4 WEEKSINDUSTRIAL TRAINING REPORT

ON

“REFRIGERATION & A/C”(Submitted to Mechanical Engineering Department for the partial fulfillment of)

DIPLOMA IN MECHANICAL ENGINEERING

SUBMITTED BY………………. SUBMITTED TO:

HOD (ME) APC, ABOHAR

ACKNOWLEDGEMENT

It is worth to do anything without mentioning the names of persons who made it possible.

I am very thankful to Our Principal Er. Tilak Raj and HOD Er Chhinder Pal who give us opportunity to go for industrial training. I am also thankful to our training Incharge Er Sudhir Bansal and Er Harvinder Singh for their value able guidance to accomplish our training on time.

I thank all those who are directly or indirectly assisted us to complete this

INDEX

Introduction to Refrigeration Methods of Refrigeration Units of Refrigeration VCR System Components Air conditioning Application of refrigeration

Introduction

Refrigeration is a process in which work is done to move heat from a low temperature to a high temperature and typically also from one location to another. The work of heat transport is traditionally driven by mechanical work, but can also be driven by heat, magnetism, electricity, laser, or other means. Refrigeration has many applications, including, but not limited to: household refrigerators, industrial freezers, cryogenics, and air conditioning. Heat pumps may use the heat output of the refrigeration process, and also may be designed to be reversible, but are otherwise similar to refrigeration units

Cycles Prof. U.S.P. Shet , Prof. T. Sundararajan and Prof. J.M . MallikarjunaIndian Institute of Technology Madras

6.2 Methods of Refrigeration:a) Natural Method:The natural method includes the utilization of ice or snow obtained naturally in coldclimate. Ice melts at 00C. So when it is placed in space or system warmer than 00C,heat is absorbed by the ice and the space is cooled. The ice then melts into water byabsorbing its latent heat at the rate of 324 kJ/kg. But, now-a-days, refrigerationrequirements have become so high that the natural methods are inadequate andtherefore obsolete.

b) Mechanical or Artificial Refrigeration:Atmosphere(T1)RefrigeratedSystem (T3)T2

δQ1

Refrigerating System (R)δWδQ2

A mechanical refrigeration system works on the principle of reversed Carnot cycle asshown in Fig.6.2. Work δw is delivered to the refrigerating system, causing it to removeRefrigeration Cycles Prof. U.S.P. Shet , Prof. T. Sundararajan and Prof. J.M . MallikarjunaIndian Institute of Technology Madrasheat δQ2 from the body or system (at lower temperature T3) and to deliver it along withwork, δw, to another body at higher temperature, T1, so that,δQ1 = δw + δQ2

There can be two methods by which the temperature T2 < T3 may be attained within therefrigerating system.i) By lowering the temperature of the working substance in the refrigeratingsystem to the level of T2. In this case, the heat will be absorbed due totemperature difference and T3 will decrease as heat δQ2 flows out.ii) By evaporating some fluid at an appropriate pressure. In this case, a constant

temperature T2 will be maintained and latent heat of fluid will be absorbed asδQ2.Depending upon the above method used, there are two types of mechanicalrefrigerating systems :i) Air systems: Uses air as a working fluid. Air does not undergo any change ofphase, but absorbs heat due to temperature difference.ii) Chemical Agent Systems: The working fluid changing its phase while boilingfrom liquid to vapor state, thereby it absorbs the latent heat.

Unit of Refrigeration:Capacity of refrigeration unit is generally defined in ton of refrigeration. A ton ofrefrigeration is defined as the quantity of heat to be removed in order to form one ton(1000 kg) of ice at 00C in 24 hrs, from liquid water at 00C. This is equivalent to 3.5 kJ/s(3.5 kW) or 210 kJ/min.

Methods of refrigeration can be classified as non-cyclic, cyclic, thermoelectric and magnetic.

Non-cyclic refrigeration

In non-cyclic refrigeration, cooling is accomplished by melting ice or by subliming dry ice (frozen carbon dioxide). These methods are used for small-scale refrigeration such as in laboratories and workshops, or in portable coolers.

Ice owes its effectiveness as a cooling agent to its melting point of 0 °C (32 °F) at sea level. To melt, ice must absorb 333.55 kJ/kg (about 144 Btu/lb) of heat. Foodstuffs maintained near this temperature have an increased storage life.

Solid carbon dioxide has no liquid phase at normal atmospheric pressure, and sublimes directly from the solid to vapor phase at a temperature of -78.5 °C (-109.3 °F), and is effective for maintaining products at low temperatures during sublimation. Systems such as this where the refrigerant evaporates and is vented to the atmosphere are known as "total loss refrigeration".

Cyclic refrigeration

Heat pump and refrigeration cycle

This consists of a refrigeration cycle, where heat is removed from a low-temperature space or source and rejected to a high-temperature sink with the help of external work, and its inverse, the thermodynamic power cycle. In the power cycle, heat is supplied from a high-temperature source to the engine, part of the heat being used to produce work and the rest being rejected to a low-temperature sink. This satisfies the second law of thermodynamics.

A refrigeration cycle describes the changes that take place in the refrigerant as it alternately absorbs and rejects heat as it circulates through a refrigerator. It is also applied toHVACR work, when describing the "process" of refrigerant flow through an HVACR unit, whether it is a packaged or split system.

Heat naturally flows from hot to cold. Work is applied to cool a living space or storage volume by pumping heat from a lower temperature heat source into a higher temperature heat sink. Insulation is used to reduce the work and energy needed to achieve and maintain a lower

temperature in the cooled space. The operating principle of the refrigeration cycle was described mathematically by Sadi Carnot in 1824 as a heat engine.

The most common types of refrigeration systems use the reverse-Rankine vapor-compression refrigeration cycle, although absorption heat pumps are used in a minority of applications.

Cyclic refrigeration can be classified as:

1. Vapor cycle, and2. Gas cycle

Vapor cycle refrigeration can further be classified as:

1. Vapor-compression refrigeration2. Vapor-absorption refrigeration

Vapor-compression cycle

(See Heat pump and refrigeration cycle and Vapor-compression refrigeration for more

details)

The vapor-compression cycle is used in most household refrigerators as well as in many large commercial and industrial refrigeration systems. Figure 1 provides a schematic diagram of the components of a typical vapor-compression refrigeration system.

Figure 1: Vapor compression refrigeration

The thermodynamics of the cycle can be analyzed on a diagram as shown in Figure 2. In this cycle, a circulating refrigerant such as Freon enters the compressor as a vapor. From point 1 to point 2, the vapor is compressed at constant entropy and exits the compressor as a vapor at a higher temperature, but still below the vapor pressure at that temperature. From

point 2 to point 3 and on to point 4, the vapor travels through the condenser which cools the vapor until it starts condensing, and then condenses the vapor into a liquid by removing additional heat at constant pressure and temperature. Between points 4 and 5, the liquid refrigerant goes through the expansion valve (also called a throttle valve) where its pressure abruptly decreases, causing flash evaporation and auto-refrigeration of, typically, less than half of the liquid.

Figure 2: Temperature–Entropy diagram

That results in a mixture of liquid and vapor at a lower temperature and pressure as shown at point 5. The cold liquid-vapor mixture then travels through the evaporator coil or tubes and is completely vaporized by cooling the warm air (from the space being refrigerated) being blown by a fan across the evaporator coil or tubes. The resulting refrigerant vapor returns to the compressor inlet at point 1 to complete the thermodynamic cycle.

The above discussion is based on the ideal vapor-compression refrigeration cycle, and does not take into account real-world effects like frictional pressure drop in the system, slight thermodynamic irreversibility during the compression of the refrigerant vapor, or non-ideal gas behavior (if any).

More information about the design and performance of vapor-compression refrigeration systems is available in the classicPerry's Chemical Engineers' Handbook.

Vapor absorption cycle

Main article: Absorption refrigerator

In the early years of the twentieth century, the vapor absorption cycle using water-ammonia systems was popular and widely used. After the development of the vapor compression cycle, the vapor absorption cycle lost much of its importance because of its low coefficient

of performance (about one fifth of that of the vapor compression cycle). Today, the vapor absorption cycle is used mainly where fuel for heating is available but electricity is not, such as in recreational vehicles that carry LP gas. It is also used in industrial environments where plentiful waste heat overcomes its inefficiency.

The absorption cycle is similar to the compression cycle, except for the method of raising the pressure of the refrigerant vapor. In the absorption system, the compressor is replaced by an absorber which dissolves the refrigerant in a suitable liquid, a liquid pump which raises the pressure and a generator which, on heat addition, drives off the refrigerant vapor from the high-pressure liquid. Some work is needed by the liquid pump but, for a given quantity of refrigerant, it is much smaller than needed by the compressor in the vapor compression cycle. In an absorption refrigerator, a suitable combination of refrigerant and absorbent is used. The most common combinations are ammonia (refrigerant) with water (absorbent), and water (refrigerant) with lithium bromide (absorbent).

Gas cycle

When the working fluid is a gas that is compressed and expanded but doesn't change phase, the refrigeration cycle is called a gas cycle. Air is most often this working fluid. As there is no condensation and evaporation intended in a gas cycle, components corresponding to the condenser and evaporator in a vapor compression cycle are the hot and cold gas-to-gas heat exchangers in gas cycles.

The gas cycle is less efficient than the vapor compression cycle because the gas cycle works on the reverse Brayton cycle instead of the reverse Rankine cycle. As such the working fluid does not receive and reject heat at constant temperature. In the gas cycle, the refrigeration effect is equal to the product of the specific heat of the gas and the rise in temperature of the gas in the low temperature side. Therefore, for the same cooling load, a gas refrigeration cycle needs a large mass flow rate and is bulky.

Because of their lower efficiency and larger bulk, air cycle coolers are not often used nowadays in terrestrial cooling devices. However, the air cycle machine is very common on gas turbine-powered jet aircraft as cooling and ventilation units, because compressed air is readily available from the engines' compressor sections. Such units also serve the purpose of pressurizing the aircraft.

Thermoelectric refrigeration

Thermoelectric cooling uses the Peltier effect to create a heat flux between the junction of two different types of materials. This effect is commonly used in camping and portable coolers and for cooling electronic components and small instruments.

Magnetic refrigeration

Main article: Magnetic refrigeration

Magnetic refrigeration, or adiabatic demagnetization, is a cooling technology based on the magnetocaloric effect, an intrinsic property of magnetic solids. The refrigerant is often aparamagnetic salt, such as cerium magnesium nitrate. The active magnetic dipoles in this case are those of the electron shells of the paramagnetic atoms.

A strong magnetic field is applied to the refrigerant, forcing its various magnetic dipoles to align and putting these degrees of freedom of the refrigerant into a state of loweredentropy. A heat sink then absorbs the heat released by the refrigerant due to its loss of entropy. Thermal contact with the heat sink is then broken so that the system is insulated, and the magnetic field is switched off. This increases the heat capacity of the refrigerant, thus decreasing its temperature below the temperature of the heat sink.

Because few materials exhibit the needed properties at room temperature, applications have so far been limited to cryogenics and research.

Other methods

Other methods of refrigeration include the air cycle machine used in aircraft; the vortex tube used for spot cooling, when compressed air is available; and thermoacoustic refrigeration using sound waves in a pressurized gas to drive heat transfer and heat exchange; steam jet cooling popular in the early 1930s for air conditioning large buildings; thermoelastic cooling using a smart metal alloy stretching and relaxing. Many Stirling cycle heat engines can be run backwards to act as a refrigerator, and therefore these engines have a niche use in cryogenics. In addition there are other types of cryo coolers such as Gifford-McMahon coolers, Joule-Thomson coolers, pulse-tube refrigerators and, for temperatures between 2 mK and 500 mK, dilution refrigerators.

VCR System

There are six main components in a refrigeration system

The Compressor The Condenser The Metering Device or expansion valve The Evaporator Piping material Refrigerant

 Compressor

It is heart of the refrigeration system as it circulates the refrigerant in the system like the heart of a human being circulating the blood in the body.

•     Two different pressures exist in the refrigeration cycle.  The evaporator or  low pressure, and the condenser, or high pressure.  These pressure areas are divided by the other two components.  On one end, is the metering device which controls the refrigerant flow, and on the other end, is the compressor.

The compressor is the heart of the system.  The compressor does just what its name is.  It compresses the low pressure refrigerant vapor from the evaporator and compresses it into a high pressure vapor.

•     The inlet to the compressor is called the “Suction Line”.  It brings the low pressure vapor into the compressor.

•     After the compressor compresses the refrigerant into a high pressure Vapor, and the outlet of the compressor is called the “Discharge Line”.

 There are three types of compressors namely reciprocating, rotary and centrifugal. The type of compressor depends on the pressure difference between the high pressure side (condenser) and low pressure side (evaporator) of the refrigeration system. This further depends on the refrigerant selected for the application under consideration.

 

Condenser

•      The “Discharge Line” leaves the compressor and runs to the inlet of the condenser.

•     Because the refrigerant was compressed, it is a hot high pressure vapor.

•    The hot vapor enters the condenser and starts to flow through the tubes.

•     Cool air is blown across the outside of the finned tubes of the condenser (usually air by a fan or water with a pump).

•     Since the air is cooler than the refrigerant, heat jumps from the tubing to the cooler air (energy goes from hot to cold – “latent heat”).

•    As the heat is removed from the refrigerant, it reaches its “saturated temperature” and starts to change state, into a high pressure liquid.

•     The high pressure liquid leaves the condenser through the “liquid line” and travels to the “metering device” through a filter dryer to remove any dirt or foreign particles.

 The condenser can be free air cooled (domestic refrigerator), forced air cooled (window air conditioner), water cooled (Central air conditioning plant in a library, cinema house and evaporative cooled (ice plant unit or a cold storage unit).

Expansion Device

•     Metering devices regulate how much liquid refrigerant enters the evaporator as per heat load on evaporator.

•     Common used metering devices are, small thin copper tubes referred to as “capillary tubes”, thermally controller diaphragm valves” (thermostatic expansion valves, called “TXV’s.  This valve has the capability of controlling the refrigerant flow. If the load on the evaporator changes, the valve can respond to the change and increase or decrease the flow accordingly. The TXV has a sensing bulb attached to the outlet of the evaporator. This bulb senses the suction line temperature and sends a signal to the TXV allowing it to adjust the flow rate. This is important because, if not all, the refrigerant in the evaporator changes state into a gas, there could be liquid refrigerant content returning to the compressor. This can be fatal to the compressor. Liquid cannot be compressed and when a compressor tries to compress a liquid, mechanical failing can happen. The compressor can suffer mechanical damage in the valves and bearings. This is called” liquid slugging”.  Normally TXV's are set to maintain 10 degrees of superheat. That means that the gas returning to the compressor is at least 10 degrees away from the risk of having any liquid. The metering device tries to maintain a preset degree of superheat at the outlet openings of the evaporator. As the metering devices regulates the amount of refrigerant going into the evaporator, the device lets small amounts of refrigerant out into the line and looses the high pressure to low pressure.

•     Now we have a low pressure, cooler liquid refrigerant entering the evaporative coil.

These are of five type namely capillary tube (domestic fridge), Automatic expansion valve (ice plant unit), Thermostatic expansion valve (Library refrigeration plant, theatre air conditioning unit and many more), Low side float valve (industrial cooling units) and high pressure float valve (industrial cooling units). These causes the required pressure drop between the high and low pressure sides and also control the flow of refrigerant as per cooling requirements.

Evaporator

The evaporator is where the heat is removed from your house, business or     products to be cooled.

•     Low pressure liquid leaves the metering device and enters the evaporator.

•     Usually, a fan will move warm air from the conditioned space across the evaporator finned coils.

•     The cooler refrigerant in the evaporator tubes, absorb the warm room air. The change of temperature causes the refrigerant to “flash” or “boil”, and changes from a low pressure liquid to a low pressure cold vapor.

•     The low pressure vapor is pulled into the compressor and the cycle starts over.

•     Evaporators are two types i.e. flooded evaporators necessitating the use of accumulators to permit only vapors to the compressor and dry expansion type evaporators. Flooded types are used in industrial units whereas dry expansion types are used in domestic and commercial refrigeration units.

 

Piping Materials

Pipe material should have high thermal conductivity, low cost, easy working and inertness with the refrigerant. Till date most commonly used pipe material is soft copper with all refrigerants except ammonia. The pipe material used with ammonia is mild steel as ammonia is highly corrosive to copper.

Refrigerant

 It is working substance in a refrigeration unit like blood in the human body. Its selection depends on many considerations like temperature to be produced, latent heat, ozone depletion potential, global warming potential, toxicity, inflammability, inertness, corrosion, erosion, action with water and lubricating oil, cost, availability, leak detection and power requirements for a certain amount of cooling needed. Various commonly used refrigerants are halogenated saturated hydrocarbons like R-134, R-22 and inorganic compounds like ammonia and air. Most common previously used refrigerants like R-12 and R-11 has been banned because of their high ozone depletion and global warming potentials. Mixed refrigerants and zoetrope’s are also in use. Refrigerants can be primary, secondary and tertiary type depending where and how these being used are. The same substance, for example, air can be primary in aircraft refrigeration; can be secondary as in a window air conditioner and tertiary in a central air conditioning plant.

ICE PLANT

COMPRESSOR

CONDENSOR

Air conditioning

Air conditioning is the process of altering the properties of air (primarily temperature and humidity) to more favourable conditions. More generally, air conditioning can refer to any form of technological cooling, heating, ventilation, or disinfection that modifies the condition of air.[1]

An air conditioner (often referred to as AC) is a major or home appliance, system, or mechanism designed to change the air temperature and humidity within an area (used for cooling and sometimes heating depending on the air properties at a given time). The cooling is typically done using a simple refrigeration cycle, but sometimes evaporation is used, commonly for comfort cooling in buildings and motor vehicles. In construction, a complete system of heating, ventilation and air conditioning is referred to as "HVAC".

The basic concept behind air conditioning is known to have been applied in ancient Egypt where reeds hung in windows had water trickling down. The evaporation of water cooled the air blowing through the window, though this process also made the air more humid. In Ancient Rome, water from aqueducts was circulated through the walls of certain houses to cool them down. Other techniques in medieval Persia involved the use of cisterns and wind towers to cool buildings during the hot season. Modern air conditioning emerged from advances in chemistry during the 19th century, and the first large-scale electrical air conditioning was invented and used in 1911 by Willis Haviland Carrier. The introduction of residential air conditioning in the 1920s helped start the great migration to the Sunbelt.

Pre-industrial cooling

The 2nd-century Chinese inventor Ding Huan (fl 180) of the Han Dynasty invented a rotary fan for air conditioning, with seven wheels 3 m (9.8 ft) in diameter and manually powered.[2] In 747, Emperor Xuanzong (r. 712–762) of the Tang Dynasty (618–907) had the Cool Hall (Liang Tian) built in the imperial palace, which the Tang Yulin describes as having water-powered fan wheels for air conditioning as well as rising jet streams of water from fountains.[3] During the subsequent Song Dynasty (960–1279), written sources mentioned the air-conditioning rotary fan as even more widely used.[4]

In the 17th century, Cornelis Drebbel demonstrated "turning Summer into Winter" for James I of England by adding salt to water.[5]

In 1758, Benjamin Franklin and John Hadley, a chemistry professor at Cambridge University, conducted an experiment to explore the principle of evaporation as a means to rapidly cool an object. Franklin and Hadley confirmed that evaporation of highly volatile liquids such as alcohol and ether could be used to drive down the temperature of an object past the freezing point of water. They conducted their experiment with the bulb of a mercury thermometer as their object and with a bellows used to "quicken" the evaporation; they lowered the temperature of the thermometer bulb down to −14 °C (7 °F) while the ambient temperature was 18 °C

(64 °F). Franklin noted that, soon after they passed the freezing point of water 0 °C (32 °F), a thin film of ice formed on the surface of the thermometer's bulb and that the ice mass was about a quarter-inch thick when they stopped the experiment upon reaching −14 °C (7 °F). Franklin concluded, "From this experiment one may see the possibility of freezing a man to death on a warm summer's day"...[6]

Mechanical cooling

Three-quarters scale model of Gorrie's ice machine. John Gorrie State Museum, Florida

In 1820, British scientist and inventor Michael Faraday discovered that compressing and liquefying ammonia could chill air when the liquefied ammonia was allowed to evaporate. In 1842, Florida physician John Gorrie used compressor technology to create ice, which he used to cool air for his patients in his hospital in Apalachicola, Florida.[7] He hoped eventually to use his ice-making machine to regulate the temperature of buildings. He even envisioned centralized air conditioning that could cool entire cities.[8] Though his prototype leaked and performed irregularly, Gorrie was granted a patent in 1851 for his ice-making machine. His hopes for its success vanished soon afterwards when his chief financial backer died; Gorrie did not get the money he needed to develop the machine. According to his biographer, Vivian M. Sherlock, he blamed the "Ice King", Frederic Tudor, for his failure, suspecting that Tudor had launched a smear campaign against his invention. Dr. Gorrie died impoverished in 1855, and the idea of air conditioning faded away for 50 years.

James Harrison's first mechanical ice-making machine began operation in 1851 on the banks of the Barwon River at Rocky Point in Geelong (Australia). His first commercial ice-making machine followed in 1854, and his patent for an ether vapor-compression refrigeration system was granted in 1855. This novel system used a compressor to force the refrigeration gas to pass

through a condenser, where it cooled down and liquefied. The liquefied gas then circulated through the refrigeration coils and vaporised again, cooling down the surrounding system. The machine employed a 5 m (16 ft.) flywheel and produced 3,000 kilograms (6,600 lb) of ice per day.

Though Harrison had commercial success establishing a second ice company back in Sydney in 1860, he later entered the debate of how to compete against the American advantage of unrefrigerated beef sales to the United Kingdom. He wrote Fresh Meat frozen and packed as if for a voyage, so that the refrigerating process may be continued for any required period, and in 1873 prepared the sailing ship Norfolk for an experimental beef shipment to the United Kingdom. His choice of a cold room system instead of installing a refrigeration system upon the ship itself proved disastrous when the ice was consumed faster than expected.

Electromechanical cooling

Willis Carrier

In 1902, the first modern electrical air conditioning unit was invented by Willis Haviland Carrier in Buffalo, New York. After graduating from Cornell University, Carrier, a native of Angola, New York, found a job at the Buffalo Forge Company. While there, Carrier began experimenting with air conditioning as a way to solve an application problem for the Sackett-Wilhelms Lithographing and Publishing Company in Brooklyn, New York, and the first "air conditioner", designed and built in Buffalo by Carrier, began working on 17 July 1902.

Designed to improve manufacturing process control in a printing plant, Carrier's invention controlled not only temperature but also humidity. Carrier used his knowledge of the heating of objects with steam and reversed the process. Instead of sending air through hot coils, he sent it through cold coils (ones filled with cold water). The air blowing over the cold coils cooled the air, and one could thereby control the amount of moisture the colder air could hold. In turn, the

humidity in the room could be controlled. The low heat and humidity helped maintain consistent paper dimensions and ink alignment. Later, Carrier's technology was applied to increase productivity in the workplace, and The Carrier Air Conditioning Company of America was formed to meet rising demand. Over time, air conditioning came to be used to improve comfort in homes and automobiles as well. Residential sales expanded dramatically in the 1950s.

In 1906, Stuart W. Cramer of Charlotte, North Carolina was exploring ways to add moisture to the air in his textile mill. Cramer coined the term "air conditioning", using it in a patent claim he filed that year as an analogue to "water conditioning", then a well-known process for making textiles easier to process. He combined moisture with ventilation to "condition" and change the air in the factories, controlling the humidity so necessary in textile plants. Willis Carrier adopted the term and incorporated it into the name of his company. The evaporation of water in air, to provide a cooling effect, is now known as evaporative cooling.

Evaporative cooling was the first real air-conditioning and shortly thereafter the first private home to have air conditioning (The Dubose House) was built in Chapel Hill, North Carolina. Realizing that air conditioning would one day be a standard feature of private homes, particularly in the South, DuBose designed an ingenious network of ductwork and vents, all painstakingly disguised behind intricate and attractive Georgian-style open moldings. Meadowmont is believed to be one of the first private homes in the United States equipped for central air conditioning.[9]

Refrigerant development

The first air conditioners and refrigerators employed toxic or flammable gases, such as ammonia, methyl chloride, or propane, that could result in fatal accidents when they leaked. Thomas Midgley, Jr created the first non-flammable, non-toxic chlorofluorocarbon gas, Freon, in 1928.

"Freon" is a trademark name owned by DuPont for any Chlorofluorocarbon (CFC), Hydrochlorofluorocarbon (HCFC), or Hydrofluorocarbon (HFC) refrigerant, the name of each including a number indicating molecular composition (R-11, R-12, R-22, R-134A). The blend most used in direct-expansion home and building comfort cooling is an HCFC known as R-22. It was to be phased out for use in new equipment by 2010, and is to be completely discontinued by 2020.

R-12 was the most common blend used in automobiles in the US until 1994, when most designs changed to R-134A. R-11 and R-12 are no longer manufactured in the US for this type of application, the only source for air-conditioning repair purposes being the cleaned and purified gas recovered from other air-conditioner systems. Several non-ozone-depleting refrigerants have been developed as alternatives, including R-410A, invented by Honeywell (formerly AlliedSignal) in Buffalo, and sold under the Genetron (R) AZ-20 name. It was first commercially used by Carrier under the brand name Puron.

Innovation in air-conditioning technologies continues, with much recent emphasis placed on energy efficiency and on improving indoor air quality. Reducing climate-change impact is an important area of innovation because, in addition to greenhouse-gas emissions associated with energy use, CFCs, HCFCs, and HFCs are, themselves, potent greenhouse gases when leaked to the atmosphere. For example, R-22 (also known as HCFC-22) has a global warming potential about 1,800 times higher than CO2.[10] As an alternative to conventional refrigerants, natural alternatives, such as carbon dioxide (CO2. R-744), have been proposed.[11]

Humidity control

Air conditioning units outside a classroom building at the University of North Carolina in Chapel Hill, North Carolina

See also: Dehumidifier

Refrigeration air-conditioning equipment usually reduces the absolute humidity of the air processed by the system. The relatively cold (below the dewpoint) evaporator coil condenses water vapor from the processed air (much like an ice-cold drink will condense water on the outside of a glass), sending the water to a drain and removing water vapor from the cooled space and lowering the relative humidity in the room. Since humans perspire to provide natural cooling by the evaporation of perspiration from the skin, drier air (up to a point) improves the comfort provided. The comfort air conditioner is designed to create a 40% to 60% relative humidity in the occupied space. In food-retailing establishments, large open chiller cabinets act as highly effective air dehumidifying units.

A specific type of air conditioner that is used only for dehumidifying is called a dehumidifier. A dehumidifier is different from a regular air conditioner in that both the evaporator and condenser coils are placed in the same air path, and the entire unit is placed in the environment that is intended to be conditioned (in this case dehumidified), rather than requiring the

condenser coil to be outdoors. Having the condenser coil in the same air path as the evaporator coil produces warm, dehumidified air. The evaporator (cold) coil is placed first in the air path, dehumidifying the air exactly as a regular air conditioner does. The air next passes over the condenser coil, re-warming the now dehumidified air. Having the condenser coil in the main air path rather than in a separate, outdoor air path (as with a regular air conditioner) results in two consequences: the output air is warm rather than cold, and the unit is able to be placed anywhere in the environment to be conditioned, without a need to have the condenser outdoors.

Unlike a regular air conditioner, a dehumidifier will actually heat a room just as an electric heater that draws the same amount of power (watts) as the dehumidifier would. A regular air conditioner transfers energy out of the room by means of the condenser coil, which is outside the room (outdoors). That is, the room can be considered a thermodynamic system from which energy is transferred to the external environment. Conversely, with a dehumidifier, no energy is transferred out of the thermodynamic system (room) because the air conditioning unit (dehumidifier) is entirely inside the room. Therefore all of the power consumed by the dehumidifier is energy that is input into the thermodynamic system (the room) and remains in the room (as heat). In addition, if the condensed water has been removed from the room, the amount of heat needed to boil that water has been added to the room. This is the inverse of adding water to the room with an evaporative cooler.

Dehumidifiers are commonly used in cold, damp climates to prevent mold growth indoors, especially in basements. They are also used to protect sensitive equipment from the adverse effects of excessive humidity in tropical countries.

The engineering of physical and thermodynamic properties of gas–vapor mixtures is called psychrometrics.

Energy

In a thermodynamically closed system, any power dissipated into the system that is being maintained at a set temperature (which is a standard mode of operation for modern air conditioners) requires that the rate of energy removal by the air conditioner increase. This increase has the effect that, for each unit of energy input into the system (say to power a light bulb in the closed system), the air conditioner removes that energy.[12] In order to do so, the air conditioner must increase its power consumption by the inverse of its "efficiency" (coefficient of performance) times the amount of power dissipated into the system. As an example, assume that inside the closed system a 100 W heating element is activated, and the air conditioner has an coefficient of performance of 200%. The air conditioner's power consumption will increase by 50 W to compensate for this, thus making the 100 W heating element cost a total of 150 W of power.

It is typical for air conditioners to operate at "efficiencies" of significantly greater than 100%.[13]

However, it may be noted that the input electrical energy is of higher thermodynamic quality (lower entropy) than the output thermal energy (heat energy).

Air conditioner equipment power in the U.S. is often described in terms of "tons of refrigeration". A ton of refrigeration is approximately equal to the cooling power of one short ton (2000 pounds or 907 kilograms) of ice melting in a 24-hour period. The value is defined as 12,000 BTU per hour, or 3517 watts.[14] Residential central air systems are usually from 1 to 5 tons (3 to 20 kilowatts (kW)) in capacity.

In an automobile, the A/C system will use around 4 horsepower (3 kW) of the engine's power

Seasonal energy efficiency ratio

Main article: Seasonal energy efficiency ratio

For residential homes, some countries set minimum requirements for energy efficiency. In the United States, the efficiency of air conditioners is often (but not always) rated by the seasonal energy efficiency ratio (SEER). The higher the SEER rating, the more energy efficient is the air conditioner. The SEER rating is the BTU of cooling output during its normal annual usage divided by the total electric energy input in watt hours (W·h) during the same period.[16]

SEER = BTU ÷ (W·h)

this can also be rewritten as:

SEER = (BTU / h) ÷ W, where "W" is the average electrical power in Watts, and (BTU/h) is the rated cooling power.

For example, a 5000 BTU/h air-conditioning unit, with a SEER of 10, would consume 5000/10 = 500 Watts of power on average.

The electrical energy consumed per year can be calculated as the average power multiplied by the annual operating time:

500 W × 1000 h = 500,000 W·h = 500 kWh

Assuming 1000 hours of operation during a typical cooling season (i.e., 8 hours per day for 125 days per year).

Another method that yields the same result, is to calculate the total annual cooling output:

5000 BTU/h × 1000 h = 5,000,000 BTU

Then, for a SEER of 10, the annual electrical energy usage would be:

5,000,000 BTU ÷ 10 = 500,000 W·h = 500 kWh

SEER is related to the coefficient of performance (COP) commonly used in thermodynamics and also to the Energy Efficiency Ratio (EER). The EER is the efficiency rating for the equipment at a particular pair of external and internal temperatures, while SEER is calculated over a whole range of external temperatures (i.e., the temperature distribution for the geographical location of the SEER test). SEER is unusual in that it is composed of an Imperial unit divided by an SI unit. The COP is a ratio with the same metric units of energy (joules) in both the numerator and denominator. They cancel out, leaving a dimensionless quantity. Formulas for the approximate conversion between SEER and EER or COP are available from the Pacific Gas and Electric Company:[17]

(1)     SEER = EER ÷ 0.9

(2)     SEER = COP × 3.792

(3)     EER = COP × 3.413

From equation (2) above, a SEER of 13 is equivalent to a COP of 3.43, which means that 3.43 units of heat energy are pumped per unit of work energy.

The United States now requires that residential systems manufactured in 2006 have a minimum SEER rating of 13 (although window-box systems are exempt from this law, so their SEER is still around 10).[18]

Construction

A typical home air conditioning unit

Refrigerants

Main article: Refrigerant

A modern R-134a hermetic refrigeration compressor

"Freon" is a trade name for a family of haloalkane refrigerants manufactured by DuPont and other companies. These refrigerants were commonly used due to their superior stability and safety properties. However, these chlorine-bearing refrigerants reach the upper atmosphere when they escape.[19] Once the refrigerant reaches the stratosphere, UV radiation from the Sun cleaves the chlorine-carbon bond, yielding a chlorine radical. These chlorine atoms catalyze the breakdown of ozone into diatomic oxygen, depleting the ozone layer that shields the Earth's surface from strong UV radiation. Each chlorine radical remains active as a catalyst unless it binds with another chlorine radical, forming a stable molecule and breaking the chain reaction. The use of CFC as a refrigerant was once common, being used in the refrigerants R-11 and R-12. In most countries the manufacture and use of CFCs has been banned or severely restricted due to concerns about ozone depletion.[20] In light of these environmental concerns, beginning on November 14, 1994, the U.S. Environmental Protection Agency has restricted the sale, possession and use of refrigerant to only licensed technicians, per Rules 608 and 609 of the EPA rules and regulations;[21] failure to comply may result in criminal and civil sanctions. Newer and more environmentally safe refrigerants such as HCFCs (R-22, used in most homes today) and HFCs (R-134a, used in most cars) have replaced most CFC use. HCFCs, in turn, are being phased out under the Montreal Protocol and replaced by hydrofluorocarbons (HFCs) such as R-410A, which lack chlorine. Carbon dioxide (R-744) is being rapidly adopted as a refrigerant in Europe and Japan. R-744 is an effective refrigerant with a global warming potential of 1. It must use higher compression to produce an equivalent cooling effect.

Types

The external section of a typical single-room air conditioning unit. For ease of installation, these are frequently placed in a window. This one was installed through a hole cut in the wall.

The internal section of the above unit. The front panel swings down to reveal the controls.

Window and through-wall

Room air conditioners come in two forms: unitary and packaged terminal PTAC systems. Unitary systems, the common one room air conditioners, sit in a window or wall opening, with interior controls. Interior air is cooled as a fan blows it over the evaporator. On the exterior the air is heated as a second fan blows it over the condenser. In this process, heat is drawn from the room and discharged to the environment. A large house or building may have several such units, permitting each room be cooled separately.

PTAC systems are also known as wall split air conditioning systems or ductless systems.[22] These PTAC systems which are frequently used in hotels have two separate units (terminal packages), the evaporative unit on the interior and the condensing unit on the exterior, with tubing passing through the wall and connecting them. This minimizes the interior system footprint and allows each room to be adjusted independently. PTAC systems may be adapted to provide heating in cold weather, either directly by using an electric strip, gas or other heater, or by reversing the refrigerant flow to heat the interior and draw heat from the exterior air, converting the air conditioner into a heat pump. While room air conditioning provides maximum flexibility, when used to cool many rooms at a time it is generally more expensive than central air conditioning.

The first practical through the wall air conditioning unit was invented by engineers at Chrysler Motors and offered for sale starting in 1935.[23]

Window unit

Evaporative coolers

Main article: Evaporative cooler

In very dry climates, evaporative coolers, sometimes referred to as swamp coolers or desert coolers, are popular for improving coolness during hot weather.

An evaporative cooler is a device that draws outside air through a wet pad, such as a large sponge soaked with water. The sensible heat of the incoming air, as measured by a dry bulb thermometer, is reduced. The total heat (sensible heat plus latent heat) of the entering air is unchanged. Some of the sensible heat of the entering air is converted to latent heat by the evaporation of water in the wet cooler pads. If the entering air is dry enough, the results can be quite cooling; evaporative coolers tend to feel as if they are not working during times of high humidity, when there is not much dry air with which the coolers can work to make the air as cool as possible for dwelling occupants. Unlike air conditioners, evaporative coolers rely on the outside air to be channeled through cooler pads that cool the air before it reaches the inside of a house through its air duct system; this cooled outside air must be allowed to push the warmer air within the house out through an exhaust opening such as an open door or window.[24]

These coolers cost less and are mechanically simple to understand and maintain.

An early type of cooler, using ice for a further effect, was patented by John Gorrie of Apalachicola, Florida in 1842. He used the device to cool the patients in his malaria hospital.

Portable units

A portable air conditioner is one on wheels that can be easily transported inside a home or office. They are currently available with capacities of about 5,000–60,000 BTU/h (1,800–

18,000 W output) and with and without electric-resistance heaters. Portable air conditioners are either evaporative or refrigerative.

Portable refrigerative air conditioners come in two forms, split and hose. These compressor-based refrigerant systems are air-cooled, meaning they use air to exchange heat, in the same way as a car or typical household air conditioner does. Such a system dehumidifies the air as it cools it. It collects water condensed from the cooled air and produces hot air which must be vented outside the cooled area; doing so transfers heat from the air in the cooled area to the outside air.

A portable split system has an indoor unit on wheels connected to an outdoor unit via flexible pipes, similar to a permanently fixed installed unit.

Hose systems, which can be monoblock or air-to-air, are vented to the outside via air ducts. The monoblock type collects the water in a bucket or tray and stops when full. The air-to-air type re-evaporates the water and discharges it through the ducted hose and can run continuously.

A single-duct unit uses air from within the room to cool its condenser, and then vents it outside. This air is replaced by hot air from outside or other rooms, thus reducing the unit's effectiveness. Modern units might have a coefficient of performance (COP, sometimes called "efficiency") of approximately 3 (i.e., 1 kW of electricity will produce 3 kW of cooling). A dual-duct unit draws air to cool its condenser from outside instead of from inside the room, and thus is more effective than most single-duct units.

Evaporative air coolers, sometimes called "swamp coolers", do not have a compressor or condenser. Liquid water is evaporated on the cooling fins, releasing the vapour into the cooled area. Evaporating water absorbs a significant amount of heat, the latent heat of vaporisation, cooling the air: humans and other animals use the same mechanism to cool themselves by sweating. They have the advantage of needing no hoses to vent heat outside the cooled area, making them truly portable; and they are very cheap to install and use less energy than refrigerative air conditioners. Disadvantages are that unless ambient humidity is low (as in a dry climate) cooling is limited and the cooled air is very humid and can feel clammy. Also, they use a lot of water, which is often at a premium in the dry climates where they work best.

A typical single hosed portable air conditioner can cool a room that is 475 sq ft (44.1 m2) or smaller and has at most a cooling power of 15,000 BTUs/h (4.3 kW). However, single hosed units cool a room less effectively than dual hosed as the air expelled from the room through the single hose creates negative pressure inside the room. Because of this, air (potentially warm air) from neighboring rooms is pulled into the room with the cooling unit to compensate.[25]

Heat pumps

Main article: Heat pump

"Heat pump" is a term for a type of air conditioner in which the refrigeration cycle can be reversed, producing heating instead of cooling in the indoor environment. They are also commonly referred to, and marketed as, a "reverse cycle air conditioner". Using an air conditioner in this way to produce heat is significantly more energy efficient than electric resistance heating. Some homeowners elect to have a heat pump system installed, which is simply a central air conditioner with heat pump functionality (the refrigeration cycle can be reversed in cold weather). When the heat pump is in heating mode, the indoor evaporator coil switches roles and becomes the condenser coil, producing heat. The outdoor condenser unit also switches roles to serve as the evaporator, and discharges cold air (colder than the ambient outdoor air).

Heat pumps are more popular in milder winter climates where the temperature is frequently in the range of 40–55°F (4–13°C), because heat pumps become inefficient in more extreme cold. This is due to the problem of ice forming on the outdoor unit's heat exchanger coil, which blocks air flow over the coil. To compensate for this, the heat pump system must temporarily switch back into the regular air conditioning mode to switch the outdoor evaporator coil back to being the condenser coil, so that it can heat up and de-ice. A heat pump system will therefore have a form of electric resistance heating in the indoor air path that is activated only in this mode in order to compensate for the temporary indoor air cooling, which would otherwise be uncomfortable in the winter. The icing problem becomes much more severe with lower outdoor temperatures, so heat pumps are commonly installed in tandem with a more conventional form of heating, such as a natural gas or oil furnace, which is used instead of the heat pump during harsher winter temperatures. In this case, the heat pump is used efficiently during the milder temperatures, and the system is switched to the conventional heat source when the outdoor temperature is lower.

Absorption heat pumps are actually a kind of air-source heat pump, but they do not depend on electricity to power them. Instead, gas, solar power, or heated water is used as a main power source. Additionally, refrigerant is not used at all in the process.[dubious – discuss] An absorption pump absorbs ammonia into water.[further explanation needed] Next, the water and ammonia mixture is depressurized to induce boiling, and the ammonia is boiled off, resulting in cooling.[26]

Some more expensive window air conditioning units have a true heat pump function. However, a window unit that has a "heat" selection is not necessarily a heat pump because some units use only electric resistance heat when heating is desired. A unit that has true heat pump functionality will be indicated its specifications by the term "heat pump".

Health issues

Air-conditioning systems can promote the growth and spread of microorganisms, such as Legionella pneumophila, the infectious agent responsible for Legionnaires' disease, or thermophilic actinomycetes; however, this is only prevalent in poorly-maintained water cooling towers. As long as the cooling tower is kept clean (usually by means of a chlorine treatment), these health hazards can be avoided.

Conversely, air conditioning (including filtration, humidification, cooling and disinfection) can be used to provide a clean, safe, hypoallergenic atmosphere in hospital operating rooms and other environments where an appropriate atmosphere is critical to patient safety and well-being. Air conditioning can have a negative effect on skin, drying it out,[33] and can also cause dehydration.[34] Air conditioning may have a positive effect on sufferers of allergies and asthma.

Prior to 1994, most automotive air conditioning systems used Dichlorodifluoromethane (R-12) as a refrigerant. It was usually sold under the brand name Freon-12 and is a chlorofluorocarbon halomethane (CFC). The manufacture of R-12 was banned in many countries in 1994 because of environmental concerns, in compliance with the Montreal Protocol. The R-12 was replaced with R-134a refrigerant, which has a lower ozone depletion potential. Old R-12 systems can be retrofitted to R-134a by a complete flush and filter/dryer replacement to remove the mineral oil, which is not compatible with R-134a.

REFERENCESwww.wikipedia.com

Refrigeration and air conditioning by Domkudwar

Refrigeration by N Singh