How to Efficiently Cool or Condense
An alternate technology, the wet-surface air cooler, has been used successfully for auxiliary fluid cooling cooling and vapor condensing. As its basic principle of operation, heat is rejected by means of latent (evaporative) heat transfer. The fluid/vapor that needs to be cooled or condensed flows through tube bundles as part of a closed-loop system. Water from the unit basin is sprayed in large quantities over the tube bundle’s exterior surface. Air is induced by fans, and latent heat transfer through evaporation takes place at the fluid film on the tubes. The saturated airstream makes two 90o turns in the unit’s plenum at a lower velocity, dropping almost all of the large water droplets back into the basin. The air then is discharged out of the unit through fan stacks.
Because of the large quantity of water sprayed over the tube bundle, the exterior of the tube surface does not dry during operation. The air and water flow over the exterior surface of the tube bundles in the same direction (co- current flow), preventing dry areas on the underside of the tubes. Because the air passes over the spray system water before and during contact with the tube bundle, the mixed water temperature remains above freezing. This protects the unit from freezing even when the ambient air temperature is below freezing.
The wet-surface air cooler does not require any additional heat transfer devices to complete the heat removal service. It has only one approach to the wet bulb temperature, so it requires less air (fan horsepower) to remove the total heat load. Typically, spray pumping horsepower also is less, which means lower operating costs.
The wet-surface air cooler systems require little maintenance. The spray system is accessible for inspection and maintenance without shutting down the unit or removing any appurtenances. And, because the cooler is closed loop, maintenance and cleaning of heat exchangers is eliminated.
Dry or Air-Cooled Systems. In the dry mode, central cooling water (CCW) loop water or oil is cooled directly in an air-cooled, fin-fan unit. Because the wet-surface air cooler operates on evaporative or latent (change of phase, from water to water vapor) heat transfer to remove the process heat, it can provide a lower fluid outlet temperature or pressure than a dry cooler.
This means that the wet-surface air cooler’s process outlet temperatures will approach the design wet bulb temperature, and the fin fan must approach the design dry bulb temperature. This is significant, especially in warmer climates. On a 100oF (38oC) day, the outlet temperature from a dry cooler will be in the range of 110 to 120oF (43 to 49oC), where the wet-surface air cooler can deliver 95oF (35oC) or lower.
Water IssuesWater use and disposal issues have become increasingly important in power and process plant siting and design. A wet-surface air cooler can use poor quality water such as that from blowdown, reverse osmosis (RO) discharge, condensate, pond water, gray water or sewage effluent for spray water makeup. Because the water does not evaporate directly off the tube surfaces, higher cycles of concentration can be achieved. The air cooler can run high cycles of concentration because the spray water only is used to wet the exterior tube surface, and the tube spacing is very wide.
In certain applications, the wet- surface air cooler actually is used as a water treatment device. Some processors use the wet-surface air cooler as not only a heat transfer device but also a first-stage evaporator. The more water that can be evaporated in the heat transfer mode, the less needs to be handled in secondary water treatment systems. This is an advantage for zero-discharge facilities.
For water-limited applications -- when not enough water is available to use evaporative cooling for the entire load -- a hybrid-type unit can be used. This type of system incorporates a dry and a wet section. The wet-surface air cooler can be designed to operate either wet or dry, further reducing the need for makeup water.