How To Choose An Air Compressor for Your Blast Cabinet
When you are purchasing a blast cabinet, it is important to make sure you have it paired up with the correct air compressor. If your air compressor is too small for the cabinet, it won’t propel the blast media with enough force to strip, clean, or finish a surface.
The statistic that you should be looking at when choosing an air compressor for your blast cabinet is the CFM, or cubic feet per minute. This is the volume of air that the air compressor generates. By matching up the CFM of the air compressor to the corresponding blast cabinet size, you will ensure you have peak productivity.
Factors to consider when choosing an air compressor
Air compressors are often rated by horsepower (HP). The twist is, there are different variations of HP, so rating compressors from one manufacturer to another is not apples to apples when it comes to HP.
Some air compressor manufacturers rate HP by peak horsepower, aka brake horsepower. Peak horsepower is the maximum output a motor can produce while the motor has start windings engaged. Peak HP can be as much as 5-7 times the rated or running HP.
What makes this interesting is that under normal operating conditions, the start windings are only engaged for a small fraction of a second. So, using peak horsepower as a HP comparison tool can be a bit misleading, seeing as the motor normally only hits this horsepower mark during startup. Using the start winding HP as a benchmark is unreliable, because if a motor drive system causes the start winding to remain engaged for too long, the motor will either overheat or prematurely fail.
Today, most manufacturers rate their electric motors by the horsepower developed after the motor has come up to the RPMs it was designed to operate continuously at, and the start windings have been disengaged. This horsepower rating is known as running horsepower or rated horsepower. Running HP is a much more accurate indicator of the horsepower a motor can maintain in continuous operation. Now, mo
st electric motor companies are eliminating any reference to peak HP and using SCFM as a more accurate indicator of performance.
Most air compressors for small garages and shops, or DIY use range between 1.5 and 6.5 HP. For industrial use applications, you may need an air compressor with larger HP capacity. As we’ll look at in the next section, horsepower ratings have changed in recent years, and older models and newer models may have different outputs with the same horsepower.
A more reliable factor to look at today is air flow rate, or CFM (cubic feet per minute).
Air Volume (CFM)
Most abrasive blast cabinet operators choose an air compressor based on the horsepower (HP) of the air compressor. In the past, the thinking was that each unit of horsepower would produce 4 CFM, (for example: a 15 HP compressor would produce 60 CFM).
This is no longer true for modern air compressors, most notably for air compressors with 10 horsepower or less. In fact, many air compressors with 5 HP or less actually produce less than 2 CFM per horsepower.
When shopping for an air compressor today, it is more important to consider the CFM than the HP.
If you are purchasing a reciprocating piston air compressor, it’s better to buy extra CFM rather than buy too little CFM. Assess your current air requirements, extrapolate your future needs, adjust for airline loss, then multiply that CFM number by 1 ½.
It is important to note there are a few different ways that CFM may be measured.
A Few Words about DCFM, SCFM and ACFM
Displaced CFM (DCFM) is a formula that calculates the RPM, stroke, and bore and transforms that into a into a CFM figure. The DCFM formula ia (Bore x stroke x RPM/2200=DCFM). Of the different CFM numbers, this one will always be greatest. This is because DCFM doesn’t take variables like humidity, local air pressure and temperature, heat dissipation, or friction into account.
Standard CFM (SCFM) is perhaps the most accurate means of measuring CFM. SCFM is defined as air at 14.696 psia and 520°R (60 °F). Sometimes other conditions are used, such as 530°R (70°F), 528°R (68°F), 0% and 36% Relative Humidity for describing the standard conditions. It is important to remember SCFM is defined by a fixed set of conditions or common reference point for comparing different compressors systems.
SCFM is an important standard of measurement because air can have a different density depending on factors such as elevation, temperature of the location.
Actual CFM (ACFM) is the volume of gas flowing anywhere in a system, independent of its temperature and pressure. If the compression system were always moving air at the exact same conditions, ACFM would always equal SCFM. But, this usually is not the case. The most important change between these two definitions is the pressure. To move air, a positive pressure or a vacuum must be created. When positive pressure is applied to a standard cubic foot of gas, it is compressed. When a vacuum is applied to a standard cubic foot of gas, it expands. The volume of gas after it is pressurized or rarefied is referred to as its “actual” volume.
ACFM can be determined a few different ways. The most common methods include measuring the volume of air that is moved through an orphic plate or measuring pump up times on large compressor tanks and running through a simple calculation. This CFM number takes in effect all the variables and will give the true output of the pump at the current working conditions (such as temperature, altitude, and humidity).
Often, CFM numbers are shown at various pressures. These numbers can be very useful to help determine if a compressor produces enough CFM for the desired job, but these can be confusing if you are comparing pressures or volumes from different compressors.
The most uniform way to compare the CFM of air compressors is through SCFM. Since all the measurements are calculated to a set of standard conditions, it standardizes comparison between the myriad of different compressor manufacturers.
There are two types of duty cycle, intermittent and continuous. The intermittent duty cycle is expressed as a percentage measuring how long an air compressor can operate uninterrupted. This is measured as a percentage of ten-minute increments. If the air compressor has a duty cycle of 50/50, and the compressor is running for a total of ten minutes, that adds up to five minutes on, and five minutes off. A compressor with a duty cycle of 60/40 can run for a total of six minutes on and four minutes off for each ten minutes it runs. The off portion of the duty cycle is for a cooling break.
It is important to note that different types of air compressors will have different duty cycles. Rotary screw air compressors will usually have longer duty cycles than reciprocating air compressors. Some types of air compressors are available with 100% duty cycles, meaning they have continuous air flow. Piston air compressors are generally a good choice for industrial blasting applications. They can run with a 100% duty rate because there is a slower compressor speed which allows for cooler compressed air production.
|Siphon Blast Cabinet CFM Consumption|
|Nozzle Size||Air Nozzle Orifice||30 PSI||40 PSI||50 PSI||60 PSI||70 PSI||80 PSI||90 PSI||100 PSI||120 PSI|
|1/4″ (#4), 5/16” (#5)||1/8”||9.8||12.03||14.26||16.39||18.62||20.76||22.99||25||30|
|5/16” (#5), 3/8” (#6)||5/32”||15.31||18.8||22.28||25.61||29.09||32.43||35.92||40||49|
|Direct Pressure Blasting CFM Consumption|
|Nozzle Size||Nozzle Size (Decimal)||20 PSI||30 PSI||40 PSI||50 PSI||60 PSI||70 PSI||80 PSI||90 PSI||100 PSI|
PSI (Air pressure in pounds per square inch)
The PSI requirements you will need for a given blast cabinet will also be affected by the size of the blast nozzle, and your desired blast pressure. The higher the PSI, and the larger the blast nozzle orifice, the higher your CFM consumption will be.
You must always make sure that your air compressor maintains the minimum air pressure required by the blast cabinet, or your abrasive blasting will be ineffective.
If the blast operation requires 80 PSI, then a single stage compressor that operates between 95-125 PSI will work, presuming the air compressor generates enough CFM to operate the blast cabinet.
Another number to consider is is the Service Factor (S.F.) rating of the motor. Service Factor is defined as the percentage of rated horsepower at which the motor can safely operate (for example 1.10 SF = 110% of rated HP). Generally, the higher the service factor a motor has, the more varied conditions a motor can handle without overheating or failing prematurely. Conditions that might cause a motor to run within its Service Factor can include low voltage, higher ambient temperatures, higher startup load, etc.
How often will you use the air compressor?
If you plan on using the air compressor several times a week, you can choose a compressor with a 50/50 duty cycle.
For commercial or industrial use, where you are using the air compressor daily, select a compressor with a higher duty cycle. Single stage, two stage, or rotary screw compressors are usually recommended for industrial use.
The larger your blast cabinet, the more air consumption it will require, and the larger air compressor you should select. This might be the most important factor in selecting an air compressor.
Portable air compressors typically operate on 115-volt, 15-amp circuits. The most common electrical power outlet is rated at 115V (120V) and 20 amps. This limits the size of most air compressors to about 2HP, unless you have upgraded to a 230V single-phase panel. Stationary, single-stage compressors and a few large portable compressors operate on 230-volt, single-phase power. Two-stage air compressors that are 10 hp and above generally operate on three-phase power only. Most 5 hp and 7.5 hp two-stage compressors can operate on single-phase and three-phase power. If no electrical power is available, you will need a gas air compressor.
An oversized compressor storage tank will be wasted on an undersized compressor (by CFM volume flow rating). Your tank capacity is only as large as the volume of air stored above your actual blasting pressure.
Consider the amount of time it takes for the compressor to fill the entire tank. Remember, the useable amount of stored air for blasting is how much air you have stored above your actual blasting pressure. In other words, the air you can use from a compressor tank is the minimum PSI + the actual air pressure you need to blast.
Blasting at a reduced pressure can reduce frictional heat created by the abrasive velocity against the blasting surface. When the you lower the blasting pressure, frictional heat is reduced, which in turn, increases the amount of time it takes to blast.
Types of Air Compressors
Now that we’ve examined factors you should consider when buying a compressor, let’s look at the different types of air compressors.
Reciprocating Air Compressors aka Piston Compressors
Reciprocating air compressors work by using positive displacement. What this means is the compressor takes in air, confines it within a small closed space, and pressurizes the air. These types of compressors use a piston within a cylinder to compress and displace the air.
A reciprocating air compressor uses automatic spring-loaded valves in each cylinder, that open only when the correct differential pressure exists across the valve. The inlet valves open when the cylinder’s air pressure is slightly below the intake pressure. Discharge valves open when the cylinder’s air pressure is slightly above the discharge pressure.
If the compression ratio (absolute discharge pressure or absolute intake pressure) is too great, this can cause excessive discharge temperature.
Single Stage Piston Compressors
When the piston air compressor uses only side of the piston, it is referred to as a single stage compressor. Single stage air compressors work by drawing in air and then compressing that air to its final pressure in a single piston stroke. Single stage air compressors can attain pressures up to 150 PSI. Generally, a single stage compressor will have a higher CFM rating than a two stage air compressor. This is because every cylinder is drawing in air and compressing air with each rotation.
Two Stage Piston Compressors
A two stage compressor works in a very similar fashion. The main difference between the two is that a two stage compressor pressurizes the air in two steps instead of one. In the first step, air is drawn in and compressed to an intermediate pressure. Then the pressurized air is piped through an intercooler (allowing the air to cool), to be compressed again in the second stage. Two stage compressors can reach pressures up to 200 PSI. Two stage compressors are more efficient at reaching higher pressures because the air is cooled between stages.
As a side note, most reciprocating air compressors over 100 HP are designed to be multi-stage compressors, with two or more steps of compression being grouped in series. Between each stage, the air is cooled, both to reduce both the air temperature and total volume of air (allowing more pressure).
Reciprocating air compressors come in a variety of configurations: as air-cooled or water-cooled, oil-lubricated or non-lubricated, in order to provide the widest range selections for air pressure and capacity.
Rotary Air Compressors
Rotary air compressors are positive displacement compressors. The most common rotary air compressor you will find is the single stage helical, or oil-flooded rotary screw air compressor. This type of compressor has two rotors within a casing, and no valves. The two rotors compress the air internally. Because of the simple design and minimal wear to parts, rotary screw air compressors are easy to maintain and operate, while providing flexible installation. Rotary air compressors can be installed on any surface that will support their freestanding weight.
In most cases, rotary air compressors lower the air temperature using air cooled or water-cooled oil coolers (The oil seals the internal clearances.) One benefit of rotary air compressors is that the cooling takes place inside the compressor, meaning the working parts never experience extreme operating temperatures. Therefore, the rotary compressor is a continuous duty, air-cooled or water-cooled compressor package.
A two-stage oil flooded rotary screw air compressor uses pairs of rotors in a combined air end assembly. The air 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 reciprocating air compressor. Two stage rotary screw air compressors are usually 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, meaning you get true oil-free compressed air. Oil-free rotary screw air compressors are available air-cooled and water-cooled and provide the same flexibility as oil-flooded rotary compressors, but where 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
Centrifugal air compressors are dynamic compressors that use the transfer of energy from a rotating impeller to the air. The rotor accomplishes this by changing the momentum and pressure of the air. The resulting momentum is converted to useful pressure by slowing the air down in a stationary diffuser.
Centrifugal units are designed to be oil-free. The oil-lubricated running gear is separated from the air by shaft seals and atmospheric vents. Centrifugal compressors are continuous duty, with few moving parts, and are well-suited for higher volume applications, especially where oil-free air is required. Most centrifugal air compressors are water-cooled, usually packaged with the after-cooler and any necessary controls.
Rotary screw compressors, rotary vane compressors, and centrifugal air compressors are most commonly found in industrial or commercial use. These types of compressors are usually operated at significantly higher horsepower and flow rates than compressors found in a small shop. This makes these types of compressors more expensive buy and to operate than the types of compressors you would find in the typical hobbyist garage.
Compressed Air Systems
Compressed air in a commercial setting can contain concentrated oil, water vapor, dirt and other contaminants, which can accelerate damage to air lines and blast machine components. Before using compressed air to operate your blast cabinet in a compressed air system, it must be cooled and dried, filtered and regulated to the desired operating pressure. Properly prepared compressed air helps prolong the life of your blast equipment, while increasing efficiency and reducing maintenance costs of your abrasive blasting equipment. Below is a recommended set up for an industrial blasting operation. Some components may not be required (be sure to consult with compressed air experts for your specific needs).
Compressed Air Supply > Aftercooler > Compressed Air Tank > Particulate Filter > Air Dryer > Inline Air Dryer > Abrasive Blast Cabinet
Using your air compressor
Once you’ve chosen the ideal air compressor, remember these helpful tips for use and maintenance.
- Avoid extension leads. If you must extend your compressor’s reach, it’s better to use a longer air hose.
- Try to avoid using the air tool while the pump is operating. This ensures the air compressor stays within its recommended duty cycle. Wait until the pump has stopped before continuing your blasting work.
Maintaining your air compressor
To keep your air compressor in running order, clean condensation from the tank daily and clean the air filter weekly.