8-136 A well-insulated heat exchanger is to heat water (c, = 4.18 kJ/kg. C) from 25C to 60C at a rate of 0.4 kg/s. The heating is to be accomplished by geothermal water (c = 4.31 kJ/kg C) available at 140C at a mass flow rate of 0.3 kg/s. The inner tube is thin-walled and has a diameter of 0.6 cm. Determine (a) the rate of heat transfer and (b) the rate of exergy destruction in the heat exchanger. Water 25C Brine 140C +60C 60C FIGURE P8-136 8-137 An adiabatic heat exchanger is to cool ethylene glycol (c, = 2.56 kJ/kg C) flowing at a rate of 2 kg/s from 80 to 40C by water (c =4.18 kJ/kg C) that enters at 20C and leaves at 55C. Determine (a) the rate of heat transfer and (b) the rate of exergy destruction in the heat exchanger. 8-138 A well-insulated, thin-walled, counter-flow heat exchanger is to be used to cool oil (c, = 2.20 kJ/kg C) from 150 to 40C at a rate of 2 kg/s by water (c = 4.18 kJ/kg. C) that enters at 22C at a rate of 1.5 kg/s. The diam- eter of the tube is 2.5 cm, and its length is 6 m. Determine (a) the rate of heat transfer and (b) the rate of exergy destruc- tion in the heat exchanger. Cold water 1.5 kg/s 22C 40C FIGURE P8-138 Hot oil 2 kg/s 150C 8-139 In a dairy plant, milk at 4C is pasteurized continu- ously at 72C at a rate of 12 L/s for 24 h/day and 365 days/yr. The milk is heated to the pasteurizing temperature by hot water heated in a natural gas-fired boiler having an efficiency of 82 percent. The pasteurized milk is then cooled by cold water at 18C before it is finally refrigerated back to 4C. To save energy and money, the plant installs a regenerator that has an effectiveness of 82 percent. If the cost of natural gas is $1.04/therm (1 therm 105,500 kJ), determine how much energy and money the regenerator will save this company per year and the annual reduction in exergy destruction. 8-140 Combustion gases enter a gas turbine at 750C and 1.2 MPa at a rate of 3.4 kg/s and leave at 630C and 500 kPa. It is estimated that heat is lost from the turbine at a rate of Exh.gas 750C 1.2 MPa TURBINE 630C 500 kPa FIGURE P8-140 30 kW. Using air properties for the combustion gases and assuming the surroundings to be at 25C and 100 kPa, deter- mine (a) the actual and reversible power outputs of the tur- bine, (b) the exergy destroyed within the turbine, and (c) the second-law efficiency of the turbine.