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Air Conditioners 1 Air Conditioners. Air Conditioners 2 Introductory Question If you operate a window air conditioner on a table in the middle of a room,

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Presentation on theme: "Air Conditioners 1 Air Conditioners. Air Conditioners 2 Introductory Question If you operate a window air conditioner on a table in the middle of a room,"— Presentation transcript:

1 Air Conditioners 1 Air Conditioners

2 Air Conditioners 2 Introductory Question If you operate a window air conditioner on a table in the middle of a room, the average temperature in the room will If you operate a window air conditioner on a table in the middle of a room, the average temperature in the room will A. become colder B. become hotter C. stay the same

3 Air Conditioners 3 Observations about Air Conditioners They cool room air on hot days They cool room air on hot days They emit hot air from their outside vents They emit hot air from their outside vents They consume lots of electric power They consume lots of electric power They are less efficient on hotter days They are less efficient on hotter days Some can be reversed so that they heat houses Some can be reversed so that they heat houses

4 Air Conditioners 4 5 Questions about Air Conditioners Why doesnt heat flow from cold to hot? Why doesnt heat flow from cold to hot? Why does an air conditioner need electricity? Why does an air conditioner need electricity? How does an air conditioner cool room air? How does an air conditioner cool room air? What role does the electricity play? What role does the electricity play? How does an air conditioner heat outdoor air? How does an air conditioner heat outdoor air?

5 Air Conditioners 5 Question 1 Why doesnt heat flow from cold to hot? Why doesnt heat flow from cold to hot? Does such heat flow violate the laws of motion? Does such heat flow violate the laws of motion? Does such heat flow violate some other laws? Does such heat flow violate some other laws?

6 Air Conditioners 6 Laws Governing Heat Flow The four laws of thermodynamics The four laws of thermodynamics are the rules governing thermal energy flow are the rules governing thermal energy flow and establish the relationships between and establish the relationships between disordered (thermal) energy and ordered energy disordered (thermal) energy and ordered energy heat and work heat and work

7 Air Conditioners 7 0th Law of Thermodynamics The law about thermal equilibrium: The law about thermal equilibrium: If two objects are in thermal equilibrium with a third object, then they are in thermal equilibrium with each other.

8 Air Conditioners 8 1st Law of Thermodynamics The law about conservation of energy The law about conservation of energy Change in internal energy equals heat in minus work out where: where: Internal energy: thermal + stored energies Internal energy: thermal + stored energies Heat in: heat transferred into object Heat in: heat transferred into object Work out: external work done by object Work out: external work done by object

9 Air Conditioners 9 Order versus Disorder Converting ordered energy into thermal energy Converting ordered energy into thermal energy involves events that are likely to occur involves events that are likely to occur is easy to accomplish and often happens is easy to accomplish and often happens Converting thermal energy into ordered energy Converting thermal energy into ordered energy involves events that are unlikely to occur involves events that are unlikely to occur is hard to accomplish and effectively never happens is hard to accomplish and effectively never happens Statistically, ordered always becomes disordered Statistically, ordered always becomes disordered

10 Air Conditioners 10 Entropy Entropy is the measure of an objects disorder Entropy is the measure of an objects disorder Includes both thermal and structural disorders Includes both thermal and structural disorders An isolated systems entropy never decreases An isolated systems entropy never decreases But entropy can move or be transferred But entropy can move or be transferred

11 Air Conditioners 11 2nd Law of Thermodynamics The law about disorder (entropy) The law about disorder (entropy) Entropy of a thermally isolated system never decreases

12 Air Conditioners 12 3rd Law of Thermodynamics The law about entropy and temperature The law about entropy and temperature An objects entropy approaches zero as its temperature approaches absolute zero

13 Air Conditioners 13 More on the 2nd Law According to the 2nd Law: According to the 2nd Law: Entropy of a thermally isolated system cant decrease Entropy of a thermally isolated system cant decrease But entropy can be redistributed within the system But entropy can be redistributed within the system Part of the system can become hotter while another part becomes colder! Part of the system can become hotter while another part becomes colder! Exporting entropy is like throwing out trash! Exporting entropy is like throwing out trash!

14 Air Conditioners 14 Natural Heat Flow One unit of thermal energy is more disordering to a cold object than to a hot object One unit of thermal energy is more disordering to a cold object than to a hot object When heat flows from hot object to cold object, When heat flows from hot object to cold object, the hot objects entropy decreases the hot objects entropy decreases and the cold objects entropy increases, and the cold objects entropy increases, so the overall entropy of the system increases so the overall entropy of the system increases and total energy is conserved and total energy is conserved Laws of motion and thermodynamics satisfied Laws of motion and thermodynamics satisfied

15 Air Conditioners 15 Unnatural Heat Flow When heat flows from cold object to hot object, When heat flows from cold object to hot object, the cold objects entropy decreases, the cold objects entropy decreases, and the hot objects entropy increases and the hot objects entropy increases so the overall entropy of the system decreases so the overall entropy of the system decreases although total energy is conserved although total energy is conserved The 2 nd law of thermodynamics is violated The 2 nd law of thermodynamics is violated To save 2 nd law, we need more entropy! To save 2 nd law, we need more entropy! Ordered energy must become disordered energy! Ordered energy must become disordered energy!

16 Air Conditioners 16 Question 2 Why does an air conditioner need electricity? Why does an air conditioner need electricity?

17 Air Conditioners 17 Heat Machines Air conditioners Air conditioners use work to transfer heat from cold to hot use work to transfer heat from cold to hot are a type of heat pump are a type of heat pump

18 Air Conditioners 18 Air conditioners (Part 1) An air conditioner An air conditioner moves heat from cold room air to hot outside air moves heat from cold room air to hot outside air moves heat against its natural flow moves heat against its natural flow must convert ordered energy into disordered energy must convert ordered energy into disordered energy so as not to decrease the worlds total entropy! so as not to decrease the worlds total entropy! uses a working fluid (chlorofluorocarbons –freon-, hydrofluorocarbons)to transfer heat uses a working fluid (chlorofluorocarbons –freon-, hydrofluorocarbons)to transfer heat This fluid absorbs heat from cool room air This fluid absorbs heat from cool room air This fluid releases heat to warm outside air This fluid releases heat to warm outside air

19 Air Conditioners 19 Air conditioners (Part 2) Evaporator Evaporator is located in room air is located in room air transfers heat from transfers heat from room air to working fluid room air to working fluid Condenser Condenser is located in outside air is located in outside air transfers heat from working transfers heat from working fluid to outside air fluid to outside air Compressor Compressor is located in outside air is located in outside air does work on working fluid does work on working fluid and produces entropy and produces entropy

20 Air Conditioners 20 Question 3 How does an air conditioner cool room air? How does an air conditioner cool room air?

21 Air Conditioners 21 The Evaporator (Part 1) The evaporator is a long, wide metal pipe The evaporator is a long, wide metal pipe pipe is heat exchanger between air and working fluid pipe is heat exchanger between air and working fluid The working fluid The working fluid arrives as a high pressure, room temperature liquid arrives as a high pressure, room temperature liquid but loses pressure passing through a constriction but loses pressure passing through a constriction and enters the evaporator as a low pressure liquid and enters the evaporator as a low pressure liquid Loss of pressure destabilizes the liquid phase Loss of pressure destabilizes the liquid phase The liquid working fluid begins to evaporate! The liquid working fluid begins to evaporate!

22 Air Conditioners 22 The Evaporator (Part 2) Working fluid evaporates in the evaporator Working fluid evaporates in the evaporator Fluid absorbs thermal energy while evaporating, Fluid absorbs thermal energy while evaporating, so it transforms into a cold gas so it transforms into a cold gas Heat flows from the hot room air to the cold gas Heat flows from the hot room air to the cold gas Working fluid leaves the evaporator Working fluid leaves the evaporator as a low density gas near room temperature as a low density gas near room temperature and carries away some of the rooms thermal energy and carries away some of the rooms thermal energy Heat has left the room! Heat has left the room!

23 Air Conditioners 23 Question 4 What role does the electricity play? What role does the electricity play?

24 Air Conditioners 24 The Compressor The compressor increases density of a gas The compressor increases density of a gas Working fluid Working fluid arrives as a low density gas near room temperature, arrives as a low density gas near room temperature, has work done on it by the compressor, has work done on it by the compressor, and experiences a rise in temperature as a result. and experiences a rise in temperature as a result. Working fluid leaves the compressor Working fluid leaves the compressor as a hot, high density gas as a hot, high density gas and carries away electric energy as thermal energy and carries away electric energy as thermal energy Ordered energy has become disordered energy! Ordered energy has become disordered energy!

25 Air Conditioners 25 Question 5 How does an air conditioner heat outdoor air? How does an air conditioner heat outdoor air?

26 Air Conditioners 26 The Condenser (Part 1) The condenser is a long, narrow metal pipe The condenser is a long, narrow metal pipe pipe is heat exchanger between air and working fluid pipe is heat exchanger between air and working fluid The working fluid The working fluid arrives as a hot, high density gas arrives as a hot, high density gas but begins to lose heat to the cooler outdoor air but begins to lose heat to the cooler outdoor air Loss of heat destabilizes the gaseous phase Loss of heat destabilizes the gaseous phase The gaseous working fluid begins to condense! The gaseous working fluid begins to condense!

27 Air Conditioners 27 The Condenser (Part 2) Working fluid condenses in the condenser Working fluid condenses in the condenser Fluid releases thermal energy while condensing, Fluid releases thermal energy while condensing, so it transforms into a hot liquid so it transforms into a hot liquid and even more heat flows from fluid into outside air and even more heat flows from fluid into outside air Working fluid leaves the condenser Working fluid leaves the condenser as high-pressure room-temperature liquid as high-pressure room-temperature liquid having released some of the rooms thermal energy having released some of the rooms thermal energy Heat has reached the outside air! Heat has reached the outside air!

28 Air Conditioners 28 Air Conditioner Overview Evaporator – located in room air Evaporator – located in room air transfers heat from room air to working fluid transfers heat from room air to working fluid Compressor – located in outside air Compressor – located in outside air does work on fluid, so working fluid gets hotter does work on fluid, so working fluid gets hotter Condenser – located in outside air Condenser – located in outside air transfers heat from working fluid to outside air, transfers heat from working fluid to outside air, including thermal energy extracted from inside air including thermal energy extracted from inside air and thermal energy added by compressor and thermal energy added by compressor

29 Air Conditioners 29 Introductory Question (Revisited) If you operate a window air conditioner on a table in the middle of a room, the average temperature in the room will If you operate a window air conditioner on a table in the middle of a room, the average temperature in the room will A. become colder B. become hotter C. stay the same

30 Air Conditioners 30 Summary about Air Conditioners They pump heat from cold to hot They pump heat from cold to hot They dont violate thermodynamics They dont violate thermodynamics They convert ordered energy to thermal energy They convert ordered energy to thermal energy


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