The Laws of Thermodynamics and Air Conditioners(your best friends during this summer)

What a Fridge and an Air Conditioner does ?

• move heat from a hot place to a cold place• these machines generically called a ‘heat pump’

Are there rules/laws that govern this Motion of Heat ?

• Yes, these are called ‘laws of thermodynamics’• Establish relationships between

Thermal quantities (Temperature T, Internal Energy U, Entropy)and

Mechanical quantities (Work)

• There are 4 laws

Zeroth Law of Thermodynamics (definition of temperature)Two objects that are each in thermal equilibrium with

a third object are also in thermal equilibrium with one another.

T=28 C

28C 28C

A B

There is no way you can do a net transfer heat from houseA to B if both are in equilibrium with the environment.

i.e. heat will not flow from A to B.

First Law of Thermodynamics: (Energy Conservation)The change in a stationary object’s internal energy

is equal to the heat transferred into that object minus the workthat object did on its surroundings. U = Q - W

piston

candle

GasMoleculesat temp T

Q

U

Internal Energy U – measure of thermal energy of systemHeat in – heat added to systemWork done – work done by the system

Work done W

Disorder vs Order – the meaning of entropy

ORDER(small entropy)

DISORDER(large entropy)

ORDER to DISORDER: Statistically Likely to happenReverse….. Very rarely happens

Converting Thermal Energy to Ordered Energy is rare

Entropy – a measure of the disorder of a system

Second Law of ThermodynamicsThe entropy of a thermally isolated system of objects

never decreases.

What does this mean ? Let represent molecules of a COLD gasand represent molecules of a HOT gas

Which of the following events are likely to happen ?

A.

B.

Natural Flow of Heat is from HOT to COLD !

Third Law of ThermodynamicsAs an object’s temperature approaches zero, its entropy

approaches zero.

Key Features of an Air conditioner1. Evaporator

• located in hot room • transfers heat from hot room air to fluid

2. Condenser• located outside room• Transfers heat from fluid to outside air

3. Compressor • located outside room• does work on the fluid by compressing it and creating entropy

What does a Fridge or Air conditioner do ? • It does the Un-natural: Moves heat from HOT to COLD• Uses a circulating fluid, usually freon, to move heat

Wall-mounted air conditioner

ResidentialCompressor/Condenser

outside Apartments: compressor/condenser Outside hospitals, etc

A Buffet of Air Conditioners

Room TempLow Pressure gas

Anatomy of an Air Conditioner

Low pressure Cold gas

Hot, highPressure gas

Hot, high Pressure liquid

12

3

4InsideBuilding

OutsideBuilding

or condenser

A compressor turns the roomtemperature freon gas intoa warm, high pressure gas.

A condenser turns the hotliquid freon into a cold lowpressure gas via free expansion.

Process 4 1: Heat is absorbed by freon gas in tubes inside a warm room. Becomes a room temperature low pressure gas before entering compressor.

Process 1 2: Room Temp low pressure freon gas enters a compressor and comes out as a high pressure hot gas. Passing thru tubes outside building, hot freon dumps heat to outside air

Process 2 3: Warm freon gas enters a constriction and is further pressurized to form a liquid in the condenser.

Process 3 4. Freon liquid undergoes free expansion into a gas and cools. The cool gas then takes in heat from the room, becoming a room temperature low pressure gasThe cycle is then repeated.

The Refrigeration Sequence in an Air conditioner

Although the airconditioner pumps heat from cold to hot regions, it doesn’t violate the 2nd law of thermodynamics. The compressor adds entropy, so that the total entropy of the system actually increases.