Discussion
A dryer removes 400 kg of water an hour. If evaporating water needs about 2.3 megajoules per kilogram, what is the minimum heat input?
Answer: D
Correct answer: about 920 megajoules an hour
\[ 400\ \mathrm{kg/h} \times 2.3\ \mathrm{MJ/kg} = 920\ \mathrm{MJ/h}. \] This is a minimum, since it accounts only for the latent heat of the water. A real dryer must also heat the ware and the air, and must vent moist air carrying sensible heat with it, so the actual input is typically half as much again or more - which is what makes waste heat recovery worth the ducting.
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