A typical window air conditioner might be rated at 10,000 BTU. For comparison, a typical 2,000-square-foot (185.8 m2) house might have a 5-ton (60,000-BTU) air conditioning system, implying that you might need perhaps 30 BTU per square foot. (Keep in mind that these are rough estimates. To size an air conditioner for your specific needs, contact an HVAC contractor.)
The cold side, consisting of the expansion valve and the cold coil, is generally placed into a furnace or some other air handler. The air handler blows air through the coil and routes the air throughout the building using a series of ducts. The hot side, known as the condensing unit, lives outside the building. In most home installations, the unit looks something like this:
The unit consists of a long, spiral coil shaped like a cylinder. Inside the coil is a fan, to blow air through the coil, along with a weather-resistant compressor and some control logic. This approach has evolved over the years because it is low-cost, and also because it normally results in reduced noise inside the house (at the expense of increased noise outside the house). Besides the fact that the hot and cold sides are split apart and the capacity is higher (making the coils and compressor larger), there is no difference between a split-system and a window air conditioner.
In warehouses, businesses, malls, large department stores, etc., the condensing unit normally lives on the roof and can be quite massive. Alternatively, there may be many smaller units on the roof, each attached inside to a small air handler that cools a specific zone in the building.
Let's take a look now at a chilled-water air conditioner.
In larger buildings and particularly in multi-story buildings, the split-system approach begins to run into problems. Either running the pipe between the condenser and the air handler exceeds distance limitations (runs that are too long start to cause lubrication difficulties in the compressor), or the amount of duct work and the length of ducts becomes unmanageable. At this point, it is time to think about a chilled-water system.
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.
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 above, 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.
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 this Web page.
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.
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