Pharos University فاروس Faculty of Engineering جامعه

الهندسة Petrochemical Department كلية البتروكيم�اويات قسم

PE 330 ENERGY CONSERVATIONLECTURES (6-7)

Insulation1-Introduction A hot reaction or storage vessel or a steam pipe will lose heat

to the atmosphere by radiation, conduction, and convection. The loss by radiation is a function of the fourth power of the absolute temperatures of the body and surroundings, and will be small for low temperature differences.

Heat loss by conduction: q=-k A (∆T/∆x)

Heat loss by convection: q=h A(Ts-T∞)

Heat loss by Radiation: q”= ε (Ts Ϭ 4 - Tsurrounding

4)

2-THERMAL RESISTANCE : In particular, there exists an analogy between the

diffusion of heat and electrical charge. Just as an electrical resistance is associated with the conduction of electricity, a thermal resistance may be associated with the conduction of heat. Defining resistance as the ratio of a driving potential to the corresponding transfer rate:

THERMAL RESISTANCE FOR CONDUCTION IN PLANE WALL :

Thermal resistance for conduction in a plane wall is:

THERMAL RESISTANCE IN COMPOSITE WALLS (BOTH CONDUCTION AND CONVECTION):

THERMAL RESISTANCE IN PARALLEL

COMPOSITE WALLS :

The equivalent resistance in case of parallel walls = (R1R2/R1+R2)

Example (1):

A house has a composite wall of wood, fiber glass insulation, and plaster board as indicated in the sketch. On a cold winter day the convection heat transfer coefficients are ho = 60 W/m2.K and hi = 30 W/m2.K. The total wall surface area is 350 m2. Determine the total heat loss through the wall.

THERMAL RESISTANCE IN CYLINDERS (BOTH CONDUCTION AND CONVECTION):

Example 2:

Solution:

Critical Thickness of insulation:

Example 3

Solution