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
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Chapter 6 Solutions
Thermodynamics: An Engineering Approach
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