May. 26, 2025
Energy
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To determine the perfect cooling tower design and size for your needs, Delta makes it easy with our downloadable sizing program. To discover Delta’s cooling tower sizing & selection program, simply click here.
If you are interested in learning the methods of determining the proper size cooling tower, rest assured that Delta is here with guidance. Explore our handy information. Click here to learn about sizing & selecting.
Whether your application is for industrial process cooling or HVAC condenser cooling, the data required is the same. The following design data is required for cooling tower sizing to properly select the appropriate model:
The Design Heat Load is determined by the Flow Rate, and the Range of cooling, and is calculated using the following formula:
Heat Load (BTU/Hr) = GPM X 500 X Range (T1 – T2) °F
If the range of cooling, Heat Load, and one of the other two factors are known (either the GPM or the ° Range of cooling), the other can be calculated using this formula.
The range of cooling is directly proportional to the Heat Load.
How comfortable are you working up a cooling tower selection?
The cooling tower selection table may look confusing, but after you have made a few selections, the process is straightforward. If you need a refresher, this may help. The following design data is required to select cooling towers:
Flow Rate in GPM
Range of cooling in °F (T1 - T2)
Area Wet Bulb Temperature in °F (Twb)
The Design Heat Load is determined by the Flow Rate, and the Range of cooling, and is calculated using the following formula: Heat Load (BTU/Hr) = GPM X 500 X ° Range of cooling.
More importantly, if the Heat Load and one of the other two factors are known, either the GPM or the ° Range of cooling, the other can be calculated using this formula.
For example: GPM = Heat Load (BTU/Hr), or 500 X ° Range of cooling ° Range of cooling = Heat Load (BTU/Hr) 500 X GPM
So, as you can see, the Design GPM and the ° Range of cooling, are directly proportional to the Heat Load.
And, 500 is the “fluid factor” which is based on water as the heat transfer fluid. The fluid factor is obtained by using the weight of a gallon of water (8.33 lbs.) multiplied by the specific heat of the water (1.0) multiplied by 60 (minutes/hour).
The first step in selecting a cooling tower is to determine the Nominal cooling tower load. Since a cooling tower ton is based on 15,000 BTU/Hr, the formula is:
Nominal Load = GPM X 500 (Constant) X ° Range of cooling, 15,000 BTU/Hr/Ton or, the more simplified version of the same formula, Nominal Load = GPM X ° Range of cooling 30
Once the Nominal cooling load has been calculated, a Correction Factor must be determined to calculate the Actual Rated cooling tower tons required for the specific conditions of service. The correction factor adjusts for the ease or difficulty of cooling based on the Theoretical Design of all cooling towers.
The Nominal Ton Correction Factor is determined by using the COUNTERFLOW COOLING TOWER SELECTION AND PERFORMANCE CHART enclosed. Note that the curves are shown as three separate sections. The WET BULB CORRECTION SECTION, the APPROACH SECTION, and the CAPACITY MULTIPLIER FACTOR SECTION. First, find the Range line in the WET BULB CORRECTION SECTION in the upper left-hand section of the chart. Move along the Range line over to the intersection of the Wet Bulb line.
Now move down along the Wet Bulb line to the APPROACH SECTION, in the lower left-hand section of the chart, and stop at the intersection of the Approach line. Move across to the CAPACITY MULTIPLIER FACTOR SECTION to the right-hand curves and stop at the intersection of the Range line and read the CAPACITY MULTIPLIER FACTOR.
The Actual Rated cooling tower tons can now be calculated by multiplying the Nominal cooling tons, which was previously calculated, by the CAPACITY MULTIPLIER FACTOR. The Actual Rated cooling tower tons is the capacity required for the specific conditions of service, and the next largest size cooling tower should be selected for the application.
Following are selection examples for three different applications. One example is based on conditions that are identified as "Theoretical Design," for reasons which will become apparent.
The second example, entitled "Actual Design" is a selection based on adjusting from Theoretical to Actual design.
The third example, "Modified Application", converts an actual once-through well water system to a cooling tower recirculation system.
The following conditions are provided for selection purposes:
The operating water flow rate is 600 GPM.
Hot water temperature (T1) to the cooling tower is 95° F.
Cold water temperature (T2) desired from the cooling tower is 85° F.
The installation location's wet bulb temperature (Twb) is 78° F.
You can now make a cooling tower selection with this information:
The water flow is 600 GPM. The Range of cooling is 10° - (T1 - T2). The Approach to the wet bulb temperature is 7° - (T2 - Twb).
First the cooling tower NOMINAL load has to be determined:
Nominal Load = GPM x 500 x ° Range, = GPM x ° Range, therefore, 15,000 BTU/Hr 30
Nominal Load = 600 gpm x 10° Range = 200 tons of cooling required.
30 Since the Heat Load = Flow (gpm) x 500 x °Range of cooling= 600 gpm x 500 x 10° = 3,000,000 BTU/Hr and a cooling tower nominal ton = 15,000 BTU/Hr, the nominal cooling tower ton is derived from the actual heat load. Therefore, a heat load of 3,000,000 BTU/Hr = 200 nominal cooling tower tons.
Now the Nominal Ton Correction Factor has to be determined for the conditions established:
A 10° Range of cooling, and a 7° Approach to the design wet bulb temperature of 78°F, using the COUNTERFLOW COOLING TOWER SELECTION AND PERFORMANCE CHART enclosed.
Find the 10° Range line in the WET BULB CORRECTION SECTION in the upper left-hand section of the chart. Move along the 10° Range line over to the intersection of the 78° Wet Bulb line.
Move down along the 78° Wet Bulb line to the APPROACH SECTION, (the lower left-hand section), and stop at the intersection of the 7° Approach line.
Move across to the CAPACITY MULTIPLIER FACTOR SECTION to the right-hand curve and stop at the intersection of the 10°Range line, and read the CAPACITY MULTIPLIER FACTOR, which is 1.0.
To select the proper cooling tower for this application, multiply the 200 Nominal tons calculated, by the 1.0 CAPACITY FACTOR. As previously stated, the correction factor adjusts for the ease or difficulty of cooling in relation to the Theoretical Design. So in this case, since the CAPACITY CORRECTION FACTOR is 1.0, the Nominal and Actual Rated tons are the same as the Theoretical Design, and a Model DT-200I cooling tower can be quoted.
Now we will select a cooling tower for the same 200-ton Nominal Load as Example #1 but is different from the Theoretical Design. The operating water flow rate is 300 GPM.
Hot water temperature (T1) to the cooling tower is 105° F.
Cold water temperature (T2) desired from the cooling tower is 85° F. The installation location's wet bulb temperature (Twb) is 76° F.
You can now make a cooling tower selection with this information:
The water flow is 300 GPM. The Range of cooling is 20° - (T1 - T2). The Approach to the wet bulb temperature is 9° - (T2 - Twb).
First, the cooling tower NOMINAL load must be determined:
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Suggested reading:Nominal Load = GPM x 500 x ° Range, = GPM x ° Range; therefore, 15,000 BTU/Hr 30. Nominal Load = 300 gpm x 20° Range = 200 cooling tons required. 30 Since the Heat Load = Flow (gpm) x 500 x °Range of cooling= 300 gpm x 500 x 20° = 3,000,000 BTU/Hr and a cooling tower nominal ton = 15,000 BTU/Hr, the Nominal cooling tower ton is derived from the actual Heat Load. Again, a 3,000,000 BTU/Hr heat load = 200 Nominal cooling tower tons.
Now the Nominal Ton Correction Factor must be determined for the conditions established; a 20° Range of cooling, and a 9° Approach to the design wet bulb temperature of 76°F, using the COUNTERFLOW COOLING TOWER SELECTION AND PERFORMANCE CHART enclosed.
First, find the 20° Range line in the WET BULB CORRECTION SECTION in the upper left-hand section of the chart. Move along the 20° Range line over to the intersection of the 76° Wet Bulb line. Move down along the 76° Wet Bulb line to the APPROACH SECTION, in the lower left-hand section of the chart, and stop at the intersection of the 9° Approach line. Move across to the CAPACITY MULTIPLIER FACTOR SECTION to the right-hand curves and stop at the intersection of the 20° Range line, and read the CAPACITY MULTIPLIER FACTOR, which in this case is 0.62.
The final step to select the proper cooling tower for this application is to multiply the 200 nominal cooling tons required, which was calculated above, by the CAPACITY FACTOR, which in this case is 0.62. The cooling tower Actual Rated tons for the conditions given are therefore 124 tons, and a Model DT-125I cooling tower can be quoted. Since the correction factor adjusts for the ease or difficulty of cooling based on the Theoretical Design, in this case, the Actual Rated tower conditions are easier than Theoretical Design.
The following is an example of modifying a "once through non-recirculating cooling application" to a recirculating cooling tower system. A cooling tower is required for heat exchanger process cooling, which is now being cooled using 55°F well water at a flow rate of (1 Million gallons/day - 300,000 sanitary = 700,000 gal per day).
Approximately 500 GPM, and discharging to a lake at 80°F. With this information we can establish the Heat Load, which is 500 GPM x 500 x 25° R (80°F - 55°F) = 6,250,000 Btu/Hr.
We can establish the cooling tower design for a 6,250,000 Btu/Hr Heat Load based on the installation location design Twb, which, for this example, we'll say is determined to be 76°F, and by establishing a reasonable cold water temperature at a 7° Approach to the Twb, at 83°F.
What we have to determine now is either the design range of cooling or the appropriate design flow rate based on the established Heat Load. Let’s select the appropriate design flow rate by using a reasonable 15° Range of cooling; 83°F cold water + 15° = 98°F hot water.
Use the Cooling Tower Heat Load Calculation to find the design flow rate as follows:
Heat Load (BTU/Hr) = GPM X 500 X ° Range of cooling, or rearranged to determine the design flow rate. GPM = Heat Load (BTU/Hr) = 6,250,000 Btu/Hr = 835 gpm 500 X ° Range of cooling 500 x 15° R Now you can make your cooling tower selection based on 835 gpm, cooling from 98°F to 83°F @ a design 76°F Twb. The cooling tower selection is = 418 Nominal Tons x .83 DCF = 347 Rated cooling tower tons, or a 350-ton cooling tower selection.
Alternate #1:
A commercial cooling tower can also be selected for this heat load based on a 25° Range of cooling. The conditions for selection would be 500 GPM, cooling from 108°F to 83°F @ 76°F Twb, which is equal to 418 Nominal tons x .62 DCF = 259 Rated cooling tower tons, for a 260 ton cooling tower requirement.
Alternate #2:
Or select for a design to cool 110°F to 83°F = 27° R of cooling, the design flow would be 6,250,000 Btu/Hr = 465 GPM. 27° R x 500
The selection for 465 GPM cooling from 110°F to 83°F @ 76°F Twb = 418 Nominal tons x .58 DCF = 242 Rated tons; so you can recommend a single Model DT-250I cooling tower.
Designing a cooling tower involves several critical considerations to ensure it meets the operational demands of the application while being energy-efficient, cost-effective, and environmentally responsible. Here are some essential design aspects:
- Natural Draft: Ideal for large-scale power plants where low operating costs and high cooling capacities are needed, though they require significant space and are typically limited to certain large-scale applications.
- Mechanical Draft:
- Induced Draft: Most commonly used in industrial and HVAC applications; they allow more precise control of air flow and cooling performance.
- Forced Draft: Preferred in confined spaces or buildings where low profile and flexibility in installation are essential.
- Heat Load: The cooling tower must be designed to handle the maximum anticipated heat load, based on the amount of heat that needs to be removed from the process.
- Temperature Range: This is the difference between the hot water temperature entering the tower and the cooled water temperature leaving the tower. A larger range typically allows for a smaller, more efficient cooling tower.
- Approach Temperature: The difference between the cooled water temperature and the wet-bulb temperature. A smaller approach temperature indicates a more efficient cooling tower, as it cools water closer to ambient wet-bulb temperature.
- Wet-Bulb Temperature: The ambient wet-bulb temperature influences cooling tower design, as it represents the lowest temperature achievable through evaporative cooling.
- Air Flow: Efficient airflow is essential for proper heat transfer. The airflow design depends on the cooling tower type (counterflow or crossflow) and the fan system (axial fans in induced draft or centrifugal fans in forced draft).
- Water Distribution: The water distribution system should provide even water coverage over the fill media to maximize heat exchange efficiency. This includes selecting nozzles and spray systems that prevent clogging and ensure uniform water flow.
- Film Fill: Uses thin sheets to create a large surface area for heat exchange; it's highly efficient but can be more susceptible to fouling.
- Splash Fill: Breaks water into droplets, creating airflow contact and maximizing cooling. It’s more resistant to fouling and suitable for water with high levels of solids.
- Choosing Fill Material: The fill material should be resistant to corrosion, biofouling, and capable of withstanding temperature fluctuations.
- Scale and Corrosion Control: Consideration must be given to water quality, as it influences the need for chemical treatment systems to prevent scaling, corrosion, and fouling.
- Biological Growth Prevention: If untreated, biological growth can reduce efficiency and pose health risks, such as Legionella. Designing for easy integration of water treatment systems, such as chemical feeders or UV disinfection, can help manage water quality.
- Corrosion Resistance: The choice of materials for the tower casing, fill, piping, and structural components depends on the water’s chemistry and environmental conditions. Options include stainless steel, fiberglass, and corrosion-resistant plastics.
- Durability: Durable materials extend the lifespan of the cooling tower, reducing maintenance costs and downtime.
- Drift Eliminators: These reduce the loss of water droplets to the atmosphere, which minimizes water wastage and reduces the risk of Legionella spread.
- Noise Control: Noise from fans and falling water can be a concern in urban or residential areas. Installing low-noise fans, using sound attenuators, and placing the tower at a strategic location can help minimize noise impact.
- Variable Frequency Drives (VFDs): VFDs on fan and pump motors allow for variable operation, reducing energy consumption by adjusting to cooling demands in real-time.
- High-Efficiency Fans and Motors: Efficient motors and fan blade designs reduce energy consumption and improve performance.
- Cycle of Concentration (COC): Increasing the COC through water treatment reduces blowdown frequency, conserving water and energy.
- Water Conservation: In areas of water scarcity, towers should be designed for optimal water usage, with systems to recycle and reduce water loss.
- Air Quality Standards: Drift and thermal emissions should comply with local environmental regulations.
- Eco-Friendly Design: Consider using materials and treatment processes that are safe for the environment, and if possible, integrate systems to capture heat or re-use water in other processes.
- Space Constraints: Cooling towers vary in footprint and height, so it’s essential to select a model that fits the site without compromising efficiency or accessibility for maintenance.
- Prevailing Winds: The tower should be positioned to minimize recirculation of warm, moist air back into the system.
- Accessibility for Maintenance: Ensure space for technicians to access and maintain all components, such as fans, pumps, and fill media.
- Hybrid and Dry Cooling: In regions with extreme temperatures or water scarcity, hybrid cooling towers that combine wet and dry cooling can adapt to changing conditions and reduce water consumption.
- Cold-Weather Operation: In colder climates, ensure the design accounts for freeze protection, such as heated basins, insulated piping, or operational strategies to prevent ice formation.
- Initial Costs: Higher efficiency and environmentally friendly designs can be more costly upfront, but may yield savings in energy, water, and maintenance over time.
- Operating and Maintenance Costs: Designs that include energy-efficient features, such as VFDs and high-efficiency fans, help reduce operating costs. Regularly scheduled maintenance should be factored into the design for ease and cost-effectiveness.
- Lifecycle Cost Analysis: Evaluating the tower’s total cost of ownership, including installation, maintenance, operational expenses, and potential downtime, is essential for long-term planning.
For more information follow Mechanical World
Designing a cooling tower requires a balance of performance, efficiency, environmental responsibility, and economic viability. Ensuring that each of these considerations is addressed will help create a cooling tower system that performs well across its lifespan, conserves resources, and meets industry standards and regulatory requirements.
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