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Air ConditionersAir Conditioners

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Introductory QuestionIntroductory Question

If you operate a window air If you operate a window air conditioner on a table in the middle conditioner on a table in the middle of a room, the average temperature of a room, the average temperature in the room willin the room will

A.A. become colderbecome colder

B.B. become hotterbecome hotter

C.C. stay the samestay the same

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5 Questions about Air 5 Questions about Air ConditionersConditioners

Why doesn’t heat flow from cold to Why doesn’t heat flow from cold to hot?hot?

Why does an air conditioner need Why does an air conditioner need electricity?electricity?

How does an air conditioner cool How does an air conditioner cool room air?room air?

What role does the electricity play?What role does the electricity play? How does an air conditioner heat How does an air conditioner heat

outdoor air?outdoor air?

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Question 1Question 1

Why doesn’t heat flow from cold to Why doesn’t heat flow from cold to hot?hot? Does such heat flow violate the laws of Does such heat flow violate the laws of

motion?motion? Does such heat flow violate some other Does such heat flow violate some other

laws?laws?

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Laws Governing Heat Laws Governing Heat FlowFlow

The four laws of thermodynamicsThe four laws of thermodynamics are the rules governing thermal energy are the rules governing thermal energy

flowflow and establish the relationships between and establish the relationships between

disordered (thermal) energy and ordered disordered (thermal) energy and ordered energyenergy

heat and workheat and work

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0th Law of 0th Law of ThermodynamicsThermodynamics

The law about thermal equilibrium:The law about thermal equilibrium:

““If two objects are in thermal If two objects are in thermal equilibrium with a third object, then equilibrium with a third object, then they are in thermal equilibrium with they are in thermal equilibrium with

each other.”each other.”

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1st Law of 1st Law of ThermodynamicsThermodynamics

The law about conservation of The law about conservation of energyenergy

““Change in internal energy equals Change in internal energy equals heat in minus work out”heat in minus work out”

where:where: Internal energy: thermal + stored Internal energy: thermal + stored

energiesenergies Heat in: heat transferred into objectHeat in: heat transferred into object Work out: external work done by objectWork out: external work done by object

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Order versus DisorderOrder versus Disorder

Converting ordered energy into thermal Converting ordered energy into thermal energyenergy involves events that are likely to occurinvolves events that are likely to occur is easy to accomplish and often happensis easy to accomplish and often happens

Converting thermal energy into ordered Converting thermal energy into ordered energyenergy involves events that are unlikely to occurinvolves events that are unlikely to occur is hard to accomplish and effectively never is hard to accomplish and effectively never

happenshappens Statistically, ordered always becomes Statistically, ordered always becomes

disordereddisordered

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EntropyEntropy

Entropy is the measure of an Entropy is the measure of an object’s disorderobject’s disorder Includes both thermal and structural Includes both thermal and structural

disordersdisorders An isolated system’s entropy never An isolated system’s entropy never

decreasesdecreases But entropy can move or be But entropy can move or be

transferredtransferred

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2nd Law of 2nd Law of ThermodynamicsThermodynamics

The law about disorder (entropy)The law about disorder (entropy)

““Entropy of a thermally isolated Entropy of a thermally isolated system never decreases”system never decreases”

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3rd Law of 3rd Law of ThermodynamicsThermodynamics

The law about entropy and The law about entropy and temperaturetemperature

““An object’s entropy approaches zero An object’s entropy approaches zero as its temperature approaches as its temperature approaches

absolute zero”absolute zero”

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More on the 2nd LawMore on the 2nd Law

According to the 2nd Law:According to the 2nd Law: Entropy of a thermally isolated system Entropy of a thermally isolated system

can’t decreasecan’t decrease But entropy can be redistributed within But entropy can be redistributed within

the systemthe system Part of the system can become hotter Part of the system can become hotter

while another part becomes colder!while another part becomes colder! Exporting entropy is like throwing Exporting entropy is like throwing

out trash!out trash!

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Natural Heat FlowNatural Heat Flow

One unit of thermal energy is more One unit of thermal energy is more disordering to a cold object than to a hot disordering to a cold object than to a hot objectobject

When heat flows from hot object to cold When heat flows from hot object to cold object,object, the hot object’s entropy the hot object’s entropy decreasesdecreases

and the cold object’s entropy and the cold object’s entropy increasesincreases,, so the overall entropy of the system so the overall entropy of the system increasesincreases and total energy is conservedand total energy is conserved

Laws of motion and thermodynamics satisfiedLaws of motion and thermodynamics satisfied

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Unnatural Heat FlowUnnatural Heat Flow

When heat flows from cold object to hot When heat flows from cold object to hot object,object, the cold object’s entropy the cold object’s entropy decreasesdecreases,, and the hot object’s entropy and the hot object’s entropy increasesincreases

so the overall entropy of the system so the overall entropy of the system decreasesdecreases although total energy is conservedalthough total energy is conserved

The 2The 2ndnd law of thermodynamics is violated law of thermodynamics is violated To save 2To save 2ndnd law, we need more entropy! law, we need more entropy! Ordered energy must become disordered Ordered energy must become disordered

energy!energy!

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Question 2Question 2

Why does an air conditioner need Why does an air conditioner need electricity?electricity?

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Heat MachinesHeat Machines

Air conditionersAir conditioners use work to transfer heat from cold to use work to transfer heat from cold to

hothot are a type of “heat pump”are a type of “heat pump”

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Air conditioners Air conditioners (Part 1)(Part 1)

An air conditionerAn air conditioner moves heat from cold room air to hot outside moves heat from cold room air to hot outside

airair moves heat against its natural flowmoves heat against its natural flow must convert ordered energy into disordered must convert ordered energy into disordered

energyenergy so as not to decrease the world’s total entropy!so as not to decrease the world’s total entropy! uses a “working fluid” (chlorofluorocarbons –uses a “working fluid” (chlorofluorocarbons –

freon-, hydrofluorocarbons)to transfer heatfreon-, hydrofluorocarbons)to transfer heat This fluid absorbs heat from cool room airThis fluid absorbs heat from cool room air This fluid releases heat to warm outside airThis fluid releases heat to warm outside air

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Air conditioners Air conditioners (Part 2)(Part 2)

EvaporatorEvaporator is located in room airis located in room air transfers heat from transfers heat from room air to working fluidroom air to working fluid

CondenserCondenser is located in outside airis located in outside air transfers heat from working transfers heat from working fluid to outside airfluid to outside air

CompressorCompressor is located in outside airis located in outside air does work on working fluiddoes work on working fluid and produces entropy and produces entropy

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Question 3Question 3

How does an air conditioner cool How does an air conditioner cool room air?room air?

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The Evaporator The Evaporator (Part 1)(Part 1) The evaporator is a long, The evaporator is a long,

wide metal pipewide metal pipe pipe is heat exchanger pipe is heat exchanger

between air and working fluidbetween air and working fluid The working fluidThe working fluid

arrives as a high pressure, arrives as a high pressure, room temperature liquidroom temperature liquid

but loses pressure passing but loses pressure passing through a constrictionthrough a constriction

and enters the evaporator as a and enters the evaporator as a low pressure liquidlow pressure liquid

Loss of pressure destabilizes Loss of pressure destabilizes the liquid phasethe liquid phase

The liquid working fluid The liquid working fluid begins to evaporate!begins to evaporate!

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The Evaporator The Evaporator (Part 2)(Part 2)

Working fluid evaporates in the Working fluid evaporates in the evaporatorevaporator Fluid absorbs thermal energy Fluid absorbs thermal energy

while evaporating,while evaporating, so it transforms into a so it transforms into a coldcold gas gas

Heat flows from the hot room Heat flows from the hot room air to the cold gasair to the cold gas

Working fluid leaves the Working fluid leaves the evaporatorevaporator as a low density gas near room as a low density gas near room

temperaturetemperature and carries away some of the and carries away some of the

room’s thermal energyroom’s thermal energy Heat has left the room!Heat has left the room!

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Question 4Question 4

What role does the electricity play?What role does the electricity play?

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The CompressorThe Compressor The compressor increases The compressor increases

density of a gasdensity of a gas Working fluidWorking fluid

arrives as a low density gas near arrives as a low density gas near room temperature,room temperature,

has work done on it by the has work done on it by the compressor,compressor,

and experiences a rise in and experiences a rise in temperature as a result.temperature as a result.

Working fluid leaves the Working fluid leaves the compressorcompressor as a hot, high density gas as a hot, high density gas and carries away electric energy as and carries away electric energy as

thermal energythermal energy Ordered energy has become Ordered energy has become

disordered energy!disordered energy!

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Question 5Question 5

How does an air conditioner heat How does an air conditioner heat outdoor air?outdoor air?

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The Condenser The Condenser (Part 1)(Part 1)

The condenser is a long, The condenser is a long, narrow metal pipenarrow metal pipe pipe is heat exchanger pipe is heat exchanger

between air and working fluidbetween air and working fluid The working fluidThe working fluid

arrives as a hot, high density arrives as a hot, high density gasgas

but begins to lose heat to the but begins to lose heat to the cooler outdoor aircooler outdoor air

Loss of heat destabilizes the Loss of heat destabilizes the gaseous phasegaseous phase

The gaseous working fluid The gaseous working fluid begins to condense!begins to condense!

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The Condenser The Condenser (Part 2)(Part 2)

Working fluid condenses in the Working fluid condenses in the condensercondenser Fluid releases thermal energy while Fluid releases thermal energy while

condensing,condensing, so it transforms into a so it transforms into a hothot liquid liquid and even more heat flows from fluid and even more heat flows from fluid

into outside airinto outside air Working fluid leaves the Working fluid leaves the

condensercondenser as high-pressure room-temperature as high-pressure room-temperature

liquidliquid having released some of the room’s having released some of the room’s

thermal energythermal energy Heat has reached the outside air!Heat has reached the outside air!

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Air Conditioner OverviewAir Conditioner Overview Evaporator – located in room airEvaporator – located in room air

transfers heat from room air to transfers heat from room air to working fluidworking fluid

Compressor – located in outside Compressor – located in outside airair does work on fluid, so working does work on fluid, so working

fluid gets hotterfluid gets hotter Condenser – located in outside Condenser – located in outside

airair transfers heat from working fluid transfers heat from working fluid

to outside air,to outside air, including thermal energy extracted including thermal energy extracted

from inside airfrom inside air and thermal energy added by and thermal energy added by

compressorcompressor

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Introductory Question Introductory Question (Revisited)(Revisited)

If you operate a window air If you operate a window air conditioner on a table in the middle conditioner on a table in the middle of a room, the average temperature of a room, the average temperature in the room willin the room will

A.A. become colderbecome colder

B.B. become hotterbecome hotter

C.C. stay the samestay the same

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Summary aboutSummary aboutAir ConditionersAir Conditioners

They pump heat from cold to hotThey pump heat from cold to hot They don’t violate thermodynamicsThey don’t violate thermodynamics They convert ordered energy to They convert ordered energy to

thermal energythermal energy