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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 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:
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
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
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