An air knife is a fancy name for using air to blow a substance from a surface. A crude example can be seen in many machine shops. A machine operator will blow metal chips and oil from a newly machined part using a nozzle attached to a high pressure (100 PSI) air line. The same effect can be achieved by using a properly designed nozzle using low pressure air to remove water, oil, dust, and even more viscous contaminants from a surface.

Most air knives use low pressure air because it is much less expensive to produce, much safer for workers, and just as or more effective than high pressure air. High pressure air is used in some applications where lower pressures do not supply enough force.

Air knife systems consist of an air supply such as a low pressure blower and a nozzle. Blower air pressures are varied depending upon application but are usually less than 15 PSI. Higher pressures supplied by an air compressor are sometimes used. Low pressure blowers typically supply a large volume of air.

Most nozzles are designed to provide a long or wide path through which air is dispersed. Outlet velocities of 35,000 feet per minute (400 miles per hour) or higher are possible, providing considerable force for cleaning or drying. Higher velocities create greater pressure drops requiring larger motors and high­pressure blowers.

Typically air knives are used on conveyor systems to clean continuously produced parts. For example, air knives might be used to remove machining chips from aluminum extrusions before anodizing or painting as the extrusions are carried past on a conveyor belt. They might be used after painting to decrease drying time by blowing air onto the parts as they pass by on a hook­chain conveyor. Air knives can also be used to clean open web conveyor belts to prevent product or debris from clogging the openings. Scrapper knives are sometimes used to remove stickies from conveyor belts. The knife scrapes the stickies off the belt and an air knife blows them into a bin.

As parts are lifted from a washing or plating bath, cleaning or plating solution adheres to the parts. This is called dragout. The wash solution can cause contamination of downstream processes or rinse tanks. Air knives can be used to blow solution from the parts back into the process tank as they are lifted from the tank. This reduces the need for rinsing further downstream and reduces contamination of downstream processes.

If air knives reduce dragout by 50 percent, rinsing requirements are reduced by 50 percent. Additionally, if wastewater from rinsing operations is treated to remove suspended solids or metals, sludge generation from wastewater treatment could be reduced by 50 percent and the costs associated with treating downstream rinsing could be reduced by 50 percent.

The effectiveness of using air knives is highly dependent upon the shape of the part being cleaned and the type of contamination being removed. A flat, smooth part is much easier to clean with air than a complex three­dimensional item. Additionally, air knives work well for cleaning internal passages, and open, flat meshes can be easily cleaned with an air knife by blowing air through the mesh.

Air knives can be used to reduce friction by blowing air beneath paper or steel sheets, thus providing an air cushion on which they can ride. Also, air knives are often used as a barrier between two areas to prevent mixing of air. For example, where doors are not practical, blowers with nozzles can be used to blow air across a doorway, such as a fork truck entrance, to prevent hot or cold outside air from mixing with conditioned air.

Operating costs consist of electric power and filter replacement. Filters are inexpensive, costing about $10 for eight square feet of inlet filter. Electric power for a five HP (horsepower) unit supplying 200 cubic feet per minute of air at five pounds per square inch pressure costs about $4.50 to operate for 24 hours assuming, an electricity cost of $0.05 per kilowatt­hour.

Equipment costs are dependent upon the installation and size of the unit. A five HP motor will cost approximately $300. A low­pressure fan rated at five HP will cost approximately $600. Higher pressure blowers will cost more. Air knife nozzles are essentially sheet metal fabrications, and their cost is dependent upon size. An air knife unit three feet in length will cost $500 to $3,000 depending upon complexity.

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