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If more than one zone is served by a CAV system, the supply air is cooled at a central location to meet the need of the zone with highest demand. The other zones get overcooled or, if comfort is to be maintained, the air is reheated at the terminal units. CAV systems with reheat are inefficient because they expend energy to cool air that will be heated again.

CAV systems with reheat, however, provide superior comfort in any zone. Constant airflow reduces pockets of "dead" air, and reheat provides close control of the space temperature. Variable air volume VAV systems vary the amount of air supplied to a zone while holding the supply air temperature constant.

This strategy saves fan energy and uses less reheat than in a CAV system. VAV systems, however, can have problems assuring uniform space temperature at low airflow rates. At times, the minimum airflow required for ventilation or for proper temperature control may be higher than is required to meet the space load.

When this occurs reheat may be required. Low-flow air diffusers in VAV systems help maintain uniform air distribution in a space at low airflows. These devices can be passive or active. Passive low flow diffusers are designed to mix the supply air with the room air efficiently at low flow. Active diffusers actually move the outlet vanes of the diffuser to maintain good mixing at low flow.

Active diffusers can also be used as VAV terminal units. Fan-powered VAV terminal units provide another method to improve air distribution at low load conditions. These units combine the benefits of a VAV system, by reducing central fan energy and reheat energy, with the benefits of a CAV system, by maintaining good airflow.

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There are two major types, series and parallel: Series fan-powered units maintain constant airflow to the zone at all times; parallel fan-powered units allow the airflow to the zone to vary somewhat, but do not allow the airflow in the zone to drop below a desired level. Both, however, allow the central fan to throttle down to the minimum airflow required for ventilation. Raised floor air distribution delivers air low in the space, at low velocity and relatively high temperature compared to traditional plenum mounted distribution systems.

Delivering air through a series of adjustable floor-mounted registers permits room air to be stratified with lower temperatures in the bottom portion of the room where people are located and high temperatures towards the ceiling. This system type is attracting increasing interest because it has the potential to save energy and to provide a high degree of individual comfort control.

These systems have historically used constant-volume air delivery. Manufacturers arenow beginning to offer VAV systems that are more easily designed, installed, and operated with raised floor plenum systems. In recent years, ventilation control systems have become more complex and, if installed and maintained properly, more dependable.

Among the advancements are:. Direct digital control DDC systems using digital-logic controllers and electrically-operated actuators are replacing traditional pneumatic controls. Pneumatic systems use analog-logic controllers and air-pressure actuators. DDC systems are repeatable and reliable,provide accurate system responses, and can be monitored from a central computer station. DDC systems also require less maintenance than pneumatic systems.

However, pneumatic controllers can be less expensive than electric actuators. Hybrid systems use a combination of digital logic controllers and pneumatic actuators. CAV systems should have controls to reset the supply air temperature at the cooling coil to provide the warmest air possible to the space with the highest cooling load.

This reduces reheat throughout the system. However, the temperature should be no higher than is necessary to properly dehumidify the air. Another option to reduce reheat is to use a bypass system. Bypass systems work like variable volume systems at the zones, but have constant airflow across the central fan. VAV systems can now be designed to serve areas with as little as six tons of cooling load.

Inlet vanes or, better yet, variable speed fans should be used to control air volume. In systems that have supply and return fans, airflow monitoring stations should be used to maintain the balance between supply and return airflow. CO 2 -based control systems control the amount of outside air required for ventilation. These systems monitor the CO 2 in the return air and modulate the outside air damper to provide only the amount of outside air required to maintain desired levels. Since CO 2 does not account for contaminants released by the building materials e. In large commercial and institutional buildings, devices used to produce cool water are called chillers.

The water is pumped to air handling units to cool the air. They use either mechanical refrigeration processes or absorption processes.

Absorption chillers are heat-operated devices that produce chilled water via an absorption cycle. Absorption chillers can be direct-fired, using natural gas or fuel oil, or indirect-fired. Indirect-fired units may use different sources for heat: Absorption chillers can be single-effect or double-effect, where one or two vapor generators are used. Double-effect chillers use two generators sequentially to increase efficiency. Evaporative coolers , also called swamp coolers, are packaged units that cool the air by humidifying it and then evaporating the moisture.

The equipment is most effective in dry climates. It can significantly reduce the peak electric demand when compared to electric chillers. Air-cooled condensers are offered on smaller, packaged systems typically from less than one ton to tons. They are initially less costly than water-cooled condensers, but do not allow the chiller to operate as efficiently. Water-cooled condensers use water that is cooled directly from the evaporative condenser or indirectly via a cooling tower.

The lower temperature achieved by evaporating water allows chillers served by water-cooled condensers to operate more efficiently. A waterside economizer consists of controls and a heat exchanger installed between the cooling tower water loop and the chilled water loop. Air is blown across copper coils to reject heat from this residential air-cooled condenser. Cogeneration is a process in which electric power is generated at the facility where the waste heat is recovered to produce service hot water, process heat, or absorption cooling.

Currently, packaged cogeneration systems between about kW are widely available. Extensive research and marketing efforts are underway for smaller systems as low as 4 kW. Fuel cells use chemical processes to generate electricity. The heat generated by fuel cells can also be recovered, as in cogeneration. Currently, the minimum size for a fuel cell in building applications is kW. Note that fuel cells need continuous, full-load operation.

The benefits of high performance, energy-efficient HVAC systems are universal. Therefore, high performance HVAC systems can be installed in all different types of buildings, including office buildings , schools , hospitals , and courthouses. Building energy simulations allow the system designer to compare different HVAC systems and control strategies. These tools vary in their scope and level of complexity. Some tools analyze individual components of HVAC systems e.

The latter tools require expertise and experience to obtain accurate results due to the detailed input required. Some building simulation packages have reduced input requirements. The trade-off is that these tools are typically not as accurate, since the programs use defaults or assumptions to replace the user inputs. However, simplified tools can be used early in the design process to investigate the influence of HVAC system selection on energy efficiency strategies such as daylighting. Skip to main content. Energy-efficient, climate responsive construction requires a whole building perspective that integrates architectural and engineering concerns early in the design process.

For example, the evaluation of a building envelope design must consider its effect on cooling loads and daylighting. An energy-efficient building envelope, coupled with a state-of-the-art lighting system and efficient, properly-sized HVAC equipment will cost less to purchase and operate than a building whose systems are selected in isolation from each other. And the difference is likely to be not only component size, but also basic system type.

If the entering air is dry enough, the results can be quite substantial. 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 other types of 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. Air conditioning can also be provided by a process called free cooling which uses pumps to circulate a coolant typically water or a glycol mix from a cold source, which in turn acts as a heat sink for the energy that is removed from the cooled space.

Common storage media are deep aquifers or a natural underground rock mass accessed via a cluster of small-diameter boreholes, equipped with heat exchanger. Some systems with small storage capacity are hybrid systems, using free cooling early in the cooling season, and later employing a heat pump to chill the circulation coming from the storage.

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The heat pump is added because the temperature of the storage gradually increases during the cooling season, thereby declining its effectiveness. Free cooling systems can have very high efficiencies, and are sometimes combined with seasonal thermal energy storage STES so the cold of winter can be used for summer air conditioning. Free cooling and hybrid systems are mature technology. 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.

Refrigeration air conditioning equipment usually reduces the absolute humidity of the air processed by the system. The relatively cold below the dewpoint evaporator coil condenses water vapor from the processed air, much like an ice-cold drink will condense water on the outside of a glass. Therefore, water vapor is removed from the cooled air and the relative humidity in the room is lowered. The water is usually sent to a drain or may simply drip onto the ground outdoors. The heat is rejected by the condenser which is located outside of room to be cooled. Most modern air-conditioning systems feature a dehumidification cycle during which the compressor runs while the fan is slowed as much as possible [ citation needed ] to reduce the evaporator temperature and therefore condense more water.

When the temperature falls below a threshold, both the fan and compressor are shut off to mitigate further temperature drops; [ clarification needed ] this prevents moisture on the evaporator from being blown back into the room. Occasionally, to thaw any ice produced, the fan runs with the compressor shut down; this function is less effective when ambient temperatures are low. Inverter air conditioners use the inside coil temperature sensor to keep the evaporator as cold as possible. When the evaporator is too cold, [ clarification needed ] the compressor is slowed or stopped with the indoor fan running.

A specialized air conditioner that is used only for dehumidifying is called a dehumidifier. It also uses a refrigeration cycle , but differs from a standard air conditioner in that both the evaporator and the condenser are placed in the same air path. A standard air conditioner transfers heat energy out of the room because its condenser coil releases heat outside. However, since all components of the dehumidifier are in the same room, no heat energy is removed. Instead, the electric power consumed by the dehumidifier remains in the room as heat, so the room is actually heated , just as by an electric heater that draws the same amount of power.

In addition, if water is condensed in the room, the amount of heat previously needed to evaporate that water also is re-released in the room the latent heat of vaporization. The dehumidification process is the inverse of adding water to the room with an evaporative cooler , and instead releases heat. Therefore, an in-room dehumidifier always will warm the room and reduce the relative humidity indirectly, as well as reducing the humidity directly by condensing and removing water.

Inside the unit, the air passes over the evaporator coil first, and is cooled and dehumidified. The now dehumidified, cold air then passes over the condenser coil where it is warmed up again. Then the air is released back into the room. The unit produces warm, dehumidified air and can usually be placed freely in the environment room that is to be conditioned. Dehumidifiers are commonly used in cold, damp climates to prevent mold growth indoors, especially in basements. They are also used to protect sensitive equipment from the adverse effects of excessive humidity in tropical countries.

In a thermodynamically closed system , any power dissipated into the system that is being maintained at a set temperature which is a standard mode of operation for modern air conditioners requires that the rate of energy removal by the air conditioner increase. This increase has the effect that, for each unit of energy input into the system say to power a light bulb in the closed system , the air conditioner removes that energy.

Air conditioner equipment power in the U. The value is defined as 12, BTU per hour, or watts. 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 electrical energy consumed per year can be calculated as the average power multiplied by the annual operating time:. Assuming hours of operation during a typical cooling season i. 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. 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. The United States now requires that residential systems manufactured in have a minimum SEER rating of 13 although window-box systems are exempt from this law, so their SEER is still around Window unit air conditioners are installed in an open window.

The interior air is cooled as a fan blows it over the evaporator. On the exterior the heat drawn from the interior is dissipated into the environment as a second fan blows outside air over the condenser. A large house or building may have several such units, allowing each room to be cooled separately. In , General Electric introduced a popular portable in-window air conditioner designed for convenience and portability.

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Packaged terminal air conditioner PTAC systems are also known as wall-split air conditioning systems. PTACs, 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 an opening passing through the wall and connecting them. This minimizes the interior system footprint and allows each room to be adjusted independently.


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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 semi-portable air conditioning unit was invented by engineers at Chrysler Motors and offered for sale starting in Split-system air conditioners come in two forms: In both types, the inside-environment evaporative heat exchanger is separated by some distance from the outside-environment condensing unit heat exchanger.

A mini-split system typically supplies air conditioned and heated air to a single or a few rooms of a building. Multi-zone systems typically offer a variety of indoor unit styles including wall-mounted, ceiling-mounted, ceiling recessed, and horizontal ducted. Multi-zone systems provide extended cooling and heating capacity up to 60, Btu's. Advantages of the ductless system include smaller size and flexibility for zoning or heating and cooling individual rooms. The inside wall space required is significantly reduced. Also, the compressor and heat exchanger can be located farther away from the inside space, rather than merely on the other side of the same unit as in a PTAC or window air conditioner.

Flexible exterior hoses lead from the outside unit to the interior one s ; these are often enclosed with metal to look like common drainpipes from the roof. In addition, ductless systems offer higher efficiency, reaching above 30 SEER. The primary disadvantage of ductless air conditioners is their cost. An additional possible disadvantage is that the cost of installing mini splits can be higher than some systems. However, lower operating costs and rebates or other financial incentives—offered in some areas—can help offset the initial expense. Central ducted air conditioning offers whole-house or large-commercial-space cooling, and often offers moderate multi-zone temperature control capability by the addition of air-louver-control boxes.

The heat-exchanger cools the air that is being forced through it by the furnace blower. As the warm air comes in contact with this cool surface the water in the air condenses. By pulling the water molecules from the air. According to the psychometric chart [42] as relative humidity decreases in order to feel cool you will have to lower the temperature even more. A common way to counteract this effect is by installing a whole-home humidifier.

A multi-split system [44] is a conventional split system, which is divided into two parts evaporator and condenser and allows cooling or heating of several rooms with one external unit. In the outdoor unit of this air conditioner there is a more powerful compressor, ports for connecting several traces and automation with locking valves for regulating the volume of refrigerant supplied to the indoor units located in the room.

A large Multi Split System is called a Variable refrigerant flow system and can be used instead of a central air conditioner system, as it allows for higher energy efficiency but it is more expensive to purchase and install. Other common types of air conditioning system are multi-split systems, the difference between separate split system and multi-split system in several indoor units. All of them are connected to the main external unit, but the principle of their operation is similar to a simple split-system. Its unique feature is the presence of one main external unit that connected to several indoor units.

Such systems might be the right solution for maintaining the microclimate in several offices, shops, large living spaces. Just few of outdoor units do not worsen the aesthetic appearance of the building. The main external unit can be connected to several different indoor types: Before selecting the installation location of air conditioner, several main factors need to be considered. First of all, the direction of air flow from the indoor units should not fall on the place of rest or work area.

Secondly, there should not be any obstacles on the way of the airflow that might prevent it from covering the space of the premises as much as possible. The outdoor unit must also be located in an open space, otherwise the heat from the house will not be effectively discharged outside and the productivity of the entire system will drop sharply. It is highly advisable to install the air conditioner units in easily accessible places, for further maintenance during operation.

The main problem when installing a multi-split system is the laying of long refrigerant lines for connecting the external unit to the internal ones.

While installing a separate split system, workers try to locate both units opposite to each other, where the length of the line is minimal. Installing a multi-split system creates more difficulties, since some of indoor units can be located far from the outside. The first models of multi-split systems had one common control system that did not allow you to set the air conditioning individually for each room. However, now the market has a wide selection of multi-split systems, in which the functional characteristics of indoor units operate separately from each other.

The selection of indoor units has one restriction: However, it is wrong to expect better performance when all indoor units are turned on at the same time, since the total capacity of the whole system is limited by the capacity of the outdoor unit. Simply put, the outdoor unit will distribute all its power to all operating indoor units in such a way that some of the rooms may not have a very comfortable temperature level. However, the calculation of the total power is not simple, since it takes into account not only the nominal power of the units, but also the cooling capacity, heating, dehumidification, humidification, venting, etc.

A portable air conditioner can be easily transported inside a home or office. Portable air conditioners are either evaporative or refrigerative. The compressor-based refrigerant systems are air-cooled, meaning they use air to exchange heat, in the same way as a car radiator or typical household air conditioner does.

Such a system dehumidifies the air as it cools it. It collects water condensed from the cooled air and produces hot air which must be vented outside the cooled area; doing so transfers heat from the air in the cooled area to the outside air. A portable system has an indoor unit on wheels connected to an outdoor unit via flexible pipes, similar to a permanently fixed installed unit. Hose systems, which can be monoblock or air-to-air , are vented to the outside via air ducts.

The monoblock type collects the water in a bucket or tray and stops when full. The air-to-air type re-evaporates the water and discharges it through the ducted hose and can run continuously. A single-hose unit uses air from within the room to cool its condenser, and then vents it outside. This air is replaced by hot air from outside or other rooms due to the negative pressure inside the room , thus reducing the unit's overall efficiency.

Modern units might have a coefficient of performance of approximately 3 i. A dual-hose unit draws air to cool its condenser from outside instead of from inside the room, and thus is more effective than most single-hose units. These units create no negative pressure in the room.

Evaporative coolers , sometimes called "swamp coolers", do not have a compressor or condenser. Liquid water is evaporated on the cooling fins, releasing the vapor into the cooled area. Evaporating water absorbs a significant amount of heat, the latent heat of vaporisation , cooling the air. Humans and animals use the same mechanism to cool themselves by sweating.

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Evaporative coolers have the advantage of needing no hoses to vent heat outside the cooled area, making them truly portable. They are also very cheap to install and use less energy than refrigerative air conditioners. Air-conditioning engineers broadly divide air conditioning applications into comfort and process applications.

Comfort applications aim to provide a building indoor environment that remains relatively constant despite changes in external weather conditions or in internal heat loads. Air conditioning makes deep plan buildings feasible, for otherwise they would have to be built narrower or with light wells so that inner spaces received sufficient outdoor air via natural ventilation. Air conditioning also allows buildings to be taller, since wind speed increases significantly with altitude making natural ventilation impractical for very tall buildings.

Women have, on average, a significantly lower resting metabolic rate than men. In addition to buildings, air conditioning can be used for many types of transportation, including automobiles, buses and other land vehicles, trains, ships, aircraft, and spacecraft. Southern European countries such as Greece have seen a wide proliferation of home air-conditioning units in recent years.

Process applications aim to provide a suitable environment for a process being carried out, regardless of internal heat and humidity loads and external weather conditions. It is the needs of the process that determine conditions, not human preference. Process applications include these:. In both comfort and process applications, the objective may be to not only control temperature, but also humidity , air quality, and air movement from space to space.

In hot weather, air conditioning can prevent heat stroke , dehydration from excessive sweating and other problems related to hyperthermia. Heat waves are the most lethal type of weather phenomenon in developed countries. Air conditioning including filtration, humidification, cooling and disinfection can be used to provide a clean, safe, hypoallergenic atmosphere in hospital operating rooms and other environments where proper atmosphere is critical to patient safety and well-being.

It is sometimes recommended for home use by people with allergies. Poorly maintained water cooling towers can promote the growth and spread of microorganisms, [55] such as Legionella pneumophila , the infectious agent responsible for Legionnaires' disease , or thermophilic actinomycetes. As long as the cooling tower is kept clean usually by means of a chlorine treatment , these health hazards can be avoided or reduced. Excessive air conditioning can have a negative effect on skin, causing it to dry out, and can also cause dehydration.

Innovation in air conditioning technologies continues, with much recent emphasis placed on energy efficiency.

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Production of the electricity used to operate air conditioners has an environmental impact, including the release of greenhouse gases. 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. Most of the homes with central air conditioning have programmable thermostats , but approximately two-thirds of the homes with central air do not use this feature to make their homes more energy efficient. Alternatives to continual air conditioning can be used with less energy, lower cost, and with less environmental impact.

The selection of the working fluids refrigerants has a significant impact not only on the performance of the air conditioners but on the environment as well. Most refrigerants used for air conditioning contribute to global warming, and many also deplete the ozone layer.

The use of CFC as a refrigerant was once common, including the refrigerants R and R sold under the brand name Freon Freon refrigerants were commonly used during the 20th century in air conditioners due to their superior stability and safety properties. When they are released accidentally or deliberately, these chlorine-bearing refrigerants eventually reach the upper atmosphere.

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These chlorine radicals 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 until it binds with another radical, forming a stable molecule and quenching the chain reaction. Prior to , most automotive air conditioning systems used R as a refrigerant. It was replaced with Ra refrigerant, which has no ozone depletion potential. Although these gasses can be recycled when air conditioning units are disposed of, uncontrolled dumping and leaking can release gas directly into the atmosphere.

The Regulation banned the use of R22 as a "top-up" fluid for maintenance between for virgin fluid and for recycled fluid. This means that equipment that uses R22 can still operate, as long as it does not leak. Although R22 is now banned, units that use the refrigerant can still be serviced and maintained.

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The manufacture and use of CFCs has been banned or severely restricted due to concerns about ozone depletion see also Montreal Protocol. Environmental Protection Agency has restricted the sale, possession and use of refrigerant to only licensed technicians, per rules under sections and of the Clean Air Act.

As an alternative to conventional refrigerants, other gases, such as CO 2 R , have been proposed. It is an effective refrigerant with a global warming potential of 1, but it must use higher compression to produce an equivalent cooling effect. In , a non-governmental organization, Greenpeace, was spurred by corporate executive policies and requested that a European lab find substitute refrigerants. This led to two alternatives, one a blend of propane R and isobutane Ra , and one of pure isobutane. From Wikipedia, the free encyclopedia. Process of altering the properties of air to more favourable conditions.

This article is about cooling of air. For air conditioning systems in vehicles, see Automobile air conditioning. For the Curved Air album, see Air Conditioning album.