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منتدي ممدوح عزت موسي MAMDOUH EZAT MOUSA FORUM

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اليومية اليومية


    Air conditioner

    شاطر

    ممدوح عزت موسي
    مدير عام المنتدي
    المشرف العام
    مشرف منتدي القصه
    مشرف منتدي العلوم الهندسيه
    مدير عام المنتدي  المشرف العاممشرف منتدي القصهمشرف منتدي العلوم الهندسيه

    عدد المساهمات : 350
    التميز : 9
    تاريخ التسجيل : 13/05/2010

    Air conditioner

    مُساهمة من طرف ممدوح عزت موسي في الأربعاء أكتوبر 13, 2010 12:54 am

    [size=18]


    An air conditioner (often referred to as AC) is a home appliance, system, or mechanism
    HistoryMain article: Air conditioning#History

    In 1758, Benjamin Franklin and John Hadley, professor of chemistry 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 to 7 °F (−14 °C) while the ambient temperature was 65 °F (18 °C). Franklin noted that soon after they passed the freezing point of water (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 7 °F (−14 °C). Franklin concluded, "From this experiment, one may see the possibility of freezing a man to death on a warm summer's day".[1]
    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.[2] 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. 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 afterward 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.
    Early commercial applications of air conditioning were manufactured to cool air for industrial processing rather than personal comfort. In 1902 the first modern electrical air conditioning was invented by Willis Carrier in Syracuse, New York. Designed to improve manufacturing process control in a printing plant, his invention controlled not only temperature but also humidity. The low heat and humidity were to help 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. 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. This evaporation of water in air, to provide a cooling effect, is now known as evaporative cooling.
    The first air conditioners and refrigerators employed toxic or flammable gases like ammonia, methyl chloride and propane, which could result in fatal accidents when they leaked. Thomas Midgley, Jr. created the first chlorofluorocarbon gas, Freon, in 1928. The refrigerant was much safer for humans but was later claimed to be harmful to the atmosphere's ozone layer. Freon is a trademark name of DuPont for any chlorofluorocarbon (CFC), hydrogenated CFC (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 is to be phased out for use in new equipment by 2010 and completely discontinued by 2020. R-12 was the most common blend used in automobiles in the United States until 1994 when most changed to R-134A. R-11 and R-12 are no longer manufactured in the United States, the only source for purchase 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, known by the brand name Puron. The most common ozone-depleting refrigerants are R-22, R-11 and R-123.
    Innovation in air conditioning technologies continues, with much recent emphasis placed on energy efficiency and improving indoor air quality. As an alternative to conventional refrigerants, natural alternatives like CO2 (R-744) have been proposed.[3]
    The increase in use of air conditioning over the years has been implicated as a contributor to increasing obesity, because appetite naturally decreases in uncomfortably high temperatures

    Air conditioning system basics and theories
    [edit] Refrigeration cycle


    A simple stylized diagram of the refrigeration cycle: 1) condensing coil, 2) expansion valve, 3) evaporator coil, 4) compressor.


    [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
    Capillary expansion valve connection to evaporator inlet. Notice frost formation.



    In the refrigeration cycle, a heat pump transfers heat from a lower-temperature heat source into a higher-temperature heat sink. Heat would naturally flow in the opposite direction. This is the most common type of air conditioning. A refrigerator works in much the same way, as it pumps the heat out of the interior and into the room in which it stands.
    This cycle takes advantage of the way phase changes work, where latent heat is released at a constant temperature during a liquid/gas phase change, and where varying the pressure of a pure substance also varies its condensation/boiling point.
    The most common refrigeration cycle uses an electric motor to drive a compressor. In an automobile, the compressor is driven by a belt over a pulley, the belt being driven by the engine's crankshaft (similar to the driving of the pulleys for the alternator, power steering, etc.). Whether in a car or building, both use electric fan motors for air circulation. Since evaporation occurs when heat is absorbed, and condensation occurs when heat is released, air conditioners use a compressor to cause pressure changes between two compartments, and actively condense and pump a refrigerant around. A refrigerant is pumped into the evaporator coil, located in the compartment to be cooled, where the low pressure causes the refrigerant to evaporate into a vapor, taking heat with it. At the opposite side of the cycle is the condenser, which is located outside of the cooled compartment, where the refrigerant vapor is compressed and forced through another heat exchange coil, condensing the refrigerant into a liquid, thus rejecting the heat previously absorbed from the cooled space.
    By placing the condenser (where the heat is rejected) inside a compartment, and the evaporator (which absorbs heat) in the ambient environment (such as outside), or merely running a normal air conditioner's refrigerant in the opposite direction, the overall effect is the opposite, and the compartment is heated. This is usually called a heat pump, and is capable of heating a home to comfortable temperatures (25° C; 70° F), even when the outside air is below the freezing point of water (0° C; 32° F).
    Cylinder unloaders are a method of load control used mainly in commercial air conditioning systems. On a semi-hermetic (or open) compressor, the heads can be fitted with unloaders which remove a portion of the load from the compressor so that it can run better when full cooling is not needed. Unloaders can be electrical or mechanical.
    Humidity
    Air conditioning equipment usually reduces the humidity of the air processed by the system. The relatively cold (below the dew point) evaporator coil condenses water vapor from the processed air, much as a cold drink will condense water on the outside of a glass. The water is drained, removing water vapor from the cooled space and thereby lowering its relative humidity. 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 retail establishments, large, open chiller cabinets act as highly effective dehumidifiers.
    Some air conditioning units dry the air without cooling it. These work like a normal air conditioner, except that a heat exchanger is placed between the intake and exhaust. In combination with convection fans, they achieve a similar level of comfort as an air cooler in humid tropical climates, but only consume about one-third the energy. They are also preferred by those who find the draft created by air coolers uncomfortable.
    Refrigerants

    Main article: Refrigerant
    [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
    A modern R-134a refirgeration 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.[6] 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 refigerants 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.[7] In light of these environmental concerns, beginning on November 14, 1994, the 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;[8] 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 of air conditioner equipment [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
    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.

    [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
    The internal section of the above unit. The front panel swings down to reveal the controls.




    Window and through-wall units


    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.[9] These PTAC systems which are frequently used in hotels have two separate units (terminal packages), the evaportive 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 cooling many rooms it is generally more expensive than central air conditioning.
    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 comfort during hot weather. This type of cooler is the dominant cooler used in Iran, which has the largest number of these units of any country in the world, causing some to refer to these units as "Persian coolers."[10] 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 comfortable; 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 a open door or window.[11]
    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 air conditioners are movable units that can be used to cool a specific region of building in a modular fashion, not requiring permanent installation. Most portable air conditioners are refrigeration based rather than evaporative,[citation needed] and it is this type that is described in this section. Portable air conditioner units are often rented in emergency situations such as power failures at warehouses or data centers.
    All refrigerated type portable air conditioners require exhaust hoses for venting. Through this process of air intake, cooling and venting, air is continually cycled through the unit until the room reaches the desired temperature setting. Also, the refrigerant works to not only cool the air but also dehumidify air in the room, owing to the temperature decrease in the air which results in the saturation of the water content of the air, causing condensation when the air is returned to the room. The air will therefore be left without this ional water content.[12]. The water loss rate is sufficiently high to require collection or drainage. The exact conditions for the condensation of the water from the air can be estimated using a Psychrometric chart for air at room pressure.
    Single hosed units


    A single hosed unit has one hose that runs from the back of the portable air conditioner to the vent kit where hot air can be released. A typical single hosed portable air conditioner can cool a room that is 475 sq. ft. (45 sq meters) or smaller and has at most a cooling power of 12,000 BTUs. 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.[13]
    Dual hosed units


    Dual hosed units are typically used in larger rooms. One hose is used as the exhaust hose to vent hot air and the other as the intake hose to draw in additional air (usually from the outside). These units generally have a cooler power of 12,000-14,000 BTUs and cool rooms that are around 500 sq ft (46 m2). The reason an intake hose is needed to draw in extra air is because with higher BTU units, air is cycled in large amounts and hot air is expelled at a faster rate. This would create negative air pressure in the room, so the intake hose eliminates reduction of room air pressure which would draw outside air into the room.[clarification needed]
    Split units


    Portable units are also available in split configuration, with the compressor and evaporator located in a separate external package and the two units connected via two detachable refrigerant pipes, as is the case with fixed split systems. Split portable units are superior to both single and dual hosed mono-portable units in that interior noise and size of the internal unit is greatly reduced due to the external location of the compressor, and no water needs to be drained from the internal unit due to the exterior location of the evaporator.
    A drawback of split portable units compared with mono-portables is that a surface exterior to the building, such as a balcony must be provided for the external compressor unit to be located.
    Unlike window ACs the split AC does not have an option of exchange of indoor and outdoor air.
    Heat and cool units


    Some portable air conditioner units are also able to provide heat by reversing the cooling process so that cool air is collected from a room and warm air is released. These units are not meant to replace actual heaters though and should not be used to cool rooms lower than 50 °F (10 °C).
    Central air conditioning


    Central air conditioning, commonly referred to as central air (U.S.) or air-con (UK), is an air conditioning system that uses ducts to distribute cooled and/or dehumidified air to more than one room, or uses pipes to distribute chilled water to heat exchangers in more than one room, and which is not plugged into a standard electrical outlet.
    With a typical split system, the condenser and compressor are located in an outdoor unit; the evaporator is mounted in the air handler unit. With a package system, all components are located in a single outdoor unit that may be located on the ground or roof.
    Central air conditioning performs like a regular air conditioner but has several added benefits:

    • When the air handling unit turns on, room air is drawn in from various parts of the building through return-air ducts. This air is pulled through a filter where airborne particles such as dust and lint are removed. Sophisticated filters may remove microscopic pollutants as well. The filtered air is routed to air supply ductwork that carries it back to rooms. Whenever the air conditioner is running, this cycle repeats continually.


    • Because the condenser unit (with its fan and the compressor) is located outside the home, it offers a lower level of indoor noise than a free-standing air conditioning unit.

    Mini (small) duct, high velocity


    A central air conditioning system using high velocity air forced through small ducts (also called mini-ducts), typically round, flexible hoses about 2 inches in diameter. Using the principle of aspiration, the higher velocity air mixes more effectively with the room air, eliminating temperature discrepancies and drafts. A high velocity system can be louder than a conventional system if sound attenuators are not used, though they come standard on most, if not all, systems.
    The smaller, flexible tubing used for a mini-duct system allows it to be more easily installed in historic buildings, and structures with solid walls, such as log homes. These small ducts are also typically longer contiguous pieces, and therefore less prone to leakage. Another added benefit of this type of ducting is the prevention of foreign particle buildup within the ducts, due to a combination of the higher velocity air, as well as the lack of hard corners.
    Thermostats

    Main article: Thermostat

    Thermostats control the operation of HVAC systems, turning on the heating or cooling systems to bring the building to the set temperature. Typically the heating and cooling systems have separate control systems (even though they may share a thermostat) so that the temperature is only controlled "one-way." That is, in cold weather, a building that is too hot will not be cooled by the thermostat. Thermostats may also be incorporated into facility energy management systems in which the power utility customer may control the overall energy expenditure. In addition, a growing number of power utilities have made available a device which, when professionally installed, will control or limit the power to an HVAC system during peak use times in order to avoid necessitating the use of rolling blackouts. The customer is given a credit of some sort in exchange, so it is often to the advantage of the consumer to buy the most efficient[citation needed] thermostat possible.
    Equipment capacity


    Air conditioner equipment power in the U.S. is often described in terms of "tons of refrigeration". A "ton of refrigeration" is defined as the cooling power of one short ton (2000 pounds or 907 kilograms) of ice melting in a 24-hour period. This is equal to 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.
    The use of electric/compressive air conditioning puts a major demand on the electrical power grid in hot weather, when most units are operating under heavy load. In the aftermath of the 2003 North America blackout locals were asked to keep their air conditioning off. During peak demand, additional power plants must often be brought online, usually expensive peaker plants. A 1995 meta-analysis of various utility studies concluded that the average air conditioner wasted 40% of the input energy. This energy is lost in the form of heat, which must be pumped out.
    In an automobile, the A/C system will use around 5 horsepower (4 kW) of the engine's power.[citation needed]
    Seasonal energy efficiency ratio (SEER)

    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.[15]

    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 (assuming 1000 hours of operation during a typical cooling season, i.e., 8 hours per day for 125 days per year)
    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
    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:[16]

    (1) SEER = EER ÷ 0.9

    (2) SEER = COP x 3.792

    (3) EER = COP x 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.
    Today, it is rare to see systems rated below SEER 9 in the United States, since older units are being replaced with higher-efficiency units. 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).[17] Substantial energy savings can be obtained from more efficient systems. For example by upgrading from SEER 9 to SEER 13, the power consumption is reduced by 30% (equal to 1 - 9/13). It is claimed that this can result in an energy savings valued at up to US$300 per year (depending on the usage rate and the cost of electricity). In many cases, the lifetime energy savings are likely to surpass the higher initial cost of a high-efficiency unit.
    As an example, the annual cost of electric power consumed by a 72,000 BTU/h air conditioning unit operating for 1000 hours per year with a SEER rating of 10 and a power cost of $0.08 per kilowatt hour (kW·h) may be calculated as follows:

    unit size, BTU/h × hours per year, h × power cost, $/kW·h ÷ (SEER, BTU/W·h × 1000 W/kW)

    (72,000 BTU/h) × (1000 h) × ($0.08/kW·h) ÷ [(10 BTU/W·h) × (1000 W/kW)] = $576.00 annual cost
    A common misconception is that the SEER rating system also applies to heating systems. However, SEER ratings only apply to air conditioning.
    Air conditioners (for cooling) and heat pumps (for heating) both work similarly in that heat is transferred or "pumped" from a cooler heat source to a warmer "heat sink". Air conditioners and heat pumps usually operate most effectively at temperatures around 10 to 13 degrees Celsius (°C) (50 to 55 degrees Fahrenheit (°F)). A balance point is reached when the heat source temperature falls below about 4 °C (40 °F), and the system is not able to pull any more heat from the heat source (this point varies from heat pump to heat pump). Similarly, when the heat sink temperature rises to about 49 °C (120 °F), the system will operate less effectively, and will not be able to "push" out any more heat. Geothermal heat pumps do not have this problem of reaching a balance point because they use the ground as a heat source/heat sink and the ground's thermal inertia prevents it from becoming too cold or too warm when moving heat from or to it. The ground's temperature does not vary nearly as much over a year as that of the air above it.
    Insulation


    Insulation reduces the required power of the air conditioning system. Thick building walls, reflective roofing, curtains and trees next to buildings also cut down on system and energy requirements.




    _________________

    "لا اله الا الله العليم الحليم،لا اله الا الله رب العرش العظيم،لا اله الا الله رب السموات والأرض ورب العرش الكريم."



     


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