Air conditioner
From Wikipedia, the free encyclopedia
An air conditioner (AC or A/C) is an appliance, system, or mechanism designed to extract heat from an area using a refrigeration cycle. The most common uses of modern air conditioners are for comfort cooling in buildings and transportation vehicles. A combined system that also provides heating and ventilation is often called an HVAC system. This article addresses air conditioner equipment primarily, rather than air conditioning in general.
[edit] Window and through-the-wall air conditioners
Many traditional air conditioners in homes or other buildings are single rectangular units. Air conditioner units need to have access to the space they are cooling (the inside) and a heat sink; normally outside air is used to cool the condenser section. For this reason, single unit air conditioners are placed in windows or through openings in a wall made for the air conditioner. There are vents on both the inside and outside parts of the unit, so inside air to be cooled can be blown in and out by a fan in the unit, and so outside air can also be blown in and out by another fan to act as the heat sink. The controls are on the inside. A large house or building may have several such units.
[edit] Portable air conditioners
A portable air conditioner or portable A/C is an air conditioner on wheels that can be easily transported inside a home or office. They are currently available with capacities of about 6,000 to 14,000 BTU/h (1800 to 4100 watts output) and with and without electric resistance heaters. Portable air conditioners come in two forms, split and monoblock:
Monoblock systems are vented to the outside via air ducts. A single duct monoblock unit draws air out of the room to cool its condenser. This air is then replaced by hot air from outside or other rooms, thus reducing efficiency.
[edit] Central air conditioners
Central air conditioning, commonly referred to as central air (US) or air-con (UK), is an air conditioning system which uses ducts to distribute cooled and/or dehumidified air to typically more than one room, or uses pipes to distribute chilled water to heat exchangers.
- With a typical split system, the condenser and compressor are located in an outdoor unit; the evaporator is mounted in the air handling unit (which is often a forced air furnace).
- With a packaged system, all components are located in a single outdoor unit that may be located on the ground or roof.
Central air conditioning has several benefits as compared to having many smaller distributed units:
- When the air handling unit turns on, room air is drawn in from various parts of the house 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 process repeats continually.
- Because the central air conditioning's condenser unit is located outside the home, it typically offers a lower level of noise indoors than window or through-the-wall air conditioning units, for example.
However, the air ducts do become dirty over time, and pose a risk of growth and spread of harmful microorganisms.<ref>[1]</ref>
[edit] "Ductless", "duct-free", or "mini-split" air conditioners
Ductless mini-split air conditioners combine some traits of central air conditioning systems with some traits of window or through-the-wall units. They were invented as an alternative to window air conditioners for buildings where the cool-air distribution ducts of a central air conditioning system could not be installed or would be prohibitively expensive to install. An outside unit including the compressor is mounted on an exterior wall of the building, and an inside unit including the evaporator is mounted high on an interior wall, or on or in the ceiling, of the room to be cooled. They are connected refrigerant tubing, condensate drain, and control wires through a hole drilled in the room's exterior wall. An outdoor unit can be connected to one, two, or three indoor units.
Like window air conditioners, a ductless mini-split system requires no air ducts throughout the building and allows separate "zones" in the building to have independent temperature controls. However, like a central air conditioning system, it does not block a window or require another window-sized hole in the wall, and it puts the main source of noise (the compressor) outside the building. Equipment to cool a given amount of inside space is more expensive than with window units but less expensive than with central systems. Customers buy them mostly for their quietness compared to window units, and their lower cost and ease of installation as compared to central systems. In a very large building, the need for ventilation and the need to cool air space that is far from the building's outer walls make ductless mini-split systems and window units impractical.
[edit] Evaporative coolers
In very dry climates, evaporative coolers are popular for improving comfort during hot weather. The evaporative cooler is a machine that draws or forces outside air through a wet pad. The sensible heat of the incoming air, as measured by a dry bulb thermometer. 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 resulting supply air can be quite comfortable. These coolers cost less than other air conditioners and are mechanically simple to understand and maintain.
[edit] Absorption air coolers and washers
There is a process called absorptive refrigeration which uses heat to produce cooling. In one instance, a three-stage absorptive cooler first dehumidifies the air with a spray of salt-water or brine. The brine osmotically absorbs water vapor from the air. The second stage sprays water in the air, cooling the air by evaporation. Finally, to control the humidity, the air passes through another brine spray. The brine is reconcentrated by distillation. The system is used in some hospitals because, with filtering, a sufficiently hot regenerative distillation removes airborne organisms.
[edit] Absorptive chillers
Some buildings use gas turbines to generate electricity. The exhausts of these are hot enough to drive an absorptive chiller that produces cold water. The cold water is then run through heat exchangers in air handlers to provide cooling and dehumidification. The dual use of the energy, both to generate electricity and cooling, makes this cogeneration technology attractive when regional utility and fuel prices are more expensive than average. When heating, in addition to power generation and cooling, are performed simultaneously by one combined system, it is known as trigeneration.
[edit] Thermostats
Thermostats control the operation of HVAC systems, turning on and off, or modulating, the heating or cooling systems to bring the building to the set temperature. Thermostats may also be incorporated into facility energy management systems in which the electrical power utility customer may control the overall energy expenditure. In addition, a growing number of electric 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.
There are many widely used thermostat technologies such as:
- Mechanical
- Electromechanical (EM)
- Pneumatic
- Digital
- Hybrid
[edit] Equipment capacity
Air conditioner equipment cooling capacity 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 <ref>[2]</ref>. Single-family residential "central air" systems are usually from 2 to 5 tons (3 to 20 kW) in capacity.
Adding insulation and other weatherization features reduces the required capacity of an air conditioning system. Thick walls, reflective roofing material, curtains, and trees next to buildings also can reduce energy requirements.
The use of electric/compressive air conditioning puts a major demand on a nation's electrical power grid in warm 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 conditioners off. During peak demand, additional power plants must often be brought online, usually natural gas fired plants because of their rapid startup. A 1995 study of various utility studies of residential air conditioning concluded that the average air conditioner wasted 40% of the input energy (citation needed). This energy is lost in the form of heat. In the U.S., the minimum efficiency requirements of new AC units has been increased periodically to address this concern.
In an automobile the A/C system will use around 5 hp (4 kW) of the engine's power.
[edit] Energy Efficiency Ratings
The performance of vapor compression refrigeration cycles is limited by thermodynamics. These AC and heat pump devices move heat rather than convert it from one form to another, so thermal efficiencies do not appropriately describe their performance. The Coefficient-of-Performance (COP) does, but this dimensionless measure does not enjoy wide use like the dimensional Energy Efficiency Ratio (EER). To more accurately describe the performance of air conditioning equipment over a typical cooling season a modified version of the EER is used, and is the Seasonal Energy Efficiency Ratio (SEER). The SEER article describes it further, and presents some economic comparisons using this useful performance measure.
[edit] Home air conditioning systems around the world
Residential air conditioning is ubiquitous in Japan, South Korea, Taiwan, Singapore and Hong Kong, especially in the latter two due to most of the population living in small high-rise flats in warm climates. In this area, with soaring summer temperatures and a high standard of living, air conditioning is considered a necessity and not a luxury. Air conditioners are usually window or split types, the latter being more modern and expensive. It is also increasing in popularity with the rising standard of living in tropical Asian nations such as India, Malaysia, and the Philippines.
In North America, home air conditioning is more prevalent in the South, Midwest, East Coast, the Great Lakes States, and Ontario Canada in most parts of which it has reached the ubiquity it enjoys in East Asia. Central air systems are most common in the United States, and are virtually standard in all new dwellings in most states. Older houses and buildings not retro-fitted with central air often still use window or through-wall units.
In Europe, home air conditioning is less common in part due to a more clement northern climate. Although in the southern European countries with a high standard of living, like Spain and Italy, air conditioning is becoming a necessity. Still, the lack of air conditioning in homes, in residential care homes, and in medical facilities was identified as a contributing factor to the estimated 35,000 deaths left in the wake of the 2003 heat wave. There were almost 15,000 deaths in France, and 2,000 in the UK; in contrast, only 141 died in Spain despite the higher temperatures, in part due to the extensive use of air conditioning. Due to the 2003 and the 2006 heatwaves, portable air conditioners have become more popular in France.
In many countries in the Persian Gulf, air conditioning is ubiquitous. This is due to the very harsh climate, and the relatively high living standards.
[edit] Car air conditioning systems around the world
Air conditioning started to become popular in American cars in the 1950s. Now, many new car models sold throughout the world are fitted with central air conditioning as standard. with plural outlets.
Early automotive air conditioning systems were independent of the heater system: ducting, controls and fans were completely separate. Later on, manufacturers integrated the two systems. Air entering an integrated system is filtered, optionally cooled and then reheated before it enters the passenger compartment. This arrangement allows car owners to take advantage of the dehumdifying effect of the evaporator coil to provide a highly effective demist capability.
Air conditioning compressors are usually driven by the engine fan-belt via an electro-magnetic clutch. It is often possible to tell whether the air conditioning is on from the outside of a car because of the periodic "click" from the compressor clutch synchronised with a change in engine note as engine load increases or decreases. Other cars may use a continuously-engaged compressor system to maintain a smooth idle. A tell-tale puddle of condensate underneath the car is also a good indication that the air conditioning is on or has been on recently.
The AC load on the engine is such that in some cars the air conditioning's compressor is disengaged during low engine vacuum conditions to improve acceleration and fuel economy.
Nowadays, cars includes automatic air conditioning, generally with four-zone climatizers.
[edit] References
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[edit] See also
[edit] External links
[edit] Servicing information
- The Association of Home Appliance Manufacturers' AC sizing worksheet
- Directory of Air Conditioning Manufacturers
- Central Air Conditioner Quality
[edit] Energy efficiency
- Consumer Guide to Home Energy Savings - Central Air Conditioners from the American Council for an Energy Efficient Economy (ACEEE)
- Energy Efficiency Program Database from the ACEEE
- Space heating and cooling from the U.S. Department of Energy's Energy Efficiency and Renewable Energy
- UK Enhanced Capital Allowance Scheme (ECA), a UK Government scheme to provide tax rebates for companies who use products which are ECA approved.
- International Energy Agency - Energy Conservation In Buildings And Community Systems
- Top-Rated Energy-Efficient Central Air Conditioners
- Online Air-Conditioning HVAC Calculator
