Laws Of Thermodynamics

Question 1

What is a thermodynamic system?

Answer 1

A thermodynamic system is the matter enclosed within an arbitrary but precisely defined system boundary. Everything external to the system is defined as the surroundings or environment.

Question 2

What is an open system?

Answer 2

A system that has mass flowing across system boundaries.

Question 3

What is a steady state system?

Answer 3

An open system that has mass entering and exiting at the same rate.

Question 4

What is a closed system?

Answer 4

A system that has no mass crossing the system boundaries. Closed systems can have variable volumes.

Question 5

What is an isolated system?

Answer 5

A system that has neither mass nor energy crossing the system boundary.

Question 6

Adiabatic Process

Answer 6

A process in which no heat energy crosses the system boundary. This includes throttling and isentropic processes.

Question 7

Isentropic Process

Answer 7

An adiabatic process in which there is no entropy production. It is reversible.

Question 8

Throttling Process

Answer 8

An adiabatic process in which there is no change in enthalpy, but for which there is a significant pressure drop.

Question 9

Isobaric Process

Answer 9

constant pressure processes

Question 10

Isothermal Process

Answer 10

constant temperature process

Question 11

Isometric/Isochoric Process

Answer 11

constant volume process

Question 12

Quasistatic/Quasiequilibrium Process

Answer 12

A process that can be divided into a series of infinitesimal deviations from equilibrium. During each step, the property changes are small, and all intensive properties are uniform throughout the system.

Question 13

Reversible Process

Answer 13

A process performed in such a way that, at the conclusion of the process, both the system and local surroundings can be restored to their initial states. Quasiequilibrium processes are considered to be reversible.

Question 14

Standard Sign Convention

Answer 14

An increase in the systems heat, enthalpy, entropy and internal energy would be positive, while a decrease would be negative. Work done by the system on the surroundings is positive and work done on the system by the surroundings is negative.

Question 15

First Law of Thermodynamics

Answer 15

Energy is not created nor destroyed. It changes forms and energy at the beginning of the process must equal the energy at the end of the process.

Question 16

FLT (Closed Systems)

Answer 16

The net heat energy, is equal to the addition of the net internal energy plus the work. This is true because heat energy entering the system can either be used to increase the temperature (internal energy) or be used to perform work on the surroundings. In most cases, the changes in potential and kinetic energies can be disregarded for closed systems.

Question 17

Reversible Boundary Work

Answer 17

The work done by or on a closed system during a process. It can be used with irreversible processes as long as the total work performed is recognized as the sum of reversible and irreversible parts.

Question 18

Special Cases of Closed Systems (Ideal Gases)

Answer 18

For a closed polytropic system. The ideal gas law and reversible boundary work will be effected by the type of process the ideal gas is going through.

Question 19

FLT (Open Systems)

Answer 19

The net heat gain is equal to the internal, potential, and kinetic energies. Plus shaft and reversible flow work. The subscripts i and e denote the conditions at the inlet and exit points of the system.

Question 20

Reversible Flow Work

Answer 20

The work required to cause the flow into the system against the exit pressure. Since reversible flow work is the work being done on the system, it is always negative.

Question 21

FLT (Non Steady Flow Open Systems)

Answer 21

If the system is not a steady flow system and more mass enters the system than exits it. Stationary mass will be stored within the system. Which will also store energy by virtue of its temperature (internal energy), pressure (flow energy), and elevation (potential energy). The rate of change in the energy storage within the system is represented by the change in product of the mass of the fluid in the system and specific internal energy storage of the system over time.

Question 22

Special Cases of Open Systems

Answer 22

For an adiabatic, steady flow polytropic system with negligible changes in potential and kinetic energies. The ideal gas law and reversible flow work will be effected by the type of process the gas or substance is going through.

Question 23

FLT (Steady Flow Open Systems)

Answer 23

If the mass flow rate is constant, the system is a steady flow system and the first law is known as the steady flow energy equation. Generally the kinetic and potential energy terms are insignificant compared with the thermal energy term. The specific enthalpy represents the combination of internal energy and reversible flow work. The work term, is the shaft work or shaft power.

Question 24

Shaft Work (Shaft Power)

Answer 24

The work that the steady flow device does on the surroundings. This device is also known as the output shaft and its used to transmit energy out of the system. Which is positive for turbines and combustion engines and negative for pumps and compressors.

Question 25

Nozzles and Diffusers

Answer 25

Flow through them is essentially adiabatic. No work is done on the fluid as it passes through, friction is minimal and potential energy changes can be neglected in most cases.

Question 26

Pumps and Compressors

Answer 26

A pump or compressor converts mechanical energy into fluid energy. Are assumed to be adiabatic and capable of isentropic compression. However due to inefficiencies, the actual exit enthalpy will deviate from the ideal isentropic enthalpy.

Question 27

Isentropic Efficiency (Pumps and Compressors)

Answer 27

The isentropic efficiency can be calculated by two methods. The first is using the ratio of the ideal work input to actual work input. The second is using the specific enthalpy. Actual properties are usually measured and known. Ideal properties are obtained assuming an isentropic process with the exit pressure not being affected by the inefficiency.

Question 28

Turbine

Answer 28

Turbines can generally be thought of as pumps operating in reverse. A turbine extracts energy from the fluid, converting fluid energy into mechanical energy. It is assumed to be adiabatic and capable of isentropic expansion. However due to inefficiencies, the actual exit enthalpy will deviate from the ideal isentropic enthalpy.

Question 29

Isentropic Efficiency (Turbine)

Answer 29

The isentropic efficiency can be calculated by two methods. The first is using the ratio of the actual work output to ideal work output. The second is using the specific enthalpy. Actual properties are usually measured and known. Ideal properties are obtained assuming an isentropic process with the exit pressure not being affected by the inefficiency.

Question 30

Evaporator

Answer 30

is a device that adds heat to the water at low (near atmospheric pressure) pressure and produces saturated steam. Evaporators add most, if not all of the heat of vaporization. Potential and kinetic energies can be ignored

Question 31

Boiler (Feedwater)

Answer 31

is a device that adds heat to water and produces superheated steam. The superheating may occur in the boiler, or there may be a separate unit known as a superheater. Boilers add most, if not all of the heat of vaporization. Superheaters add additional energy known as superheat or superheat energy. Potential and kinetic energies can be ignored.

Question 32

Condenser

Answer 32

A condenser is a device that removes heat from the saturated or superheated steam. Usually, all of the superheated energy is removed, leaving the steam as a saturated liquid. Some of the heat of vaporization may also be removed, resulting in a subcooled liquid. Potential and kinetic energies can be ignored.

Question 33

Heat Exchangers

Answer 33

A heat exchanger transfers heat from one fluid to another through a wall separating them. They have two working fluids, two entrances and two exits. No work is done within a heat exchanger, it’s usually well insulated (adiabatic) and the potential and kinetic energies of fluids can be ignored.

Question 34

Feedwater Heater

Answer 34

A feedwater heater uses steam to increase the temperature of water entering the steam generator. The steam can come from any waste steam source but is usually bled off from a turbine, while the water that is heated usually comes from a condenser. These operations can be adiabatic and have two or more input streams (a bleed steam input and a condenser liquid input).

Question 35

Open Feedwater Heaters

Answer 35

A open feedwater heater, also known as direct contact heater or mixing heater, physically mixes the steam and water.

Question 36

Closed Feedwater Heater

Answer 36

A closed feedwater heater, is a traditional closed heat exchanger, that can operate at either high or low pressures. There is no mixing of the water and steam in the closed feedwater heater and the cooled stream leaves the feedwater heater as a liquid.

Question 37

Second Law of Thermodynamics (Environment)

Answer 37

The entropy of the environment always increases in real processes.

Question 38

Second Law of Thermodynamics (Working Fluid)

Answer 38

A substance can be restored to its original state without increasing the entropy of the environment only in a reversible system.

Question 39

Second Law of Thermodynamics (Equipment)

Answer 39

A machine that restores the working fluid to its original state, requires a heat sink, or a machine rejects more energy than the useful work it performs.

Question 40

Second Law of Thermodynamics (Natural Processes)

Answer 40

A natural process that starts in one equilibrium state and ends in another will go in the direction that causes the entropy of the system and the environment to increase.

Question 41

Thermal Reservoir

Answer 41

A thermal reservoir is an infinite mass with a constant temperature. Since the reservoir has infinite mass, it’s temperature, does not change when energy is supplied or absorbed.

Question 42

Heat Source

Answer 42

When the thermal reservoir supplies energy, it is known as a heat source.

Question 43

Heat Sink

Answer 43

When the thermal reservoir absorbs energy, it is known as a heat sink.

Question 44

Entropy Production (Isothermal Process)

Answer 44

The total change in entropy when heat is transferred to or from the reservoir/system.

Question 45

What is the entropy production equal to?

Answer 45

Is the ratio of heat, that enters the reservoir/system, to the temperature of the reservoir.

Question 46

What are the entropy production

Answer 46

Entropy production cannot be less than zero. If a process is reversible then it can be equal to zero. If its irreversible then it has to be greater than zero.