Evaporative cooler report

1. DEPARTMENT OF MECHANICAL ENGINEERING
Report Writing
Name of Subject: Heat Transfer
Name of the Topic: Cooler
Name of Student: Suman kumar Gop
Roll No.: 43
Semester: VI
Section: B
Name of Faculty
Prof. K. S. Chaudhari
Session: 2017-2018

2. Introduction
An evaporative cooler (also swamp cooler, desert cooler and wet air cooler) is a device that
cools air through the evaporation of water. Evaporative cooling differs from typical air
conditioning systems which use vapour-compression or absorption refrigeration cycles.
Evaporative cooling works by employing water's large enthalpy of vaporization. The
temperature of dry air can be dropped significantly through the phase transition of liquid water
to water vapour (evaporation), which can cool air using much less energy than refrigeration. In
extremely dry climates, evaporative cooling of air has the added benefit of conditioning the air
with more moisture for the comfort of building occupants.
The cooling potential for evaporative cooling is dependent on the wet bulb depression, the
difference between dry-bulb temperature and wet-bulb temperature. In arid climates,
evaporative cooling can reduce energy consumption and total equipment for conditioning as
an alternative to compressor-based cooling. In climates not considered arid, indirect
evaporative cooling can still take advantage of the evaporative cooling process without
increasing humidity. Passive evaporative cooling strategies offer the same benefits of
mechanical evaporative cooling systems without the complexity of equipment and ductwork.
WORKING PRINCIPLE
Evaporative coolers lower the temperature of air using the principle of evaporative cooling,
unlike typical air conditioning systems which use vapour-compression refrigeration or
absorption. Evaporative cooling is the addition of water vapour into air, which causes a
lowering of the temperature of the air. The energy needed to evaporate the water is taken from
the air in the form of sensible heat, which affects the temperature of the air, and converted into
latent heat, the energy present in the water vapour component of the air, whilst the air remains
at a constant enthalpy value. This conversion of sensible heat to latent heat is known as an
adiabatic process because it occurs at a constant enthalpy value. Evaporative cooling therefore
causes a drop in the temperature of air proportional to the sensible heat drop and an increase in
humidity proportional to the latent heat gain. Evaporative cooling can be visualized using a
psychrometric chart by finding the initial air condition and moving along a line of constant
enthalpy toward a state of higher humidity.
THEORY
As water is evaporated, energy is lost from the air, reducing the temperature. Two temperatures
are important when dealing with evaporative cooling systems.
Dry Bulb Temperature
This is the temperature that we usually think of as air temperature, measured by a regular
thermometer exposed to the air stream.
Wet Bulb Temperature
This is the lowest temperature that can be reached by the evaporation of water only. When
considering water evaporating into air, the wet-bulb temperature, as compared to the air's dry-
bulb temperature is a measure of the potential for evaporative cooling. The dry and wet bulb
temperature can be used to calculate the relative humidity.
Evaporation will take place when the humidity is below 100% and the air begins to absorb
water. Any given volume of air can hold a certain amount of water vapour and the degree of
absorption will depend on the amount it is already holding.
The term humidity describes how much water is already in the air; relative to the amount it
is capable of holding. Air is saturated when it cannot hold any more water. Imagine it as a
sponge, if the sponge held half as much water as it was capable of holding, it would be 50%

3. saturated. In the case of air, we would describe the Relative Humidity as being 50%. Energy is
required to change water from liquid to vapour. This energy is obtained in an adiabatic process
from the air itself. Air entering an evaporative air cooler gives up heat energy to evaporate
water. During this process, the dry bulb temperature of the air passing through the cooler is
lowered
What is an evaporative cooler?
An evaporative cooler is a box-shaped appliance with one or more porous surfaces that enable
air to pass through. A fan inside the unit pulls outside air through the sides and into the house.
To produce cool air, each porous side is fitted with a pad of water-absorbing material. Water
is stored in a pan at the bottom of the cooler and a small pump lifts the water to the top of each
side.
To effectively cool your home, each pad needs to remain damp, but not soaked. Dampness
creates the most evaporation and, therefore, the most cooling. The amount of water the pump
moves may need to be adjusted from time to time to properly dampen the pads.
Adjusting the air flow
Climate control inside a home with an evaporative cooler depends on proper air balance. To
limit humidity, you need to make sure that the same volume of air flows out of your home as
is pumped in. You can attain balanced air flow by installing ducts in each room or opening
windows when the cooler is in use. A window should be open just enough to allow air pressure
inside a room to slowly and quietly close the door to that room. If the door closes forcefully,
there is too little exhaust and the window should be opened wider. However, the window is
open too far if the door doesn't move at all.
A simple example of natural evaporative cooling is perspiration, or sweat, secreted by the body,
evaporation of which cools the body. The amount of heat transfer depends on the evaporation
rate, however for each kilogram of water vaporized 2,257 kJ of energy (about 890 BTU per
pound of pure water, at 95 °F) are transferred. The evaporation rate depends on the temperature
and humidity of the air, which is why sweat accumulates more on hot,humid days, as it does
not evaporate fast enough.
Vapour-compression refrigeration uses evaporative cooling, but the evaporated vapor is within
a sealed system, and is then compressed ready to evaporate again, using energy to do so. Simple
evaporative coolers water is evaporated into the environment, and not recovered. In an interior
space cooling unit, the evaporated water is introduced into the space along with the now-cooled
air; in an evaporative tower the evaporated water is carried off in the airflow exhaust.
A closely related process, sublimation cooling differs from evaporative cooling in that a phase
transition from solid to vapour, rather than liquid to vapour occurs. Sublimation cooling has
been observed to operate on a planetary scale on the planetoid Pluto, where it has been called
an anti-greenhouse effect. Another application of a phase change to cooling is the "self-
refrigerating" beverage can. A separate compartment inside the can contains a desiccant and a
liquid. Just before drinking, a tab is pulled so that the desiccant comes into contact with the
liquid and dissolves. As it does so it absorbs an amount of heat energy called the latent heat of
fusion. Evaporative cooling works with the phase change of liquid into vapour and the latent
heat of vaporization, but the self-cooling can uses a change from solid to liquid, and the latent
heat of fusion to achieve the sameresult.
Evaporative air conditioning uses evaporation to cool the air. In an evaporative cooler, a pump
circulates water from the reservoir on to a cooling pad, which in turn becomes very wet. A fan
draws air from outside the unit through the moistened pad. As it passes through the pad the air
is cooled by evaporation. The key to effective evaporative cooling is ensuring that each of the
cooling pads are completely saturated at all times during operation and that the systems fan &
motor are sized and designed to deliver the appropriate airflow for the home.

4. Fig.1.1 Schematic Diag. Of Evaporative Cooler
Evaporative coolers, often called "swamp coolers", are cooling systems that use only water
And a blower to circulate air. When warm, dry (unsaturated) air is pulled through a watersoaked
pad, water is evaporated and is absorbed as water vapour into the air. The air is cooled in the
process and the humidity is increased.
The evaporator cooling technology is an energy-efficient alternative to compressor-based
Cooling. In dry and arid regions, evaporative cooling can meet most or all building cooling
loads using one-fourth the energy of conventional equipment. It can also be applied
costeffectively when integrated with conventional chiller systems, which can greatly improve
a facility's load profile. Unfortunately, evaporative cooling requires an abundant water source
and is only effective when the relative humidity is low, restricting its efficient use to dry
climates (most of the south-western USA and other dry-climate areas worldwide).
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 cool ing 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. 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 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.

5. 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 cooling

6. 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. Like
the sensible cooling, the sensible heating process is also represented by a straight horizontal
line on the psychometric 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.
sensible heating

7. Conclusion
They are energy efficient. Evaporative coolers use up to 75 percent less electricity than a
standard air conditioner. This can result in significant savings, especially in hot desert
climates. They are environmentally friendly. They do not use refrigerants like CFCs and HCFCs
for the cooling process, so there are no greenhouse gases emitted. They don’t need much
repair. The technology for evaporative coolers is very simple, and there are fewer working
parts, which means maintenance and repair costs are low, They use common household
current. Evaporative coolers do not require high amperage circuits — they operate on 120-
volt electricity, which is less than a standard air conditioner uses. That means evaporative
coolers can be plugged into any household outlet .They filter the air effectively. Evaporative
cooling systems use moist pads as part of the cooling process, and these pads are very good
air filters, effectively trapping dust and pollen. They are also very inexpensive to replace,
compared to air conditioner filters. They add moisture to the house. These coolers naturally
add moisture to a house, which keeps wood furniture and fabrics from drying out. They do
not require ductwork. Because they fit in a window, you do not need ductwork to distribute
the air. Smaller units can be placed on the window with very little installation.