Air Compressor Sizing:
Questions about air compressors are very important ... Large tanks are only as large as the storage capacity above the desired blasting pressure. With most blasting pressures occurring at 80-100 psi the tank volume above this pressure is very small. Never rely on tank capacity to make up for less than required cfm capacity.
Important Note about
220 Volt Single Phase Electric Air Compressors:
Never consider motor horsepower when purchasing smaller air compressors. Volume of air delivered is all about amperage used and 220-volt single-phase electrical is capable of supplying a maximum of 23-24 cfm. Amounts above this require lots of copper windings and lots of amperage.
1. Always look for the compressed air volume figure at or above the blasting pressure or 100 psi. Nobody uses an air compressor unless the compressor tank has compressed air inside and the greater the pressure inside the tank the harder the pumps works. If the pump is poor quality the volume will be greatly reduced as the tank pressure increases. Never accept pump displacement for cfm being supplied and never look solely at the motor horsepower!
2. Make sure to look at the duty cycle for any compressor. Often the pump is operating faster than it would normally be operating in order to show a larger volume of compressed air delivery. Often duty cycle appears as a % of 100 ... Duty cycle takes into account the amount of time the compressor needs to be off to cool. Word to the buyer, compressor duty cycle is measured in 10 minute time periods. If the compressor is 60% duty cycle this means the compressor must be off for 4 minutes of every 10...this is fairly loose but used when warranty occurs.
The best duty cycle is 100% and this compressor pump is usually larger and slower which means piston heat is not a problem. Don't let lower duty cycles prevent you from purchasing the unit if the other features are positive.
3. Cast iron heads are best...just as you think about cast iron and engine blocks the same applies to compressors.
In review the more air volume the better...the best duty cycle is 100%...cast iron is better than aluminum...and the final determining factor is electrical power available. If you only have 220 single phase, the best possible air volume you can get is 23-24 cfm at 175 psi. Keep in mind that screw compressors and dual piston heads on a single tank are exceptions ... everything else is a compromise with price being the sole contributing factor in making your final decisions on choice ...
GUIDE TO AIR COMPRESSOR SELECTION
Satisfaction with the blast cabinet goes hand in hand with the air compressor and support equipment. It is necessary for the air compressor to produce enough volume of compressed air (CFM – cubic feet per minute) to operate the blast cabinet. The most critical factor in choosing a compressor if you're an owner of an abrasive blast cabinet 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 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 break. Typically, rotary screw air compressors have longer duty cycles than reciprocating air compressors. Piston air compressors are available with 100% duty cycles.
- Air Volume (CFM): Users commonly size air compressors based on the compressor's horsepower (HP) rating. The historical rule of thumb in the abrasive blast industry related to air compressors 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 air compressor (see definition below) will be used, it is always better to oversize the machine than to undersize it. Determine the current requirements, take into consideration future requirements and airline loss, and 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): This pressure is determined by the desired blast pressure of the blast cabinet. It is important that the air compressor maintains 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 that 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 panel is added to upgrade a circuit to 208V-230V, single phase. To keep energy costs in line, it is always recommended to operate the air compressor on 230V-460V, 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 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 - 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. 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 packaged.
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.
Never consider an oversized compressor tank size for an undersized 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. 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" nozzle would 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):