Approach temperature is the difference between the cooled water leaving the tower and the wet bulb temperature of the ambient air entering the tower. This critical parameter represents how closely the tower can cool water to the theoretical saturation point. Lower approach temperatures require larger towers and greater air flow, directly impacting both capital cost and operating efficiency. Typical approach values range from 3-10 degrees Fahrenheit in commercial systems.
Technical Factors
Approach temperature depends on fill media effectiveness, air and water flow rates, air velocity, water distribution uniformity, and operating conditions. A 3-degree approach requires approximately 25% larger tower volume compared to a 7-degree approach. Counterflow towers achieve tighter approaches than crossflow designs using equivalent footprints. Higher air velocities and better water distribution improve approach temperature but increase fan horsepower requirements and operating costs.
Design Implications
Specifying tighter approach temperatures reduces chiller lift requirements and improves HVAC system efficiency, lowering electrical consumption by 2-3% per degree improvement. However, capital costs increase significantly with each degree reduction. Engineers balance cooling performance requirements against installation budgets and operating energy consumption. Seasonal variations in wet bulb temperature affect actual approach performance, with summer peaks often exceeding design approaches by 2-4 degrees.