Steam Engineering

Enthalpy Of Evaporation

2009-04-05T05:33:00.000-07:00

Enthalpy of evaporation or latent heat (hfg)

This is the amount of heat required to change the state of water at its boiling temperature, into steam. It involves no change in the temperature of the steam/water mixture, and all the energy is used to change the state from liquid (water) to vapour (saturated steam).

The old term latent heat is based on the fact that although heat was added, there was no change in temperature. However, the accepted term is now enthalpy of evaporation.

Like the phase change from ice to water, the process of evaporation is also reversible. The same amount of heat that produced the steam is released back to its surroundings during condensation, when steam meets any surface at a lower temperature.

This may be considered as the useful portion of heat in the steam for heating purposes, as it is that portion of the total heat in the steam that is extracted when the steam condenses back to water.Reference:Google Search

Enthalpy

2009-04-05T05:29:00.000-07:00

Enthalpy
A few quantities called "thermodynamic potentials" are useful in the chemical thermodynamics of reactions and non-cyclic processes. They are internal energy, the enthalpy, the Helmholtz free energy and the Gibbs free energy.
But lets consider enthalpy and internal energy
Enthalpy is defined by

H = U + PV
where P and V are the pressure and volume, and U is internal energy. Enthalpy is then a precisely measurable state variable, since it is defined in terms of three other precisely definable state variables. It is somewhat parallel to the first law of thermodynamics for a constant pressure system

Q = ΔU + PΔV since in this case Q=ΔH

It is a useful quantity for tracking chemical reactions. If as a result of an exothermic reaction some energy is released to a system, it has to show up in some measurable form in terms of the state variables. An increase in the enthalpy H = U + PV might be associated with an increase in internal energy which could be measured by calorimetry, or with work done by the system, or a combination of the two.

The internal energy U might be thought of as the energy required to create a system in the absence of changes in temperature or volume. But if the process changes the volume, as in a chemical reaction which produces a gaseous product, then work must be done to produce the change in volume. For a constant pressure process the work you must do to produce a volume change ΔV is PΔV. Then the term PV can be interpreted as the work you must do to "create room" for the system if you presume it started at zero volume.

Reference: Google Search

Entropy

2009-04-05T05:26:00.000-07:00

Source http://www.engineeringtoolbox.com/steam-entropy-d_99.html

Entropy is the quantitative measure of disorder in a system. The concept comes out of thermodynamics, which deals with the transfer of heat energy within a system. Instead of talking about some form of "absolute entropy," physicists generally talk about the change in entropy that takes place in a specific thermodynamic process.
Calculating Entropy
In an isothermal process, the change in entropy (delta-S) is the change in heat (Q) divided by the absolute temperature (T):

delta-S = Q/T
In any reversible thermodynamic process, it can be represented in calculus as the integral from a processes initial state to final state of dQ/T.
The SI units of entropy are J/K (joules/degrees Kelvin).

Entropy as Time's Arrow

Reference:Google Search

Reversible Process

2009-04-05T05:19:00.000-07:00

A reversible process ia a process that once taken place can be reversed ao that the system and surroundings are returned back to original conditions. In reality, there is no reversible process but for analysis purpose, reversible is use to make the analysis simpler.

Reference :http://www.engineersedge.com/thermodynamics/reversible_process.htm

Irriversible Process

2009-04-04T22:50:00.000-07:00

Irreversible Process is a process that cannot return both the system and surroundings to their original condition, even though the process is reversed. Example an engine does not return back the fuel it consumed during climbing the hill,when it coasting back down the hill.There are a number of factors that contribute the the irreversibility of a process, among them are frictions,unrestrained expansion of fluid,heat transfer through a finite temperature change, mixing of two substances.

Reference:http://www.engineersedge.com/thermodynamics/irreversible_process.htm

Polytropic

2009-04-04T22:46:00.000-07:00

From roymech.co.uk.

From Wikipedia
A polytropic process is a thermodynamic process that obeys the relation:

PVn = C,
where P is pressure, V is volume, n is any real number (the polytropic index), and C is a constant. This equation can be used to accurately characterize processes of certain systems, notably the compression or expansion of a gas, but in some cases, possibly liquids and solids.

Under standard conditions, most gases can be accurately characterized by the ideal gas law. This construct allows for the pressure-volume relationship to be defined for essentially all ideal thermodynamic cycles, such as the well-known Carnot cycle. (Note however that there may be instances where a polytropic process occurs in a non-ideal gas.)

For certain indices n, the process will be synonymous with other processes:

if n = 0, then PV0=P=const and it is an isobaric process (constant pressure)
if n = 1, then for an ideal gas PV=NkT=const and it is an isothermal process (constant temperature)
if n = γ = cp/cV, then for an ideal gas it is an adiabatic process (no heat transferred)
Note that , since (see: adiabatic index)
if n = , then it is an isochoric process (constant volume)
When the index n occurs between any two of the former values (0,1,gamma, or infinity), it means that the polytropic curve will lie between the curves of the two corresponding indices.

The equation is a valid characterization of a thermodynamic process assuming that:

The process is quasistatic
The values of the heat capacities,cp and cV, are almost constant when 'n' is not zero or infinity. (In reality, cp and cV are a function of temperature, but are nearly linear within small changes of temperature).

Reference:From Google Search

Isothermal

2009-04-04T22:45:00.001-07:00

From http://commons.wikimedia.org/wiki/File:Isothermal_process.png
An isothermal process is a change in which the temperature of the system stays constant: ΔT = 0. This typically occurs when a system is in contact with an outside thermal reservoir (heat bath), and the change occurs slowly enough to allow the system to continually adjust to the temperature of the reservoir through heat exchange. An alternative special case in which a system exchanges no heat with its surroundings (Q = 0) is called an adiabatic process. In other words, in an isothermal process, the value ΔT = 0 but Q ≠ 0, while in an adiabatic process, ΔT ≠ 0 but Q = 0.

From Wikipedia

Reference :From Google Search

Isochoric

2009-04-04T22:43:00.000-07:00

From Wikipedia
An isochoric process, also called an isovolumetric process, is a process during which volume remains constant. The name is derived from the Greek isos, "equal", and khora, "place."

If an ideal gas is used in an isochoric process, and the quantity of gas stays constant, then the increase in energy is proportional to an increase in temperature and pressure. Take for example a gas heated in a rigid container: the pressure and temperature of the gas will increase, but the volume will remain the same.

Isochoric Process in the Pressure volume diagram. In this diagram, pressure increases, but volume remains constant.
In the ideal Otto cycle we found an example of an isochoric process when we assume an instantaneous burning of the gasoline-air mixture in an internal combustion engine car. There is an increase in the temperature and the pressure of the gas inside the cylinder while the volume remains the same.
Reference:From Google search

Isobaric

2009-04-04T22:42:00.000-07:00

From Wikipedia
An isobaric process is a thermodynamic process in which the pressure stays constant: Δp = 0 The term derives from the Greek isos, "equal," and barus, "heavy." The heat transferred to the system does work but also changes the internal energy of the system:

According to the first law of thermodynamics, where W is work done by the system, U is internal energy, and Q is heat. Pressure-volume work (by the system) is defined as: (Δ means change over the whole process, it doesn't mean differential)

but since pressure is constant, this means that

An isobaric process is shown on a P-V diagram as a straight horizontal line, connecting the initial and final thermostatic states. If the process moves towards the right, then it is an expansion. If the process moves towards the left, then it is a compression.
Reference: Fom Google Search

Isentropic

2009-04-04T22:41:00.000-07:00

From Wikipedia
In thermodynamics, an isentropic process (iso = "equal" (Greek); entropy = "disorder") is one during which the entropy of the system remains constant. [1][2] It can be proved that any reversible adiabatic process is an isentropic process.

Second law of thermodynamics states that,

where δQ is the amount of energy the system gains by heating, T is the temperature of the system, and dS is the change in entropy. The equal sign will hold for a reversible process. For a reversible isentropic process, there is no transfer of heat energy and therefore the process is also adiabatic. For an irreversible process, the entropy will increase. Hence removal of heat from the system (cooling) is necessary to maintain a constant internal entropy for an irreversible process so as to make it isentropic. Thus an irreversible isentropic process is not adiabatic.

For reversible processes, an isentropic transformation is carried out by thermally "insulating" the system from its surroundings. Temperature is the thermodynamic conjugate variable to entropy, and so the conjugate process would be an isothermal process in which the system is thermally "connected" to a constant-temperature heat bath.

An isentropic flow is a flow that is both adiabatic and reversible. That is, no energy is added to the flow, and no energy losses occur due to friction or dissipative effects. For an isentropic flow of a perfect gas, several relations can be derived to define the pressure, density and temperature along a streamline.

Derivation of the isentropic relations
For a closed system, the total change in energy of a system is the sum of the work done and the heat added,

The work done on a system by changing the volume is,

where p is the pressure and V the volume. The change in enthalpy () is given by,

Since a reversible process is adiabatic (i.e. no heat transfer occurs), so .

This leads to two important observations,
,
and

or

=>

Reference:From Google Search

Isenthalpic

2009-04-04T22:39:00.000-07:00

From Wikipedia

An isenthalpic process is one that proceeds without any change in enthalpy, H; or specific enthalpy, h.[1]

In a steady-state, steady-flow process, significant changes in pressure and temperature can occur to the fluid and yet the process will be isenthalpic if there is no transfer of heat to or from the surroundings, no work done on or by the surroundings, and no change in the kinetic energy of the fluid.[2] (If a steady-state, steady-flow process is analysed using a control volume everything outside the control volume is considered to be the surroundings.[3])

The throttling process is a good example of an isenthalpic process. Consider the lifting of a relief valve or safety valve on a pressure vessel. The specific enthalpy of the fluid inside the pressure vessel is the same as the specific enthalpy of the fluid as it escapes from the valve.[2] With a knowledge of the specific enthalpy of the fluid, and the pressure outside the pressure vessel, it is possible to determine the temperature and speed of the escaping fluid.

In an isenthalpic process:

h1 = h2
dh = 0
Isenthalpic processes on an ideal gas follow isotherms since dh = Cp0(T2 − T1)

Reference: from Google Search

Adiabatic

2009-04-04T22:38:00.000-07:00

From Wikimedia

In thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which no heat is transferred to or from the working fluid. The term "adiabatic" literally means impassable (from Greek ἀ-διὰ-βαῖνειν, not-through-to pass), corresponding here to an absence of heat transfer. Conversely, a process that involves heat transfer (addition or loss of heat to the surroundings) is generally called diabatic.

For example, an adiabatic boundary is a boundary that is impermeable to heat transfer and the system is said to be adiabatically (or thermally) insulated; an insulated wall approximates an adiabatic boundary. Another example is the adiabatic flame temperature, which is the temperature that would be achieved by a flame in the absence of heat loss to the surroundings. An adiabatic process that is reversible is also called an isentropic process. Additionally, an adiabatic process that is irreversible and extracts no work is in an isenthalpic process, such as viscous drag, progressing towards a nonnegative change in entropy.
From Wikipedia

Reference: From Google Search

Saturated Steam Table

2009-04-04T22:37:00.001-07:00

The saturated steam tables
The steam tables list the properties of steam at varying pressures. They are the results of actual tests carried out on steam. Table 2.2.1 shows the properties of dry saturated steam at atmospheric pressure - 0 bar g.

Table 2.2.1 Properties of saturated steam at atmospheric pressure

Example 2.2.1
At atmospheric pressure (0 bar g), water boils at 100°C, and 419 kJ of energy are required to heat 1 kg of water from 0°C to its saturation temperature of 100°C. Therefore the specific enthalpy of water at 0 bar g and 100°C is 419 kJ/kg, as shown in the steam tables (see Table 2.2.2).

Another 2 257 kJ of energy are required to evaporate 1 kg of water at 100°C into 1 kg of steam at 100°C. Therefore at 0 bar g the specific enthalpy of evaporation is 2 257 kJ/kg, as shown in the steam tables (see Table 2.2.2).
However, steam at atmospheric pressure is of a limited practical use. This is because it cannot be conveyed under its own pressure along a steam pipe to the point of use.

Note: Because of the pressure/volume relationship of steam, (volume is reduced as pressure is increased) it is usually generated in the boiler at a pressure of at least 7 bar g. The generation of steam at higher pressures enables the steam distribution pipes to be kept to a reasonable size.

As the steam pressure increases, the density of the steam will also increase. As the specific volume is inversely related to the density, the specific volume will decrease with increasing pressure.

Figure 2.2.2 shows the relationship of specific volume to pressure. This highlights that the greatest change in specific volume occurs at lower pressures, whereas at the higher end of the pressure scale there is much less change in specific volume.

Fig. 2.2.2 Steam pressure/specific volume relationship

The extract from the steam tables shown in Table 2.2.2 shows specific volume, and other data related to saturated steam.

At 7 bar g, the saturation temperature of water is 170°C. More heat energy is required to raise its temperature to saturation point at 7 bar g than would be needed if the water were at atmospheric pressure. The table gives a value of 721 kJ to raise 1 kg of water from 0°C to its saturation temperature of 170°C.

The heat energy (enthalpy of evaporation) needed by the water at 7 bar g to change it into steam is actually less than the heat energy required at atmospheric pressure. This is because the specific enthalpy of evaporation decreases as the steam pressure increases.

However, as the specific volume also decreases with increasing pressure, the amount of heat energy transferred in the same volume actually increases with steam pressure.

Table 2.2.2 Extract from the saturated steam tables

Reference: Google Search

Dryness Fraction

2009-04-04T22:35:00.000-07:00

Dryness fraction
Steam with a temperature equal to the boiling point at that pressure is known as dry saturated steam. However, to produce 100% dry steam in an industrial boiler designed to produce saturated steam is rarely possible, and the steam will usually contain droplets of water.

In practice, because of turbulence and splashing, as bubbles of steam break through the water surface, the steam space contains a mixture of water droplets and steam.

Steam produced in any shell-type boiler (see Block 3), where the heat is supplied only to the water and where the steam remains in contact with the water surface, may typically contain around 5% water by mass.

If the water content of the steam is 5% by mass, then the steam is said to be 95% dry and has a dryness fraction of 0.95.

The actual enthalpy of evaporation of wet steam is the product of the dryness fraction () and the specific enthalpy (hfg) from the steam tables. Wet steam will have lower usable heat energy than dry saturated steam.

Equation 2.2.2
Therefore:

Equation 2.2.3
Because the specfic volume of water is several orders of magnitude lower than that of steam, the droplets of water in wet steam will occupy negligible space. Therefore the specific volume of wet steam will be less than dry steam:

Equation 2.2.4
Where vg is the specific volume of dry saturated steam.

Example 2.2.2
Steam at a pressure of 6 bar g having a dryness fraction of 0.94 will only contain 94% of the enthalpy of evaporation of dry saturated steam at 6 bar g. The following calculations use figures from steam tables:

The steam phase diagram
The data provided in the steam tables can also be expressed in a graphical form. Figure 2.2.3 illustrates the relationship between the enthalpy and temperature of the various states of water and steam; this is known as a phase diagram.

Fig. 2.2.3 Temperature enthalpy phase diagram
As water is heated from 0°C to its saturation temperature, its condition follows the saturated water line until it has received all of its liquid enthalpy, hf, (A - B).

If further heat continues to be added, the water changes phase to a water/vapour mixture and continues to increase in enthalpy while remaining at saturation temperature ,hfg, (B - C).

As the water/vapour mixture increases in dryness, its condition moves from the saturated liquid line to the saturated vapour line. Therefore at a point exactly halfway between these two states, the dryness fraction () is 0.5. Similarly, on the saturated steam line the steam is 100% dry.

Once it has received all of its enthalpy of evaporation, it reaches the saturated steam line. If it continues to be heated after this point, the pressure remains constant but the temperature of the steam will begin to rise as superheat is imparted (C - D).

The saturated water and saturated steam lines enclose a region in which a water/vapour mixture exists - wet steam. In the region to the left of the saturated water line only water exists, and in the region to the right of the saturated steam line only superheated steam exists.

The point at which the saturated water and saturated steam lines meet is known as the critical point. As the pressure increases towards the critical point the enthalpy of evaporation decreases, until it becomes zero at the critical point. This suggests that water changes directly into saturated steam at the critical point.

Above the critical point the steam may be considered as a gas. The gaseous state is the most diffuse state in which the molecules have an almost unrestricted motion, and the volume increases without limit as the pressure is reduced.

The critical point is the highest temperature at which water can exist. Any compression at constant temperature above the critical point will not produce a phase change.

Compression at constant temperature below the critical point however, will result in liquefaction of the vapour as it passes from the superheated region into the wet steam region.

The critical point occurs at 374.15°C and 221.2 bar a for steam. Above this pressure the steam is termed supercritical and no well-defined boiling point applies.

Reference:From Google Search

Properties of Steam

2009-04-04T21:38:00.001-07:00

A better understanding of the properties of steam may be achieved by understanding the general molecular and atomic structure of matter, and applying this knowledge to ice, water and steam.

A molecule is the smallest amount of any element or compound substance still possessing all the chemical properties of that substance which can exist. Molecules themselves are made up of even smaller particles called atoms, which define the basic elements such as hydrogen and oxygen.

The specific combinations of these atomic elements provide compound substances. One such compound is represented by the chemical formula H2O, having molecules made up of two atoms of hydrogen and one atom of oxygen.

The reason water is so plentiful on the earth is because hydrogen and oxygen are amongst the most abundant elements in the universe. Carbon is another element of significant abundance, and is a key component in all organic matter.

Most mineral substances can exist in the three physical states (solid, liquid and vapour) which are referred to as phases. In the case of H2O, the terms ice, water and steam are used to denote the three phases respectively.

The molecular structure of ice, water, and steam is still not fully understood, but it is convenient to consider the molecules as bonded together by electrical charges (referred to as the hydrogen bond). The degree of excitation of the molecules determines the physical state (or phase) of the substance.

Reference:Google Search :Steam Properties

Thermodynamic Cycles

2009-04-04T21:36:00.000-07:00

From Wikipedia
A thermodynamic cycle is a series of thermodynamic processes which returns a system to its initial state. Properties depend only on the thermodynamic state and thus do not change over a cycle. Variables such as heat and work are not zero over a cycle, but rather are process dependent. The first law of thermodynamics dictates that the net heat input is equal to the net work output over any cycle. The repeating nature of the process path allows for continuous operation, making the cycle an important concept in thermodynamics. Thermodynamic cycles often use quasistatic processes to model the workings of actual devices.

Example of P-V diagram of a thermodynamic cycle.A thermodynamic cycle is a closed loop on a P-V diagram. A P-V diagrams X axis shows volume (V) and Y axis shows pressure (P). The area enclosed by the loop is the work (W) done by the process.

If the cyclic process moves clockwise around the loop, then it represents a heat engine, and W will be positive. If it moves counterclockwise then it represents a heat pump, and W will be negative.

The Carnot cycle
The Carnot cycle when acting as a heat engine consists of the following steps:

Reversible isothermal expansion of the gas at the "hot" temperature, TH (isothermal heat addition). During this step (A to B on Figure 1, 1 to 2 in Figure 2) the expanding gas causes the piston to do work on the surroundings. The gas expansion is propelled by absorption of quantity Q1 of heat from the high temperature reservoir.
Isentropic (Reversible adiabatic) expansion of the gas. For this step (B to C on Figure 1, 2 to 3 in Figure 2) we assume the piston and cylinder are thermally insulated, so that no heat is gained or lost. The gas continues to expand, doing work on the surroundings. The gas expansion causes it to cool to the "cold" temperature, TC.
Reversible isothermal compression of the gas at the "cold" temperature, TC. (isothermal heat rejection) (C to D on Figure 1, 3 to 4 on Figure 2) Now the surroundings do work on the gas, causing quantity Q2 of heat to flow out of the gas to the low temperature reservoir.
Isentropic compression of the gas. (D to A on Figure 1, 4 to 1 in Figure 2) Once again we assume the piston and cylinder are thermally insulated. During this step, the surroundings do work on the gas, compressing it and causing the temperature to rise to TH. At this point the gas is in the same state as at the start of step 1.

Ideal cycle

An illustration of an ideal cycle heat engine (arrows clockwise).An ideal cycle is constructed out of:

TOP and BOTTOM of the loop: a pair of parallel isobaric processes
LEFT and RIGHT of the loop: a pair of parallel isochoric processes

Otto Cycle
The Otto cycle

The four-stroke engine was first patented by Eugenio Barsanti and Felice Matteucci in 1854, followed by a first prototype in 1860. It was also conceptualized by French engineer, Alphonse Beau de Rochas in 1862.

However, the German engineer Nicolaus Otto was the first to develop a functioning four-stroke engine, which is why the four-stroke principle today is commonly known as the Otto cycle and four-stroke engines using spark plugs often are called Otto engines. The Otto Cycle consists of adiabatic compression, heat addition at constant volume, adiabatic expansion and rejection of heat at constant volume.

Stirling Cycle
A Stirling cycle is like an Otto cycle, except that the adiabats are replaced by isotherms.

TOP and BOTTOM of the loop: a pair of quasi-parallel isothermal processes
LEFT and RIGHT sides of the loop: a pair of parallel isochoric processes
Heat flows into the loop through the top isotherm and the left isochore, and some of this heat flows back out through the bottom isotherm and the right isochore, but most of the heat flow is through the pair of isotherms. This makes sense since all the work done by the cycle is done by the pair of isothermal processes, which are described by Q=W. This suggests that all the net heat comes in through the top isotherm. In fact, all of the heat which comes in through the left isochore comes out through the right isochore: since the top isotherm is all at the same warmer temperature TH and the bottom isotherm is all at the same cooler temperature TC, and since change in energy for an isochore is proportional to change in temperature, then all of the heat coming in through the left isochore is cancelled out exactly by the heat going out the right isochore.

Rankine Cycle
The Rankine cycle is a thermodynamic cycle which converts heat into work. The heat is supplied externally to a closed loop, which usually uses water as the working fluid. This cycle generates about 80% of all electric power used throughout the world.[1], including virtually all solar thermal, biomass, coal and nuclear power plants. It is named after William John Macquorn Rankine, a Scottish polymath.

There are four processes in the Rankine cycle, each changing the state of the working fluid. These states are identified by number in the diagram to the right.

Process 1-2: The working fluid is pumped from low to high pressure, as the fluid is a liquid at this stage the pump requires little input energy.
Process 2-3: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapor.
Process 3-4: The dry saturated vapor expands through a turbine, generating power. This decreases the temperature and pressure of the vapor, and some condensation may occur.
Process 4-1: The wet vapor then enters a condenser where it is condensed at a constant pressure and temperature to become a saturated liquid. The pressure and temperature of the condenser is fixed by the temperature of the cooling coils as the fluid is undergoing a phase-change.
In an ideal Rankine cycle the pump and turbine would be isentropic, i.e., the pump and turbine would generate no entropy and hence maximize the net work output. Processes 1-2 and 3-4 would be represented by vertical lines on the Ts diagram and more closely resemble that of the Carnot cycle. The Rankine cycle shown here prevents the vapor ending up in the superheat region after the expansion in the turbine [1], which reduces the energy removed by the condensers.

Reference: Google search with Thermodynamic Cycles as key words

Law

2009-04-04T21:34:00.000-07:00

Zeroth Law of Thermodynamics
Zeroth law as equivalence relation
A system is said to be in thermal equilibrium when its temperature does not change over time. Let A, B, and C be distinct thermodynamic systems or bodies. The zeroth law of thermodynamics can then be expressed as:

"If A and B are each in thermal equilibrium with C, A is also in thermal equilibrium with B."

Then T(A)=T(C)=====Thermal equilibrium of many systems

The preceding sentence asserts that thermal equilibrium is a Euclidean relation between thermodynamic systems. If we also grant that all thermodynamic systems are (trivially) in thermal equilibrium with themselves, then thermal equilibrium is also a reflexive relation. Relations that are both reflexive and Euclidean are equivalence relations. One consequence of this reasoning is that thermal equilibrium is a transitive relation between the temperature T of A, B, and C:

First Law of Thermodynamics
In thermodynamics, the first law of thermodynamics is an expression of the more universal physical law of the conservation of energy. Succinctly, the first law of thermodynamics states:

“ The increase in the internal energy of a system is equal to the amount of energy added by heating the system, minus the amount lost as a result of the work done by the system on its surroundings.

The first law of thermodynamics is the application of the conservation of energy principle to heat and thermodynamic processes:

The first law makes use of the key concepts of internal energy, heat, and system work. It is used extensively in the discussion of heat engines.

It is typical for chemistry texts to write the first law as ΔU=Q+W. It is the same law, of course - the thermodynamic expression of the conservation of energy principle. It is just that W is defined as the work done on the system instead of work done by the system. In the context of physics, the common scenario is one of adding heat to a volume of gas and using the expansion of that gas to do work, as in the pushing down of a piston in an internal combustion engine. In the context of chemical reactions and process, it may be more common to deal with situations where work is done on the system rather than by it.

Second Law of Thermodynamics
The second law of thermodynamics is an expression of the universal law of increasing entropy, stating that the entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium.

The second law of thermodynamics is a general principle which places constraints upon the direction of heat transfer and the attainable efficiencies of heat engines. In so doing, it goes beyond the limitations imposed by the first law of thermodynamics. It's implications may be visualized in terms of the waterfall analogy.
The second law traces its origin to French physicist Sadi Carnot's 1824 paper Reflections on the Motive Power of Fire, which presented the view that motive power (work) is due to the fall of caloric (heat) from a hot to cold body (working substance). In simple terms, the second law is an expression of the fact that over time, ignoring the effects of self-gravity, differences in temperature, pressure, and density tend to even out in a physical system that is isolated from the outside world. Entropy is a measure of how far along this evening-out process has progressed.

Third Law Of Thermodynamics
The third law of thermodynamics is a statistical law of nature regarding entropy and the impossibility of reaching absolute zero of temperature. The most common enunciation of third law of thermodynamics is:

“ As a system approaches absolute zero, all processes cease and the entropy of the system approaches a minimum value.

The third law was developed by Walther Nernst, during the years 1906-1912, and is thus sometimes referred to as Nernst's theorem or Nernst's postulate. The third law of thermodynamics states that the entropy of a system at zero is a well-defined constant. This is because a system at zero temperature exists in its ground state, so that its entropy is determined only by the degeneracy of the ground state; or, it states that "it is impossible by any procedure, no matter how idealised, to reduce any system to the absolute zero of temperature in a finite number of operations".

An alternative version of the third law of thermodynamics as stated by Gilbert N. Lewis and Merle Randall in 1923:

“ If the entropy of each element in some (perfect) crystalline state be taken as zero at the absolute zero of temperature, every substance has a finite positive entropy; but at the absolute zero of temperature the entropy may become zero, and does so become in the case of perfect crystalline substances. ”

This version states not only ΔS will reach zero at 0 Kelvin, but S itself will also reach zero, at least for perfect crystalline substances. (This statement is now known to have some rare exceptions.
Reference:
Google Search with keywords Thermodynamic Laws

Thermodynamic

2009-04-04T21:20:00.000-07:00

PROPERTIES OF STEAM AND WATER

INTRODUCTION

The process by which we convert water into steam and use the steam to turn a propulsion shaft encompasses the generation and expansion phases of the steam cycle. A study of the properties of water and steam at these critical phases is necessary to understand the steam cycle. This lesson defines terms associated with these properties and processes, and explains the use of steam tables to calculate the work and efficiency created by steam.

REFERENCE:
http://www.massengineers.com/Documents/properties_of_steam_and_water.htm