Chemical Engineering Thermodynamics3Second Law of
Thermodynamics
Aspects of the Second Law
We conclude our introduction to the second law by observing that
the second law and deductions from it have many important uses,
including means for:1. predicting the direction of processes.2.
establishing conditions for equilibrium.3. determining the best
theoretical performance of cycles, engines, and other devices.4.
evaluating quantitatively the factors that preclude the attainment
of the best theoretical performance level.Other uses of the second
law include:5. defining a temperature scale independent of the
properties of any thermometric substance.6. developing means for
evaluating properties such as u and h in terms of properties that
are more readily obtained experimentally.
Statements of the Second Law
Clausius statement of the second law asserts that:
It is impossible for any system to operate in such a way that
the sole result wouldbe an energy transfer by heat from a cooler to
a hotter body.
The Clausius statement does not rule out the possibility of
transferring energy by heat from a cooler body to a hotter body,
for this is exactly what refrigerators and heat pumps accomplish.
However, as the words sole result in the statement suggest, when a
heat transfer from a cooler body to a hotter body occurs, there
must be other effects within the system accomplishing the heat
transfer, its surroundings, or both. If the system operates in a
thermodynamic cycle, its initial state is restored after each
cycle, so the only place that must be examined for such other
effects is its surroundings.
cooling of food is most commonly accomplished by refrigerators
driven by electric motors requiring power from their surroundings
to operate. The Clausius statement implies it is impossible to
construct a refrigeration cycle that operates without a power
input.
KelvinPlanck statement of the second law:It is impossible for
any system to operate in a thermodynamic cycle and delivera net
amount of energy by work to its surroundings while receiving energy
byheat transfer from a single thermal reservoir.
The KelvinPlanck statement does not rule out the possibility of
a system developing a net amount of work from a heat transfer drawn
from a single reservoir. It only denies this possibility if the
system undergoes a thermodynamic cycle.
The entropy statement of the second law states:
It is impossible for any system to operate in a way that entropy
is destroyed.
It follows that the entropy production may be positive or zero
but never negative. Thus, entropy production is an indicator of
whether a process is possible or impossible.
Thermodynamic Cycles
cyclepurposeinputoutput
1. Power cycleProduce workQHQc
2. Refrigeration cycleRemove heat from cold reservoirWQH
3. Heat pump cycleTransfer heat to hot reservoirWQH
Power Cycles
Evaluating a Power Cycle Performance ClaimAn inventor claims to
have developed a power cycle capable of delivering a net work
output of 410 kJ for an energyinput by heat transfer of 1000 kJ.
The system undergoing the cycle receives the heat transfer from hot
gases at atemperature of 500 K and discharges energy by heat
transfer to the atmosphere at 300 K. Evaluate this claim.
Refrigeration and Heat Pump Cycles
Evaluating Refrigerator PerformanceBy steadily circulating a
refrigerant at low temperature through passages in the walls of the
freezer compartment,a refrigerator maintains the freezer
compartment at 258C when the air surrounding the refrigerator is at
228C.The rate of heat transfer from the freezer compartment to the
refrigerant is 8000 kJ/h and the power input required to operate
the refrigerator is 3200 kJ/h. Determine the coefficient of
performance of the refrigeratorand compare with the coefficient of
performance of a reversible refrigeration cycle operating between
reservoirsat the same two temperatures.
Evaluating Heat Pump PerformanceA dwelling requires 6 3 105 Btu
per day to maintain its temperature at 708F when the outside
temperature is328F. (a) If an electric heat pump is used to supply
this energy, determine the minimum theoretical work inputfor one
day of operation, in Btu/day. (b) Evaluating electricity at 8 cents
per kW ? h, determine the minimumtheoretical cost to operate the
heat pump, in $/day.
