The Science

Air conditioning includes the cooling and heating of air. It also cleans the air and controls the moisture level. An air conditioner is able to cool a building because it removes heat from the indoor air and transfers it outdoors. A chemical refrigerant in the system absorbs the unwanted heat and pumps it through a system of piping to the outside coil. The fan, located in the outside unit, blows outside air over the hot coil, transferring heat from the refrigerant to the outdoor air.

Components of the system

Most air conditioning systems have five mechanical components:

– A compressor
– A condenser coil and fan
– A metering device or an expansion valve
– An evaporator coil and blower
– A chemical refrigerant

How it works

Most central air conditioning units operate by means of a split system. That is, they consist of a ‘hot’ side, or the condensing unit—including the condensing coil, the compressor and the fan—which is situated outside your home, and a ‘cold’ side that is located inside your home. The cold side consists of an expansion valve and a cold coil, and it is usually part of your furnace or some type of air handler. The furnace blows air through an evaporator coil, which cools the air. Then this cool air is routed throughout your home by means of a series of air ducts. A window unit operates on the same principal, the only difference being that both the hot side and the cold side are located within the same housing unit.

The compressor (which is controlled by the thermostat) is the ‘heart’ of the system. The compressor acts as the pump, causing the refrigerant to flow through the system. Its job is to draw in a low-pressure, low-temperature, refrigerant in a gaseous state and by compressing this gas, raise the pressure and temperature of the refrigerant. This high-pressure, high-temperature gas then flows to the condenser coil.

The condenser coil is a series of piping with a fan that draws outside air across the coil. As the refrigerant passes through the condenser coil and the cooler outside air passes across the coil, the air absorbs heat from the refrigerant which causes the refrigerant to condense from a gas to a liquid state. The high-pressure, high-temperature liquid then reaches the expansion valve.

The expansion valve is the ‘brain’ of the system. By sensing the temperature of the evaporator, or cooling coil, it allows liquid to pass through a very small orifice, which causes the refrigerant to expand to a low-pressure, low-temperature gas. This ‘cold’ refrigerant flows to the evaporator.

The evaporator coil is a network of piping connected to a furnace or air handler that blows indoor air across it, causing the coil to absorb heat from the air. The cooled air is then delivered to the house through ducting. The refrigerant then flows back to the compressor where the cycle starts over again.

Glossary of terms

Ambient Temperature – The temperature, usually of the air, that surrounds operating equipment.

Compressor – A machine used to compress gasses. The function of the compressor is to maintain a pressure difference between the high and low sides of the system. It is considered the heart of an air conditioning or heat pump system. The compressor is the part of the outdoor equipment that pumps the refrigerant. On the cooling side, it draws in the low-pressure refrigerant and compresses it into high-pressure on the condensing side of the cycle. The compressor maintains adequate pressure within the system to cause refrigerant to flow in sufficient quantities to meet the cooling requirements. There are four types of compressors: reciprocating, rotary, scroll, and centrifugal. Most residential systems have reciprocating or scroll compressors.

Condenser – A device that transfers unwanted heat out of a refrigeration system to a medium (either air, water, or a combination of air and water) that absorbs the heat and transfers it to a disposal point. There are three types of condensers: air-cooled condensers, water-cooled condensers, and evaporative condensers. The evaporative condenser uses a combination of air and water as its condensing medium. Most residential systems have an air-cooled condenser.

Enthalpy – Heat content or total heat, including both sensible and latent heat. The amount of heat contained in a refrigerant at any given temperature with reference to -40°F.

Evaporator – Absorbs heat from the surrounding air or liquid and moves it outside the refrigerated area by means of a refrigerant. It is also know as a cooling coil, blower coil, chilling unit or indoor coil.

HVAC – Heating, ventilation, and air conditioning.

Insulation – Any material that slows down the transfer of heat.

Kilowatt (KW) – Equal to 1,000 watts.

Latent Heat – The heat energy needed to change the state of a substance (i.e.: from a liquid to a gas) but not it’s temperature.

Sensible Heat – Heat energy that causes a rise or fall in the temperature of a gas, liquid or solid when added or removed from that material. Sensible heat changes the temperature by changing the speed at which the molecules move.

Split System – A refrigeration or air conditioning system that consists of a “hot” side, or the condensing unit-including the condensing coil, the compressor and the fan-which is situated outside your home. The hot side is connected to the “cold” side-located inside your home-via supply and return refrigerant lines. The cold side consists of an expansion valve and a cold coil, and it is usually part of the furnace or some type of air handler. The furnace blows air through an evaporator coil, which cools the air. Then this cool air is routed throughout your home by means of a series of air ducts. This type of set up is also used with heat pump installations.

Supercooled Liquid – Liquid refrigerant cooled below its saturation point.

Subcooling – Creating a drop in temperature by removing sensible heat from a refrigerant liquid.

Superheated vapor – Refrigerant vapor heated beyond its saturation point.

Superheating – Creating a rise in temperature by adding heat energy to a refrigeration vapor.