Air conditioning

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AIR CONDITIONING

Volkov A.V.

Vladimirskiy State University named after the Stoletov brothers

Vladimir, Russia

Air conditioning is a process which regulates temperature, humidity, air consumption and, sometimes dust content and smell in a confined space. Air is cooled with a cooling system or it is heated with hot steam or water. Smells and dust are removed by filtration. Humidifiers regulate humidity before air is blown by ventilator which also removes stagnated smells.

Air conditioning systems often function as forced ventilation. At warm periods they cool and drain air, and when it's cold they heat and humidify it. They also can function as heating systems or work with them jointly.

AC is usually used in public and administrative buildings, public transport to provide comfortable conditions. In industrial buildings it helps with obtaining of high-quality products and also with ensuring optimal working conditions and increases productivity. It is also needed in mines and in cabins of industrial machines for greatest comfort. Moreover some scientific researches require certain climate, so air conditioning systems are essential. Storage of food and agricultural production can't be imagined without air conditioning as well.

The first modern air conditioning unit was invented by Willis Carrier in the beginning of the 20^th century. The first air conditioners and refrigerators employed toxic or flammable gases, such as ammonia, methyl chloride, or propane, that could result in fatal accidents when they leaked. Most of further researches of air conditioning were devoted to solving such problems.

Air conditioners employ the same operating principles and basic components as a refrigerator. Refrigerators transfer heat from the cool interior of the refrigerator to the relatively warm surroundings of a room with electricity, the same way an air conditioner uses energy to transfer heat from the interior of a room to the relatively warm outside environment.

An air conditioner cools a room with an evaporator (a cold indoor coil). The condenser, a hot outdoor coil, releases the collected heat outside. The evaporator and condenser coils are serpentine tubing surrounded by aluminum fins. This tubing consists of copper.

A pump, called the compressor, moves a heat transfer fluid (or refrigerant) between the evaporator and the condenser. The pump pushes the refrigerant through the circuit of tubing and fins in the coils.

The liquid refrigerant evaporates in the indoor evaporator coil, pulls heat out of indoor air and cooling a room. The hot refrigerant gas pumps outdoors into the condenser where it reverts back to a liquid, gives up its heat to the outside air flowing over the condenser's metal tubing and fins.

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 systems promote the growth and spread of microorganisms, such as Legionella pneumophila, the infection responsible for Legionnaires' disease, or thermophilic actinomycetes; however, this is only prevalent in poorly maintained water cooling towers. As long as the cooling tower is kept clean (usually by means of a chlorine treatment), these health hazards can be avoided.

Conversely, air conditioning (including filtration, humidification, cooling and disinfection) can be used to make a clean, safe, hypoallergenic atmosphere in hospitals and other environments where proper atmosphere is critical to patient safety and well-being. Excessive air conditioning can have a negative effect on skin, causing drying, and can also cause dehydration.

Installation types

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.

Packaged terminal air conditioner systems are also known as wall-split air conditioning systems. They are ductless 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. 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

Split-system air conditioners come in two forms: mini-split and central systems. 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. Mutli-zone systems are a common application of ductless systems and allow up to 8 rooms (zones) to be conditioned from a single outdoor unit. Multi-zone systems typically offer a variety of indoor unit styles including wall-mounted, ceiling-mounted, ceiling recessed, and horizontal ducted.

Mini-split systems typically produce 9,000 to 36,000 Btu (9,500-38,000 kJ) per hour of cooling. Multi-zone systems provide extended cooling and heating capacity up to 60,000 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. Such systems cost about US$1,500 to US$2,000 per ton (12,000 BTU per hour) of cooling capacity. This is about 30% more than central systems (not including ductwork) and may cost more than twice as much as window units of similar capacity.

An additional possible disadvantage that the cost of installing mini splits can be higher than some systems, although lower operating costs and rebates or other financial incentives - offered in some areas - can help offset the initial expense

Central 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.

In central air conditioning, the inside heat-exchanger is typically placed inside the central furnace/AC unit of the forced air heating system which is then used in the summer to distribute chilled air throughout a residence or commercial building.

A portable air conditioner can be easily transported inside a home or office. They are currently available with capacities of about 5,000-60,000 BTU/h (1,500- 18,000 W) and with or without electric-resistance heaters. 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 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 effectiveness

Modern units might have a coefficient of performance of approximately 3 (i.e., 1 kW of electricity will produce 3 kW of cooling). 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.

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.

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.

In an automobile, the A/C system will use around 4 horsepower (3 kW) of the engine's power, thus increasing fuel consumption of the vehicle. Most refrigerants used for air conditioning contribute to global warming, and many also deplete the ozone layer.

Nowadays engineers innovate these technologies, with much recent emphasis placed on energy efficiency. Production of the electricity used to operate air conditioners has an environmental impact, including the release of greenhouse gases.

air conditioning filtration humidity ventilation

References:

1.McDowall Robert ,Fundamentals of HVAC Systems, Elsevier, 2006.

2.Daou K.; Wang Xia, "Desiccant cooling air conditioning: a review", 2005.

3.Nagengast Bernard, "A History of Comfort Cooling Using Ice", February 1999.

4.Stefanov E. V., Ventilation and Air Conditioning, 2005.

5.http://www.carelrussia.com/what-s-air-conditioning-

6.https://en.wikipedia.org/wiki/Air_conditioning

7.http://energy.gov

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