At their core, cooling towers are ingeniously designed structures engineered to dissipate heat from industrial processes or air conditioning systems to the surrounding atmosphere. Their operation hinges on the principle of evaporative cooling, a natural process that exploits the latent heat of vaporization to lower the temperature of water. Within the confines of a cooling tower, warm water from industrial processes or HVAC systems is pumped to the tower’s upper reaches and distributed over a network of fill media. As the water cascades downward, it encounters ambient air moving upward, creating a counter-current flow that facilitates evaporation. Through this process, heat is transferred from the water to the air, effectively cooling the water in the process. The cooled water is then collected at the base of the tower and recirculated back into the system, while the heat-laden air is expelled into the atmosphere.
Cooling Tower Types
Cooling towers are categorized into two primary types: Natural draft and Mechanical draft.
Natural draft towers: These rely on very large concrete chimneys to facilitate air movement through the media. Typically, these towers are reserved for applications with water flow rates exceeding 45,000 m3/hr, primarily utilized by utility power stations due to their substantial size.
Mechanical draft towers: These employ substantial fans to either propel or draw air through the circulated water. As the water descends over fill surfaces, it enhances the interaction time between the water and air, thus optimizing heat transfer. The cooling efficiency of Mechanical draft towers is contingent upon factors such as the diameter and operational speed of their fans. Given their widespread usage, this article will primarily focus on mechanical draft cooling towers.
Mechanical draft towers are further classified based on their airflow arrangements.
Counter flow induced draft: In this design, hot water enters from the top, while air is introduced from the bottom and exits from the top. Both forced and induced draft fans are employed in this configuration.
Counter flow forced draft: Similar to induced draft, hot water enters from the top, but air is forcefully introduced from the bottom.
Cross flow induced draft: Here, water enters from the top and flows over the fill, while air is introduced from the side(s), either single-flow (on one side) or double-flow (opposite sides). An induced draft fan draws air across the wetted fill, expelling it through the top of the structure.
Mechanical draft towers are versatile in terms of airflow arrangements and are available in a wide range of capacities, from approximately 10 tons, 2.5 m3/hr flow to several thousand tons and m3/hr. These towers can be either factory-built or field-erected, with concrete towers typically being field-erected.
Many cooling towers are designed for modular assembly, allowing for scalability and flexibility to achieve the desired capacity. These multi-cell towers are often assemblies of two or more individual cooling towers or ‘cells’, with the number of cells determining their capacity, such as an eight-cell tower. Multiple-cell towers can vary in shape, including lineal, square, or round, depending on the configuration of the individual components.
Assessing Cooling Tower Performance
This segment outlines the methods for evaluating the effectiveness of cooling towers. The evaluation aims to compare the current approach and range levels with their initial design specifications, pinpoint any energy inefficiencies, and propose enhancements. Portable monitoring devices are employed during the evaluation process to measure the following parameters:
- Wet bulb temperature of air
- Dry bulb temperature of air
- Cooling tower inlet water temperature
- Cooling tower outlet water temperature
- Exhaust air temperature
- Electrical readings of pump and fan motors
- Water flow rate
- Air flow rate
These recorded parameters are subsequently utilized to assess the performance of the cooling tower in various manners. (Note: CT refers to cooling tower; CW refers to cooling water). These include:
Range: This represents the variance between the inlet and outlet temperatures of the cooling tower water. Refer to Figure 2. A larger CT Range indicates efficient temperature reduction by the cooling tower, signifying effective performance. The formula is:
CT Range (°C) = CW inlet temp (°C) – CW outlet temp (°C)
Approach: This is the difference between the cooling tower outlet coldwater temperature and ambient wet bulb temperature. The lower the approach the better is the cooling tower performance. Although, both range and approach should be monitored, the `Approach’ is a better indicator of cooling tower performance. See Figure 2.
CT Approach (°C) = CW outlet temp (°C) – Wet bulb temp (°C)
Effectiveness. This is the ratio between the range and the ideal range (in percentage), i.e., the difference between cooling water inlet temperature and ambient wet bulb temperature, or in other words it is = Range / (Range + Approach). The higher this ratio, higher is the cooling tower effectiveness.
CT Effectiveness (%) = 100 x (CW temp – CW out temp) /
(CW in temp – WB temp)
Cooling capacity: This is the heat rejected in kCal/hr or TR, given as product of mass flow rate of water, specific heat and temperature difference.
Evaporation loss: This is the water quantity evaporated for cooling duty. Theoretically, the evaporation quantity works out to 1.8 m3 for every 1,000,000 kCal heat rejected. The following formula can be used (Perry):
Evaporation loss (m3/hr) = 0.00085 x 1.8 x circulation rate
(m3/hr) x (T1-T2)
T1 – T2 = temperature difference between inlet and outlet water
Cycles of concentration (C.O.C): This is the ratio of dissolved solids in circulating water to the dissolved solids in make up water.
Blow down losses: These depend upon cycles of concentration and the evaporation losses and is given by formula:
Blow down = Evaporation loss / (C.O.C. – 1)
Liquid/Gas (L/G) ratio: The L/G ratio of a cooling tower is the ratio between the water and the air mass flow rates. Cooling towers have certain design values, but seasonal variations require adjustment and tuning of water and air flow rates to get the best cooling tower effectiveness. Adjustments can be made by water box loading changes or blade angle adjustments. Thermodynamic rules also dictate that the heat removed from the water must be equal to the heat absorbed by the surrounding air. Therefore the following formulae can be used:
L(T1 – T2) = G(h2 – h1)
L/G = (h2 – h1) / (T1 – T2)
Where:
L/G = liquid to gas mass flow ratio (kg/kg)
T1 = hot water temperature (0C)
T2 = cold-water temperature (0C)
h2 = enthalpy of air-water vapour mixture at exhaust wet-bulb temperature (same units as above)
h1 = enthalpy of air-water vapour mixture at inlet wet-bulb temperature (same units as above)
Energy Efficiency Improvement Opportunities
The primary areas for energy conservation encompass:
- Optimal selection of cooling tower (as structural modifications are not feasible post-installation)
- Utilization of appropriate fill materials
- Optimization of pumps and water distribution systems
- Efficiency enhancements in fans and motors
Selecting the Appropriate Cooling Towers
Once a cooling tower is installed, significant improvements to its energy performance become challenging. Several factors influence the cooling tower’s performance, necessitating careful consideration during the selection process: capacity, range, approach, heat load, wet bulb temperature, and their interrelationships.
Capacity: Heat dissipation (measured in kCal/hour) and circulated flow rate (m3/hr) serve as indicators of cooling tower capacity. However, relying solely on these design parameters is insufficient for gauging cooling tower performance. For instance, a cooling tower designed to handle a flow rate of 4540 m3/hr across a 13.9°C range may appear larger than one tasked with the same flow rate but across a 19.5°C range. Therefore, additional design parameters are essential for accurate evaluation.
Range: The range is not determined by the cooling tower itself but rather by the process it serves. It is entirely dependent on the heat load and water circulation rate through the exchanger and into the cooling water. The range is calculated as a function of the heat load and flow circulated through the system:
Range (°C) = Heat load (kCal/hour) / Water circulation rate
(l/hour)
Cooling towers are typically specified to cool a specific flow rate from one temperature to another at a designated wet bulb temperature. For instance, a cooling tower might be designated to cool 4540 m3/hr from 48.9°C to 32.2°C at a wet bulb temperature of 26.7°C.
Approach: Typically, the closer the approach to the wet bulb temperature, the higher the cost of the cooling tower due to its larger size. Cooling tower manufacturers typically ensure a minimum approach of 2.8°C to the design wet bulb as the coldest water temperature guaranteed. When selecting the size of the cooling tower, the approach holds paramount importance, followed closely by the flow rate, with the range and wet bulb temperature considered of lesser significance.
The approach is calculated as follows:
Approach (5.5°C) = Cold-water temperature (32.2°C) –
Wet bulb temperature (26.7°C).
The Benefits of Cooling Towers
The utilization of cooling towers in HVAC systems offers a plethora of benefits, ranging from enhanced energy efficiency to improved system reliability and longevity. Let’s explore some of the key advantages:
Energy Efficiency: By harnessing the natural process of evaporation, cooling towers significantly reduce the energy consumption associated with traditional air conditioning methods. Compared to air-cooled systems, which rely solely on forced convection for heat dissipation, cooling towers offer superior heat rejection efficiency, especially in applications with high heat loads.
Cost Savings: The energy savings achieved through the use of cooling towers translate directly into cost savings for facility operators and building owners. Moreover, the longevity and reliability of cooling towers contribute to lower maintenance costs and reduced downtime, further enhancing their economic viability.
Water Conservation: While cooling towers consume water for evaporation and makeup purposes, they typically require significantly less water than alternative cooling methods such as once-through cooling systems. Additionally, advances in water treatment technologies and the adoption of water-saving strategies such as drift eliminators and basin covers help minimize water consumption and mitigate environmental impact.
Flexibility and Scalability: Cooling towers are inherently modular and scalable, allowing for easy expansion or modification to accommodate changing cooling requirements. Whether it’s retrofitting an existing system or designing a new installation from scratch, cooling towers offer unparalleled flexibility and adaptability, ensuring optimal performance under varying operating conditions.
Environmental Sustainability: As the global push for sustainability intensifies, cooling towers emerge as key enablers of environmentally responsible practices. By reducing energy consumption, minimizing water usage, and mitigating the environmental impact of industrial processes, cooling towers play a vital role in promoting sustainability across diverse sectors.
Innovations Shaping the Future
As the quest for sustainability intensifies and technological advancements continue to accelerate, the landscape of cooling tower engineering is undergoing rapid transformation. From the integration of smart sensors and predictive analytics to optimize performance and energy consumption to the development of eco-friendly materials for construction, the future of cooling towers is characterized by innovation and adaptation. Let’s explore some of the emerging trends and technologies shaping the future of cooling towers:
Smart Monitoring and Control Systems: The advent of IoT (Internet of Things) technology has revolutionized the way cooling towers are monitored and controlled. Smart sensors embedded within cooling towers continuously collect data on key parameters such as water flow rate, temperature, and pressure, enabling real-time monitoring and analysis. Advanced analytics algorithms leverage this data to optimize system performance, identify potential issues before they escalate, and improve energy efficiency.
Water Treatment and Conservation: Water scarcity and environmental concerns have spurred innovation in water treatment technologies aimed at optimizing water usage and quality in cooling tower systems. Advanced filtration systems, chemical treatments, and biological controls help maintain water quality, prevent microbial growth, and minimize the formation of scale and corrosion, thus extending the lifespan of cooling tower equipment and reducing maintenance requirements.
Energy-Efficient Designs: With energy efficiency at the forefront of design considerations, cooling tower manufacturers are investing in innovative design features and technologies to minimize energy consumption while maximizing heat rejection efficiency. This includes the use of high-efficiency fan designs, variable speed drives, and aerodynamic enhancements to optimize airflow and reduce power consumption. Additionally, advancements in thermal insulation materials and coatings help minimize heat losses and improve overall system efficiency.
Hybrid Cooling Systems: Hybrid cooling systems, which combine the benefits of evaporative cooling with air-cooled or dry cooling technologies, offer an attractive alternative for applications where water availability or quality is limited. By integrating evaporative cooling towers with supplemental air-cooled or dry coolers, hybrid systems provide greater flexibility and resilience, ensuring reliable operation under diverse environmental conditions.
Material Innovations: The use of advanced materials such as fiberglass-reinforced plastics (FRP), stainless steel, and corrosion-resistant alloys has revolutionized the construction of cooling towers, enabling longer service life, reduced maintenance requirements, and enhanced durability. Additionally, the adoption of sustainable and recyclable materials aligns with the growing emphasis on environmental stewardship and circular economy principles.
Conclusion
Overall, cooling towers play a vital role in unlocking efficiency in HVAC systems. By efficiently removing heat from buildings and maintaining optimal indoor temperatures, cooling towers help reduce energy costs, improve sustainability, and enhance the overall comfort of building occupants. Incorporating cooling towers into HVAC systems is a smart choice for building owners and operators looking to improve energy efficiency and sustainability.
Tarun Kumar Kushwaha has a sixteen-year tenure dedicated to central air conditioning systems, specializing in Service, Operations, and Retrofit. Presently, he serves as the RM-Service for North India at Dunham-Bush India. His professional journey commenced at Blue Star Ltd. in 2008, where he honed his skills managing various chillers anfd related equipment. Transitioning to Thermax, Tarun focused on Vapour Absorption Chillers, delving into specialized expertise in this domain. His tenure at Johnson Controls saw him spearheading Retrofit operations in North India, overseeing projects aimed at energy conservation. Tarun holds accreditation as an IGBC-AP and is a Level 2 inspector for NDT Testing.
