Physical properties of raw materials

The majority of fans have their components manufactured from materials supplied by others. Thus many casings and impellers will be fabricated from sheet steel, but have cast iron or steel hubs, die cast aluminium blades, plastic casings and/or impel­lers, etc. Even stainless steel or nickel chrome alloys may be appropriate in applications where the air or gas contains corro­sive elements or is at high temperature.

Most fans have their major components manufactured from sheet steel whilst other components may be of cast iron or ma­chined from an alloy steel. Iron ore is the basis of all these mate­rials and can be converted into iron by these methods:

• blast furnace,

• sintering or pelletised/blast furnace,

• direct reduction.

In a blast furnace, iron ore reacts with hot coke to produce pig iron. The sintering or pelletising process prior to the blast fur­nace operation is added to allow blending of iron ores and also to control the size of the blast furnace feed. Sintering or pelletising improves the blast furnace operation and reduces energy consumption. Direct reduction produces sponge iron from iron ore pellets by using natural gas. Most iron is produced from sintered iron ore and coke. The steel maker controls the sintering process to produce a consistent iron quality.

Modern blastfurnaces are fitted with many instruments and, to­gether with computer modelling, enable in-process control. Iron is taken from the blast furnace as finished material for iron foundries. Iron is transferred to the oxygen steel process for conversion to various grades of steel. Iron from direct reduction plants is mixed with scrap steel in an electric arc furnace to pro­duce various grades of steel.

Standard tests are applied, solely to assess compliance with the published specifications. Some materials are characterized only by their physical properties or chemical composition, oth­ers by both. Grey cast iron is specified by its physical proper­ties. Some low grades of carbon steel are specified by their chemical composition, no physical properties are necessary. Most materials are described by both.

For the physical properties defined in Section 17.2, standard test pieces are stretched in a machine which simultaneously measures the increase in length and the applied load. There are several different test piece sizes which give slightly different results. One standard test piece is very small, this fits a ma­chine called a Hounsfield Tensometer. Very small test pieces are useful when samples must be taken from castings or finished parts.

The various tests undertaken are now outlined.

Tensile strength

The strength of the material when it fractures. See Chapter 7, for typical values.

Limit of proportionality

The strength of the material when the relationship between stress and strain ceases to be linear. In low carbon steel this is classified as the yield point, the onset of plastic deformation, the material does not return to its original length when the load is removed. Most designs do not stress materials beyond the limit of proportionality.

Elongation

How much the material has increased in length when it frac­tured. Different test pieces have different gauge lengths, each gauge length gives a slightly different result. Good elongation properties, 15 to 20%, are required for complex components which are highly stressed. Good elongation indicates ductility. Ductility is necessary so that components can deform very slightly to spread the load. A good cast iron may be 4%.

Reduction in area

Ductile materials “thin” slightly as they are stretched. When the material fractures, the cross-sectional area of the fracture is less than the original test piece. Reduction in area is reported in most American standards but not used very much in Europe.

Hardness

The ability of the material to withstand surface indentation. No special test piece is required, raw material and finished parts can be tested. Several scales of hardness are used; Brinell Hardness Number, Vickers Pyramid Hardness and Rockwell. Approximate conversions are available between scales (see Chapter 23). In carbon steels, the hardness is directly related to the strength.

Impact strength

The ability of the material to withstand shock or impact. A spe­cial test piece is required to fit the test machine. Most materials lose impact strength as the temperature reduces. Depending upon the material, impact properties should be checked when operating below 0 °C. Two different tests are used which give different results, very approximate conversions are available. Charpy and Izod are the most popular. A benchmark for off­shore equipment is 27 J at the design temperature. It is normal to check three test pieces.

Fatigue strength

All the tests defined so far can be performed fairly quickly; “test the pieces today, get the results tomorrow”. Fatigue is very dif­

Ferent. A special test piece is either subjected to repeated ten­sile loads or repeated bending loads.

For repeated tensile loads, the test piece experiences cyclic loads from 0 to + value. A bending test piece is loaded from — value to +value. To find the endurance limit the test piece must not fail. Atest piece may appear satisfactory if it lasts five million cycles. If the machine runs at 3000 r/min this will take 1667 min­utes, i. e. 28 hours. Of course, it will not be possible to guess the correct stress so several tests must be run.

Testing for fatigue in clean air is the most simple. However, these results may not be applicable to the actual fan environ­ment. Valid conclusions may only be drawn by conducting the tests in air/gas containing the actual contaminants.

It s not common for fatigue strength of materials to be checked on a contractual basis. Such tests would take too long to reach any valid conclusions. It should be especially noted that the fa­tigue strength of aluminium products continues to fall with the number of stress reversals. The asymptotic curve assumed in may specifications just does not exist.

Most centrifugal fan designs are not based on fatigue but axial fan blades are cantilevered. An important factor in their design is therefore due to fluctuating stresses and hence fatigue fail­ure. The manufacturer should state if the life of the blades, or any other component is limited by running at the rated condi­tions, or indeed any other likely situation, such as running in reverse.

Creep resistance

Creep is the permanent distortion of the material after being subjected to a stress for a long period of time. This is not many a problem in fans, although those built of GRP, PVC, PTFE or other engineering plastic, may suffer at any temperature. It must however be considered for fans operating at gas/air tem­peratures above about 400 °C. Creep testing is similar to fa­tigue testing but creep tests can last for years. Published re­search data is therefore often used when necessary.

Limitations

Many mechanical properties of a material are dependent on its grain direction. Unless specified otherwise all these values re­late to the longitudinal direction. Properties in the transverse di­rection or the through direction may well be lower, dependent on the physical treatment of the material and its grain structure.

Posted in Fans Ventilation A Practical Guide


Добавить комментарий

Ваш e-mail не будет опубликован. Обязательные поля помечены *