Residential central air conditioning systems that need to cool and dehumidify uses a condenser, evaporator coil, and refrigerant piping to operate. The means of moving the air might differ (furnace, air handler, etc.), but the principal is the same for all split systems. The term "split" refers to the physical separation of the condenser and the evaporator. This is in contrast to a window or wall air conditioner in which they are "packaged" together. The refrigerant is compressed and run through a series of tubes to remove as much heat as possible, then piped to an evaporator coil as a warm liquid. Expansion of the compressed liquid causes it to cool, and as the air passes over the coil, heat is extracted. The cool liquid becomes a cool gas as it gathers heat from the air, and is drawn back to the compressor to start the procedure again. As the air passes over the evaporator coil and cools, moisture in the air condenses and drains off as condensate. Some hybrid and geothermal systems use cold water instead of refrigerants until the temperature gets to a certain point, but the majority of systems use refrigerants for the entire cooling cycle. In arid climates when dehumidification is not needed, only cooling, evaporative air conditioning is practical and economical. Evaporative systems can be as simple as a pond of water on a flat roof, or more involved with the use of air handling equipment and special fabrics. By running water down loose material that is suspended and blowing air through the wet fabric, the evaporation of water will lower the air temperature and raise the humidity. The water that does not evaporate will be cooled down enough to contain in a vessel and become a secondary source of cooling by moving air over the vessel to create air conditioning. The conventional central air system is considered low velocity. In this type of system, the air temperature will drop some 15 to 18 degrees F when it passes through the cooling coil, and a 6 inch round duct will deliver approximately 2,500 BTU's of cooling. Alternatives include the ductless mini-split, and the high velocity system. The latter operates at an approximate 30 degree F temperature drop and delivers about 2,500 BTU's of cooling through a 2 inch round duct. This compact delivery system adapts well to retro fits and installations in older homes where duct space is limited. The ductless mini-split, as the name suggests, requires no ductwork at all; and works well with open floor plans or where no ductwork at all can be installed. The fan unit is mounted on the wall or ceiling and piped to the outside condenser. Split systems that use refrigerants are of the same design, no matter what the source of air handling equipment used (furnace, heat pump, hydro-air, etc.). 
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