Table of Contents

Introduction To Psychometric Processes – Learnmech

INTRODUCTION: –

The psychrometry is that branch of engineering science, which deals with the study of moist air i.e., dry air mixed with water vapour or humidity. It also includes the study of behaviors of dry air and water vapour mixture under various sets of conditions. Thought the earth’s
atmosphere is a mixture of gases including nitrogen, oxygen, argon and carbon dioxide, yet for the purpose of psychrometry, it is considered to be a mixture of dry air and water vapour only.

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PSYCHROMETRIC PROCESSES: –

The various psychometric process involved in air conditioning to vary the psychometric properties of air according to the requirements are as below: –

1. Sensible heating,
2. Sensible cooling,
3. Humidification and dehumidification,
4. Cooling and adiabatic humidification,
5. Cooling and humidification by water injection,
6. Heating and humidification,
7. Humidification by steam injection,
8. Adiabatic chemical dehumidification,
9. Adiabatic mixing of air streams.

SENSIBLE HEATING: –

The heating air without any change in its specific humidity is known as sensible heating. Let air at temperature tdl passes over a heating coil temperature td3 . It may be noted that the temperature of air leaving the heating coil will be less than td3. The process of sensible heating, on the psychometric chart, is shown by a horizontal line 1-2 extending from left to right as. The point 3 presents the surface temperature of the heating coil.

SENSIBLE COOLING: –

The cooling of air, without any change in its specific humidity, is known as sensible cooling. Let air at temperature tdl passes over a cooling coil of temperature td3 . It may be noted that the temperature of air leaving the cooling coil will be more than td3.The process of sensible cooling on the psychometric chart, is shown by a horizontal line 1-2 extending from right to left. The point 3 represents the surface temperature of the cooling coil.

HUMIDIFICATION AND DEHUMIDIFICATION: –

The addition of moisture to the air, without change in its dry bulb temperature, is known as humidification. Similarly, removal of moisture from the air, with out change in its dry bulb temperature is known as dehumidification process is shown on the psychometric chart.

It may be noted that in humidification, the relative humidity increases from Φ1 to Φ2 and specific humidity also increases from W1 to W2 as shown. Similarly, in dehumidification, the relative humidity decreases from Φ1 to Φ2 and specific humidity also decreases from W1 to
W2.
It may be noted that in humidification, change in enthalpy is shown by the intercept ( h2 – h1) on the psychometric chart. Since the dry bulb temperature of air during the humidification remains constant, therefore, its sensible heat also remains constant. It is thus obvious that the
change in enthalpy per kg of dry air due to the increased moisture content equal to (W2 – W1) kg per kg of dry air is considered causes a latent heat transfer ( LH) .

COOLING AND DEHUMIDIFICATION: –

This process is generally used in summer air conditioning to cool and dehumidify the air,. The air is passed over a cooling coil through a cold-water spray. In this process, the dry bulb temperature as well as the specific humidity of air decreases. The final relative humidity of
the air is generally higher than that of the entering air. The dehumidification of air is only possible when the effective surface of the cooling coil is less than the dew point temperature of the entering the coil. The effective surface of the coil is known as apparatus dew point.

COOLING WITH ADIABATIC HUMIDIFICATION: –

When the air is passed through an insulated chamber having sprays of water maintained at a temperature higher than the dew point temperature of entering air, but lower than its dry bulb temperature of entering air equal to the wet bulb temperature of the entering air then the air is said to be cooled and humidified. Since no heat is supplied or rejected from the spray water as the same water is recirculated again and again. Therefore, in this case, a condition of adiabatic saturation will be reached. The temperature of spray water will reach the
thermodynamic wet bulb temperature of the air entering the spray water. This process is shown by line 1-3 on the psychometric chart. The path along the constant wet bulb temperature line or constant enthalpy line.

In an ideal case i.e when the humidification is perfect, the final condition of the air will be point 3. In actual practice, perfect humidification is never achieved. Therefore, the final condition of air at outlet is represented by point 2 on the line 1-3.

COOLING AND HUMIDIFICATION BY WATER INJECTION (EVAPORATIVE COOLING): –

Let water at a temperature tl is injected into the flowing stream of dry air. The final condition of air depends upon the amount of water evaporation. When the water is injected at a temperature equal to the wet bulb temperature of the entering air. When the process follows
the path of constant wet bulb temperature line.

HEATING AND HUMIDIFICATION: –

The process is generally used in winter air conditioning to warm and humidify the air. It is the reverse process of cooling and dehumidification. When air is passed through a humidifier having spray water temperature higher than the dry bulb temperature of the entering air, the unsaturated air will reach the condition of saturation and thus the air becomes hot. The heat of vaporization of water is absorbed from the spray water itself and hence it gets cooled. In this way, the air becomes heated and humidified . The process of heating and humidification is shown by line 1-2 on the psychometric chart as shown. The air enters at condition 1 and leaves at condition 2. In this process, the dry bulb temperature as well as specific humidity of air increases. The final relative humidity of the air can be lower or higher than that of air increases. The final relative humidity of the air can be lower or higher than that of the entering air.

HEATING AND HUMIDIFICATION BY STEAM INJECTION: –

The steam is normally injected into the air in order to increase its specific humidity as shown. This process is used for the air conditioning of textile mills where high humidity is to be maintained. The dry bulb temperature of air changes very little during this process, as
shown on the psychometric chart.

HEATING AND DEHUMIDIFICATION –ADIABATIC CHEMICAL DEHUMIDIFICATION: –

This process is mainly used in industrial air conditioning and can also 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 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. The process, which is the reverse of adiabatic saturation process, is shown by the line 1-2 on the psychometric chart as shown. The path followed during the process is along the constant wet bulb temperature line or constant enthalpy line.

ADIABATIC MIXING OF TWO AIR STREAMS: –

When two quantities of air having different enthalpies and different specific humilities 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 mixtutre.
Now consider two air streams 1 and 2 mixing adiabatically as shown.

Let m1 = Mass of air entering at 1,
h1 = Enthalpy of air entering at 1,
W1 = Specific humidity of air entering at 1,
m2 , h2 , W2 = Corresponding values of air entering at 2, and
m3 , h3 , W3 = Corresponding values of the mixture leaving at 3.

m1 h3 – h2 W3 – W2
________ = ______________ = ___________
m2 h1 – h3 W1 – W3