What formula is used to solve this?

Question

Spent steam from an electric generating plant leaves the turbines at 120 degrees C and is cooled to 90 degrees C liquid water by water from a cooling tower in a heat exchanger. How much heat is removed by the cooling tower for each kg of spent steam?

Answer 1

specific heat water is 4186 J/kgC

heat of vaporization of water is 2.256 MJ/kg

specific heat of steam is 2100 J/kgK

energy is removed to cool it to 100 C (assuming atmospheric pressure), then energy to condense it to liquid, then more to cool it from 100 to 90 C

part 1) E = 2100 J/kgK x 1kg x 20C = 42000 J

part 2) E = 2.256 MJ/kg x 1kg = 2256000 J

part 3) E = 4186 J/kgC x 1 kg x 10 = 41860 J

adding them up

total = 2339900 J