How Air Conditioners Work

The foremost modern air conditioning system was developed in 1902 by a young electrical engineer known as Willis Haviland Carrier. It was designed to solve a humidity problem at the Sackett-Wilhelms Lithographing and Publishing Company in Brooklyn, N.Y. Paper stock at the plant would at times absorb moisture from the warm summer air, making it cumbersome to apply the layered inking techniques of the time. Carrier conditioned the air inside the building by blowing it over chilled pipes. The air is cooled as it is passed over the cold pipes, and since cool air is not able to carry as much moisture as warm air, the process reduced the humidity in the plant and stabilized the moisture content of the paper. Reducing the humidity also had additional benefit of lowering the air temperature and a new technology was born.

Carrier realized he had developed something with extra potential, and it wasn't long before air-conditioning systems started appearing in theaters and stores, making the long, hot summer months extra comfortable.

The actual process air conditioners use to reduce the ambient air temperature in a room is based on a very simple scientific principle. The rest is achieved with the application of a few ingenious mechanical techniques. Actually, an air conditioner is very similar to another appliance in your home - the refrigerator. Air conditioners does not come with the exterior housing a refrigerator depends on to insulate its cold box. Instead, the walls in your home keep cold air in and hot air out.

Air-conditioning Basics

Air conditioners use refrigeration to cool indoor air, applying the knowledge the physical law: When a liquid changes to a gas (in a process called phase conversion), it absorbs heat. Air conditioners use this feature of phase conversion by forcing special chemical compounds to evaporate and condense repeated in a closed system of coils.

The compounds used are known as refrigerants with properties that allow them to change at relatively low temperatures. Air conditioners also have fans that move warm interior air over these cold, refrigerant-filled coils. In fact, central air conditioners have a complete system of ducts designed to channel air to and from these curly, air-chilling coils.

When hot air flows over the cold, low-pressure evaporator coils, the refrigerant within absorbs heat as it changes from liquid to gaseous state. To continue cooling efficiently, the air conditioner must convert the refrigerant gas back to a liquid again. To achieve that, a compressor applies high pressure to the gas, a process that generates unwanted heat. All the extra heat generated by compressing the gas is later evacuated to the outdoors with the assistance of an additional set of coils known as condenser coils, and a second fan. As the gas cools down, it then transforms to a liquid, and the process starts all over repeatedly. Think of it as a continuous cycle: liquid refrigerant, phase conversion to a gas/ heat absorption, compression and phase transition back to a liquid again.

It is quite easy to notice that there are two different processes going on in an air conditioner. Refrigerant is cooling the air indoor, and the resulting gas is continuously compressed and cooled for conversion back to a liquid again. 

Cool the Green Way

The chemical constituents of modern refrigerant compounds has changed over the years as a result of environmental concerns and international treaty agreements like the Montreal Protocol. Older refrigerant formulas containing chlorine atoms with the potential to damage the ozone layer have been gradually phased out in favor of more environmentally friendly coolants.

The Parts of an Air Conditioner
The greatest task an air conditioner does is to cool the indoor air. Air conditioners monitor and regulate the air temperature using a thermostat. They also come with an inbuilt filter that screens airborne particles from the circulating air. Air conditioners behave like as dehumidifiers. Temperature is the main concern for relative humidity, scaling down the temperature of a volume of humid air makes it to release a part of its moisture. That is why there are drains and moisture collecting pans near or fitted to air conditioners, and why air conditioners discharge water when they work on very humid days.

The major parts of an air conditioner manage refrigerant and move air in two directions: indoors and outside:

1.Evaporator : collects the liquid refrigerant.
2.Condenser : where heat transfer takes place.
3.Expansion valve : controls flow of refrigerant into the evaporator.
4.Compressor : pump that compresses and pressurizes the refrigerant.

The chilled side of an air conditioner houses the evaporator and a fan that blows air over the chilled coils and into the room. The hot side houses the compressor, condenser and an additional fan to funnel hot air from the compressed refrigerant to the outdoors. In between the two sets of coils, is an expansion valve. It controls the amount of compressed liquid refrigerant flowing into the evaporator. In the evaporator, the refrigerant undergoes a pressure drop, expands and transforms back into gas. The compressor is a huge electric pump that pressurizes the refrigerant gas as part of the process of transforming it back into liquid. There are other additional sensors, timers and valves, but the evaporator, compressor, condenser and expansion valve are the main functional components of an air conditioner.

Even though this is a conventional setup for an air conditioner, there are several other variations. Window air conditioners come with all these components fitted into a relatively small metal container that can be installed in a window opening. The hot air discharges from the rear of the unit, while the condenser coils and a fan cool and re-circulate air indoor. Bigger air conditioners operate differently: Central air conditioners have the following components: a control thermostat with a home's heating system,the compressor and condenser-the hot side of the unit. In very large buildings, like hotels and hospitals, the exterior condensing unit is usually mounted on the roof.

Window and Split-system AC Units

A window air conditioner unit is a complete air conditioner within a confined space. The units are usually small enough to fit into a standard window frame. You close the window down on the unit, plug it in and switch it on to have cool air. If you dismantle a window unit, you will see the following components:

1.A compressor.
2.An expansion valve.
3.A hot coil (on the exterior).
4.A chilled coil (on the interior).
5.Two fans.
6. A control unit.

The fans blow air over the coils to improve their ability to evacuate heat to the outside and cold into the room.When you peep into larger air-conditioning units, its time to begin viewing split-system units. A split-system air conditioner separates the hot side from the cold side of the system.

The cold side, consist of an expansion valve and the cold coil, is normally placed into a furnace or any other air handler. The air handler blows air through the coil and circulates the air throughout the building via a series of ducts. The hot side, known as the condensing unit is located outside the building.The unit consists of a long, spiral coil shaped like a cylinder. Within the coil is a fan, to circulate air through the coil, together with a weather resistant compressor and few control logic. This approach evolved over the years because of it minimal cost, and also because it usually leads to lower noise within the house.Besides the fact that the hot and cold sides are separated and the capacity is higher,making the coils and compressor larger, there is no  major difference between a split-system and a window unit.In warehouses, large business offices, malls, big department stores and other enormous buildings, the condensing unit normally resides on the roof and be enormous in size. Alternatively, there can be several smaller units on the roof, each connected to a small air handler that cools a particular area in the building.

In bigger buildings,especially in multi-story buildings, the split-system approach starts to run into problems. Either channeling the pipe between the condenser and the air handler exceeds distance proportions runs that are too long begin to cause lubrication problems within the compressor or the quantity of duct work and the length of ducts becomes unmanageable.

Chilled-water and Cooling-tower AC Units

Even though standard air conditioners are very common,they use huge of energy and generate a lot of heat. For big installations like office buildings, air handling and conditioning is at times monitored differently.

Some systems use water as part of the cooling process. The two well-known types are chilled water systems and cooling tower air conditioners.

1.Chilled water systems :In a chilled-water system, the whole air conditioner is installed on the roof or behind the building. It cools water to about 40 to 45 degrees Fahrenheit (4.4 and 7.2 degrees Celsius).The chilled water is then pumped in pipes through the building and linked to air handlers.This can be a flexible system where the water pipes act as the evaporator coils of a standard air conditioner. If properly insulated, there is no practical distance limit to the length of the cold water pipe.
2.Cooling tower system : In all of the air conditioning systems explained earlier, air is used to evacuate heat emanating from the compressor coils. In some bigger systems, a cooling tower is used in its place. The tower generates a stream of cold water running through a heat exchanger, cooling the hot condenser coils. The tower circulates air through a stream of water making some quantity of it to evaporate, and the evaporation cools the water stream. The main draw back of this system is that water has to be continuously added intermittently to make up for liquid lost through evaporation. The level of cooling an air conditioning system can obtain from a cooling tower depends on the level of relative humidity of the air and the barometric pressure.

Due to increasing electrical power costs and environmental concerns, other means of cooling air are continuously being explored. One of them is the off-peak or ice-cooling technology. An off-peak cooling system utilizes frozen ice during the evening hours to cool interior air during the hottest part of the day. Even though the system uses energy, the biggest energy drain is when community demand for power is at its lowest. Energy is less expensive during off-peak hours, and the lowered consumption during peak times reduces the demand on the power grid.

Another means is geo-thermal heating. At about 6 feet (1.8 meters) below ground, the earth's temperature varies from 45 to 75 degrees Fahrenheit (7.2 to 23.8 degrees Celsius). The concept of geo-thermal cooling is to use this steady temperature as a heat/cold source instead of using electricity to generate heat/cold. The commonly used type of geo-thermal unit for the home is known as the closed-loop system. Polyethylene pipes containing mixture of liquids is buried below ground. During winter, the fluid absorbs heat from the earth and conveys it via the system,into the building. During summer, the system reverses itself ,cooling the building by evacuating heat via the pipes to move it below ground.

For energy efficiency, solar powered air conditioners are also being used. There are still hurdles to cross, but nearly 5 percent of all electricity consumed in the U.S. is used to power air conditioning of one type or another, so there is an enormous market for energy-friendly air conditioning options.

BTU and EER

Nearly all air conditioners have their capacity rated in British thermal units (Btu). A Btu is the quantity of heat needed to increase the temperature of 1 pound (0.45 kilograms) of water through one degree Fahrenheit (0.56 degrees Celsius). One Btu is equivalent to 1,055 joules. In heating and cooling terminology, one ton is equivalent to 12,000 Btu.

A common window air conditioner unit may be rated as 10,000 Btu.A common 2,000-square-foot (185.8 square meters) house may have a 5-ton (60,000-Btu) air conditioning system, meaning that you may need about 30 Btu per square foot. These are rough estimates. To choose an air conditioner precisely for your particular use, you need to get in touch with an HVAC contractor.

The energy efficiency rating (EER) of an air conditioner is its Btu rating over its wattage. As an example, if a 10,000-Btu air conditioner consumes 1,200 watts, its EER is 8.3 (10,000 Btu/1,200 watts). Clearly, you would want the EER to be very high, but a higher EER is followed by a higher cost.