Thermodynamic Processes

Question 1

Explain the relationship between real and ideal processes.

Answer 1

All real processes are irreversible; they are not done infinitely slowly. For any given volume, there is no well-defined pressure and temperature, since the system is not always in equilibrium.

Real processes can be compared to ideal to see how well the process or system is designed or built.

Question 2

Explain the shape of the temperature-entropy (T-s) diagram process line for a typical boiler.

Answer 2

Because feedwater is normally subcooled, point 1 begins in the compressed liquid region. For a non-superheated plant, the steam exits with a quality greater than ninety-nine percent, so the process line would end on the isotherm (isobar) in the wet vapor region before reaching the Saturated Vapor Line.

Question 3

Solve problems applying the General Energy Equation to secondary processes.

Answer 3

KE₁ + PE₁ + P₁V₁ + U₁ + Q = KE₂ + PE₂ +P₂V₂ +U₂ + W

Q= H₂ - H₁

Question 4

Describe nozzle processes.

Answer 4

Nozzles are used to change the energy of a working fluid from one form of energy to another. Depending on the type of nozzle, the kinetic energy of the fluid will increase or decrease. There are several types of nozzles that perform different functions.

Nozzle Equation:

KE₂ - KE₁ = H_in-H_out

Question 5

Solve nozzle process problems, applying the General Energy Equation.

Answer 5

Nozzle Kinetic Energy to Enthalpy Equation:

KE₂ - KE₁ = H_in - H_out

Question 6

Describe the functions of nozzles in flow restrictors.

Answer 6

One simple use is as a flow restriction. The venturi-type nozzle is located in the main steam piping. The purpose of this nozzle is to limit the flow of steam to the atmosphere in the event of a downstream, main steam line (MSL) rupture. The nozzle serves as a flow restrictor by reducing the area available for the steam flow. During normal operation, this nozzle is sufficiently sized to permit normal steam flow rates.

Question 7

Describe the functions of nozzles in air ejectors.

Answer 7

In the convergence of the first nozzle steam pressure falls and velocity increases, it passes through the throat with no change and exits the divergent zone into the suction box, here the steam velocity goes supersonic and pressure drops further which sucks non-condensable gases from the condenser. The steam and gases enter the convergent diffuser where pressure increases and velocity decreases. The combined flow exits the divergent divuser nozzle pressure increasing and velocity falling subsonic.

Question 8

Describe the functions of nozzles in air ejectors.

Answer 8

In the convergence of the first nozzle steam pressure falls and velocity increases, it passes through the throat with no change and exits the divergent zone into the suction box, here the steam velocity goes supersonic and pressure drops further which sucks non-condensable gases from the condenser. The steam and gases enter the convergent diffuser where pressure increases and velocity decreases. The combined flow exits the divergent divuser nozzle pressure increasing and velocity falling subsonic.

Question 9

Describe the functions of nozzles in turbines.

Answer 9

In the turbine, the steam velocity is increased as it passes through a nozzle. This converts the steam thermal energy to kinetic energy by expanding the steam from a higher to a lower pressure.

Question 10

What is the Continuity Equation for steady flow processes?

Answer 10

Question 11

What happens to steam pressure and velocity in a converging nozzle?

Answer 11

An increase in velocity in a converging nozzle is accompanied by a decrease in pressure. (area ↓, velocity ↑, and pressure ↓)

But under supersonic flow conditions: (area ↓, velocity ↓, and pressure ↑)

Question 12

What happens to steam pressure and velocity in a diverging nozzle?

Answer 12

Velocity and kinetic energy decrease, and the pressure must have increase in a diverging nozzle. (area ↑, velocity ↓, and pressure ↑)

But in supersonic flow conditions: area ↑, velocity ↑, and pressure ↓

Question 13

Describe the principals of operation of an air ejector.

Answer 13

Steam under pressure enters the convergent nozzle. The effect is expected (area ↓, velocity ↑, and pressure ↓).

As the sonic steam passes through the constant area of the throat of the nozzle, the velocity and pressure remain constant. At the exit of the throat, the area begins to increase. Here the velocity of the steam is dependent on the pressure downstream which is the same as the vacuum pressure in the main condenser. Flow velocity becomes supersonic. This effect is different from the expected responses of subsonic flow in a nozzle. Here the energy conversion does not follow expected subsonic responses (area ↑, velocity ↓, and pressure ↑) but instead the area ↑, velocity ↑, and pressure ↓.

The air and other gases are mechanically entrained with the high velocity steam. The high velocity steam molecules impinge the gas molecules, providing kinetic energy to the gas molecules. The energetic gas molecules travel with the steam out of the suction chamber and into the next convergent-divergent nozzle. This
reduces the number of molecules in the suction chamber area and allows other molecules from the condenser to be drawn into the suction chamber.

The pressure seen downstream of the convergent-divergent nozzle is now the off-gas system pressure. Off-gas system pressure is slightly higher than atmospheric pressure. To slow the supersonic flow before exiting the ejector, it is slowed by use of a convergent nozzle. Contrary to expected results in subsonic flow, as the supersonic steam enters the convergent nozzle, as the nozzle area decreases
the velocity of the steam and entrained molecules decrease and the pressure increases (area ↓, velocity ↓, and pressure ↑). Again this is opposite of expected results in subsonic flow conditions.

As the mixture flows through the constant area of the throat the velocity becomes sonic. The steam then enters the throat at sonic speed. The velocity and pressure remain constant through the throat. The steam mixture enters the final divergent
nozzle (also known as the subsonic diffuser). As the area of the nozzle increases, the velocity of the steam decreases to subsonic speeds and the pressure increases to the pressure of the off gas system. This energy conversion is as expected (area ↑, velocity ↓, and pressure ↑).

Question 14

Draw the SJAE and Pressure/Velocity Changes through the system.

Answer 14

Question 15

What are the two types of turbines?

Answer 15

impulse turbines and reaction turbines.

Impulse turbines require high velocity steam for efficient operation.

Reaction turbines are more efficient when using a lower velocity steam.

Question 16

Explain the function of nozzles, fixed blading and moving blading in the turbine.

Answer 16

The convergent nozzles in the turbine casing accelerate and direct the steam into the blades, causing them to move. Impulse turbines have dished blades or buckets.

When more than one set of moving blades follows, fixed impulse blades are located between the moving blades to redirect the steam.

Question 17

Explain the reason turbines are multi-staged.

Answer 17

Staging and compounding of impulse turbines are methods of removing a greater fraction of the energy stored in the inlet steam.

Question 18

What is a pressure compounded turbine?

Answer 18

A pressure compounded turbine can extract energy by successive pressure drops. Since pressure decreases occur only through nozzles, a pressure compounded
turbine must consist of two or more stages.

Question 19

What is a velocity compounded turbine?

Answer 19

A velocity compunded turbine can extract energy by successive velocity decreases, A velocity-compounded turbine may consist of only one stage: a nozzle followed by a set of moving blades, a set of fixed blades, and another set of moving blades.

Question 20

Distinguish between actual turbine performance and ideal thermal efficiency, using a temperature-entropy (T-s) diagram.

Answer 20

The solid line, from point 1 to point 2, corresponds to the ideal process in which steam is isentropically expanded through the turbine rotors. The dashed line, from point 1 to point 2’, corresponds to the real irreversible process. Shown is the actual exhaust enthalpy of the steam where Point 2’ is greater than the ideal exhaust enthalpy, point 2. This demonstrates that the work output of a real turbine is less than that of an ideal turbine.

Question 21

Distinguish between actual turbine performance and ideal thermal efficiency, using an enthalpy-entropy (h-s) diagram.

Answer 21

The solid line, from point 1 to point 2, corresponds to the ideal process in which steam is isentropically expanded through the turbine rotors. The dashed line, from point 1 to point 2’, corresponds to the real irreversible process. Shown is the actual exhaust enthalpy of the steam where Point 2’ is greater than the ideal exhaust enthalpy, point 2. This demonstrates that the work output of a real turbine is less than that of an ideal turbine.

Question 22

What is turbine efficiency and what is the formula?

Answer 22

Turbine efficiency (η_t) is the ratio of work done by a real turbine to the work done by an ideal turbine in accomplishing the same gas expansion.

Question 23

What is the general energy equation for a pump?

Answer 23

The General Energy Equation for a pump can be reduced to:
P₁V₁ + U₁ = P₂V₂ + U₂ + W
The equation can be further simplified by combining terms into enthalpy and dividing
through by mass to make the equation:
W = H_in - H_out

To obtain the total rate of work, multiply both sides by the mass flow rate, but leave the enthalpy terms as specific by not multiplying through yet.

W_p= M(H_in - H_out)

Question 24

Distinguish between ideal and real pumping processes

on an enthalpy-entropy (h-s) diagram.

Answer 24

The solid lines (point 1 to point 2) represent the ideal isentropic pumping process, and the dashed lines (point 1 to point 2′) represent the real irreversible process.

Question 25

Distinguish between ideal and real pumping processes on a

temperature-entropy (T-s) diagram.

Answer 25

The solid lines (point 1 to point 2) represent the ideal isentropic pumping process, and the dashed lines (point 1 to point 2′) represent the real irreversible process.

Question 26

Define pump efficiency.

Answer 26

In order to relate a real pump to an ideal pump, pump efficiency (η_p) is defined as the ratio of the work required by the pump if it were an ideal pump ( W _{<span>p, ideal</span>}) and the real work required by the pump ( W_{p, real} ).

Question 27

Solve condensing process problems, applying the General Energy Equation.

Answer 27

General Energy Equation for a condenser/heat exchanger:

q = h_out - h_in

Question 28

Explain vacum formation in condenser processes.

Answer 28

Steam condenses on the tubes, and its specific volume
decreases, helping to maintain a vacuum in the
condenser shell.

Question 29

Explain the condensing process using an enthalpy-entropy (h-s) diagram.

Answer 29

Steam enters the condenser from the turbine as a wet vapor. The steam gives
up its latent heat of vaporization to the circulating water and changes phase to a
saturated liquid. The temperature of the circulating water increases, and the
temperature of the steam remains the same until all the steam is converted into
a liquid. From point 1 to point 2, the steam condenses on the tubes, and its
specific volume decreases. Some additional heat is removed once the steam is condensed. This is represented as difference between point 2 and point 2′. This causes the temperature to decrease below saturation temperature.

Question 30

Explain the condensing process using an temperature-enetropy (t-s) diagram.

Answer 30

Steam enters the condenser from the turbine as a wet vapor. The steam gives
up its latent heat of vaporization to the circulating water and changes phase to a
saturated liquid. The temperature of the circulating water increases, and the
temperature of the steam remains the same until all the steam is converted into
a liquid. From point 1 to point 2, the steam condenses on the tubes, and its
specific volume decreases. Some additional heat is removed once the steam is condensed. This is represented as difference between point 2 and point 2′. This causes the temperature to decrease below saturation temperature.

Question 31

Describe subcooling.

Answer 31

• Cooling the condensate below saturation temperature is called subcooling and is
• desirable to a small extent.
• A few degrees subcooling are necessary to prevent cavitation in the condensate pumps.
• Excessive condensate depression decreases the operating efficiency of the plant since the subcooled condensate must be reheated in the feedwater heaters and steam generators, thus requiring more heat from the primary loop and reactor.
• Excessive condensate depression also allows for an increased absorption of air by the condensate because cooler water can hold more dissolved oxygen. This results in accelerated oxygen corrosion of plant materials.

Question 32

What is the formula for condensate depression.

Answer 32

Condensate Depression = T_sat − T_actual

Where:

Condensate Depression = number of degrees of condensate depression (°F)
T_sat = temperature of saturated liquid at the given pressure or vacuum (°F)
T_actual = temperature of subcooled liquid being considered (°F)

Question 33

Describe throttling

Answer 33

A throttling process is one in which the fluid is made to flow through a restriction (for example, a partially opened valve, or orifice, although the concept also applies to a pipe break) causing a considerable drop in the pressure of the fluid. The main effect is a significant pressure drop without any work interactions or changes in kinetic or potential energy. This may also result in a small increase in the internal energy.

Question 34

Explain the reduction of process pressure from

throttling using an enthalpy-entropy (h-s) diagram.

Answer 34

Throttling is essentially a constant enthalpy process, as shown in the h-s and T-s diagrams in Figure 4-18. Enthalpy remains constant while entropy increases. No work is done and no heat is added. The result is a pressure drop and a slight increase in velocity. The change in specific volume depends on the fluid being processed. Throttling is inherently irreversible with no ideal equivalent. For this reason, point 2′ is used in Figure 4-18. As the pressure decreases from P₁ to P₂′, enthalpy is constant and entropy increases. If the pressure decreases to a value found above or to the right of the saturated vapor line (point 3), then the process fluid (steam) will be in the superheated region. The value of throttling lies in the ability to provide a simple means of controlling equipment and processes.

Question 35

Explain the reduction of process pressure from throttling using a temperature-entropy diagram.

Answer 35

Throttling is essentially a constant enthalpy process, as shown in the h-s and T-s diagrams in Figure 4-18. Enthalpy remains constant while entropy increases. No work is done and no heat is added. The result is a pressure drop and a slight increase in velocity. The change in specific volume depends on the fluid being processed. Throttling is inherently irreversible with no ideal equivalent. For this reason, point 2′ is used in Figure 4-18. As the pressure decreases from P1 to P2′, enthalpy is constant and entropy increases. If the pressure decreases to a value found above or to the right of the saturated vapor line (point 3), then the process fluid (steam) will be in the superheated region. The value of throttling lies in the ability to provide a simple means of controlling equipment and processes.