GIVE DETAILED ANSWERS AND EXPLANATION

1. A rigid tank contains a hot fluid that is cooled while being stirred by a paddle wheel. Initially, the internal energy of the fluid is 800 kJ. During the cooling process, the fluid loses 500 kJ of heat, and the paddle wheel does 100 kJ of work on the fluid. Determine the final internal energy of the fluid. Neglect the energy stored in the paddle wheel.

2) A fan that consumes 20 W of electric power when operating is claimed to m discharge air from a ventilated room at a rate of 1.0 kg/s at a discharge m velocity of 8 m/s (Fig. 2-50). Determine if this claim is reasonable

3). A room is initially at the outdoor temperature of 25C. Now a large fan that consumes 200 W of electricity when running is turned on (Fig. 2-51). The heat transfer rate between the room and the outdoor air is given as Q = UAiT, - T0) where U = 6 W/m2-C is the overall heat transfer coefficient, A = 30 m2 is the exposed surface area of the room, and 7} and T0 are the indoor and outdoor air temperatures, respectively. Determine the indoor air temperature when steady operating conditions are established.

4) The lighting needs of a classroom are met by 30 fluorescent lamps, each consuming 80 W of electricity (Fig. 2-52). The lights in the classroom are kept on for 12 hours a day and 250 days a year. For a unit electricity cost of 77 CHAPTER 2 7 cents per kWh, determine annual energy cost of lighting for this classroom. Also, discuss the effect of lighting on the heating and air-conditioning requirements of the room

5) The motion of a steel ball in a hemispherical bowl of radius h shown in Fig. 2-53 is to be analyzed. The ball is initially held at the highest location at point A, and then it is released. Obtain relations for the conservation of energy of the ball for the cases of frictionless and actual motions.

6) The efficiency of cooking appliances affects the internal heat gain from them since an inefficient appliance consumes a greater amount of energy for the same task, and the excess energy consumed shows up as heat in the living space. The efficiency of open burners is determined to be 73 percent for electric units and 38 percent for gas units (Fig. 2-59). Consider a 2-kW electric burner at a location where the unit costs of electricity and natural gas are $0.09/kWh and $1.20/term, respectively. Determine the rate of energy consumption by the burner and the unit cost of utilized energy for both electric and gas burners

7)Electric power is to be generated by installing a hydraulic turbine-generator at a site 70 m below the free surface of a large water reservoir that can supply water at a rate of 1500 kg/s steadily (Fig. 2-62). If the mechanical power output of the turbine is 800 kW and the electric power generation is 750 kW, determine the turbine efficiency and the combined turbine-generator efficiency of this plant. Neglect losses in the pipes.

8) A 60-hp electric motor (a motor that delivers 60 hp of shaft power at full load) that has an efficiency of 89.0 percent is worn out and is to be replaced by a 93.2 percent efficient high-efficiency motor (Fig. 2-63). The motor operates 3500 hours a year at full load. Taking the unit cost of electricity to be $0.08/kWh, determine the amount of energy and money saved as a result of installing the high-efficiency motor instead of the standard motor. Also, determine the simple payback period if the purchase prices of the standard and high-efficiency motors are $4520 and $5160, respectively

9)A geothermal power plant in Nevada is generating electricity using geothermal water extracted at 180C, and reinjected back to the ground at 85C. It is proposed to utilize the reinjected brine for heating the residential and commercial buildings in the area, and calculations show that the geothermal heating system can save 18 million therms of natural gas a year. Determine the amount of NO* and C02 emissions the geothermal system will save a year. Take the average NO* and C02 emissions of gas furnaces to be 0.0047 kg/therm and 6.4 kg/therm, respectively.

10)Consider a person standing in a breezy room at 20C. Determine the total rate m of heat transfer from this person if the exposed surface area and the average B outer surface temperature of the person are 1.6 m2 and 29C, respectively, and the convection heat transfer coefficient is 6 W/m2-C

11)The lighting needs of a storage room are being met by 6 fluorescent light fixtures, each fixture containing four lamps rated at 60 W each. All the lamps are on during operating hours of the facility, which are 6 a m to 6 p m 365 days a year. The storage room is actually used for an average of 3 h a day. If the price of electricity is $0.08/kWh, determine the amount of energy and money that will be saved as a result of installing motion sensors. Also, determine the simple payback period if the purchase price of the sensor is $32 and it takes 1 hour to install it at a cost of $40. 2-49 A university campus has 200 classrooms and 400 faculty offices. The classrooms are equipped with 12 fluorescent tubes, each consuming 110 W, including the electricity used by the ballasts. The faculty offices, on average, have half as many tubes. The campus is open 240 days a year. The classrooms and faculty offices are not occupied an average of 4 h a day, but the lights are kept on. If the unit cost of electricity is $0.082/kWh, determine how much the campus will save a year if the lights in the classrooms and faculty offices are turned off during unoccupied periods. 2-50 Consider a room that is initially at the outdoor temperature of 20C. The room contains a 100-W lightbulb, a 110-W TV set, a 200-W refrigerator, and a 1000-W iron. Assuming no heat transfer through the walls, determine the rate of increase of the energy content of the room when all of these electric devices are on.

12)Consider a 24-kW hooded electric open burner in an area where the unit costs o f electricity and natural gas are $0.10/kWh and $1.20/therm (1 therm = 105,500 kJ), respectively. The efficiency of open burners can be taken to be 73 percent for electric burners and 38 percent for gas burners. Determine the rate of energy consum ption and the unit cost of utilized energy for both electric and gas burners. 2-61 A 75-hp (shaft output) motor that has an efficiency of 91.0 percent is worn out and is replaced by a high-efficiency 75-hp motor that has an efficiency of 95.4 percent. Determine the reduction in the heat gain of the room due to higher efficiency under full-load conditions. 2-62 A 90-hp (shaft output) electric car is powered by an electric motor mounted in the engine compartment. If the motor has an average efficiency of 91 percent, determine the rate of heat supply by the motor to the engine compartment at full load. 2-63 A 75-hp (shaft output) motor that has an efficiency of 91.0 percent is worn out and is to be replaced by a high-efficiency motor that has an efficiency of 95.4 percent. The motor operates 4368 hours a year at a load factor of 0.75. Taking the cost of electricity to be $0.08/kWh, determine the amount of energy and money saved as a result of installing the high-efficiency motor instead of the standard motor. Also, determine the simple payback period if the purchase prices of the standard and high-efficiency motors are $5449 and $5520, respectively. 2-64E The steam requirements of a manufacturing facility are being met by a boiler whose rated heat input is 5.5 X 106 Btu/h. The combustion efficiency of the boiler is measured to be 0.7 by a hand-held flue gas analyzer. After tuning up the boiler, the combustion efficiency rises to 0.8. The boiler operates 4200 hours a year intermittently. Taking the unit cost of energy to be $4.35/106 Btu, determine the annual energy and cost savings as a result of tuning up the boiler

13) 4) The lighting needs of a classroom are met by 30 fluorescent lamps, each consuming 80 W of electricity (Fig. 2-52). The lights in the classroom are kept on for 12 hours a day and 250 days a year. For a unit electricity cost of 77 CHAPTER 2 7 cents per kWh, determine annual energy cost of lighting for this classroom. Also, discuss the effect of lighting on the heating and air-conditioning requirements of the room. Soln The lighting of a classroom by fluorescent lamps is considered. The annual electricity cost of lighting for this classroom is to be determined, and the lighting's effect on the heating and air-conditioning requirements is to be discussed. Assumptions The effect of voltage fluctuations is negligible so that each fluorescent lamp consumes its rated power. Analysis The electric power consumed by the lamps when all are on and the number of hours they are kept on per year are Lighting power = (Power consumed per lamp) X (No. of lamps) = (80 W/lamp)(30 lamps) = 2400 W = 2.4 kW Operating hours = (12 h/day)(250 days/year) = 3000 h/year Then the amount and cost of electricity used per year become Lighting energy = (Lighting power) (Operating hours) = (2.4 kW) (3000 h/year) = 7200 kWh/year Lighting cost = (Lighting energy) (Unit cost) = (7200 kWh/year) ($0.07/kWh) = $504/year

5) The motion of a steel ball in a hemispherical bowl of radius h shown in Fig. 2-53 is to be analyzed. The ball is initially held at the highest location at point A, and then it is released. Obtain relations for the conservation of energy of the ball for the cases of frictionless and actual motions.

Soln the bottom of the bowl, and moves up toward point C on the opposite side. In the ideal case of frictionless motion, the ball will oscillate between points A and C. The actual motion involves the conversion of the kinetic and potential energies of the ball to each other, together with overcoming resistance to motion due to friction (doing frictional work). The general energy balance for any system undergoing any process is F F = A F in ^ o u t LXL-' system N et energy transfer Change in internal