PSYCHROMETRY

PSYCHROMETRY
PROPERTIES, processes AND charts
-Presented by-PRABHAV JOSHI
(ENROLL NO-120070125031)
-Guided by-PURVI MA’M

CONTENTS
 What is PSYCHROMETRY?
 Properties of
PSYCHROMETRY
 PSYCHROMETRIC Processes
 PSYCHROMETRIC CHART

Psychrometry
OPsychrometry is the science dealing
with the physical laws of air – water
vapour
mixtures.
OWhen designing an air conditioning
system, the temperature and moisture
content of the air to be conditioned,
and the same properties of the air
needed to produce the desired air
conditioning effect.
OIn other words, we can say that
Psychrometry is the study of MOIST
AIR or mixture of dry air and water
vapour.

PROPERTIES OF
PSYCHROMETRY
 DEW POINT TEMPERATURE
 RELATIVE HUMIDITY
 HUMIDITY RATIO
 DRY-BULB TEMPERATURE
 WET BULB TEMPERATURE
 HUMIDITY
• Specific humidity
• Absolute humidity
 PSYCHROMETRIC RATIO

Dew Point
O The temperature at which moisture starts to
condense out of the air is known as DEW POINT.
O Dew point is also known as saturation temperature.
O Dew point temperature is determined by moving
from a state point horizontally to the left along lines
of constant humidity ratio until the upper, curved,
saturation temperature boundary is reached.

Dew Point
OCondensatio
n occurs at
the Dew
Point
Temperature

Relative Humidity
Relative
Humidity
Amount of moisture that a given amount
of air is holding
=
Amount of moisture that a given amount
of air can hold
50% RH 100% RH -
Saturated
(percentage)

Humidity Ratio
OActual weight of
water in an air –
water vapour
mixture
OAmount of moisture
per unit of dry air
OCan be defined as,
W=m/G

DRY -BULB
TEMPERATURE
 The dry-bulb temperature is the
temperature indicated by a
thermometer exposed to the air in a
place sheltered from direct solar
radiation. The term dry-bulb is
customarily added to temperature to
distinguish it from wet-bulb and dew
point temperature.

WET BULB
TEMPERATURE
 Wet bulb temperature is the temperature
recorded by thermometer when the bulb is
enveloped by cotton wick saturated with
water.
 The accuracy of a simple wet-bulb
thermometer depends on how fast air
passes over the bulb and how well the
thermometer is shielded from the radiant
temperature of its surroundings.

THERMOMETERS
Dry Bulb
Wet Bulb
Measured in degrees
Fahrenheit (oF)

HUMIDITY
• Specific Humidity:
 Specific humidity is defined as the
proportion of the mass of water vapour
per unit mass of the moist air sample (dry
air plus the water vapour); it is closely
related to humidity ratio and always lower
in value.
• Absolute Humidity:
 The mass of water vapor per unit volume
of air containing the water vapor. This
quantity is also known as the water
vapour density.

PSYCHROMETRIC RATIO
 The psychrometric ratio is the ratio of the heat
transfer coefficient to the product of mass transfer
coefficient and humid heat at a wetted surface. It
may be evaluated with the following equation:
r = hc / ky cs
where:
 r= Psychrometric ratio, dimensionless
 hc = convective heat transfer coefficient, W m-2 K-1
 ky= convective mass transfer coefficient, kg m-2 s-1
 cs= humid heat, J kg-1 K-1

PSYCHROMETRIC PROCESSES
 SENSIBLE HEATING
 SENSIBLE COOLING
 DEHUMIDIFICATION
 HUMIDFIYING
 ADIABATIC COOLING
 ADIABATIC MIXING
(MOIST AIR & WATER VAPOR)

Psychrometric Processes or
Air conditioning Processes
1. Sensible Heating : it is addition of heat to moist air
without the addition of moisture. It follows a constant
humidity ratio line on the psychrometric chart.
m m
1 2
tt2
1
hh1
2
Q

Sensible heating
Q mc t p   

2. Sensible Cooling: It is the removal of heat from moist air
without the removal of moisture. It also follows a constant
W on the psychrometric chart.
m m
1 2
tt2
1
hh1
2
Q

Sensible Heat & Cooling Processes
O Heating & Cooling
process – addition or
removal of sensible heat
without change in
absolute moisture
content.
O NOTE: RH changes as
temperature changes.
18 KChatterjea, 2008

3. Cooling and Dehumidifying: It is the removal of heat and
moisture from moist air, it involves sensible and latent
heat transfer.
m m
1 2
tt2
1
hh1
2
Q
mw
hw

Psychrometry - Dehumidification
O Dehumidification by
cooling – in cooling
coils temp of air
reduces and the
saturation point (dew
point) is reached.
O Further cooling
results in reduction of
absolute humidity.

4. Heating and Humidifying: It is the addition of heat and
moisture to moist air, it also involves sensible and latent
heat transfer.
m m
1 2
tt2
1
hh1
2
Q
mw
hw

5. HUMIDIFYING : It is the addition of moisture to moist
air without the addition of heat.
m m
1 2
tt2
1
hh1
2
mw
hw

Removing Sensible Heat and
Moisture

Adiabatic or evaporative
cooling
 A psychrometric process which invovles
the cooling without heat loss or gain.
Sensible heat lost by air is converted to
latent heat in the added water vapor.

ADIABATIC MIXING OF
MOIST AIR & STREAM
OA psychrometric process that
invovles no net heat loss or
gain during the mixing of two
air streams.

Adiabatic mixing
     
O Governing equation
mh Q mh
in out

Psychrometry – Climate Classification
O Climate
Classification
shows the various
approximate zones
of temperature and
humidity on the
psychrometric
chart.

Breakdown of the Lines
Dry
Blub
Relative
Humidity
Dew
Point
100%
Saturation
Enthalpy
Grains
of
Moisture
Wet
Blub
Specific
Volume

Psychrometric chart
description
O Vertical lines are dry-bulb temperature
O Horizontal lines are humidity ratio (right axis) or
dew point temp (left axis)
O Slanted lines are wet-bulb temp and enthalpy
O Specific volume are the “other” slanted lines

REFERENCES
 I have collected information regarding PSYCHROMETRY topic
from the book “ENGINEERING THERMODYNAMICS” by
Mr. P. K. NAG...
 Overview of Psychometrics:
http://www.handsdownsoftware.com/Overview_of_
Psychrometrics.pdf
 For some topics I have taken a help of GOOGLE and
WIKIPEDIA…..

-THANK YOU FOR YOUR
ATTENTION….

PSYCHROMETRY

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