The maximum cooling by an air-cooled condenser is around 20°F difference between the air and refrigerant. And so, the refrigerant temperature approaches 115°F on a 95°F day in midsummer. Further cooling is not possible unless the dry-bulb temperature drops.
Cooling towers, on the other hand, are limited by the wet-bulb temperature. A stock tower can achieve a 10°F difference between the outside wet-bulb temperature and the condenser water returned to the chiller. Even on a 100 percent humidity day, the tower can cool more than an air-cooled chiller, and in areas of low humidity, the performance is even better. The payoff comes from increased chiller efficiency, and working with colder condenser temperatures. On a retrofit project, some of this thermal advantage is lost in a water-to-refrigerant heat exchanger, which transfers heat to the cooling-tower circuit. Careful selection of equipment and controls, however, minimizes these losses.
An advantage of this system is that the heat rejected from the chiller is available for use as waste heat. The shell-and-tube exchanger can be used as a desuperheater to heat domestic water or hydronic water, instead of spending fan energy to reject the heat at the cooling tower.
Additional savings accrue from the use of a small circulating pump and the cooling-tower fans, in lieu of a bank of fans on the air-cooled condenser. The tremendous latent heat of evaporating water enhances this thermal advantage. A further bonus, from modifying the air-cooled chiller, is that the cooling tower can be used later, when the present unit is replaced with an efficient water-cooled chiller.
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Excerpt from Energy Conservation Projects.