Mechanical Engineering - Heat Transfer, Refrigeration and Air Conditioning - Discussion

q(heat transfer) should be maximum.
So thermal resistance should be minimum.

Is it talking about heat flow to surrounding or through pipe?

The heat transfer at critical radius is maximum and after that it decreases. This concept is used for wires, steam pipe insulation. So at critical temp the thermal resistance is minimum while heat transfer is maximum.

Adding insulation to a cylindrical pipe or a spherical shell, however, is a different matter than normal insulation on the wall. Because additional insulation increases the conduction resistance of the insulation layer, But decreases the convection resistance of the surface, because of increase in the outer surface area for the convention.

The thickness up to which heat flow increase and after that heat flow decrease is termed as "Critical thickness" For pipes, ri is taken higher than (ro) cr, so that any insulation added will only decrease the heat loss from the pipe. And For wire s and cables "ri" kept lower than (ro) cr, so that add of insulation increases the heat loss from the wire/ cable.

Yes, right @Amal.

If heat transfer should be maximum, then why they use a thermal insulation in a system?

According to the question, Is Critical radius is the Critical radius of insulation?

Because if it is so then as 'critical Radius of insulation' is there then it will offer maximum resistance to heat flow and minimum heat flow. So option A would be right one.

A is the correct answer.

A critical radius is one in which heat flow is maximum. Thus resistance will be obviously minimum. The answer is B.

As insulation is added to reduce heat loss pipe so at critical insulation the resistance to heat flow (means heat loss) is maximum above critical insulation heat loss increased due to increase in conduction and below critical radius heat loss increase due to convection.