In recent years, water collecting systems, with the associated advantages of energy saving and noise reduction, have become the foundation for the development of a scheme to optimize the structure of cooling towers. To explore the feasibility of this approach for mechanical draft cooling towers, a small-scale experimental device has been built to study the resistance and splash performances of three U-type water collecting devices (WCDs) for different water flow rates and wind speeds. The experimental results show that within the considered ranges of wind speed and water flow rate, the pressure drop of the different WCDs can vary significantly. The resistance and local splash performances can also be remarkably different. Some recommendations about the most suitable system are provided. Moreover, a regression analysis of the experimental data is conducted, and the resulting fitting formulas for resistance and splash performance of WCD are reported.

Energy saving and water-consumption reduction of a circulating cooling water system is an important way to achieve the goal of “emission peak, carbon neutrality”, as its cooling capacity has a great impact on the efficiency and carbon-emission reduction of served power units [

Natural draft cooling towers with high-level WCDs were first proposed by Electricity France and Harmon Cooling Towers Belgium in the late 1970s. In China, a high-level water collecting cooling tower with a spray area of 4750 m^{2} was first independently designed in the early 1990s [

Previous studies mainly focus on exploring WCDs for natural draft cooling towers in terms of thermal and resistance performances by wind tunnel test or numerical simulation. However, mechanical draft cooling towers with the features of high turbulent air, small size and high-water spray density have been widely used in the fields of industrial process and house heating and cooling process (e.g., heating, ventilation and air conditioning, chillers, and heat pump) [

The current study investigates the feasibility of the application of high-level water collecting in a mechanical draft cooling tower, especially for the resistance and splash performances with respect to different WCDs, with the aim of providing guidance for the optimization of mechanical draft cooling towers using the WCDs. The innovations in the current study are: (1) a small-scale test rig is built to study the resistance and splash performances of three U-type WCDs within the considered ranges of wind speed and water flow rate; (2) the type of WCD with better resistance and splash performances is determined; (3) the fitting formulas of resistance and splash performances of the WCD are reported.

The aim of the current study is mainly to investigate the trend in the change of resistance and splash performances with respect to three different configurations of WCDs. It is difficult to conduct experimental design based on similarity criterion. Therefore, the model scale was designed as 1:10 based on the relative positions of the media, the WCD, the spray nozzle, the fan and other equipment of an original tower along the direction of the tower height.

The experiments were conducted with an industrial mechanical draft cooling tower as the prototype, whose basic parameters are listed in ^{3}/(h⋅m^{2}), where m^{2} meant the water flow rate was for unit spray area. Therefore, the water flow rate of the model tower was 13.27 × 1.075 × 1.075 = 15.34 m^{3}/h. Since the WCD was scaled by a 1:10 ratio, the water flow rate of the model tower was reduced by 10 times. Therefore, the water flow of the model tower was 1.534 m^{3}/h. The air flow of the model tower was 1.534 × 0.75 × 1000 ÷ 1.2 = 958.75 m^{3}/h. Generally, the similarity criteria

Air flow rate/(m^{3}/h) |
Water flow rate/(m^{3}/h) |
Tower cross-sectional area/m^{2} |
Wind speed in the tower/(m/s) | Air-water ratio |
---|---|---|---|---|

3,651,480 | 5850 | 440.75 | 2.3 | 0.75 |

The water flow rate of the prototype tower was designed to match the cooling capability of a natural draft case. However, since the dimension of the mechanical draft cooling tower was much smaller than that of a natural draft cooling tower, it was necessary to increase water flow rate to make the water spray density match its cooling capability. To carry out study on the changing trend of resistance and splash performances of the WCDs within the considered ranges of wind speed and water flow rate, the water flow rates used in the experiments were set as 3.07 and 4.60 m^{3}/h (i.e., 51.17 and 76.67 L/min), which were two and three times of the originally designed water flow rate (1.53 m^{3}/h), respectively. The air speeds were set as 0.23, 0.35, 0.46, 0.69, 1.0, 1.5 and 2.0 m/s, among which the first four values of air speeds were equal to the air-water ratio of the original tower, and the last three values of air speeds were designed for large air-water ratio conditions to explore the changing trend of WCDs’ performance under large water spray densities.

The test rig mainly included an experimental box, a water circulation system, an air system, a circuit control system, and a measurement system. The cross-sectional dimension of the box was 1075 mm × 1075 mm and the total height was 2650 mm. To observe water distribution uniformity and measuring point arrangement, one sidewall of the box was encapsulated with an acrylic glass plate and the other three sidewalls were 304 steel plates. Media layer was replaced by a PVC material with triangular grids, and the thickness of the media layer was set as 0.5 cm. According to the dimension of the original WCD, U-shape WCD was designed based on the model scale of 1:10 with an inclined slab angle of 44°, a channel depth of 62 mm, and a channel width of 80 mm. The slab spacing of WCD type 1 was 200 mm, and the slab length was 223 mm. To ensure that the top point of the inclined plate of a WCD was in the same perpendicular line as the central point of the water collecting channel of the adjacent WCD, WCD type 2 and WCD type 3 were designed by reducing the row number of the water collector and adjusting the length of the inclined plate. The detailed dimensions are listed in

Configurations | Inclined angle/° | Slab spacing/mm | Slab length/mm |
---|---|---|---|

WCD type 1 | 44 | 200 | 223 |

WCD type 2 | 240 | 334 | |

WCD type 3 | 250 | 292 |

The circulating water system was mainly composed of a water pump, nozzle, pond and catch basin and other components, which were driven by a 100∼600 W (model: YL25WHBL2-20) pump and a 1500 W (model: DBZ15-50-1.5) pump as shown in

The air flow was mainly provided by an axial fan installed on the top of the experimental box. The maximum air speed was 5 m/s. The fan power was adjusted by using a variable frequency control box to achieve the required air speeds.

The measuring system included flow meters, measuring cylinders, pitot tubes, differential pressure sensors and hot wire anemometers, etc. Detailed information on experimental instruments is listed in ^{2} = 2Δ

No. | Instruments | Descriptions | Models/Values |
---|---|---|---|

1 | Water pump | Model | YL25WHBL2-20 |

Power range | 100 to 600 W | ||

Model | DBZ15-50-1.5 | ||

Power | 1500 W | ||

2 | Differential pressure sensor | Model | Testo 512 |

Measurement range | 0 to 200 Pa | ||

Accuracy | ±0.5% full scale | ||

3 | Hot wire anemometer | Model | AR866 |

Measurement range | 0.3 to 30 m/s | ||

Accuracy | ±1% full scale | ||

4 | Measuring cylinder | Range | 0 to 200 mL |

Accuracy | ±2 mL | ||

5 | Turbine flowmeter | Model | K24 |

Range | 10∼100 L/min | ||

Accuracy | ±0.5% full scale | ||

6 | Tray | Dimension | 700 mm × 400 mm |

The resistance performance of the WCD referred to the air-side pressure drop when air flowed through the WCD model. Twelve measuring points were arranged above the WCD, and twelve measuring points were arranged below the WCD. The measuring points below the WCD was 1100 mm high from the bottom of the model and the measuring points above the WCD was 1425 mm high from the bottom of the model. The average value of the twelve measuring points was taken as the final value for the air-side pressure drop. The measuring points of inlet air speed were located at the height of 940 mm from the bottom of the model, and the inlet air speed was calculated by the average value of six measuring points. The measuring points of outlet air speed were located at the height of 1960 mm from the bottom of the model, and the outlet air speed was calculated by the average value of six measuring points.

The splash performance of the WCD referred to the splash water volume of the WCD. To distinguish the splash water volume from the water volume flowing down from the wall, a rectangular tray was placed at the bottom center of the box as shown in

It was ensured that the box was sealed, water was sufficient, and the electrical connection was checked before test. The fan was started and adjusted using the fan frequency control box to control the air speed inside the box. The pump was started and the control valve was used to adjust the water flow rate to the required values. Ensuring that the experimental conditions were stable, the relevant measurements, including air-side pressure drop, local splash water and wind speed and other important experimental parameters were then taken.

The effect of air speed on resistance performance of the studied WCDs under two large water flow conditions (i.e., 51.17 and 76.67 L/min) is experimentally studied and reported in

The resistance of the WCDs of type 2 and type 3 are similar at the same air speed, and they are both greater than that of the WCD type 1, especially within the air speed range of 1.0–1.5 m/s. The pressure drop values of the WCD types 1, 2 and 3 at the air speed of 1.0 m/s are 2.4, 2.9 and 3.6 Pa, respectively for a water flow rate of 51.17 L/min. The pressure drop values of the WCD types 1, 2 and 3 at the air speed of 1.5 m/s are 4.4, 4.9 and 4.9 Pa, respectively. The pressure drop values of the WCD types 1, 2 and 3 at the air speed of 1.0 m/s are 3.0, 3.6 and 3.6 Pa respectively for a water flow rate of 76.67 L/min. The pressure drop values of the WCD types 1, 2 and 3 at the air speed of 1.5 m/s are 4.9, 5.8 and 5.2 Pa, respectively. Considering the structural dimension of the WCD, the spacing of the water collecting slab of the WCD type 1 is 200 mm and the length of the water collecting slab is 223 mm. The water collecting slab of the WCD type 2 is 240 mm and the length of the water collecting slab is 334 mm. The water collecting slab of the WCD type 3 is 250 mm and the length of the water collecting slab is 292 mm. Although the spacing of the water collecting slab of the WCD type 1 is small, and results in a small airflow channel and relatively large flow resistance, the length of the collecting slab is also small, and the frictional resistance of the collecting slab is small, which finally leads to its smallest resistance among the studied WCDs. The resistance of the WCD types 2 and 3 is similar under the combined effect of the spacing and the length of the water collecting plate. Both the spacing and the length of the water collecting slab have effects on the resistance performance to some extent.

Comparing

As mentioned above, a small rectangular tray was placed at the bottom center of the experimental box to measure the local splash volume for a duration of 5 min.

Based on comparative study of the resistance and splash performances of the three configurations of U-shape WCDs within the considered ranges of wind speed and water flow rate. It can be seen that the WCD type 1 is better than the WCD types 2 and 3 in terms of the resistance and local splash performances. Therefore, the configuration of the WCD type 1 is recommended when the mechanical draft cooling tower is using WCDs.

The experiment consisted of six measuring points of air speed and twelve measuring points of differential pressure. To reduce the measuring error, the arithmetic mean was taken as the experimental data, which had a smaller dispersion degree when compared with a single measurement [

Parameters | Sensor | Measuring range | Sensor accuracy | Maximum standard deviation of mean |
---|---|---|---|---|

Inlet air speed | Hot wire anemometer | 0.3∼30 m/s | 1% full scale | 0.207 m/s |

Exit air speed | Hot wire anemometer | 0.3∼30 m/s | 1% full scale | 0.183 m/s |

Differential pressure | Differential pressure sensor | 0∼200 Pa | 0.5% full scale | 1.02 Pa |

where _{mean}_{mean}_{mean}_{1} + _{2} +…+ _{n}

The measuring parameters include water flow rate, air speed inside the tower, air-side pressure drop and splash water volume. There is no propagation of uncertainty of a variable to another. According to Baloch et al. [_{1}) of each part. The summary of test uncertainty is reported in

Equipment | Uncertainty |
---|---|

Air speed | 0.3 m/s |

Pressure | 1 Pa |

Water flow rate | 0.5 L/min |

Splash water volume | 2 mL |

The air-side pressure drop Δ^{2})/2.

where _{1}_{2}_{w}_{a}^{2}, respectively, kg/(m^{2}⋅s).

The pressure drop correlation of the WCD was obtained based on the dimensional analysis and regression analysis of the experimental data. The effect of water conditions on the resistance performance was also considered in _{ps}_{w}^{3}/s. _{a}^{3}/s.

The local splash water _{sw}^{3}/s) of the WCDs was mainly subject to the geometric parameter of WCDs, air speed and water flow rate. Besides, the effect of Reynolds number was also considered for the dimensional analysis. The correlation equation of the local splash performance of the WCD was obtained based on the concept of air-water ratio commonly used in cooling tower analyses._{sw}^{3}/s. ^{2}, its value is 0.28 m^{2}.

The correlations of resistance and splash performances were validated by calculating the pressure drop (Δ_{sw}_{sw}

A small-scale tower was built taking a real cooling tower as the prototype based on the criteria of relative positions of the main components along the tower height and the same air-water ratio. The resistance and splash performances of three U-type WCDs within the considered ranges of wind speed and water flow rate were experimentally studied. The correlations of resistance and splash performances were obtained. The following conclusions can be drawn:

The pressure drop values of the WCDs of type 2 and type 3 are similar and both are greater than that of the type 1. Within the considered ranges of wind speed and water flow rate, the pressure drop of the WCD of type 2 can be up to 0.8 Pa higher than that of the type 1, and the pressure drop of type 3 can be up to 1.0 Pa higher than that of the type 1.

The local splash volume of the WCD type 2 is the highest, followed by the type 3, and the type 1 is the lowest. Within the considered ranges of wind speed and water flow rate, the local splash volume of the WCD of type 2 can be up to 0.23 L higher than that of the type 1, and the local splash volume of the type 3 can be up to 0.16 L higher than that of the type 1.

The WCD type 1 is superior to the WCD types 2 and 3 with the consideration of both resistance and local splash performances. Therefore, the WCD type 1 is recommended when the mechanical draft cooling tower is using WCDs.

The fitting formulas of resistance and splash performances of WCD are reported based on the regression analysis of experimental data. The fitting formulas provide a guide for the optimal design of U-type WCDs applied in mechanical draft cooling towers.

Although the prediction correlations have been validated, the limitations of engineering applications are still unresolved. It is recommended to conduct future work on filed tests to validate or modify the correlations.

Spacing of collecting slab, m

Vertical height of collecting slab, m

Length of collecting slab, m

_{ps}

Distance between nozzle and collecting tank, m

Mean absolute percentage error

Number of measurement points

Air-side pressure drop, Pa

_{a}

Air flow rate, m^{3}/h

_{w}

Water flow rate, m^{3}/h

_{sw}

Local splash water, L

Reynolds number

Standard deviation of experiment

Splash performance measurement area, m^{2}

Wind speed, m/s

Water collecting device

_{mean}

Arithmetic mean

_{mean}

Standard deviation of mean

Air density, kg/m^{3}

The authors would like to thank the reviewers and editors for their useful suggestions for the improvement in the quality of our manuscript.

This work was supported by the

The authors declare that they have no conflicts of interest to report regarding the present study.

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