In an industrial process, 10 kg of water per hour is to be heated from 200C to 800C. To do this, steam at 1500C 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 900C, how many kg of steam is required per hour? (Take, specific heat of steam = 1 cal per g0C, latent heat of vaporisation = 540 cal g-1)

Suppose m gm steam is required per hour.Heat is released by steam in following three steps: (i) When steam at  150∘C→ steam at 100∘C    Q1=mcsteam Δθ=m×1(150−100)=50m cal (ii) When steam at   100∘C→Q2 water at 100∘C      Q2=mLV=m×540=540mcal(iii) When water at 100∘C→Q2 water at 90∘C    Q3=mcwΔθ=m×1×(100−90)=10mcal     Hence, total heat given by the steam,     Q=Q1+Q2+Q3=600mcal               …(i)    Heat taken by 10 kg water,           Q′=mcwΔθ=10×103×1×(80−20)=600×103 cal     As,   Q=Q′⇒600m=600×103⇒m=103g=1kg