Direct Expansion Air Conditioning

Direct Expansion Air Conditioning What It Is A direct expansion air conditioning (DX) system uses a refrigerant vapour expansion/compression (RVEC) cycle to directly cool the supply air to an occupied space. DX systems (both packaged and split) directly cools the air supplied to the building because the evaporator is in direct contact with the supply air, Expansion refers to the treatment of the refrigerant (a valve reduces its pressure and temperature) prior to it entering the evaporator. DX systems can come equipped with all the components in the unit (packaged system) intended for installation on the rooftop or by the side of a building; or it may have some components installed inside the building and some outside (split system). DX systems require a ventilation fan to distribute the cool air and resupply/re-circulate it. How It Works A RVEC cycle has four basic components; an evaporator, compressor, condenser, and thermal expansion control device. The evaporator (located inside the supply air ductwork) absorbs heat through the process of expanding the refrigerant flowing within it. The refrigerant then flows to a compressor which compresses it causing it to condense in the condenser and release the heat it removed from the supply air. The condensed liquid refrigerant then flows through the thermal expansion control device which controls the flow and pressure of the refrigerant back into the evaporator. Packaged Systems – Packaged DX units contain all 4 parts of the RVEC system, as well as fans and internal ducting. These units are designed to be installed easily to serve local zones cooling needs; multiple units can be installed to service multiple zones in a building. Split Systems – These systems generally have the evaporator and fans inside the building, while the rest of the RVEC system components are a separate unit placed outside the building. This allows system designs that are more flexible, allowing performance that can satisfy greater variations on system demands. Split units are made to an incremental performance scale, meaning only certain working load sizes are available. Benefits DX systems are less expensive to install, and uses less space in mechanical and electrical rooms than centralized cooling systems DX systems can be expanded in an incremental fashion to match changing building requirements Packaged Systems have standardized operating performances per unit, allowing more precise system sizing Packaged Systems generally require less ventilation, and do not require dedicated condensate lines Packaged Systems occupy less space than comparable split systems Split Systems tend to be larger allowing for fewer units, and therefore less maintenance costs than a comparable Packaged system Split Systems have lower noise levels because the compressor unit is located further away from the cooling load area Split Systems may allow vertical duct shafts to be smaller in size. ------by------------- Mohammad Imran HVAC Engineer