Air Knife Operating Principle and Applications

Airknives provide a high-intensity stream of directed air. This can be a cost-effective and efficient solution for many industrial processes:

• Cleaning (e.g. component cleaning) and particle recovery. In manufacturing, they typically blow debris or liquid off products being transported on conveyors using an intense sheet of laminar airflow (or streamline flow). Materials removed include oil and swarf, leather dust from belting and debris from conveyor belts.
• Drying (e.g. coat drying, drying of hands, fruit and vegetables; cans, jars and bottles, plastic crates; circuit boards; printing plates; printed circuit boards; castings; extruded sections and billets; rolled sheet and strip; dewatering)
• Liquid film and solids spreading and shaping (e.g. paint coating, oil film thickness reduction, chocolate on biscuits)
• Cooling
• Bonding techniques
• Static electricity removal (using anti-static bars)

Since an airstream is used, no mechanical contact with the process is needed.

Air Knife Mechanism

A blower is used to deliver air to a plenum chamber through a duct. A narrow slot orifice in the plenum converts static pressure into kinetic energy in the form of a controlled curtain of high-velocity air. When an object is placed in the way, the “knife” of air blows off gross contamination and shatters any residual film of moisture, dispersing it in the form of atomised water and vapour and leaving the surface clean and dry.

If the product has absorbed moisture, has a complex shape with intricate holes or a high water surface tension, it may be necessary to heat the air for thorough drying. The heat naturally produced by blowers can sometimes be used for this purpose by recirculating the air from a closed drying space. This technique saves energy and may also reduce the load on the HVAC system. For some products, an oven may be needed for complete drying.

Benefits of Air Knives

There are many advantages to using an air knife:

• Efficiency, especially for removing unwanted matter from surfaces
• Low electricity consumption: blower-type air knives are particularly low in energy-use and can result in reduced power consumption of up to 80% when replacing conventional drying systems. There are no additional loadings on conveyor-belt drive motors, as would be the case for mechanical belt scrapers
• Less downtime: The wear and damage associated with mechanical systems is eliminated
• Compact: Easily installed in areas where space is at a premium and where complex shapes are being handled. In film spreading and thicknessing processes the airknife system uses only 10% of the space required by conventional systems.
• Rapid start-up
• Low capital investment
• Increased process line speeds
• Minimum maintenance requirements

Principles of Air Knife Selection

To obtain the most effective and efficient unit, many engineering factors need to be considered, many of which depend on the application. Optimal results are not always obtained from the solution that appears most convenient to implement at the time of acquisition, or that is most standard.

Reducing pressure loss and creating laminar air flow are two key goals for maximizing the efficiency of an air knife. Pressure loss is dependent on the airflow exit shape, while laminar flow can be achieved by making use of the Coanda effect. According to this principle, air striking a flat surface has a tendency to flow closely in line with that surface. These aspects of design make it possible to apply the force of the air jet in a more directed way.

To estimate the necessary air velocity for a particular application, the shape and speed of the product being subjected to the air jet must be known, as well as its proximity to the air knife and how it is being held in place. The required velocity having been determined, the necessary pressure is calculated to specify the right blower. Pressure drops from the blower to the air knife must also be accounted for to be certain that the blower is not undersized. Performance and service life depends heavily on the choice of blower, whether regenerative or centrifugal.

Blower and Compressed-air Driven Air Knives

In choosing an air knife, the choice of blower-driven and compressed air systems is a fundamental one. Compressor air knives use airflows at higher pressure and lower volumes when taking only compressed air into account, though greater volumes (up to 40 times greater) are achieved by the entrainment of additional ambient air into the air jet. However, the velocity is not increased, and blower systems can produce higher velocities for the same amount of power, which can cut power consumption costs significantly.

High-efficiency blower type air knives have significantly lower operating costs than compressor air knives. Under laboratory conditions, compressor air knives may be quieter, but surface shear forces can render them equally noisy, or even worse, than blower-type air knives. Hidden maintenance costs associated with the compressor can add significantly to the total costs of operating compressor air knives. Blowers can also be located closer to the air knife plenum, since their size and noise levels are smaller than those of a compressor, further decreasing accessory costs relative to those of compressor driven systems.

Air knives are a particularly inefficient use of compressed air, since pressure drops from the compressor to the delivery site are usually high. Even without taking the use of separators and refrigerant dehumidifiers for the compressed air system into account, not to mention maintenance, the pressure drop issue can make blower air knives much more efficient to operate. The use of compressed air by other equipment will affect the performance of compressor-driven air knives, since pressure will fluctuate unavoidably. Likewise, valves and other equipment in pneumatic systems will be adversely affected by the use of the air knife. For this reason additional compressors are often installed, increasing equipment and maintenance costs.

Compared to compressed-air systems, blower-powered air knives are quieter and safer, and are more cost effective for most applications. With lateral channel blowers, the air is oil-free and requires no additional filtering.

CFW Products

When we supply an air knife, we consider:

• the mass and nature of liquid to be removed or spread
• the presence of solid materials
• the product composition, shape and surface finish
• throughput
• the method of product transportation

Careful, customised design of an air knife assembly can result in substantial energy savings and better-running processes compared to conventional solutions. Air knives are supplied in robust stainless steel or aluminium versions.

We also provide blowers, centrifugal fans and accessories for your drying system or other industrial process. Our V blower models are suitable for low-pressure air knives. For further information please contact one of our sales engineers.