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.
How Cooling Towers Work: The Fundamentals
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
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).
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
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.
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 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.
Recirculation Rate (gpm) x ΔT x Ef / 1,000
MU = Evap (gpm) x (C/(C-1))
C = cycles of concentration, commonly determined by either:
cooling tower chloride/makeup water chlorides concentration
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)
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:
- 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!