Short Notes: Basic Psychrometric Processes | Short Notes for Mechanical Engineering PDF Download

 Page 1


Basic Psychrometric Processes  
For comfort air conditioning, properties of air should be known so that we can 
control the environment in the space. The study of properties of ambient air is 
known as psychrometric.
Pressure: Pressure is the effect of a force applied to a surface. Pressure is the 
amount of force acting per unit area. The symbol of pressure is 7 7 - 
Total pressure of air p=pv+pa 
where, pv = partial pressure of water vapour 
ps = partial pressure of dry ai 
Partial pressure of water vapour
A = (PvJ.»' 
or A =(P„)
~[P-(P ) ^
1527.4 - U r .
U 5
I S ¦ pyt - r v ¦ 
2700
where, (p vs)wb = saturation pressure of water vapour corresponding to wet bulb 
temperature
p = atmospheric pressure of moist air 
Tw b = wet bulb temperature 
Tdb = dry bulb temperature
Dry Air: It is the mixture of gases. Generally O2 and N2 make up the major part of 
the combustion.
It consists 21 % 02 and 79% N2 by volume.
It consists 23% 02 and 77% N2 by mass.
Page 2


Basic Psychrometric Processes  
For comfort air conditioning, properties of air should be known so that we can 
control the environment in the space. The study of properties of ambient air is 
known as psychrometric.
Pressure: Pressure is the effect of a force applied to a surface. Pressure is the 
amount of force acting per unit area. The symbol of pressure is 7 7 - 
Total pressure of air p=pv+pa 
where, pv = partial pressure of water vapour 
ps = partial pressure of dry ai 
Partial pressure of water vapour
A = (PvJ.»' 
or A =(P„)
~[P-(P ) ^
1527.4 - U r .
U 5
I S ¦ pyt - r v ¦ 
2700
where, (p vs)wb = saturation pressure of water vapour corresponding to wet bulb 
temperature
p = atmospheric pressure of moist air 
Tw b = wet bulb temperature 
Tdb = dry bulb temperature
Dry Air: It is the mixture of gases. Generally O2 and N2 make up the major part of 
the combustion.
It consists 21 % 02 and 79% N2 by volume.
It consists 23% 02 and 77% N2 by mass.
Moisture: The water vapour present in the air is called moisture.
Moist Air: It is the mixture of dry air and water vapour.
Absolute Humidity: The weight of water vapour present in unit volume of air is 
called absolute humidity.
Weiaht of water vapour
Absolute hum iditv = ------ -----------------------£ -----
' Volume of air (mixture)
Specific Humidity: It is the ratio of mass of water vapour in air (mixture) to mass of 
dry air in air mixture.
„ , ... Mass of water vapour in air
Specific hum iditv = -------------------------- £ --------------
Mass o f drv air in air
if = 0.622
— = 0.622 — ——
P, P ~PT.
Relative Humidity: It is the ratio of mass of water vapour in a given volume to mass 
of water vapour in the same volume of saturated air at same temperature.
’ Pv,
where mv and ms are the mass of water vapour and mass of saturated air pv and pvs 
are the partial pressure of water vapour and partial pressure of water vapour in 
saturated air at same temperature for saturated air relative humidity is 1 or 100%.
Degree of Saturation (p): It is the ratio of mass of water vapour in a sample of air to 
mass of water vapour in saturated air at the same temperature.
m m. i f
P = ----- —
n\. m a if,
where w and ws are specific humidities of air and saturated air.
Dry Bulb Temperature (DBT): It is the normal temperature of air measured by an 
ordinary thermometer.
Wet Bulb Temperature: It is the temperature measured by a thermometer when bulb 
is covered by wet cloth.
Wet Bulb Depression (WBD) = DBT - WBT
Dew Point Temperature (DPT): It is temperature at which water vapour in air start 
condensing.
For unsaturated air, DBT > WBT > DPT 
For saturated air, DBT = WBT = DPT
Page 3


Basic Psychrometric Processes  
For comfort air conditioning, properties of air should be known so that we can 
control the environment in the space. The study of properties of ambient air is 
known as psychrometric.
Pressure: Pressure is the effect of a force applied to a surface. Pressure is the 
amount of force acting per unit area. The symbol of pressure is 7 7 - 
Total pressure of air p=pv+pa 
where, pv = partial pressure of water vapour 
ps = partial pressure of dry ai 
Partial pressure of water vapour
A = (PvJ.»' 
or A =(P„)
~[P-(P ) ^
1527.4 - U r .
U 5
I S ¦ pyt - r v ¦ 
2700
where, (p vs)wb = saturation pressure of water vapour corresponding to wet bulb 
temperature
p = atmospheric pressure of moist air 
Tw b = wet bulb temperature 
Tdb = dry bulb temperature
Dry Air: It is the mixture of gases. Generally O2 and N2 make up the major part of 
the combustion.
It consists 21 % 02 and 79% N2 by volume.
It consists 23% 02 and 77% N2 by mass.
Moisture: The water vapour present in the air is called moisture.
Moist Air: It is the mixture of dry air and water vapour.
Absolute Humidity: The weight of water vapour present in unit volume of air is 
called absolute humidity.
Weiaht of water vapour
Absolute hum iditv = ------ -----------------------£ -----
' Volume of air (mixture)
Specific Humidity: It is the ratio of mass of water vapour in air (mixture) to mass of 
dry air in air mixture.
„ , ... Mass of water vapour in air
Specific hum iditv = -------------------------- £ --------------
Mass o f drv air in air
if = 0.622
— = 0.622 — ——
P, P ~PT.
Relative Humidity: It is the ratio of mass of water vapour in a given volume to mass 
of water vapour in the same volume of saturated air at same temperature.
’ Pv,
where mv and ms are the mass of water vapour and mass of saturated air pv and pvs 
are the partial pressure of water vapour and partial pressure of water vapour in 
saturated air at same temperature for saturated air relative humidity is 1 or 100%.
Degree of Saturation (p): It is the ratio of mass of water vapour in a sample of air to 
mass of water vapour in saturated air at the same temperature.
m m. i f
P = ----- —
n\. m a if,
where w and ws are specific humidities of air and saturated air.
Dry Bulb Temperature (DBT): It is the normal temperature of air measured by an 
ordinary thermometer.
Wet Bulb Temperature: It is the temperature measured by a thermometer when bulb 
is covered by wet cloth.
Wet Bulb Depression (WBD) = DBT - WBT
Dew Point Temperature (DPT): It is temperature at which water vapour in air start 
condensing.
For unsaturated air, DBT > WBT > DPT 
For saturated air, DBT = WBT = DPT
Dew Point Depression: It is the difference between Dry Bulb Temperature (DBT) ani 
Dew Point Temperature (DPT), i.e.,
DPD = DBT - DPT
Enthalpy of Air (h): Enthalpy of air is defined as
H = Ha + Hv , H=maha+mv hv
ha = Cpa(f)
hv = 2500 + 1800f
f = Dry Bulb Temperature in °C,
h = C , . v - i t [2500 -lS C T r^ J k J kg dry air
Or h = Cpm tab w(hg — Cp v t^ b)
where, Cpm = Cpia + CpvW
Cpa, Cp v , Cp m are the specific heat of dry air water vapour and moist air respectively.
hg -? enthalpy of saturated water vapour at DPT in kJ/kg.
Psychrometric Chart
• Enthalpy is more or less constant with wet bulb temperature. Enthalpy lines 
are also downward from lelt to right.
• Constant specific Volume lines are more steeper inclined lines than wet bulb 
temperate lines or enthalpy lines
• Adiabatic lines on psychrometric chart are represented along constant 
enthalpy lines.
/
k J
Relative 
i humidity
Dew i 
po5h! ^
Moisture
content
f06T--- ?
Psychrometric chart
Sensible Heating: It is process of heating air at constant w.
Air in — Air out
Heating cod 
Heating process f, tj
Sensible heating
0 = k - i { = C rm J
where, Cpm = specific heating moist
Sensible Cooling: It is process of cooling air at constant w. for sensible cooling, the 
coil temperature must be greater than dew point temperature and less than dbt.
Page 4


Basic Psychrometric Processes  
For comfort air conditioning, properties of air should be known so that we can 
control the environment in the space. The study of properties of ambient air is 
known as psychrometric.
Pressure: Pressure is the effect of a force applied to a surface. Pressure is the 
amount of force acting per unit area. The symbol of pressure is 7 7 - 
Total pressure of air p=pv+pa 
where, pv = partial pressure of water vapour 
ps = partial pressure of dry ai 
Partial pressure of water vapour
A = (PvJ.»' 
or A =(P„)
~[P-(P ) ^
1527.4 - U r .
U 5
I S ¦ pyt - r v ¦ 
2700
where, (p vs)wb = saturation pressure of water vapour corresponding to wet bulb 
temperature
p = atmospheric pressure of moist air 
Tw b = wet bulb temperature 
Tdb = dry bulb temperature
Dry Air: It is the mixture of gases. Generally O2 and N2 make up the major part of 
the combustion.
It consists 21 % 02 and 79% N2 by volume.
It consists 23% 02 and 77% N2 by mass.
Moisture: The water vapour present in the air is called moisture.
Moist Air: It is the mixture of dry air and water vapour.
Absolute Humidity: The weight of water vapour present in unit volume of air is 
called absolute humidity.
Weiaht of water vapour
Absolute hum iditv = ------ -----------------------£ -----
' Volume of air (mixture)
Specific Humidity: It is the ratio of mass of water vapour in air (mixture) to mass of 
dry air in air mixture.
„ , ... Mass of water vapour in air
Specific hum iditv = -------------------------- £ --------------
Mass o f drv air in air
if = 0.622
— = 0.622 — ——
P, P ~PT.
Relative Humidity: It is the ratio of mass of water vapour in a given volume to mass 
of water vapour in the same volume of saturated air at same temperature.
’ Pv,
where mv and ms are the mass of water vapour and mass of saturated air pv and pvs 
are the partial pressure of water vapour and partial pressure of water vapour in 
saturated air at same temperature for saturated air relative humidity is 1 or 100%.
Degree of Saturation (p): It is the ratio of mass of water vapour in a sample of air to 
mass of water vapour in saturated air at the same temperature.
m m. i f
P = ----- —
n\. m a if,
where w and ws are specific humidities of air and saturated air.
Dry Bulb Temperature (DBT): It is the normal temperature of air measured by an 
ordinary thermometer.
Wet Bulb Temperature: It is the temperature measured by a thermometer when bulb 
is covered by wet cloth.
Wet Bulb Depression (WBD) = DBT - WBT
Dew Point Temperature (DPT): It is temperature at which water vapour in air start 
condensing.
For unsaturated air, DBT > WBT > DPT 
For saturated air, DBT = WBT = DPT
Dew Point Depression: It is the difference between Dry Bulb Temperature (DBT) ani 
Dew Point Temperature (DPT), i.e.,
DPD = DBT - DPT
Enthalpy of Air (h): Enthalpy of air is defined as
H = Ha + Hv , H=maha+mv hv
ha = Cpa(f)
hv = 2500 + 1800f
f = Dry Bulb Temperature in °C,
h = C , . v - i t [2500 -lS C T r^ J k J kg dry air
Or h = Cpm tab w(hg — Cp v t^ b)
where, Cpm = Cpia + CpvW
Cpa, Cp v , Cp m are the specific heat of dry air water vapour and moist air respectively.
hg -? enthalpy of saturated water vapour at DPT in kJ/kg.
Psychrometric Chart
• Enthalpy is more or less constant with wet bulb temperature. Enthalpy lines 
are also downward from lelt to right.
• Constant specific Volume lines are more steeper inclined lines than wet bulb 
temperate lines or enthalpy lines
• Adiabatic lines on psychrometric chart are represented along constant 
enthalpy lines.
/
k J
Relative 
i humidity
Dew i 
po5h! ^
Moisture
content
f06T--- ?
Psychrometric chart
Sensible Heating: It is process of heating air at constant w.
Air in — Air out
Heating cod 
Heating process f, tj
Sensible heating
0 = k - i { = C rm J
where, Cpm = specific heating moist
Sensible Cooling: It is process of cooling air at constant w. for sensible cooling, the 
coil temperature must be greater than dew point temperature and less than dbt.
fj f,
Sensible cooling
Humidification: The addition of moisture to air at a constant, Dry Bulb Temperature 
(DBT) is knows as humidification. It is process of increasing w at constant DBT.
DBT humidification
Dehumidification: The removal of moisture from air at a constant Dry Bulb 
Temperature is called dehumidification. It is process of decreasing w at constant 
DBT.
Heating with Humidification: It is process in which air heated with humidification. 
The heating and humidification process line (1-2) as shown in following figure. In 
this process, Dry Bulb Temperature and specific humidity of incoming air increases.
Addition of heat Q7 .2 = mg[(hr h2)-(wr w2 )h j 
h2 - hi = Sensible heat (Sh) 
h2 - h2 = latent heat (LH )
Sensible heat factor =
Sensible heat added 
Total heat added
= 5 * - k ~ } \
SH — Lh h Ii
Heating with Dehumidification: In this process, air is heated with dehumidification.
Heating with dehum idrftcation graph
Page 5


Basic Psychrometric Processes  
For comfort air conditioning, properties of air should be known so that we can 
control the environment in the space. The study of properties of ambient air is 
known as psychrometric.
Pressure: Pressure is the effect of a force applied to a surface. Pressure is the 
amount of force acting per unit area. The symbol of pressure is 7 7 - 
Total pressure of air p=pv+pa 
where, pv = partial pressure of water vapour 
ps = partial pressure of dry ai 
Partial pressure of water vapour
A = (PvJ.»' 
or A =(P„)
~[P-(P ) ^
1527.4 - U r .
U 5
I S ¦ pyt - r v ¦ 
2700
where, (p vs)wb = saturation pressure of water vapour corresponding to wet bulb 
temperature
p = atmospheric pressure of moist air 
Tw b = wet bulb temperature 
Tdb = dry bulb temperature
Dry Air: It is the mixture of gases. Generally O2 and N2 make up the major part of 
the combustion.
It consists 21 % 02 and 79% N2 by volume.
It consists 23% 02 and 77% N2 by mass.
Moisture: The water vapour present in the air is called moisture.
Moist Air: It is the mixture of dry air and water vapour.
Absolute Humidity: The weight of water vapour present in unit volume of air is 
called absolute humidity.
Weiaht of water vapour
Absolute hum iditv = ------ -----------------------£ -----
' Volume of air (mixture)
Specific Humidity: It is the ratio of mass of water vapour in air (mixture) to mass of 
dry air in air mixture.
„ , ... Mass of water vapour in air
Specific hum iditv = -------------------------- £ --------------
Mass o f drv air in air
if = 0.622
— = 0.622 — ——
P, P ~PT.
Relative Humidity: It is the ratio of mass of water vapour in a given volume to mass 
of water vapour in the same volume of saturated air at same temperature.
’ Pv,
where mv and ms are the mass of water vapour and mass of saturated air pv and pvs 
are the partial pressure of water vapour and partial pressure of water vapour in 
saturated air at same temperature for saturated air relative humidity is 1 or 100%.
Degree of Saturation (p): It is the ratio of mass of water vapour in a sample of air to 
mass of water vapour in saturated air at the same temperature.
m m. i f
P = ----- —
n\. m a if,
where w and ws are specific humidities of air and saturated air.
Dry Bulb Temperature (DBT): It is the normal temperature of air measured by an 
ordinary thermometer.
Wet Bulb Temperature: It is the temperature measured by a thermometer when bulb 
is covered by wet cloth.
Wet Bulb Depression (WBD) = DBT - WBT
Dew Point Temperature (DPT): It is temperature at which water vapour in air start 
condensing.
For unsaturated air, DBT > WBT > DPT 
For saturated air, DBT = WBT = DPT
Dew Point Depression: It is the difference between Dry Bulb Temperature (DBT) ani 
Dew Point Temperature (DPT), i.e.,
DPD = DBT - DPT
Enthalpy of Air (h): Enthalpy of air is defined as
H = Ha + Hv , H=maha+mv hv
ha = Cpa(f)
hv = 2500 + 1800f
f = Dry Bulb Temperature in °C,
h = C , . v - i t [2500 -lS C T r^ J k J kg dry air
Or h = Cpm tab w(hg — Cp v t^ b)
where, Cpm = Cpia + CpvW
Cpa, Cp v , Cp m are the specific heat of dry air water vapour and moist air respectively.
hg -? enthalpy of saturated water vapour at DPT in kJ/kg.
Psychrometric Chart
• Enthalpy is more or less constant with wet bulb temperature. Enthalpy lines 
are also downward from lelt to right.
• Constant specific Volume lines are more steeper inclined lines than wet bulb 
temperate lines or enthalpy lines
• Adiabatic lines on psychrometric chart are represented along constant 
enthalpy lines.
/
k J
Relative 
i humidity
Dew i 
po5h! ^
Moisture
content
f06T--- ?
Psychrometric chart
Sensible Heating: It is process of heating air at constant w.
Air in — Air out
Heating cod 
Heating process f, tj
Sensible heating
0 = k - i { = C rm J
where, Cpm = specific heating moist
Sensible Cooling: It is process of cooling air at constant w. for sensible cooling, the 
coil temperature must be greater than dew point temperature and less than dbt.
fj f,
Sensible cooling
Humidification: The addition of moisture to air at a constant, Dry Bulb Temperature 
(DBT) is knows as humidification. It is process of increasing w at constant DBT.
DBT humidification
Dehumidification: The removal of moisture from air at a constant Dry Bulb 
Temperature is called dehumidification. It is process of decreasing w at constant 
DBT.
Heating with Humidification: It is process in which air heated with humidification. 
The heating and humidification process line (1-2) as shown in following figure. In 
this process, Dry Bulb Temperature and specific humidity of incoming air increases.
Addition of heat Q7 .2 = mg[(hr h2)-(wr w2 )h j 
h2 - hi = Sensible heat (Sh) 
h2 - h2 = latent heat (LH )
Sensible heat factor =
Sensible heat added 
Total heat added
= 5 * - k ~ } \
SH — Lh h Ii
Heating with Dehumidification: In this process, air is heated with dehumidification.
Heating with dehum idrftcation graph
Cooling with Humidification: In this process, an air is cooled with humidification.
Cooling with humidification giaph
• V f
Cooling with Dehumidification: In this process air is cooled below the down point 
with dehumidification. It is necessary to maintain. In a cooling with 
dehumidification, both the humidity ratio and temperature of most air decrease. 
Some water vapour is condensed in the form of liquid water, called a condensate. 
This process is shown by a curve in psychrometric chart.
Q7-2 = ma [(hr h2)-(wr W 2)hf]
Since, (wr w2 )h 1 =0 
So, Q7.2 = ma(hr h2)
Sensible heat factor
h ; - h,
ht- \
where, hi and h2 = enthalpy of moist air entering and leaving the coil 
ma = mass flow rate of the condensate 
h2 = enthalpy of condensate.
Adiabatic Mixing of Air Streams: Streams of air at different condition are mixed as 
shown in figure by mass balance.
* 1
Mixing ot air streams