Analysis of the Effect of Packing Materials (Fills) and Flow Rate on the Range and Efficiency of a Forced Draft Evaporative Cooling Tower

Abstract

In the present study, experimental investigation is carried out on two different kinds of packing materials (fills). PVC fills that are traditionally used in the industry are compared and analyzed against the cellulose-based paper fills. Different mass flow rates of air are used to study the effect of the flow rate of air on the forced draft cooling tower. The volume flow rate of water also varied, and the range of the cooling tower, along with efficiency, was analyzed. Along with these two parameters, the effect of inlet water temperature on the performance of the tower was studied. Cooling tower efficiency was plotted against different L/G ratios ranging from 0.95 to 7.67. Results showed that the type of packing has a significant impact on the cooling tower performance. Paper fills gave a maximum cooling tower efficiency and range equal to 93.12% and 16.5 °C, respectively. The optimal L/G ratio range of 0.96 to 1.44 was identified as the point at which the cooling tower demonstrated its highest performance. The effect of the mass flow rate of water on the performance of the tower was far greater compared to the volume flow rate of water and inlet water temperature. The paper fills are found to be most effective under current experimental conditions, and the same can be implemented in the industrial towers under a wide spectrum of inlet water temperatures, mass flow rates of air, and volume flow rates of water.

1. Introduction

A counterflow wet cooling tower is a device used in power generation plants which rejects heat from the water and sends it instead to the atmosphere [1]. The tower is used based upon the principle of evaporative cooling. The towers are also used in various industrial applications like refrigeration, etc. [2] The air comes into direct contact with the hot water, causing the evaporation of some water from the stream. As a result, it utilizes the latent heat of the water and cools it [3,4]. These types of cooling towers in which air and water are in direct contact with each other are called wet cooling towers. An external blower is used to create a forced draft situation, and this type of tower is called a forced draft cooling tower. The location of the blower or fan determines whether it is an induced or forced draft tower.

Packing materials or ‘fills’ are used in between the air and water flow streams, in order to increase the contact area between air and water. The fills increase the contact time of the air and water, which allows a transfer of heat from water to air. These fills should be strong, lightweight, and long-lasting where these are held together by a steel frame. Fills are one of the most relevant dominating factors influencing the performance of the tower. Pooriya [5] conducted the experimental work on wet cooling towers. The towers’ performance was parametrically investigated in order to understand the effects of water as well as air flow rate, water temperature at inlet, and type of packing. Three different types of PVC packings were studied separately to investigate the influence of rib numbers. Navarro [6] conducted an experimental study on a counter flow forced draft, inverted cooling tower to evaluate its performance. Through optimization efforts, he identified that the thermal performance of the inverted cooling tower is optimized when a uniform flow pattern is maintained along with a higher surface area. This combination was suggested as the best approach to achieve superior performance in the inverted cooling tower. Mehdi [7] experimentally investigated the thermal performance of mechanical draft cooling towers by changing parameters such as hot water temperature, air mass flow rate, water flow rate, and stage numbers of fills. Farhad [8] performed an experimental study, and a comparative study in terms of tower characteristic ratio, water to air flow ratio, and efficiency for two film type packings were presented for a wide range of (L/G) ratios from 0.2 to 4 where vertical corrugated fills and horizontal corrugated fills were analyzed. Lemouari [9] experimentally investigated the performance characteristics of a counter flow wet cooling tower represented by the heat rejected by the tower and its thermal effectiveness. The tower is filled with a Vertical Grid Apparatus type fill which is 0.42 m high and contains four galvanized sheets having a zigzag form, between which are disposed three metallic vertical grids in parallel having a cross-sectional test area of 0.15 m × 0.148 m. A first regime, called the Pellicular Regime (PR), exists with low water flow rates. A second regime, called Bubble and Dispersion Regime (BDR), appears with relatively larger water flow rates. Kong [10,11] investigated a counterflow forced draft wet cooling tower where foamed corrugated type fills were used and range and efficiency considerably decreased, whereas with increase in length there was an increase in heat rejected. Singla [12] used the expanded wire mesh of 120 cm height and 0.09 m² cross-section and found that the effectiveness increased when air flow rate increased and water flow rate decreased. Shinde and Gulhane [13] studied the cellulose type fills and PVC fills which show a similar trend of rise in efficiency with increases in the air flow rate. In the research conducted by Alimoradi [14], the focus was on packing compaction and Vertical grid PVC with three different levels of compaction. The packing had a cross-section of 15 cm × 15 cm. The study revealed that there was an increase in efficiency with increases in the inlet water temperature and packing compaction.

R. Ramkumar [15] studied the optimization of the cooling tower using the L9 orthogonal Taguchi method and established that the Taguchi method is a reliable tool for optimization. He also established that packing configuration (PVC plates) was the more dominant factor, followed by the L/G ratio. Kariem [16] analyzed the counterflow WCT using high-density polyethylene fills and concluded that the cooling tower performed its best at the lowest L/G ratio. Jourdan [17] visualized the water film thickness of the falling water on the fills, and the wetting rate of the fills were studied. They observed the mean water film thickness in the range of 300–600 μm. However, there are recuperations with increases in the packing density, such as heavy maintenance related to fouling in the tower [18,19]. Sampath [20] conducted experimental research on the humidification system using ducts to study different fills using crossflow and counterflow methods. The flow rate of air varied from 0.062 kg/s to 0.083 kg/s, and the volume flow rate of water was kept constant at 450 L/hour. The maximum heat transfer rate was observed in the cases of counterflow setup and paper-based cellulose fills. Malli et al. [21] conducted an experiment to examine two variations of cellulosic pad cooling (5090 and 7090). They discovered that the pressure decreased and the rate of water evaporation increased as the frontal velocity and pad thickness were elevated. Conversely, the efficiency and humidity variation decreased with higher frontal velocity. Bishoyi et al. [22] performed an experimental inquiry on two forms of evaporative pad cooling in the climatic conditions of India and reported that Honeycomb paper-based evaporative pad cooling exhibited a higher energy efficiency ratio and cooling capacity compared to Aspen-based pad cooling. Gao et al. [23] conducted an empirical investigation concerning the thermal efficiency of wet cooling towers, examining five distinct configurations of fill layout. They concluded that non-uniform layout patterns have the potential to enhance thermal performance by up to 30% when compared to uniform layouts.

An extensive research work is conducted by researchers in this area where parameters affecting the performance of cooling tower are varied along with the fill densities. Material-specific fills are not studied extensively in the literature. Also, researchers have generally overlooked the utilization of cellulose-based packings in forced draft wet cooling towers, despite their extensive exploration in the literature as evaporative cooling pads [24] and as pre-cooling media for conventional cooling towers [25,26]. However, given the availability of cellulose-based packings and their satisfactory performance in other evaporative cooling applications [20], it is justifiable to investigate their impact on these systems.

In order to address this research gap, a counterflow wet cooling tower (forced draft) was designed and fabricated for experimental purposes. The experiments focused on two types of fills: PVC-based fills and cellulose paper-based fills, both having the same specific area. The primary objective of the experimental work was to evaluate and compare the performance of these fills in terms of cooling tower range, efficiency, and inlet temperature, considering various air and water flow rates. Additionally, the study aimed to determine the optimal packing type. The investigation also involved the optimization of parameters and the identification of the best operating conditions.

2. Experimental Setup

In Figure 1, the setup can be divided into four parts consisting of: (a) A cooling tower which is made with an insulated acrylic sheet of 10 mm thickness; (b) An electrical panel control system which houses all the electrical controls along with the temperature sensors, heater input controls, data acquisition system, and system main switches; (c) A heating water tank and the collection tank that houses the heaters and temperature and humidity measuring sensors, respectively; and (d) Blower, air flow system and water flow systems.

The tower here is housed with PVC-based fills and paper-based cellulose fills, as shown in Figure 1. In the experimental setup, direct contact between air and water is established, with the air flowing from the bottom of the tower and the water being sprinkled from the top using a shower system. Pt-100 sensors are employed to measure the temperature at both the inlet and outlet of the tower. This allows for accurate monitoring of the temperature changes throughout the cooling process. The relative humidity and temperature sensor are used at the inlet and the outlet of air so as to record the temperature and humidity of air. Drift eliminators are housed in the tower before the air exits the tower, which removes the entrained droplets of water from the air stream. These eliminators induce a certain amount of pressure differential, which helps in having uniform pressure inside the tower. The cross-section area of the tower is 150 × 150 mm and the length of the tower is 750 mm, but the length of fills used is restricted to 600 mm so as to house the sensors and eliminators in the tower. The tower is insulated with the help of transparent acrylic sheets of 10 mm thickness, which shield the tower from any atmospheric interference with respect to the heat transfer. Electrical panel control systems house all the sensory connections to the data acquisition system along with the control knobs for the water flow, air flow, heater on/off, and the master switch. It also houses an electrical metering system to monitor the heat supplied by the heater in the form of KWhr units. The screen showed the live data of the sensors at the time of experiments. The water used in the setup is deionized water, which is stored in the water tank consisting of heaters. A 2 KW system of heaters is used for heating and maintaining the water temperature at 40 °C, 42.5 °C, and 45 °C. A digital controller is used to control these temperatures (±0.5 °C in each case). The collection tank provides an inlet to the air through a blower outlet which has temperature and humidity sensors. The Pt-100 sensors are also housed here to record the outlet temperature of water. This collection tank is then connected to the water tank with heaters.

A centrifugal blower is used to force the air into the tower, which is controlled by a gate valve and measured by the help of an anemometer. The air travels through a steel pipe with a diameter of 25.4 mm which exits the air into the tower. Figure 2a shows the schematic line diagram of the tower along with the sensor locations and the flow of fluids. The heated water is pumped towards the top of the tower with the help of a water pump. A rotameter (capacity: 0–200 L/h) is installed between the water pump and the shower and measures the water flow rate. The shower (diameter of holes: 1 mm, 64 holes) then sprinkles the water in the tower onto the fills. Air flowing from the bottom of the tower then comes in contact with the hot water, and heat and mass transfer takes place. The cooled water is collected by the collecting tank where the temperature of the water is recorded.

The method proposed by National Instruments [27] was faithfully adhered to for the adjustment of thermocouples. Two designated temperatures were accurately sustained using a chilled basin [28] and vigorously boiling water for the adjustment objective. The factory-calibrated flow meter was diligently readjusted by allowing water to pass through it at a precise flow rate for 10 min. The water gathered in a reservoir from the flow meter was precisely measured to determine the flow rate using the provided duration and density [29]. Moreover, the wind gauge was aligned at different velocities at assorted intervals with the assistance of a wind tunnel. The instruments that are used to measure the parameters at the input to cooling tower are as per

Table 1. Further information on measuring devices utilized during the tests.

Parameter	Sensor Type	Range of Sensor	Accuracy	Resolution
Air (DBT)	Pt-100	−20 °C to 100 °C	±0.1 °C	0.1 °C
RH	Hygrometer	0 to 99%	±1%	0.1%
Air Velocity	Vane Anemometer	0.4 to 30 m/s	±0.1 m/s	0.1 m/s
Water Flow Rate	Rotameter	0–200 L/h	±2%	5 L/h

.

Whenever measurements of any parameter are conducted, there will inevitably be some degree of error associated with the instrument used or the measurement procedure itself. To ensure the reliability of the results, it is essential to assess and study the uncertainty involved in the measurements. By understanding and quantifying the uncertainty, researchers can evaluate the validity of the obtained results and account for any potential discrepancies or variations in the data. Properly considering uncertainty helps to enhance the credibility and accuracy of the study’s findings. The equations used to calculate the uncertainty are as below:

Range of the cooling tower: Equation (1) provides the calculation for the range in a cooling tower, which is determined as the difference between the inlet hot water temperature and the exit cold-water temperature:

$$\left(\Delta T\right)=T_{w1}-T_{w2}$$

(1)

$$\frac{\partial \left(\Delta T\right)}{\Delta T}=\sqrt{{\left(\frac{{\partial T}_{w1}}{T_{w1}}\right)}^2+{\left(\frac{{\partial T}_{w2}}{T_{w2}}\right)}^2}$$

(2)

Equation (2) is used to calculate the uncertainty in the Range of the cooling tower.

Cooling Efficiency: Equation (2) defines the calculation for the efficiency of a cooling tower. It is obtained by dividing the range of the cooling tower by the difference between the hot water temperature and the inlet wet bulb temperature:

$$\epsilon =\frac{T_{w1}-T_{w2} }{T_{w1}-T_{wb} }=\frac{{\Delta T}_1}{{\Delta T}_2}$$

(3)

$$\frac{\partial \left(\epsilon \right)}{\epsilon }=\sqrt{{\left(\frac{{\partial \Delta T}_1}{{\Delta T}_1}\right)}^2+{\left(-\frac{{\partial \Delta T}_2}{{\Delta T}_2}\right)}^2}$$

(4)

Equation (4) is used to calculate the uncertainty in the cooling efficiency (ε) of the cooling tower.

Table 2. Uncertainty values of the dependent variables.

Parameter	Percentage of Uncertainty (%)
Range of cooling tower	1.72
Cooling efficiency	2.86

presents the uncertainties obtained from the measurements. These uncertainties are relatively small, indicating that the results obtained from the study are reliable.

Figure 3 shows the types of fills used in these experiments. The cellulose-based paper fill and PVC fill are both stacked in a metal frame which is divided into five parts. This metal frame provides support to the fills to hold their position against the air and water pressures. Five decks of each are used in the experiments, both having a surface area of packing of 0.405 m² and a dimension of the metal frame of 600 × 150 × 150 mm.

3. Results and Discussion

In this experimental study we have investigated various parameters that affect the performance of cooling towers. There are two types of parameters: one is controlling parameters, such as water flow rate, mass flow rate of air, and types of fills, and the other is performance parameters, like temperature range, mass of evaporation, and efficiency.

The initial phase of this study involves collecting empirical data for both types of fills (PVC-based and cellulose-based paper fills) installed in a cooling tower. Just prior to starting the experimentation, the calibration of temperature sensors, flowmeter, and anemometer was performed. In this empirical investigation, a comprehensive factorial design was utilized, incorporating seven different levels of water mass flow rate and four different levels of air flow rate for each fill. Initially, the water was heated using an electric heater to achieve and maintain a consistent temperature throughout the experiment, with three temperature options: 45 °C, 42.5 °C, and 40 °C. This temperature was controlled using a temperature controller. Once the desired water temperature was reached, the water pump and the blower were activated. Subsequently, hot water was introduced into the cooling tower from the top and evenly distributed across the fills, allowing it to interact with the incoming air. As the air ascended, it underwent heating and humidification, and the resulting humid and warm air was then discharged into the environment, while the cooled water was directed into the storage tank.

3.1. Effect of Inlet Water Temperature

Dependence on the temperature differential between the water and cooling efficiency on the water volume flow rate was experimentally evaluated for various inlet water temperatures, as Figure 4 and Figure 5 depict. In the experimental investigation, the relationship between the water temperature difference and cooling efficiency was examined, focusing on the dependence on the volume flow rate of water. The study considered various temperatures (45, 42.5, and 40 °C) for the inlet water while keeping the mass flow rate of air fixed at 0.0146 kg/s. The experiment specifically utilized cellulose-based paper fills as the packing material. By varying the volume flow rate of water and observing the resulting changes in water temperature difference and cooling efficiency, the study aimed to gain insights into the performance characteristics of the cooling system. Based on Figure 4 and Figure 5, several observations can be deduced. Firstly, when examining the same temperature of the incoming water, it becomes evident that both the ΔT of the water and the effectiveness of cooling decline as the volume flow rate of water amplifies. This implies that augmented water flow rates lead to diminished ΔT and decreased cooling efficiency.

Additionally, both figures illustrate a distinct association between the ΔT of the water and the temperature of the incoming water, as well as between the cooling efficiency and the temperature of the incoming water. Precisely, heightened temperatures of the incoming water correspond to amplified ΔT of the water and increased cooling efficiency, but it is clearly observed here that this relation is not directly proportional. For example, at 0.042 kg/s of volume flow rate of water, the increase in ΔT is 14.28% when the temperature is raised to 42.5 °C from 40 °C, whereas, for the same volume flow rate of water, when the temperature is raised to 45 °C from 42.5 °C, the increase in ΔT is 22.22%. The reason for this behavior is the heat transfer that takes place between the air and water, as the temperature of air entering the tower is constant in all the cases.

3.2. Effects of the Mass Flow Rate of Water

Figure 6 describes the variation of temperature range with an increasing volume flow rate of water at different air flow rates. Four levels of mass flow rate of air are investigated here, i.e., 0.0073, 0.0097, 0.0122 and 0.0146 kg/s, where the inlet water temperature to the tower is fixed at 42.5 °C and cellulose-based paper fills were used. Temperature range decreases with an increasing flow rate of water for a particular mass flow rate of air. Moreover, the temperature range increases with an increasing mass flow rate of air. In Figure 6, certain trends are observed at different flow rates of air. Increasing flow rate also increases the capacity of air to hold the water vapour along with the heat from the hot water, and heat transfer efficiency goes up. At a particular temperature of inlet water and air flow rate, efficiency of cooling decreases by increasing the volume flow rate of water. The trend shows that the mass flow rate of air is the most effective parameter in the efficiency enhancement. It is also observed in Figure 6 and Figure 7 that the steep slopes are gradually being converted to gentle slopes as we increase the volume flow rate of water. This occurs because the capacity of the air to carry the energy is the same whereas the amount of energy in form of volume flow rate of water pumped into the tower is increasing. This also suggests that the water holding capacity of the cellulose fill in its pores is at its maximum, reaching a point where it no longer contributes significantly to the heat and mass transfer process.

3.3. The Effects of the Mass Flow Rate of Air

In Figure 8, as the air flow rate increases, there is an improvement in the efficiency of the cooling tower. This is because the increased air flow rate enhances the interaction between the air and water, resulting in larger drops in the water temperature. Consequently, the range, which is the temperature difference between the hot water and the cold water, increases, leading to a higher cooling tower efficiency. Similar observations have been made for different water flow rates. When the water flow rate is excessive, it hampers the effective interaction between the air and the water, resulting in a decrease in the range and ultimately the cooling efficiency.

Therefore, finding the optimal water flow rate is crucial to ensure efficient interaction between the two fluids and maximize the performance of the cooling tower.

When efficiency is compared with inlet water temperature at different levels of volume flow rate of water, it is evident that efficiency will increase with decrease in water flow rate, but Figure 9 clearly states that there is less dependency on inlet water temperature while comparing with the water flow rate. Clearly, a dominant parameter is established here.

3.4. Comparison of Fills Using the Range and Cooling Efficiency of the Cooling Tower

In Figure 10 and Figure 11, the cellulose-based paper fills are compared with the PVC fills. The temperature of inlet water is kept constant at 42.5 °C and the mass flow rate of air is maintained at 0.0122 kg/s. All other atmospheric conditions considering the inlet humidity of air and the wet bulb temperature of the surroundings are kept constant.

The influence of the filling material on cooling efficiency can also be observed in Figure 10 and Figure 11. The increase in range is at a maximum of 7 °C and a minimum of 3 °C, when the water flow rate continues increasing. Around 80% of increase in range is achieved by using paper-based fills. In Figure 11, we observe that the maximum rise in efficiency of 79% is achieved at a water flow rate of 0.035 kg/s, whereas the least rise in efficiency is at a water flow rate of 0.014 kg/s, followed by 0.021 kg/s. The efficiencies are 41.17% and 60%, respectively. As we continue increasing the water flow rate, the rise in efficiency because of the paper-based fills first goes on increasing and then gradually decreases. Of the two categories of fills used, cellulose-derived paper fills exhibit the utmost effectiveness in cooling tower operations. This superiority arises from the property of the cellulose material that has the capacity to hold water for longer durations. This leads to increases in the contact time of the air and water, which further helps in the exchange of heat and mass transfer. These fills are composed of rigid paper containing cellulose as one of its main ingredients. This unique composition grants it superior natural cooling capabilities through the process of evaporation. The packing can consistently lower air temperatures and provides pure air due to its properties of material and design. Furthermore, the enhanced wettability or surface density and superior material properties contribute to the exceptional performance of these fills, surpassing that of other alternatives like PVC.

To analyze the optimal mass flow rates of air and water and their impact on the cooling tower’s performance, a graph comparing the L/G ratio to the cooling tower efficiency was plotted. Figure 12 presents a comparative study between PVC fills and paper fills, specifically examining the efficiency of the tower at different L/G ratios. As previously discussed, it is evident from this figure that the efficiency of paper fills is higher than that of PVC fills. In the given scenario with a mass flow rate of water at 0.014 kg/s, the efficiencies of paper fills and PVC fills are 91.65% and 47.41%, respectively, when the L/G ratio is 0.96. As the L/G ratio increases to 1.44, the efficiency of paper fills decreases to 82.17%, and for PVC fills, it drops to 44.9%. While the mass flow rate of water remains constant, the mass flow rate of air decreases from 0.0146 kg/s to 0.0097 kg/s. Further increasing the L/G ratio leads to a significant decline in efficiency until reaching an L/G ratio of 3, beyond which the efficiency drop becomes more gradual. Thus, the optimal L/G ratio range of 0.96 to 1.44 was identified as the point at which the cooling tower demonstrated its highest performance. Conversely, the cooling tower showed its lowest efficiency when the L/G ratio exceeded 5.75.

4. Conclusions

The experimental exploration centered on two distinct fillings, possessing varied material properties and employed as packing substances in a cooling tower. The examination involved scrutinizing parameters such as the initial water temperature, airflow and water flow rates, and the types of fills implemented. The performance of cellulose-based paper fills and PVC fills was evaluated by modifying the rates of airflow and water flow. The primary findings can be summarized as follows:

• There exists a direct relationship between the temperature difference of the water, the cooling efficiency, the temperature of the incoming water, the fills utilized as packing, and the airflow rate.
• Increasing the water flow rate results in a reduction in the temperature difference of the water and cooling efficiency.
• Optimal cooling efficiency is attained with lower water flow rates, higher airflow rates, and higher initial water temperatures.
• Cellulose-based paper fills exhibit superiority in terms of augmenting the cooling tower’s range and efficiency, making them ideal for industrial applications.
• The optimal L/G ratio range of 0.96 to 1.44 was identified as the point at which the cooling tower demonstrated its highest performance. Conversely, the cooling tower showed its lowest efficiency when the L/G ratio exceeded 5.75.
• Enhancing the specific packing areas leads to improved performance in terms of cooling tower range and cooling tower efficiency, as well as heat and mass transfer rates.
• These deductions provide valuable insights into the performance characteristics of cellulose-based paper fills and PVC fills in cooling tower applications, highlighting the advantages of cellulose-based paper fills in terms of heat transfer, cooling efficiency, and range.