SNS COLLEGE OF ENGINEERING Coimbatore-107 Subject: Thermal Engineering Psychrometric Processes BY SIVAKUMAR P AP/MECH

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

Psychrometry Psychrometry is the science dealing with the physical laws of air – water vapour mixtures. 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. 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 The temperature at which moisture starts to condense out of the air is known as DEW POINT. Dew point is also known as saturation temperature. 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. To remove moisture from the air, an air conditioner evaporator coil must operate below the dew point temperature. Moisture will collect on the cold surface of an evaporator coil and then drain through the condensate line.

Dew Point Condensation 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 (percentage) The fourth property, relative humidity, is a comparison of the amount of moisture that a given amount of air is holding, to the amount of moisture that the same amount of air can hold, at the same dry-bulb temperature. 100% RH - Saturated 50% RH

Humidity Ratio Actual weight of water in an air – water vapour mixture Amount of moisture per unit of dry air Can be defined as, W=m/G Finally, humidity ratio describes the actual weight of water in an air – water vapor mixture. In other words, if one pound of air were wrung completely dry, comparing the weight of the water to the weight of the dry air would yield its humidity ratio. Humidity ratio can be expressed as pounds of moisture per pound of dry air, or as grains of moisture per pound of dry air. There are 7000 grains of water in a pound. To appreciate the magnitude of these units of measurement, at sea level one pound of 70°F air occupies approximately 13.5 cubic feet, and one grain of water in that air weighs about two-thousandths (0.002) of an ounce.

HUMIDITY Specific 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 PROCESSES SENSIBLE HEATING SENSIBLE COOLING DEHUMIDIFICATION HUMIDFIYING ADIABATIC COOLING ADIABATIC MIXING (MOIST AIR & WATER VAPOR)

Air conditioning Processes 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 1 2 t1 h1 t2 h2 Q

Sensible heating

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 1 2 t1 h1 t2 h2 Q

Sensible Heat & Cooling Processes Heating & Cooling process – addition or removal of sensible heat without change in absolute moisture content. NOTE: RH changes as temperature changes. 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 1 2 t1 h1 t2 h2 Q mw hw

Psychrometry - Dehumidification Dehumidification by cooling – in cooling coils temp of air reduces and the saturation point (dew point) is reached. Further cooling results in reduction of absolute humidity. KChatterjea, 2008

Dehumidification by Cooling

Heating and Humidifying: It is the addition of heat and moisture to moist air, it also involves sensible and latent heat transfer. m 1 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 1 2 t1 h1 t2 h2 mw hw

Removing Sensible Heat and Moisture Put all of these changes together on one chart and they show the direction the air condition will move when the dry-bulb temperature or moisture content is altered.

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 A psychrometric process that invovles no net heat loss or gain during the mixing of two air streams.

Adiabatic mixing Governing equation

Psychrometry – Climate Classification Climate Classification shows the various approximate zones of temperature and humidity on the psychrometric chart. KChatterjea, 2008

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

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

Any Questions???

-THANK YOU FOR YOUR ATTENTION….