How Air Conditioners Work

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.

Basic Operations
Most air conditioning systems have five mechanical components:

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 series 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. 
 

Below are some commonly used terms in the heating, ventilation and air conditioning industry. These may be useful when dealing with contractors or assessing a solution to your problem.

Glossary Of Terms

Air Conditioner - Assembly of equipment for the simultaneous control of air temperature, relative humidity, purity, and motion.

Air Cooled - Uses a fan to discharge heat from the condenser coil to the outdoors.

Air-Source - Air is being used as the heat source or heat sink for a heat pump.

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

BTU (British Thermal Unit) - The standard of measurement used for measuring the amount of heat required to raise the temperature of one pound of water by one degree (Fahrenheit).

BTUH - The number of BTUs in an hour.

Central Air Conditioner System -System in which air is treated at a central location and carried to and from the rooms by one or more fans and a system of ducts.

Compressor - The pump that moves the refrigerant from the indoor evaporator to the outdoor condenser and back to the evaporator again. The compressor is often called "the heart of the system" because it circulates the refrigerant through the loop.

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.

Condenser Coil - A series or network of tubes filled with refrigerant, normally located outside the home, that removes heat from the hot, gaseous refrigerant so that the refrigerant becomes liquid again.

Cooling Capacity - A measure of the ability of a unit to remove heat from an enclosed space.

COP - Coefficient of Performance of a heat pump means the ratio of the rate of useful heat output delivered by the complete heat pump unit (exclusive of supplementary heating) to the corresponding rate of energy input, in consistent units and under operating conditions.

DOE -The Department of Energy. A government agency that sets industry efficiency standards and monitors the use of various energy sources.

EER - Energy Efficiency Ratio means the ratio of the cooling capacity of the air conditioner in British Thermal Units per hour, to the total electrical input in watts under ARI-specified test conditions.

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 known as a cooling coil, blower coil, chilling unit or indoor coil.

Evaporator Coil - A series or network of tubes filled with refrigerant located inside the home that take heat and moisture out of indoor air as liquid refrigerant evaporates.

Free Delivery - There are no ducts and the unit may be installed in the field without ducts if needed.

Ground-Source - The ground or soil below the frost line is being used as the heat source or heat sink for a heat pump.

Ground Water-Source - Water from an underground well is being used as the heat source or heat sink for a heat pump.

HVAC - Heating, ventilation, and air conditioning.

Heat Pump - An air conditioner capable of heating by refrigeration. It may or may not include a capability for cooling. Outside air or water is used as a heat source or heat sink, depending upon whether the system is heating or cooling.

Heating Capacity - A measure of the ability of a unit to add heat to an enclosed space.

HSPF - Heating Seasonal Performance Factor means the total heating output of a heat pump in British Thermal Units during its normal usage period for heating divided by the total electrical energy input in watt-hours during the same period.

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

(K) Factor - The insulating value of any material. Also known as conductivity.

Kilowatt (kW) - Equal to 1,000 watts.

Kilowatt-hour (kWh) - A common unit of electrical consumption measured by the total energy created by one kilowatt in one hour.

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.

Single Package -A central air conditioner which combines both condenser and air handling capabilities in a single packaged unit.

Split System - A central air conditioner consisting of two or more major components. The system usually consists of a compressor-containing unit and condenser, installed outside the building and a non-compressor -containing air handling unit installed within the building. This is the most common type of system installed in a home.

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.

Ton -The unit of measurement for air conditioning system capacity. One ton of air conditioning removes 12,000 Btu's of heat energy per hour from a home. Central air conditioners are sized in tons. Residential units usually range from 1 to 5 tons.

Water-Source -Water is being used as the heat source or heat sink for a heat pump. Sources of underground water are wells and sources of surface water are lakes, large ponds, and rivers.

Watt - A unit of power that equals one joule per second. Named after James Watt.

Air Conditioner Filters
The most important maintenance task that will ensure the efficiency of your air conditioner is to routinely replace or clean its filters. Clogged, dirty filters block normal airflow and reduce a system's efficiency significantly. With normal airflow obstructed, air that bypasses the filter may carry dirt directly into the evaporator coil and impair the coil's heat-absorbing capacity. Filters are located somewhere along the return duct's length. Common filter locations are in walls, ceilings, furnaces, or in the air conditioner itself.

Some types of filters are reusable; others must be replaced. They are available in a variety of types and efficiencies. Clean or replace your air conditioning system's filter or filters every month or two during the cooling season. Filters may need more frequent attention if the air conditioner is in constant use, is subjected to dusty conditions, or you have fur-bearing pets in the house. If you use a disposable type filter, it's always wise to keep several spares inside the house.  

Willis Haviland Carrier --- Father of Cool
"I fish only for edible fish, and hunt only for edible game even in the laboratory." - Willis Haviland Carrier on being practical.

In 1902, only one year after Willis Haviland Carrier graduated from Cornell University with a Masters in Engineering, the first air (temperature and humidity) conditioning was in operation, making one Brooklyn printing plant owner very happy.

Fluctuations in heat and humidity in his plant had caused the dimensions of the printing paper to keep altering slightly, enough to ensure a misalignment of the colored inks.
The new air conditioning machine created a stable environment and aligned four-color printing became possible. All thanks to the new employee at the Buffalo Forge Company, who started on a salary of only $10.00 per week. The 'Apparatus for Treating Air' (U.S. Pat# 808897) granted in 1906, was the first of several patents awarded to Willis Carrier. The recognized 'father of air conditioning' is Carrier, but the term 'air conditioning' actually originated with textile engineer, Stuart H. Cramer. Cramer used the phrase 'air conditioning' in a 1906 patent claim filed for a device that added water vapor to the air in textile plants - to condition the yarn.

In 1911, Carrier disclosed his basic Rational Psychrometric Formulae to the American Society of Mechanical Engineers. The formula still stands today as the basis in all fundamental calculations for the air conditioning industry. Carrier said he received his 'flash of genius' while waiting for a train. It was a foggy night and he was going over in his mind the problem of temperature and humidity control. By the time the train arrived, Carrier had an understanding of the relationship between temperature, humidity and dew point.

Industries flourished with the new ability to control the temperature and humidity levels during and after production. Film, tobacco, processed meats, medical capsules, textiles and other products acquired significant improvements in quality with air conditioning. Willis and six other engineers formed the Carrier Engineering Corporation in 1915 with a starting capital of $35,000 (1995 sales topped $5 billion). The company was dedicated to improving air conditioning technology.

In 1921, Carrier patented the centrifugal refrigeration machine. The 'centrifugal chiller' was the first practical method of air conditioning large spaces. Previous refrigeration machines used reciprocating-compressors (piston-driven) to pump refrigerant (often toxic and flammable ammonia) throughout the system. Carrier designed a centrifugal-compressor similar to the centrifugal turning-blades of a water pump. The result was a safer and more efficient chiller.

Cooling for human comfort, rather than industrial need, began in 1924, noted by the three Carrier centrifugal chillers installed in the J.L. Hudson Department Store in Detroit, Michigan. Shoppers flocked to the 'air conditioned' store. The boom in human cooling spread from the department stores to the movie theaters, most notably the Rivoli theater in New York, whose summer film business skyrocketed when it heavily advertised the cool comfort. Demand increased for smaller units and the Carrier Company obliged.

In 1928, Carrier developed the first residential 'Weathermaker', an air conditioner for private home use. The Great Depression and then WW2 slowed the non-industrial use of air conditioning. After the war, consumer sales started to grow again. The rest is history, cool and comfortable history.

Photo:Early Home Unit