, a 20 horsepower compressor should theoretically produce 80 CFM of compressed air. However this no longer holds true, especially with air compressors that are 10 horsepower or less. Currently, it is not unusual for small 5 horsepower air compressors to produce less than two CFM for every horsepower. Therefore when shopping for an air compressor pay more attention to the CFM than the HP.

If a reciprocating piston air compressor (see definition below) is to be used, it is always better to oversize the machine than to undersize it. Determine your current requirements, take into consideration future requirements and airline loss then multiply the total CFM by 1.5. This will provide enough compressed air for a 50% duty cycle.

Air Pressure (PSI - pounds per square inch): The pressure is determined by the desired blast pressure in the blast cabinet. It is important that the air compressor maintain air pressures higher than required by the blast cabinet. If the blast operation requires 80 psi, then a single stage compressor (see air compressor definitions) that operates between 95-125 psi will work assuming the compressor produces enough air volume (CFM) to operate the blast cabinet.

Power Source: Oftentimes, the electrical power available to operate the air compressor is the limiting factor. The most common electrical power outlet is rated at 115V (120V) and 20 amps. This limits the size of the air compressor motor to about 2 HP unless a new 208V-230V single-phase panel is added to upgrade the circuit. To keep energy costs in line it is always recommended operating the air compressor on 230V-460V, single or three-phase power when it is available.

TYPES OF AIR COMPRESSORS

Reciprocating Air Compressors: Sizes at 100 PSI --1/2 HP & 1 CFM to 1,250 HP & 6,300 CFM

Reciprocating air compressors are positive displacement compressors. This means, they take in successive volumes of air, which are confined within a closed space, and elevate 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 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.

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. 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 and they may be packaged to provide a wide range of pressure and capacity selections.

Rotary Air Compressors: Sizes 30 CFM to 3000 CFM

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

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.

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 in air-cooled and water-cooled 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.

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: Sizes 400 CFM to 15,000 CFM

The centrifugal air compressor is a dynamic compressor that 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.

It is an oil-free compressor by design. The oil-lubricated running gear is separated from the air by shaft seals and atmospheric vents. It is continuous duty, 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 are typically packaged to include the after cooler and all controls.

Tank Size:

Never consider an oversized compressor storage tank size for undersized compressor CFM volume flow rating. Your tank capacity is only as large as the volume of air stored above your actual blasting pressure. Keep in mind how long it takes for the compressor to fill the entire tank and remember, the only useable amount for blasting is that amount above your actual blasting pressure. Blasting at a reduced pressure reduces the frictional heat created by the abrasive velocity against the part. When the blasting pressure drops, the frictional heat is reduced creating increased blasting time periods. Example...Your vehicle tire is low on air. The tire pressure measures 25 psi. You want to use the compressed air stored in your very large, big as a house, compressor tank. The tank pressure is 20 psi, how much of the air in the tank is useable? "NONE."

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 cooled and 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).