Refrigerating System – Evaporative condensers
Evaporative condensers are apparently water-cooled designs and work on the principle of cooling by evaporating water into a moving air stream. The effectiveness of this evaporative cooling process depends upon the wet bulb temperature of the air entering the unit, the volume of air flow and the efficiency of the air/water interface.
Evaporative condensers use water sprays and air flow to condense refrigerant vapors inside the tubes. The condensed refrigerant drains into a tank called a liquid receiver. Refrigerant subcooling can be accomplished by piping the liquid from the receiver back through the water sump where additional cooling reduces the liquid temperature even further.
In an evaporative condenser (Figure 3.19a), the fluid to be cooled is circulated inside the tubes of the unit’s heat exchanger. Heat flows from the process fluid through the coil tubes to the water outside, which is cascading downward over the tubes. Air is forced upward through the coil, evaporating a small percentage of the water, absorbing the latent heat of vaporization, and discharging the heat to the atmosphere. The remaining water falls to the sump to be recirculated by the pump, while water entrained in the air stream is reclaimed and returned to the sump by the mist eliminators at the unit discharge. The only water consumed is the amount evaporated plus a small amount which is intentionally bled off to limit the concentration of impurities in the pan. With the optional extended surface coil the recirculating water pump can be shut off and the unit operated dry during periods of belowdesign ambient temperatures. Air is still forced upward through the coil, but the heat is now dissipated to the atmosphere by sensible cooling alone.
The following are some characteristics of these condensers:
• reduced circulating water for a given capacity,
• water treatment is necessary,
• reduced space requirement,
• smal piping sizes and short overall lengths,
• smal system pumps, and
• availability of large capacity units and indoor configurations.
The volume of water used by evaporative condensers is significant. Not only does water evaporate just to reject the heat, but water must be added to avoid the buildup of dissolved solids in the basins of the evaporative condensers. If these solids build up to the point that they foul the condenser surfaces, the performance of the unit can be greatly reduced.