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**pickleboy** · 07-12-2005, 05:58 PM

here is a little info for you to digest, maybe it will help. this was pertaining to blasting cabinets but it still has the info you need to size. the easiest is to add up the cfm of all the tools that will / can be run at the same time and x by 1.5. this will give you the min cfm needed. then consider tank size as it is relative to recovery time, and also the main point is duty cycle, if this is 50% or less stay away. the closer to 100% the better, meaning it can run continously if needed.

GUIDE TO AIR COMPRESSOR SELECTION

The most critical factor in 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.
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): The pressure is determined by the desired blast pressure in 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.

Tank Size
Never consider an oversized compressor storage tank size for an undersize compressor cfm volume flow rating. Your tank capacity is only as large as the volume of air stored above your actual blasting pressure. Please remember 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 frictional heat created by the abrasive velocity against the part. When the blasting pressure drops the frictional heat lessons creating greatly increased blasting time periods. Dope Slap Example...Your auto 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"

**mpierich** · 07-13-2005, 10:29 AM

Thanks pickle, I gotta digest all that but it's a good start. Was thinking it might be cheaper to go all electric but the air motors last so much longer etc. I think I'll stick with it and try to find an old compressor.

Thanks again.

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