Fresh water is a vital resource to industry, especially when used for cooling. Fresh water also is a scarce resource: Only 2.5 percent of all water on Earth is fresh; the rest is saltwater. Of the fresh water, less than 1 percent is readily accessible for use. The balance, more than 99 percent, is frozen in glacial ice or trapped below ground.
Faced the reality of limited water supply, plant operators are searching for technologies that allow the cooling of manufacturing processes with less fresh water or alternative sources of water. Wet surface air cooling (WSAC) is one technology that is meeting those challenges in power, refining and natural gas production applications, among others.
A wet surface air cooler uses evaporation to remove heat (figure 1). The liquid, gas or vapor to be cooled flows through a closed-loop tube bundle that is flooded with water. As water drenches the tubes, fans induce air over the bundle in a co-current direction. Heat rejection occurs on the tube’s exterior, where heat is transferred from the process stream to the airstream.
The saturated airstream is pulled through the tube bundle, making two 90° turns into the fan plenum. This motion and reduction in velocity causes large water droplets to leave the airstream and fall into a basin. Air is discharged through the fan stacks at a high velocity, and the water collected in the basin is recycled through the system.
Closed-loop evaporative cooling offers an inherent advantage over dry cooling methods: dry air coolers (DAC) use sensible (no phase change) heat transfer while wet surface air coolers employ latent (phase change) heat transfer. This fundamental difference means that the wet surface air cooler’s process outlet temperature will approach the higher design wet bulb temperature; by contrast, a dry cooler’s outlet temperature approaches the design dry bulb temperature (figure 2). There usually is a large difference in these two temperatures: a typical 85°F (29°C) dry bulb day at a 60 percent relative humidity would have a corresponding wet bulb temperature of 65°F (18°C), allowing a much lower process outlet temperature with the wet surface air cooling system.
Wet surface air cooling systems can reduce the total amount of water needed for cooling by maximizing water reuse. The wet surface air cooling system’s dual 90° angle turns reclaim a significant amount of cooling water that would otherwise be lost as droplets entrained within the exhaust airstream drift. Utilizing wide tube spacing and large orifice free-flow spray nozzles allows for higher cycles of concentration and the use of poor quality water for cooling. Potential sources include:
• Blowdown from other cooling towers.
• Reverse osmosis.
• Plant discharge.
• Produced water from drilling and mining operations.
• Brackish or sea water.
• Sewage plant effluent.
The Department of Energy and the Electric Power Research Institute (EPRI) recently funded a study to assess water quality in a closed-loop evaporative system. Study variables included various sources of spray water makeup as well as different tube materials such as 304 stainless steel, 316 stainless steel and titanium. Results showed that existing cooling tower blowdown could be used as makeup to the wet surface air cooler running 50-plus cycles of concentration without thermal performance loss or maintenance issues.
Some wet surface air coolers utilizing wastewater as makeup are simultaneously performing two important tasks. The wet surface air cooler conserves fresh water by using wastewater for cooling. At the same time, the evaporative process reduces the amount of wastewater that eventually needs to be treated before it is sent to the municipal water system. In some applications, the wet surface air cooler primarily is used as a first-stage evaporator for wastewater.
Limited Water Availability
What about wet surface air cooling applications where water availability is limited? Hybrid systems that combine wet and dry coolers are designed for operation in these conditions. One option is to place a dry air cooler upstream of the wet surface air cooler, allowing the dry air cooler to accomplish the first part of the cooling process. The process stream then is sent to the wet surface air cooling unit for removal of the remaining heat load to achieve the desired outlet temperature. This configuration saves water consumption and delivers lower outlet temperatures than the dry air cooler is capable of providing alone. When the ambient weather is cold enough, the spray water is shut off, allowing the dry air cooler to reject all the necessary heat without any use of water.
A more integrated solution is possible as well. A hybrid wet/dry system can save as much as 50 percent of annual water use when there is not enough water available to handle the entire heat load. The hybrid design differs from the traditional wet surface air cooler by adding widely spaced fins (four to five fins per inch) to the exterior of the tubes in the tube bundle. The fins allow the wet surface air coolers to operate in a dry mode (no spray water) when the ambient temperature is cold enough.
Wet surface air cooling systems help plant operators meet the challenges of increasing fresh water scarcity. These closed-loop evaporative coolers optimize water use for cooling and offer the flexibility of functioning in dry and wet modes, depending on ambient temperature conditions.