A pack countercurrent water cooling tower m2 is to cool the water from 43.3°C to

Question

A pack countercurrent water cooling tower m2 is to cool the water from 43.3°C to 29.4°C. The water flow rate of L = 1.356 kg water/s.m2. The air at 29.4°C has a wet bulb temperature of 23.9°C. The mass transfer coefficient KGa is estimated as 1.207 x 10-7 kg mol/s.m3.Pa and HLa/MBkGaP as 4.187 x 104 j/kg.K. The tower operates at a pressure of 1.013 x 105 Pa.

a. Calculate the minimum air rate.

b. If the gas flow rate is takes as a 1.75 times the minimum air rate, calculate the height of packed tower z.

Explanation / Answer

Y1 =0.0165 kg/kg dry air [From Psychrometric chart] / H1 =71.7 kJ/kg / H2 =154.8 kJ/kg From Graph, at Gs, min, / H2 =197 kJ/kg at 43.30C NPTEL – Chemical – Mass Transfer

(a) Gs, min=4546.4 kg/h

NtoG=Area under the curve= (154.8-71.7)×0.036148=3.004

HtoG=2.734 m

(b)Tower height= 2.734×3.004 m=8.213 m (Ans.)

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