Cooling efficiency of hybrid cooling tower with finned tube radiator

. Hybrid cooling towers are a new type of equipment that can be used in cooling water circuits. In this work, a hybrid cooling tower is developed, in which inclined corrugated plates are used as a fill, and the circulating liquid flows through an internal tubular radiator, the finned surface of which is continuously washed by the coolant water. The purpose of the work is to perform an experimental evaluation of the cooling efficiency of the developed hybrid system used for circulating water cooling. It was obtained that at the wetting density of the coolant water equal to 28.3 m 3 /(m 2 ·h) and the mass flow rate of the circulating water in the tubular radiator of 0.0135 kg/s, the cooling efficiency of the developed cooling tower can reach 38.6%.


Introduction
Cooling towers are used in circulating water supply systems to cool water flowing in closed circuits.Cooling towers are used ubiquitously at thermal and nuclear power plants, in metallurgical, chemical, food, and other industries.Today, there are two main types of cooling towers: wet and dry [1,2].In wet cooling towers, the process water comes into direct contact with the ambient air.They are characterized by high capacity, high operating costs, and require water pretreatment, since the formation of deposits on heat transfer surfaces instantly leads to a decrease in the efficiency of heat removal.Dry cooling towers do not provide direct contact of the circulating water with ambient air, which contributes to minimal water pollution, but they have low capacity.
Therefore, it is urgent to improve the cooling efficiency of existing cooling towers, as well as to develop new cooling towers [3,4].This is how hybrid cooling towers appearthese are water coolers in which the heat and mass transfer processes inherent in the wet and dry cooling towers are combined.
The authors [5] compared three cooling towers and concluded that the hybrid type has minimal energy consumption and can be used in humid or dry environments.In the work [6], a mathematical model for predicting the operation mechanism of dry, wet, and hybrid cooling towers has been developed.The hybrid cooling system model was based on a combination of dry and wet cooling tower models.They found that the performance of the hybrid cooling tower is between the dry and wet cooling towers, depending on the water-to-air ratio.The same conclusion was reached in the experimental work [7].The authors [8] modelled a wet cooling tower with dry units added to assess the amount of water loss.It was shown that by using hybrid towers, water consumption decreases by 34-38% for the plants in the study.Recently, a paper [9] presented a study of the behavior of a hybrid mechanical draft wet cooling tower.The results showed that with an increase in the liquid flow rate, the cooling capacity enhancement of the cooling tower and water savings were observed.
Previously, a hybrid cooling tower was developed, consisting of inclined corrugated plates as a filling and a tubular radiator for the flow of circulating water.To intensify heat exchange, the surface of the radiator tubes is finned [10].The purpose of this work is to evaluate the cooling efficiency of cooling water in a hybrid cooling tower with the finned radiator tubes and the inclined corrugated contact elements.

Experimental design
The hybrid cooling system developed to cool the circulating water is shown in Figure 1.The operating principle of the system is as follows: the water heated in the process equipment is divided into two streams before being supplied to the hybrid cooling tower.Besides, the water flow rate of the first stream is significantly greater than that for the second stream.The first flow is directed to the tubular radiator to avoid direct contact with atmospheric air.The second water flow is supplied through a distributor to the upper part of the cooling tower.This stream of water comes into contact with ambient air and partially evaporates and discharges the collecting reservoir.Cooling of the first (main) water stream occurs due to heat transfer through the radiator wall to atmospheric air, as well as washing liquid, which consists of the cooled second stream and fresh makeup fluid.Then, this liquid is fed back to the top of the cooling tower by the pump.Thus, the washing liquid stream acts as a coolant, forming a closed coolant circulation circuit.
The cooling efficiency of the circulating water in the tubular radiator depends mainly on the cooling process in the fill volume, which is influenced by its type.The fill consists of corrugated metal plates arranged vertically in several rows as contact elements.The liquid enters the fill through the inlet pipe located on the side surface of the tower wall.Plates are arranged perpendicular to each other, forming a zigzag profile.Plates with horizontal corrugations of a round profile provide turbulence in the flowing liquid film.Radiator tubes are installed across the inclined corrugated plates.Earlier studies [11] showed that the main thermal resistance is concentrated during heat transfer from the wetting liquid to the outer wall of the radiator tubes.To intensify the processes of heat and mass transfer inside the cooling tower, the radiator tubes are made with transverse finning, which also increases the overall surface of heat transfer [12].
The liquid inside the fill volume is distributed as follows: water moves from top to bottom and washes the surface of the finned tubes, removes heat, and flows down in the form of a film over the perforated plates, forming a zigzag trajectory.Part of the liquid falls through the holes in the plate and breaks into droplets, which, hitting the surface of the liquid film of the underlying plate and tubes, break into new water droplets.The air is supplied by the fan from bottom to top, and passes through the holes in the plates, pushing the water droplets into different directions, thereby spraying liquid throughout the entire fill volume and along the outer surface of the finned tubes.

Test conditions
To evaluate the cooling efficiency of the hybrid cooling tower with the finned radiator tubes, experimental studies were carried out on a water-air system.Figure 2 represents the experimental setup in which the cooling tower fill consists of four inclined corrugated plates with a total height of 340 mm.The cross-section dimensions of the cooling tower under study are 100x100 mm.Contact elements are the metal corrugated plates with a thickness of 0.6 mm and a radius of curvature of 7.5 mm.Through round holes with a pitch of 11-12 mm are drilled on the side surfaces and in the upper part of corrugations for the passage of liquid and gas.Round holes with a diameter of 3 mm and a pitch of 10 mm are also made in the recesses of the corrugated plates.The radiator is made of 30 copper tubes with a diameter of 8 mm and a length of 125 mm each, interconnected by silicone hoses.The finning of the radiator tubes is performed by contact welding on each pipe 19 of steel rings with a thickness of 0.5 mm (fin thickness) with a pitch of 5 mm, while an outer diameter of the tubes, considering the transverse finning, is 15 mm.The finned tubes are mounted across the inclined corrugated plates.Various sensors were used to measure and control the parameters during the experiment, the characteristics of which are given in Table 1.One of the criteria for evaluating the performance of heat and mass transfer processes in various cooling towers is the cooling efficiency, which characterizes the perfection of the contact elements (fill) used.Thus, the cooling efficiency of the hybrid cooling tower operating in a wet mode can be estimated by changing the temperature difference of the main liquid flow according to the equation: where T1 is the water temperature at the inlet to the tubular radiator, °С; T2 is the water temperature at the outlet from the tubular radiator, °С; T* is the equilibrium water temperature, i.e., the dew point temperature (theoretical liquid cooling limit), °С.
The cooling of the main flow of liquid entering the tubular radiator is accompanied by the simultaneous cooling of the washing liquid due to the counter flow of atmospheric air and heat transfer through the wall of the finned radiator.The efficiency of the heat transfer process from the washing liquid to the main stream can be estimated by the following formula: where T4 is the water temperature at the outlet of the fill, °С.

Results and discussion
The results of processing the experimental data show that the cooling efficiency of the circulating water in the wet part of the hybrid cooling tower depends significantly on the mass flow rate of the coolant liquid, the air flow rate, and the flow rate of the main water flow in the finned tubular radiator.An increase in the cooling efficiency is observed with an increase in the wetting density of the coolant liquid, the mass flow rate of air, as well as a decrease in the mass flow rate of water in the tubular radiator of the hybrid cooling tower.Thus, a decrease in the mass flow rate of liquid in finned tubes from 0.0236 to 0.0135 kg/s leads to an increase in the cooling efficiency by 35.8% with an average air velocity of 1.88 m/s and a wetting density of 28.3 m 3 /(m 2 •h) (Fig. 3 a, c).In addition, it can be seen that the cooling efficiency of the hybrid cooling tower is increased when the fill is operated in a film mode with an increase in the wetting density of the coolant liquid (the change in efficiency can reach 23.3 and 34.4% with a mass flow rate of water in the tubular radiator of 0.0236 and 0.0135 kg/s, respectively).The effect of air velocity on the cooling efficiency of the hybrid cooling tower is due to the change in the average temperature of the coolant liquid due to turbulence of the two-phase gas-liquid flow in the cooling tower fill.However, an excessive increase in the average air velocity can lead to the breakdown of liquid droplets and their further entrainment from the device [12].The efficiency of the heat transfer process through the wall of the finned radiator in the hybrid cooling tower is described by the following regularities: when the fill operates in the film mode, the change in efficiency with an increase in the average air velocity does not exceed 5.4%.When loading mode begins, characterized by an inflection point, a sharp increase in the heat transfer efficiency is observed.This is due to the high cooling of the washing liquid.In this mode, the efficiency of the heat transfer process can reach 73.3% with a wetting density of 44.5 m 3 /(m 2 •h) (Fig. 4 a, b). the tubular radiator of 0.0135 kg/s, the cooling efficiency of the developed cooling tower can reach 38.6%.The high value of the cooling efficiency is achieved due to the uniform distribution of the liquid phase in the fill, consisting of the inclined corrugated contact elements, and the formation of a developed heat and mass transfer surface.In addition, the use of finning of the radiator tubes leads to a more uniform distribution of the thermal resistance of heat transfer.Thus, the use of a hybrid cooling system used for the circulating water with elements for intensifying heat and mass transfer processes when operating in "dry" and "wet" modes will eliminate the development of microorganisms, reduce biofouling of the working surfaces of cooling towers, thereby ensuring high operating performance of the circulating water supply system of various industrial enterprises.