Description of Operation
Air knives on a production line commonly range from 0.25 to 200 inches (6.3 to 5,100 mm) in length with a discharge air slot or holes ranging from 0.001 to 0.25 in (0.025 to 6.3 mm). A stationary air knife configuration can require from one to a dozen air knives depending on the application criteria. Air is blasted through the air knife slots via an air generator, either an industrial blower or air compressor, to deliver the predetermined exit air volume and velocity needed.
There are many application, environmental, efficiency and duty cycle aspects to consider when choosing between compressors and blowers. Compressed air, which is least efficient when used for air knives discharging into free air, allows for use of primary plant air. The piping sizes supplying the air knives can be as little as 1⁄4 in (6.4 mm) diameter so they are ideal for confined spaces. Blower powered air knives must be larger in size along with larger diameter supply piping, but the efficiency improvement over compressed air is easily justified with the electrical power cost savings.
Air knife designs today have evolved to where some manufacturers produce a very efficient “teardrop” shape with a .95 coefficient of discharge. These blower powered air knife designs typically have a profile of approximately 3.5 in (89 mm) wide x 5.5 in (140 mm) tall x any length, but the teardrop profile can range from 1.5 to 10 in (38 to 250 mm) tall depending on the criteria of the product for which the impact air velocity must be engineered. With construction ranging from 1⁄8 in (3.2 mm) thick aluminum extrusion to 11 gage fabricated stainless steel, air knives can weigh 1 lb/ft to 25 lbs/ft. Depending on the width and speed of the product, the air knife can provide effective blow off performance from 0.5 to 12 in (13 to 300 mm) or more away from the surface of the product. Round air nozzles of 1 to 4 in (25 to 100 mm) diameter can be effective against surfaces which are up to several feet (1 to 2 meters) from the product surface when engineered for such applications.
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