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  • Writer's pictureAnne Lee

What is a Direct Expansion (DX) System?

Updated: Jun 13, 2023

DX system

Due to its capacity to do away with the majority of ductwork and piping, the direct expansion system in HVAC has been expanding quickly.

This system is popular since the installation process has been simplified, lowering the overall system cost.

Standard air conditioning components can be configured in a variety of ways so that the system can heat or cool a structure.

All arrangements can be broken down into one of two categories: central air conditioning systems or direct expansion systems.

Direct expansion is referred to as DX. Air is directly chilled in, at the edge of, or immediately next to the room being conditioned in DX units.

Direct heat exchange occurs as refrigerant travels through a nearby cooling coil. An expansion valve controls the refrigerant, which expands during the process. The name "direct expansion" was born.

The most often utilised form of air conditioning is direct expansion. Room air conditioners, split systems, ducted systems, and package type units are examples of Dx units.

A conventional residential air conditioner or commercial HVAC system is commonly referred to as a Dx air conditioning system.

It's interesting to note that a variety of Dx air conditioners are available for usage in both business and household settings.

For a variety of reasons, DX type air conditioners are frequently used in tiny structures, one-story buildings, and private residences.

The length of refrigeration piping has a practical limit, even if it is adequately insulated. As a result, big structures or complexes might not be appropriate for direct expansion cooling.

Secondly, single thermal zone air conditioning is where DX units excel. An area of a building or complex where the HVAC needs are sufficiently similar is referred to as a thermal zone.

A single sensor, such as a temperature sensor or thermostat, can be used to maintain conditions (such as temperature and humidity) in these zones.

Examples of various thermal zones are as follows:

  • In a business, the room that houses electronic office equipment like printers, copiers, fax machines (yes, they are still in use), etc. will have a higher heat load than the other areas of the office.

  • A separate variable controller will be needed for a meeting room that might hold 30 people for a weekly meeting but only a few individuals at random the rest of the week.

  • Different interior conditions (such as temperature, pressure, filtration, etc.) are needed in different areas of a hospital, forming separate zones. Examples include the operating room, testing lab, isolation room, etc.

  • A restaurant will have various zones because the eating room for customers will require more mild circumstances (temperature, humidity, etc.) than the kitchen or storage facilities.

In essence, every area that needs distinct conditions in terms of temperature, humidity, pressure, filtration, and/or is prone to significant fluctuations in heat loads should be treated as a separate zone.

To guarantee that the HVAC system can sufficiently function and deliver the desired comfort, each thermal zone should be managed separately. This is the reason why each of these zones will need a separate DX air conditioning system.

Benefits of Direct Expansion System

  • Low cost of installation.

  • The system is simple to test, modify, and balance.

  • Minimum wall or ceiling space is required.

  • Minimal energy usage.

  • Low cost of upkeep.

  • Individual sections can be operated without turning on the building's complete system.

  • Comfort under a range of load scenarios.

  • Low noise (NC 35) level.

  • Effective relative humidity regulation.

Components of direct expansion cooling system

Direct Expansion (DX) System

Any DX cooling system consists of four major parts, plus a heat-absorbing substance (often referred to as the refrigerant), which controls the flow of heat energy by generating the temperature and pressure differentials necessary for DX cooling:


It is the substance that moves through the system, absorbing heat and dissipating it in various places.


It provides the energy to move the refrigerant through the system and is a load on an electrical motor


Which enhances the refrigerant's boiling by capturing heat from the environment


This allows the refrigerant to revert to a liquid condition, dissipating heat into the surrounding air

Expansion valve

It controls the pressure difference between the high and low pressure sides of the system and permits the drop in temperature and pressure required for DX cooling.

The motor-driven compressor powers the entire system, which is a closed loop. This device, which is typically a single-phase electrical motor load, provides the energy needed to operate the cooling system.

The refrigerant, which is the second most important element, is transported throughout the system by the compressor. It is a requirement that the refrigerant has the ability to boil below room temperature.

Today's refrigerants come in many different forms, each with their own special qualities, but they all have one thing in common: they all evaporate at low temperatures.

The ability of the refrigerant to change phase, or boil, at low temperatures allows it to maximise its capacity to absorb heat energy from a space.

The evaporator, which is positioned in the area to be cooled, and the condenser, which is often outside the building, are the two components that make it easier for heat to be transferred to or from the refrigerant.

The condenser is where the majority of the heat energy is lost, and for heat transfer to take place, the air around the condenser coil needs to be cooler than the refrigerant.

To ensure that there is a sufficient heat gradient between the outside air and the refrigerant, the compressor increases the temperature and pressure of the refrigerant.

The expansion valve, which controls refrigerant pressure, is located between the condenser and the evaporator.

By allowing the refrigerant to expand, we can make more room for the gas and reduce the amount of collisions between the atoms. As a result, kinetic thermal energy decreases.

temperature formula

The expansion valve causes the gas to expand into the larger volume of the intake pipe feeding the evaporator, which decreases the pressure of the gas and lowers the temperature of the refrigerant.

We can purposefully create cooler than room temperature in this way.

Refrigerants are created with a boiling point that can evaporate and gasify at room temperature. There must be a temperature difference between the room and the refrigerant for heat to move.

The colder refrigerant will absorb the heat, or thermal energy, from the air pushed across the evaporator coils if it is now flowing through them and the ambient air temperature is higher. This will have the same effect as cooling the air.

Heat can be continuously removed from the desired location and discharged outside the building by repeatedly cycling in a cooler refrigerant.

Cooling Equipment Type

Field-Assembled DX System

This kind is used for DX systems that are assembled on the construction site from individually manufactured components rather than being produced (and rated) as a single package.

Direct expansion cooling is known as DX. A refrigerant coil is immediately positioned in the supply air stream of DX cooling equipment. The refrigerant absorbs energy as it expands and evaporates, bringing the supply air stream's temperature down.

A hydronic coil is a network of tubing that is put in a supply air stream and through which hot or cold water flows. This process heats or cools the supply air stream, which then heats or cools the room. Different heating and cooling system layouts use hydronic coils, central furnaces, and DX coils.

Packaged Terminal DX Air Conditioner

A packaged terminal DX air conditioner (PTAC) is a self-contained air conditioning unit that is commonly mounted through a wall. Without the need for ducts, it releases cool air into the room directly.

In addition to occasionally having hydronic or steam heating coils, PTACs frequently have electric resistance heating components.

Rooftop Packaged DX Unit

A rooftop packaged DX unit is often referred to as a unitary single-packaged air conditioner. The self-contained DX air conditioner in question is often mounted on a building's roof and used to circulate cool air into the conditioned area.

It has a constant-volume fan or a variable-volume fan and can be utilised in single-zone or multiple-zone applications. These devices frequently come together with a central boiler, hydronic coils or steam coils within the same assembly.

An air economizer may frequently be included with rooftop packaged DX units, depending on the cooling capability and climate.

Vertical Single-package Air Conditioner

A vertical single-package air conditioner that is pre-assembled in the factory and offers cooling, dehumidification, and temperature control. The device is designed to be mounted externally on, next to, or through an outside wall.

The main parts of the unit are oriented vertically and may have ductwork among other ventilation methods.

split DX system

Split DX System

A split DX system is also referred to as a split system or a unitary split system air conditioner. It is made up of two prefabricated assemblies: an interior DX coil with an integrated supply fan and a condensing unit that utilises outside air as a heat sink.

The indoor unit frequently sits beside a central boiler, hydronic coils or steam coils in the same assembly. It is challenging to add an air economizer to a split system since the indoor unit is typically placed far from exterior walls.

In A Nutshell

A crucial part of the central air conditioning system are direct expansion (DX) systems. There are three primary categories of DX systems: split, packaged, and VRF systems, each with a particular set of benefits and drawbacks.

Anyone planning to install a central air conditioning system in their house or place of business must have a thorough understanding of the several types of DX systems.

You may decide which kind of DX system is appropriate for your particular needs by taking into account aspects like cost, available space, and energy efficiency.

No matter which DX system you select, you can be sure that it will keep you cool during the hot summer months and give you a comfortable indoor environment.

As an OEM HVAC manufacturer, TCW Group creates chilled water systems and DX systems that are appropriate for commercial and industrial HVAC systems. The corrosive, maritime, and hazardous environments were included.

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