Molecular Physics | Thermodynamics Answers

. A furnace wall consists of 200 mm layer of refractory bricks, 6 mm layer of

steel plate and a 100 mm layer of insulation bricks. The maximum temperature of the wall is

1150°C on the furnace side and the minimum temperature is 40°C on the outermost side of the

wall. An accurate energy balance over the furnace shows that the heat loss from the wall is

400 W/m2. It is known that there is a thin layer of air between the layers of refractory bricks and

steel plate. Thermal conductivities for the three layers are 1.52, 45 and 0.138 W/m°C respec￾tively. Find :

(i) To how many millimetres of insulation brick is the air layer equivalent ?

(ii) What is the temperature of the outer surface of the steel plate ?

5. Find the heat flow rate

through the composite wall as shown in

Fig. 15.12. Assume one dimensional flow.

kA = 150 W/m°C,

kB = 30 W/m°C,

kC = 65 W/m°C and

kD = 50 W/m°C.

mild steel tank of wall thickness 12 mm contains water at 95°C. The

thermal conductivity of mild steel is 50 W/m°C, and the heat transfer coefficients dfor the inside

and outside the tank are 2850 and 10 W/m2°C, respectively. If the atmospheric temperature is

15°C, calculate :

(i) The rate of heat loss per m2 of the tank surface area ;

(ii) The temperature of the outside surface of the tank.

20 g of gas at 20oC and 1 bar pressure is compressed to 9 bar by the law pV1.4 = C.

Taking the gas constant R = 287 J/kg K calculate the work done. (Note that for a compression

process the work will turn out to be positive if you correctly identify the initial and final

conditions)

A gas is compressed from 120 kPa and 15oC to 800 kPa. Calculate the final temperature when

the process is

i. Isothermal (n=1)

ii. Polytropic (n=1.3)

iii Adiabatic (γ=1.4)

iv. Polytropic (n= 1.5)

a car of mass M is moving with speed v. the brake of mass m and specific heat capacity c, is used to stop the car. if half of the K.E is absorbed by the brake thenwhat is the increase in the temperature of the brake

The interior of a refrigerator having inside dimensions of 0.5 m × 0.5 m

base area and 1 m height, is to be maintained at 6°C. The walls of the refrigerator are constructed

of two mild steel sheets 3 mm thick (k = 46.5 W/m°C) with 50 mm of glass wool insulation (k =

0.046 W/m°C) between them. If the average heat transfer coefficients at the inner and outer

surfaces are 11.6 W/m2°C and 14.5 W/m2°C respectively, calculate :

(i) The rate at which heat must be removed from the interior to maintain the specified

temperature in the kitchen at 25°C, and

(ii) The temperature on the outer surface of the metal sheet.

A furnace wall is made up of three layers of thicknesses 250 mm, 100 mm

and 150 mm with thermal conductivities of 1.65, k and 9.2 W/m°C respectively. The inside is

exposed to gases at 1250°C with a convection coefficient of 25 W/m2°C and the inside surface is

at 1100°C, the outside surface is exposed air at 25°C with convection coefficient of 12 W/m2°C.

Determine :

(i) The unknown thermal conductivity ‘k’ ;

(ii) The overall heat transfer coefficient ;

(iii) All surface temperatures.

A 150 mm steam pipe has inside dimater of 120 mm and outside diam￾eter of 160 mm. It is insulated at the outside with asbestos. The steam temperature is 150°C and

the air temperature is 20°C. h (steam side) = 100 W/m2°C, h (air side) = 30 W/m2°C, k (asbestos)

= 0.8 W/m°C and k (steel) = 42 W/m°C. How thick should the asbestos be provided in order to

limit the heat losses to 2.1 kW/m2 ?