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Abstract: Based on the experiments on spray induction and spray cooling conducted in a laboratory-scale jet cooling tower, the air induction capacity of the tower under different nozzle arrangements and spray pressures was investigated, as well as the thermal performance of the tower at various tower lengths, spray pressures, inlet air parameters, and inlet water temperatures. Some meaningful conclusions were drawn. Keywords: Jet cooling tower, Spray induction, Heat and mass transfer, Spray cooling Abstract: Based on a series of experiments, this study investigates the relationship between air induction capacity and the arrangement of spray nozzles as well as the pressure of the spray water. It also examines the effects of tower length, spray water pressure, inlet air properties, and water temperature on the thermal performance of the tower, and draws some important conclusions. Keywords: Spray cooling tower, Spray induction, Heat and mass transfer, Spray cooling 1. Introduction With the rapid development of the national economy and the improvement of people’s living standards, the water consumption in industrial and commercial buildings has increased significantly, with cooling water accounting for a large portion of this usage. Installing a cooling tower to achieve the recycling of cooling water is an economical and effective measure for saving water and energy, bringing significant economic and social benefits. Cooling towers come in wet and dry types. In dry cooling towers, air and water do not come into direct contact; there is only heat exchange, no mass exchange. In a wet cooling tower, air comes into direct contact with water, allowing for heat and mass exchange, which results in a better cooling effect. In wet cooling towers, counterflow and cross-flow mechanically ventilated cooling towers were widely used in the past; their main components are fans and wetting fillers, which results in high noise levels, significant vibration, and complicated maintenance. In the early 1970s, another type of cooling tower emerged abroad—the jet cooling tower. Its structure is similar to that of the spray chambers used in window installation, but it does not rely on a fan for power nor does it have any wetting fillers; instead, it draws in the surrounding air into the tower through the effect of the sprayed water flow, where it comes into contact with the spray droplets to undergo heat and moisture exchange, thereby achieving the purpose of cooling the water. Figure 1 is a schematic diagram of the construction principle of a jet cooling tower. http://www.studa.net/Newspic/20051226/10572859.gif Figure 1: Schematic diagram of a jet-type cooling tower 1. Water distributor; 2. Nozzles and spray pipes; 3. Water outlet pipes; 4. Filter; 5. Water collection tank; 6. Water collector. Since a jet-type cooling tower has no fans nor packing, its structure is simple, making it easy to use and maintain. It also generates less noise and vibration, and its appearance is more compatible with buildings. The earliest jet cooling towers were developed by the American company BAC; currently, there are 12 different models available, with a cooling capacity ranging from 3.9 to 663 m3/h. In our country, work in this area was also carried out between 1977 and 1980, but the prototypes developed did not achieve the desired results. Between 1988 and 1990, the China Electronics Engineering Design Institute developed jet cooling towers. In collaboration with the Department of Environment at Tsinghua University, it conducted extensive testing on nozzle selection, and developed the CE-1-PL type jet cooling tower based on Japanese models as references. During the development process, they also made improvements to the flow-stabilizing device at the air inlet and the gas-water separation device at the outlet. However, despite the fact that jet cooling towers have been in use for over 20 years, there has been insufficient research on their operating principles both domestically and internationally. Few improvements have been made to these towers over the years; therefore, it is necessary to conduct more in-depth studies on this type of tower. Since 1092, the Department of Thermal Energy at Tsinghua University has collaborated with the Beijing Energy Conservation Office to conduct experimental research and mathematical simulations on jet coolers. The results obtained have provided guidance for the improvement of such coolers, and this paper mainly focuses on the experimental research work. 2 Structure of the experimental jet tower and test bench To study the aerodynamic performance (spray induction performance) and thermal performance (spray cooling performance) of jet coolers, a test bench was built in the Air Conditioning Laboratory at Tsinghua University. The unit consists of a test jet tower, an air handling system, a water system, and measuring instruments. The cross-sectional dimensions of the experimental spray tower can be changed; there are various possible arrangements for the nozzles within the tower. The length of the spray tower can be adjusted, and the water pressure used for spraying, as well as the parameters of the air and water entering the tower, can all be controlled. http://www.studa.net/Newspic/20051226/10572859.gif Figure 2: Nozzle arrangement and boundary dimensions. http://www.studa.net/Newspic/20051226/1057299511.gif Figure 3: Air handling system. Figure 2 is a cross-sectional view of the spray tower used in the experiment. The tower has 3 rows of nozzles, with 5 to 7 nozzles per row. Figure 3 shows the air handling system, which can treat a mixture of outdoor fresh air and a portion of return air to meet the parameters required for the tests. This system includes an exhaust fan, a supply fan, a humidifier, and a heater. Figure 4 is the schematic diagram of the water system, which supplies hot water at a constant temperature to the spray tower; the water temperature is controlled by electric heating, and if the heat generated by the electric heating is insufficient, the fuel-fired hot water boiler in the water system can be activated. http://www.studa.net/Newspic/20051226/1057291084.gif Figure 4: Flow diagram of the water system 1. Spraying tower 2. Float flow meter 3. Electric heater 4. Water pump 5. Mixing tank 6. Fuel-fired hot water boiler The instruments used in the experiment mainly include a glass mercury thermometer with a resolution of 0.1℃ ； Ventilated wet and dry bulb thermometer ； LZB-40 type rotameter ； QDF-2 thermoball anemometer ； Copper-constantan thermocouple and UJ31 potentiometer ； Y-150 type Bourdon tube pressure gauge, range 0～0.4MPa. 3 Test Content and Test Conditions 3.1 Aerodynamic Tests To analyze the various factors that affect the ability to induce air within the spray tower, tests were conducted under different conditions. For each test condition, the amount of air entering the tower, L, and the water flow rate at the same time, Q, were measured; thereby the air-to-water ratio λ could be calculated, where λ = L/Q. In the test conditions: ① The water spray pressure p was varied 7 times, with values of 0.05, 0.075, 0.1, 0.125, 0.15, 0.175, and 0.2 MPa respectively ； ②The horizontal spacing S between the nozzles was changed 4 times, to 40, 50, 60, and 70 mm respectively. ③ The vertical spacing K between the nozzles was changed 4 times, to 200, 230, 250, and 280 mm respectively. ④ The boundary dimensions changed accordingly, with distance A being 70, 90, and 105 mm respectively ； Distances from B are 125, 150, 175, and 205 mm respectively. 3.2 Thermal performance tests: In order to identify the factors that affect the temperature drop of the water inside the tower, tests were conducted under various operating conditions. For each condition, the inlet and outlet water temperatures, tw1 and tw2, were measured. The temperature drop Δt of the water was calculated, while the dry-bulb temperature t and wet-bulb temperature ts of the air entering the tower were also measured. The test conditions are as follows: ① The water temperature entering the tower was changed 3 times, at 37, 40, and 45 respectively℃ ； ②The wet-bulb temperature ts of the air entering the tower varies between 23 and 29°C ； ③The tower length XL changes 3 times, being 1700, 2060, and 2500 mm respectively ； ④The spray pressure p was varied 5 times, at 0.05, 0.075, 0.1, 0.125, and 0.15 MPa respectively ； 4 Test Results and Analysis 4.1 Aerodynamic Tests 4.1.1 When the nozzle spacing and boundary conditions remain constant, the air-water ratio in the spray tower varies with the spraying pressure, as shown in Figure 5-7, where the boundary dimension A stays unchanged. Two points can be observed from these graphs: first, the gas-to-water ratio increases as the nozzle spacing K and S increase ； Second, the gas-to-water ratio increases as the injection pressure rises, and remains unchanged once it reaches a certain value. Although an increase in the gas-to-water ratio λ will improve the cooling effect, it leads to higher investment costs, and an excessive increase in the spray pressure will also raise energy consumption. Furthermore, as can be seen from Figures 5 to 7, as K×S increases from 250×50 (mm) to 280×60 (mm), the increase in λ slows down, and once the injection pressure p reaches 0.15 MPa, the increase in λ becomes insignificant. Therefore, in the design of the spray tower, K×S should be set at 280×60 (mm), and during operation, p should be set between 0.1 and 0.15 MPa. http://www.studa.net/Newspic/20051226/1057311694.gif Figure 5: Relationship between air-water ratio and spray pressure, S=60mm, K=280, 250, 200mm http://www.studa.net/Newspic/20051226/1057318260.gif Figure 6: Relationship between air-water ratio and spray pressure, S=60mm, K=280, 250, 230, 200mm http://www.studa.net/Newspic/20051226/105731248.gif Figure 7: Relationship between air-water ratio and spray pressure, S=40mm, K=280, 250, 230, 200mm 4.1.2 The variation in the air-water ratio in the spray tower as the spray pressure changes, for different boundary dimensions, is shown in Figure 8. Figure 8 shows the test curves obtained under the conditions of K=250 mm, S=60 mm, with the A value remaining constant at 90 mm, and the B value being changed twice (to 155 mm and 125 mm). This figure shows that, when the cross-sectional dimensions of the spray tower and the spraying pressure remain constant, a larger boundary size results in a higher gas-to-water ratio. However, as can be seen from Figure 8, within the range where the injection pressure p changes from 0.1 to 0.2 MPa, the rate of increase in the gas-to-water ratio decreases significantly; in other words, increasing A or B at this point does not result in a large increase in the λ value, because when dimensions A or B are too large, the inducing effect of the injected water stream becomes ineffective. So there is no need to increase A or B. http://www.studa.net/Newspic/20051226/1057328595.gif Figure 8 Relationship between air-water ratio and spraying pressure 4.2 Thermal performance tests 4.2.1 The variation in water temperature drop as a function of spraying pressure is shown in Figure 9. Figure 9 also illustrates the effect of tower length changes on the water temperature drop. As can be seen from the graph, under constant conditions of tower height, inlet water temperature, and inlet air wet-bulb temperature, the decrease in water temperature decreases slightly as the spray pressure increases. This seems to contradict the conclusion that an increase in spray pressure leads to an increased gas-to-water ratio, which facilitates heat and moisture exchange. However, as the spray pressure increases and the droplet velocity speeds up, the contact time between air and the droplets is reduced, which can indeed suppress the upward trend in water temperature drop; as a result, the water temperature drop not only stops increasing but may even decrease. http://www.studa.net/Newspic/20051226/1057332500.gif Figure 9 Relationship between water temperature drop and spray pressure 4.2.2 The variation of water temperature drop with the wet-bulb temperature of the air entering the tower is shown in Figures 10 and 11. http://www.studa.net/Newspic/20051226/1057348314.gif Figure 10: Relationship between water temperature drop and the wet-bulb temperature of the air entering the tower. XL=2060mm, p=0.075MPa, tw1=37℃. http://www.studa.net/Newspic/20051226/1057345167.gif Figure 11: Relationship between water temperature drop and the wet-bulb temperature of the air entering the tower. XL=2500mm, p=0.05MPa, tw1=37℃. As can be seen from the figures, under constant conditions of tower length, spray pressure, and water temperature entering the tower, the water temperature drop decreases as the wet-bulb temperature of the air entering the tower increases. The theoretical explanation for this phenomenon is as follows. Since the jet cooling tower operates on a co-current heat transfer principle, the driving force for the heat and moisture exchange within the tower is the difference between the enthalpy of the saturated air layer on the surface of the liquid droplets and the enthalpy of the air entering the tower; the enthalpy of the air entering the tower depends primarily on its wet-bulb temperature. As the wet-bulb temperature of the air entering the tower increases, its enthalpy rises; as a result, the aforementioned enthalpy difference decreases. The driving force for heat and moisture exchange between the air in the tower and the water also decreases, which leads to a smaller drop in water temperature. Conversely, the decrease in water temperature becomes greater. Generally speaking, the dry-bulb temperature of the air entering the tower has little effect on the water temperature drop. However, when the dry and wet-bulb temperatures of the air in the tower are close to each other, that is, when the air entering the tower is near saturated, the water temperature drop within the tower is also small, as in this case only heat dissipation through temperature difference occurs, with no heat dissipation via evaporation. This situation has occurred many times during the testing process. It can be seen that jet towers are more advantageous when used in areas with relatively low air humidity than in humid areas. 4.2.3 The variation of the water temperature drop inside the tower with the inlet water temperature is shown in Figure 12. As can be seen from the graph, under constant conditions of tower height, spray velocity, and wet-bulb temperature at the inlet, as the water temperature at the inlet increases, the rate of temperature drop also increases. This is because when the water temperature is high, the temperature of the saturated air layer surrounding the water droplets is also high, resulting in a higher vapor pressure. As a result, the difference in vapor pressure between this air and the air entering the tower increases, which facilitates the evaporation of water and the release of heat. http://www.studa.net/Newspic/20051226/1057341095.gif Figure 12 Relationship between water temperature drop and inlet water temperature. Therefore, jet coolers are more suitable for cooling hot or moderately warm water. 4.2.4 The effect of the tower height on the water temperature drop can be seen from both Figure 9 and Figure 12. Generally speaking, the longer the tower, the greater the drop in water temperature. However, tests have shown that when the tower length is increased from 2060 mm to 2500 mm, the increase in water temperature drop is significantly lower than when the tower length is increased from 1700 mm to 2060 mm; therefore, the tower length should not exceed 2500 mm. 4.2.5 Since the water distributors of existing spray towers (with water filters in between) are all located on one side of the tower body, modular assembly is not possible. Therefore, if the water distributors could be placed at the top of the tower, allowing the water distribution pipes to be arranged vertically, it would enable standard modular assembly of spray towers of different capacities. To investigate the feasibility of this approach, the nozzle arrays were modified to be arranged vertically with a spacing of 280 mm, while the longitudinal spacing between the nozzles remained at 60 mm. Thermal tests were conducted at tw1 = 37°C and tw2 = 27°C, and the results showed that the water temperature drop not only did not decrease but increased slightly, reaching over 5°C in both cases. 5 Conclusions 5.1 The main factors affecting the air-water ratio in jet cooling towers are nozzle spacing and spray pressure; a higher air-water ratio facilitates heat and moisture exchange between air and water within the tower, resulting in a greater decrease in water temperature. However, excessively increasing the nozzle spacing and injection pressure is not beneficial, as it will lead to an increase in tower size, initial investment, and energy consumption. Based on the comprehensive test results, it is recommended that the K×S value for the spray tower be set at 280×60 (mm), with a spraying pressure of 0.1 to 0.15 MPa. 5.2 The height of the tower has a certain impact on the temperature drop in the spray tower; when the tower is not long enough, air and water do not have enough time to exchange heat and moisture, so the temperature drop is not significant ； If the tower is too long, the decrease in water temperature will not be significant, and both the initial investment and the floor space required will increase. Tests show that a tower length of no more than 2500 mm is appropriate. 5.3 For a spray tower with fixed structural dimensions, the main factors affecting the temperature drop of the water are the parameters of the air entering the tower, the temperature of the water entering the tower, and the spraying pressure. When the water temperature entering the tower and the spray pressure remain constant, the lower the wet-bulb temperature of the air entering the tower, the greater the decrease in water temperature. Therefore, to achieve a sufficiently large temperature drop in water, spray towers cannot be used in areas with too high outdoor air wet-bulb temperatures. Tests have shown that the wet-bulb temperature of the air entering the tower should preferably not exceed 28°C; otherwise, the water temperature drop will not meet the design requirements. 5.4 Tests have shown that for a spray tower with fixed structural dimensions, within a certain range, adjusting the spray pressure is sufficient to meet the requirements arising from variations in load (cooling water volume), and the change in water temperature drop can also meet the requirements. This characteristic of the spray tower gives it great flexibility in use. Under special circumstances, to reduce the size and floor area of the tower, a higher spray pressure can be used. This characteristic of the spray tower is also known as “equivalent expansion capacity”. 5.5 Tests have shown that a water divider can be placed at the top of the tower to enable the creation of modular spray cooling towers.