Air Conditioner Facts

Have you ever wondered how an air conditioner works? A common misconception is that an air conditioner lowers the temperature of a room simply by pumping cooler air in. Warm air is actually removed from your home or office and cycled back as cooler air. A thermostat will signal the end of this cycle once a desired temperature is reached.

An air conditioner can be likened to a refrigerator, but without the insulated box. Just like in a refrigerator, cooling is provided by the evaporation of a refrigerant, like Freon. The term Freon® refers to “any of various nonflammable fluorocarbons used as refrigerants and as propellants for aerosols”, according to the Merriam-Webster Dictionary Online.

Diagram of a typical air conditioner.

• Cool Freon gas is compressed by means of a compressor, causing it to become hot, high pressure Freon gas (seen in red in the diagram above)
• Its heat is dissipated by running the hot gas through a set of coils, condensing it into a liquid
• An expansion valve causes the Freon liquid to evaporate and become cold low-pressure Freon gas (seen in light blue in the diagram above)
• When the cold gas runs through another set of coils, heat is absorbed by the gas, cooling down the inside of the building
• A small amount of lightweight oil is mixed in with the Freon to lubricate the compressor

Popular indoor air con units have filters to catch dust, pollen, mould spores and other allergens as well as smoke and everyday pollution in the air, thereby cleaning the air you breathe. Most air conditioners also act as dehumidifiers by taking excess water from the air and using it to cool the unit. This water is then either disposed off via a hose to the outside, or routed back into the system to be reused efficiently.

Various different air conditioner models and types are available to suit your budget and needs.

Window and Split-system AC Units
Window air conditioner units house a complete air conditioner in a limited space, typically fitting into a standard window frame. Inside the unit you will find the following components:

• Two fans
• A compressor
• An expansion valve
• A hot coil (on the outside)
• A chilled coil (on the inside)
• A control unit

When the fans blow air over the coils, warm air is dissipated to the outside and cold air is blown into the room being cooled.

Split system units are used for larger air-conditioning applications. As seen below, there is a split between the hot side and the cold side of these units.

The hot side, referred to as the condensing unit is situated outside the building. Inside the condensing unit you will find a long, spiral coil, shaped like a cylinder, surrounding a fan used to blow air through the coil. Apart from the coil and fan, you will also see a weather resistant compressor and control logic. This system is favoured because of its relatively low cost and low noise level inside the building. Compared to a window air conditioner, the split system air conditioner has separate hot and cold sides and the capacity is much higher (with bigger coils and compressor).

The condensing unit, which can be quite massive, is normally placed on the roof of warehouses, shopping centers and large offices. Another option is to have several smaller units on the roof, each attached to small air handlers inside the building covering specific zones.

The split system approach is not effective for larger buildings and multi-story buildings, as lubrication becomes impossible when the pipe between the condenser and the air handler is too long. The length and amount of ducts also become unmanageable. A chilled-water system is the answer for this scenario.

Chilled-water and Cooling-tower AC Units
The entire air conditioner in a chilled-water system resides behind a building or on the roof. Water is cooled to between 4,4°C and 7,2°C and then piped throughout the building to air handlers. Well-insulated chilled-water pipes have no practical limit as far as the length of pipe is concerned.

As can be seen on the diagram, the air conditioner on the left is completely standard. Cold Freon chills the water in the heat exchanger, before the cold water runs through to various air handlers inside the building.

All of the systems explained above have one thing in common; heat is dissipated from the outside coil using air. Cooling towers are used for significantly improved efficiency in large systems. Inside the cooling tower a stream of lower temperature water is created which then runs through a heat exchanger, cooling the hot coils of the air conditioner unit. The initial cost to buy this system is considerably higher, but the system pays for itself fairly quickly thanks to significant energy savings (especially in low humidity areas).

There are numerous different shape and size cooling towers, but all operate on the same principle:

• Air is blown through a stream of water in a cooling tower causing some of the water to evaporate
• Normally, the water trickles through a thick sheet of open plastic mesh with air blowing at right angles to the water flow
• The stream of water is cooled by the resulting evaporation
• Water is constantly added to the cooling tower to replace water lost to evaporation

The degree of cooling obtainable from a cooling tower is dependant on the barometric pressure and relative humidity of the air.

The temperature of the water in a cooling tower will be about 3,36 degrees lower than the outside temperature when the humidity is at 80% with a barometric pressure of 29,92 inches (normal sea-level pressure). So on a typical 35°C day at the coast under these conditions, the water in the cooling tower will be about 31,7°C. Should the humidity drop to 50%, the water temperature could drop by 8,4 degrees resulting in a water temperature inside the cooling tower of 26,7°C. A humidity of 20% would cause the water temperature to drop 15,7 degrees, so the cooled water will be about 19,4°C. Similarly, small drops in temperature have a positive effect on energy consumption.

Cooling towers often found behind buildings consist of sheets of plastic mesh with copious amounts of water running through it.

Cooling towers and air conditioning equipment are often centralized in office complexes and campuses with chilled water routed to all of the buildings by means of underground pipes.

BTU and EER
Air conditioner capacity is usually rated in British Thermal Units (BTU). A BTU is equal to 1 055 joules - the amount of energy needed to raise the temperature of one pound (0,45kg) of water by 1 degree Fahrenheit (0,56°C). In heating a cooling terms, 12 000 BTU equals 1 ton. For help in calculating which air conditioner you need, please contact us.

The Energy Efficiency Rating (EER) of an air conditioner refers to its BTU rating in relation to its wattage. For instance, if a 10 000 BTU air conditioner operates on 1 200 watts, its EER is 8,3 (10 000 BTU/1 200 watts). You want your EER to be as high as possible, but bear in mind that the higher the EER, the higher the price.

Let us compare two 10 000 BTU air conditioners, one consumes 1 200 watts and has an EER of 8,3, and the other consumes 1 000 watts and has an EER of 10. The one unit is roughly R 1 000,00 more expensive than the other. Which unit would you choose? In order to calculate the payback period on the more expensive unit, you need to estimate how many hours per year the unit will be in use and find out how much you pay, on average per kilowatt hour (kWh).

In the long South African summers air conditioners can typically be used for seven months per year and at least six hours daily. The cost for electricity in South Africa is more or less 42c per kWh. The 200 watts higher electricity consumption of the one unit means that every five hours the cheaper unit will consume 1 additional kWh (or 42c) more than the more expensive unit.

In total over seven months of average 30 days each:

7 months x 30 days/month x 6 hours/day = 1 260 hours
[(1260 hours x 200 watts) / (1000 watts/kW)] x R0,42/kWh = R105,84

Under these operating conditions it will take about nine and a half years for the unit costing R 1 000,00 more, to break even.