Question

Due to a faulty door contact the small light bulb (25 W) inside a refrigerator is kept on and limited insulation lets 50 W of energy from the outside seep into the refrigerated space. How much of a temperature difference to the ambient at 20°C must the refrigerator have in its heat exchanger with an area of 1 m² and an average heat transfer coefficient of 15 W/m² K to reject the leaks of energy. 

Answer 1

Q_tot  = 25 + 50 = 75 W to go out

Q = hA∆T = 15 x 1 x ∆T = 75

∆T = Q / hA = 75/(15 X 1) = 5 °C

OR   T must be at least 25 °C