Value Engineered Solutions to Environmental Control
Environmental air handling systems required for close tolerance high efficiency cleanroom environmental control are commonly either chilled-water applied systems(CW) or direct-expansion unitary systems(DX)
A chilled-water applied system uses chilled water to transport heat energy between the airside, chillers and the outdoors. These systems are more commonly found in large HVAC installations, given their cooling efficiency advantages.
The components of the chiller (evaporator, compressor, an air- or water-cooled condenser, and expansion device) are often manufactured, assembled, and tested as a complete package within the factory. These packaged systems can reduce field labor, speed installation and improve reliability.
Alternatively, the components of the refrigeration loop may be selected separately. While water-cooled chillers are rarely installed as separate components, some air cooled chillers offer the flexibility of separating the components for installation in different locations. This allows the system design engineer to position the components where they best serve the space, acoustic, and maintenance requirements of the building owner.
Another benefit of a chilled-water applied system is refrigerant containment. Having the refrigeration equipment installed in a central location minimizes the potential for refrigerant leaks, simplifies refrigerant handling practices, and typically makes it easier to contain a leak if one does occur.
Factors affecting the decision to select Direct Expansion Unitary or Chilled Water Applied systems include:
In a direct-expansion (DX) unitary system, the evaporator is in direct contact with the air stream, so the cooling coil of the airside loop is also the evaporator of the refrigeration loop. The term “direct” refers to the position of the evaporator with respect to the airside loop.
The term "expansion" refers to the method used to introduce the refrigerant into the cooling coil. The liquid refrigerant passes through an expansion device (usually a valve) just before entering the cooling coil (the evaporator). This expansion device reduces the pressure and temperature of the refrigerant to the point where it is colder than the air passing through the coil.
The components of the DX unitary system refrigeration loop (evaporator, compressor, condenser, expansion device and even some unit controls) may be packaged together, which provides for factory assembly and testing of all components, including the electrical wiring, the refrigerant piping, and the controls. This is called a Packaged DX system.
Alternatively, the components of the refrigeration loop may be split apart, allowing for increased flexibility in the system design. This is called a Split DX system. Separating the elements has the advantage of providing the system design engineer with complete flexibility to match components in order to achieve the desired performance.
One of the most common reasons for selecting a DX system, especially a packaged DX system, is that, in a smaller building, it frequently has a lower installed cost than a chilled-water system because it requires less field labor and has fewer materials to install. Packaged DX systems that use air-cooled condensers can be located on the roof of a building, in a small equipment room, or even within the perimeter wall of the building.
Additionally, if the tenants are paying the utility bills, multiple packaged DX units may make it easier to track energy use, as only the specific unit serving that tenant would be used to meet the individual cooling or heating requirements.
Factors affecting the decision to select DX Unitary or Chiller-Based Applied systems include:
Florida Cleanroom Systems recommends Trane chilled water applied systems(CW) or direct-expansion unitary systems(DX) when considering the environmental systems required for close tolerance high efficiency cleanroom air handling systems.
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