A

Project report

On

Entitled

“EVAPORATIVE COOLER”

Submitted in partial fulfilment for the award of degree of

Bachelor of Technology

In

Department of mechanical engineering

Jagan Nath University

Jaipur

Submitted by: Submitted to:

Amit Prasad Mahendra Pratap Singh

(H.O.D)

(Mechanical engineering)

CANDIDATE’S DECLARATION

I hereby declare that the work which is being presented in this dissertation titled “EVAPORATIVE COOLER” Towards partial fulfillment of requirements for the award of the degree of “Bachelor of Technology in Mechanical Engineering” submitted in the department of mechanical engineering Jagannath University, Jaipur in the supervision of Mr. Mahendra Pratap Singh H.O.D of Mechanical Engineering Department, Jagannath University, Jaipur

Date:

Place:Jaipur

CERTIFICATE

This is to verify that the above statement made by the student is correct to the best of my knowledge.

(Mahendra Pratap Singh)

H.O.D

Dept. of mechanical engineering

Jagannath University

ABSTRACT

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.

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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 TemperatureThis is the temperature that we usually think of as air temperature, measured by a regular thermometer exposed to the air stream.

Wet Bulb TemperatureThis 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% 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 same result.

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.

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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 water- soaked 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 cost- effectively 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).

Psychrometric chart’s Terminology

Dry-bulb temperature (DBT)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. In meteorology and psychrometrics the word temperature by itself without a prefix usually means dry-bulb temperature. Technically, the temperature registered by the dry-bulb thermometer of a psychrometer.

Wet-bulb temperature (WBT)

The thermodynamic wet-bulb temperature is a thermodynamic property of a mixture of air and water vapour. The value indicated by a wet-bulb thermometer often provides an adequate approximation of the thermodynamic wet-bulb temperature.

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. Speeds up to 5,000 ft/min (~60 mph) are best but it may be dangerous to move a thermometer at that speed. Errors up to 15% can occur if the air movement is too slow or if there is too much radiant heat present (from sunlight, for example).

A wet bulb temperature taken with air moving at about 1–2 m/s is referred to as a screen temperature, whereas a temperature taken with air moving about 3.5 m/s or more is referred to as sling temperature.

A psychrometer is a device that includes both a dry-bulb and a wet-bulb thermometer. A sling psychrometer requires manual operation to create the airflow over the bulbs, but a powered psychrometer includes a fan for this function. Knowing both the dry-bulb temperature (DBT) and wet-bulb temperature (WBT), one can determine the relative humidity (RH) from the psychometric chart appropriate to the air pressure.

Relative humidityThe ratio of the vapour pressure of moisture in the sample to the saturation pressure at the dry bulb temperature of the sample.

Dew point temperatureThe saturation temperature of the moisture present in the sample of air, it can also be defined as the temperature at which the vapour changes into liquid (condensation). Usually the level at which water vapour changes into liquid marks the base of the cloud in the atmosphere hence called condensation level. So the temperature value that allows this process (condensation) to take place is called the 'dew point temperature'. A simplified definition is the temperature at which the water vapour turns into "dew"

Humidity & Specific HumiditySpecific 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 humidityThe mass of water vapour per unit volume of air containing the water vapour. This quantity is also known as the water vapour density.[

Humid heatHumid heat is the constant-pressure specific heat of moist air, per unit mass of dry air

Locating parameters on chart

* Dry bulb temperature: These lines are drawn straight, not always parallel to each other, and slightly inclined from the vertical position. This is the t–axis, the abscissa (horizontal) axis. Each line represents a constant temperature.

* Dew point temperature: From the state point follow the horizontal line of constant humidity ratio to the intercept of 100% RH, also known as the saturation curve. The dew point temperature is equal to the fully saturated dry bulb or wet bulb temperatures.

* Wet bulb temperature: These lines are oblique lines that differ slightly from the enthalpy lines. They are identically straight but are not exactly parallel to each other. These intersect the saturation curve at DBT point.

* Relative humidity: These hyperbolic lines are shown in intervals of 10%. The saturation curve is at 100% RH, while dry air is at 0% RH.

* Humidity ratio: These are the horizontal lines on the chart. Humidity ratio is usually expressed as mass of moisture per mass of dry air (pounds or kilograms of moisture per pound or kilogram of dry air, respectively). The range is from 0 for dry air up to 0.03 (lbmw/lbma) on the right hand ω-axis, the ordinate or vertical axis of the chart.

* Specific enthalpy: These are oblique lines drawn diagonally downward from left to right across the chart that are parallel to each other. These are not parallel to wet bulb temperature lines.

Specific volume: These are a family of equally spaced straight lines that are nearly parallel.

The region above the saturation curve is a two-phase region that represents a mixture of saturated moist air and liquid water, in thermal equilibrium.

The protractor on the upper left of the chart has two scales. The inner scale represents sensible-total heat ratio (SHF). The outer scale gives the ratio of enthalpy difference to humidity difference. This is used to establish the slope of a condition line between two processes.

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 chart is 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.

Fig.1.2 psychrometric process

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. 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.

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.

Fig 1.3 sensible cooling

Sensible Heating of the AirSensible 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.

Fig 1.4 sensible heating

What is Humidification Process?The process in which the moisture or water vapour 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 ProcessCooling 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 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.

Fig 1.5 cooling and humidification

Heating and Humidification ProcessIn 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).

Fig 1.6 heating and humidification

What is Dehumidification?

The process in which the moisture or water vapour 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 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 ProcessThe 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 vapour 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 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.

Fig 1.7 cooling and dehumidification

Heating and Dehumidification ProcessThe 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 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.

fig 1.8 heating and dehumidification

Advantages of Evaporative Cooling Systems 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.

Disadvantages of Evaporative Cooling Systems They do not work as well in humid climates. Evaporative cooling works best in dry

climates. Too much moisture in the outside air makes the system work inefficiently. Moisture can build up, causing condensation and corrosion.

They need some attention. The moisture pads in an evaporative cooler must be kept moist with water. If they dry out the cooler will not work well. This can be a problem in desert areas where the dry air will take a toll on the pads.

Evaporative cooling systems use a time-tested technique for temperature control. If you live in a dry climate, they are a viable option to explore.

Bibliography

Air conditioning & refrigeration by R.S.Khurmi R&AC by R K Rajput http://en.wikipedia.org/wiki/Evaporative_cooler http://www.ijergs.org/

http://www.brighthubengineering.com/