Understanding Central Station Systems
In large commercial buildings central station systems are used to provide the conditioned air for the building. Smaller split systems are not practical due to the higher initial equipment cost, future maintenance/repair costs and increased energy consumption as the split system tries to keep up with the demands of cooling a large space.
Central station systems can range in size from 25 tons up to multi-thousand ton systems that are very complex. The large units incorporate sophisticated electronics and mechanical components and that is why there are very few manufacturers in the world that specialize in this type of equipment. These systems are typically located in a mechanical room or on the roof of the building.
There are two types of central air conditioning systems for large buildings and they are listed below with the description of how they operate:
Chilled Water – In this operation the water chiller refrigerates the water to 43-45 degrees Fahrenheit and the chilled water is pumped to the air handler on the individual building floors. The air handler draws in the hot air from the room and blows it across the cooling coil. The heat absorbed by cooling coil warms the water 8-12 degrees Fahrenheit and thus water temperature in the coil is around 55 degrees Fahrenheit. Now when the blower blows the hot air across this coil, cooler air will be supplied to the area.
Direct Expansion or DX – The blower sucks the warmer air out of the building and returns it to the air mixing chamber where it can be mixed with a percentage of outside air based upon outside air temperature and humidity. The air is then passed over the cooling coil where it is cooled to the system set point and the air is then supplied through the duct work to the building to cool the space.
Proper maintenance of these systems will provide improved indoor air quality and building comfort for employees, reduce energy consumption and extend the life of these expensive pieces of equipment.