Are closed-loop, evaporative wet-surface air coolers a solution for your heat transfer process applications?

The closed-loop design ensures that the process liquid, vapor or gas flows through the inside of the heat exchanger tubes, with the cooling air and the spray water flow in the same direction on the outside of the tubes.

Fluid cooling and vapor/gas condensing can be accomplished by several methods, with the most common being an open tower with plate-and-frame or shell-and-tube heat exchangers (wet), or air-cooled, fin-fan coolers (dry).

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.

Wet-surface air cooling units can be custom designed and manufactured to include spray pumps, expansion tank, fittings, instrumentation and controls. Generally, little field assembly is required.

In an open tower configuration, there are two loop systems -- one open and one closed -- that require two heat transfer devices to complete the same duty. Open-loop water flows through a heat exchanger, where it is warmed, and then to the cooling tower, where it is cooled via evaporation. This means there are two approach temperatures, a sensible approach in the heat exchanger and a second (latent) approach to the wet bulb in the cooling tower.

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.

The wet-surface air cooler has a single point of heat transfer to directly cool the fluid or vapor. It is a direct replacement for open cooling towers or dry, air-cooled systems.

Due to the efficient heat removal and lower air requirements, the wet-surface air cooler takes up a much smaller footprint -- typically 25 percent of the area required for a fin fan. Also, its heat transfer surface requires little maintenance. The tubes of the wet-surface air cooler are all prime surface and do not require fins. Also, in fin-fan units, cold air passes directly over the tubes, which can lead to freezing inside the tubes during low ambient operation. In the air cooler, the recirculating spray water is kept warm by the direct contact with the heat source, acting as a buffer between the tubes and cold air.

Water Issues

Water 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.