🌊 Cooling Tower Design Guide: How Cooling Towers Work, Sizing, and Key Engineering Considerations

What a Cooling Tower Does

A cooling tower rejects heat from a building's refrigeration or process system to the atmosphere using evaporative cooling. Hot condenser water from water-cooled chillers, industrial process equipment, or other heat sources flows through the tower, where a portion of it evaporates. The evaporation absorbs heat (approximately 1,000 BTU per pound of water evaporated), cooling the remaining water. The cooled water returns to the chiller condenser to absorb more heat, completing the loop.

Water-cooled chiller systems with cooling towers are significantly more efficient than air-cooled chillers because the wet-bulb temperature of outdoor air (the theoretical minimum temperature the tower water can reach) is substantially lower than the dry-bulb temperature that limits air-cooled condenser performance. In most U.S. climates, this advantage translates to chiller efficiency improvements of 20–40% — a major driver for using cooling towers in large commercial and industrial facilities.

How Evaporative Cooling Works

When water evaporates, it absorbs latent heat from the remaining liquid, cooling it. The rate of evaporation — and thus the cooling effect — depends on the difference between the water temperature and the wet-bulb temperature of the surrounding air. The wet-bulb temperature represents the lowest temperature that water can reach through evaporative cooling in a given air condition.

A cooling tower can theoretically cool water to the ambient wet-bulb temperature but practically achieves approach temperatures of 5–10°F above wet-bulb. This means a tower can cool condenser water to temperatures that are impossible for air-cooled systems, which are limited by the dry-bulb temperature (always higher than wet-bulb in unsaturated air).

Cooling Tower Types

Counterflow towers: Air flows upward through the fill media while water flows downward — the two streams move in opposite directions. This counterflow arrangement provides the most efficient heat and mass transfer because the coldest water contacts the driest incoming air. Counterflow towers have a smaller footprint than crossflow towers of equal capacity but are taller.

Crossflow towers: Air flows horizontally through the fill while water falls vertically. The two streams cross at right angles. Crossflow towers are lower-profile, making them better suited for sites with height restrictions, and are generally easier to maintain because the fill media is more accessible. Slightly less thermodynamically efficient than counterflow for the same conditions.

Open (wet) towers: The most common type — the condenser water directly contacts the air. Highly efficient but the open water system is exposed to airborne contaminants, dust, and biological growth. Legionella risk management is a critical concern for open towers.

Closed-circuit (fluid) coolers: Condenser water flows through a closed coil inside the tower; only the spray water contacts the air. The closed loop eliminates direct contamination of the process water but is less efficient than open towers (water-to-coil-to-air heat transfer adds thermal resistance). Used where water quality or contamination control is critical.

Induced draft vs. forced draft: Induced draft towers have fans at the top pulling air upward through the fill. Forced draft towers have fans at the air inlet pushing air through. Induced draft is more common in commercial HVAC — it provides more uniform airflow distribution and recirculation of exhaust air back into the tower inlet is less of a concern.

Sizing a Cooling Tower: Range and Approach

Two key parameters define cooling tower performance:

Range: The temperature difference between the hot water entering the tower and the cold water leaving. Range = entering water temperature (EWT) − leaving water temperature (LWT). A typical chiller condenser water system might have EWT = 95°F and LWT = 85°F, giving a range of 10°F. Range is determined by the heat load and condenser water flow rate: Range = Q / (500 × GPM), where Q is in BTU/hr and GPM is condenser water flow.

Approach: The temperature difference between the cold water leaving the tower (LWT) and the ambient wet-bulb temperature (WBT). Approach = LWT − WBT. If LWT = 85°F and WBT = 76°F, the approach is 9°F. Smaller approach means the tower is cooling water closer to the thermodynamic limit — which requires more tower size and cost. Approach values below 5°F are rarely economical; 7–10°F is typical for commercial applications.

Cooling tower manufacturers publish performance data as a function of range, approach, and wet-bulb temperature. Specify the tower by its thermal duty: GPM to be cooled, hot water temperature entering, cold water temperature leaving, and design wet-bulb temperature. Use the ASHRAE 0.4% or 1% design wet-bulb temperature for your location.

Condenser Water Flow Rates

The standard rule of thumb for cooling tower condenser water flow is 3 GPM per ton of chiller capacity for a 10°F range (95°F entering, 85°F leaving) at 76°F wet-bulb. This corresponds to roughly 15,000 BTU/hr of heat rejection per ton — the 12,000 BTU/ton of cooling capacity plus approximately 25% for compressor heat input.

For high-efficiency chillers or systems with higher condenser water temperatures, the heat rejection rate per ton increases slightly and condenser water flow must be recalculated from the actual chiller heat rejection data, not the rule of thumb.

Makeup Water Requirements

An open cooling tower loses water through three mechanisms: evaporation (the cooling mechanism — approximately 0.1% of circulation flow per degree of cooling), drift (fine water droplets carried out by the air — typically less than 0.005% of flow with modern drift eliminators), and blowdown (intentional discharge to control dissolved solids concentration).

Total makeup water for a cooling tower system is typically 1.5–2.5 GPM per 100 tons of cooling capacity under full load. For a 500-ton chiller plant, this means 7.5–12.5 GPM of city water consumption during peak operation. Water cost and availability should be considered in system selection, particularly in water-scarce regions.

Blowdown is required to control cycles of concentration (CoC) — the ratio of dissolved solids in the tower water to dissolved solids in the makeup water. As water evaporates, dissolved minerals remain and concentrate. Excessive concentration leads to scaling on heat transfer surfaces and accelerated corrosion. Typical design CoC is 3–6 cycles, balancing water consumption against chemical treatment cost.

Legionella Risk Management

Cooling towers are a known risk factor for Legionella pneumophila, the bacterium that causes Legionnaires' disease. Warm stagnant water in the 77–108°F range, with biofilm and scale providing nutrients, creates conditions where Legionella can multiply. Infected aerosols from tower drift can be inhaled by people in and around the building.

ASHRAE Guideline 12-2020 (Managing the Risk of Legionellosis Associated with Building Water Systems) and ANSI/ASHRAE Standard 188 provide the framework for cooling tower Legionella risk management. Key elements include: maintaining biocide treatment (chlorine, bromine, or approved alternative), controlling tower water chemistry (pH 7.0–8.0, appropriate inhibitor levels), minimizing dead legs and stagnant zones, maintaining water temperature outside the Legionella growth range when feasible, and conducting routine sampling and testing.

Many jurisdictions now require a written Water Management Plan (WMP) for cooling towers as a condition of operation. High-profile Legionella outbreaks traced to cooling towers have driven significant regulatory attention to this issue.