Advances in Closed-Circuit Evaporative Cooling
In a conventional closed-circuit evaporative cooling tower or evaporative condenser (figure 1), water is circulated from an integral basin to a spray system that evenly distributes water over the outside of a heat exchange coil. Excess or waste heat from the process fluid or refrigerant flowing inside the coil is transferred through the tube walls of the coil to the circulated water. Heat then is transferred from the water to air, which is simultaneously blown over the coil counter-current to the circulating water. The heated air ultimately is rejected into the atmosphere through an evaporative heat transfer process. The heated water is returned to the basin, where it is recirculated over the coil.
While conventional technology has served industry well for more than 50 years, its effectiveness is constricted by the circulating water temperature and the heat exchange coil's available surface area. During unit operation, the circulating water's temperature approaches that of the process fluid or refrigerant, limiting the rate of sensible heat transfer.
An evaporative heat transfer technology successfully employed in open crossflow cooling towers is now being incorporated into evaporative condensers as a secondary step of heat transfer (figure 2). The secondary process reduces the circulating water temperature over the heat transfer coil by 6 to 8°F (3 to 4°C). The reduction enhances the sensible heat transfer rate, and the additional surface area augments the evaporative heat transfer process.
Reducing Scale TendencyThe design of the evaporative cooling coil with secondary cooling reduces the tendency for scale to accumulate on the heat transfer coil surface (see sidebar). Scale is the buildup of solids (often calcium and magnesium compounds) resulting from the evaporation of water. By reducing scale tendency, the coil design enables equipment to sustain peak heat transfer capability over the life of the unit, reducing the amount of energy required to operate the system.
Even minimal amounts of scale on a heat transfer coil surface will affect the unit's thermal performance and increase operating costs. Figure 3 illustrates the impact of scale buildup on thermal performance for an evaporative condenser. With only 0.03125" thick scale, the heat transfer capacity of the evaporative condenser is reduced by 27%. As scale thickness increases, capacity losses increase significantly.
The formation of scale is not always noticed by the system operator. Unfortunately, an evaporative condenser with a scaled coil forces other system components to work harder to make up for its shortcomings. In a refrigeration system, the burden is placed on the compressor. With scale buildup on the condenser coil, the compressors must work against higher head pressures and, therefore, consume greater energy. This increases system operating costs year-round, although the impaired performance may go unnoticed until the system operates on hot days.
With 0.03125" thick scale, compressor power increases 7% and compressor tonnage decreases 1%. Maximum plant throughput and associated revenue are reduced by a similar percentage. Figure 4 illustrates further penalties as scale increases.
In addition to lost throughput, the energy costs for 0.03125" of scale on the coil are significant. For example, consider a 1,000 ton refrigeration (TR) freezer application with 0°F (-17.8°C) suction temperature and 95°F (35°C) condensing temperature. The additional energy costs amount to 25% of the initial cost of a new condenser every year.
Reduced Installation TimeThe heat transfer characteristics achieved with the secondary heat transfer step reduce the heat transfer coil surface area required to provide the same thermal performance as with conventional technology. Reduced surface area results in less unit weight and reduced coil volume. The reduced refrigerant charge decreases the operating charge of the overall system and can be helpful in meeting certain local code requirements. Additionally, fewer coil connections means fewer isolation valves and fittings, less liquid drain lines and fewer purge lines. In turn, installation time is reduced.
The parallel and crossflow air and water paths employed with the secondary heat transfer approach places the air-handling system at the top of the unit and limits air entry to one side. Single-side air entry enhances layout flexibility by permitting up to three sides of the unit to be placed close to building walls, thereby minimizing the size of any required enclosures. Conventional, induced-draft configurations require air entry from all four sides, limiting layout flexibility and potentially increasing enclosure requirements.
Simplified Maintenance RequirementsConventional coil technology employs counterflow air and water paths over the heat transfer coil. The upward flow of air through a conventional unit requires that discharge eliminators be located above the water distribution system to prevent water from blowing out the top of the unit onto nearby surfaces. By contrast, downwardly directed coil intake airflow eliminates the need for discharge eliminators above the water distribution system. This allows direct access to the spray water nozzles and branches for routine inspection and maintenance.
Crossflow air and water paths utilized in the secondary heat transfer section allow a spacious internal plenum and facilitates routine cleaning and maintenance of the cold water basin and the air-handling system. Addition-ally, the negative plenum pressure created by the induced draft configuration during unit operation minimizes potential leak-producing stresses placed on watertight seams. This reduces routine maintenance of field-installed seams and increases winter operating reliability.
Advances in heat transfer technology continue to be developed. Owners and operators embracing new technologies are reaping rewards through lower energy consumption, longer system and equipment life, reduced maintenance downtime and increased system throughput. Sys-tem designers and purchasers should thoroughly evaluate all available technology before making process cooling and refrigeration equipment buying decisions.
SIDEBARFour facets of the secondary heat transfer step contribute to the reduced propensity to scale.
Adding a Secondary Heat Transfer Surface Improves Performance
1. Air and Water Flow in Parallel Paths. Better water coverage around the coil tubes is due to air and spray water flowing in a smooth, parallel, downward path over the heat transfer coil, maintaining full tube coverage. With this parallel flow, spray water is not stripped from the underside of the tubes by the airflow. This eliminates scale producing dry-spots typical in older technology designs (figure A).
2. Increased Water Flow Over Coil. Spray water flow rate over the heat transfer coil area is more than twice that of conventional technology. The secondary coil approach sprays water over the coil at 10 gal/min/ft2 of coil face area compared to 4 to 5 gal/min/ft2 of coil face area on conventional designs. The increased water flow rate provides continuous flooding of the primary heat transfer surface, decreasing scaling potential without increasing pump horsepower.
3. Evaporative Cooling Occurs Primarily in Secondary Heat Transfer Surface. The secondary heat transfer step incorporates combined flow technology using both primary and secondary heat transfer surfaces. The primary heat transfer surface - the heat transfer coil - is an important system component. Because it relies primarily on sensible heat transfer, the coil is protected from detrimental scale. More than 80% of latent heat transfer occurs in the secondary surface, moving evaporation away from the heat transfer coil.
4. Colder Spray Water. Spray water at colder temperatures has a lower propensity to form scale because scaling compounds remain in the solution rather than being deposited as solids on heat transfer surfaces. With the secondary design, the spray water is commonly 6 to 8°F (3 to 4°C) colder than conventional designs due to the addition of the secondary heat transfer surface. Based on the Langelier Index, colder spray water reduces the scaling potential by 25%.