1. The document discusses various psychrometric processes including sensible cooling and heating of air, humidification, dehumidification, cooling and humidification, and heating and humidification. 2. Sensible cooling involves reducing the dry bulb temperature of air without changing its moisture content, represented as a horizontal line moving left on a psychrometric chart. Sensible heating is the opposite. 3. Cooling and humidification reduces dry bulb temperature while increasing moisture content, shown as an angled line moving up and left. Heating and humidification increases both temperature and moisture.

1. 1 PSYCOMETRY Notes By: Muhammad Sheeraz
Psychrometric Processes: Sensible Cooling and Sensible Heating of Air
 Psychrometric Processes
In the domestic and industrial air conditioning applications some psychrometric
processes have to be performed on the air to change the psychrometric properties
of air so as to obtain certain values of temperature and humidity of air within the
enclosed space. Some of the common psychrometric processes carried out on air
are: sensible heating and cooling of air, humidification and dehumidification of air,
mixing of various streams of air, or there may be combinations of the various
processes.
Illustrating and analyzing the psychrometric properties and psychrometric
processes by using the psychrometric chartis very easy, convenient and time
saving. In the next few paragraphs we shall see some of the most commonly
employed psychrometric processes in the field of HVAC and how they are
represented on the psychrometric chart.
 Sensible Cooling of the Air
Cooling of the air is one of the most common psychrometric processes in the air
conditioning systems. The basic function of the air-conditioners is to cool the air
absorbed from the room or the atmosphere, which is at higher temperatures. The
sensible cooling of air is the process in which only the sensible heat of the air is
removed so as to reduce its temperature, and there is no change in the moisture
content (kg/kg of dry air) of the air. During sensible cooling process the dry bulb
(DB) temperature and wet bulb (WB) temperature of the air reduces, while the
latent heat of the air, and the dew point (DP) temperature of the air remains
constant. There is overall reduction in the enthalpy of the air.
In the ordinary window or the split air conditioner the cooling of air is carried out
by passing it over the evaporator coil, also called as the cooling coil. The room air
or the atmospheric air passes over this coil carrying the refrigerant at extremely
low temperatures, and gets cooled and passes to the space which is to be
maintained at the comfort conditions.
In general the sensible cooling process is carried out by passing the air over the
coil. In the unitary air conditioners these coils are cooled by the refrigerant passing
through them and are called also called evaporator coils. In central air conditioners

2. 2 PSYCOMETRY Notes By: Muhammad Sheeraz
these coils are cooled by the chilled water, which is chilled by its passage through
the evaporator of the large air conditioning system. In certain cases the coil is also
cooled by the some gas passing inside it.
The sensible cooling process is represented by a straight horizontal line on
the psychrometric chart. The line starts from the initial DB temperature of the air
and ends at the final DB temperature of the air extending towards the left side
from high temperature to the low temperature (see the figure below). The sensible
cooling line is also the constant DP temperature line since the moisture content of
the air remains constant. The initial and final points on the psychrometric chart
give all the properties of the air.
 Sensible Heating of the Air
Sensible heating process is opposite to sensible cooling process. In sensible heating
process the temperature of air is increased without changing its moisture content.
During this process the sensible heat, DB and WB temperature of the air increases
while latent of air, and the DP point temperature of the air remains constant.
Sensible heating of the air is important when the air conditioner is used as the heat
pump to heat the air. In the heat pump the air is heated by passing it over the
condenser coil or the heating coil that carry the high temperature refrigerant. In
some cases the heating of air is also done to suit different industrial and comfort
air-conditioning applications where large air conditioning systems are used.
In general the sensible heating process is carried out by passing the air over the
heating coil. This coil may be heated by passing the refrigerant, the hot water, the
steam or by electric resistance heating coil. The hot water and steam are used for
the industrial applications.

3. 3 PSYCOMETRY Notes By: Muhammad Sheeraz
Like the sensible cooling, the sensible heating process is also represented by a
straight horizontal line on the psychrometric chart. The line starts from the initial
DB temperature of air and ends at the final temperature extending towards the
right (see the figure). The sensible heating line is also the constant DP temperature
line.
 What is Humidification Process?
The process in which the moisture or water vapor or humidity is added to the air
without changing its dry bulb (DB) temperature is called as humidification process.
This process is represented by a straight vertical line on the psychrometric chart
starting from the initial value of relative humidity, extending upwards and ending
at the final value of the relative humidity. In actual practice the pure humidification
process is not possible, since the humidification is always accompanied by cooling
or heating of the air. Humidification process along with cooling or heating is used
in number of air conditioning applications. Let us see how these processes are
obtained and how they are represented on the psychrometric chart.
This article describes psychrometric processes like humidification, cooling and
humidification, and heating and humidification. The article describes how these
processes are achieved and how they are represented on the psychrometric chart.
 Cooling and Humidification Process
Cooling and humidification process is one of the most commonly used air
conditioning application for the cooling purposes. In this process the moisture is
added to the air by passing it over the stream or spray of water which is at

4. 4 PSYCOMETRY Notes By: Muhammad Sheeraz
temperature lower than the dry bulb temperature of the air. When the ordinary air
passes over the stream of water, the particles of water present within the stream
tend to get evaporated by giving up the heat to the stream. The evaporated water
is absorbed by the air so its moisture content, thus the humidity increases. At the
same time, since the temperature of the absorbed moisture is less than the DB
bulb temperature of the air, there is reduction in the overall temperature of the
air. Since the heat is released in the stream or spray of water, its temperature
increases.
One of the most popular applications of cooling and humidification is the
evaporative cooler, also called as the desert cooler. The evaporative cooler is the
sort of big box inside which is a small water tank, small water pump and the fan.
The water from the tank is circulated by the pump and is also sprayed inside the
box. The fan blows strong currents of air over the water sprays, thus cooling the air
and humidifying it simultaneously. The evaporative cooler is highly effective
cooling devise having very low initial and running cost compared to the unitary air
conditioners. For cooling purposes, the cooling and humidification process can be
used only in dry and hot climates like desert areas, countries like India, China,
Africa etc. This cooling process cannot be used in hot and high humidity climates.
The cooling and humidification process is also used in various industries like textile,
where certain level of temperature and moisture content has to be maintained. In
such cases large quantity of water is sprayed, and large blowers are used to blow
the air over the spray of water.
During the cooling and humidification process the dry bulb of the air reduces, its
wet bulb and the dew point temperature increases, while its moisture content and
thus the relative humidity also increases. Also, the sensible heat of the air reduces,
while the latent heat of the air increases resulting in the overall increase in the
enthalpy of the air.
Cooling and humidification process is represented by an angular line on the
psychrometric chart starting from the given value of the dry bulb temperature and
the relative humidity and extending upwards toward left.

5. 5 PSYCOMETRY Notes By: Muhammad Sheeraz
 Heating and Humidification Process
In heating and humidification psychrometric process of the air, the dry bulb
temperature as well as the humidity of the air increases. The heating and
humidification process is carried out by passing the air over spray of water, which
is maintained at temperature higher than the dry bulb temperature of air or by
mixing air and the steam.
When the ordinary air is passed over the spray of water maintained at
temperature higher than the dry bulb temperature of the air, the moisture
particles from the spray tend to get evaporated and get absorbed in the air due to
which the moisture content of the air increase. At the same time, since the
temperature of the moisture is greater than the dry bulb temperature of the air,
there is overall increase in its temperature.
During heating and humidification process the dry bulb, wet bulb, and dew point
temperature of the air increases along with its relative humidity. The heating and
humidification process is represented on the psychrometric chart by an angular
line that starts from the given value of the dry bulb temperature and extends
upwards towards right (see the figure below).

6. 6 PSYCOMETRY Notes By: Muhammad Sheeraz
 What is Dehumidification?
The process in which the moisture or water vapor or the humidity is removed from
the air keeping its dry bulb (DB) temperature constant is called as the
dehumidification process. This process is represented by a straight vertical line on
the psychrometric chart starting from the initial value of relative humidity,
extending downwards and ending at the final value of the relative humidity. Like
the pure humidification process, in actual practice the pure dehumidification
process is not possible, since the dehumidification is always accompanied by
cooling or heating of the air. Dehumidification process along with cooling or
heating is used in number of air conditioning applications. Let us see how these
processes are obtained and how they are represented on the psychrometric chart.
This article describes psychrometric processes like dehumidification, cooling and
dehumidification, and heating and dehumidification. The article describes how
these processes are achieved and how they are represented on the psychrometric
chart.
 Cooling and Dehumidification Process
The process in which the air is cooled sensibly and at the same time the moisture is
removed from it is called as cooling and dehumidification process. Cooling and
dehumidification process is obtained when the air at the given dry bulb and dew
point (DP) temperature is cooled below the dew point temperature.
Let us understand the cooling and dehumidification process in more details. When
the air comes in contact with the cooling coil that is maintained at the temperature
below its dew point temperature, its DB temperature starts reducing. The process
of cooling continues and at some point it reaches the value of dew point
temperature of the air. At this point the water vapor within the air starts getting
converted into the dew particles due to which the dew is formed on the surface of
the cooling and the moisture content of the air reduces thereby reducing its
humidity level. Thus when the air is cooled below its dew point temperature, there
is cooling as well as dehumidification of air.
The cooling and dehumidification process is most widely used air conditioning
application. It is used in all types of window, split, packaged and central air
conditioning systems for producing the comfort conditions inside the space to be

7. 7 PSYCOMETRY Notes By: Muhammad Sheeraz
cooled. In the window and split air conditioners the evaporator coil or cooling coil
is maintained at temperature lower than the dew point temperature of the room
air or the atmospheric air by the cool refrigerant passing through it. When the
room air passes over this coil its DB temperature reduces and at the same time
moisture is also removed since the air is cooled below its DP temperature. The dew
formed on the cooling coil is removed out by small tubing. In the central air
conditioning systems the cooling coil is cooled by the refrigerant or the chilled
water. When the room air passes over this coil, it gets cooled and dehumidified.
In the general the cooling and dehumidification process is obtained by passing the
air over coil through which the cool refrigerant, chilled water or cooled gas is
passed.
During the cooling and dehumidification process the dry bulb, wet bulb and the
dew point temperature of air reduces. Similarly, the sensible heat and the latent
heat of the air also reduce leading to overall reduction in the enthalpy of the air.
The cooling and dehumidification process is represented by a straight angular line
on the psychrometric chart. The line starts from the given value of the DB
temperature and extends downwards towards left.
 Heating and Dehumidification Process
The process in which the air is heated and at the same time moisture is removed
from it is called as heating and dehumidification process. This process is obtained
by passing the air over certain chemicals like alumina and molecular sieves. These
elements have inherent properties due to which they keep on releasing the heat
and also have the tendency to absorb the moisture. These are called as the
hygroscopic chemicals.
In actual practice the hygroscopic elements are enclosed in the large vessel and
the high pressure air is passed inside the vessel through one opening. When the air

8. 8 PSYCOMETRY Notes By: Muhammad Sheeraz
comes in contact with the chemicals the moisture from the air is absorbed and
since the chemicals emit heat, the DB temperature of the air increases. The hot
and dehumidified air comes out from the vessel through other opening in the
vessel. The inlet and outlet openings of the vessel are controlled by the valve.
The heating and humidification process is commonly used for reducing the dew
point temperature of air. There are number of automatic valves in the chemical
plants that are operated by the compressed air at high pressure. If the dew point
temperature of this air is high, there are chances of formation of dew inside the
valves which can lead to their corrosion and also faulty their operation. Thus it is
very important that the air passing to such automatic valves have very low dew
point temperature. The heating and dehumidification process by using hygroscopic
materials is used often in the air drying units.
During the heating and dehumidification process dry bulb temperature of the air
increases while its dew point and wet bulb temperature reduces. On the
psychrometric chart, this process is represented by a straight angular line starting
from the given DB temperature conditions and extending downwards towards
right to the final DB temperature conditions.