Chilled-water and Cooling-tower AC Units

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 F (4.4 and 7.2 C). This chilled water is then piped throughout the building and connected to air handlers as needed. There is no practical limit to the length of a chilled-water pipe if it is well-insulated.

Chilled Water AC

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.

Cooling Tower AC
Cooling tower
Cooling Towers
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:

  1. A cooling tower blows air through a stream of water so that some of the water evaporates.
  2. Generally, the water trickles through a thick sheet of open plastic mesh.
  3. Air blows through the mesh at right angles to the water flow.
  4. The evaporation cools the stream of water.
  5. Because some of the water is lost to evaporation, the cooling tower constantly adds water to the system to make up the difference.

Cooling Tower
Cooling towers

The amount of cooling that you get from a cooling tower depends on the relative humidity of the air and the barometric pressure.

For example, assuming a 95 F (35 C) 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 F (3.36 degrees to 31.7 C).

If the humidity is 50 percent, then the water temperature will drop perhaps 15 degrees to 80 F (8.4 degrees to 26.7 C).

If the humidity is 20 percent, then the water temperature will drop about 28 degrees to 67 F (15.7 degrees to 19.4 C). Even small temperature drops can have a significant effect on energy consumption.

To understand how the relative humidity and atmospheric pressure control the temperature drop in a cooling tower on any given day, check out USA Today: How a sling psychrometer works.

Whenever you walk behind a building and find a unit that has large quantities of water running through a plastic mesh, you will know you have found a cooling tower.

In many office complexes and college campuses, cooling towers and air conditioning equipment are centralized, and chilled water is routed to all of the buildings through miles of underground pipes.

For more information about air conditioners and related topics, check out the links on the next page.