Air Mixers

Where are Air Mixers Used?

Air mixers are available in several sizes, with the capability to operate over a wide range of speeds and provide low shear simple blending to high speed and high shear mixing when required. The mixers can be used for laboratory applications, for pilot-scale operations and for large-scale production. Air mixers are used extensively with drums (called drum mixers), fabricated in sizes of 30, 55 and 100 gallons from either a stainless steel alloy, aluminum or high density polyethylene. Other air mixers (designated as tote mixers) are used for mixing processes in totes, fabricated as 250, 500 and 750 gallon containers from metal or plastic. In recent years Intermediate Bulk Containers have become widely used in the pharmaceutical and biopharmaceutical industries. This type of container can be custom designed and fabricated, usually with volumes ranging from 3 to 300 liters, including charging and discharging systems. These containers are fabricated from stainless steel, usually the 304 and 316 alloys, to provide the required sanitary conditions. Some intermediate bulk containers are fabricated from high density polyethylene or high density polypropylene and are then supported by a tubular steel frame.

The air mixer may be attached to drums, totes or intermediate bulk containers using a clamp mount, stand mount, drum lid mount or bracket (plate) mount. Air mixers set into the lid of a drum or tote are also available. For use with tote mixing systems, mixers can be obtained with shafts having adjustable length and impeller configuration and are then capable of consistently blending in very deep totes. If necessary the mixer can be mounted through the bung that is set in the lid of the drum or tote, e.g., through the standard 2-inch bung on a 55-gallon drum. Bung-mounted mixers necessarily incorporate an impeller with collapsible (folding) blades for easy installation, but the choice of impeller becomes limited. Folding props, or folding impellers are not the best mixing impellers but allow decent mixing to be done in drums with the 2? bung port. The so-called ?manual drum mixers? are designed to provide easy placement and attachment to the drum or tote.

Some common air mixers are:

Sanitary Air Mixers

Sanitary air mixers are becoming quite popular, due to the availability of all stainless air motors. Today, the drive assemblies, shafts and impellers are fabricated from stainless steel, providing hygienic conditions and meeting the sanitary regulations for the processing industries. The sanitary conditions required by today?s processing industries, particularly the pharmaceutical and bio-pharmaceutical industries, has resulted in the development of air mixers fabricated from stainless steel, not only the shaft and impeller that are in contact with the fluids being mixed, but also the air motor. The use of the stainless steel alloys significantly minimizes the microbial contamination of the fluids being processed. Although the 304 stainless steel alloy has usually been selected for the fabrication of air mixers, with sanitary guidelines becoming more stringent, the 316 and 316L alloys are now preferred.

The smoothness of the surfaces of the equipment is important, allowing more efficient cleaning and disinfection, inhibiting erosion corrosion and significantly reducing the sites for particulates to collect and contaminate the products. Smooth surfaces may be obtained by a mechanical, a chemical treatment or by electropolishing and these treatments may then be followed by a passivation process. Mechanical surface treatments involve the use of abrasive compounds, buffing and grinding processes. The different grits provide surfaces with different smoothness, with the higher numbers giving the smoother surfaces, e.g., 200 and 220 grit abrasives. Heat treatments, used in producing the stainless steels, form surface scale and tend to discolor the surface of the alloy. A chemical treatment, e.g., the immersion in a bath containing nitric and hydrofluoric acids or citric acid, usually at a temperature of 35-45?C, is required to remove the scale. Electropolishing requires the passage of an electric current to dissolve away the surface layer of the metal, that current flowing through a conductive, aqueous solution to a counter electrode. The rate of removal of the metal is dependent upon the electrolyte, temperature, current density and time. The ?art? in electropolishing requires the careful configuration of the counter electrode (the second electrode juxtapositioned to the alloy piece to complete the electrical circuit). This method is particularly suitable for components that have complex geometries.

Passivation is the formation of a film of chromium oxide at the surface of the alloy, the chromium being one of the components of the alloy. This film provides a protective layer on the surface and inhibits general corrosion reactions to enhance long term performance. Since impurities in or on the surface on the steel alloy interfere with the formation of this passive film, the process is usually carried out after the other polishing treatments.

Surface smoothness or surface roughness is conveniently measured using a profilometer and is expressed as the arithmetic mean of the departure of the peak heights and valley depths from a center line. This average is referred to as the Ra value and is usually given in microns, with low values indicative of the smoother surfaces. The surfaces of the components of air mixers are polished to Ra values ranging from 10 to 20, which may be obtained by mechanically polishing with #240 and #320 grit. By way of comparison, electropolishing will give Ra values from 4 to 8.

Types of Air Motors

Of the several types of air motors that are available commercially, the vane motor is often the preferred choice, e.g., the air motors manufactured by Gast and by Atlas Copco. The design of the vane motor is relatively simple, incorporating a front end plate, rotor, vanes, cylinder and a back plate. The motor contains a slotted rotor, fitted into the cylinder to provide a crescent-shaped chamber as it rotates. Vanes, which move to form separate working chambers within the crescent- shaped chamber, are fitted to the slots in the rotor. The vanes are pressed to the walls of the cylinder, by the centrifugal force generated as the rotor rotates, to effectively seal each working chamber as it is formed. Vane motors are available in which the direction of rotation is either clockwise or counter-clockwise and there are models available in which the direction of rotation can be reversed. The number of vanes fitted into the motor varies from three to ten and is regarded as an important design criterion. Motors with only a few vanes have smaller losses due to friction but can be difficult to start. The reverse is true when more vanes are introduced, i.e., starting the motor is easier but frictional losses are greater.

Stainless Air Motors

For food, beverage, biotech, and pharmaceutical mixing applications, it is common to use a stainless motor (no paint, and all stainless steel motors). These stainless air motors are compact, powerful, lube-free, very reliable, biopharm and sanitary friendly, and are inherently variable speed and intrinsically safe. Stainless Motors are and excellent choise where sanitary or aseptic conditions must be maintained.

Features/Benefits of Stainless Air Motors for Pneumatic mixers

The Performance of Air Mixers

The performance of the air motor is dependent upon (i) the inlet air pressure, (ii) the starting torque and (iii) the stall torque. The starting torque is obtained when the full pressure is applied to a motor with a blocked shaft, whereas the stall torque is that given when the motor just stops after the brakes are applied. The stall torque is not usually quoted in the tabulated data for an air motor, but it is estimated to be twice the torque at the maximum power for the motor. Air motors are able to operate from free speed to standstill without damage to the motor and therefore cover the complete torque curve. The power produced is the product of the torque and the speed and air-driven motors exhibit a characteristic power curve, with a maximum at approximately 50% of the free speed.

It is possible to modify the performance of an air motor by either throttle adjustment or pressure regulation. A throttle is usually fitted to the inlet to the motor and can effectively reduce the air consumption of the motor Modification of the performance using the throttle is favored when it is necessary to maintain a high starting torque while running at a lower speed. Pressure regulation is preferred when control of the stall torque is required and the starting torque is less important.

Factors in the Selection of an Air Motor

Selection of an air motor for a given application initially involves establishing the working point for the motor, i.e., the combination of the required operating speed for the motor and the torque at that point. It is also necessary to determine the amount of air that is consumed by the motor, which, as would be expected, increases with the speed. It is important to realize that, due to internal leaks, air is consumed even when the motor is stationary. The air consumption for the motors is measured in volumes (Vs), as it is for all pneumatic equipment and it is volume that would be occupied by the air at atmospheric pressure.

It is also essential to determine what is required of the motor in the particular process being considered. It is important to recognize that different air motors are able to operate at the same working point. The motor is at its? maximum efficiency when it runs at the speed giving the maximum power, so it is advantageous to select a motor that produces maximum power close to the working point. If it is important to maintain stable speeds during the mixing process, it is necessary to avoid operating the motor at speeds below the point of maximum power. In this way the motor has ?reserve power? if it is needed. Air motors that operate at low speed with high torque place a high load on the gears, whereas the lifetimes of the vanes in the motor are shortened by operating at high speeds. The life of an air motor is highly dependent upon the conditions of operation. The gears and parts in the motor, other than the vanes, can be expected to provide 3000-5000 hours of service in a mixture of free running, operation at maximum power and braking down to stall. Under such conditions the vanes are shorter lived, operating only 500 hours if not lubricated, but the lifetime is extended to approximately 1500 hours when the vanes are lubricated by adding a small amount of oil to the compressed air. Although there are motors available that do not require lubrication, the vanes being fabricated from a low friction material, when long service life is important then a lubricated motor should be selected. Extended service life may also be realized by selecting an oversized motor for the air mixer. Applications engineers at sales@wmprocess.com can answer any technical questions you have on air mixers or pneumatic motors.

Important Note on Explosion Proof Motor Ratings

Over the last several decades certain uses of electric power in manufacturing processes released flammable gases and vapors or combustible dusts into the surroundings that created potential hazards. The National Electric Code (NFPA Number 70, ANSI-C1) defines and classifies hazardous locations, i.e., locations at which the atmosphere surrounding electrical equipment is flammable or ignitable. In this code the term ?Class? defines the physical characteristics of the hazardous materials, e.g., Class I refers to gases, vapors and liquids that can produce ignitable or explosive mixtures with air and Class II includes combustible powders and dusts, as well as dust that is electrically conductive. The term ?Group? indicates the explosive properties of the materials, with Group A being the most stringent and Groups B,C, etc progressively less stringent. A third term used is ?Division? and Division 1 indicates that a potentially explosive atmosphere is or could always be present at a particular location. An excellent summary of the Classes, Groups and Divisions is given on the following website, www.reliance.com/mtr/eplprfmn.htm.

Underwriters, Laboratories, Inc. (U/L) have used these classifications as a basis for establishing test protocols that have been an important part of the development of safe electrical equipment. It was also realized that explosions or fires could be caused by the temperature of the surfaces of the equipment, as well as by arcing or sparks within the equipment. In 1974 the requirements for explosion-proof electric motors were revised and new maximum temperatures that may be attained at the surfaces of the motor were specified for the various atmospheres.

According to the National Electric Code, to be considered ?explosion-proof,? equipment such as an electric motor must be contained in an enclosure that can withstand an explosion and prevent the ignition of the surrounding atmosphere. It is also recommended that, during operation, particularly at full load or overload conditions, the temperatures attained by the surfaces of the equipment cannot ignite that atmosphere. Explosion-proof equipment is necessarily constructed with enclosures capable of withstanding an explosion without bursting apart and without joints being loosened.

Any questions please contact us at sales@wmprocess.com

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