What Is a Cooling Tower and How Does It Work?

A cooling tower is a structure that uses water evaporation or airflow to remove excess heat from industrial equipment and HVAC systems. They’re essential for power plants, factories, refineries, processing plants, hotels and other facilities that depend on water circulation to reject heat from the building or machinery.

Cooling towers come in a wide range of configurations so that they can adapt to each facility’s cooling needs and geographic location. In this guide, we’ll explore key aspects you need to know about cooling towers, including:

How cooling towers work

  • What “fill” is and what it does
  • Types of cooling towers
  • Why cooling towers use so much water
  • Cooling tower maintenance
  • How to lower the upkeep cost of your cooling tower

For starters, let’s look at how they keep your equipment and facilities cool.

How do cooling towers work?

Every cooling tower performs the same fundamental process: hot water goes in, cool water comes out. The heat transfer method depends on the type of cooling tower your facility uses, but a lot of the underlying principles and functions are similar.

Wet cooling towers (also known as open-circuit cooling towers) are some of the most common, and they work like this:

Pipes feed hot water from industrial machinery or HVAC systems into the cooling tower.

The hot water drains or gets sprayed into a “spill area,” where some of it evaporates and the rest collects in a basin, reducing the overall temperature of the water in the tower.

Air circulates through the cooling tower from either an induced draft, forced draft, or natural draft, which cools the water that doesn’t evaporate.
The cooled water pools in the bottom of the tower, where it gets recirculated back into your industrial equipment or air conditioning system.

Throughout this process, wet cooling towers lose water in three ways: evaporation, drift (also called “windage”), and blow-down (also called “draw-off”). This lost water is then replaced with make-up water.

Wet cooling towers reduce heat through evaporative cooling. As the hot water evaporates, it gets dispersed into the atmosphere instead of recirculating into your system, so water is lost, but the overall water temperature goes down.

As air flows through the cooling tower, some of the water gets blown out of the tower, so there’s some water loss due to drift.

When the water evaporates, it leaves behind any minerals it contains, such as calcium, magnesium, chloride, and silica. This increases the concentration of those minerals in the remaining water. If unaddressed, this can cause corrosion or build-up in your cooling tower, reducing its effectiveness and lifespan. So wet cooling towers include a system for removing some water as well, and this component is known as “blow-down” or “draw-off.”

Obviously, if this process ran continually like this, you’d eventually lose all your water. So wet cooling towers also require a steady supply of fresh make-up water, which is equivalent to the water loss from evaporation + drift + draw-off.

What about other cooling towers?

Evaporative cooling isn’t the only way cooling towers can remove excess heat. Closed-circuit cooling towers keep the water from your industrial processes in a tube, spray the tube with cooler water, and use a fan to help transfer heat. This protects your water from potential contamination in the cooling tower or make-up water.

What is “fill”?

“Fill” is a material that’s designed to increase the surface area of the water when it arrives in the spill area, and usually increases the time it’s exposed to the air by forcing the water to flow across or through the fill (sort of like a sponge, or a sloped waterfall). Some fill materials are more likely to collect dirt, dust, and minerals from the water, so water quality, your environment, and your needs can impact what type of fill is most suitable for your situation.

You may sometimes hear fill referred to as “splash fill” or “film fill.” This describes the primary way the fill interacts with your water. Film fill is a sheet of material (such as PVC) which forces the water to spread into a thin film. Splash fill disrupts the flow of water to prevent it from splashing. Both types of material increase your hot water’s exposure to the air and improve cooling.

Types of water cooling towers

Classifying cooling towers is a little complicated. A single cooling tower may have multiple classifications, and there are often multiple names for the same classification. It’s important to remember that each classification or category refers to a specific aspect of the cooling process.

Each type of cooling tower has pros and cons for your water management system and is designed for particular applications. To guide you through the different classifications and categories, we’ve divided the types of cooling towers into:

  • The heat transfer method they use
  • The airflow process they use
  • The method of assembly
  • Heat transfer method

You may hear cooling towers referred to by the heat transfer method they use: wet, dry, fluid, closed circuit, open circuit. Here’s what that all means.

Wet (or open-circuit) cooling towers

Wet cooling towers, also known as open-circuit cooling towers, primarily use evaporative cooling to remove heat. Water drains or sprays into a spill area which may or may not use a fill material to increase evaporation and cooling. Evaporative cooling requires lost water to be replaced with make-up water.

They’re called “open-circuit” towers because the cooling process happens within your “water circuit.” Water flows from your machinery into the cooling tower, then back to your machinery.

Fluid coolers

Fluid coolers are a type of closed-circuit cooling tower. They’re called “closed circuit” towers because the cooling process happens externally, without directly touching your water or exposing it to the air. Hot water is still diverted to the cooling tower through pipes, but when it enters the tower it remains in some form of tubing. Fluid coolers then spray this tubing with cold water and use a mechanical draft to increase cooling.

Closed-circuit cooling towers don’t rely on evaporative heating or expose your water to the open air, so there’s no water loss within your supply.

Dry cooling towers (or air-cooled condensers)

Dry cooling towers are another type of closed-circuit tower. They function similarly to a fluid cooler, but without spraying water on the tubing—which means this cooling tower doesn’t lose or use any water at all. These towers rely on airflow (either mechanical draft or natural draft) to cool the tubing. This allows the heat to transfer from the hot water to the tubing and then to the air.

Dry cooling towers are best suited for facilities in arid locations, where water is scarce, expensive, or too essential to the environment.

Hybrid cooling towers

Hybrid cooling towers are highly versatile, and can switch between dry and wet cooling. This prevents a facility from becoming dependent on water-based cooling in climates where the availability or cost of water fluctuates drastically with the seasons.

Airflow system

Cooling towers are sometimes referred to by the interaction between the air flow and water flow: crossflow and counterflow. They may also be called by the method they use to create that airflow: mechanical draft, induced draft, forced draft, natural draft, and fan-assisted natural draft.

Here’s what those categories mean.

Crossflow cooling towers

Crossflow cooling towers collect your hot water in basins on top of the tower, then use gravity-fed distribution to drain it into your fill material. As water passes through the fill, air flows horizontally, perpendicular to the water flow. The water drains down, and the air flows across—hence, crossflow.

Since crossflow towers rely on gravity to disperse hot water into the fill material, they simplify variable flow and can easily adjust to “low flow” events while still evenly distributing your water. Uneven distribution can create channels in the fill material, where water flows in fixed streams and has a greater risk of freezing in cold weather or scaling from mineral deposits.

These towers generally have a larger plenum space (the area air flows through), and as a result they’re easier to conduct maintenance on. In a crossflow cooling tower, specialists have plenty of room to maneuver and inspect your cool water basin, drift eliminators, and other key components. They may even have full-size access doors and mechanical platforms for working on the fan, gearbox, and other hard-to-reach parts.

Perhaps most importantly, the gravity-fed distribution system allows you to use smaller pumps and requires less maintenance, so crossflow cooling towers tend to cost less—both initially and over time.

Counterflow cooling tower

Counterflow cooling towers draw in air from underneath the fill material and direct it vertically, so the air flows up and the water flows down—hence, counterflow. Since the air flows up through the tower, you can’t rely on gravity to distribute the hot water, and counterflow cooling towers use pressurized nozzles to spray the water onto the fill.

The main advantage of counterflow cooling towers is that they’re more efficient and tend to take up less space (compared to crossflow towers with the same cooling capacity). The pressurized spray system breaks up the water into finer drops, so it’s easier to cool. They may also give you more options for fill material—which your water quality may require.

The downside is that despite being more efficient, counterflow cooling towers actually cost more because of the greater pump requirements and maintenance related to the pressurized water distribution system (the spray nozzles). Additionally, when water flow is low, it disrupts the spray from the nozzles, causing uneven distribution and channeling in the fill material—thus increasing the risk of ice build-up in cold weather or scaling from high mineral concentrations. So regular maintenance becomes more essential, and problems are harder to spot (because counterflow cooling towers aren’t as accessible as crossflow towers). They also produce more noise as the water sprays from the tower ceiling and tends to have farther to fall.

Induced draft

“Induced draft” is a type of mechanical draft which uses a large fan on top of the tower to draw air up toward the ceiling. While the fan is larger than what you’d use on a forced draft tower, it uses a smaller motor to produce the same water-cooling capacity. With induced draft, air enters the cooling tower slowly and exits quickly. This reduces the risk that air will recirculate, which makes the cooling process less effective because the air exiting the tower is warmer.

Forced draft

“Forced draft” is a type of mechanical draft which uses a fan or fans at the bottom or side of the tower to drive air into the cooling system. This air enters quickly and exits slowly, which increases the risk of recirculation. Since this fan is exposed to the entering air (which is cooler), it’s also at a greater risk of problems in freezing temperatures. However, forced draft generates more static pressure, which makes it more suitable for confined (and sometimes indoor) spaces.

Natural draft

Natural draft uses the architecture of the cooling tower and the principles of air density to create a natural air flow. Natural draft cooling towers are tall, hyperboloid structures, often used in power plants. As air enters the cooling tower, it gets hotter and more humid, and naturally rises up through the tower like smoke through a chimney.

Fan-assisted natural draft

A fan-assisted natural draft looks a lot like a forced draft setup, but it doesn’t rely on the fan or fans to propel the air up through the cooling tower. The fans help circulate air into the tower, but the system primarily uses architecture and the changes in air density to create the draft.

Method of assembly

While cooling towers come in many different configurations, there are two main ways they’re actually built: in a factory, or on-site. Each offers advantages for particular situations.

Factory-assembled towers

Some cooling towers are put together in a factory and then shipped wherever they’re needed. These preassembled units are convenient, but have less capacity—they have to fit on trucks, after all. Due to their size, they’re more commonly used in food processing plants, hotels, and other facilities that don’t generate as much heat as say, a power plant. (They’re very common for cooling in HVAC systems.) Since packaged cooling towers are often used in residential areas, they tend to place greater emphasis on noise-reduction features such as a gravity-fed water distribution system or induced draft, but they can be configured to meet the needs of your facility.

Field-erected towers

Facilities that produce more heat often need towers with greater capacity than what manufacturers can preassemble and ship. So power plants, refineries, steel processing plants, and other large industrial facilities use field-erected towers, which can be as large as they need to be. The manufacturer typically provides labor and expertise to assemble cooling towers on-site.

Why do cooling towers use so much water?

Most cooling towers use water to operate. And some cooling towers use a lot of water.

Depending on the cooling needs of the building or process, the size of the cooling tower dictates how much water is used and heat rejection can be provided. Cooling towers use tonnage as a rating. Typically, the larger the tonnage, the more water the cooling tower will use.

A fluid cooler constantly sprays water on the tubing that contains your hot water, so the heat transfers from the hot water inside the tubing to the cool water outside the tubing. The sprayed water never enters your water supply (which is why these are also called closed-circuit cooling towers), and it continually needs to be replenished.

Wet cooling towers use evaporative cooling and expose your hot water to the open air, so your system is constantly losing water through evaporation and drift as vapor and droplets escape through the top of the tower. Additionally, as the water evaporates, it leaves behind minerals and increases their concentration in your water supply, which—if left alone—could be harmful to your cooling tower and other equipment. To address this issue, wet cooling towers also remove some of the water from your system. As mentioned previously, this water is called blow-down or draw-off.

To compensate for these three ways in which your wet cooling tower loses water, it uses more. Wet cooling towers constantly add fresh water—called make-up water—to ensure that the same amount of water always returns to your water supply. The amount of make-up water always has to equal your evaporation + drift + blow-down.

Common problems with cooling towers

Cooling towers are a vital part of your water management system. If a single component stops working, it can drastically reduce your ability to efficiently reject heat and keep your equipment or facility cool. When your cooling tower works less efficiently, it uses more power and costs more money. These problems can also quickly snowball into disasters, interfering with your industrial processes and damaging or destroying your machinery.

So it’s important to be familiar with the problems you can run into with cooling towers and what all goes into maintaining them.

Scaling

As minerals like calcium, magnesium, silica, and chloride build-up in your cooling tower, it can form layers called “scaling.” This is more common in open-circuit cooling towers which use evaporative cooling, as the water (and therefore the minerals it contains) come into contact with your heat exchange surfaces. Scaling must be removed regularly, or your system has to work harder to transfer heat. A key way to avoid scale is to ensure a proper and continuous flow of water into the system. In the event of an obstruction or blockage to the make-up water, low water flow can quickly cause scale and damage to the cooling media.

Contamination

Due to their high humidity and mineral concentrations, cooling towers can become a breeding ground for harmful bacteria that can damage or reduce the effectiveness of your equipment. In your cold water basin, you may sometimes find a thin film of bacteria. This “biofilm” inhibits the heat transfer process, making your tower less efficient. When this happens, you need a specialist to take a water sample, then purge the bacteria from your water supply by flushing it with chlorine. You’ll repeat this process until all the bacteria is gone, or it will just come back.

Fouling

As water and outside air passes through your cooling tower, it may bring dirt, debris, and dust into contact with your fill. As it builds up, it can clog your fill material and prevent water from flowing through the way it was designed. This can create channeling, which can lead to freezing in cold temperatures, but at the very least, it reduces your cooling tower’s efficiency.

Freezing

Cooling towers that operate in extremely cold weather run the risk of ice build-up. As ice forms, it adds weight to your tower. Too much ice can damage the structural integrity of your tower or even cause it to collapse altogether. To prevent ice build-up, it’s important to constantly monitor your tower’s heat, use variable-speed motors, and keep water flowing over the fill as designed. Even though your cooling tower is intended to cool water, having a basin heater is important to keep that cool water from freezing.

Corrosion

As sodium and other minerals pass through your pipes, they can gradually break down the metal. This is a bigger concern in closed-circuit cooling towers (such as fluid coolers), where it’s harder to spot corrosion early on. Corrosion in your pipes can cause leaks in your cooling tower or other parts of your water supply. Protective lining on your pipes will increase their lifespan by preventing corrosion, but you can also add chemicals to your water treatment or increase the pH balance of your water.

Leaks

Leaks are often hard to spot, which means they can be extremely expensive. Imagine how much water can leak in a single day of operation, and how much more water your cooling tower may have to pump in order to compensate for the leak. You should check your basin float levels and system valves regularly—don’t wait for your monthly water bill to learn you’ve been losing thousands of gallons of water due to a leak! (Check out how much one leak cost this company.)

Did you know?

Apana’s water management solution uploads data from your water network to the cloud in real-time. Our analytics tools will send you alerts and actionable guidance if there’s unusual activity. You’ll know the moment there’s an issue associated with your cooling tower, and have next steps to resolve the problem.

Loss of water flow

Fouling, corrosion, freezing, and other issues can lead to loss of water flow, where water doesn’t pump into or move through your cooling tower the way it’s designed. Loss of water flow can cause major disasters like emergency shutdowns of your equipment, hydrocarbon leaks, and even fires when machinery doesn’t have enough cool water circulating through it. If you don’t detect it soon enough, loss of water flow can cause irreparable damage to some of your most expensive equipment. This also inevitably disrupts your regular industrial operations, as you have to perform unscheduled maintenance or outright replace major pieces of equipment.

Apana’s water management system doesn’t just detect leaks—it can also notify you when water isn’t flowing properly. A single one of these early alerts can save you thousands of dollars in prevented damage and lost time.

How to lower the upkeep cost of your cooling tower

Cooling towers are a key component of most buildings, industrial processes and major HVAC systems. They’re highly efficient, but with the amount of water they go through, they also have high upkeep costs. Some configurations are less cost-effective than others—forced draft and pressurized water distribution systems require more power, for example—but all of them require regular maintenance to operate efficiently.

If you have a wet cooling tower or fluid cooler, however, you may be able to lower your corporate water bill by tracking how much water your cooling tower uses. Part of your water bill is a sewer fee, which assumes all the water your facility uses is being drained into the sewers. But with cooling towers that use evaporative cooling, a lot of the water your cooling tower uses never makes it to the sewer—it’s released into the atmosphere.

As long as you regularly document how much water is passing through your cooling tower’s make-up and draw-off lines, your provider will likely reimburse part of your sewer fee. You could send someone around to manually record the numbers from your water meters. But with Apana’s intelligent water management platform, our technology uploads real-time data from your water meters, so you get automatic readings of how much water is passing through each line, which you can conveniently access in our analytics software.

Posted in Blog