11.20:
A brass boiler has a base area of 0.15 m2 and thickness 1.0 cm. It boils water at a rate

of 6.0 kg/min when placed on a gas stove. Estimate the temperature of the part of the

flame in contact with the boiler. Thermal conductivity of brass = 109 J s -1 m-1 K-1;

Heat of vaporisation of water = 2256 × 103 J/kg.

It is given in the question that:

Thermal conductivity of brass, K = 109 J s -1 m–1 K-1

Heat of vaporisation, L = 2256 × 103 J kg-1

The amount of heat flowing into the water through the brass base of the boiler:

Q1=KA(T1-T2)tl

Where,

T1 = Temperature of the flame in contact with the boiler

T2 = Boiling point of water = 100°C

The heat required for boiling the water:

Q2=mL

Amount of heat flowing into the water = Amount of heat required to boil the water

mL=LA(T1-T2)tl
T1-T2=mLlKAt
=6×2256×103×0.01109×0.15×60
=137.98°c

Therefore, the temperature of the part of the flame in contact with the boiler is 237.98 °C.