Cooling towers play a pivotal role as the ultimate heat rejection devices for processed water from power plants, industrial processes, HVAC&R (heating, ventilation, air conditioning, and refrigeration) systems, and the like. There are two primary kinds of cooling towers. The first type is the open cooling tower or direct-contact cooling tower (shown in Figure 1 ), which allows water to come into direct contact with the cooling atmosphere, enabling the heat load source to be directly transferred to the air. The second type is the closed-circuit cooling tower (shown in Figure 2 ), which involves indirect contact between the heated fluid and the atmosphere. This type essentially combines a heat exchanger and a cooling tower into one relatively small unit.
Wet and dry cooling towers (shown in Figure 3 and Figure 4 ) are two subcategories of cooling towers that differ in their method of heat transfer. Wet cooling towers utilize evaporative cooling, where water is sprayed onto the surface of heat exchange media, such as packing or tubes, and the heat is transferred through evaporation. In contrast, dry cooling towers use air as the cooling medium, and heat is transferred from the hot water passing through the tubes to the ambient air through a heat exchanger. Dry cooling towers, known for their ability to minimize water consumption, are often chosen in the case of water scarcity or environmental considerations. While wet cooling towers are more efficient in terms of cooling, they consume more water than dry cooling towers. On the other hand, dry cooling towers consume less water but are less efficient [ 1 ].
The wet cooling towers operate through direct contact between air and processed water, causing water to evaporate and thereby lowering the temperature of the processed water. By employing direct evaporation for heat removal, wet cooling towers use only about 5% of the water consumption rate of a once-through system, making them the most cost-effective option. Moreover, the discharge of heated water (blowdown) is minimal, which greatly reduces the ecological impact. Normally, cooling towers can cool water to within 2.22 to 2.78 °C of the ambient wet-bulb temperature, which is always lower than the ambient dry-bulb temperature and approximately 19.4 °C lower than air-cooled systems of a reasonable size (between 879 to 1758 kW). This lower temperature improves the entire system’s efficiency, thereby significantly reducing energy use and increasing process output [ 2 ].
In various HVAC&R systems, one or multiple chillers are utilized to meet the cooling demands of a building. These chillers typically adopt water-cooled condensers, where the water absorbs heat from the condenser, followed by dumping the heat into the atmosphere. This essential heat dissipation process is carried out by a designated cooling tower. Cooling towers employ a combined process of heat and mass transfer to effectively cool the water. The water designated for cooling is uniformly distributed within the tower using spray nozzles, splash bars, or film-type fill. The fill offers a substantial contact surface area with water and the atmospheric air to facilitate effective heat transfer. Upon the direct-contact heat transfer between air and water, a fraction of the water undergoes a phase change from liquid to vapor at a certain water vapor pressure, absorbing heat from the water itself and lowering the temperature accordingly. The driving forces include heat transfer and mass transfer (in this case, water vapor) which are associated with the temperature difference and vapor pressure difference, respectively. The effectiveness of heat and mass transfer is a function of heat and mass transfer coefficients, as well as the extent of the surface area exposed to heat and mass transfer.
Several factors determine the classification of cooling towers. One of the key factors is whether the tower has a fan (mechanical draft) or not (natural draft). Natural draft cooling towers are very large and rely on the natural convection currents created by the temperature difference between the warm air inside the tower and the cooler outside air. They are typically used in large power plants and industrial facilities where large amounts of waste heat need to be dissipated. The size of natural draft towers is significant due to their reliance on buoyancy-driven air flow.
The mechanical draft cooling towers use fans to actively move air through the tower, which can increase the heat exchange rate. They are suitable when precise control over the cooling process is needed or when environmental conditions (such as high ambient temperatures) require extra air flow. Fan-assisted cooling towers are often used in industries with varying heat loads or when the heat load is relatively high. In the case of mechanical draft, the tower can either be forced or induced draft, depending on the location of the fan relative to the filled bed. In forced draft cooling towers, fans are used to force air through the tower, enhancing the heat exchange process. They are often used in applications where a specific amount of cooling needs to be achieved regardless of ambient conditions. Forced draft towers are commonly found in HVAC systems for commercial and industrial buildings. Induced draft towers, on the other hand, have fans located at the top of the tower that create a negative pressure, drawing air upwards through the tower. These towers are versatile and can handle a wide range of cooling needs. They are often used in power plants, refineries, and other industrial processes.
Another important factor is the method of heat transfer. Heat transfer in cooling towers involves various mechanisms such as conduction, convection, and evaporation, but the most significant transfer method is via evaporation or mass transfer. One of the most important factors in enhancing the efficiency of wet cooling towers is to increase the surface area for more contact area and residence time for heat and mass transfer, which is achieved through the use of fill media. Fill media in wet cooling towers are designed to maximize the contact area between the air and water and the resident time for contact, facilitating efficient heat and mass transfer. Since the filling section contributes to 70% of the heat transfer within wet cooling towers [ 3 ], careful selection and maintenance of the packing material are essential for maximizing the overall performance of the cooling tower. The characteristics of the fillings directly impact the heat transfer efficiency and operating costs of the cooling towers. The design, arrangement, and material properties of the fillings play a vital role in their heat transfer capabilities.
Over the years, researchers and engineers have been dedicated to improving the design, materials, and functionality of fill media. These improvements are driven by the need to optimize heat and mass transfer processes, reduce energy consumption, and ensure the sustainable operation of wet cooling towers. In this regard, this paper aims to provide a comprehensive review of the recent progress in fill media, including innovations in design and material, progress in manufacturing techniques, and their impact on heat transfer efficiency, and aggregate and present data that can be practically applied in the design of cooling towers. Based on a comprehensive review of the recent literature, an overview on the efficacy of fill media in cooling towers is provided in this paper. Special attention will be given to the incorporation of advanced materials with enhanced thermal properties. Additionally, the utilization of novel geometries and configurations in fill media designs will be explored. Furthermore, the review will examine the impact of fill media on the functioning of the cooling tower and pressure drop issues. The impact of these advancements on the overall cost-effectiveness and reliability of wet cooling towers will also be discussed.
In industrial applications involving heat and mass transfer, fill media play a critical role in enhancing efficiency and optimizing performance. By utilizing appropriate fill media, cooling towers can achieve higher cooling efficiencies, reduce water consumption, and enhance overall system performance. The selection and design of fill media play a crucial role in maximizing the heat transfer capabilities of cooling towers while minimizing operational costs. This section provides an in-depth overview of fill media, exploring the various types and characteristics available. Understanding the different types of fill media and their unique properties is critical for selecting the most suitable option for specific industrial processes.
Fill media, also known as fill packing or filling, contain a large surface area per unit volume to facilitate effective heat transfer and mass transfer. The cooling tower’s effectiveness is enhanced by the fill media, which optimize the heat exchange surface area. When water is distributed over the fill media, it forms thin films or droplets that contact the air passing through the tower. Fill media are typically composed of inexpensive materials with low thermal conductivity, such as plastic, metal, or wood. The use of high thermal conductivity materials like metals is usually not necessary as far as effective heat transfer is concerned; this is because the main objective of the filling is to provide more surface/time for direct contact with water and air. The amount of heat that is transferred across the filling between air and water is usually insignificant. The fillings are structured in a way that provides a large surface area for the water–air interaction.
5,6,2/m3 [5,It should be noted that fill media increase the surface area without significantly altering the volume. This expansion in surface area is expected to enhance both heat and mass transfer, as well as increase the pressure drop. It is customary to define a compactness parameter, called specific area or wettability [ 4 7 ]. This parameter represents the ratio of the total surface area of the fill media to their volume, typically falling within the range of 200 to 500 m/m 8 ]. However, with an increase in specific area, there is also a corresponding rise in pressure drop, leading to higher fan power consumption.
When dealing with high specific area values, it is important to consider the impact of increased air velocity within the fill media, as excessive noise generation must be avoided. Furthermore, cooling towers are typically situated at a considerable distance from populated areas to minimize potential disturbances caused by noise. They are strategically placed in cool areas, preferably in close proximity to the chillers. Whether located on the building’s rooftop or adjacent to the structure on the ground, efforts are made to minimize the cooling tower’s volume and height. This is carried out to align with architectural designs and to optimize land usage, considering that land area can be expensive and valuable.
In summary, the addition of a cost-effective fill media results in a decrease in the required volume of the cooling tower, making it smaller while improving heat and mass transfer. However, it also leads to an increase in pressure drop, subsequently impacting fan power consumption. A trade-off exists between these parameters, which are all subject to engineering rules, regulations, and cost considerations.
Cooling tower fill is typically made up of thin, closely spaced sheets or elements, often arranged in a honeycomb or corrugated pattern. The two main types of fill are splash-type and film-type [ 2 ]. As shown in Figure 5 , splash-type fill is designed to enhance the contact area and duration of falling water. This is achieved by directing the water over staggered splash bars arranged at various levels, causing it to cascade through each elevation. This specific type of fill causes the descending water droplets to break up into smaller droplets, which increases the droplet surface area for heat transfer between the water and air. This adds to the surface area provided by the wet film surrounding the fill. Another advantage of the smaller droplets is their propensity to separate dirt particles from the water. Furthermore, this fill type offers the advantage of generating a low-pressure drop across the cooling tower [ 9 ].
The film-type fill (shown in Figure 6 ) functions in a similar manner by directing the water over closely spaced vertically arranged sheets, typically made of polyvinyl chloride (PVC). This causes the water to form a thin film over the surface of the sheets to facilitate falling film evaporation [ 2 ].
The choice of fill media can significantly impact the performance of a cooling tower, water consumption, and overall system performance. Various types of fill exhibit varying characteristics that impact heat transfer efficiency, pressure drop, and resistance to fouling. Careful selection of fill media based on the specific cooling tower system requirements is vital to optimize its performance.
Numerous studies have examined the effect of various fill types on the thermal/hydraulic performance of cooling towers, along with comprehensive analyses of energy consumption and operating costs. Table 1 presents a concise summary of various fill media based on experimental research. While numerous studies focus on modeling and numerical simulation, this table only includes experimental investigations conducted after the year 2016. The table comprises 14 distinct experiments, which can be classified into two main categories based on the types of fill media: splash and film fill media.
The availability of data from these experimental studies enables a comparative analysis of the fill media’s effectiveness, facilitating the selection of suitable fill types for specific cooling tower applications. Furthermore, the inclusion of experiments conducted after 2016 ensures that the table reflects the most recent advancements in cooling tower technology.
To comprehensively evaluate the effectiveness of various fill media in cooling towers, a variety of performance indices have been developed. These indices act as valuable tools for quantitatively analyzing and comparing the effects of fill media on heat and mass transfer efficiency, pressure drop, fan power consumption, and other crucial factors. This section aims to explore and explain the various performance indices applied in the study of fill media in cooling towers, providing insights into their significance and relevance in optimizing cooling tower performance. Understanding these indices enables researchers and engineers to make informed decisions when choosing the most appropriate fill media for particular cooling tower applications, ensuring enhanced cooling performance and energy efficiency.
M e = h d a fi A fr L fi m w = h d a fi L fi G w = ∫ T w , out T w , in c p , w d T w ( i masw − i ma )
(1)
hd
is the mass transfer coefficient;afi
is the interfacial surface area between air and water per unit volume of fill zone;Afr
is the frontal area of fill perpendicular to air flow direction;Lfi
is the fill length,mw
is the mass flow rate of water;Gw
is the mass velocity of water;cp,w
is the specific heat of water at constant pressure;i
ma is the specific enthalpy (per kg dry air);i
maew is the specific enthalpy of saturated water;Tw,in
andTw,out
are the temperatures of the inlet and outlet water, respectively.In order to evaluate and compare the thermal performance of fills, Merkel [ 24 ] introduced a non-dimensional coefficient of performance, known as the Merkel number (Me). This is defined as the ratio of the actual heat transfer rate to the maximum possible heat transfer rate:in whichis the mass transfer coefficient;is the interfacial surface area between air and water per unit volume of fill zone;is the frontal area of fill perpendicular to air flow direction;is the fill length,is the mass flow rate of water;is the mass velocity of water;is the specific heat of water at constant pressure;is the specific enthalpy (per kg dry air);is the specific enthalpy of saturated water;andare the temperatures of the inlet and outlet water, respectively.
Kfdm
as follows:K fdm = 2 [ Δ p fi − ( ρ avo v avo 2 − ρ avi v avi 2 ) + ( ρ avi − ρ av , avg ) g L fi , z ] ρ av , avg A fr 2 / m av , avg 2
(2)
pfi
is the pressure drop over fill;ρavo
andρavi
are the outlet and inlet densities of the air–vapor mixture, respectively;vavo
andvavi
are the outlet and inlet velocities of the air–vapor mixture, respectively;ρav,avg
is average density of the air–vapor mixture;g
is the gravitational acceleration;Lfi,z
is the fill length in the vertical direction; andmav,avg
is the average mass flow rate of the air–vapor mixture.In cooling towers equipped with splash or trickle fills, the resistance to air flow can vary, either being uniform in all directions (isotropic) or directionally dependent (anisotropic). The air flow resistance can be quantified using a loss coefficientas follows:where Δis the pressure drop over fill;andare the outlet and inlet densities of the air–vapor mixture, respectively;andare the outlet and inlet velocities of the air–vapor mixture, respectively;is average density of the air–vapor mixture;is the gravitational acceleration;is the fill length in the vertical direction; andis the average mass flow rate of the air–vapor mixture.
26,Δ p = k × 1 2 × ρ u 2
(3)
p
is the static pressure drop measured across the fill andk
is the pressure loss coefficient of a cooling tower packing that is determined by measuring the pressure drop across the packing.The hydraulic characteristics of wet cooling towers containing fill media can be described either by the loss coefficient or the pressure drop across the fill. The pressure drop across a fill media is related to the loss coefficient through the following standard equation [ 25 27 ]:in which Δis the static pressure drop measured across the fill andis the pressure loss coefficient of a cooling tower packing that is determined by measuring the pressure drop across the packing.
C T C = m w c p , w ( T w , in − T w , out )
(4)
Another important parameter is the capacity of a cooling tower (CTC), which refers to its ability to handle a specific cooling load, which is the amount of thermal energy that the cooling tower can remove from the system and can be determined according to [ 28 ]:
Capacity represents the cooling tower’s capability to absorb and dissipate heat. A higher capacity indicates that the cooling tower can handle more significant cooling loads.
Consider a scenario in which a condenser must dissipate waste heat into the atmosphere. During the design or simulation stage, the heat load on the condenser is known. The coefficient of performance (COP) for the chiller is typically assumed to be 4, and the condenser’s capacity can be determined based on the capacity of the chiller [ 2 ]. Consequently, the thermal capacity of cooling towers available on the market is designed to match the thermal capacity of the chillers.
As depicted in Figure 7 , both the air and water temperatures experience changes while passing through the cooling tower. It is important to note that the vertical axis in the figure represents the wet-bulb temperature for air. The variation in air and water temperature follows a pattern similar to that of a counterflow heat exchanger. The change in water temperature is known under standard conditions. Typically, and presumably, the system has been well designed to achieve this objective with a well-posed controller in place.
The temperature difference between the water inlet
Tw,in
and outletTw,out
in the cooling tower, which must be equal to the temperature difference in the condenser, is referred to as “Range”. On the other hand, the temperature difference between the wet-bulb temperature of the airTwb,in
and the water outlet temperatureTw,out
is known as “Approach”. The Approach value depends on the specific cooling tower. Apparently, the Approach value can approach zero if the tower size is sufficiently large. As mentioned earlier, the use of fill media can increase the contact area and residence time at a constant volume, resulting in a decrease in the Approach value.For a more comprehensive understanding of the impact of wet-bulb temperature, dry-bulb temperature, and specific humidity, one can refer to the ASHRAE handbook [ 2 ]. However, for brevity, only a few key points will be highlighted. The thermal performance of a cooling tower is primarily determined by the entering air wet-bulb temperature. The incoming air dry-bulb temperature and relative humidity have a negligible individual effect on the thermal performance of mechanical draft cooling towers. However, they do affect the rate of water evaporation within the cooling tower. The ratio of latent to sensible heat is crucial in evaluating the water consumption of a cooling tower. Evaporation, involving mass transfer, occurs only in the latent portion of heat transfer and is directly influenced by changes in specific humidity. The incoming air dry-bulb temperature and relative humidity affect the ratio of latent to sensible heat transfer, consequently influencing the evaporation rate.
In any case, it is necessary to calculate the Approach and fan power consumption when studying the fill media. It is important to mention that in order to calculate the Approach and fan power consumption, it is essential to develop thermal and fluid mechanical models of the fill media.
It should be noted that not all of these considerations and points may be explicitly mentioned in the available published papers, and some studies may examine fill media under unknown or vague scenarios. The goals and objectives of these studies might not be clearly defined. For instance, as it will be discussed in more detail, it is crucial to keep the Range and mass flow rate of the cooling tower constant while studying the fill media, but some studies may overlook this aspect.
To distinguish these studies and uncover valuable insights from the available literature, it is essential to provide an overview of the overall performance of cooling towers. By doing so, all the necessary and useful data can be extracted and reported, enabling a comprehensive analysis of the available papers.
Engineering societies provide standard conditions for conducting experiments and reporting condenser performance. This ensures uniformity in experimental analysis and facilitating designers in selecting appropriate condensers and cooling towers. AHRI is one such reputable pioneer in this field [ 29 ].
Based on the standard conditions proposed by AHRI [ 29 ], the water temperature at the condenser outlet is set at 35 °C, and the inlet temperature is 27 °C. The mass flow rate is defined as 59 mL/s per 1.25 kW of refrigeration capacity. Furthermore, the air dry-bulb temperature is 35 °C, and the air wet-bulb temperature is 25 °C. It is clear that the specified conditions are more closely linked with the climate conditions in the USA. These standardized conditions provide a consistent basis for experimentation and capacity reporting, eliminating concerns about the impact of diverse experimental conditions on reported outcomes.
Whether following the standards proposed by AHRI or those of other societies and associations, the common aspect is knowing the inlet and outlet temperatures of water, along with the dry-bulb and wet-bulb temperatures of the air. These temperatures are clearly labeled in Figure 7 . The outlet water from the condenser is then directed to the inlet of the cooling tower, where it is expected to reach the same temperature as the condenser’s inlet temperature.
It is important to note that, due to the advantages of standardized testing methods, the performance of the fill media should ideally be studied under standard conditions. Typically, the experiments are conducted under these standard conditions, and the results are then extrapolated or interpolated to other conditions using linear correlations. However, it should be acknowledged that not all papers may strictly adhere to this procedure.
The fill media in a cooling tower can be considered its central component, and its heat transfer capability is crucial to the overall efficiency of the tower. The material of fill media plays a significant role in its thermal performance by providing a large surface area for heat transfer between the water and the surrounding air, as well as increasing the contact time between the two fluids. Consequently, materials science and manufacturing techniques have been employed to develop various types of filling over the years, and research has been conducted to improve the transport phenomena in these devices and, consequently, enhance their performance characteristics. Ultimately, the choice of fill material depends on factors such as cost, durability, thermal performance, and maintenance requirements. In the case of a filling material that is thermodynamically inefficient within a cooling tower, it will require a larger tower space. This not only results in increased pressure drop but also contributes to higher electricity consumption.
The following presents the pros and cons of typical materials used for filling packages in cooling towers:
PVC. PVC is a widely used material used for filling packages due to its durability and cost-effectiveness. It is resistant to corrosion, chemicals, and UV radiation. However, it has a lower heat transfer coefficient than other materials, and its thermal performance deteriorates over time due to fouling.
Polypropylene. Polypropylene is another common material used for fill packing due to its high resistance to corrosion, chemicals, and high temperatures. It has a higher heat transfer coefficient than PVC, which leads to better thermal performance. However, it is more expensive than PVC and can be prone to clogging if not properly maintained.
Wood. Wood was traditionally used for filling packages, and it is still used in some older cooling towers. Wood has a high heat transfer coefficient and is biodegradable, making it an environmentally friendly option. However, it is prone to decay and requires regular maintenance, which can be costly.
Metal. Metal filling packages, such as stainless steel or aluminum, demonstrate remarkable resistance to corrosion and can withstand high temperatures. They also have a high heat transfer coefficient, which leads to good thermal performance. However, they are more expensive than other materials and can be prone to scaling and fouling if not properly maintained.
There are also other materials being investigated in addition to these commonly used ones. Table 2 includes various innovative materials used in recent research. While most researchers and practical applications primarily employ metal, wood, and PVC materials for experimental and academic investigations, other materials provided in the table could offer valuable insights for future designs and utilization by researchers.
This section investigates several experiments conducted by previous researchers on the topic of cooling towers, specifically focusing on different types of fillings. To provide detailed explanations and delve deeper into the subject matter, it is recommended to refer to reputable academic papers. These papers will offer comprehensive insights into the conducted experiments, which provide more detailed descriptions of the experimental set-ups, fill types, and outcomes.
In scenarios where the fill loss coefficient is negligible or when the fill reaches the air inlet area, the air moves obliquely or in cross-counterflow to the water flow. As a result, a specific Me or transfer characteristic emerges in the case of cross-counterflow, which falls between the values between purely counterflow and crossflow fills. In order to assess the overall fill performance, considering the isotropic or anisotropic fill resistance, one can use computational fluid dynamics (CFD) as a powerful tool for simulation. However, 2D or 3D numerical models and characteristics of fill media are needed to accurately model the performance of natural-draught wet cooling towers and evaluation of the trickle fill effect.
Grobbelaar et al. [ 32 ] presented an experimental set-up, measuring methods, and analytical approaches to specify the fill performance characteristics in both counterflow and crossflow configurations. Specifically, the results obtained for a particular fill are presented and analyzed, providing valuable insights for evaluating the performance of fills under cross-counterflow conditions.
In the comparative experimental tests, the trickle fill employed had cross-fluted channels intended to facilitate air flow in a specific direction. Typically, during installation, the fill is aligned with the expected air flow direction. Specifically, in counterflow installations, the channels are oriented vertically. Three different fill configurations were examined, as shown in Figure 8
Grobbelaar et al. [ 32 ] conducted 376 experimental tests for the three fill configurations to determine fill performance characteristic relations, as given in Table 3 and Table 4
The fill characteristic equations reveal that the Merkel number shows minimal sensitivity to variations in the water inlet temperature across all tested fill configurations. When the cross-fluted channels are misaligned with the air flow direction, the loss coefficient per meter of fill increases, resulting in an approximately 25% increase in the Merkel number. This increase is attributed to greater turbulence arising from the steeper pressure gradient across the fill. The counterflow configuration generally exhibits a higher Merkel number compared to crossflow, mainly because water droplets spend more time in the cooling zone in counterflow. However, at higher ratios of water mass velocity to air mass velocity, crossflow performance starts to outperform counterflow. The authors speculate that the difference lies in the micro flow pattern of water, with water in counterflow mostly adhering to the perimeter of the channels, causing little channel obstruction, while in crossflow, more water drips through the channels, causing a slightly higher loss coefficient. For this particular fill, the term
hdafi
(related to heat transfer) is almost independent of water mass velocity in counterflow, indicating a similar behavior to a film fill where water flows mainly in a film. However, in crossflow, the termhdafi
does show some weak dependence on water mass velocity, indicating significant dripping within the fill. The researchers suggest that describing fill performance in terms ofhdafi
rather than the Merkel number may be more appropriate, especially when given characteristic equations in terms of air and water mass velocities. Such an approach would help differentiating between different fill types and their response to varying water flow rates.Fan et al. [ 33 ] proposed a practical experimental set-up for a parallel counterflow Maisotsenko-cycle cooling tower (MCT) with fillings. The Maisotsenko cycle, known as the M-Cycle, holds the promise of decreasing electricity generation expenses by enabling water to be cooled below the wet-bulb temperature of the inlet air, thereby offering potential cost savings. However, traditional wet cooling towers are limited in their cooling capacity, particularly in hot and wet climates. The objective was to gain insights into the ideal length for dry channels, to evaluate the thermal performance of the tower under different conditions, and to analyze pressure drops within MCT. The findings revealed that the optimum length for dry channel is 2.4 m, and effectiveness of the wet-bulb temperature reached a maximum of 180%. Furthermore, the study also examined how the velocity of air flow within the wet channels affects the pressure drops across the innovative fills. These results affirm the significant potential of implementing the M-Cycle technique in thermally wet cooling towers and provide valuable guidelines for industrial applications and enhancing the performance of MCTs.
Based on the review result, very few examples in the literature demonstrate the hydraulic performance of fill media in wet cooling towers. Researchers have explored various configurations of fill media and their impact on cooling tower thermal performance.
In the case of mechanical draft cooling towers, appropriate fans are selected primarily based on the loss coefficient of the fill (i.e., the pressure drop across the fill). In natural draft cooling towers, the draft is also strongly influenced by the loss coefficient of the fill. Overall, understanding the relationship between pressure drop, loss coefficient, and flow rates is crucial for evaluating the hydraulic performance of filled cooling towers and selecting appropriate fans to ensure optimal cooling tower operation.
26,27,34,35,k = C 1 ( L ′ ) C 2 ( G ′ ) C 3 + C 4 ( L ′ ) C 5 ( G ′ ) C 6
(5)
ci
coefficients are constants that can be determined experimentally for a specific fill media.To correlate the pressure drop across the cooling tower fill, empirical relations are used, which depend on the water and air mass flowrates as well as other relevant parameters. These correlations are derived based on experimental data obtained by testing the cooling tower under different operating conditions. By incorporating the air and water flowrates along with relevant constants into these empirical correlations, engineers and researchers can predict the pressure drop in the cooling tower fill media. This helps in the design and analysis of cooling tower systems. Several equations have been proposed for this purpose [ 8 36 ]. According to Kloppers and Kröger [ 26 ], the following equation is more accurate but is only valid for specific ranges and may not accurately represent a wide range of operating conditions for cooling towers:wherecoefficients are constants that can be determined experimentally for a specific fill media.
2. The study primarily examined the effect of water and air flow rates on the hydraulic characteristics of the cooling tower at several inlet water temperatures. Two hydrodynamic operating regimes were observed during the tower’s air–water contact operation: the pellicular regime (PR) characterized by low pressure drop values during low water flow rates, and the bubble and dispersion regime (BDR) marked by relatively higher pressure drop values associated with larger water flow rates. These regimes revealed two distinct states of pressure drop characteristics. It is anticipated that these two distinct hydrodynamic regimes will give rise to two different thermal regimes within the cooling tower. It is widely recognized that hydrodynamics significantly influence the heat and mass transport processes in such equipment.Lemouari et al. [ 37 ] conducted an experiment focusing on the hydraulic performance of a counterflow wet cooling tower. The tower employed a “vertical grid apparatus” (VGA) type of packing, consisting of four galvanized zigzag-shaped sheets and three metallic vertical grids arranged in parallel within a measurement zone of 0.15×0.148 m. The study primarily examined the effect of water and air flow rates on the hydraulic characteristics of the cooling tower at several inlet water temperatures. Two hydrodynamic operating regimes were observed during the tower’s air–water contact operation: the pellicular regime (PR) characterized by low pressure drop values during low water flow rates, and the bubble and dispersion regime (BDR) marked by relatively higher pressure drop values associated with larger water flow rates. These regimes revealed two distinct states of pressure drop characteristics. It is anticipated that these two distinct hydrodynamic regimes will give rise to two different thermal regimes within the cooling tower. It is widely recognized that hydrodynamics significantly influence the heat and mass transport processes in such equipment.
Ka
) within the investigated cooling tower. These correlations, which are listed inThe proposed relationship between pressure drop characteristics and combined heat and mass transport (air–water) through the packing inside the wet cooling tower for the PR and BDR regimes are listed in Table 5 37 ]. Lemouari et al. [ 37 ] also proposed empirical correlations to emphasize the relationship between the hydraulic characteristics and the combined heat and mass transport process represented by a global heat and mass transport coefficient () within the investigated cooling tower. These correlations, which are listed in Table 6 , are developed for each specific hydrodynamic operating regime of the wet cooling tower.
The results obtained from the experiment indicated that this particular type of cooling tower demonstrated notably favorable hydraulic characteristics, resulting in energy savings. It was concluded that the inlet water temperature had no significant impact on the pressure drop, which aligns with earlier research by Kloppers [ 27 ], and Kloppers and Kröger [ 26 ]. Moreover, the “VGA” fill type showed lower pressure drop values compared to other types of cooling tower fill media [ 38 39 ]. This indicates that cooling towers equipped with “VGA” packing have favorable momentum transport characteristics, leading to a reduction in pressure loss and potential energy savings [ 37 ].
Table 7 presents a compilation of previous research findings concerning wet cooling towers, with a focus on various aspects. Many researchers have explored wet cooling towers, given their incorporation of fill media. These investigations delve into relevant data and ranges for novel designs or alterations. Moreover, the table includes studies investigating the impact of fill configurations on cooling tower performance.However, it is essential to acknowledge that the research output can offer practical ranges, even though each cooling tower operates under specific conditions and constraints. Therefore, during the design process, it is advisable to first consider the specific limitations and applications of the cooling tower and then refer to the table to identify the closest operating conditions for better performance prediction. This approach ensures that the design aligns with the tower’s unique operating requirements, leading to more efficient and effective cooling solutions.
In the context of cooling towers, fouling is the phenomenon characterized by the accumulation of unwanted substances, including biological growth on the surface of the plastic water film flow area. This deposition hinders the cooling process and can lead to an excessive build-up of material on the packing. In more severe cases, fouling can even lead to a decrease in the overall efficiency of the cooling tower. An indicative sign of this is when fill fouling disrupts the flow of air and water through the tower. It is noteworthy that plastic fills are more susceptible to fouling compared to the conventional splash bars. The fouling of cooling tower fills stands out as a pivotal determinant influencing the thermal performance of the cooling tower. As time goes by, this fouling progressively reduces the effectiveness and capability of the tower [ 55 ]. Among the many reviewed papers concerning wet cooling towers, only a few of them took the effects of fouling into consideration, particularly through experimental approaches. The processes involved in cooling tower fouling are indeed quite complex. Due to the intricate nature of the fouling mechanisms, quantifying the fouling process using conventional models becomes challenging. As a result, there is a rising demand to understand the actual influence of various applications [ 56 ]. Qureshi and Zubair [ 55 ] conducted an experimental research and simulation study and found that the tower performance experiences some 18% decline compared to the clean condition of fill media. Despite the ongoing increase in weight caused by fouling, the tower’s performance reached a stable state after this decline and can cause an increase in water outlet temperature. In an alternative modeling approach to fouling, Guo et al. [ 57 ] analyzed the impact of fouling using neural networks. They focused on PVC fill media and investigated how fouling affects the tower’s performance and efficiency. The study spanned a 250-day period, during which the effectiveness (defined as the ratio of the actual temperature difference to the maximum achievable) decreased from 0.51 to 0.14 for a water inlet temperature of 40 °C, and from 0.41 to 0.11 for a temperature of 50 °C.
In summary, according to what is explained in this section, as the wettability of fill media increases, there is a simultaneous augmentation of the heat and mass transfer area, along with an associated rise in pressure drop. The quantitative aspects of these increments represent critical parameters on which engineers and researchers need to focus. It is essential to note that an increase in the effective heat and mass transfer area results in a reduction in the approach, subsequently leading to a smaller cooling tower size for a constant load. Conversely, an escalation in pressure drop contributes to higher fan power consumption and the potential for noise generation. Furthermore, the potential challenges of fouling and fill media failure, especially at low temperatures, are to be considered. To bridge this knowledge gap, experiments should be conducted by researchers under well-defined standard conditions or tailored to their specific climate. Regrettably, a substantial portion of research papers reviewed did not present data based on such defined conditions. Therefore, not only is this research deficiency highlighted by this paper, but it also provides a comprehensive review of recent studies on fill media in cooling towers, offering data that can be pragmatically applied in design practices.
The arrangement of fill packing is a critical parameter influencing a cooling tower’s performance due to its substantial impact on heat and mass transfer. Different fill packing configurations can lead to varying degrees of pressure drop in cooling towers, which can affect the overall performance. The type and size of fill media employed can also affect the optimal fill packing configuration for a given cooling tower. Additionally, the orientation of fill media can impact both the packing configuration and the overall performance of cooling towers.
A well-designed fill media configuration can enhance cooling tower efficiency and minimize operating costs. Optimal fill packing configurations vary and depend on the specific application and operating conditions of a cooling tower. Proper maintenance and cleaning of fill media is essential to maintain the desired packing configuration and performance of cooling towers.
Advances in fill media technology are resulting in novel and innovative fill packing configurations that improve the performance in cooling towers. These designs incorporate high-efficiency media, improved air and water mixing, and reduced pressure drop, resulting in enhanced cooling tower performance and reduced operating costs.
59,Since the 1940s, researchers have been primarily focused on investigating the thermal efficiency of fill media used in cooling towers [ 58 60 ]. This section aims to provide an overview of the latest research conducted on this topic. Nevertheless, the current studies presented in Table 8 offer brief explanations of some of these different configurations, most of which have been experimentally investigated over the past few decades.
This review study serves as a base of reference for recent efforts, providing a comprehensive overview of the recent advancements and progress in fill media technology used in cooling towers with different materials and configurations. By analyzing existing research and empirical test data, detailed insights into the potential benefits of these innovations are studied. As detailed within this research, the majority of the experimental literature is related to cooling towers concentrated on operations conducted within the Range of 2.5 °C to 25 °C (temperature difference between liquid inlet and outlet) and within an Approach of 1 °C to 9 °C (difference between liquid outlet temperature and air inlet temperature). In conclusion, the investigations revealed that the performance of the fill area in a wet cooling tower is influenced by multiple parameters and operating conditions, such as the fill material type, fill geometry, water flow rate, water temperature, air flow rate, and ambient conditions. Understanding and considering these factors are crucial for designing and maintaining the fill area to optimize the cooling tower’s overall efficiency and to extend its lifespan. The incorporation of improved fill materials and configurations has the potential to significantly enhance cooling tower performance and efficiency. In some reported cases, by utilizing new fill media configurations and material, the cooling range, effectiveness, and Merkel number experienced a substantial increase of 28%, 85%, and 131%, respectively. As industries continue to seek environmentally sustainable and energy-efficient solutions, the proper selection and utilization of fill media technology are of paramount importance. Cooling towers that capitalize on these innovations can contribute to reduced energy consumption and lower environmental impact, thus aligning with the broader goals of sustainable industrial practices. Overall, this study summarizes and emphasizes the importance of ongoing research and development in the field regarding fill media technology for wet cooling towers. By analyzing the potential of novel materials and optimized configurations, engineers and designers can understand the enhanced cooling tower performance, leading them to select a more suitable filling prior to achievement of more efficient industrial processes which cast a positive impact on energy consumption and environmental sustainability.
In terms of future research directions, this study identified a notable gap in the field of filling media investigation. To advance this area, it is advisable to initially explore the impact of fill media under a consistent cooling capacity and subsequently assess the influence of implementing the chosen fill media under well-defined conditions based on available standard like ASHRAE or AHRI or defined by researchers based on their specific climate. Moreover, there is a clear need for comprehensive research focused on the pressure drop caused by the fill media within cooling towers. This aspect is crucial, as it can potentially jeopardize overall performance and may affect other influential parameters.