In a chilled-water system, the entire air conditioner lives on the roof or behind the building. It cools water to between 40 and 45 degrees Fahrenheit (4.4 and 7.2 degrees Celsius). This chilled water is then piped throughout the building and connected to air handlers as needed. There's no practical limit to the length of a chilled-water pipe if it's well-insulated.
You can see in this diagram that the air conditioner (on the left) is completely standard. The heat exchanger lets the cold Freon chill the water that runs throughout the building.
In all of the systems described earlier, air is used to dissipate the heat from the outside coil. In large systems, the efficiency can be improved significantly by using a cooling tower. The cooling tower creates a stream of lower-temperature water. This water runs through a heat exchanger and cools the hot coils of the air conditioner unit. It costs more to buy the system initially, but the energy savings can be significant over time (especially in areas with low humidity), so the system pays for itself fairly quickly.
- Cooling towers come in all shapes and sizes. They all work on the same principle:
- A cooling tower blows air through a stream of water so that some of the water evaporates.
- Generally, the water trickles through a thick sheet of open plastic mesh.
- Air blows through the mesh at right angles to the water flow.
- The evaporation cools the stream of water.
Because some of the water is lost to evaporation, the cooling tower constantly adds water to the system to make up the difference.
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The amount of cooling that you get from a cooling tower depends on the relative humidity of the air and the barometric pressure.
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For example, assuming a 95-degree Fahrenheit (35-degree Celsius) day, barometric pressure of 29.92 inches (sea-level normal pressure) and 80-percent humidity, the temperature of the water in the cooling tower will drop about 6 degrees to 89 degrees Fahrenheit (3.36 degrees to 31.7 degrees Celsius). If the humidity is 50 percent, then the water temperature will drop perhaps 15 degrees to 80 degrees Fahrenheit (8.4 degrees to 26.7 degrees Celsius). And, if the humidity is 20 percent, then the water temperature will drop about 28 degrees to 67 degrees Fahrenheit (15.7 degrees to 19.4 degrees Celsius). Even small temperature drops can have a significant effect on energy consumption.
Whenever you walk behind a building and find a unit that has large quantities of water running through a thick sheet of plastic mesh, you will know you have found a cooling tower!
