Chapter 18, Problem 057 (a) What is the rate of energy loss in watts per square

Question

Chapter 18, Problem 057 (a) What is the rate of energy loss in watts per square meter through a glass window 3.2 mm thick if the outside temperature is -6.0°F and the inside temperature is +81°F? (b) A storm window having the same thickness of glass is installed parallel to the first window, with an air gap of 6.4 cm between the two windows. What now is the rate of energy loss if conduction is the only important energy-loss mechanism? (The termal conductivity of window glass is 1.0 W/m K, and of air is 0.026 W/m-K.) (a) Number Units (b) Number Units

Explanation / Answer

(a) Here, rate of heat loss is given by H= kT/L= thermal conductivity*temperature difference in K/ thickness= 1*(300.37-252.04)/ 0.0032 = 15103.125 watts per square meter.

(b) Now, heat loss is through inner window, air layer and storm window. So,  rate of heat loss is given by H= kT/L { where the L/k is now given by 0.0032/1 + 0.0064/0.026 + 0.0032/1= 0.2525 } = (300.37-252.04)/(L/k)= 48.33/ .2525= 191.4 watts per square meter.

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