Pharos University جامعه فاروس Faculty of Engineering كلية الهندسة Petrochemical Department قسم البتروكيماويات PE 330 ENERGY CONSERVATION LECTURES (6-7) Insulation 1-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 differen ces.

Heat loss by conduction: q=-k A (∆T/∆x) Heat loss by convection: q=h A(Ts-T∞) Heat loss by Radiation: q”= ε Ϭ (Ts 4 - T surrounding 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 = (R 1 R 2 /R 1 +R 2 )

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 h o = 60 W/m 2.K and h i = 30 W/m 2.K. The total wall surface area is 350 m 2. 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