ASHRAE Journal - October 2011 - 84

EMERGING TECHNOLOGIES

Cooling on a Small Scale
By Alissa Cooperman; John Dieckmann, Member ASHRAE; and James Brodrick, Ph.D., Member ASHRAE

Evaporative Cooling for Residential and Light-Commercial

W

ith evaporative cooling of the condenser, the outdoor air wetbulb temperature, rather than the dry-bulb temperature,

becomes the heat sink temperature. This results in a lower condensing temperature, saving energy. Evaporative cooling of condensers is common in large chillers and in industrial-scale refrigeration equipment, where some level of maintenance cost is easily offset by the energy cost savings. However, in smaller systems, evaporative cooling of condensers generally has been avoided (in favor of air cooling), primarily due to maintenance issues.

Depending on the climate, signiﬁcant energy savings could be obtained through the use of evaporative condenser cooling in residential and light commercial airconditioning systems.

Basics
Three basic ways (and probably others and variants on these) to implement evaporative condenser cooling are: • Directly evaporatively precool the cooling air before it goes through a conventional air-cooled ﬁnned condenser coil; • Flood water over the condenser coil while air is blown through it; and • Use a cooling tower to evaporatively cool cooling water, which in turn cools a water-cooled condenser. The ﬁrst method can be compared to air cooling with equal cooling airﬂow rates through identical air cooled ﬁnned condenser coils, at Air-Conditioning, Heating and Refrigeration Institute (AHRI)-prescribed conditions (outdoor dry and wet bulb temperatures of 95°F
84 ASHRAE Journal

[35°C] and 75°F [23°C], respectively, with the cooling air evaporatively precooled in one case and not evaporatively precooled in the other). As shown in Figure 1, in the air cooling case (no evaporative precooling), cooling air enters the condenser at 95°F (35°C) and warms to 110°F (43°C) (a 15°F [8°C] rise) as it removes heat from the condensing refrigerant (this temperature rise corresponds to a cooling airﬂow rate of approximately 900 cfm [425 L/s] per ton of cooling capacity). The condensing temperature must be above the leaving air temperature. In this case it is assumed to be 120°F (49°C). With the air evaporatively precooled, the dry-bulb temperature approaches the wet-bulb temperature. For this example, it is assumed that the dry-bulb temperature is reduced to 80°F (27°C) (reduced by three-fourths of the difference between the dry- and wet-bulb temperatures). To a ﬁrst order, everything else is the same, so the leaving air temperature is 95°F (35°C), which is 15°F (8°C) higher than
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the entering air temperature. The condensing temperature is 105°F (41°C), which is 10°F (6°C) higher than the leaving air temperature and 15°F (8oC) lower than the condensing temperature with air cooling. This difference in condensing temperature typically results in about a 30% increase in the compressor energy efﬁciency ratio (EER). With Method 2, as the air goes through the condenser and warms up, more water can evaporate, effectively increasing the speciﬁc heat of the air several-fold (the combined sensible heat capacity and incremental latent heat capacity as the saturation humidity ratio increases as the air temperature increases). If the air does not have to get as warm to carry away the heat, the condensing temperature can be even lower. Figure 2 compares this method of evaporative condenser cooling with air cooling using the same assumptions that are the basis for Figure 1 (air drybulb temperature within 5°F [3°C] of the wet-bulb temperature, 10°F [6°C] temperature difference between the leaving air and condensing temperatures, 900 cfm [425 L/s] per ton cooling airﬂow rate). With evaporative cooling throughout the condenser air side, the air temperature rise is only 4°F (2°C) and the resulting condensing temperature is 94°F (34°C), 26°F (14°C) less than the air-cooled case, with an increase in compressor EER of more than 50%. Method 3 (cooling tower + water cooled condenser) is the most common conﬁguration for large systems. Following standard design assumptions for cooling towers and water cooled condensers, the cooling water will leave the cooling
October 2011
ASHRAE Journal - October 2011

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