Question

Water entering a wet cooling tower at 35°C at a rate of 100 kg/s is cooled to 22°C in the cooling tower by air that enters the tower at 1 atm, 20°C, and 60% relative humidity and leaves saturated at 30°C. The makeup water of 22°C is supplied to the outlet to make the mass flow rate of the cool water the same as that at the inlet, see Figure Q5.

Q.5Water entering a wet cooling tower at 35°C at a rate of 100 kg/s is cooled to 22°C in the cooling tower by air that enters the tower at 1 atm, 20°C, and 60% relative humidity and leaves saturated at 30°C. The makeup water of 22°C is supplied to the outlet to make the mass flow rate of the cool water the same as that at the inlet, see Figure Q5 (a) Define the absolute humidity, relative humidity, and dew point temperature in words. Where appropriate express the definition in equation form (6 marks) (b) Neglecting the power input to the fan, write down equations for the conservation of the mass (both air and water) and energy for the cooling tower flow (6 marks) (c) Determine the volume flow rate of air into the cooling tower. Use the psychrometric (9 marks) chart and property table of saturated water (d) Calculate the mass flow rate of the required makeup water (4 marks) 30°C o, = 100% WARM WATER 3 35 C 100 kg/s System boundary OAIR 1 atm 20°C ф1-60% 100 kg/s 4 COOL |22°C WATER Makeup water Figure Q5.

Answer 1

Substituting values in ma expression ma 100 C146 64-3228) 96.9 Ks/sec (lo0-42 2) CO 0213-0 0087) 32-28 . Volume flono rate of air into cooling tower 0.842 m3/Kg of dre g6.9KE/sec x 81.6m/sec d) Mass lonw rate oF make-up roater VO) 969 Co o273-0-oos7.8o ks/sec m mate up ma (8a