CONSTRUCTA Fridge Manual do Utilizador descarregar pdf (Página 3)

Refrigerator and heat pump

A heat engine run in reverse is a refrigerator and heat pump.

Work is done to move heat from a cold temperature source to a

hot sink.

This device and be used for cooling or heating.

Clausius Statement of the Second Law

of Thermodynamics.

It is impossible to construct a refrigerator

operating in a cycle whose sole effect is to

transfer heat from a cooler object to a hotter

one.

Heat always flows from high temperature

to low temperature.

Equivalence of the Kelvin-Planck and

Clausius statements.

If a perfect refrigerator were possible (Clausius) then a perfect

heat engine could be constructed (Kelvin-Planck). Thus, the

impossibility one implies the impossibility of the other.

Proof of the Carnot Principle

Carnot engine

reversed

higher efficiency

engine

If a heat engine with a higher efficiency than a Carnot engine

could exist. Then it could convert heat to work with 100% efficiency.

The Carnot engine has the highest efficiency for any heat engine

acting between two temperatures.

Real Heat Engines

Power Plant

heat must be removed

to condense the gas

work done in expansion

Example

In a wood burning power plant the steam in the turbine operates

between the high temperature of 810 K and a low temperature of

366 K. What is the Carnot efficiency for this plant.

−

Compare this to the efficiency calculated from the electrical power

output of 59 MW and heat power input of 165 MW (see prob. 60)

WP59MW

Q H 165MW

0.35=== =

The actual efficiency is less than the Carnot efficiency.

366

05=− =

1 2 3 4

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