Open Circuit Cooling Tower Explained: How It Works, Where It's Used, and How to Maintain It

What Is an Open Circuit Cooling Tower and How Does It Work?

An open circuit cooling tower — also commonly referred to as an open loop cooling tower — is a heat rejection device that removes excess heat from a process or building by transferring it to the atmosphere through direct contact between the hot process water and ambient air. Unlike a closed circuit cooling tower where the process fluid is isolated in a coil, the water in an open circuit system flows directly over fill media, exposing it to a stream of moving air. This direct contact causes a portion of the water to evaporate, and since evaporation is an endothermic process, it draws heat away from the remaining water, cooling it down before it is recirculated back to the process equipment.

The basic operating cycle is straightforward. Hot water from a chiller condenser, industrial process, or HVAC system is pumped to the top of the cooling tower and distributed evenly over a fill — a structured or random packing material that maximizes the surface area of water exposed to air. Air is drawn or forced through the fill simultaneously, either from the side or from the bottom, depending on the tower design. As the water trickles down through the fill, evaporation and convective heat transfer cool it by typically 5–15°C. The cooled water collects in the cold water basin at the bottom and is then pumped back to the heat source to repeat the cycle. A small percentage of water — usually 1–3% of total circulation rate — is lost through evaporation, drift, and blowdown, and this must be continuously replenished through a makeup water supply.

Key Components of an Open Circuit Cooling Tower

Understanding the individual components of an open loop cooling tower helps operators diagnose performance issues, plan maintenance, and evaluate system upgrades. Each part plays a specific role in the overall heat rejection process.

• Fill Media (Packing): The fill is the heart of the open circuit cooling tower. It breaks up the water flow into thin sheets or droplets, dramatically increasing the air-water contact surface area and residence time. Fill comes in two main types — film fill, where water flows in thin films over closely spaced corrugated PVC sheets, and splash fill, where water droplets are repeatedly broken up by horizontal splash bars. Film fill is more thermally efficient but more prone to clogging in dirty water applications.
• Drift Eliminators: Positioned above the fill, drift eliminators are sinusoidal or chevron-shaped baffles that force the air stream to change direction multiple times, causing entrained water droplets to impinge on the baffle surfaces and drain back into the tower rather than being carried out with the exhaust air. Modern high-efficiency drift eliminators reduce water carryover to less than 0.0005% of the circulation flow rate.
• Water Distribution System: The distribution system delivers hot water evenly across the entire fill surface. It typically consists of a main header pipe, lateral distribution pipes, and spray nozzles or gravity-fed orifices. Uneven water distribution creates dry spots in the fill that reduce thermal performance and can lead to accelerated biological growth.
• Fan and Motor Assembly: Fans move the required volume of air through the fill to sustain evaporative cooling. In mechanical draft towers, axial propeller fans are the most common choice for their high airflow capacity and relatively low energy consumption. Fan motors are typically totally enclosed and fan-cooled (TEFC) to withstand the humid, corrosive environment inside the tower.
• Cold Water Basin: The basin at the base of the tower collects the cooled water before it is returned to the process. The basin also serves as the sump for the circulation pump suction, and its design affects water residence time, sediment accumulation, and biological growth risk. Most basins include a make-up water inlet with a float valve, an overflow outlet, a blowdown connection, and an access point for cleaning.
• Tower Structure and Casing: Open circuit cooling towers are constructed from a range of materials depending on the application. Galvanized steel is standard for general industrial use. Fiberglass-reinforced plastic (FRP) is preferred in corrosive environments such as chemical plants or coastal installations. Concrete is used for very large utility-scale towers due to its durability and low long-term maintenance cost.

Types of Open Circuit Cooling Towers

Open loop cooling towers are categorized by the direction of airflow relative to the falling water and by the mechanism used to move air through the system. Each configuration has distinct performance characteristics, installation requirements, and maintenance considerations.

Counterflow vs. Crossflow

In a counterflow cooling tower, air moves vertically upward through the fill while water falls downward — the two flows travel in opposite directions. This arrangement creates the most efficient air-water contact because the coldest water at the bottom meets the driest incoming air, maximizing the driving force for evaporation. Counterflow towers tend to be taller and more compact in plan area, making them well-suited for sites with limited footprint.

In a crossflow cooling tower, air moves horizontally through the fill while water falls vertically. Hot water is distributed from a gravity-fed basin at the top of the fill rather than sprayed under pressure. Crossflow towers are generally wider and lower in profile than counterflow designs, which can simplify installation, maintenance access, and pump head requirements. They are commonly used in large HVAC applications and light industrial processes where head pressure is a constraint.

Induced Draft vs. Forced Draft

In an induced draft cooling tower, the fan is located at the top of the tower and pulls air upward through the fill. This is by far the most common arrangement for open circuit towers because the fan operates in relatively clean, low-humidity air, improving fan and motor reliability. The negative pressure created inside the tower also reduces the risk of hot, humid exhaust air being recirculated back into the air inlet.

In a forced draft cooling tower, the fan is positioned at the air inlet — typically at the base or side of the tower — and pushes air through the fill. Forced draft fans can be located away from the humid tower environment, which simplifies mechanical maintenance. However, the positive pressure inside the tower makes recirculation more likely, and the fan handles saturated inlet air, increasing the risk of icing in cold climates.

Natural Draft Cooling Towers

Natural draft open circuit cooling towers — the iconic hyperboloid concrete structures seen at power plants — use the buoyancy of warm, humid exhaust air to drive airflow without any mechanical fans. The hyperbolic shape creates a tall chimney effect that generates a consistent upward draft. These towers are only economical at very large scales, typically above 100 MW of heat rejection, due to the high civil construction cost of the concrete shell. They have no fan energy cost and extremely low maintenance requirements once constructed.

Open Circuit vs. Closed Circuit Cooling Towers: Which One Do You Need?

Choosing between an open circuit and a closed circuit (fluid cooler) cooling tower is one of the first major decisions in any cooling system design. Each type has a fundamentally different relationship between the process fluid and the environment, with significant implications for system performance, water quality management, and capital cost.

The open circuit cooling tower's direct-contact evaporative process makes it inherently more thermally efficient than a closed circuit system, as it can cool water to within a few degrees of the ambient wet-bulb temperature. Closed circuit towers are preferred when the process fluid must remain uncontaminated — such as in food processing, pharmaceutical manufacturing, or data center cooling — or when the fluid itself is expensive or hazardous and cannot risk exposure to the atmosphere.

Common Industrial and Commercial Applications

Open loop evaporative cooling towers are among the most widely deployed heat rejection systems across heavy industry and commercial building services. Their ability to reject large quantities of heat at low operating cost makes them the default choice in a wide range of applications.

• HVAC Chiller Condensers: The most common application of open circuit cooling towers is rejecting heat from the condenser side of water-cooled chillers in large commercial buildings, hospitals, hotels, and shopping centers. Water-cooled chiller systems paired with open circuit towers are significantly more energy-efficient than air-cooled alternatives, with COP values typically 30–50% higher.
• Power Generation: Thermal power plants — including coal, gas, nuclear, and concentrated solar — use large-scale open circuit cooling towers to condense steam after it passes through the turbine. The cooling tower is a critical component of the Rankine cycle thermodynamic efficiency, and its performance directly affects plant output and water consumption.
• Steel and Metal Processing: Cooling towers serve blast furnaces, electric arc furnaces, continuous casting equipment, and rolling mill hydraulic systems. These applications demand high-flow, high-temperature-differential towers capable of handling process upsets and variable loads.
• Petrochemical and Refining: Refineries and chemical plants use cooling tower water extensively to condense process vapors, cool heat exchangers, and remove heat from reactors. These facilities often operate multiple large cooling tower cells in a central utility area serving dozens of process units simultaneously.
• Injection Molding and Plastics: Plastic molding machinery requires precise mold temperature control. Open circuit cooling towers provide the bulk cooling capacity, with the tower water typically passed through a heat exchanger before entering the mold circuits to maintain water quality and temperature stability.
• Food and Beverage Processing: Breweries, dairy plants, and food processing facilities use cooling towers to remove heat from refrigeration condensers, pasteurizers, and process coolers — though in most cases an intermediate heat exchanger is used to keep the open circuit tower water separated from any food-contact circuits.

How to Size and Select an Open Circuit Cooling Tower

Proper sizing of an open circuit cooling tower requires a clear understanding of the thermal load, the available ambient conditions, and the required leaving water temperature. Undersizing results in inadequate heat rejection and elevated process temperatures; oversizing wastes capital and increases operating costs unnecessarily.

Define the Thermal Duty

The starting point is calculating the total heat rejection rate, expressed in kilowatts (kW), tons of refrigeration (TR), or megawatts (MW) depending on the industry. For an HVAC chiller application, the cooling tower must reject both the building cooling load and the compressor heat of rejection — typically 20–30% more than the chiller's rated cooling capacity. For industrial processes, the heat load is determined from mass and energy balances across the process equipment being cooled.

Establish the Design Wet-Bulb Temperature

Since open circuit cooling towers reject heat primarily through evaporation, their performance is governed by the ambient wet-bulb temperature (WBT) rather than the dry-bulb temperature. The design WBT is typically selected at the 1% or 0.4% summer design condition from ASHRAE climate data for the project location — meaning the WBT is exceeded only 1% or 0.4% of the total annual hours. Selecting too conservative a WBT increases tower size unnecessarily; selecting too aggressive a value results in insufficient cooling during peak summer conditions.

Set the Range and Approach

Two parameters define the thermal performance of an open circuit cooling tower. The range is the temperature difference between the hot water inlet and the cold water outlet — typically 5–10°C for HVAC applications and up to 15°C for some industrial systems. The approach is the difference between the cold water outlet temperature and the ambient wet-bulb temperature. A smaller approach requires a larger tower and more fill surface area. Approach temperatures below 3°C are generally not economically practical for standard open circuit towers and may require specialized designs.

Account for Site-Specific Constraints

Beyond thermal calculations, site constraints play a major role in tower selection. Available footprint determines whether a single large cell or multiple smaller cells are needed. Building height restrictions, noise sensitivity of neighboring areas, prevailing wind direction (which affects recirculation risk), seismic zone requirements, and local water quality all influence the final tower configuration, material specification, and ancillary equipment selection.

Water Treatment for Open Circuit Cooling Towers

Water treatment is one of the most critical and often underestimated aspects of operating an open loop cooling tower system. Because the circulating water is in continuous contact with the atmosphere, it is subject to evaporative concentration of dissolved minerals, contamination by airborne particles, biological growth, and corrosion of metal system components. Without proper treatment, all of these issues degrade system performance, damage equipment, and increase operating costs.

Cycles of Concentration and Blowdown

As water evaporates from the tower, the dissolved minerals it contained remain in the circulating water, causing their concentration to increase over time. The ratio of mineral concentration in the circulating water to that of the makeup water is called the cycles of concentration (COC). Most open circuit systems are operated at 3–6 COC. Exceeding this range increases the risk of scale deposition and corrosion. Blowdown — intentionally discharging a controlled flow of concentrated water from the basin and replacing it with fresh makeup water — is used to maintain the COC within the target range. Automatic blowdown controllers using conductivity measurement are standard practice in well-managed systems.

Scale and Corrosion Inhibitors

Scale inhibitors — typically phosphonate or polymer-based compounds — are dosed continuously to prevent calcium carbonate, calcium sulfate, and silica from depositing on heat exchanger surfaces and fill media. Corrosion inhibitors protect steel components, copper alloys, and galvanized surfaces by forming a thin protective film on metal surfaces. The correct inhibitor chemistry is selected based on the makeup water analysis, system metallurgy, and operating COC. pH is maintained in the range of 7.0–8.5 to balance scale and corrosion tendencies.

Biological Control and Legionella Prevention

Open circuit cooling towers are recognized as potential amplification sites for Legionella pneumophila, the bacterium responsible for Legionnaires' disease. The warm, nutrient-rich circulating water provides ideal growth conditions if not properly managed. Biocide programs combining oxidizing biocides (such as chlorine or bromine compounds dosed to maintain 0.5–1.0 ppm free residual) with non-oxidizing biocides (such as isothiazolinone or DBNPA used periodically for shock dosing) are the industry standard for biological control. Physical control measures — including regular basin cleaning, drift eliminator maintenance, and deadleg elimination — complement the chemical program. Regulatory requirements for Legionella risk assessments and cooling tower water management plans are now mandated in many jurisdictions, including the United States (ASHRAE 188), the United Kingdom (L8 ACoP), and the European Union.

Maintenance Best Practices for Open Circuit Cooling Towers

A structured, proactive maintenance program is essential to keep an open loop cooling tower operating at design efficiency and to maximize its service life — typically 15–25 years for well-maintained FRP or galvanized steel units. The following practices represent industry best standards for cooling tower maintenance.

• Basin Cleaning: Sediment, biological slime, and debris accumulate in the cold water basin over time, providing nutrients for microbial growth and blocking the suction strainer. Basins should be physically cleaned and disinfected at least annually — typically during a planned shutdown — or more frequently if biological activity is high. Basin sweepers or side-stream filtration systems can reduce sediment accumulation between full cleanings.
• Fill Media Inspection: Inspect fill for biological fouling, scaling, sagging, or physical damage at least annually. Blocked or collapsed fill reduces airflow and water distribution, significantly degrading thermal performance. PVC fill that has become brittle with age or has suffered UV degradation should be replaced before it fails structurally and causes a system shutdown.
• Fan and Drive System Maintenance: Inspect fan blades for erosion, pitting, or imbalance. Check fan blade pitch settings and adjust as needed to maintain design airflow. Lubricate fan shaft bearings according to the manufacturer's schedule. On gear-drive towers, check gearbox oil level and quality annually and change oil per the recommended interval. On belt-drive towers, inspect belt tension and wear every 3–6 months.
• Distribution System Checks: Inspect spray nozzles or gravity distribution holes for clogging, wear, or misalignment. Partially blocked nozzles create dry areas in the fill that reduce performance and promote biological growth. Clean or replace nozzles as part of the annual service. Check lateral pipe connections and hot water basin partitions for cracks or corrosion.
• Drift Eliminator Assessment: Check drift eliminators for proper seating, cracks, and warping. Damaged or improperly fitted drift eliminators allow unacceptable water carryover, increasing makeup water consumption and — critically — the potential for Legionella-laden aerosol to be discharged into the surrounding environment.
• Structural Inspection: Inspect the tower casing, louvers, basin walls, and support structure for corrosion, cracks, and fastener failure. For galvanized steel towers, check the condition of the galvanized coating and apply cold galvanizing compound or epoxy coating to any areas showing bare metal or rust spots. Address any structural deficiencies promptly to prevent progressive deterioration.

Common Performance Problems and How to Diagnose Them

When an open circuit cooling tower is not meeting its design leaving water temperature, several possible causes need to be systematically evaluated before committing to equipment replacement or major remediation work.

When diagnosing thermal performance shortfalls, always start by verifying the actual ambient wet-bulb temperature against the design condition. A cooling tower that appears to be underperforming during an unusually hot and humid summer may actually be operating correctly — it is simply being asked to perform beyond its design envelope. Comparing normalized performance data (adjusted for actual versus design wet-bulb temperature and water flow rate) provides a much more reliable picture of true tower condition than raw temperature readings alone.