period three
System Variations
notes
“Free” Cooling
▲Airside economizer
▲Waterside economizer
◆Strainer cycle
◆Plate-and-frame heat exchanger
◆Refrigerant migration
Figure 83
“Free” Cooling
There are a number of methods that use cool outdoor conditions to reduce cooling energy costs. They are often referred to as “free cooling” because they reduce or eliminate the energy consumed by the compressor. They are not truly free, but really reduced-cost cooling options.
The most prevalent method is the use of an airside economizer. When the temperature, or enthalpy, of the outdoor air is low enough, the outdoor-air and return-air dampers in the air handler are modulated and the cooler outdoor air is used to reduce the temperature of air entering the cooling coil. This can reduce or totally eliminate the requirement for mechanical cooling for much of the year in many climates.
In water-cooled systems, there are also several types of waterside economizers. The most direct method, but typically the least desirable, is to use a strainer cycle. In this system, the condenser- and chilled-water systems are connected. When the outdoor wet-bulb temperature is low enough, cold water from the cooling tower is routed directly into the chilled-water loop. Although the strainer cycle is the most efficient waterside economizer option, it greatly increases the risk of fouling in the chilled-water system and cooling coils with the same type of contamination that is common in open-cooling-tower systems. A strainer or filter can be used to minimize this contamination, but the potential for fouling prevents widespread use of the strainer-cycle system.
