Solution: Q2

2) An adiabatic heat exchanger is used to heat cold water at 15⁰C entering at a rate of 5 kg/s by hot air at 90⁰C entering also at rate of 5 kg/s. If the exit temperature of hot air is 20⁰C, the exit temperature of cold water is

Solution:

Heat is being transferred  between two fluids

Heat transfer (Q)              =  m c ∆T

m = Mass of fluid
c = Specific Heat

Heat difference of Hot Air           =  90 - 20 = 70⁰C
Heat difference of Cold Water    =  X - 15

Specific heat of air                      = 1.005  KJ/kg K

Specific heat of Water                =  4.187 KJ/kg K

Heat transferred from Hot to Cold fluids   =  Heat absorbed from Cold to Hot fluid

                       m c_w ∆T               =          m c_A ∆T

                       c_w ∆T                   =           c_A ∆T

               4.187 x (X - 15)           =          1.005 x 70

                             X                    =           31.8⁰C

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