How Does Air Conditioning Work?

Contrary to popular perception, Air Conditioning (AC) is not about adding cool air to the room, but more about drawing heat away from it. The end result is a space with significantly less heat, which makes it feel cooler to occupants. Air conditioning takes advantage of the effects of evaporation, much like a swab of alcohol makes a person's skin feel cooler as the liquid evaporates. The alcohol doesn't lower the person's skin temperature, but rather draws away heat from the air as it turns to a gas.

Air conditioning units contain a special chemical called a refrigerant, which has the unique ability to change from a gas to a liquid in a short amount of time. A refrigerant called freon is commonly used in AC units, although there are other commercial refrigerants available. The refrigerant is pumped into the unit at the factory, along with a small amount of lubricating oil for the compressor.

The parts of a typical AC unit usually form a closed system consisting of a compressor, a condenser, an expansion valve and a thermostat. Motorized fans help to circulate the conditioned air, while thin metal fins allow heat to dissipate quickly. The heaviest part of a typical air conditioning is often the compressor, since it must be strong enough to withstand a significant amount of pressure.

The process of cooling an area begins with the refrigerant entering the compressor, usually located at the bottom of the unit. At this point, the refrigerant is a cool gas. As the gas enters the compressor's inner chamber, the compressor squeezes the refrigerant and the gas becomes a very hot gas under high pressure. This hot gas goes through a series of condensing coils placed outside of the room being cooled. The heat dissipates into the outside air, much like a car's radiator dissipates heat from the engine coolant. Once the refrigerant reaches the end of these coils, it is significantly cooler and in liquid form.

This liquid is still under high pressure, like the contents of an aerosol can. In the case of air conditioning, the liquid refrigerant is forced through a very tiny opening called an expansion valve. The liquid refrigerant comes out of the other end of the expansion a very small amount at a time. Because the refrigerant evaporates at a much lower temperature than water, it begins to evaporate while traveling through another set of coils. It is this evaporation action that draws heat out of the surrounding air, including the air contained in the room. The unit's fan blows across metal fins placed over these coils, causing the sensation of cooling in the room. 

At this point, the liquid refrigerant has become a cold gas again and re-enters the compressor, where the entire process begins again until a thermostat registers a specific temperature and shuts off the compressor. When the room warms up, the thermostat senses the added heat and the compressor kicks back on to create more of the hot pressurized gas. At some point, the temperature of the room may equal the cooling power of the air conditioner and the compressor will shut off again. The air conditioning systems of most houses do benefit from energy-saving steps such as using window shades and keeping doors closed, since they don't have to work as hard to keep the room at an acceptable level of cool.

Do You Know Why Air Conditioner Capacity Is Measured in Tons?

Tonnage in the HVAC profession refers to a specific amount of heat energy the air conditioner must extract from your home every hour to keep it cool and comfortable. It’s measured in BTUs (British Thermal Units) and a “ton” of cooling equals 12,000 BTUs extracted per hour. Your contractor will use both his own expertise as well as sophisticated, industry-standard software to determine your home’s BTU requirements, known as "sizing the house," then recommend an air conditioner that offers that cooling capacity.

You shouldn’t accept anything less. 

In the past, calculating the required BTU tonnage per hour with precision was often not a high priority, particularly in new construction. In an era when energy costs were much lower than today, builders frequently installed oversized A/C units as a matter of course to standardize the make and model and cover for all variables in a home. This standard procedure was so widespread that contractors who perform air conditioning upgrades of older units nowadays report that more than 50 percent of the existing installed air conditioners are larger than they need to be. Not only do these oversized units consume unnecessary amounts of energy and produce high utility costs, ironically, they also cool less effectively and often have higher rates of component failure and shorter service life, too. Undersized units aren’t a bargain, either. They won’t extract enough BTU tonnage to adequately cool the home. In addition, a unit that’s too small for the thermal characteristics of the house will run extended “On” cycles, using excessive amounts of electricity.

Getting a Handle on the Cooling Load

In the summer, your home carries a heavy burden. It’s called the cooling load and it’s the sum total of a range of factors that influence how much cooling power in BTU tonnage is required to keep your home at a comfortable indoor temperature. This comfort goal is called the “design temperature” and it’s usually around 78 degrees. To maintain that temperature, the contractor has to know the number of BTUs of heat energy that need to be taken out of the house every hour by the air conditioner.

The cooling load is influenced by factors including the:

• Local climate

• Square footage of the house

• Orientation of the home and the amount of solar exposure it receives

• Total area of windows and the type of glass

• Amount and type of insulation

• Condition of the ductwork

• Number of major appliances

• Amount of lighting and the type of bulbs

• Number and age of occupants

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What Is Cooling Capacity?

Cooling capacity refers to the rate heat is removed from a space, under standard conditions. These conditions take into account such factors as the volume of the space being cooled and the air temperatures both inside and outside the space. Typically, the cooling capacity of a modern air conditioner is printed on the outside of the unit.

Generally, cooling capacities are rated in British Thermal Units (BTUs) per hour, or in tons. A ton of cooling capacity is the same as 12,000 BTUs per hour, and the term has its origin in the cooling effect of a ton of ice. Often, capacity measurements are given in tonnage for cooling units and BTUs for heating units. Broken down, a BTU is based on the amount of energy it takes to raise the temperature of a pound of water by just one degree Fahrenheit.

Cooling capacity is directly related to the size of the area being cooled. As an illustration, a typical 2,000-square-foot home, depending on the climate it is in, may require an air conditioning system with a cooling capacity of 48,000 BTUs per hour. In contrast, an average 150-square-foot bedroom may be able to get by with a unit rated at 5,000 BTUs per hour.

Air conditioners cool a heated room by transferring the heat to the fluid-filled evaporator coils of the unit. The units condense some of the moisture in the air, too. That makes the atmosphere in the room not only cooler, but less humid and more comfortable as well.

Cooling capacity ratings are usually printed directly on the sticker of newer air conditioning units. On older models, the capacity is sometimes coded within the serial numbers. These ratings apply not only to residential and commercial air conditioning systems – including heat pumps that operate in the air conditioning mode – but also to industrial chillers, cooling towers, and other cooling equipment.

The Air-Conditioning, Heating and Refrigeration Institute (AHRI) independently certifies heating, ventilating, air conditioning, and refrigeration (HVACR) equipment and components to ensure these systems meet industry efficiency standards. Generally, the Institute – known as the Air-Conditioning and Refrigeration Institute (ARI) prior to 2009 – works with manufacturers to develop standards and guidelines that result in uniform ratings, including cooling capacity ratings, to give consumers a basis of comparison. An ARI number on a heating or air conditioning unit is a guarantee that its cooling or heating capacity has been properly tested.

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