Read on to learn the basics of choosing DX units for your environmental control. The immediate and most noteworthy difference between these two systems is that the DX units cool air using refrigerant and CHW units cool air utilizing chilled water. A DX unit uses refrigerant-based cooling and cools indoor air using a condensed refrigerant liquid. Direct Expansion means that the refrigerant expands to produce the cooling effect in a coil that is in direct contact with the conditioned air that will be delivered to the space.
The DX unit uses a refrigerant vapor expansion and compression cycle to cool air coming in through a supply plenum and returns it to the area that needs cooling through the return. This central air conditioning system comes in either a split-system or a packaged unit. In a split system the components are separated with the evaporator coil located in an indoor cabinet and condenser coil located in an outdoor cabinet.
The compressor can either be in the indoor or outdoor cabinet. A packaged unit has the entire cooling system self-contained in one unit, with the evaporator coil, condenser, and compressor all located in one cabinet. This allows for flexibility in the installation since the unit can be either outside or indoors depending on system specifications without too large of a footprint. In the direct expansion or DX types of air central conditioning plants the air used for cooling space is directly chilled by the refrigerant in the cooling coil of the air handling unit.
Since the air is cooled directly by the refrigerant the cooling efficiency of the DX plants is higher. However, it is not always feasible to carry the refrigerant piping to the large distances hence, direct expansion or the DX type of central air conditioning system is usually used for cooling the small buildings or the rooms on the single floor. There are three main compartments of the DX type of central conditioning systems please refer the fig below :.
The plant room comprises of the important parts of the refrigeration system, the compressor and the condenser. The compressor can be either semi-hermetically sealed or open type. The semi-hermetically sealed compressors are cooled by the air, which is blown by the fan, while open type compressor is water cooled. The open compressor can be driven directly by motor shaft by coupling or by the belt via pulley arrangement. The condenser is of shell and tube type and is cooled by the water.
The refrigerant flows along the tube side of the condenser and water along the shell side, which enables faster cooling of the refrigerant. The water used for cooling the compressor and the condenser is cooled in the cooling tower kept at the top of the plant room, though it can be kept at other convenient location also.
The refrigerant leaving the condenser in the plant room enters the thermostatic expansion valve and then the air handling unit, which is kept in the separate room. The air handling unit is a large box type of unit that comprises of the evaporator or the cooling coil, air filter and the large blower. 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.
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. 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. Each thermal zone should be controlled separately to ensure the HVAC system can adequately perform and provide intended comfort.
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.
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