Chapter: 6 Refrigeration and Psychrometry

I. Introduction to Refrigeration
Refrigeration refers to the use of mechanical devices or heat activated devices for
producing and maintaining the temperature in a region less than that of
surroundings.

American Society of Refrigerating Engineers defines refrigeration as “the science

of providing and maintaining temperature below that of the surrounding
atmosphere”.

Refrigeration is extensively used for increasing the storage life of perishable items
specially food products, vegetables, fruits, milk, beverages, chilling of water, ice
formation etc. along with industrial applications in chemical manufacturing,
petroleum refinery, petrochemical plants, paper and pulp industry etc.

II. Performance Parameter

Refrigerator’s performance is given by coefficient of performance (COP), COP for
a refrigerator shown in block diagram, Fig. 6.1 can be given as under.
COP is the ratio of refrigeration effect and net work done upon refrigerator.
COP is dimensionless quantity so the two quantities should have same units.

Desired effect
COP of refrigeration =
Net work done

Here the desired effect is the continuous removal of heat at the rate of Q 2 for
maintaining body at lower temperature T 2. Net work done for getting this
refrigeration effect is W.
Q2 Q2
= COP= =
W Q 1−Q2
COP of refrigerator may have any magnitude i.e. less than unity or greater than
unity.

Fig 6.1 Refrigerator

III. Unit Of Refrigeration
Refrigeration effect is the amount of heat extracted by refrigerator from the
refrigerated space. The higher capacity of refrigerator means higher shall be the
refrigeration effect. This refrigeration effect is defined by the unit of refrigeration
called ‘Ton’ of refrigeration.

‘Ton’ as unit of refrigeration has been defined based on formation of ice as

described ahead.“One ‘Ton’ of refrigeration can be defined by the amount of heat
being removed from one ton of water at 0ºC to form one ton of ice at 0ºC within 24
hours.”

“One ‘Ton’ of refrigeration can be defined by the amount of heat being removed
from one ton of water at 0ºC to form one ton of ice at 0ºC within 24 hours.” Thus,
a Ton of refrigeration shall quantify the latent heat required to be removed for
solidification of water at 0ºC.

Latent heat of solidification from water to ice at 0ºC = 334.5 kJ/kg

1 Ton of refrigeration = mass of water × latent heat at 0ºC from water to ice (in SI
units)
1000× 334.5 KJ 1000 ×334.5 KJ KJ
¿ = =3.87
24 hr 24 × 3600 sec sec

Generally, 1 Ton is taken as 3.5 kJ/sec. This deviation from 3.87 to 3.5 kJ/sec is
there because Ton was originally defined in fps units and the approximations
during conversion yielded numerical value of 3.5 kJ/sec which is now universally
accepted.

1000× 80 Kcal Kcal

1 Ton of refrigeration (in MKS units) ¿ =55.4 =50
24 ×60 min min

In MKS units this is commonly taken as 50 kcal/min.

Therefore, 1 Ton = 3.5 kJ/sec

And 1 Ton = 50 kcal/min

IV. Carnot Refrigeration Cycle
Carnot cycle being the most efficient and ideal cycle can also be used for getting
the refrigeration effect upon its reversal.
Here, the refrigerated space/body is to be maintained at low temperature T1 for
which heat Q1 should be removed at constant rate and rejected to surroundings at
high temperature, Th.
Amount of heat rejected to surroundings is Q h. While the net work done upon
refrigerator is W

Different processes in reversed Carnot cycle are,

1–2 Reversible adiabatic compression
2–3 Reversible isothermal heat rejection, Qh at temperature, Th
3–4 Reversible adiabatic expansion
4–1 Reversible isothermal heat absorption, Q1 at temperature, T1

Fig 6.2 Carnot Refrigeration Cycle

 The coefficient of performance of refrigerator depends upon the two temperature
values i.e. low temperature T1 and high temperature Th. For COP value to be
high the low temperature T1 should be high while higher temperature T h should
be small.
With increasing low temperature value the COP goes on increasing, while with
decreasing high temperature Th value the COP goes on increasing.

Practically, the lower temperature requirement is decided by the user while

higher temperature is generally fixed by the atmospheric temperature value.
Thus, it could be said that for certain low temperature to be maintained,
COP of refrigerator shall be more during cold days as compared to hot days.

COPcold days > COPhot days because Th, cold days < Th, hot days

Similar to the carnot power cycles, reversed carnot cycle for refrigeration is not
feasible in practice.
Therefore, number of modified cycles are practically used for refrigeration as
discussed ahead. But the reversed carnot cycle provides the basis for
comparison and provides the benchmark for achieving maximum COP.

V. Vapor Compression Refrigeration Cycles

Vapour compression cycle based refrigeration systems are extensively used in
refrigeration systems.
These cycles are used for most of small domestic and large industrial
applications.
The vapour compression cycle has the refrigerant being circulated in closed
circuit through compressor, condenser, throttle valve or expansion valve &
evaporator as shown in Fig. 6.3 Refrigerant (in gas/vapour) is compressed
isentropically in compressor from state 1 to 2.
 Fig 6.3 Vapour Compression Refrigeration Cycle

High pressure and high temperature refrigerant enters the condenser at state 2
where its condensation occurs and refrigerant is available in liquid form at state 3.

Refrigerant in the form of saturated liquid at high pressure is passed through

expansion valve where isenthalpic expansion occurs. Refrigerant leaving
expansion valve at state 4 is in the form of low pressure wet mixture of liquid and
vapour.
 Fig 6.4 Vapour Compression Cycle

1–2 or 1'–2' : - Isentropic compression

2–3 or 2'–3' : - Isobaric heat rejection
3–4 or 3'–4' : - Isenthalpic expansion or throttling process
4–1 or 4'–1' : - Isobaric heat absorption

Low pressure liquid-vapour mixture is passed through evaporator section in which

refrigerant picks up heat from surroundings thereby showing refrigeration effect.

As a result of this heat absorption the liquid-vapour mixture refrigerant gets

transformed into dry gaseous refrigerant in case of dry compression as shown in
Fig. 6.4
Compression of dry and saturated or superheated refrigerant is called dry
compression.Compression of dry refrigerant yields superheated state of
refrigerant as shown by state 2. (Fig.6.4)

Wet compression:
It is also possible that the refrigerant is in wet state i.e. liquid-vapour mixture at
inlet of compressor, state 1'.

Compression of wet mixture gets transformed into dry refrigerant (gaseous form)
as shown by state 2'.

Dry refrigerant at high pressure and high temperature is passed through

condenser where refrigerant gets condensed and condensate is available in
saturated liquid form at high pressure.

Subsequently refrigerant is throttled from high pressure to low pressure inside

expansion valve from state 3' to 4'.

Low pressure refrigerant in wet state is passed through evaporator from state 4' to
1' where it picks up heat from surroundings and some of its liquid fraction gets
transformed into vapour but it does not become dry (gas) refrigerant at inlet to
compressor.

This wet refrigerant is compressed inside compressor.

In case of dry compression the compressor efficiency is found to be more than

that of wet compression due to higher volumetric efficiency with dry refrigerant
and also chances of damage by liquid refrigerant are absent.

During dry compression the temperature of compressed refrigerant becomes

more than condensation temperature.
Due to this high temperature after compression the compressor becomes very hot
therefore requiring cooling.

Coolant used in condenser may also be used for cooling compressor as its
temperature is lower than compressor temperature.

This cooling of compressor reduces compression work.

T-s and p-h diagram for vapour compression cycle shows that the refrigeration
capacity can be increased by sub cooling the condensate before it enters the
expansion valve and also by increasing the degree of expansion in expansion
valve.

Here in second option the condensation temperature is fixed and evaporator

temperature is lowered which increases expansion ratio and compression ratio.

This increase in compression ratio shows poor volumetric efficiency in single

stage dry compression.

The temperature after compression also gets increased with increased

compression ratio which may be harmful to refrigerant properties.
To regulate the excessive rise in refrigerant temperature the intercooling during
compression with multistage compression is used.

Thermodynamic analysis:
For simple vapour compression cycle, the COP, refrigeration effect and work input
can be estimated based on following assumptions:
1. All process of refrigeration cycle are internally reversible except the
expansion through valve which is throttling process and is irreversible.
2. Compressor and expansion valve have no heat interaction with
surroundings during their operation i.e. they operate adiabatically.
3. Refrigerant leaving condenser is saturated liquid.
4. Refrigerant entering compressor is saturated vapour in case of ‘dry
compression’ and liquid vapour mixture in case of ‘wet compression’.
5. Changes in kinetic energy and potential energy are negligible.
VI. Refrigerants
Refrigerant is the working fluid used in refrigeration/air conditioning equipment's
having capability of carrying heat/rejecting heat in the form of sensible heat or
latent heat.

Refrigerants carrying heat/rejecting heat in the form of latent heat are more
effective and for this refrigerant should possess suitable properties to get
transformed from liquid to gas and vice-versa.

During selection of refrigerant its chemical, physical and other general properties
are being looked into along with refrigeration cycle requirements and application

Refrigerants can be classified into two main types as

‘Primary refrigerants’ and ‘Secondary refrigerants’.

The primary refrigerants are those refrigerants which are directly involved in
refrigeration system.The primary refrigerants are used in vapour compression
systems

Secondary refrigerants are first cooled by primary refrigerants and then used for
imparting refrigeration. Secondary refrigerants are liquids used for transporting
low-temperature heat energy from one place to another as done by ‘brine’,
antifreeze agents etc.
Commonly used refrigerants are air, ammonia, carbon dioxide, sulphur-dioxide,
fluorinated hydrocarbons and Freons etc.
All refrigerants are assigned with internationally acceptable number such as R–
12, R–717 etc.

Primary refrigerants: Primary refrigerants can be further classified into following

categories depending upon their characteristics.
 Halocarbon compounds
 Inorganic compounds
 Hydrocarbons
 Azeotropes
 Unsaturated organic compounds

Halocarbon group includes refrigerants containing one or more of three halogens

such as chlorine, fluorine and bromine.

These refrigerants are traded in market under the brand names of Freon,
Genetron, Isotron and Arctron.

But due to disastrous effect of chlorine in refrigerant upon the earth’s protective
ozone layer the efforts are being made to replace the use of CFCs by the class of
refrigerants having hydrogen in place of chlorine.

Such new ecofriendly refrigerants are called hydrofluoric carbons (HFCs) for
example R–134a (CF3 CH2F) can replace R-12 (CCl2F2).

Halocarbon compounds

Halocarbon group includes refrigerants containing one or more of three halogens

such as chlorine, fluorine and bromine.

These refrigerants are traded in market under the brand names of Freon,
Genetron, Isotron and Arctron.

But due to disastrous effect of chlorine in refrigerant upon the earth’s protective
ozone layer the efforts are being made to replace the use of CFCs by the class of
refrigerants having hydrogen in place of chlorine.

Such new ecofriendly refrigerants are called hydrofluoro carbons (HFCs) for
example R–134a (CF3 CH2F) can replace R-12 (CCl2F2).
 Fig 6.5 Halocarbon compounds

The numbering of halocarbon group refrigerants is based on;

 first digit on right gives number of fluorine atoms.

 second digit from right is one more than the number of

hydrogen atoms.

 third digit from right is one less than the number of carbon
atoms.

Note : Third digit is not given when it is zero.

Inorganic compounds
Refrigerants used in olden days were inorganic compounds.

Some of such refrigerants are still used in different applications such as ice
plants, steam refrigeration, aeroplanes and ship refrigeration etc. due to their
inherent thermodynamic and physical properties.
 Fig 6.6 Inorganic Compound
Hydrocarbons
Hydrocarbons: Some of hydrocarbons are also used as refrigerants particularly in
petroleum and petrochemical industry.

Fig 6.6 Hydrocarbons

Azeotropes
Azeotropes are those mixture of different refrigerants which do not separate into
their components by distillation or with the change in pressure and temperature.

Azeotrope refrigerants behave differently from their constituents such as

azeotrope evaporates and condenses as single substance having properties that
are different from those of either constituents.

Examples of azeotrope are; R–502 is azeotrope which is mixture of 48.8%, R–22

and 51.2% of R–115. R–500 is azeotrope which is mixture of 73.8% R–12 and
26.2% of R–152.

Unsaturated organic compounds

These are refrigerants from hydrocarbon group and have ethylene or propylene
as the base.
 Fig 6.7 Unsaturated Organic Compound

DESIRED PROPERTIES OF REFRIGERANTS

A refrigerant being used in refrigeration should have following thermodynamic
properties, physical properties and working properties.
 Boiling temperature of refrigerant should be quite low at atmospheric
conditions for effective refrigeration.
 For refrigerants having higher boiling temperatures at atmospheric
conditions the compressor is run at higher vacuum.
 For an ideal refrigerant the freezing temperature of refrigerant should be
quite low so as to prevent its freezing at evaporator temperature.
 Freezing point temperature should be less than evaporator temperature.
For example, refrigerant R–22 has freezing point of – 160ºC and normally
most of refrigerants have freezing point below – 30ºC.
 Critical temperature of the ideal refrigerant should be higher than the
condenser temperature for the ease of condensation.
 Refrigerant should have large latent heat at evaporator temperature as
this shall increase the refrigerating capacity per kg of refrigerant.
 Refrigerant should have small specific volume at inlet to compressor as
this reduces compressor size for same refrigeration capacity.
 Specific heat of refrigerant in liquid form should be small and it should be
large for refrigerant in vapour form, since these increase the refrigerating
capacity per kg of refrigerant.
 Thermal conductivity of refrigerant should be high.
 Viscosity of refrigerants should be small for the ease of better heat transfer and
small pumping work requirement.
 Refrigerant should be chemically inert and non toxic.
 Refrigerant should be non-flammable, non explosive and do not have any
harmful effect upon coming in contact with material stored in refrigeration space.
 Refrigerant may have pleasant distinct odour so as to know about its leakage.
 Refrigerant should be readily available at lesser price.

CHOICE OF REFRIGERANTS
 Selection of suitable refrigerant depends upon the number of parameters as
there cannot be single refrigerant well suited for all kinds of refrigeration systems.

Thermodynamic properties, physical properties and other properties are

considered in reference to the refrigeration system during choice of suitable
refrigerant.

Apart from these properties, due consideration should be given to the working
pressure and temperature range and pressure ratio, space restrictions,
corrosiveness and in flammability, oil miscibility etc. before selecting refrigerant
with suitable compressor and other equipment's of refrigeration system.

VII. PSYCHROMETRY
Psychrometry or Psychrometrics refers to the study of system involving dry air
and water.

Properties of mixture of air and water are called psychrometric properties.

Psychrometry becomes very important from air conditioning point of view as in air
conditioning the human comfort conditions defined in terms of temperature,
humidity and air circulation/ventilation are being considered.

Different psychrometric properties, charts and processes generally used are

defined ahead.

Dry air:
Atmospheric air having 79% nitrogen and 21% oxygen by volume is considered
dry air. Its, molecular weight is taken as 29.

Moist air:
Moist air is the mixture of dry air and water vapour in which dry air is treated as if
it were pure component.

Quantity of water vapour present in the mixture depends upon the temperature of
air and it may vary from zero in dry air to the maximum quantity when mixture is
saturated of water vapour (called saturation capacity of air).

Moist air is assumed to behave as ideal gas for the purpose of analysis.

Mixture pressure is the sum of partial pressures of dry air and water vapour.
When the partial pressure of water vapour corresponds to the saturation pressure
of water at mixture temperature then mixture is said to be saturated.

Saturated air is the mixture of dry air and saturated water vapour.

When the temperature of mixture of air and vapour is above the saturation
temperature of water vapour then the vapour is called superheated vapour.

Humidity ratio or specific humidity:

This is given by the ratio of the mass of water vapour to the mass of dry air. It can
also be defined as mass of water vapour present in per kg of dry air.
It is given in terms of grams per kg of dry air. Mathematically, humidity ratio or
specific humidity,
m
ω= v
ma
Absolute humidity: Absolute humidity refers to the weight of water vapour
present in unit volume of air.

Relative humidity:
Relative humidity gives an account of moisture content in an actual mixture as
compared to the mixture in saturated state at same temperature and pressure. It
can be given by the ratio of actual mass of water vapour in given volume to the
mass of water vapour if the air is saturated at the same temperature and
pressure.
Dry bulb temperature:
Dry bulb temperature refers to the temperature of air measured with ordinary
thermometer having its bulb open.

Wet bulb temperature:

Wet bulb temperature refers to the temperature measurement with the
thermometer having its bulb covered with wet cloth and exposed to air stream
whose temperature is being measured.
For getting wet bulb temperature thermometer bulb is covered with water wet
cloth/wick moistened with water and then temperature is measured.

Wet bulb depression:

Difference between dry bulb temperature and wet bulb temperature is called wet
bulb depression. Wet bulb depression is zero in case of saturated air as the dry
bulb temperature and wet bulb temperatures are equal.
Dew point temperature:
It is the temperature at which condensation of water vapour present in moist air
just begins. It could be understood from different examples that if the moist air
temperature is reduced then water vapour gets condensed. Examples for such
condensation is formation of dew on grass, condensation of water vapour on
exterior surface of steel tumbler having cold water inside, condensation of water
vapour on glass window-panes during winter season etc.
Dew point temperature shall be equal to the saturation temperature corresponding
to partial pressure of water vapour in moist air.

Dew point depression:

Difference between dew point temperature and dry bulb temperature is called dew
point depression.

Humid specific volume:

It refers to the volume of mixture per kg of dry air in mixture and is given by m 3/kg
of dry air.

Psychrometric charts:
Psychrometric chart gives the graphical representation of different important
properties of moist air. These charts are readily available for different mixture
pressure. Fig. 6.8

shows psychrometric chart. Abscissa (x-axis) of chart gives the dry bulb
temperature (ºC) and
the ordinate (y-axis) has humidity ratio (ω) in kg or gram of water vapour per kg of
dry air
 Fig 6.8 Psychrometric charts

Since humidity ratio can be directly related to partial pressure of water vapour so
vapour pressure (pv) can also be shown on ordinate.

Constant relative humidity (φ) curves are also shown on psychrometric chart for
different φ values such as φ = 10%, 60%, 100%. Mixture enthalpy per unit mass
of dry air (kJ/kg of dry air) is also available on psychrometric chart.

Dew point temperature for the moist air can be known by following the line of
constant ω (or constant pv) up to saturation line, φ = 100% as the dew point
refers to the state where mixture becomes saturated when cooled at constant
vapour pressure.

Wet bulb temperature is also available on psychrometric chart where constant wet
bulb temperature is also available on psychrometric chart where constant wet
bulb temperature lines run from upper left to the lower right of chart.

These constant wet bulb temperature lines approximate to the lines of constant
mixture enthalpy per unit mass of dry air.
Psychrometric chart also has lines representing volume per unit mass of dry air
(m3/kg). These specific volume lines can be approximated as state giving volume
of dry air or water vapour per unit mass of dry air since each component of
mixture i.e. air and water vapour occupy the same volume.
Psychrometric processes:
Different psychrometric processes commonly encountered in air conditioning are
as follows.

1. Sensible heating or sensible cooling

2. Cooling with dehumidification

3. Humidification

4. Evaporator cooling

5. Adiabatic mixing of moist air stream

Sensible heating or sensible cooling

Sensible heating or cooling refers to the heating or cooling without phase change
i.e. heating or cooling of air without increase or decrease of moisture content.

For sensible heating the air is passed over heating coils (electrical resistance
type or steam type) while for sensible cooling the air is passed over cooling coils
(such as evaporator coil of refrigeration cycle), as shown in Fig. 6.9

This heating and cooling may be for complete amount of air being passed over
the coil as shown in Fig. 6.9 a and b, or the portion of air flowing can be by
passed through by-pass passage thus, only a fraction is passed over coil.

Later arrangement is called sensible heating or sensible cooling with by pass as

shown in Fig. 6.9 e and f.
 Fig 6.9 Sensible Heating and Sensible Cooling
Sensible heating and sensible cooling without by pass is shown on

Psychrometric charts Fig. 6.9 c and d, respectively between states 1 and 2. Thus, for
inlet air temperature being T 1, the heating/cooling causes change in temperature up to
T2 When some amount of air is by passed then the temperature of air coming out shall
be different from T2.

In case of sensible heating with by pass, temperature of air coming out is less than
temperature of air without by pass i.e. T3 < T2.

Similarly in case of sensible cooling with bypass temperature of air leaving is more than
temperature of air without by-pass i.e. T3 > T2.
Heat added can be quantified by the amount of heat gained by dry air and water vapour
between dry bulb temperature T1 and T2

Qsensible heating = Cpa (T2 – T1) + ω · Cpv (T2 - T1)

= (Cpa + ω Cpv) (T2 – T1)

Qsensible heating = Cpm · (T2 – T1), Hence Cpm is mean specific heat.

In case of sensible heating with by pass, if the amount of air by passed is B kg per kg of
air then heat balance can be given as below. Here (1 – B) kg of air passes over the coil
and later on mixes with by passed air at exit.

B · (Cpa + ω Cpv) T1 + (1 – B) (Cpa + ω Cpv) T2 = 1 × (Cpa + ω Cpv) · T3

or,

T 2−T 3 Length2−3 on psychrometric chart

By pass factor, B = =
T 2−T 1 Length2−1 on psychrometric chart

This amount of air by passed per kg of air flowing is also called as By-pass factor of the
coil. Factor (1 – B) is called contact factor.

Humidification:

This is the process of adding moisture to the air. Humidification may be required during
air conditioning when air being circulated in occupied space may have little moisture in
it.

Available means for increasing humidity are to inject steam or spray liquid water into air.
Normally the steam being injected shall be at temperature more than that of air and
humidification shall be accompanied by increase in humidity ratio and dry-bulb
temperature.

When liquid water is sprayed then the moist air shall leave humidification section with
lower temperature than at inlet and increased humidity ratio.

Figure 6.10 shows schematic for humidification process along with representation on
psychrometric chart for humidification using steam and liquid water spray both.
 Fig 6.10 Humidification
Evaporator cooling

This type of cooling is needed in hot and dry climates.

Evaporative cooling has arrangement for spray of spraying liquid water into air or
passing air through a pad soaked with water.

Due to less humidity of air it shall evaporate some amount of water in its contact and
thus reduce its temperature because of heat extracted for evaporation of water.

This air leaving evaporative cooler shall have temperature less than inlet air
temperature and also due to moisture being picked up the humidity ratio gets increased.

Arrangement for such type of cooling and its representation on psychrometric chart is
shown in Fig. 6.11

Fig 6.11 Evaporator Cooling

VIII. Fuel And Combustion
Every real life system requires energy input for its’ performance.
Energy input may be in the form of heat. Now question arises from where shall we
get heat? Traditionally heat for energy input can be had from the heat released by
fuel during combustion process.
Fuels have been provided by nature and the combustion process provides a fluid
medium at elevated temperature.
During combustion the energy is released by oxidation of fuel elements such as
carbon C, hydrogen H2 and sulphur S, i.e. high temperature chemical reaction of
these elements with oxygen O2 (generally from air) releases energy to produce
high temperature gases.
These high temperature gases act as heat source.

Air fuel ratio:

It refers to the ratio of amount of air in combustion reaction with the amount of
fuel. Mathematically, it can be given by the ratio of mass of air and mass of fuel.
Mass of air Molecular wt . of air ×no . of moles of air
AF= =
Mass of fue l Molecular wt .of fuel ×no . of moles of fuel

Fuel-air ratio
Fuel-air ratio is inverse of Air-fuel ratio. Theoretical air-fuel ratio can be estimated
from stoichiometric combustion analysis for just complete combustion.

Equivalence ratio:
It is the ratio of actual fuel-air ratio to the theoretical fuel-air ratio for complete
combustion.
Fuel-air mixture will be called lean mixture when equivalence ratio is less than
unity while for equivalence ratio value being greater than unity the mixture will be
rich mixture.

Theoretical air:
Theoretical amount of air refers to the minimum amount of air that is required for
providing sufficient oxygen for complete combustion of fuel. Complete combustion
means complete reaction of oxygen present in air with C, H 2, S etc. resulting into
carbon dioxide, water, sulphur dioxide, nitrogen with air as combustion products.

Stoichiometric air
At the end of complete reaction there will be no free oxygen in the products. This
theoretical air is also called “stoichiometric air”.

Excess air:
Any air supplied in excess of “theoretical air” is called excess air. Generally
excess air is 25 to 100% to ensure better and complete combustion.
Flash point and Fire point:
Flash point refers to that temperature at which vapour is given off from liquid fuel
at a sufficient rate to form an inflammable mixture but not at a sufficient rate to
support continuous combustion.

Fire point refers to that temperature at which vaporization of liquid fuel is sufficient
enough to provide for continuous combustion.

These temperatures depend not only on the fuel characteristics but also on the
rate of heating, air movement over fuel surface and means of ignition. These
temperatures are specified in reference to certain standard conditions.

Although flash point and fire point temperatures are defined in relation with
ignition but these temperatures are not measure of ignitability of fuel but of the
initial volatility of fuel.

Adiabatic flame temperature:

Adiabatic flame temperature refers to the temperature that could be attained by
the products of combustion when the combustion reaction is carried out in limit of
adiabatic operation of combustion chamber.

Limit of adiabatic operation of combustion chamber means that in the absence of

work, kinetic and potential energies the energy released during combustion shall
be carried by the combustion products with minimum or no heat transfer to
surroundings.

This is the maximum temperature which can be attained in a combustion chamber

and is very useful parameter for designers.

Actual temperature shall be less than adiabatic flame temperature due to heat
transfer to surroundings, incomplete combustion and dissociation etc.

Wet and dry analysis of combustion:

Combustion analysis when carried out considering water vapour into account is
called “wet analysis” while the analysis made on the assumption that vapour is
removed after condensing it, is called “dry analysis”.

Volumetric and gravimetric analysis:

Combustion analysis when carried out based upon percentage by volume of
constituent reactants and products is called volumetric analysis.

Combustion analysis carried out based upon percentage by mass of reactants

and products is called gravimetric analysis.
Pour point:
It refers to the lowest temperature at which liquid fuel flows under specified
conditions.

Cloud point:
When some petroleum fuels are cooled, the oil assumes cloudy appearance.

This is due to paraffin wax or other solid substances separating from solution.

The temperature at which cloudy appearance is first evident is called cloud point.

Composition of air: Atmospheric air is considered to be comprising of nitrogen,

oxygen and other inert gases.

For combustion calculations the air is considered to be comprising of nitrogen and

oxygen in following proportions.

Molecular weight of air is taken as 29.

Composition of air by mass = Oxygen (23.3%) + Nitrogen (76.7%)
Composition of air by volume = Oxygen (21%) + Nitrogen (79%)

Enthalpy of combustion: Enthalpy of combustion of fuel is defined as the

difference between the enthalpy of the products and enthalpy of reactants when
complete combustion occurs at given temperature and pressure.

It may be given as higher heating value or lower heating value.

Higher heating value (HHV) of fuel is the enthalpy of combustion when all the
water (H2O) formed during combustion is in liquid phase.

Lower heating value (LHV) of fuel refers to the enthalpy of combustion when all
the water (H2O) formed during combustion is in vapour form.

The lower heating value will be less than higher heating value by the amount of
heat required for evaporation of water.

HHV = LHV + (Heat required for evaporation of water)

It is also called calorific value of fuel and is defined as the number of heat units
liberated when unit mass of fuel is burnt completely in a calorimeter under given
conditions.
 Enthalpy of formation:
Enthalpy of formation of a compound is the energy released or absorbed when
compound is formed from its elements at standard reference state.

Thus enthalpy of formation shall equal heat transfer in a reaction during which
compound is formed from its’ elements at standard reference state.

Enthalpy of formation will have positive (+ ve) value if formation is by an

endothermic reaction and negative (– ve) value if formation is by an exothermic
reaction.

Standard reference state:

It refers to thermodynamic state at which the enthalpy datum can be set for study
of reacting systems. At standard reference state, zero value is assigned arbitrarily
to the enthalpy of stable elements.

Generally, standard reference state is taken as 25°C and 1 atm,

i.e. Tref = 25°C = 298.15 K, pref = 1 atm

Dissociation:
It refers to the combustion products getting dissociated and thus absorbing some
of energy. Such as, the case of carbon dioxide getting formed during combustion
and subsequently getting dissociated can be explained as below,

Combustion: C + O2 CO2 + Heat

Dissociation: Heat + CO2 C + O2

Thus generally, dissociation has inherent requirement of high temperature and

heat.
IX.