Cooling towers are an essential component of most industrial and commercial facilities. They cool water used for a variety of processes and applications. In a future post, we will be discussing the role cooling towers play in your facility’s sustainability efforts, but today we want to answer the question: how do cooling towers work? Let’s start with the basics.

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The Fundamentals of How Cooling Towers Work

Cooling towers are specialized heat exchangers that are designed to remove waste heat by creating steam. Their function is to pull heat from the water and return cold water to cool down industrial equipment. In a cooling tower, heat is transferred via sensible heat and latent heat.

• Sensible heat: heat transfer related to changes in water temperature
• Latent heat: heat transfer related to changes in physical state

With cooling towers, most of the heat is transferred to the atmosphere through recirculating cooling water evaporation. Evaporative cooling is frequently used to remove large quantities of heat from processes, equipment, and living spaces. This is shown in the following Heat Transfer Equation:

Q = LHe x m

Where:   Q = evaporative heat (heat loss)

                LHe = latent heat of vaporization/evaporation of water (Btu/lb.)

                m = mass of water

The Role of Water in the Cooling Process

Water is used as the medium in cooling towers. A relatively large quantity of heat is released (1,000 Btu’s) for every pound of water evaporated. When dealing with water, heat transfer is approximately 1,000 times more efficient through evaporation versus sensible heat. The actual value is slightly lower (970.4 Btu’s), but throughout the industry, the 1,000 Btu per pound value is universally accepted.

Cooling towers are designed to create effective heat transfer by connecting air and water in an efficient and expedient manner. Anywhere from 75–95 percent of the heat from the process, equipment, or buildings is removed through evaporation and only 5–25 percent is removed via convection.

The concept of wet bulb and dry bulb temperatures should be considered. Wet bulb temperature is defined as the lowest water temperature at which heat can be removed via evaporation. The lower the wet bulb temperature, the lower the relative humidity, and the more efficient the cooling tower is at heat removal.

Cooling Tower Designs

There are three different tower designs. The right type is determined based on the size of the system/application, geographical location/climate, water quality, and local utility costs.

• Natural draft: large hyperbolic, typically found at power plants
• Cross-flow: induced draft or forced draft
• Counter-flow: induced draft or forced draft

Evaporation

When the recirculating water in a cooling tower evaporates, it theoretically exits as a pure vapor. An extremely small percentage of this vapor carries tiny water droplets containing dissolved solids. This is known as cooling tower drift. At this early point in our discussion, cooling tower drift will be considered out of scope.

Cycles of Concentration

The sequential diagrams in Figure 1 illustrate what we mean by cycles of concentration. As more water is evaporated, the quantity of dissolved solids stays consistent, so their concentration increases.

Cooling Tower Approach

• The difference between the cold sump temperature and the wet bulb temperature is called the cooling tower approach.
• The temperature difference between the hot return water and the cold sump water is referred to as the cooling range (ΔT).

Evaporation Equation

The equation in Figure 2 helps determine how much water is being evaporated (E) in terms of gallons per minute.

Note: The term ΔT is the actual measured temperature drop across the tower and not the design temperature.

Cooling Tower Internals

Counter-Flow

Hot return water and air flow are in direct opposition to each other. Drift eliminators introduce a difficult path for water droplets to navigate. Most water droplets cannot find their way to the atmosphere and fall back into the cooling tower basin, leading to decreased annual water consumption.

Splash-Fill

Although it is not considered the most efficient, splash-fill is still used in many applications, including poor quality makeup water or towers serving applications with a high potential for cooling water process contamination. Under these conditions, the higher efficiency fill may foul and require frequent chemical cleanings.

Film-Fill

Film-fill is most frequently used and widely known. It is the familiar PVC-type construction, with the characteristic waffle pattern that spreads the cooling water over a larger surface area for contact with the circulating air flow. Film-fill can be used in applications where the makeup water quality is good and hot return water temperatures never exceed 140°F. 

Cooling Tower Mass Balance Calculations

In addition to the equations below, cycles of concentration (COC) can also be calculated by using the ratio of makeup water to blowdown.

Evaporation (gpm)

Recirculation Rate (gpm) x ΔT x Ef / 1,000

Makeup (gpm)

MU = Evap (gpm) x (C/(C-1))

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C = cycles of concentration, commonly determined by either:

cooling tower chloride/makeup water chlorides concentration

or

Cooling tower magnesium/makeup water magnesium concentration

Use magnesium if chlorine, hypochlorite, bromine, or chlorine dioxide is used as an oxidizing biocide.

• Blowdown = Makeup – Evaporation (all in gpm)

• Bleed (Intentional Water Loss) = Blowdown – Drift
  • Drift = 0.01 to 0.3% for mechanical draft towers
  • Drift = 0.3 to 1.0% for natural draft towers

• Half Life (Holding Time Index) (hr.) = 0.693 x V/BD (time to 50% depletion)

• Half Life (hr.) = 2.303 x V/B x Log10 Ci/Cf
  • V = System Volume
  • BD = Blowdown
  • Ci = Initial additive concentration

Cf = Final additive concentration

Choosing the Right Location for Your Cooling Tower

If possible, adhere to the following recommendations when choosing a cooling tower location:

• Do not place near contaminant sources
• Place where sump can be easily cleaned
• Avoid dead legs to reduce potential deposits and corrosion
• Minimize sunlight to reduce potential algae growth
• Add side-stream filters to remove solids (5–7% of flow)

Free-Cooling System

Through manipulation of condenser and chilled water header isolation valves, the system can be set up to have cooling tower water flow bypass the chiller (which is secured/not running) and flow directly through the chilled water piping/coil to the air handler.

The water from the air handler is then returned to the cooling tower to repeat the cycle. Not running the chiller compressor and chilled water loop circulation pumps helps conserve electrical power usage. The required outside ambient air temperature for free cooling is 40–45°F.

General Operational Guidelines

• Maintain flow through off-line equipment or remove it from service properly
• Stop process leaks quickly
• Aim to stay within recommended cooling water treatment control parameters

It’s important to stay vigilant and keep the following in mind:

• Operation
• Water treatment
• Preventive and corrective maintenance (documentation)
• Statistical trending of key performance indicators and testing parameters
• Automated system monitoring, control, and communication
• Well-designed and executed calibration program

Many factors contribute to the efficiency of your cooling system. As with all other technologies, due diligence is necessary when determining the feasibility of utilizing new methods. Always consult your equipment manuals and guides, and don’t forget to contact ChemTreat’s experienced team for assistance!

5 Steps to Better Cooling Tower Maintenance

on August 3, (last updated: February 15, ) in Cooling Tower Cleaners, HVAC, Legionnaires Disease

What are the Five Steps to Better Cooling Tower Maintenance?

These five steps can lead you to better Cooling Tower maintenance. Cleaning cooling towers is a complete process, not just one task. To prevent these massive systems from failing, you must prioritize regular maintenance. To maximize your maintenance efforts, you will have to make sure to provide attention to all components of the system. To help facilities optimize the process of cooling tower maintenance, here are some steps to ensure adequate cleaning of your cooling tower system.

1. Inspect towers

Inspect towers at least monthly. Sediment, scale, and slime can lead to buildup and help Legionella grow and thrive. Regular inspections will help determine when to schedule cleaning.

2. Disinfect the Tower

Cooling towers can be a dangerous breeding ground for bacteria, making it essential to reduce the nutrients available for Legionella growth. BioSpray® Tower can kill 99.9% of Legionella and is on the EPA list of disinfectants for use against SARS-COV-2 when appropriately applied on hard non-porous surfaces. It offers a labor-saving, no-rinse formula. Protocols for the disinfection of Legionella are well defined by the Occupational Safety and Health Administration, the Centers for Disease Control and Prevention, and the U.S. Environmental Protection Agency.

3. Remove Sludge In Tower Basin

The most common area is the basin where sludge can accrue and can impact the longevity and performance of the cooling tower system. A cooling tower fill cleaner will remove dirt and debris and inhibit the growth of other bacteria when utilized in conjunction with appropriate biocides. You can use specialized vacuums to remove the sludge in cooling towers. 

4. Fix Clogs and Fouling in the Fill

The most predominant part of the heat exchange within a cooling tower system occurs in the cooling tower fill. The fill is what leads the passing air in the system to produce cold water and remove heat efficiently. When the fill becomes fouled or clogged, the cooling doesn’t take place because the air can’t touch the heat source as planned. As a result, it generates higher electricity bills. If sediment is visible, the basin needs to be cleaned, and cooling tower vacuums make it easier to remove contaminants without shutting down or draining your system. 

5. Descale the Fill

The tower fill provides the perfect environment for bacterial growth, especially Legionella. A descaler will dissolve the scale and mineral deposits that Legionella likes to hide in, leaving the surface ready for cleaning.

What Are The Results?

Routine maintenance of cooling towers reduces the risk of system damage and health issues and increases safety and health, durability, performance, and efficiency. Failure to do so can incur high costs and, even worse, someone’s life is put in danger due to dangerous bacterial growth. Regular checks help you use your equipment to the fullest while also creating a safe, compliant, and clean environment.

1. Safety And Health

Legionella or other microbe-borne diseases can spread rapidly through air conditioning systems. One of the most common sources for Legionnaires’ disease is an infected cooling tower, and poorly maintained towers can lead to a potential outbreak. The warm waters in these buildings create perfect conditions for bacteria like algae or mold, which help grow dangerous pathogens such as Legionella that could spread through the air released when a cooling tower is in use. 

2. Performance

A fully functioning cooling tower maintenance program ensures detection of problems early on and assures optimum performance during its lifetime. Frequent maintenance enables you to discover issues at an early stage that can cause excessive or undue wear and tear on your cooling tower systems, allowing for prompt correction.

3. Efficient And Economical

A cooling tower working at optimum performance delivers the desired results. However, minor faults or damages can affect the undertaking, and if overlooked, it can lead to expensive repairs and high running costs.

The Bottom Line

Whether you require reactive cooling tower maintenance or are planning to develop a routine preventative maintenance schedule, it’s imperative to have sound knowledge about cleaning cooling towers. What is going to make your maintenance easier to complete is the right tools. Our Cooling tower basin cleaning systems provide a solution that allows cleaning without having to shut down towers. When cleaning cooling tower fills, industrial descaling chemicals paired with cooling tower fill cleaning systems will provide complete product from efficiency-robbing foul and dangerous bacteria. Our cooling tower products help you keep cooling towers clean and meet ANSI/ASHRAE Standard 188-, a standard for maintenance and the remediation of Legionella in commercial building water.

Next Steps:

Click here to use our Cooling Tower Cleaner Buying Guide to choose the right system for your cooling tower needs.