Air Conditioner Full Definition
An air conditioner (often referred to as AC) is a home appliance, system, or mechanism 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”.
SYSTEM BASICS AND THEORIES
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.
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.
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 coolness as an air cooler in humid tropical climates, but only consume about one-third the energy.
“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. 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. 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; 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.
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. 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.
The following are the basic parts for a window unit air conditioner.
- Adjustable louvers
- Control panel
- Front grill
- Thermostat sensor
- Condenser coil
- Evaporator coil
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
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 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 (~2.9 kW) 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.
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 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.
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 cooling power of 12,000–14,000 BTUs/h (3.5–4.1 kW) 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. Without the intake hose, the high exhaust flow rate would create negative air pressure in the room resulting in less cooling potential due to insufficient air flow. The intake hose therefore allows for a higher air flow rate resulting in higher cooling capacity.
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. Most split system air conditioners do not introduce outside air for ventilation, and only perform a temperature control function. It is, however, possible to add ventilation ducting to some split systems, so that they do bring fresh air into the room.
Heat and cool
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 10 °C (50 °F).
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 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
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 they are 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. For large buildings, ground source-coldness is successfully used to reduce energy consumption of central air conditioner systems, often in combination with a heat pump based heating system.
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 4 horsepower (3 kW) of the engine’s power.
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 sum of the rate of heat transfer from the building into the ambient environment, and the rate of heat generation by anything in the building that is a heat source.
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 of condensation on the pipe surface that would otherwise accelerate corrosion.
Home systems around the world
This especially applies to capitals and urbanized areas in hot parts of the world where most of the population lives in small high-rise flats. 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 very common. 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 the United States, but still quite prevalent. This is especially true of the Great Lakes regions of Southern Ontario and Southern Quebec, where there are especially high humidity levels. The majority of modern urban high-rise condominiums built in Canadian cities have air conditioning systems. While energy is comparatively 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 often hot, but rarely reach the dangerous temperatures experienced in the southern United States. As such, some Canadians, especially in older homes, simply choose to forgo air conditioning and use simple fans and evaporative coolers instead. 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 the Enwave 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. Southern European countries such as Greece have seen a wide proliferation of home air-conditioning units in recent years. In another southern European country, Malta, it is estimated that around 55% of households have an air conditioner installed.In India AC sales have dropped by 40% due to higher costs and upgradation of energy efficiency norms.