ENERGY SYSTEM TYPES:
Isolated System
Isolated:
- An isolated system is one that is not
influenced in any way by the surroundings
- No energy in the form of heat or
work may cross the boundary of the system
- No mass may cross the boundary of the system
Open:
- An open system is one that may have a transfer of both
mass and energy with its surroundings
Closed:
- A closed system has no transfer of mass with its
surroundings, but may have a transfer of energy
Q
(either heat or work) with its surroundings
Closed System
Work
Energy
Changes
Energy
Changes
Heat
W
Energy Mass
Changes
PROPERTIES:
SYSTEMS/PROCESSES:
Intensive:
- Independent of mass and does not depend on how much of
substance is present
- For example: temperature, pressure
Extensive:
- Depends on mass (or how much of a substance is present)
- For example: volume, weight
- If you know two properties of a substance, the remaining
can be determined
- We always try to use INTENSIVE properties
- In order to make an extensive property intensive, divide
it by its mass
- Volume (V) is extensive; Volume / Mass = Specific Volume (v)
We will discuss in detail in future lessons the five (5) following “systems” individually to
determine the types of WORK On/By the system, or HEAT In/Out of the system:
- Reactor Vessel: RCS comes in with a certain amount of energy and
leaves with more energy due to the heat input from the fuel (Tc and Th)
- Steam Generator: Feedwater enters the SG with a certain amount of energy
and leaves as a saturated steam with more energy due to the heat IN from the RCS
- Turbine: Saturated steam enters the turbine with a certain amount of energy and leaves
as a wet vapor with less energy due to imparting its energy to the turbine blades to turn
the generator and make electricity (work BY)
- Condenser: Wet vapor enters the condenser and heat is removed to the Circ Water
system so a subcooled liquid can now be pumped back to the SG (heat OUT)
- Pump(s): A subcooled liquid (after loosing energy to the Circ Water System) has work
done on the system from secondary pumps (FW, Cond, Htr Drain) by increasing its
pressure to be used in the SG again (work ON)
Open System
TEMPERATURE SCALES/CONVERSIONS:
ABSOLUTE TEMPERATURE SCALES:
- Temperature is a measure of the molecular activity of a substance. The greater the
movement of molecules, the higher the temperature.
- The two temperature scales normally used are the Fahrenheit (F) and Celsius (C) scales
- These scales are based on a specification of the number of increments between the freezing
point and boiling point of water at standard atmospheric pressure
- Celsius scale has 100 units between these points
- Fahrenheit scale has 180 units
- The freezing point of water was selected as the zero point of the Celsius scale
- The temperature at which water boils was set at 100 on the Celsius scale and 212 on the
Fahrenheit scale
- The relationship between the scales is represented by the following equations:
- It is necessary to define an absolute temperature scale having
only positive values
- The absolute Celsius scale is called the Kelvin (K) scale
- The absolute Fahrenheit scale is called the Rankine (R) scale
- The zero points on both absolute scales represent where there
is no molecular motion of individual atoms
- The relationships between the absolute and relative
temperature scales are shown in the following equations:
°F = 32.0 + (9/5)°C
°C = (°F - 32.0)(5/9)
PRESSURE SCALES:
Pabs = Patm + Pgauge
Pabs = Patm - Pvac
- Each of these four scales is represented by the figure to the
right
0 psig
(0 psiv)
Atmospheric pressure
14.7 psiv
14.7 psia
Vacuum - negative
gauge pressure
Absolute
pressure
212
672
100
373
32
492
0
273
-460
0
– 273
0
Absolute
pressure
1.5 psia
0 psia
EXAMPLE:
What is the Rankine equivalent of 80°C?
- °F
= (9/5) °C + 32
= (9/5)(80) + 32
= 176 °F
- °R
= °F + 460
= 176 + 460
= 636 °R
What is the Kelvin equivalent of 80°F?
- °C
= (5/9) (°F - 32)
= (5/9) (80 - 32)
= 26.7°C
-°K
= °C + 273
= 26.7 + 273
= 299.7 °K
EXAMPLE:
PRESSURE RELATIONSHIPS:
24.7 psia
10 psig
Gauge
pressure
K = °C + 273
°K
°C
°K = °C + 273
* Some examples/calculations might show Absolute Pressure
as 15 psia instead of 14.7 psia (or Absolute Vacuum as 15 psiv)
Pressure
- Pressure is a measure of the force exerted per unit area on the boundaries
of a substance (or system)
- It is caused by the collisions of the molecules of the substance with the
boundaries of the system
- All values of absolute pressure are positive, because a negative value
would indicate tension, which is considered impossible in any fluid
- When pressure is measured relative to a perfect vacuum, it is called
absolute pressure (psia); when measured relative to atmospheric
pressure (14.7 psi), it is called gauge pressure (psig)
- Pressure gauges measure the difference between the pressure of the
fluid to which they are connected and that of the surrounding air
- The following equations show the relationships between absolute, gauge,
vacuum, and atmospheric pressures (as does the figure to the right):
°R = °F + 460
°R = °F + 460
°R
°F
- In addition to pounds per square inch,
pressure can be measured with reference
to the force that exists in a column of fluid
at a certain height (which is the case with
Condenser Vacuum)
- The most common of these are inches of
water or inches of mercury (Hg)
- Conversion factors for these two are:
- 14.7 psia = 408 inches of water, or,
- 0.433 psi = 1 foot of water
- 14.7 psia = 29.9 inches of Hg, or,
- 0.491 psi = 1 inch of Hg
- Assuming absolute pressure is 15 psia and
absolute vacuum is 30 inches Hg, you can
see that 1 psia = 2 inches Hg
A pressure gauge on a condenser reads
27 inches of mercury (Hg) vacuum. What
is the absolute pressure corresponding
to this vacuum? (Assume atmospheric
pressure of 15 psia and perfect vacuum
is 30 in Hg)
- 0 psia = 15 psiv (perfect vacuum)
- 15 psiv = 30 inches Hg
- 13.5 psiv = 27 inches Hg
- 30 in Hg - 27 in Hg = 3 in Hg
- 3 in Hg = 1.5 psia
* This point is shown on the figure
to the left
ENERGY, WORK, POWER, and HEAT TERMS:
- Energy is defined as the capacity of a system to perform work or produce heat
- Potential energy (PE) is defined as the energy of position
- Kinetic energy (KE) is the energy of motion
- Potential energy and kinetic energy are macroscopic forms of energy
- A substance can contain microscopic forms of energy include those due to the rotation,
vibration, translation, and interactions among the molecules of a substance
- These microscopic forms of energy are collectively called internal energy
- The specific internal energy (u) of a substance is its internal energy per unit mass
- A substance can also perform work due to the pressure and volume of the substance
(called P-V energy). If a substance is pressurized and allowed to expand, work will be done
- The specific P-V energy of a substance is the P-V energy per unit mass
- Enthalpy is the combination of 1) internal energies, and 2) P-V energies
- Specific enthalpy (h) is defined as h = u + Pν
- Even though Enthalpy cannot be directly measured, the Steam Tables
provide values for different pressures/temperatures of a fluid, therefore,
the difference in enthalpies can be easily determined
- For example, the difference between the enthalpy of the saturated steam
entering the turbine and the enthalpy of the wet vapor entering the
condenser, is the amount of work done by the turbine
- Kinetic energy, potential energy, internal energy, and P-V energy are
forms of energy that are properties of a system
- Work is not a property of a system, but a process done by or on a
system (defined as the force on an object through a distance; W = FD)
- Heat, like work, is energy in transit. The transfer of energy as heat occurs
at the molecular level as a result of a temperature difference
- The best way to quantify the definition of heat is to consider the
relationship between the amount of heat added to (Qin) a system or
removed from (Qout) a system and the change in the temperature of the system
- Power is defined as the time rate of doing work (equivalent to the rate of the
energy transfer)
- In the English system, the mechanical units of power are foot-pounds-force per
second or per hour (ft-lbf/sec or ft-lbf/hr) and horsepower (hp)
- The thermal units of power are British Thermal Units per hour (Btu/hr), and the
electrical units of power are watts (W) or kilowatts (kW)
OPS Generic Fundamentals - PWR
193001 - Thermodynamics - Units/Properties
Rev Date: Apr28, 2016
For Training Use Only