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Direct Expansion (DX) Air Conditioning – Frequently Asked Questions

When talking about at air conditioning, there are a lot of technical terms thrown around.  “DX units”, “direct expansion”, “dx split system” and “dx cooling” are just some examples of terms we here from time-to-time from our AC Technicians.  People in-the-know about heating, ventilation and air conditioning (HVAC) may also use these terms.

Customers will occasionally mention DX air conditioning, or ask a question about a DX cooling etc.  Sometimes the terminology is used correctly, sometimes it is used incorrectly.  While it is not essential to know exactly what air conditioning is, or know all the right words to use, it certainly helps when having a conversation with an HVAC professional.

While a good air conditioning contractor should explain things in a logical way, a little knowledge really does go a long way.  Some basic knowledge allows for a comprehensive conversation with an aircon technician and ensures you truly understand what is being said.  It also helps the decision-making process as you have more information on board if/when looking to buy an air conditioner.

In this article, we will help answer some frequently asked questions;

We’ll help provide answers to these key questions, and more, in order to help your understanding on the topic in order to know exactly what DX air conditioning is.

What is a DX unit for air conditioning?

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.

A Dx air conditioning system is often just a technical term for a standard home air conditioner or commercial HVAC system.  Interestingly, there is a wide range of Dx air conditioners that can be used for both commercial or residential application.

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.

Direct Expansion DX Air Conditioning planning for multiple thermal zones

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).

Examples of different thermal zones include:

  • The room containing electronic office equipment such as printers, copiers, fax machines (yes, they are still around), etc. in a business will have a larger heat load than the rest of the office.
  • A meeting room that may have 30 people for a weekly meeting, but only a few people randomly throughout the rest of the week will require a separate variable controller for this space.
  • Various rooms in a hospital, including operation theatre, testing lab, isolation room, etc. will require different indoor conditions (including temperature, pressure, filtration, etc) compared to other spaces, thus constituting separate zones.
  • A customer dining area will call for more moderate conditions (temperature, humidity, etc.) than the kitchen or storage areas, therefore a restaurant will have different zones.

Essentially any space that requires different conditions in terms of temperature, humidity, pressure, filtration and/or is subject to large variations in heat loads should be considered a separate zone.

Each thermal zone should be controlled separately to ensure the HVAC system can adequately perform and provide intended comfort.  Hence why each of these zones will require it’s own dedicated DX air conditioning system.

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:

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.  Mission accomplished!

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

DX air conditioning system diagram

What are the different types of DX air conditioning?

The most common types of DX air conditioning are unitary air conditioning systems; factory assembled, pre-made, self-contained units.  Each DX system consists of refrigeration units with fans, filters and controls.

DX AC systems come in a wide range of types and capacity.   Depending on specific requirements, DX units are available in the form of room air conditioners, split systems, multi-head units, cassette systems, heat pumps, ducted systems and packaged units, with additional air-cooled or water-cooled condensing options.

Window/Room Air Conditioner (RAC)

Dx room air conditioners (RAC’s) provide heating or cooling to an individual room rather than a whole building.  RAC’s are cost-effective AC units that provide hot or cold air only when/where required.

Window box unit installed

These box units are ductless and installed in the framed or unframed external opening in a building (window or wall) without any duct work.  They consist of the DX air conditioning basics; evaporator, condenser, expansion valve and refrigerant.

Boxed room air conditioners are basic DX units with limited control and capacity.  They are an entry-level option for providing conditioned air to a single small room or space.  They are typically found in older style homes and rental properties.

Split Systems

A split system is the most common form of ductless DX air conditioning.  They are also a cost-effective, energy-efficient, and versatile option as a home or commercial air conditioner.

DX unit wall split system

Split air conditioners also contain all of the DX system basics, and more, split over two unit.  The condensing unit outside contains the condenser coil, compressor, condenser fan motor, electrical printed circuit (PC) boards and capacitor/s.  The evaporator fan coil unit inside contains the evaporator coil, expansion valve, evaporator fan motor, electrical PC board/s and filters.  The 2 units are connected by pipework and interconnect cable.

Dx split systems are very flexible in terms of indoor unit location, outdoor unit placement and direction and length of pipe run.  Furthermore, the indoor unit may be a wall-mounted, floor-sitting, ceiling-hung, or bulkhead-hidden unit.  These options are all customisable, which makes the split system an excellent DX aircon option for most homes and businesses, especially given the array of layouts that exist.

Heat Pumps

DX systems operating as reverse cycle air conditioning are often known as heat pumps.  Heat pumps provide both heating and cooling from the same device.  This is a great option for anyone wanting hot and cold air from just one system.

A heat pump allows hot or cold air to be produced by the inclusion of a 4-way reversing valve.  Heat flow in the refrigeration cycle can be reversed so that heat is extracted from the outside air and transferred (“rejected”) into the building.

It is worth noting that a Dx heat pump system is more energy efficiency in heating than cooling due to the added heat of compression during the heating cycle.  However, this would just be one of many factors influencing a decision to purchase a Dx dual heating and cooling system.

Ducted Systems

Ducted aircon is a popular form of Dx air conditioning system used for residential and commercial application.  Ducted Dx units are similar to split Dx units, except they are typically larger, serve multiple spaces and there is ductwork with outlets/vents involved.

Ducted Air Conditioning

Ducted Dx units contain all the basic functional elements of a Dx cooling and/or heating system.  Ducted Dx outdoor units are fairly consistent in design, apart from variation in air-blow direction (i.e. top-blow or side-blow).  Ducted Dx indoor units have greater variation, which can be useful when customising an HVAC system for a building in terms of space, aesthetics and design.

Ductwork that channels air from the indoor unit to the room/s or spaces can be hidden behind the ceiling or exposed to create an industrial look.  Outlets/vents that connect to the ductwork come in a wide range of shapes, colours and designs, adding to the degree of customisation available with ducted Dx air conditioning.

Packaged Units

Package units could be viewed as a combination of a box room air conditioner and a ducted air conditioning system; all components are housed within one main unit, and ductwork transports air to the relevant indoor space/s.

Daikin Packaged Unit

Packaged units are designed to serve large areas.  They are mostly used for commercial application, and typically sit up on the roof.  Hence the term “rooftop packaged units”.

While package units can come in a range of forms and set up, Dx package units follow the basic principles of a DX air conditioning system; air is heated or cooled by heat transfer directly with refrigerant in the system.  

Air-Cooled and Water-Cooled Systems

Larger packaged and ducted DX system can be either air-cooled (condenser fan) or water-cooled (cooling tower).  This refers to the heat rejection method used by the condenser.

This option adds to the multitude of Dx HVAC system set ups available.  It also allows for greater customisation of DX systems for any residential or commercial complex.

Air Cooled Condenser

For air-cooled refrigeration systems, heat is expelled into the outside air by the condenser via air cooling.  Refrigerant is cooled by air being blown over the condenser coil/s by a fan, causing it to be rejected into the atmosphere.  Most DX systems use air-cooled finned tube condensers to remove heat from refrigerant in this manner.

Water Cooled Condenser

For water-cooled refrigeration systems, heat is expelled by the condenser using water-cooling.  Refrigerant is cooled down by water passing over the condenser coil/s.  That water is then circulated through a cooling tower to cool it down.  The water re-circulated and the process repeats.  The water-cooled systems use shell and tube type condensers, as opposed to the usual finned type units.

Cooling Tower

No matter which DX air conditioning system you have, or which you are looking to purchase, there are pros and cons for each one.  Hopefully this article has provided answers to some of the questions you may have about DX system.  For any specific questions or guidance, please don’t hesitate to contact us.