It is often stated that the refrigerator door should be opened as few times as possible for the shortest duration of time to save energy. Consider a household refrigerator whose interior volume is 0.9 m3 and average internal temperature is 4°C. At any given time, one-third of the refrigerated space is occupied by food items, and the remaining 0.6 m3 is filled with air. The average temperature and pressure in the kitchen are 20°C and 95 kPa, respectively. Also, the moisture contents of the air in the kitchen and the refrigerator are 0.010 and 0.004 kg per kg of air, respectively, and thus 0.006 kg of water vapor is condensed and removed for each kg of air that enters. The refrigerator door is opened an average of 20 times a day, and each time half of the air volume in the refrigerator is replaced by the warmer kitchen air. If the refrigerator has a coefficient of performance of 1.4 and the cost of electricity is $0.115/kWh, determine the cost of the energy wasted per year as a result of opening the refrigerator door. What would your answer be if the kitchen air were very dry and thus a negligible amount of water vapor condensed in the refrigerator?
The cost of the energy wasted per year.
The cost of the energy in the room of dry air.
Answer to Problem 124P
The cost of the energy wasted per year is
The cost of the energy in the room of dry air is
Explanation of Solution
Determine the total volume of refrigerated air replaced by room air per year.
Determine the density of the air.
Here, the air pressure is
Determine the mass of the air.
Determine the amount of moisture condensed and removed by the refrigerator.
Determine the sensible heat gain of the refrigerated space.
Determine the latent heat gain of the refrigerated space.
Here, the heat of vaporization of water is
Determine the total heat gains of the refrigerated space.
Determine the amount of electrical energy the refrigerator will consume to remove this heat from the refrigerated space.
Determine the cost of energy wasted per year.
Determine the amount of electrical energy the refrigerator will consume to remove this heat from the refrigerated space in dry air of the room.
Determine the cost of energy in the room of dry air.
Conclusion:
From the Table A-1, “Ideal-gas specific heats of various common gases” to obtain the value of universal gas constant and specific heat of air at 300 K temperature as
Refer to Table A-4, “Saturated water-Temperature”, to obtain the value heat of vaporization of water at
Write the formula of interpolation method of two variables.
Here, the variables denote by x and y are temperature and enthalpy of vaporization.
Show the temperature at
S. No |
Temperature, |
enthalpy of vaporization |
1 | ||
2 | ||
3 |
Calculate heat of vaporization of water at
Substitute
From above calculation the heat of vaporization of water at
Substitute
Substitute 95 kPa for
Substitute
Substitute
Substitute
Substitute
Substitute
Substitute
Substitute
Thus, the cost of the energy wasted per year is
Substitute
Substitute
Thus, the cost of the energy in the room of dry air is
Want to see more full solutions like this?
Chapter 6 Solutions
Thermodynamics: An Engineering Approach
- Calculate the amount of energy required in BTU to heat the air in a 10°F house 30 by 50 by 40 ft and increase the temperature by 70F at constant pressure. 75600 65700 84600 64800arrow_forward1. It is commonly recommended that hot foods be cooled first to room temperature by simply waiting a while before they are put into the refrigerator to save energy. Despite this common sense recommendation, a person keeps cooking a large pan of stew twice a week and putting the pan into the refrigerator while it is still hot, thinking that the money saved is probably too little. The average mass of the pan and its contents is 5-kg. The average temperature of the kitchen is 20°C, and the average temperature of the food is 95°C when it is taken off the stove. The refrigerated space is maintained at 3°C, and the average specific heat of the food and the pan can be taken to be 3.9 kJ/kg-K. If the refrigerator has a coefficient of performance of 1.2 and the cost of electricity is Php 21.59 per kW-hr. Determine how much will be saved a year by waiting for the food to cool to room temperature before putting it into the refrigerator. Ans: Php 170.66 20°C Hot food 5°Carrow_forwardIn adiabatic process, the system has Q=max. value O W=0 O Q=0 O U-0 O * A vertical piston-cylinder device contains water and is being heated on top of a range. During the process, 65 Btu of heat is transferred to the water, and heat losses from the side walls amount to 8 Btu. The piston rises as a result of evaporation, and 5 Btu of work is done by the vapor. Determine the change in the energy of the water .for this process 61 Btu O 52 Btu 55 Btu 60 Btu * hp compressor in a facility that operates at full load-75 for 2500 h a year is powered by an electric motor that has an efficiency of 93 percent. If the unit cost of electricity is $0.11/kWh, the annual electricity cost of this :compressor is 16,540 $ 19,180 $ O 5,380 $ O 14,300 $ O barometric pressure or 1 atmospheric 1 pressure is equal to 1.019 kgf/cm2 1 kgf/cm2 0.9 kgf/cm2 0 kgf/cm2 Oarrow_forward
- If H is the work output of a heat engine and Q is the total heat input of an engine, then determine its thermal efficiency.arrow_forwardHow would you define a system to determine the temperature rise created in a lake when a portion of its water is used to cool a nearby electrical power plant?arrow_forwardA frictionless piston fitted inside a cylinder containing gas at a pressure of 200 kPa, volume of 1 litre and temperature of 25 °C. Carbon monoxide is to be compressed in the system and the final volume is 0.5 litre. Work is done to move the piston and heat is transferred to the cooling system around the cylinder. Illustrate the importance of expressions for work done in a thermodynamic process by applying the first principles while calculating the work done when the expansion is ADIABATIC, ISOTHERMAL and POLYTROPIC for n=1.5.arrow_forward
- a) A basement refrigerator is used to store cool beverages during the long hot summer. The refrigerator is maintained at 2°C and the ambient basement temperature is 20°C. Glass bottles filled with cola at 25°C are placed in the refrigerator. Each bottle weighs 0.20 kg and there is 0.25 kg of cola in each bottle. Assume the glass bottles are in thermal equilibrium with the cola at all times (i.e. the temperature of the bottles and cola are equal). The available energy (work) to cool the bottles of cola in the refrigerator space is 110 kJ. The coefficient of performance of the refrigerator is 0.20 % of that of a Carnot refrigerator. Assume the bottle has the properties of plate glass, and the cola has the properties of saturated liquid water. How many bottles containing cola can be placed in the refrigerator and cooled to 2°C? b) Define the reversible (Q /Q.)rev ratio in terms of TH and T. Prove that the coefficient of performance of a reversible heat pump () TH TH-TL" revarrow_forwardA piston-cylinder device as shown in Figure 1-7, initially contains 0.75 kg of refrigerant- 134a at 120 kPa and 20°C. Heat is now transferred to the refrigerant from a source at 150°C, and the piston, which is resting on a set of stops, starts moving when the pressure inside reaches 140 kPa. Heat transfer continues until the temperature reaches 90°C. Assuming the surroundings to be at 25°C and 100 kPa, determine: (a) the work done, (b) the heat transfer, (c) the exergy destroyed, (d) the second law efficiency of this process. Environment 100 kPa 25°C R-134a R-134a 0.75 kg 0.75 kg Нeat 120 kPa 140 kPa 20°C 90°C Initial State Heat Source Final State 150°C Figure 1-7 A piston-cylinder devicearrow_forwardTHERMODYNAMICS 11 kg of air is heated from 24 to 84 degrees Celsius in a piston-cylinder device by passing electricity via a resistive heater within the cylinder. The pressure within the cylinder is remained constant at 300kPa during the procedure, and a 78kJ heat loss occurs. What is the kWh value of the supplied electric energy?arrow_forward
- The U.S. Department of Energy estimates that 570,000 barrels of oil would be saved per day if every household in the United States lowered the thermostat setting in winter by 6°F (3.3°C). Assuming the average heating season to be 180 days and the cost of oil to be $55/barrel, determine how much money would be saved per year.arrow_forwardThermodynamics: A rigid cylinder is filled with refrigerant-134a at 18°C. At this state, 40 percent of the mass is vapor. A valve at the bottom is opened, and refrigerant is withdrawn outside.Heat is transferred to the tank such that the temperature in the tank remains constant and the refrigerant fills the tank all the time (V1= V2). Determine the amount of heat that must be transferred by the time one-fourth of the total mass has been withdrawn. The tank volume is 0.1-m3. Clearly show work.arrow_forwardConsider two identical rooms, one with a 2-kW electric resistance heater and the other with three couples fast dancing. In which room will the air temperature rise more quickly?arrow_forward
- Elements Of ElectromagneticsMechanical EngineeringISBN:9780190698614Author:Sadiku, Matthew N. O.Publisher:Oxford University PressMechanics of Materials (10th Edition)Mechanical EngineeringISBN:9780134319650Author:Russell C. HibbelerPublisher:PEARSONThermodynamics: An Engineering ApproachMechanical EngineeringISBN:9781259822674Author:Yunus A. Cengel Dr., Michael A. BolesPublisher:McGraw-Hill Education
- Control Systems EngineeringMechanical EngineeringISBN:9781118170519Author:Norman S. NisePublisher:WILEYMechanics of Materials (MindTap Course List)Mechanical EngineeringISBN:9781337093347Author:Barry J. Goodno, James M. GerePublisher:Cengage LearningEngineering Mechanics: StaticsMechanical EngineeringISBN:9781118807330Author:James L. Meriam, L. G. Kraige, J. N. BoltonPublisher:WILEY