Corrosion Resistant Fans – An Axair Guide

Plastic fans have come a long way from their origins as a somewhat Corrosive fume extract application
‘home made’ product. The principal manufacturers, including our partner, SEAT Ventilation, are now offering powerful, low noise, low vibration products with very good mechanical isolation. Here, we offer an overview of the key principles and benefits of plastic centrifugal fans.

Laboratory fume cupboards are by far the most popular application for corrosion resistant plastic fans. There are several other important applications such as the ventilation of fumes from chemical stores, from industrial process tanks, of hydrogen gas which is both corrosive and explosive, and of odours from water processing plant. Nevertheless, laboratories form the dominant user market.

Current fume extraction standards now recommend the use of plastic centrifugal fans. They also suggest a minimum air speed at an open fume cupboard sash (the counterbalanced sliding window), define the air speed in-duct for a ‘quiet’ ventilation system, and specify a design discharge air speed to atmosphere to prevent the ingestion of fumes back into the building. Thus the ventilation system and a fan of a desired performance (normally 10% over the specified minimum flow rate) are virtually selected from the criteria contained within the Standards.

The capacity to handle corrosive or explosive fumes will always be the principal reason for specifying a plastic fan, but it is not the exclusive one. There are many instances where the ability of the plastic fan to endure outdoor climatic conditions is reason enough for its selection.

While plastic fans can be made in a variety of materials to resist specific chemical fumes, the most versatile and widely requested plastic is polypropylene.

A useful characteristic of polypropylene is its ability to endure temperatures as high as 90°C when static. This is fine for the scroll housing but the fan impeller must remain stable at speeds close to 3000/mm. when driven by 2 pole electric motors on 50Hz supply, and 3600/mm. on 60Hz. It ¡s therefore prudent to limit the airstream temperature to 50°C to cater for the worst operating condition. This, of course, prevents the fans from being used for handling air at elevated temperatures or as direct steam extractors.

On the subject of ‘high’ speed operation, the centrifugal forces on the impeller can be resisted, and the impeller prevented from breaking-up, by incorporating carbon fibre into the plastic moulding compound. A carbon mix can also be used to provide a necessary level of conductivity in Ex flameproof fan mouldings.

Virtually all plastic fans are made with circular inlet spigots that are matched in diameter with the range of commercially available PVC and polypropylene ventilation tubes, adaptors, ‘T’ joints, socket flanges etc. Some also have circular discharge ducts to facilitate connection to a stack of height and diameter specified in the popular Standards previously referred to.

S20 polypropylene fanImpellers can be of the backward-curved type, favoured and even recommended for their efficiency and ‘non-overloading’ characteristics. In practice, most of the leading manufacturers sell the more compact and competitively priced forward-curved multi-vane impeller. The motor may then lack the ideal level of efficiency but it can, in most cases, be selected to endure lower system static pressures than a ducted centrifugal fan would be likely to encounter.

The motors used to drive plastic fans must, of necessity, be located outside of the scroll housing in order to be protected from the corrosive fumes. As with metal fans, motors can be selected in a variety of asynchronous speeds and for use on any specified electrical supply.

Plastic fans have always been a popular choice for use in Ex flameproof zones because there is no danger of contact sparking. Since the ATEX Directive became a legal requirement in July 2003 it has also become necessary to demonstrate that sparks cannot be caused by static electricity. The static avoidance method favoured for the plastic fan is to mix carbon into the plastic moulding compound. This provides a conductive path for the static electricity to discharge along. It is also important that the temperature rise of the ATEX approved electric motor is not compromised by the way it is integrated within the design of the fan. Thus, not only the motor, but the entire product must now meet the flameproof criteria set-out in the ATEX Directive.

In all cases, fans that are now produced for use in flameproof zones, as designated by the end-user, must demonstrably meet the requirements of the ATEX Directive. Manufacturers may self-certify their products for use in Zone 2 areas, but for Zone I must obtain certification though a European Notified Body.

Historically, plastic fans were made in small quantities to satisfy niche markets. Consequently manufacturers could afford to invest time and skill in cutting sheet material, fabricating and welding plastic component parts. Where larger sizes of fan are concerned this remains the only economical solution, especially where bespoke features are called for. However, demand for smaller sizes, often to ventilate a single fume cupboard or hood, has grown so much in recent years that plastic moulding is the only viable technique for the manufacturer to remain competitive and responsive.

This has enabled suppliers to offer substantial quantities of fans for major projects on realistic time scales and has ensured consistent quality through lack of welded joints. Plastic fans now have much improved impeller balance, and tend to weigh less than they previously did through optimisation of the plastic content.

In the past, many plastic fans were belt-driven for the purposes of
making impeller speed adjustment on-site to meet the practical operating point of the ventilation system. Final trimming of the air flow rate would then be by damper control.

In recent times, variable frequency inverter drives have become much more affordable. Most systems can now benefit from their use in the control of directly driven fans, which are cheaper to buy and are virtually maintenance-free except for occasional wash-down.

The result is a fan that can be simply controlled by a combination of inverter and air speed monitor to maintain the ideal air flow rate with minimum noise, least consumption of electrical energy, minimum heat loss from the building and a low-draught passage of air through the equipment.

Where a single fan is required to ventilate a number of laboratory fume cupboards it becomes necessary for each cupboard to have an electronically controlled motorised damper that responds to air flow demand in its branch of the system.

To find out more about our range of corrosion resistant fans, Contact  one of our Product Engineers now.


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