Air knife

Last updated June 16, 2021

An air knife is a tool used to blow off liquid or debris from products as they travel on conveyors. Air knives are normally used in manufacturing or as the first step in a recursive recycling process to separate lighter or smaller particles from other components for use in later or subsequent steps, post manufacturing parts drying and conveyor cleaning, part of component cleaning. The knife consists of a high-intensity, uniform sheet of laminar airflow sometimes known as streamline flow.

An industrial air knife is a pressurized air plenum containing a series of holes or continuous slots through which pressurized air exits in a laminar flow pattern. The exit air velocity then creates an impact air velocity onto the surface of whatever object the air is directed. This impact air velocity can range from a gentle breeze to greater than Mach 0.6 (40,000 ft/min) to alter the surface of a product without mechanical contact.

Air knives remove liquids, control the thickness of liquids, dry the liquid coatings, remove foreign particles, cool product surfaces or create a hold down force to assist in the mechanical bonding of materials to the surface. Electrical currents from anti-static bars can also be injected into the exit air knife stream to neutralize the static electricity charge on some surfaces.

In the majority of manufacturing applications for air knives, the air knives are stationary while the product passes through the air velocity air stream. In other circumstances, the product is stationary and the air knives move (reciprocate or rotate) over the surface of the stationary product. Although there are very few applications where an air knife can actually cut a product (break mechanical bonds between two points), air knives are often the most efficient method of removing or controlling unwanted or foreign substances on any surface. In reference to the galvanizing industry, air knives are used to precisely control the amount of zinc coating by wiping away the coating while it is still in a liquid state on the steel surface. In most hot dip applications both top and bottom coated surfaces can be independently controlled by computer via feedback loop as accurately as plus or minus 5 grams/meter squared. Most air knife systems for this application use heated nitrogen gas as the wiping agent, not atmospheric air. Nitrogen is used to reduce oxidation of the base metal.

History

In the 1950s and 60s, the term "air doctor" was first used to refer to the non-contact method of debris blow-off using compressed air. The printing and textile industries were some of the largest users of air doctors at that time, named by analogy with "doctor rolls" and "doctor blades". They often needed wide paths of air from a compressed air system to control the thickness of liquids on a surface, or to blow debris off the surface of materials prior to the next process. Other terms used were air bar, air squeegee, air curtain, air jet, air blast, air blow off, air nozzle, air comb, air blade and air doctor blade. Today the most commonly used term is simply "air knife".

Although air knives powered by compressed plant air are used in a wide variety of industrial applications, industrial blower-powered air knives have proven to reduce the energy usage versus compressed air knives by 50–75% for most applications.^[1] Blower-powered air knife systems came of age with the advent of the 1987 Montreal Protocol, which started the clock on the worldwide phase-out of atmospheric ozone depleting CFC’s (chlorofluorocarbons) then used as cleaning agents in many industries.

Most of these solvent-based cleaning agents simply evaporated which required no blow-off or other drying methods. Although the printed circuit board industry was still in its infancy, it was among the first to initiate the conversion to aqueous and semi-aqueous-based parts cleaning systems.

With nearly every existing and all future circuit board factories using the new environmentally friendly cleaning technology, they also needed a new method of drying the p.c. boards following their water-based cleaning to remove solder fluxes and other contaminants. The trend away from other types of solvent-based parts cleaning to water-based cleaning for other industries began soon thereafter. Additionally, the conversion to water-based inks, paints, coatings, adhesives and other solutions used in various manufacturing sectors has resulted in the need for air knife dryers where none had previously existed. As a result of the Montreal Protocol and worldwide industry compliance with environmental stewardship mandates, the former niche business of air knives became an industry.

Description of operation

Air knives on a production line commonly range from 0.25 to 200 inches (6.4 to 5,080.0 mm) in length with a discharge air slot or holes ranging from 0.001 to 0.25 in (0.025 to 6.350 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 254 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 gauge 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 305 mm) or more away from the surface of the product. Round air nozzles of 1 to 4 in (25 to 102 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.

Types and applications

The most common use of air knives is to contain or remove free-standing materials (liquids or solids) from the surface of material. The applications include drying bottles and cans after filling and rinsing, printed circuit boards following the conveyorized wash to remove solder paste and flux, metals castings after automatic machining and many more. They can also deliver heated or cooled air to a surface, or create an invisible air barrier to separate heated or cooled environments from one another in industrial applications such as continuous metal heat treating ovens, cold process or storage areas in food processing or dust containment for the entrance to clean rooms.

There is a variety of uses for air knives in many different industries, applications and environments. The invisible-high velocity air streams can be discharged by air knife designs of numerous shapes and sizes. These range from “garage built” devices with a low level of precision to the most exotic metals of construction used in air knives for class 100 clean rooms.

In instances where noise reduction and moisture containment around a conveyorized air knife installation becomes important, some manufacturing facilities have installed air knives within an enclosure. These enclosures keep water contained, reduce the amount of air knife noise and even eliminate any liquid that could create safety concerns.

Basic design features

Compressed air-powered air knives

There has always been a wide assortment of blow-off appliances. Air knives and nozzles for compressed air blow-off range from home made round pipes with holes to engineered high pressure air knives. In order to achieve the highest efficiency using compressed air, many manufacturers of compressed air knives utilize the Coandă effect to improve compressed air knife design over other types of knives and nozzles. Although the efficiency of compressed air for low-pressure blow-off air is much lower than blowers, the Coanda-inspired air knives entrain ambient air into the high-velocity stream to enhance the blow off effect.

Blower-powered air knives

The teardrop-shaped air knife has a bulbous plenum which tapers down to a precise air discharge slot as the standard of the blower driven air knife industry. Whereas a round pipe with holes drilled has an average coefficient of discharge of 0.6 (60% efficient), the teardrop-shaped air knife is commonly 0.95 (95% efficient), which provides much higher-impact air velocity to the surface at which the air is directed with the lowest blower motor power demand. These teardrop designs are available in extruded aluminum shapes as well as fabricated carbon and stainless steels.

OSHA compliance

Information is needed regarding the Occupational Safety and Health Administration (OSHA) and their standards and directives on Compressed Air, especially when used for cleaning.

Related Research Articles

Spray painting is a painting technique in which a device sprays coating material through the air onto a surface. The most common types employ compressed gas—usually air—to atomize and direct the paint particles. Spray guns evolved from airbrushes, and the two are usually distinguished by their size and the size of the spray pattern they produce. Airbrushes are hand-held and used instead of a brush for detailed work such as photo retouching, painting nails, or fine art. Air gun spraying uses generally larger equipment. It is typically used for covering large surfaces with an even coating of liquid. Spray guns can be either automated or hand-held and have interchangeable heads to allow for different spray patterns. Single color aerosol paint cans are portable and easy to store.

Gas duster Product used for cleaning or dusting sensitive devices that cannot be cleaned using water

Gas duster, also known as canned air or compressed air, is a product used for cleaning or dusting electronic equipment and other sensitive devices that cannot be cleaned using water.

Powder coating is a type of coating that is applied as a free-flowing, dry powder. Unlike conventional liquid paint which is delivered via an evaporating solvent, powder coating is typically applied electrostatically and then cured under heat or with ultraviolet light. The powder may be a thermoplastic or a thermoset polymer. It is usually used to create a hard finish that is tougher than conventional paint. Powder coating is mainly used for coating of metals, such as household appliances, aluminium extrusions, drum hardware, automobiles, and bicycle frames. Advancements in powder coating technology like UV curable powder coatings allow for other materials such as plastics, composites, carbon fiber, and MDF to be powder coated due to the minimum heat and oven dwell time required to process these components.

Fusion bonded epoxy coating, also known as fusion-bond epoxy powder coating and commonly referred to as FBE coating, is an epoxy-based powder coating that is widely used to protect from corrosion steel pipe used in pipeline construction, concrete reinforcing bars and on a wide variety of piping connections, valves etc. FBE coatings are thermoset polymer coatings. They come under the category of protective coatings in paints and coating nomenclature. The name fusion-bond epoxy is due to resigning cross-link and the application method, which is different from a conventional paint.

A dust collector is a system used to enhance the quality of air released from industrial and commercial processes by collecting dust and other impurities from air or gas. Designed to handle high-volume dust loads, a dust collector system consists of a blower, dust filter, a filter-cleaning system, and a dust receptacle or dust removal system. It is distinguished from air purifiers, which use disposable filters to remove dust.

A lobe pump, or rotary lobe pump, is a type of positive displacement pump. It is similar to a gear pump except the lobes are designed to almost meet, rather than touch and turn each other. An early example of a lobe pump is the Roots Blower, patented in 1860. to blow combustion air to melt iron in blast furnaces, but now more commonly used as an engine supercharger.

Thermal spraying techniques are coating processes in which melted materials are sprayed onto a surface. The "feedstock" is heated by electrical or chemical means.

A Rotary Vacuum Filter Drum consists of a drum rotating in a tub of liquid to be filtered.

A spray nozzle is a precision device that facilitates dispersion of liquid into a spray. Nozzles are used for three purposes: to distribute a liquid over an area, to increase liquid surface area, and create impact force on a solid surface. A wide variety of spray nozzle applications use a number of spray characteristics to describe the spray.

Abrasive blasting, more commonly known as sandblasting, is the operation of forcibly propelling a stream of abrasive material against a surface under high pressure to smooth a rough surface, roughen a smooth surface, shape a surface or remove surface contaminants. A pressurised fluid, typically compressed air, or a centrifugal wheel is used to propel the blasting material. The first abrasive blasting process was patented by Benjamin Chew Tilghman on 18 October 1870.

Ultra-low volume (ULV) application of pesticides has been defined as spraying at a Volume Application Rate (VAR) of less than 5 L/ha for field crops or less than 50 L/ha for tree/bush crops. VARs of 0.25 – 2 l/ha are typical for aerial ULV application to forest or migratory pests.

Ultrasonic nozzles are a type of spray nozzle that uses high frequency vibration produced by piezoelectric transducers acting upon the nozzle tip that will create capillary waves in a liquid film. Once the amplitude of the capillary waves reaches a critical height, they become too tall to support themselves and tiny droplets fall off the tip of each wave resulting in atomization.

Electrostatic coating is a manufacturing process that employs charged particles to more efficiently paint a workpiece. Paint, in the form of either powdered particles or atomized liquid, is initially projected towards a conductive workpiece using normal spraying methods, and is then accelerated toward the work piece by a powerful electrostatic charge.

Nitrogen generators and stations are stationary or mobile air-to-nitrogen production complexes.

A spray is a dynamic collection of drops dispersed in a gas. The process of forming a spray is known as atomization. A spray nozzle is the device used to generate a spray. The two main uses of sprays are to distribute material over a cross-section and to generate liquid surface area. There are thousands of applications in which sprays allow material to be used most efficiently. The spray characteristics required must be understood in order to select the most appropriate technology, optimal device and size.

The peeler centrifuge is a device that performs by rotating filtration basket in an axis. A centrifuge follows on the principle of centrifugal force to separate solids from liquids by density difference. High rotation speed provides high centrifugal force that allows the suspended solid in feed to settle on the inner surface of basket. There are three kinds of centrifuge, horizontal, vertical peeler centrifuge and siphon peeler centrifuge. These classes of instrument apply to various areas such as fertilisers, pharmaceutical, plastics and food including artificial sweetener and modified starch.

A conical plate centrifuge is a type of centrifuge that has a series of conical discs which provides a parallel configuration of centrifugation spaces.

Screen/Scroll centrifuge is a filtering or screen centrifuge which is also known as worm screen or conveyor discharge centrifuge. This centrifuge was first introduced in the midst of 19th century. After developing new technologies over the decades, it is now one of the widely used processes in many industries for the separation of crystalline, granular or fibrous materials from a solid-liquid mixture. Also, this process is considered to dry the solid material. This process has been some of the most frequently seen within, especially, coal preparation industry. Moreover, it can be found in other industries such as chemical, environmental, food and other mining fields.

Carbon dioxide cleaning (CO₂ cleaning) comprises a family of methods for parts cleaning and sterilization, using carbon dioxide in its various phases. Due to being non-destructive, non-abrasive, and residue-free, it is often preferred for use on delicate surfaces. CO₂ cleaning has found application in the aerospace, automotive, electronics, medical, and other industries. Carbon dioxide snow cleaning has been used to remove particles and organic residues from metals, polymers, ceramics, glasses, and other materials, and from surfaces including hard drives and optical surfaces.

Ice blasting is a form of non-abrasive blasting where frozen water particles are combined with compressed air and propelled towards a surface for cleaning purposes. Ice is one of several different medias commonly used for blast cleaning. Another common method of non-abrasive blasting is dry ice blasting, which uses the solid form of CO2 as a blast media. Forms of abrasive blasting use mediums such as sand, plastic beads, and baking soda.

References

↑ VanderPyl, Daniel J. (1 August 2007). "Advances in Blow-Off Drying Systems and Air Knives". PCI Magazine. BNP Media. Archived from the original on 2015-09-12. Retrieved 28 November 2016.

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.

[1] VanderPyl, Daniel J. (1 August 2007). "Advances in Blow-Off Drying Systems and Air Knives". PCI Magazine. BNP Media. Archived from the original on 2015-09-12. Retrieved 28 November 2016.

[1]