Pb. 11-131 [Çengel]
11–131 In a chemical plant, a certain chemical is heated by hot
water supplied by a natural gas furnace.
The hot water (Cp= 4180 J/kg ºC) is then discharged at 60 ºC
at a rate of 8 kg/min. The average temperature of the cold water
entering the furnace throughout the year is 14 ºC.
The plant operates 8 h a day, 5 days a week, 52 weeks a year.
The furnace has an efficiency of 78 percent, and the cost of the
natural gas is $1.00 per therm (1 therm = 105,500 kJ).
In order to save energy, it is proposed to install a water-to-water
heat exchanger to preheat the incoming cold water by the drained hot water.
Assuming that the heat exchanger will recover 72 percent of the available heat in
the hot water, determine the heat transfer rating of the heat exchanger
that needs to be purchased and suggest a suitable type.
Also, determine the amount of money this heat exchanger will
save the company per year from natural gas savings.