By DR. DF Duvenhage

1. Introduction: The Cornerstone of Optimal Cooling Tower Performance

Evaporative cooling, a well-established and effective method for removing heat from various systems, has been utilized for over half a century. Its popularity stems from its numerous advantages, chief among which is its superior heat rejection efficiency. Moreover, evaporative cooling systems are simple to operate and maintain, ensuring a relatively simple integration with other plant operations and systems.

At the heart of evaporative cooling lies a natural principle. In open-circuit cooling towers, water to be cooled is distributed over a fill pack while air is drawn or blown through the packing. A small portion of the water evaporates, effectively cooling the remaining water. The cooled water then falls into the tower’s sump, and the outgoing air stream carries away the heat extracted from the water.

Closed-circuit evaporative cooling towers, also known as evaporative condensers, employ a heat exchanger or coil within the tower instead of a fill pack. Water is circulated over the heat exchange coil, and heat is extracted from the refrigerant or primary fluid passing through the coil through the same evaporative process.

Evaporative cooling stands out for its remarkable combination of high thermal efficiency and cost-effectiveness. It achieves low cooling temperatures with minimal energy and water usage. These low cooling temperatures are crucial for many processes to achieve optimal system efficiency. By reducing energy consumption, evaporative cooling contributes to preserving natural resources and protecting the environment.

Evaporative cooling towers play a pivotal role in industrial and commercial applications by dissipating heat from various processes. Accurately sizing a cooling tower is crucial for ensuring it effectively meets the specific cooling demands of the application. Improper sizing can lead to a range of detrimental consequences, including:

• Inefficient heat transfer: An undersized cooling tower will struggle to remove the necessary heat, resulting in elevated process temperatures and potential equipment damage or production loss. Conversely, an oversized cooling tower will result in excessive energy and water consumption, thereby increasing operating costs.
• Compromised system performance: Improper cooling tower sizing can disrupt the overall performance of the process it serves. For instance, an undersized cooling tower in power plants can lead to reduced power generation efficiency, while an oversized tower can contribute to system instability.
• Environmental concerns: An oversized cooling tower can contribute to increased water consumption and, depending on the water treatment in place, higher discharge rates of harmful chemicals, raising environmental concerns.

The challenges faced in selecting the proper size cooling tower are relatively simple; however, ineffective sizing can cause various problems during operation if they are not carefully considered, as discussed above. A critical factor when sizing a cooling tower, which is easily over-simplified, is the seasonal variation in ambient conditions. If a cooling tower is sized for an annualised average condition, it may not be effective in the peak of summer or the dead of winter. Air temperature and humidity impact evaporative towers the most. They need to pull sufficient airflow in any condition, and the selected cooling tower must have enough surface area and soling water circulation rate to achieve that. Sizing your tower right for the extremes from the beginning is the best way to avoid this issue.

2. Understanding Cooling Load: The Foundation of Sizing

The primary factor governing cooling tower sizing is the cooling load, which represents the total amount of heat that the tower must remove from the system or process. Cooling load is typically measured in British thermal units per hour (Btu/h) or kilowatts (kW). Accurately determining the cooling load is essential for selecting the appropriate cooling tower size. The two units measure the same thing but are based on different historical approaches. However, 1 kW (1,000 watts) is equal to 3,412.14 Btu/h, and conversely, 1 Btu/h equates to 0.0002931 kW. Btu/h are typically used with empirical units, in America, while kW is used with metric units, in Europe and many other countries.

Selecting the appropriate size for an evaporative cooling tower requires careful analysis of four primary factors: heat load, range, approach, and wet-bulb temperature. Mastering these considerations ensures optimum cooling efficiency and avoids costly over- or under-sizing.

1. Heat Load: Accurately determining the process heat load is paramount. Overestimating leads to unnecessary capital expenditure and energy consumption, while underestimating risks inadequate cooling. Standard engineering methods or manufacturer-provided tools can facilitate this calculation. The heat load that needs to be dissipated is unique to each process that requires cooling, and is based on the amount of undesirable heat generated during that process. For example, when compressing air in a multi-stage compressor, the air is heated up, loosing density and ultimately will reach a point where that air will damage materials it needs to interact with. To address this, a cooling tower may be employed to cool the air by circulating cooled water from an evaporative cooling tower through an air-water heat exchanger after each stage. This value might vary under changing process conditions or plant outputs, so the maximum heat load generated must be accounted for when sizing a cooling tower.
2. Range: The desired temperature reduction of the hot process water to the desired cooled temperature, defines the range. Higher ranges demand larger towers and increased energy input, necessitating a judicious balance between desired cooling level and operational efficiency. This value is typically an input and is based on the heat load that needs to be dissipated, as well as the heat-exchange efficiency of the heat exchangers being used to dissipate the heat. If the overall heat transfer coefficient of the heat exchangers in place is known, then the range can be calculated, and vice-versa.
3. Approach: This parameter measures the difference between the outlet cold water temperature and the ambient wet-bulb temperature. A smaller approach implies higher efficiency but necessitates a larger tower to accommodate the required heat transfer surface area. The closer the outlet cold water temperature is to the ambient wet-bulb temperature, the more efficient the cooling tower is.
4. Wet-Bulb Temperature: This crucial environmental factor represents the minimum achievable water temperature via evaporation at a given humidity. High wet-bulb temperatures limit cooling potential and necessitate larger towers or alternative cooling methods to meet process requirements. This value varies seasonally and between locations. The seasonal mean, maximum and minimum must be taken into account when sizing a wet-cooling tower. As an example; if the seasonal maximum wet-bulb temperature coincides with the peak plant or process demand, then the cooling tower will be pushed to its limits to reject the resulting high heat load with a lower achievable minimum temperature due to the higher wet-bulb temperatures.

Figure 1: A graphical representation of Range and Approach. https://www.researchgate.net/publication/345463378/figure/fig3/AS:958892817276932@1605629271660/Diagram-Showing-Cooling-Tower-Range-Approach.jpg

Selecting an evaporative cooling tower demands a comprehensive understanding of these four interconnected factors. Utilizing accurate heat load calculations, optimizing range and approach for efficiency, and accounting for local wet-bulb temperature variations are key to achieving optimal cooling performance and cost-effectiveness.

Each of these has an impact on the size and design of the cooling tower:

• Heat load: The cooling tower size requirement increases directly with heat load.
• Range: The cooling tower size requirement decreases inversely with range.
• Approach: The cooling tower size requirement decreases inversely with approach temperature.
• Wet bulb temperature (WBT): The cooling tower size requirement decreases inversely with wet bulb temperature.

When three of these four sizing factors are held constant, the requirement for cooling tower size varies directly with heat load, but inversely with range, approach, and wet bulb temperature. This highlights the interplay between these factors and the importance of considering how they might vary with plant operation and atmospheric conditions to optimize cooling tower size.

Cooling load comprises two primary components: sensible heat and latent heat. Sensible heat refers to the energy required to raise the temperature of a substance without causing a change in phase, while latent heat is the energy absorbed or released during a phase change, such as from liquid to vapor.

Heat gain, the total amount of heat entering a system, contributes to the overall cooling load. Sources of heat gain include process heat generated by industrial equipment, solar radiation, and heat transfer from the surrounding environment

Several factors influence the cooling load of a system, including:

• Process heat requirements: The heat generated by the industrial process being cooled is a major contributor to the cooling load.
• Ambient conditions: Temperature, humidity, and wind speed significantly impact cooling tower performance and, consequently, the cooling load.
• Equipment efficiency: The efficiency of the equipment being cooled directly affects the heat generated and the cooling load required.

Calculating cooling load involves considering the various factors mentioned above. Manual calculation methods, while effective, can be time-consuming and prone to errors. Alternatively, specialized software tools offer a more convenient and accurate approach to cooling load calculations.

3. Sizing Considerations for Different Applications

Selecting the optimal size for an evaporative cooling tower requires astute consideration of the specific application. While the core principles remain constant, the nuances of heat rejection demands vary across industries. Let’s delve into the unique sizing considerations for three key domains: power plants, petrochemical industries, and HVAC systems.

Power Plants

The colossal heat loads generated by power plants pose unique challenges. Accurately estimating condenser heat rejection and factoring in seasonal ambient variations are crucial. Additionally, optimizing range and approach for maximum efficiency under variable load conditions is paramount. Often, multiple tower arrangements are employed to ensure seamless operation across all load profiles.

The cooling towers used in power plants are typically natural-draft cooling towers, using large towering structures that exploit the occurrence of upwards airflow due to their curved shapes and hot, moist air, to expel heat to the atmosphere. Cooling is critical for steam-turbine power plants, where immense volumes of steam are expanded over a turbine to generate vast amounts of electrical energy. This steam, however, needs to be condensed once it has past through the turbine, in order for the vapour to return to a liquid form as well as to maintain a vacuum over the turbine for increased pressreu drop and efficiency.

During the operation of power plants, the most criticalproblem is the use of cooling equipment

for circulating water, particularly in the summer hot period of the year, when the difference between the air temperature and that of the water supplied from the turbines for cooling is smaller. The changes in climatic conditions and trends appearing in recent years; characterised by longer periods high temperatures and greater stress on available water resources, have had a negative impact on the sustainability of standard operating conditions cooling systems of power plants. This makes the selection of cooling technology and the sizing thereof even more important in order to avoid resulting envrionmental penalties and increased water-related costs.

Petrochemical Industries

The diverse and often exothermic nature of petrochemical processes necessitates a flexible and extensive cooling approach. Precise heat load calculations for individual process streams and accurate wet-bulb temperature data become critical. Material selection for the tower and fill media must take into account potential exposure to corrosive chemicals. Redundancy in tower configurations is often employed to ensure uninterrupted process operation both to ensure plant-uptime and production, but also to guarantee the safety of the hazardous nature of the reactions taking place.

Due to the variety of cooling requirements at a petrochemical plant, a centralised approach to cooling is often not possible. As such, various cooling tower clusters are often incorporated. Each cluster will need its own load determination and sizing calculations in order to select the right size and amount of cooling towers. The make, model and technology used for each application might vary, so a cookie-cutter approach to sizing will not be advisable in this sector.

HVAC Systems

Chilling requirements for buildings present a different set of sizing challenges. Fluctuating cooling demands due to occupancy and weather necessitate careful consideration of part-load performance. Smaller tower footprints and noise emission constraints come into play. Additionally, water treatment and drift loss management become crucial factors in selecting the optimal tower size and configuration.

Since HVAC systems use refrigerants to provide chilled water, and air, to the various demand centres in a building, evaporative condensers will be used alongside chillers. This, therefore, adds an additional complication in sizing that needs to be taken into consideration. The chiller will determine the cooling tower or condenser size. Some Chillers have built-in condensers, and therefore only need standard evaporative cooling towers on the exterior of the building. Some chillers need the condenser to be the cooling tower itself, with refrigerant gasses flowing on the inside of tube bundles, and cooling water flowing on the exterior of the tubes in order to condense the gasses.

4. Conclusion

Despite their individual nuances, certain principles bind these diverse applications. Accurately determining heat load, carefully selecting range and approach, and accounting for local wet-bulb conditions remain foundational to effective tower sizing. Additionally, understanding the importance of tower material selection, drift loss control, and maintenance considerations are crucial for long-term operational success.

Selecting the optimal evaporative cooling tower size is an intricate dance between process demands, environmental conditions, and cost considerations. Each application presents a unique set of challenges, necessitating careful analysis and a tailored approach. By understanding the nuances of each domain and applying fundamental engineering principles, we can craft an oasis of efficient cooling that caters to the specific needs of any thirsty process.