Surface protection

To give the basic materials of a fan protection against tempera­ture, corrosion and erosion or to improve its appearance, it is important to provide a good surface finish. The possibilities are endless, but may be considered under five basic headings:

• painting

• galvanising

• plating

• lining

• coating

These will now be discussed in a little more detail, although it is important to emphasise the necessity of discussion with a repu­table supplier or specialist sub-contractor.

It is unfortunate that everyone seems to believe that he has a God-given right to specify his own unique finish. Thus the fan manufacturer may be burdened with non-standard paint sys­tems or even unusual colours. The consequent increased workload in just substituting one paint for another has to be imagined — change of brushes or applicators, cleaning of pipe­lines etc., etc. Wherever possible, users are recommended to study Eurovent document 1 /9 on the surface treatment of fans.

Painting

The number of paints in existence, and the methods by which they are applied, must total many thousands. Correct surface preparation, choice of paint system and careful application must all be right to give satisfactory protection and good ap­pearance. Paints may be categorised into the following types:

• primers e. g. zinc phosphate or zinc chromate

• air drying e. g. alkyd resins, chlorinated rubbers or esters

• two pack e. g. epoxy or polyurethanes

Some of the types detailed above may be restricted in their use by local or national ordinances, especially where they are likely to end up being poured into the drains. There is a trend towards water-based paints, as apposed to oil or lead bases, for such reasons.

There are a number of national and international Standards which are relevant including BS 381, BS 5493 and BS 7079.

Galvanising

This is the term used for the coating of iron and steel compo­nents with zinc. It is probably a more robust surface than paint in protecting the underlying metal from corrosion. The initial bright finish (often enhanced by the inclusion of a small amount of alu­minium in the molten zinc tank), however, rapidly “dulls” in ser­vice.

The coating is usually defined by its weight per unit area in ac­cordance with the grades specified in BS 729. See also ISO 1459 and subsequent revisions. Weights can vary from around 300 to 800 g/m2, the heavier coatings being applicable to thicker materials, or where the ambient atmosphere is aggres­sive e. g. an oil refinery close to the sea.

Plating

Perhaps the most commonly recognised plating is that using chrome. Not only can it give an excellent surface and appear­ance, but it also gives a measure of protection against many ad­verse environments. Many plating systems are quite complex and have a layer of copper beneath the chrome.

Nickel can also be used for electroplating and the relevant stan­dards for both materials are BS1224, IS0 1456 and IS01458.

Lining

Perhaps more popular in the past than nowadays, is the lining of industrial fans with thick rubber to all surfaces in contact with the gas being handled. The lining is applied in sheets up to about 6 mm thick to the casing of either cast iron or sheet steel. Impellers also may be lined. These are usually of the open pad­dle bladed type, although it is possible with shrouded types pro­vided sufficient clearance is maintained at the interface be­tween the shroud lip and the fan inlet cone.

There are two main types of rubber used:

• Natural rubbers for ambient temperatures where the air/gas is oil-free

• Synthetic rubbers such as nitryl, butyl or neoprene for gas temperatures up to 120 °C and/or when fumes are present.

Both natural and synthetic rubbers are available as hard or soft grades. The hardness scales used are the Shore scale or the IRHD (International Rubber Hardness Degrees).

Hard rubber or ebonite is 60-80 Shore D scale or 80-100 IRHD. Soft rubber (India rubber if natural) is 40-80 Shore A scale or 40-80 IRHD. For further information see ISO 7619.

The design of rubber-lined components is especially important to ensure that there is adhesion (see BS 6374). The procedure requires that:

• metal surfaces are shot blasted to Sa 2ЛА

• adhesive is applied to all the surfaces to be lined

• rubber sheets are manually laid with overlapping joints

• rubber is vulcanised by heating to 120 °С using steam or hot water.

Testing of the lined components is essential to ensure their in­tegrity and the following are commonly specified:

• Spark testing at 20kV to guarantee continuity

• Rap testing with a special hammer to check the adhesion between rubber and metal

• Hardness testing using a hand-held gauge to measure this hardness and to ensure that the vulcanising process has been completed.

Other materials which have been used for thick lining of fans in­clude many other polymers and organic materials. PVC and other plastics have also been employed.

Coating

The term coating is used to apply to thin coating perhaps of only 150 цт thick. Typically these are of glass-like appearance and are baked on. To ensure continuity they require all sharp edges to be rounded and welds ground smooth over and above the re­quirements of Sa 3.

Posted in Fans Ventilation A Practical Guide


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