Q1 (a) The temperature at the inner and outer surfaces of a boiler wall made of 35

mm thick steel and covered with an insulating material of 20 mm thickness

are 450 °C and 700 °C respectively. If the thermal conductivities of steel and

insulating material are 78 W/m°C and 0.156 W/m°C respectively, determine

the rate of flow through the boiler wall.

Q =

(AT)overall

ERth

(b) Air at 25 °C flows past a flat plate at 2.5 m/s. The plate measures 600 mm

(length) x 300 mm (breadth) and is maintained at a uniform temperature at

95 °C. Calculate the heat loss from the plate, if the air flows parallel to the

plate.

Nuv

Pr = 0.896

y = 1797 x 10-6 m²/s

k = 0.04896 W/m K

= 0.332Re,º 0.5 pp-0.333

[10]

[10]/n(c) Water flows at the rate of 85 kg/min through a double pipe counter flow heat

exchanger. Water is heated from 50 °C to 95 °C by an oil flowing through the

tube. The specific heat of the oil is 1.680 kJ/kg.K. The oil enters at 150 °C and

leaves at 85 °C. The overall heat transfer co-efficient is 450 W/m² K. Calculate

heat exchanger area and rate of heat transfer for counter flow.

ATIM =

[(T₁ t₂) (T₂-t₁)

In [124]

- t₁

Q=UXA (AT)m

[10]