An air conditioner (often referred to as AC) is a home appliance, system, ormechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle. In construction, a complete system of heating,ventilation and air conditioning is referred to as "HVAC". Its purpose, in a building or anautomobile, is to provide comfort during either hot or cold weather.
History
Main 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".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. 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 campaignagainst 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 likeammonia, 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.
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 applications
Main article: Air conditioning#Air conditioning applications
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Air conditioning system basics and theories
Refrigeration cycle
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 aheat 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 vaporfrom 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
Reverse-cycle
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For more details on this topic, see Air conditioning#Heat pumps.
Types of air conditioner equipment
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 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. 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 cooling many rooms 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.
Basic parts
The following are the basic parts for a window unit air conditioner.Exterior
- Adjustable louvers
- Control panel
- Front grill
- Thermostat sensor
Interior
- Blower
- Partition
- Fan
- Compressor
- Condenser coil
- Evaporator coil
- Filter
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." 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 an open door or window.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 ofApalachicola, Florida in 1842. He used the device to cool the patients in his malariahospital.
Absorptive chillers
Main article: Absorption refrigerator
Portable air conditioners
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Portable air conditioners are movable units that can be used to cool a specific region of a building or home in a modular fashion, not requiring permanent installation. They are used for much the same purposes and in much the same ways as traditional "window a/c" units (cooling an overly hot room, cooling rooms in older homes without central a/c, providing a general "boost" in capacity to a home with an undersized central a/c e.g. "a hot upstairs bedroom", cooling a room that never had a/c before but is now being used for living or work space i.e. an attic converted into a play room or a shed converted into a workshop, etc). Portable a/c units provide a cleaner looking end product (no bulky unit hanging out of the window) which may allow installation in areas with stricter neighborhood ordinances/association rules, and are generally easier to install (the window design and installation part itself becomes much less of an obstacle for the average person); for this reason they are a popular alternative to traditional "window units" but do have some disadvantages. For example, they generally cost more than for an equally powerful (capacity) window unit e.g. a 10,000 BTU/h (~2.9kW) portable a/c with a standard feature set may sell for $300 retail versus the same capacity/featured window a/c unit at $150-$200 and they are somewhat noisier, since the compressor and condenser fan components are now inside the occupied space (although modern portable a/c units are fairly quiet and unobtrusive). Older portable a/c units also required periodic emptying of a condensate water tank (basically the water/humidity removed from the air) but modern units are designed in such a way that they rarely need to be emptied or maintained other than periodically cleaning the air filter.
Most portable air conditioners are refrigeration based rather than evaporative, and it is this type that is described in this section. Another application for portable air conditioner units is for the temporary rental in emergency situations such as power failures at warehouses, offices, or data centers.
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 (44.1 m2) or smaller and has at most a cooling power of 12,000 BTUs/h (3.5kW). 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.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/h (3.5-4.1kW) 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.Split units
Portable units are also available in split configuration, often 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 can be greatly reduced due to the external location of the compressor, and the water collected can be pumped to the outdoor unit using a pump, avoiding the need to drain water from the indoor unit periodically when running in cooling mode.. 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 heat rooms that are below 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 orroof.
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 filterwhere 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 often consumes more electricity to pump around air, and 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 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.
Passive ground source-based cooling
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If underground conditions are suitable, then by far the most energy-efficient way to chill air, is to pump up the coldness of ground water or from underground soil or rock formations, and use that coldness directly (without a heat pump compressor) to chill indoor air. Unless next to open water, they require a high initial investment: drilling deep holes and fitting them with pipes or a filter and pump. But after that, such systems consume five to twenty times less energy than heat pump-based systems. These systems have the disadvantage that they can not chill below or even near the temperature of the deeper underground, so they only work well if winters or nearby mountains cool groundwater below roughly 16 °C (60 °F). Also, in the longer run such systems have a tendency to 'deplete' underground coldness, which makes them less efficient. This can be fixed in the winter months, by collecting winter coldness from the air through a roof top heat exchanger and pumping it into the underground cold-source. Unfortunately, such systems are as yet hardly developed One factor is that some of the world's leading manufacturers of air conditioners also manufacture the boilers and turbines for large electricity plants. Therefore they have little incentive to reduce electricity use of air conditioners. For large buildings, ground source-coldness is successfully used to reduce energy consumption of central air conditioner systems, often in combination with heat pump based heating systems.
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 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 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. 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'spower.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.- SEER = BTU ÷ (W·h)
- SEER = (BTU / h) ÷ W, where "W" is the average electrical power in Watts, and (BTU/h) is the rated cooling power.
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
- 5,000,000 BTU ÷ 10 = 500,000 W·h = 500 kWh
- (1) SEER = EER ÷ 0.9
- (2) SEER = COP x 3.792
- (3) EER = COP x 3.413
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).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
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'sthermal 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
An air-conditioning unit is only able to cool a building to a given temperature if the cooling capacity of the air-conditioning unit is greater than the rate of heat transfer from the building into the ambient environment.Additional cooling capacity can be supplied by increasing the size, and most likely the energy consumption, of the air-conditioning unit. Restricting the rate of heat transfer is achieved by measures such as increasing structural insulation thickness's and improving air tightness. Since the rate of heat transfer through the building fabric has such a direct influence on air-conditioning requirements the level of insulation in the building fabric should be considered when selecting an air-conditioning unit.
Pipe insulation is applied to air-conditioning distribution pipework. This is partly to reduce the heat gain to the distribution pipework but also to prevent the formation ofcondensation on the pipe surface that would otherwise accelerate corrosion.
Home air conditioning systems around the world
Domestic air conditioning is most prevalent and ubiquitous in developed Asian and Middle Eastern nations and territories. This especially applies to capitals and urbanized areas where most of the population lives in small high-rise flats. In these areas, with high summer temperatures and a somewhat high standard of living, air conditioning is considered a necessity and not a luxury. Japanese-made domestic air conditioners are usually window or split types, the latter being more modern and expensive. In Israel, virtually all residential systems are split types.In the United States of America, home air conditioning is most prevalent in theSouth/Southwest and on the East Coast.[citation needed] Central air systems are most common in the United States of America, and increasingly a standard design factor.
In Canada, home air conditioning is less common than in East Asia and the United States, but it still quite prevalent. This is especially true of the Great Lakes regions of southern Ontario and Quebec, where there are especially high humidity levels. While window and split units are common in these regions, central air systems are the most widespread in Western Canada. Most Western Canadian homes are built with already-compatible central forced air natural gas heating systems, making installing a central air system very simple. In Central Canada separate room-based hydro powered heating is more common, leading to the higher cost of retrofitting a central air system. The majority of modern urban high-rise condominiums built in Canadian cities have air conditioning systems. It is also offered as a relatively low-cost option on most new built homes. While energy is comparatively very cheap in Canada, the large size of the average Canadian home and cold winters make heating and cooling one of the largest household expenses. Canadian summers are uncomfortably hot, but rarely reach the dangerous temperatures experienced in the United States or Asia. As such, many Canadians, especially in older homes, simply choose to forgo air conditioning in lieu of simple fans and evaporative coolers. Cost of operation (as a factor of efficiency) of air conditioning is often considered an environmentally unfriendly mitigation to poor thermal design. There have been a number of advances in more environmentally friendly technologies, including insulation advancement, geothermal cooling, and theEnwave deep lake system in Toronto that cools a number of office towers using cold water from Lake Ontario.
In Europe, home air conditioning is generally less common, in part due to higher energy costs and moderate summer temperatures. Southern European countries such asGreece, on the other hand, have seen a wide proliferation of home air-conditioning units in recent years.[18]In another southern European country, Malta, it is estimated that around 55% of households have an air conditioner installed.[19] The lack of air conditioning in residences, residential care homes and medical facilities was identified as a contributing factor to the estimated 35,000 deaths – mostly in Germany, Franceand Italy – left in the wake of the 2003 heat wave.
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