The Commercial Cooling Tower Guide: How Evaporative Heat Rejection Actually Works

Fill is the heart of a cooling tower's performance — a large pack of plastic sheets (film fill) or splash bars that maximizes the surface area and contact time between the falling water and the moving air. The more efficiently the fill spreads water into a thin film, the more evaporation and heat transfer you get from a given tower size. Film fill is compact and efficient but clogs if water isn't kept clean; splash fill is more fouling-tolerant but bulkier. Fouled, collapsed, or scaled fill is one of the most common causes of a tower that can no longer hit its design temperatures.

A fan moves air through the tower to drive evaporation — pulling air across the fill in an induced-draft design (fan at the top) or pushing it through in a forced-draft design (fan at the side). The fan, its motor, gearbox or belt drive, and bearings are the tower's main mechanical wear items and its largest electrical load. Variable-frequency drives that modulate fan speed to the actual load are a major energy-saver and reduce wear; failed bearings, slipping belts, or out-of-balance fans are frequent service calls.

The basin at the bottom of the tower collects the cooled water before it returns to the chiller's condenser, and it houses the controls that manage water chemistry: a makeup valve that adds fresh water to replace what's lost, and a blowdown (bleed) valve that intentionally dumps a portion of concentrated water to keep dissolved solids in check. A dirty basin is a breeding ground for sediment, biofilm, and bacteria, which is why basin cleaning is central to both performance and water safety.

The distribution system spreads warm return water evenly across the top of the fill — through pressurized spray nozzles in a counterflow tower or gravity-fed hot-water basins and metering orifices in a crossflow tower. Even distribution is everything: clogged nozzles or fouled distribution basins create dry spots in the fill, which slash heat-transfer capacity and waste fan energy. Keeping the distribution clean and unobstructed is a high-value, frequently neglected maintenance task.

Drift eliminators are baffles near the air discharge that capture water droplets carried along by the exiting air, draining them back into the tower before they escape as 'drift.' They matter for two reasons: water conservation, and — critically — public health, because drift is how aerosolized water (and any bacteria in it) leaves the tower and travels. Modern high-efficiency PVC drift eliminators cut drift to a tiny fraction of the circulating flow. Damaged, missing, or fouled eliminators increase water loss and are a direct Legionella-exposure concern.

In an open-circuit (direct) tower, the condenser water itself is exposed to the air as it cascades over the fill — simpler and lower in first cost, but the process water picks up airborne dirt and requires diligent treatment. In a closed-circuit tower (also called a fluid cooler), the process fluid stays sealed inside a coil while separate spray water and air cool the coil from outside — this keeps the process loop clean, reduces fouling and chemical treatment in that loop, and protects against freezing, at a higher first cost. Which one a building has shapes its treatment burden, freeze strategy, and maintenance routine.

• •Lower first cost and simpler construction
• •Condenser water directly contacts the air
• •Requires diligent water treatment of the process loop
• •Available in crossflow and counterflow designs

• •Process fluid stays sealed and clean
• •Simplified freeze protection for the process loop
• •Lower fouling and treatment in the process loop
• •Higher first cost than open-circuit towers

• •Crossflow: easier access to fill and distribution
• •Counterflow: typically more compact footprint
• •Both available open- or closed-circuit
• •Choice balances serviceability against footprint

Commercial Heat Rejection Is What We Do

Com+ Mechanical services water-cooled plants and cooling towers across the NYC metro — open-circuit and closed-circuit, crossflow and counterflow, on rooftops and at grade. This is a commercial focus on real condenser-water systems, so the diagnosis and recommendation are grounded in how towers actually run and fail.

Water Safety Taken Seriously

Cooling towers carry genuine Legionella risk, and we treat the water-management and cleaning side of tower service as central, not an afterthought — coordinating with your water-treatment program so the tower stays both efficient and compliant with the framework in ASHRAE Standard 188.

Built for Occupied, Multi-Site Buildings

We coordinate roof or yard access, certificates of insurance, tenant and building notice, and crane or rigging windows for component and tower work, and we support property-management companies running one plant or a portfolio across multiple buildings and boroughs — one vendor, consistent documentation, coordinated scheduling.

Efficiency- and Compliance-Minded

We weigh fill condition, fan-drive controls, water chemistry, and approach performance against plant energy use and Local Law 97 emissions exposure, so the tower work we do helps your building's efficiency and compliance position — not just a like-for-like fix.

Inspection & Repair

Tower assessment, root-cause diagnosis, and a scoped repair to the failed components — documented before any work proceeds.

Preventive Maintenance

Planned tower maintenance scoped to your equipment and run hours to protect efficiency, water safety, and equipment life through the season.

Replacement / Project

Turnkey tower replacement or upgrade — equipment selection, rigging, basin and piping tie-ins, controls, and commissioning.

Own a Commercial Property?

Business+ plans start at $499/year — includes 2 rtu tune-ups, 10% off all services, and priority scheduling.

How does a cooling tower actually cool the water?

By evaporation. Warm condenser water from the chiller is spread over a large surface inside the tower called fill while a fan moves air across it. A small fraction of the water evaporates, and because evaporation absorbs a large amount of heat, the water left behind is cooled. The cooled water collects in the basin and returns to the chiller's condenser to absorb more building heat. The important detail is that a tower can cool water close to the outdoor wet-bulb temperature — usually well below the air's dry-bulb temperature — which is what makes water-cooled plants efficient. If you're comparing water-cooled towers against air-cooled equipment, see our air-cooled vs. water-cooled chillers guide.

What's the difference between an open-circuit and a closed-circuit cooling tower?

In an open-circuit (direct) tower, the condenser water itself cascades over the fill and is directly exposed to the air — it's simpler and lower in first cost, but the process water picks up airborne dirt and needs careful treatment. In a closed-circuit tower, also called a fluid cooler, the process fluid stays sealed inside a coil, and separate spray water and air cool that coil from the outside. A closed circuit keeps the process loop clean, reduces fouling and chemical treatment in that loop, and helps with freeze protection — at a higher first cost. Which one fits depends on water quality, freeze exposure, and how clean the process loop needs to stay.

What's the difference between crossflow and counterflow towers?

It refers to how the air and water move relative to each other. In a counterflow tower, water falls straight down while air is drawn straight up against it, through pressurized spray nozzles — typically a more compact footprint for a given capacity. In a crossflow tower, water falls vertically through the fill while air is pulled horizontally across it, usually with gravity-fed hot-water distribution basins that are easier to inspect and clean. Counterflow tends to be more compact; crossflow tends to offer easier maintenance access. The right pick balances footprint against serviceability for your site.

Why does a cooling tower use so much water?

A tower loses water three ways. Evaporation is the largest and is the whole point — roughly on the order of 1% to 2% of the circulating flow for about every 10°F of cooling, which for a busy plant is a lot of water. Drift is a small amount of water droplets carried out with the air, minimized by drift eliminators. Blowdown (or bleed) is water intentionally dumped to keep dissolved solids from concentrating to the point of scaling, governed by the 'cycles of concentration' the treatment program targets — commonly in the range of three to six cycles. Makeup water replaces all of it. Tight treatment and good controls minimize blowdown and conserve water without risking scale or corrosion.

Why are cooling towers a Legionella concern, and what is ASHRAE 188?

Cooling towers are the textbook environment for Legionella bacteria: they hold warm water — the bacteria grow fastest in roughly the 77°F to 113°F range, which overlaps normal tower operation — and they aerosolize a fine mist that can carry bacteria into the air, where drift can spread it. That's why towers are tied to public-health rules. ASHRAE Standard 188 is the U.S. standard that establishes the requirement for a formal written Water Management Program — identifying hazards, setting control measures and limits, monitoring, and documenting corrective action — to manage Legionella risk in building water systems, cooling towers included. Many jurisdictions also impose registration, testing, and reporting requirements on top of it.

What maintenance does a cooling tower need?

Towers need diligent, year-round attention. The core tasks are keeping the water chemistry controlled through a treatment program (biocide, scale and corrosion control, and blowdown), cleaning the basin to remove sediment and biofilm, clearing the distribution nozzles or hot-water basins so water spreads evenly across the fill, inspecting the fill and drift eliminators for fouling and damage, and servicing the fan, motor, gearbox or belts, and bearings. Skipping water treatment or basin cleaning is what turns a tower into both an efficiency problem and a health hazard. A planned program, like the tower coverage in Com+ Mechanical's cooling-tower service, is the reliable way to keep a tower safe and at capacity.

How long does a commercial cooling tower last?

Service life varies widely with construction material and water chemistry, but a well-maintained commercial cooling tower commonly runs in the range of 15 to 20 years or more, with mechanical components like fans, motors, and drives typically replaced sooner along the way. Galvanized steel, stainless steel, and engineered-plastic (HDPE) towers age very differently, and water quality is the single biggest factor — aggressive or poorly treated water shortens the life of basins and fill dramatically. Fill, drift eliminators, and drives are routinely refurbished to extend a tower's useful life.

Can Com+ service or replace just the cooling tower in my plant?

Yes. We service, repair, and replace cooling towers as a standalone component of a water-cooled plant — basin cleaning, fan and drive work, fill and drift-eliminator replacement, distribution repair, and full tower changeouts — and we coordinate with your water-treatment program and the rest of the chiller plant so the whole system runs efficiently and safely. For repair and emergency tower work specifically, see our commercial cooling tower service and cooling tower repair pages, and for the broader plant decision, our chiller guide.