A cooling tower is a heat rejection device that transfers process waste heat to the atmosphere through the evaporative cooling of a water stream. As water cascades over internal fill media, a portion evaporates, and the latent heat of vaporization absorbs thermal energy from the remaining water, lowering its temperature. Cooling towers are fundamental components in large-scale HVAC systems, serving as the primary means of heat rejection for water-cooled chiller plants and condenser water loops.
Technical Details and Performance Metrics
Cooling tower performance is defined by two critical measurements. Approach is the difference between the cold water outlet temperature and the ambient wet-bulb temperature, typically ranging from 5°F to 15°F (2.8°C to 8.3°C). Range is the temperature difference between the hot water inlet and the cold water outlet, commonly falling between 10°F and 20°F (5.6°C to 11.1°C). A lower approach indicates higher tower efficiency but requires a larger, more costly unit.
Cooling towers are rated in nominal tons of cooling capacity, where one cooling tower ton equals 15,000 BTU/hr (accounting for the compressor heat added beyond the evaporator load). Key design variables include:
- Fill media type: Film fill uses thin, closely spaced sheets to spread water into a thin layer for maximum air-water contact. Splash fill uses horizontal bars or grids to break water into droplets. Film fill offers greater thermal efficiency per unit volume, while splash fill is more tolerant of poor water quality.
- Airflow configuration: Counterflow towers direct air upward against the downward flow of water, achieving higher thermal performance in a smaller footprint. Crossflow towers move air horizontally across the falling water, offering easier maintenance access and lower fan static pressure.
- Draft type: Mechanical draft towers use fans (forced or induced) to move air. Natural draft towers rely on the buoyancy of warm, moist air within a hyperbolic shell structure, typically found in power generation rather than HVAC applications.
Applications in HVAC Systems
In commercial and institutional HVAC, cooling towers reject condenser heat from water-cooled centrifugal, screw, or absorption chillers. Water-cooled systems paired with cooling towers consistently achieve higher energy efficiency than air-cooled alternatives, with chiller-tower combinations often reaching system efficiencies below 1.0 kW per ton. Cooling towers also serve data centers, hospitals, manufacturing facilities, and district cooling plants where large, sustained cooling loads demand reliable and economical heat rejection.
Codes, Standards, and Maintenance Considerations
Cooling tower selection and installation are governed by ASHRAE Standard 90.1, which sets minimum efficiency requirements for heat rejection equipment. The Cooling Technology Institute (CTI) provides certification programs (CTI STD-201) that validate manufacturer performance claims through independent testing. ASHRAE Guideline 12 addresses the minimization of Legionella risk in building water systems, which is directly relevant to cooling tower operation.
Proper maintenance includes regular water treatment to control scale, corrosion, and biological growth; periodic inspection of fill media, drift eliminators, and fan assemblies; and monitoring of makeup water consumption. Drift eliminators limit water loss to less than 0.001% of circulating flow in modern designs, reducing both water waste and the potential for airborne contaminant dispersion.