What happens when the thickness of insulation on a pipe exceeds the critical value?

This question was previously asked in

TNTRB 2012 ME Official Question Paper

Option 2 : Heat transfer rate decreases

NTPC Diploma Trainee 2020: Full Mock Test

12108

120 Questions
120 Marks
120 Mins

**Concept:**

Pipe with insulation can be assumed as cylinder.

- The value of Router for which the heat transfer rate is maximum is called the critical radius of insulation.
- The thickness up to which heat flow increases and after which heat flow decreases is termed as Critical thickness.
- The insulation radius at which resistance to heat flow is minimum and consequently heat flow rate is maximum is called “critical radius”. And it starts decreasing after the critical thickness of insulation.

Note that the critical radius of insulation depends on the thermal conductivity of the insulation k and the external convection heat transfer coefficient h.

The critical radius of insulation for a cylindrical body:

\({r_{cr,cylinder}} = \frac{k}{h}\)

Critical radius of insulation for a spherical shell:

\({r_{cr,sphere}} = \frac{{2k}}{h}\)

The rate of heat transfer from the cylinder increases with the addition of insulation for r2 < rcr, reaches a maximum when r2 = rcr, and starts to decrease for r2 > rcr. Thus, insulating the pipe may increase the rate of heat transfer from the pipe instead of decreasing it when r2 < rcr.