Discussion
A boiler raises 5000 kg/h of steam with an enthalpy rise of 2700 kJ/kg, burning 400 kg/h of oil of net calorific value 42 MJ/kg. Its efficiency is about
Answer: C
Correct answer: 80 per cent
\[ \eta = \frac{5000 \times 2700}{400 \times 42{,}000} = \frac{13{,}500{,}000}{16{,}800{,}000} = 0.804 \approx 80\%. \] This is the direct method: useful heat in the steam over energy in the fuel. It is quick and needs only two measurements, but it says nothing about where the missing 20 per cent went - which is what the indirect method is for.
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