Air cannons, also known as air blasters or just “blasters” belong to a family of products known as flow aid devices. For over 30 years, air cannons have been used widely in industries such as cement manufacturing, electric power generation, coal, metal, and non-metal mining, and pulp and paper manufacturing.
Applications for air cannons range from emptying stagnant bulk-material from storage vessels to cleaning high-temperature gas ducts. Air cannon systems have proven over time to be an effective solution to problems in material flow.
In the cement industry, air cannons have found a number of applications, including the elimination of buildups and blockages in preheater towers at points such as the riser ducts, feed pipes, kiln inlets, and cyclones. They are also used extensively on clinker coolers and in material transfer chutes and storage bunkers.
The Basics of Air Cannon Operation
In its simplest form, an air cannon consists of a fast-acting, high-flow valve and a pressure vessel, or tank. Work is performed when compressed air (or other inert gas) in the tank is instantaneously released by the valve.
In a typical air cannon application, 700 liters (25 cubic feet) of free air is compressed to 7 bar (100 psi) inside a 100-liter (3.5 cubic feet) tank. The air cannon valve is designed so that the pressure of the air supply holds the valve closed, until a solenoid valve releases the holding pressure. When triggered, the fast-acting valve releases this tank volume in less than 300 milliseconds creating a high-magnitude force at the exit of the nozzle that is installed through the wall into the vessel or duct. This force is used to breakdown buildups and blockages to improve material or gas flow through the vessel or duct. Different blast characteristics are achieved by varying the operating pressure, tank volume, valve design, and nozzle design.
The general rules of air cannon application are:
- More efficient valve designs deliver higher blast forces,
- Higher operating pressures deliver higher blast forces,
- Larger tanks deliver longer blast durations,
- Nozzles spread or direct the blast to suit the application.
The peak forces generated by an air cannon blast occur during the first few thousandths of a second after the valve is opened. In applications where high-force output is required to perform the needed task, the useful energy is released during these initial few thousandths of a second, and the subsequent discharge of compressed air is wasted energy.
The Cost of Operating Air Cannon Systems
Air cannon systems are most often connected to the plant air system, sharing this compressed air resource with other equipment and processes. In other operations, the air cannon system is supplied by a dedicated compressor. This may be because the plant has no plant air system, or because the air requirements of the air cannons exceed the existing plant air system’s capacity. Blast forces vary directly with the air cannon’s supply pressure; therefore, an air cannon system’s performance suffers when the compressed air system cannot deliver the required pressure and volume.
Compressed air is a relatively expensive source of energy. While it is difficult to know the exact cost of compressed air used within a given plant, a US government estimate from 2000 calculates it at up to \$0.30 USD per 1000 cubic feet of air. Energy costs have gone up in the eight years since that estimate and in all likelihood will continue to rise. As energy prices continue to rise, it becomes increasingly valuable to find ways to reduce the compressed air consumption of air cannon systems.
For New System Installations
Use Air More Efficiently with Advanced Valve Technology
One way to minimize the use of compressed air is to select an air cannon that uses efficient valve technology. When supply pressures are equal, more efficient valves mounted on smaller tanks can deliver higher blast forces than less efficient valves mounted on larger tanks. This results in a compressed air savings equal to the difference in tank volumes. These compressed air savings are increased further when the operating pressure of the more efficient valve is reduced to the point where it will still deliver the same peak blast force as the less efficient valve.
A real-world example involves two of Martin Engineering’s air cannon models: the BIG BLASTERÆ HURRICANE Air Cannon, which was brought to market in 2007 and uses Martin Engineering’s newest valve design, produces roughly twice the blast force output while using about half the volume of compressed air, as the BIG BLASTERÆ XHV design, which was introduced over a decade ago. If the two designs were set to deliver the same blast force output, the BIG BLASTERÆ HURRICANE design would operate at roughly half the air pressure, further reducing the compressed air consumption to about one-quarter of what the BIG BLASTERÆ XHV Air Cannon uses.
Selecting the air cannon system with the more advanced valve will cost a little more up front, but the air savings will more than pay back the difference over the life of the system.
To Upgrade Existing Air Cannon Systems
Save Air by Closing the Valve Mid-Blast
Air cannon systems that are currently in operation may also benefit from new air-saving technology. Martin Engineering’s BIG BLASTERÆ XHV TORNADO Air Cannons can be equipped with a Piston Return Reservoir that will close the valve after approximately 50% of the volume has been discharged. The Piston Return Reservoir, also known as a “Canister”, is a pneumatic device that momentarily interrupts the exhaust of the air cannon’s quick exhaust valve. This interruption allows the valve’s pressure to equalize with the tank’s pressure, enabling the return spring to close the piston while the tank is still half-pressurized. The air cannon’s peak force is unaffected by this process, meaning that the same amount of work can be done with using only one-half the compressed air.
In other words, when equipped with a Piston Return Reservoir, the air cannon produces an initial blast of the same strength, but the duration of that blast (the aftershock, if you will,) is reduced. Instead, some air is retained in the air cannon tank, minimizing the overall air consumption without reducing the initial output force. Because the air cannon’s main air tank never empties completely, the amount of air required to fill that tank after every discharge is reduced.
The performance of air cannon system before and after the installation of the Piston Return Reservoir is shown in Figures 1 and 2.
Force and pressure curves of a typical air cannon blast. The blast force output reaches its peak in the moments just after the valve is opened and is reduced as the tank pressure approaches zero.
Force and pressure curves of an air cannon blast using the Piston Return Reservoir (canister). As the canister pressure approaches the tank pressure, the valve is closed with approximately 50% of the original pressure remaining in the tank. Closing the valve mid-blast does not affect the peak force output.
Figure 1 shows the force and pressure curves of a typical air cannon blast. The blast force output reaches its peak in the moments just after the valve is opened and is reduced as the tank pressure approaches zero.
Figure 2 shows the force and pressure curves of an air cannon blast using the Piston Return Reservoir (Canister). As the canister pressure approaches the tank pressure, the valve is closed with approximately 50% of the original pressure remaining in the tank. The peak force output has remained the same; but the addition of the Piston Return Reservoir has reduced the overall air consumption by 50 percent. This reduces the drain on the plant air system, and saves the plant money.
The return on investment derived from the compressed air savings for installing Piston Return Reservoir on a typical BIG BLASTERÆ XHV TORNADO air cannon system is approximately 18 months. Plants operating standard BIG BLASTERÆ XHV air cannons can also take advantage of the air savings by upgrading their valves with a TORNADO Exhaust Valve and Canister combination.
An Upgrade Case History
A Midwestern US cement plant operated a system of four BB4-2030 BIG BLASTERÆ XHV TORNADO Air Cannons on their limestone quarry’s primary crusher. This system, which had a fast cycle time of 30 seconds between firings, demanded 7 cubic meters of free air per minute (250 cfm) at 6.5 bar (95 psi).
When operating conditions required additional cleaning capacity, four BIG BLASTERÆ HURRICANE Air Cannons were added to the crusher’s system. However, the plant’s compressed air system could not support the 35% increase in air flow demanded by the new air cannons, and the system’s pressure was reduced to 4.5 bar (65 psi). The force output from the air cannon system was correspondingly reduced by 30%, negatively affecting the cleaning performance of the system. In short, the plant had added additional air cannons, but was suffering reduced performance of the entire air cannon system because of the added air consumption.
BIG BLASTERÆ XHV TORNADO (left) and HURRICANE Air Canons (two on right) installed on a cement plant’s primary crusher.
To overcome this challenge, Piston Return Reservoirs were added to each of the four original BB4-2030 BIG BLASTERÆ XHV TORNADO Air Cannons, cutting their compressed air demand by half. As a result, the operating pressure was raised back to 6.5 bar (95 psi), and the force output of air cannons was raised by 30%. This allowed the system to remove the tough material buildups and keep production running smoothly.
Piston Return Reservoirs installed on BIG BLASTERÆ XHV TORNADO Air Cannons.
Improving Air Cannon Efficiency Pays Off
To reduce air costs, reduce air consumption.
On new air cannon installations, the selection of an efficient valve such as the BIG BLASTERÆ HURRICANE Air Cannon Valve from Martin Engineering will provide effective material movement with reduced compressed air usage.
To reduce air consumption of existing air cannon system —without reducing peak output force—minimize the length of the blast, by installing a Piston Return Reservoir on BIG BLASTERÆ XHV TORNADO Air Cannons.
Other models of air cannons can take advantage of air savings by upgrading with MARTINÆ XHV TORNADO Valve and Canister technology.
The benefits of selecting efficient valve technology for new air cannon projects, and of upgrading existing air cannon systems include: reduced energy costs, improved air cannon performance, and increased compressed air capacity for other processes within the plant.
Footnote: Department of Energy Document DOE/GO-102000-0986, December 2000, Compressed Air Tip Sheet #1
Chris Schmelzer is Flow Aid Products Manager for Martin Engineering USA. He holds a BS and an MBA from Western Illinois University.
Martin Engineering (Neponset, Illinois) is the originator and leader in air cannon technology. Founded in 1944, the company has operations around the world to make bulk materials handling cleaner, safer, and more productive. www.martin-eng.com.