# Critical Thickness of Insulation - MCQS with Answers

## Critical Thickness of Insulation - MCQS with Answers

Q1. What is the effect of change in outer radius of the hollow cylinder on the thermal resistance of conduction?

a. the thermal resistance of conduction increases with increase in outer radius of the hollow cylinder
b. the thermal resistance of conduction decreases with increase in outer radius of the hollow cylinder
c. the thermal resistance of conduction remains same with change in outer radius of the hollow cylinder
d. unpredictable

ANSWER: a. the thermal resistance of conduction increases with increase in outer radius of the hollow cylinder

Q2. What is the effect of change in outer radius of the hollow cylinder on the thermal resistance of convection?

a. the thermal resistance of convection increases with increase in outer radius of the hollow cylinder
b. the thermal resistance of convection decreases with increase in outer radius of the hollow cylinder
c. the thermal resistance of convection remains same with change in outer radius of the hollow cylinder
d. unpredictable

ANSWER: b. the thermal resistance of convection decreases with increase in outer radius of the hollow cylinder

Q3. What is the formula for thermal resistance for convection at the outer surface of hollow cylinder?
Where,
h = convection heat transfer coefficient
l = length of hollow cylinder
r = outer radius of the cylinder

a. Rc = 1 / h r l
b. Rc = 2 π h r l
c. Rc = 1 / 2 π h r l
d. none of the above

ANSWER: c. Rc = 1 / 2 π h r l

Q4. Consider a plane wall of area A, having a layer of insulation on it. What will happen to the thermal resistance for convection of wall if the thickness of the insulation is increased?

a. the thermal resistance for convection increases with increase in thickness of insulation
b. the thermal resistance for convection decreases with increase in thickness of insulation
c. the thermal resistance for convection remains same with increase in thickness of insulation
d. unpredictable

ANSWER: c. the thermal resistance for convection remains same with increase in thickness of insulation

#### Discussion

• Continue: Critical Thickness of Insulation - MCQS with Answers -Pankaj (06/24/15)
• 5. What is the reason behind the fact that the thermal resistance of hollow cylinder for convection decreases with increase in the thickness of the insulation?

a. volume of insulating material increases and it transfer less heat energy
b. increase in surface area of the cylinder leads to less convective heat transfer
c. increase in surface area of the cylinder leads to more convective heat transfer
d. none of the above

ANSWER: c. increase in surface area of the cylinder leads to more convective heat transfer

6. What is the critical thickness of insulation for hollow cylinder?

a. a thickness of insulation at which heat transfer rate from cylinder is minimum
b. a thickness of insulation at which heat transfer rate from cylinder is maximum
c. a thickness of insulation at which heat transfer from cylinder does not change with change in outside temperature
d. none of the above

ANSWER: b. a thickness of insulation at which heat transfer rate from cylinder is maximum

7. When the thickness of an insulation is greater than the critical thickness of insulation, then the heat transfer rate of insulated pipe

a. will be more than the heat transfer rate from the same pipe without insulation
b. will be less than the heat transfer rate from the same pipe without insulation
c. will become equal to the heat transfer rate from the same pipe without insulation
d. none of the above

ANSWER: b. will be less than the heat transfer rate from the same pipe without insulation

8. When the thickness of an insulation is less than the critical thickness of the insulation, then the heat transfer rate of insulated pipe

a. will be more than the heat transfer rate from the same pipe without insulation
b. will be less than the heat transfer rate from the same pipe without insulation
c. will become equal to the heat transfer rate from the same pipe without insulation
d. none of the above

ANSWER: a. will be more than the heat transfer rate from the same pipe without insulation

9. If ri and ro are the inner and outer radii of the the cylinder with insulating layer of some thickness and k is the thermal conductivity of the insulating material and hi and ho are the convective heat transfer coefficients of inside and outside fluids respectively. What is the formula for critical radius of insulation (rc)?

a. rc = (ro – ri) / k
b. rc = k / (ro – ri)
c. rc = k / hi
d. rc = k / ho