Basics terms used in Thermodynamics & related calculations


It is an axiomatic science which deals with the relations among heat, work and properties of equilibrium system.It is a science, which deals with interaction between energy and material system.

Terms used in Thermodynamic science:

System:It is a finite quantity of matter or a prescribed region of space.

Boundary:It is the actual or hypothetical envelope enclosing the system.

Closed system: If the boundary of the system is impervious to the flow of matter is called  closed system.

Open system: there is  flow of matter into and out of the system

Isolated system:An isolated system doesnot exchange energy  & matter with any of the other system

Homogeneous system: A system which contains single phase is called homogeneous system

Hetergeneous system : Here system consists of more than two phases

Pure substance: A substance which is homogeneous in compossition &  chemical aggregation

State: It is the condition of the system at an instant of time as described or measured by its properties

Cycle:Any process or series of processes whose end states are identical is termed as a cycle

Process: It occurs when the system undergoes a change in state or n energy transfer at a steady state.

Reversible process: A process is one which can be stopped at any stage and reversed so that the system & surroundings are exactly restored to their initial states.

Example:Exapnsion & compression of springs, Electrolysis

Irrreversible process: In thisprocess heat is transferred through a finite temperature.

Example:Combustion, free expansion, heat transfer

Temperature: It is state of a body  which distinguishes a hot body from a cold body.It is measured in Fahrenheit, Degree centi gare, Kelvin etc

Pressure: It is a force per unit area.Pressure is exerted by gases, vapours & liquids.It is measured in Kg/cm2,Bar, Pascal, N/m2, N/mm2, mm of water column.

Thermal Equilibrium: Temperature of the system does not change with time and has same value at all the points of the system.

Mechanical Equilibrium: It is the condition where there are no unbalanced forces within the system, pressure in the system is same at all the points.

Chemical Equilibrium: Composition of chemicals does not change with time and no chemical reaction takes place in the system.

Enthalpy:It is the total heat content of the steam. Expressed in h…kcal/kg.

Entropy: is a function of a quantity of heat, which shows the possibility of conversion that heat into work. That is, if Entropy is more then there is minimum availability for conversion of heat into work and for minimum entropy there is maximum availability for conversion of heat into work. It is measured in kcal/kg.

Boyle’s law:At constant temperature, pressure of a perfect gas is inversely proportional to its volume.

P = 1/V or PV = Constant

Charles law:At constant pressure, volume of a given mass of a gas is directly varies as its temperature.

V = T
(V1/T1) = (V2/T2) = …… Constant

Gay-Lussac law:The pressure of a given mass of a perfect gas varies directly as its temperature, when the volume remains constant.
P = T
(P1/T1) = (P2/T2) = Constant

Relation among pressure, temperature, enthalpy and specific volume of steam

  • At constant temperature enthalpy decreases with increase in pressure
  • At constant pressure enthalpy increases with increase in temperature.
  • Specific volume increases with decrease in pressure and increase in temperature
  • Enthalpy of evaporation decreases with increase in pressure and temperatures.
Specific heat: Amount of heat required to raise the temperature of unit mass of substance by 1 degree centigrade.It is measured in constant pressure Cp & Constant volume Cv.Mesured in kcal/kg

Specific volume: It is the volume occupied by the unit mass of the system.It is measured in m3/kg.

Intensive property:It is the property of steam, whose value for the entire system is not equal to the sum of their values for the individual parts of the system.

Example: Temperature, pressure and density.

Extensive property:It is the property of steam, whose value for the entire system is equal to the sum of their values for the individual parts of the system.

Example: Volume and mass.

Zeroth law of thermodynamics:The law states that, when two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other.

First law of thermodynamics: Law states that when a system undergoes a thermodynamic cycle then the net heat supplied to the system from surroundings is equal to net work done by the system on its surroundings.

Second law of thermodynamics:It states that, it is impossible to construct an engine working on a cyclic process, to transfer heat from a body at a lower temperature to a body at a higher temperature without the aid of an external energy.

Different types of heat given to water to convert it into steam

Sensible heat : Amount of heat given to water to bring water from its normal temperature to Boiling temperature or saturation temperature. It is denoted by hf & measured in kcal/kg.

Latent heat: Amount of heat iven to saturated water to convert into saturated steam.It is denoted by hf g& measured in kcal/kg.

Total heat : It is the quantity of heat required to convert 1 kg of water into wet steam at constant pressure.It is the sum of total heat of water and the latent heat.It is also called as enthalpy.Denoted as h for wet steam and hg for dry steam

h = hf + x hfg

x is the dryness fraction of steam.

Dryness fraction of steam (x) : It is the ratio of the mass of actual dry steam to the mass of the steam containing it.

x = Ms/(Ms + Mw)

Superheated steam: when steam is heated after it has become dry and saturated.It is called superheated steam.

hsup = hf + hfg + Cps (Tsup-Ts)

Whwre, Cps = Specific heat of super heated steam = 0.5 kcl/kg

Tsup = Temperature of superheated steam
Ts = Saturated temperature at saturated pressure
(Tsup-Ts) is called as Degree of superheat.

Basic calculations:

1-Convert the pressure 80 cm of Hg to Kpa

Pressure at 760 mm of Hg = Density X g X h

                                           = 13.596 X 1000 X 9.81 X 760/1000
                                           = 101325 Pa =101.325 Kpa

2-Convert 2mm of water column pressure to Pascal

Pressure due to 2 mm of water column =1000 X 9.81 X 2 =19620 Pa

3-On a piston of 15 cm diameter of a force of 2000 N is applied uniformely caluclate the pressure on piston

Pressure = Force applied/Area of the piston

P = 2000 / ((3.142 X 0.152/4).....Area of piston =Pi X D2/4

P = 113636.363 N/M2

3-Atube contains an oil of specific gravity 0.88 to a depth of 1500 mm, find the gauge pressure at this depth

SG of oil =0.88
Depth of the oilin tube =1500 mm
We know that,
P = Density X g X h

P = 0.88 X1000 X  9.81 X 1.5...As SG =Density of oil / Density of water, Density of water is 1000 kg/m3

P = 12,949.2 N=/m2

1 N/M2 =1 X 10-5 kg/cm2

P =0.13 kg/cm2

4-Determine the dryness fraction of steam which has 2 kg of water in suspension with 60 kg of steam

Mass of dry steam Ms = 60 kg

Mass ofwater in suspension = 2 kg

Dryness fraction of steam x = 60/(60+2) =0.967

5-Determine the amount of heat given to 3 kg of water at 25 deg c to convert it into steam at 7 kg/cm2G and 0.9 dry.

Mass of water =Mw = 3 kg

Temperature of water = tw = 25 deg c

Pressure of steam = 7 kg/cm2

Fom the steam table at 7 kg/cm2 pressure (take absolute pressure i.e at 8 kg/cm2),

hf =172.73 kcal/kg

hfg = 488.22 kcal/kg

We have the formula for enthalpy of 1 kg of steam at 0 deg c is

h = hf + xhfg =172.73 + 0.9 X 488.22 =612.12 kcal/kg

Sensible heat of 1 kg of water at 25 deg c is = Mw X Cpw X (tw-t0)

=3 X 1 X (25-0) =75 kcal/kg

Net quantity of heat supplied per kg of water =612.12-75 =537.12 kcal/kg

Therefore total amount of heat to be supplied =3 X 537.12 =1611.38 kcal

6-Determine the mass of 0.2 m3 of wet steam at a pressure of 5 kg/cm2G and dryness fraction of 0.85. Also calculate heat of 1 m3 of steam.

Steam pressure P = 5 kg/cm2

Dryness fraction x =0.85

Fro steam table at 5 kg/cm2G

hf = 160.64 kcal/kg

hfg =497.47 kcal/kg
 Vg = 0.312 m3/kg

Density = 1/(x X Vg) =1/(0.85 X 0.312) =3.77 kg/m3

Mass of 0.2 m3 of steam = 0.2 X 3.77 =0.754 kg

Total heat of 1m3 of steam which has 3.77 kg of mass
=3.77 X (hf + xhfg) = 3.77 X (160.64 + 0.85 X 497.47) = 2199.7 kcal

7-What amount of heat would be required to produce 100 MT of steam at pressure of 67 barA and temperature 490 deg c from 150 deg c? Consider Cp of super heated steam =0.5 kcal/kg

Mass of steam = 100 MT =100 X 1000 =100000 kg

Steam pressure P=67 kg/cm2 absolute

Temperature of steam tsup =490 deg C

Temperature of water t =150 deg C

Refer steam table at 67 kg/cm2A & 490 deg c 

hf =300 kcal/kg

hfg =362.91 kcal/kg

ts = 283.75 kcal/kg

hsup = hf + hfg + Cps X (tsup-ts) =300 + 362.91 + 0.5 X (490-283.75) =766.035 kcal

Amount of heat already associated with 1 kg of water = 1 X 1 X (150-25) =125 kcal..Specifc heat of water is 1 kcal/kg

So net heat required = 766.035-125 =641.035 kcal

Total amount of heat required =641.035 X 100000 =64,103,500‬ kcal

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