State and Equilibrium
State Postulate: State of a simple, compressible
system is completely defined by two
independent, intensive properties
Keywords: State, simple compressible,
independent, intensive
Process and Cycles
Quasi-equilibrium
Isothermal, isobaric, adiabatic
Forms of Energy
Microscopic or macroscopic
Internal energy – sensible, latent, chemical, nuke
Kinetic and potential energies
Heat and Work
Heat – temperature difference (3 forms)
Work – anything else (shaft, electricity,
boundary, spring, etc.)
Sign conventions, path vs. point functions
Characteristics of heat and work
Energy Efficiency
Desired output / required input
Heating value of a fuel
Propagating efficiencies
Example: Water Heater
Gas:
55%
Electric: 90%
Why would gas be better?
Example
Your boss is inspired to “do something” about
global warming. He decides to replace all of the
100 W lightbulbs in your company’s building
(800 total) with compact florescent bulbs (19 W).
The cfl’s cost $2.00 each, plus $0.75 labor to
install. Electricity costs your company $0.09 /
kW-hr and the bulbs are on for an average of 12
hours per day, five days per week. The bulbs are
expected to last 5 years each. Does the energy
savings justify the added cost of the bulbs?
Phase Change Processes
Lets boil some water…
Property Tables
See handout…
Given
Determine
80oC, 200 kPa
v
225oC, 100 kPa
h
150oC, X = 0.57, 2 kg
P, V
500 kPa, 0.200 m3/kg
u
The evaporator of a refrigerator operates
with R-134a at a pressure of 120 kPa.
Assuming that the fluid in the evaporator is a
saturated mixture, what is the temperature in
the evaporator?
One kg of R134a fills a 0.14 m3 weighted
piston cylinder device at a temperature of 26.4oC. The container is heated until the
temperature is 100oC. Determine the final
volume of the R-134a.
A piston-cylinder device initially contains 1.4
kg saturated liquid water at 200oC. Now heat
is transferred to the water until the volume
quadruples and the cylinder contains
saturated vapor only. Determine
(a) the initial volume of the tank
(b) the final temperature and pressure
(c) the internal energy change of the water
Determine the specific volume of
superheated water vapor at 10 MPa and
400oC, using
(a) the ideal gas equation
(b) the generalized compressibility chart
(c) the steam tables
A person boils water in a 30 cm diameter pot
that is covered with a well fitting lid. The
rising water vapor displaces all of the air in
the pot. Then, it is removed from the heat
and allowed to cool to the room temperature
of 20oC. The total mass of water and pot is 4
kg. Now, the person tries to open the pot by
lifting the lid up. Assuming that no air has
leaked into the pot during cooling, determine
if the lid will open or if the pot will move up
with the lid.