Chemical Engineering - Heat Transfer - Discussion

Option A is correct.

Here 2 factors are acting. The desired answer depends upon the dominating one. The 2 factors are, 1st HT increases as the area for HT increases due to the insulating material, 2nd the insulating material itself impose a resistance to HT. Now it determines the critical thickness. Up to critical thickness HT increases due to the area of the insulating material. Beyond that resistance of the insulating material more dominant and HT decrease.

A 10 cm dia steam pipe, carrying steam at 180°C, is covered with insulation (conductivity = 0.6 W/m.°C). It loses heat to the surroundings at 30°C. Assume a heat transfer co-efficient of 0.8 W/m2.°C for heat transfer from the surface to the surroundings.

Neglect wall resistance of the pipe and film resistance of steam. If the insulation thickness is 2 cm, the rate of heat loss from this insulated pipe will be.

The concept is if the insulation exceeds (rc) critical radius of insulation then the insulation not worthy i.e. we are wasting money. And here if at all the thickness is less than the rc but still it is obvious thing that heat transfer is less in a insulated pipe as compared to non-insulated.

Upto critical radius HT increases, beyond that the resistance of the insulating material is dominant.

And also with increase in radius there is increase in thermal resistance and decrease in convective resistance here the h value greater than k.

From the formula critical Radius = Thermal conductivity/heat transfer coefficient.

We can say that heat loss is minimum when it is insulated.

Radius of insulation is less than that of critical radius. Hence heat loss from ins pipe is more than that in without insulated pipe.

k/d = 0.75 mts.
So critical radius = 75 cm.

But here the radius is 7 cm.
So, ans A must be right.

RC>RO so it is undesirable to have inaulatiin addition of insulation increses heat loss.

Also calculated by the formula.

1) without insulation.

2) with insulation.

A must be the right answer according to the critical radius concept.