In an industrial process, 10 kg of water per hour is to be heated from 20°C to 80°C. To do this, steam at 150°C is passed from a boiler into a copper coil immersed in water. The steam condenses in the coil and is returned to the boiler as water at 90°C. How many kg of steam is required per hour?(Take specific heat of steam = 1 calorie per g°C, Latent heat of vaporization = 540 cal/g)

Suppose m kg of steam is required per hour.Heat is released by steam in following three steps(i) When 150∘C steam ⟶Q1100∘C steam Then Q1=mcSteam Δθ=m×1(150−100)=50 m cal(ii) When 100∘C steam ⟶Q2100∘C water Then Q2=mLV=m×540=540 m cal (iii) When 100∘C water ⟶Q390∘C water Then Q3=mcWΔθ=m×1×(100−90)=10 m calHence, total heat given by the steam,Q=Q1+Q2+Q3=600 m cal Hence Q=Q′⇒600m=10×103×60 ⇒m=103g=1kg