There are many ways for standard air conditioning components to be arranged in order for the equipment to provide heating or cooling to a building.  All arrangements essentially come down to two primary methods;  Direct Expansion systems or Central Air Conditioning systems.

DX stands for direct expansion.  In Dx units, air is directly cooled in, at the boundary of, or directly adjacent to the room being conditioned.  Heat is directly exchanged with refrigerant passing through a local cooling coil.  Refrigerant expands during the process and refrigerant is controlled by an expansion valve.  Hence the term “direct expansion”.

Direct expansion is the most commonly used type of air conditioning.  Dx units include room air conditioners, split systems, ducted systems and package type units.

DX type air conditioners are typically used for small buildings, single-level complexes, and homes for many reasons.  Firstly, there is a practical limitation to the length of refrigeration pipework, even if it is well insulated.  Therefore, direct expansion cooling may not be suitable for large buildings or complexes.

Secondly, DX units are most suitable for air conditioning single thermal zones.  A thermal zone is a single space, or number of spaces, within a building or complex where the HVAC requirements are sufficiently similar.  In these zones, conditions (such as temperature and humidity) can be maintained using a single sensor (such as a temperature sensor or thermostat).

How do DX systems work?

As mentioned, DX cooling is a result of air being cooled directly by refrigerant in or close to the area being conditioned.  The figure below is a diagram showing the typical DX refrigeration cycle:

At the condenser, heat is removed and rejected (released) into the outside air.  Once the heat is removed, the refrigerant turns from a gas into a high-pressure liquid.  Heat is removed by air (condenser fan) or water (cooling tower).

Interestingly, the compressor in a DX system may use pistons (reciprocating) or rollers (rotary screw).  It can be air cooled (semi-hermetically sealed compressor) or water cooled (open type compressor).  An open compressor may further vary by being driven by a motor shaft (direct drive) or pulley set up (belt drive).

As shown in the diagram above, refrigerant leaves the condenser in a high-pressure liquid form.  Refrigerant reaches a thermostatic expansion (TX) or electronic expansion (EE) valve at the fan coil unit (FCU) which houses the evaporator heat exchange coil.

The expansion valve controls the flow of refrigerant into the FCU’s evaporator coil.  Refrigerant enters the coil in a low pressure low temperature state as shown above.

Hot air is extracted from the conditioned room and blown over the heat exchange coil by the evaporator/indoor fan.  Refrigerant in the coil absorbs the heat and expands.  This lowers the temperature of air being blown into the room.

The result of this process?  Cool air is blown into the room and the temperature of the room decreases. 

The figure below is a schematic diagram of a typical air-cooled DX air conditioning system: