Mechanical fitness of a fan at high temperatures

The strength of metals and plastics varies according to their temperature. When handling air or gas at temperatures other

28000	<M E
24000	E O>
20000	JC <A 3
16000	3 TJ 0
12000	S Tft
8000	0» C 3
4000	Ј HI

A. Elastic limit N very B. Limit of proportionality I close C. Yield stress (extension increases with no increase in load) ) together 0. Maximum nominal stress Ј. Breaking stress

Ј 55

0 SO 100 150 200 250 300 350 400 450 Metal temperature °C

WL3 KEI

Extension original length

Figure 7.20 Stress/strain relationship for a typical steel

Than ambient, the materials of construction may need to be de-rated from the values normally given in textbooks.

As noted in Chapter 8, Section 8.6.2, all elements of the fan must be satisfactory. Those within the gas stream are likely to take up the same temperature, but elements outside may take up a temperature somewhere between that of the gas stream and the ambient air around the fan.

It is important to note the stress/strain relationship for the typi­cal steel used in the fan construction as shown firstly as Figure

7.1 and repeated as Figure 7.20 with more detail. This diagram is applicable to a given temperature. The general shape This di­agram is applicable to a given temperature. The general shape of the relationship between load and extension however re­mains similar. At increased temperatures, the values of A, B, C, D and E all reduce together with the value of the extension to failure

Load

Stress =

Cross — sectional area

Strain =

Figure 7.21 Reduction in fan running speed due to gas temperature Safe rev / minTemp =

= Safe rev/min, n

Steel strength at temperature

Steel strength at 20° C e. g. at 315°C = 86% of rpm at 20°C

Shaft — Usually the most important factor affecting the shaft is its critical speed (i. e. whirling takes place).

„ … . … constant

Critical speed Nc =

^deflection

Deflection A =

All factors are constant except Young’s Modulus E which falls with increasing temperature.

Therefore for the shaft:

Safe rev/mintemp =

= Safe rev/min, n<,

E at temperature E at 20°C

In the past, factors of safety were applied to the ultimate stress (i. e. D) in determining the design stress. Nowadays, with the common use of Finite Element Analysis, it is frequently the case that a design stress within the elastic limit or yield is speci­fied. Account must be taken of any shock loadings.

It should be noted that above 400°C creep stresses become im­portant. At high temperatures under stress it is found that the ordinary condition of elasticity of metals changes to a state of viscous flow whereby continuous deformation or creep pro­ceeds at slow rates. Above about 535°C any stress however small would cause continuous flow or creep in carbon steels. A molybdenum content is of value in reducing the rate of creep. It is therefore necessary to decide a creep rate for reason­able impeller life.

The choice of steel has to be carefully considered and must be related to the exact range of working temperatures. Stainless steel is not always the answer — some grades are weaker at high temperatures than carbon steels. The reduction in strength with temperatures of a typical carbon steel is shown in Figure 17.21, together with the variation in the modulus of elas­ticity.

Impeller — Forces acting on the impeller are centrifugal stresses (air forces generally negligible).

Centrifugal force oc(rev/min)2

Metal temperature Figure 17.22 Reduction in fan speed due to metal temperature

Thus all factors may be combined on a single graph as shown in Figure 17.22. It will be seen that the impeller is usually the most important item. The drastic fall-off in safe operating speed for a carbon steel impeller above 400 °C will be noted.

Posted in Fans Ventilation A Practical Guide