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Dr-Fix-It ! General Air Conditioning Definitions With links to More Specific Terminology, Suppliers as well as Maintenance and Repair Information. A Comprehensive Resource for Owners, Managers, Builders, and Operators of Commercial and Residential Buildings Air Conditioning.
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In general, there are four separate methods in common use to supply conditioned air to a building.

The first is the Direct Expansion Method (DX).

A refrigerant is circulated from a Compressor as hot gas, through a Condensing Unit where it becomes liquid, to an Evaporator Unit where it is allowed to 'flash' into a gas and then return to the compressor. A fan blows air across the cold coil to cool the air. This is the method that is most commonly brought to mind when referring to Air Conditioning.

A Central DX "Split-System" Air Conditioner typically consists of a Condensing unit matched to an evaporative unit in either a fan-coil or a furnace-coil unit. A Central DX "Package" Air Conditioner typically has all of the components housed in a single unit.  Smaller self-contained DX units are the familiar window and wall units. Commercial buildings such as motels and hotels utilize the Package Terminal Air Conditioner (PTAC) extensively.

Comparing Efficiency  Comparing the energy efficiency of different heating and cooling equipment brands is relatively easy. Although the rating systems are standardized - allowing one to make a fair comparison within the rating system - there are three different rating systems used. Each rating is used for a specific product type (in other words, all furnaces use the same system, heat pumps use a different system, and so on). They are: AFUE (gas heating) SEER (cooling) HSPF (heat pump heating). The second is the Hydronic Method.
Water is cooled by a chiller and is pumped throughout the building. The chilled water is piped to flow through coils in various fan-coil units and returned to the chiller to be cooled again. Again, a fan blows air across the cold coil to cool the air.

Large AC compressor rack

No matter, if the Coil is DX or Hydronic, an air conditioning unit containing only a fan and a coil, is simply called a Fan-Coil Unit. Alternate names for the same unit are Air Handling Unit (AHU), and Air Handler. Fan-coil units on roofs are frequently labeled Roof-Top Units (RTU).

The Third is the Evaporative Method.
Water is sprayed or allowed to fall into an enclosure through which a fan is also directing an airflow. The in-rush of air causes some of the water to evaporate which has a cooling effect. Cooling Towers utilize the Evaporative Method to cool water for use as a cooling medium for chillers or water-source air conditioning. Smaller evaporative coolers sometimes called "Swamp Coolers" use the Evaporative Method to cool the air which is directed into the occupied space.

The Fourth is the Absorption Method.
A source of heat is used to liberate the refrigerant (such as ammonia or lithium bromide) from a water solution. Absorption Systems were once popular for air conditioning applications. Today, they are more frequently found in industrial cooling applications.

HVAC Equations, Data, and Rules of Thumb A fat handbook for designing, installing, repairing, or working around heating, ventilation, and cooling systems. A veteran designer himself, author A.A.Bell Jr. assembles the most frequently used design data and "rules of thumb".

www.ari.org  Air Conditioning and Refrigeration Institute

www.ashrae.org American Society of Heating, Refrigeration & Air Conditioning Engineers.

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