RAC Psychrometry Guide

RAC MODULE 5, ARUN JOSE TOM, CCET pg. 1
REFRIGERATION
&
AIR
CONDITIONING
MODULE 5

Syllabus
Air conditioning – meaning and utility, comfort and
industrial air conditioning.
Psychometric properties- saturated and unsaturated air,
dry, wet and dew point temperature – humidity, specific
humidity, absolute humidity, relative humidity and degree
of saturation- thermodynamic equations- enthalpy of
moisture- adiabatic saturation process -psychrometers.
Thermodynamic wet bulb temperature, psychometric
chart- Psychometric processes- adiabatic mixing- sensible
heating and cooling- humidifying and dehumidifying, air
washer – bypass factor- sensible heat factor-RSHF and
GSHF line- Design condition- Apparent dew point
temperature – Choice of supply condition, state and mass
rate of dehumidified air quantity – Fresh air supplied –air
refrigeration.
Comfort air conditioning- factors affecting human
comfort. Effective temperature – comfort chart.
Summer air conditioning- factors affecting-cooling load
estimation

PSYCHROMETRY
Dry air is a mechanical mixture of the gases: nitrogen, oxygen,
carbon dioxide, hydrogen, argon, neon, krypton, helium, etc. For
practical purposes dry air is considered to consist of 79% by
volume (77% by mass) nitrogen and 21% by volume (23% by
mass) oxygen. Completely dry air does not exist in nature. Water
vapour in varying amounts is always present in air. The study of
humid air (air-water vapour mixture) is known as psychrometry.
The source of water vapour in air is mainly the evaporation of
water from large water bodies like lakes and sea. The amount of
water vapour varies widely in air with locality and weather
conditions and is normally 1% to 3% by mass of the mixture.
Terms used in Psychrometry
Dry Air: Air contains no water vapour. It is a mixture of gases
such as nitrogen, oxygen, carbon dioxide, hydrogen, argon, neon,
helium…etc
Moist Air: Mixture of dry air and water vapour
Dry Bulb Temperature (td): Temperature of air measured by
ordinary thermometer, when it is not affected by the moisture
present in the air
Wet Bulb Temperature (tw): Temperature Recorded by a
thermometer, when its bulb is covered by a wet cloth exposed to
the air
Wet Bulb Depression: It is the difference between dry bulb
temperature and wet bulb temperature at any point. The wet bulb
temperature indicates relative humidity of the air

Dalton’s law of Partial Pressures
The total pressure exerted by the mixture of air and water vapour
is equal to the sum of the pressures, which each constituent would
exert, if it occupied the same space by itself
Pb = Pa + Pv
Pa = partial pressure of dry air
Pv = partial pressure of water vapour
The total pressure exerted by air and water vapour mixture is
equal to the barometric pressure
Partial Pressure of Water Vapour (Pv)
Pv is very small, therefore the saturation temperature by water
vapour at Pv is also low. Thus the water vapour in air exists in the
superheated state
Pv = Pw -
(𝐏𝐛− 𝐏𝐰)(𝐭𝐝− 𝐭𝐰)
𝟏𝟓𝟒𝟒−𝟏.𝟒𝟒𝐭𝐰
Pb = Barometric Pressure
Pw = Saturation pressure corresponding to wet bulb temperature
td= Dry bulb temperature
tw = Wet bulb temperature
Dew Point Temperature (tdp): It is the temperature of air
recorded by a thermometer, when the water vapour present in it
begins to condense
Dew point temperature is the saturation temperature
corresponding to the partial pressure of water vapour (Pv)
Dew Point Depression: It is the difference between the dry bulb
temperature and dew point temperature of air

Saturated Air: Air which contains maximum amount of water
vapour which air can hold at a given temperature and pressure. It
is a mixture of dry air and water vapour, when the air has diffused
the maximum amount of water vapour into it
Saturation Pressure (Ps): The water vapour pressure
corresponds to saturated state. Saturation Pressure is the pressure
corresponding to the dry bulb temperature
Note: For a saturated air, the dry bulb temperature, wet bulb
temperature and dew point temperature is same
Specific Humidity or Humidity Ratio or Moisture Content
(W)
Ratio of the mass of water vapour to the mass of dry air in a given
volume of the air-vapour mixture. It is generally expressed in
kg/kg of dry air
W = 0.622 (
𝐏𝐯
𝐏𝐛−𝐏𝐯
)
For a saturated air, Ws = 0.622 (
𝐏𝐬
𝐏𝐛−𝐏𝐬
)
Degree of Saturation or Percentage Humidity (μ)
Ratio of actual specific humidity to the specific humidity of
saturated air at the same dry bulb temperature
μ =
𝐖
𝐖𝐬
Relative Humidity (Φ): Ratio of mass of water vapour in a
given volume of moist air at a given temperature to the mass of
water vapour contained in the same volume of moist air at the
same temperature, when the air is saturated

Φ =
𝐦𝐯
𝐦𝐬
The relative humidity may also be defined as the ratio of actual
partial pressure of water vapour in moist air at a given
temperature to the saturation pressure of water vapour at the same
temperature
Φ =
𝐏𝐯
𝐏𝐬
Φ =
𝐦𝐯
𝐦𝐬
=
𝐏𝐯
𝐏𝐬
 If Φ = 0; W =0; μ = 0
 If Φ = 1; W =Ws; μ = 1
Vapour Density
Mass of water vapour present in 1 m3
of dry air
ρv = Wρa =
𝐖(𝐏𝐛− 𝐏𝐯)
𝐑𝐚𝐭𝐝
Ra = Gas constant for air = 287 J/kgK
Enthalpy of Moist Air
The enthalpy of moist air is numerically equal to the enthalpy of
dry air and the enthalpy of water vapour associated with dry air
h = 1.022td + W (hfgdp + 2.3tdp) kJ/kg

Psychrometric Chart
It is a graphical representation of the various thermodynamic
properties of moist air. In a psychrometric chart, dry bulb
temperature is taken as abscissa and specific humidity as ordinate.
The saturation curve represents 100% relative humidity at various
dry bulb temperatures.
Dry Bulb Temperature Lines

The dry bulb temperature lines are vertical, parallel to the ordinate
and uniformly spaced. The values of dry bulb temperatures are
also shown on the saturation curve
Specific Humidity Lines
The specific humidity lines are horizontal, parallel to the abscissa
and are uniformly spaced
Dew Point Temperature Lines

Dew point temperature lines are horizontal, parallel to the
abscissa and non-uniformly spaced. At any point on the saturation
curve, the dry bulb and dew point temperatures are equal
Wet Bulb Temperature Lines
Wet bulb temperature lines are inclined straight lines and non-
uniformly spaced. At any point on the saturation curve, the dry
bulb and wet bulb temperatures are equal
Enthalpy Lines

Enthalpy lines are inclined straight lines and uniformly spaced.
These lines are parallel to the wet bulb temperature lines, and are
drawn up to the saturation curve. Some of these lines coincide
with the wet bulb temperature lines also
Specific Volume Lines
The specific volume lines are obliquely inclined straight lines and
uniformly spaced. These lines are drawn up to the saturation
curve
Vapour Pressure Lines

The vapour pressure lines are horizontal and uniformly spaced.
Generally, the vapour pressure lines are not drawn in the main
chart. A scale showing vapour pressure in mm of Hg is given on
the extreme left side of the chart
Relative Humidity Lines
The relative humidity lines are curved lines and follow the
saturation curve. The saturation curve represents 100% relative
humidity
Psychrometric Process

 Sensible Heating(C): Increase the dry bulb temperature of air
without change in its specific humidity(moisture content),
using heating coil
 Sensible Cooling(G): Decreases dry bulb temperature of air
without change in its specific humidity(moisture content),
using cooling coil
 Humidification(A): Increase the specific humidity without
change in dry bulb temperature, using humidifier
 Dehumidification(E): Decreases the specific humidity
without change in dry bulb temperature, using dehumidifier
 Heating with Humidification(B): Increases both dry bulb
temperature and specific humidity of air, using high
temperature steam
 Heating with Dehumidification(D): Increases the dry bulb
temperature and decreases the specific humidity of air, using
silica gel or alumina
 Cooling with Humidification(H): Decreases the dry bulb
temperature and increases the specific humidity of air, using
cold water
 Cooling with Dehumidification(F): Decreases both dry bulb
temperature and specific humidity of air, using cooling coil
with very low temperature
1.Sensible Heating
Heating of air without any change in its specific humidity

Air at temperature td1 passes over a heating coil of temperature
td3. The temperature of air leaving the heating coil td2 will be less
than td3. The point 3 represents the surface temperature of the
heating coil.
 Specific humidity remains constant
 Dry bulb temperature increases
 Relative humidity decreases
For heating process, Sensible Heat, SH = h2 – h1
2.Sensible Cooling
Cooling of air without any change in its specific humidity
Air at temperature td1 passes over a cooling coil of temperature
td3. The temperature of air leaving the heating coil td2 will be more
than td3. The point 3 represents the surface temperature of the
cooling coil.
 Specific humidity remains constant
 Dry bulb temperature decreases
 Relative humidity incerases
For cooling process, Sensible Heat, SH = h1 – h2

By-pass factor of Heating and Cooling Coil
When air passes over a coil, some of it (say x kg) just by-passes
unaffected while the remaining (1-x) kg comes in direct contact
with the coil. This by-pass process of air is measured in terms of
a by-pass factor
By-pass factor for heating coil,
BPF =
𝐭𝐝𝟑−𝐭𝐝𝟐
𝐭𝐝𝟑−𝐭𝐝𝟏
By-pass factor for cooling coil,
BPF =
𝐭𝐝𝟐−𝐭𝐝𝟑
𝐭𝐝𝟏−𝐭𝐝𝟑
Efficiency of heating & cooling coil
The term (1-BPF) is known as efficiency of coil or contact
factor
Efficiency of the heating coil, ηH = 1-BPF
Efficiency of the cooling coil, ηc = 1-BPF

3.Humidification & Dehumidification
The addition of moisture to the air, without change in its dry bulb
temperature, is known as humidification
The removal of moisture from the air, without change in its dry
bulb temperature, is known as dehumidification
Humidification
 Relative humidity increases
 Specific humidity increases
Dehumidification
 Relative humidity decreases
 Specific humidity decreases
For humidification process, Latent Heat, LH = h2 – h1
For dehumidification process, Latent Heat, LH = h1 – h2

Sensible Heat Factor (SHF)
The heat added during a psychrometric process may be split up
into sensible heat and latent heat. The ratio of the sensible heat to
the total heat is known as sensible heat factor (SHF)
SHF =
𝐒𝐞𝐧𝐬𝐢𝐛𝐥𝐞 𝐇𝐞𝐚𝐭
𝐓𝐨𝐭𝐚𝐥 𝐇𝐞𝐚𝐭
=
𝐒𝐞𝐧𝐬𝐢𝐛𝐥𝐞 𝐇𝐞𝐚𝐭
𝐒𝐞𝐧𝐬𝐢𝐛𝐥𝐞 𝐇𝐞𝐚𝐭+𝐋𝐚𝐭𝐞𝐧𝐭 𝐇𝐞𝐚𝐭
=
𝐒𝐇
𝐒𝐇+𝐋𝐇
4.Cooling and Dehumidification
This process is generally used in summer air conditioning. In this
process, the dry bulb temperature as well as the specific humidity
of air decreases. The dehumidification and cooling is possible
when the effective surface temperature of the cooling coil is less
than the dew point temperature of the air entering the coil. The
effective surface temperature of the coil is known as apparatus
dew point (ADP)
td1 = dry bulb temperature of air entering the cooling coil
td2 = dry bulb temperature of air leaving the cooling coil
tdp1 = dew point temperature of the entering air = td3
td4 = effective surface temperature or ADP of the cooling coil

Under ideal conditions, the dry bulb temperature of the air leaving
the cooling coil should be equal to the surface temperature of the
cooling coil.
By-pass factor for cooling coil,
BPF =
𝐭𝐝𝟐−𝐭𝐝𝟒
𝐭𝐝𝟏−𝐭𝐝𝟒
=
𝐭𝐝𝟐−𝐀𝐃𝐏
𝐭𝐝𝟏−𝐀𝐃𝐏
Sensible heat removed, SH = hA – h2
Latent heat removed, LH = h1 – hA
SHF =
𝐒𝐇
𝐒𝐇+𝐋𝐇
=
hA – h2
(hA – h2) +(h1 – hA)
=
hA – h2
(h1 – h2)
The line 1-4 is known as sensible heat factor line
5.Cooling and Humidification
In this process, the dry bulb temperature decreases and the
specific humidity of air increases. The humidification and cooling
follows the path along the constant wet bulb temperature line or
constant enthalpy line

Effectiveness or the humidifying efficiency of the spray chamber
=
𝐀𝐜𝐭𝐮𝐚𝐥 𝐝𝐫𝐨𝐩 𝐢𝐧 𝐃𝐁𝐓
𝐈𝐝𝐞𝐚𝐥 𝐝𝐫𝐨𝐩 𝐢𝐧 𝐃𝐁𝐓
=
𝐭𝐝𝟏−𝐭𝐝𝟐
𝐭𝐝𝟏−𝐭𝐝𝟑
=
𝐀𝐜𝐭𝐮𝐚𝐥 𝐝𝐫𝐨𝐩 𝐢𝐧 𝐬𝐩𝐞𝐜𝐢𝐟𝐢𝐜 𝐡𝐮𝐦𝐢𝐝𝐢𝐭𝐲
𝐈𝐝𝐞𝐚𝐥 𝐝𝐫𝐨𝐩 𝐢𝐧 𝐬𝐩𝐞𝐜𝐢𝐟𝐢𝐜 𝐡𝐮𝐦𝐢𝐝𝐢𝐭𝐲
=
𝐖𝟐−𝐖𝟏
𝐖𝟑−𝐖𝟏
Cooling and humidification by water injection

Mass balance, W2 = W1 +
𝐦𝐰
𝐦𝐚
Heat balance, h2 = h1 +
𝐦𝐰
𝐦𝐚
hw
Here, h2 = h1 ; constant enthalpy process
• mw = mass of water supplied
• ma = mass of dry air
• W1= Specific humidity of entering air
• W2 = Specific humidity of leaving air
• hfw or hw = Enthalpy of water injected into the air
6.Heating and Humidification
This process is generally used in winter air conditioning to warm
and humidify the air. It is the reverse process of cooling and
dehumidification. In this process, the dry bulb temperature as
well as the specific humidity of air increases. The final relative
humidity of the air can be lower or higher than that of the entering
air

SHF =
𝐒𝐇
𝐒𝐇+𝐋𝐇
=
hA – h1
(hA – h1) +(h2 – hA)
=
hA – h1
(h2 – h1)
Heating and humidification by steam injection
Mass balance, W2 = W1 +
𝐦𝐬
𝐦𝐚
Heat balance, h2 = h1 +
𝐦𝐬
𝐦𝐚
hs
• Ms = mass of steam supplied
• ma = mass of dry air entering
• W1= Specific humidity of entering air
• W2 = Specific humidity of leaving air
• h1 = Enthalpy of air entering
• h2 = Enthalpy of air leaving
• hs = Enthalpy of steam injected into the air

7.Heating and Dehumidification- Adiabatic
chemical Dehumidification
This process is mainly used in industrial air conditioning and can
also be used for some comfort air conditioning installations
requiring either a low relative humidity or low dew point
temperature in the room
In this process, the air is passed over chemicals which have an
affinity for moisture. As the air comes in contact with these
chemicals, the moisture gets condensed out of the air and gives
up its latent heat. Due to the condensation, the specific humidity
decreases and the heat of condensation supplies sensible heat for
heating the air and thus increasing its dry bulb temperature
Effectiveness or efficiency of the dehumidifier
=
𝐀𝐜𝐭𝐮𝐚𝐥 𝐢𝐧𝐜𝐫𝐞𝐚𝐬𝐞 𝐢𝐧 𝐃𝐁𝐓
𝐈𝐝𝐞𝐚𝐥 𝐢𝐧𝐜𝐞𝐫𝐚𝐬𝐞 𝐢𝐧 𝐃𝐁𝐓
=
𝐭𝐝𝟑−𝐭𝐝𝟏
𝐭𝐝𝟐−𝐭𝐝𝟏

Adiabatic Mixing of Two Air Streams
When two quantities of air having different enthalpies and
different specific humidities are mixed, the final condition of the
air mixture depends upon the masses involved, and on the
enthalpy and specific humidity of each of the constituent masses
which enter the mixture
Mass balance, m1 + m2 = m3
Energy balance, m1h1 + m2h2 = m3h3
Mass balance of water vapour, m1W1 + m2W2 = m3W3
𝐦𝟏
𝐦𝟐
=
𝐡𝟑− 𝐡𝟐
𝐡𝟏− 𝐡𝟑
= =
𝐖𝟑− 𝐖𝟐
𝐖𝟏− 𝐖𝟑

Comfort Conditions
Human body maintains its thermal equilibrium with the
environment by means of 3 modes of heat transfer
 Evaporation
 Radiation
 Convection
Factors Affecting Human Comfort
 Effective temperature
 Heat production and regulation in human body
 Heat and moisture losses from the human body
 Moisture content of air
 Quality and quantity of air
 Air motion
 Hot and cold surfaces
 Air stratification
1. Effective Temperature
The degree of warmth or cold felt by a human body depends
mainly on the following 3 factors
 Dry bulb temperature
 Relative humidity
 Air velocity
In order to evaluate the combined effect of these factors, the term
effective temperature is employed
The numerical value of effective temperature is made equal to
the temperature of still (5 to 8 m/min air velocity) saturated air,
which produces the same sensation of warmth or coolness as
produced under the given conditions

In the comfort chart, the dry bulb temperature is taken as abscissa
and the wet bulb temperature as ordinates. The relative humidity
lines are replotted from the psychrometric chart. The statistically
prepared graphs corresponding to summer and winter season are
also superimposed
A close study of the chart reveals that the several combinations
of wet and dry bulb temperatures with different relative
humidities will produce the same effective temperature. All
points located on a given effective temperature line do not
indicate conditions of equal comfort or discomfort

 For summer air conditioning: 98% people felt comfortable for
an effective temperature of 21.60C
 For winter air conditioning: 97.7% people felt comfortable for
an effective temperature of 200C
The same effective temperature is observed at higher dry bulb and
wet bulb temperatures with higher velocities
2. Heat production and regulation in human body
Human body acts like a heat engine which gets its energy from
the combustion of food within the body. The process of
combustion (called metabolism) produces heat and energy due to

oxidation of products in the body by oxygen obtained from
inhaled air. The rate of heat production depends upon the
individuals health, his physical activity and his environment
The human body has a thermal efficiency of 20%, therefore the
remaining 80% of the heat must be rejected to the surrounding
environment. The rate and the manner of rejection of heat is
controlled by the automatic regulation system of a human body.
The heat loss from the skin is take place by radiation, convection
and by evaporation
When surrounding air temperature is above the blood
temperature, the heat is removed by evaporation. When
surrounding air temperature is below the blood temperature, the
heat is removed by radiation and convection. In case the body
fails to throw off the requisite amount of heat, the blood
temperature rises. This results in the accumulation of heat which
will cause discomfort
3. Heat and moisture losses from the human body
The heat is given off from the human body as either sensible or
latent heat or both. In order to design any air-conditioning system
for spaces which human bodies are to occupy, it is necessary to
know the rates at which these two forms of heat are given off
under different conditions of air temperature and bodily activity
Curve A – men working at the rate of 90 KNm/h
Curve B – men working at the rate of 45 KNm/h
Curve C – men working at the rate of 22.5 KNm/h
Curve D – men at rest

At the lower effective temperature, the heat dissipation increases
which results in a feeling of coolness. At higher effective
temperature, the ability to lose heat rapidly decreases resulting in
severe discomfort
4. Moisture content of air
The moisture content of outside air during winter is generally low
and it is above the average during summer, because the capacity
of the air to carry moisture is dependent upon its dry bulb
temperature
 In Winter moisture is added to the air by the process of
humidification
 In summer, the moisture is removed from the air by the
dehumidification process

5. Quality and quantity of air
The air should be free from toxic, unhealthful or disagreeable
fumes. It should be free from dust and odour. Clean air must
always be supplied to an occupied space. In general, when there
is no smoking in a room, 1 m3
/min per person of outside air will
take care of all the conditions. When smoking takes place in a
room, 1.5 m3
/min per person of outside air is necessary. For
general application, a minimum of 0.3 m3
/min of outside air per
person, mixed with 0.6 m3
/min of recirculated air is good
6. Air Motion
The distribution of air is very important to maintain uniform
temperature in the conditioned space. The air velocity in the
occupied zone should not exceed 8 to 12 m/min. The flow of air
should be preferably towards the faces of the individuals rather
than from the rear in the occupied zone. For the proper and perfect
distribution of air in the air conditioned space, down flow should
be preferred instead of up flow
7. Cold and hot surfaces
The cold or hot objects in a conditioned space may cause
discomfort to the occupants. In the designing of an air
conditioning system, the temperature of the surfaces to which the
body may be exposed must be given considerable importance
8. Air Stratification
When air is heated, its density decreases and thus it rises to the
upper part of the confined space. This results in a considerable
variation in the temperatures between the floor and ceiling levels.
The movement of the air to produce the temperature gradient
from floor to ceiling is termed as air stratification. Air

conditioning system must be designed to reduce the air
stratification to a minimum
Room Sensible Heat Factor
It is defined as the ratio of the room sensible heat to the room total
heat
RSHF =
𝐑𝐒𝐇
𝐑𝐓𝐇
=
𝐑𝐒𝐇
𝐑𝐒𝐇+𝐑𝐋𝐇
RSH = Room sensible heat
RLH = Room latent heat
RTH = Room total heat
The conditioned air supplied to the room must have the capacity to take up
simultaneously both the room sensible heat and room latent heat loads. The
point S on the psychrometric chart represents the supply air condition and
the point R represents the required final condition in the room (i.e. room
design condition). The line SR is called the room sensible heat factor line
(RSHF line)

The slope of RSHF line gives the ratio of the room sensible heat
(RSH) to the room latent heat (RLH). Thus the supply air having
its conditions given by any point on this line will satisfy the
requirements of the room with adequate supply of such air. The
supply air having conditions marked by points S1 S2, S3 S4 etc.,
will satisfy the requirement but the quantity of air supplied will
be different for different supply air points. The supply condition
at S requires minimum air and at point S4, it is maximum of all
the four points.
Procedure for drawing RSHF Line
When the supply air conditions are not known, the room sensible
heat factor line may be drawn from the calculated value of room
sensible heat factor (RSHF)
 Mark point ‘a’ on the sensible heat factor scale given on the
right hand corner of the psychrometric chart as shown in the
Figure. The point ‘a’ represents the calculated value of RSHF.
 Join point ‘a’ with the alignment circle (260
C DBT, 50%
relative humidity) or the reference point ‘b’. The line ab is
called base line.

 Mark point ‘R’ on the psychrometric chart to represent the
room design conditions.
 Through point ‘R’ draw a line RR' parallel to the base line ab.
This line is the required room sensible heat factor line.
 Note: In a cooling and dehumidification process, the
temperature at which the room sensible heat factor line
intersects the saturation curve is called room apparatus dew
point (ADP).
Grand Sensible Heat Factor
It is defined as the ratio of the total sensible heat to the grand total
heat which the cooling coil or the conditioning apparatus is
required to handle
GSHF =
𝐓𝐒𝐇
𝐆𝐓𝐇
=
𝐓𝐒𝐇
𝐓𝐒𝐇+𝐓𝐋𝐇
=
𝐑𝐒𝐇+𝐎𝐀𝐒𝐇
(𝐑𝐒𝐇+𝐎𝐀𝐒𝐇)+(𝐑𝐋𝐇+𝐎𝐀𝐋𝐇)

TSH = Total Sensible Heat = RSH + OASH
TLH = Total Latent Heat = RLH + OALH
GTH = Grand Total Heat
= (TSH + TLH)
= (RSH + RLH + OATH)
= (RSH + RLH + OASH + OALH)
Outside air sensible heat, OASH = 0.02044V1 (td1-td2) kW
Outside air latent heat, OALH = 50V1 (W1-W2) kW
Outside air total heat, OATH = OASH + OALH
= 0.02V1 (h1-h2) kW
• V1 = volume of outside air or ventilation in m3
/min
• td1 = dry bulb temperature of outside air in o
C
• W1 = specific humidity of outside air in kg/kg of dry air
• h1 = enthalpy of outside air in kJ/kg of dry air
• td2 = dry bulb temperature of room air in o
C
• W2 = specific humidity of room air in kg/kg of dry air
• h2 = enthalpy of room air in kJ/kg of dry air
Line 3-4: GSHF Line
Line 4-2: RSHF Line
Generally, the air supplied to the air conditioning plant is a
mixture of fresh air (or outside air or ventilation) and the
recirculated air having the properties of room air. On the
psychrometric chart, as shown in the Figure, the point 1
represents the outside condition of air, the point 2 represents the
room air condition and the point 3 represents the mixture

condition of air entering the cooling coil. When the mixture
condition enters the cooling coil or conditioning apparatus, it is
cooled and dehumidified. The point 4 shows the supply air or
leaving condition of air from the cooling coil or conditioning
apparatus. When the point 3 is joined with the point 4, it gives a
grand sensible heat factor line (GSHF line). This line, when
produced up to the saturation curve, gives apparatus dew point
(ADP).
Effective Room Sensible Heat Factor
It is defined as the ratio of the effective sensible heat to the
effective room total heat
ERSHF =
𝐄𝐑𝐒𝐇
𝐄𝐑𝐓𝐇
=
𝐄𝐑𝐒𝐇
𝐄𝐑𝐒𝐇+𝐄𝐑𝐋𝐇

Effective room sensible heat, ERSH = RSH + (OASH X BPF)
Effective room latent heat, ERLH = RLH + (OALH X BPF)
Effective room total heat, ERTH = ERSH + ERLH
Outside air sensible heat, OASH = 0.02044V1 (td1-td2) kW
Outside air latent heat, OALH = 50V1 (W1-W2) kW
The line joining the point 2 and point 6 gives the effective room
sensible heat factor line (ERSH Line)
BPF =
𝐭𝐝𝟒−𝐀𝐃𝐏
𝐭𝐝𝟑−𝐀𝐃𝐏
Summer Air Conditioning Systems
1. Simple system with 100 % re-circulated air

In this simple system, there is no outside air and the same air is
recirculated. It can be seen that cold and dry air is supplied to the
room and the air that leaves the condition space is assumed to be
at the same conditions as that of the conditioned space. The
supply air condition should be such that as it flows through the
conditioned space it can counteract the sensible and latent heat
transfers taking place from the outside to the conditioned space,
so that the space can be maintained at required low temperature
and humidity.
The Room Sensible Cooling load (Qs,r), Room Latent Cooling
Load (Ql,r) and Room Total Cooling load (Qt,r) are given by
Room Sensible Heat, Qs,r or RSH = mscp(ti-ts)
Room Total Heat, Qt,r or RTH = ms (hi-hs)
Room Latent Heat, Ql,r or RLH = TH –SH
i = room design condition
s = supply condition
ms = mass of air supplied to the room
ts = dry bulb temperature of supply air
hs = enthalpy of supply air
ti = dry bulb temperature of room air
hi = enthalpy of room air
From cooling load calculations, the sensible, latent and total
cooling loads on the room are obtained. Hence one can find the
Room Sensible Heat Factor (RSHF) from the equation
RSHF =
𝐑𝐒𝐇
𝐑𝐓𝐇
=
𝐑𝐒𝐇

It should be noted that for the given room sensible and latent
cooling loads, the supply condition must always lie on this line so
that the it can extract the sensible and latent loads on the
conditioned space in the required proportions.
Since the case being considered is one of 100 % re-circulation,
the process that the air undergoes as it flows through the cooling
coil (i.e. process i-s) will be exactly opposite to the process
undergone by air as it flows through the room (process s-i). Thus,
the temperature and humidity ratio of air decrease as it flows
through the cooling coil, and temperature and humidity ratio
increase as air flows through the conditioned space. Assuming no
heat transfer due to the ducts and fans, the sensible and latent heat
transfer rates at the cooling coil are exactly equal to the sensible
and latent heat transfer rates to the conditioned space; i.e.,
2. System with outdoor air for ventilation
In actual air conditioning systems, some amount of outdoor
(fresh) air is added to take care of the ventilation requirements.
Normally, the required outdoor air for ventilation purposes is
known from the occupancy data and the type of the building (e.g.
operation theatres require 100% outdoor air). Normally either the
quantity of outdoor air required is specified in absolute values or
it is specified as a fraction of the re-circulated air.
Case i) By-pass factor of the cooling coil is zero
Figure shows the schematic of the summer air-conditioning
system with outdoor air and the corresponding process on
psychrometric chart, when the by-pass factor is zero

Here s-i line is called RSHF line. The intersection of RSHF line
with the saturation curve gives the room ADP. As shown on the
psychrometric chart, when the by-pass factor is zero, the room
ADP is equal to coil ADP, which in turn is equal to the
temperature of the supply air.
ms = mass of supply air or total mass of air supplied to the room
mo = mass of outside fresh air
mrc= mass of recirculated air from the room
ho= enthalpy of outside air fresh air
hm = enthalpy of mixture before entering the cooling coil (Fresh
air + Recirculated air)
hi = enthalpy of room or inside condition = enthalpy of
recirculated air
to= dry bulb temperature of fresh outside air
tm = dry bulb temperature of mixture before entering the cooling
coil (Fresh air + Recirculated air)
ts =dry bulb temperature of supply air or cooling coil
temperature(ADP)

ti =dry bulb temperature of room or inside air = dry bulb
temperature of recirculated air
Where mrc is the re-circulated air flow rate and mo is the outdoor
air flow rate. Since either mo or the ratio mo:mrc are specified ,one
can calculate the amount of re-circulated air
Where ‘m’ refers to the mixing condition which is a result of
mixing of the recirculated air with outdoor air. Applying mass
and energy balance to the mixing process one can obtain the state
of the mixed air from the equation:
Energy balance, moho + mrchi = mshm
𝐦𝐨
𝐦𝐫𝐜
=
𝐡𝐦− 𝐡𝐢
𝐡𝐨− 𝐡𝐦
Since hm>hi the load on the cooling coil is greater than the load
on the conditioned space .This is of course due to the fact that
during mixing , some amount of hot and humid air is added and
the same amount of relative cool and dry air is exhausted
Fresh Air Load
Fresh Air Sensible Heat, Qs,f = mocp(to-ti)

Fresh Air Total Heat, Qt,f = mo (ho-hi)
Fresh Air Latent Heat, Ql,f = TH –SH
Room Load
RSHF =
𝐑𝐒𝐇
𝐑𝐓𝐇
=
𝐑𝐒𝐇
The difference between the cooling coil and conditioned space
increases as the amount of outdoor air (mo) increases and /or the
outdoor air becomes hotter and more humid.
The sensible, latent and total heat transfer rate, Qs,c, Ql,c and Qt,c
at the cooling coil are given by:
Cooling Coil Sensible Heat, Qs,c = mscp(tm-ts)
Cooling Load or Cooling Coil Total Heat,
Qt,c = ms (hm-hs)
Cooling Coil Latent Heat, Ql,c = TH –SH

The line joining the mixed condition ‘m’ with the coil ADP is the
process line undergone by the air as it flows through the cooling
coil .The slope of this line depends on the Coil Sensible Heat
Factor (CSHF)given by:
CSHF =
𝐐𝐒,𝐂
𝐐𝐭,𝐂
=
𝐐𝐒,𝐂
𝐐𝐒,𝐂+ 𝐐𝐥,𝐂
Case ii) By-pass factor of the cooling coil is greater than zero
Figure shows the schematic of the summer air-conditioning
system with outdoor air and the corresponding process on
psychrometric chart, when the by-pass factor is greater than zero

For actual cooling coils ,the by pass factor will be greater than
zero ,as a result the air temp at the exit of the cooling coil will be
higher than the coil ADP
Here s-i line is called RSHF line. As shown on the psychrometric
chart, when the by-pass factor is greater than zero, then the supply
condition is not equal to ADP or supply condition never on the
saturation curve
ms = mass of supply air or total mass of air supplied to the room
mo = mass of outside fresh air
mrc= mass of recirculated air from the room
ho= enthalpy of outside air fresh air
hm = enthalpy of mixture before entering the cooling coil (Fresh
air + Recirculated air)
hi = enthalpy of room or inside condition = enthalpy of
recirculated air
to= dry bulb temperature of fresh outside air
tm = dry bulb temperature of mixture before entering the cooling
coil (Fresh air + Recirculated air)
ts =dry bulb temperature of supply air
ti =dry bulb temperature of room or inside air = dry bulb
temperature of recirculated air
Where mrc is the re-circulated air flow rate and mo is the outdoor
air flow rate. Since either mo or the ratio mo:mrc are specified ,one
can calculate the amount of re-circulated air

Where ‘m’ refers to the mixing condition which is a result of
mixing of the recirculated air with outdoor air. Applying mass
and energy balance to the mixing process one can obtain the state
of the mixed air from the equation:
Energy balance, moho + mrchi = mshm
𝐦𝐨
𝐦𝐫𝐜
=
𝐡𝐦− 𝐡𝐢
𝐡𝐨− 𝐡𝐦
Since hm>hi the load on the cooling coil is greater than the load
on the conditioned space .This is of course due to the fact that
during mixing , some amount of hot and humid air is added and
the same amount of relative cool and dry air is exhausted
Fresh Air Load
Fresh Air Sensible Heat, Qs,f = mocp(to-ti)
Fresh Air Total Heat, Qt,f = mo (ho-hi)
Fresh Air Latent Heat, Ql,f = TH –SH
Room Load

RSHF =
𝐑𝐒𝐇
𝐑𝐓𝐇
=
𝐑𝐒𝐇
The difference between the cooling coil and conditioned space
increases as the amount of outdoor air (mo) increases and /or the
outdoor air becomes hotter and more humid.
The sensible, latent and total heat transfer rate, Qs,c, Ql,c and Qt,c
at the cooling coil are given by:
Cooling Coil Sensible Heat, Qs,c = mscp(tm-ts)
Cooling Load or Cooling Coil Total Heat,
Qt,c = ms (hm-hs)
Cooling Coil Latent Heat, Ql,c = TH –SH
The line joining the mixed condition ‘m’ with the coil ADP is the
process line undergone by the air as it flows through the cooling
coil .The slope of this line depends on the Coil Sensible Heat
Factor (CSHF)given by:
CSHF =
𝐐𝐒,𝐂
𝐐𝐭,𝐂
=
𝐐𝐒,𝐂
𝐐𝐒,𝐂+ 𝐐𝐥,𝐂

Prepared By
ARUN JOSE TOM
ASSISTANT PROFESSOR
DEPARTMENT OF MECHANICAL
ENGINEERING
CARMEL COLLEGE OF ENGINEERING &
TECHNOLOGY

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