Carnot

Thermodynamics

Professor Lee Carkner

Lecture 12

PAL # 11 Second Law Refrigerant 134a flowing through a condenser Heat output of condenser is equal to the

change in enthalpy of fluid QH =

h1 = 271.22 kJ/kg (superheat vapor, Table A-13)

h2 = 95.47 kJ/kg (saturated liquid, Table A-12)

QH = (0.018)(271.22-95.47) =

COP = QH/W = 3.164/1.2 =

QL = QH – W = 3.164 – 1.2 =

Reversible

A reversible process:

has a net heat and work exchange for all systems as zero

is the theoretical limits for a process

Irreversible

An irreversible process can be due to: Friction

Unrestrained expansion of a gas into a vacuum

Heat transfer through temperature difference

Achieving Reversibility

Heat transfer through a very small temperature differential dT becomes reversible as dT approaches zero

Example Isothermal Work:

dT always very small

The Carnot Cycle

The Carnot engine consists of all reversible processes and thus is the most efficient

Carnot Cycle An adiabatic fall from TH to TL

Adiabatic process is frictionless and isothermal process has very small temperature differentials

Carnot Cycle

Carnot Principles

All Carnot engines operating between two heat reservoirs have the same efficiency

While we cannot build a real Carnot engine, it gives us the upper limit for the efficiency of a real engine

Carnot Efficiency

The efficiency of a reversible engine depends only on the temperatures of the reservoirs

th,rev = 1 – (TL/TH)

Maximum efficiency for any real engine Can increase the efficiency of any engine by:

Kinds of Engines

Quality of Energy

Since work is what we want, we can say that high temperature sources have higher quality energy than low temperature sources

Quality is different from quantity

Efficiency and Temperature

Carnot Refrigerator

We can also make the same determination for the efficiency of the Carnot refrigerator or heat pump

COPR = 1 / (TH/TL -1)

COPHP = 1 / (1 – TL/TH) Smaller temperature difference means more

efficiency

Carnot Refrigeration Cycle

Kinds of Refrigerators

Heating a House

Thermodynamic Temperature Scale

The efficiency of any engine depends on the ratio of the heats

Thus we can determine the temperature of two reservoirs by measuring the heat flow in and out of an ideal engine operating between them

Kelvin Scale

If assign a magnitude to the degree size we get

a complete temperature scale, independent of any substance in a thermometer

Note that we don’t actually use an engine to find T

Perpetual Motion 1st kind:

Machine that creates energy

2nd kind: Machine that converts heat completely into work

3rd kind: Machine with no dissipation

Next Time

Read: 7.1-7.6 Homework: Ch 6, P: 131, 138, Ch 7, P: 29,

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