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

O Psychrometry is the science dealing with the physical laws of air – water vapourmixtures.

O When 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.

O In 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 PointOCondensati

on occurs at the Dew Point Temperature

Relative Humidity

RelativeHumidity

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 RatioOActual 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 m1 2

t1

h1

t2

h2

Q

Sensible heatingtcmQ 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 m1 2

t1

h1

t2

h2

Q

18 KChatterjea, 2008

Sensible Heat & Cooling Processes 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.

3. Cooling and Dehumidifying: It is the removal of heat and moisture from moist air, it involves sensible and latent heat transfer.

m m1 2

t1

h1

t2

h2

Q

mw

hw

20 KChatterjea, 2008

Psychrometry - DehumidificationPsychrometry - 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.

Dehumidification by Cooling

4. Heating and Humidifying: It is the addition of heat and moisture to moist air, it also involves sensible and latent heat transfer.

m m1 2

t1

h1

t2

h2

Q

mw

hw

5. HUMIDIFYING : It is the addition of moisture to moist air without the addition of heat.

m m1 2

t1

h1

t2

h2

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 mixingO Governing equation

hmQhmoutin

29 KChatterjea, 2008

Psychrometry – Climate Classification

Psychrometry – Climate Classification

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

DryBlub

Relative

Humidity

DewPoint

100%Saturatio

n

Enthalpy

WetBlub

Grains of

Moisture

SpecificVolume

Breakdown of the Lines

Psychrometric chart description

O Vertical lines are dry-bulb temperatureO Horizontal lines are humidity ratio (right

axis) or dew point temp (left axis)O Slanted lines are wet-bulb temp and

enthalpyO 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…..

Any Questions???

-THANK YOU FOR YOUR ATTENTION….