WO2019112491A1

WO2019112491A1 - Cooling tower and method for preventing development of contamination on cooling tower heat exchanger

Info

Publication number: WO2019112491A1
Application number: PCT/SA2018/000013
Authority: WO
Inventors: Yousef AL YOUSEF; Ahmed AL MUBARAK; Filip HERMAN; Marcin Malicki; Shafi AL HSAIEN; Abdullah AL KATHIRI
Original assignee: King Abdulaziz City For Science And Technology; Saudi Electricity Company; New Energy Transfer Ltd
Priority date: 2017-12-08
Filing date: 2018-12-05
Publication date: 2019-06-13
Also published as: PL423807A1; PL235691B1

Abstract

The subject of the invention is the apparatus and the method of adiabatic cooling of water and preventing the development of contaminations on the cooling tower heat exchanger, where the cooling tower comprises the fan system (17), the heat exchanger (5), the heat exchanger sprinkling system (3), the water discharge outlet (11) equipped with the valve (13), the water supply system with the pipeline (10) equipped with the pump (16), where the gas preparation station (1) is connected by the gas pipeline (6) with the micro-nano bubbles generator (2) which is connected to the water source (4) by the pipeline (12), where the micro-nano bubbles generator (2) is connected by the pipeline (7) with the cooling tower sprinkling system (3) and the heat exchanger (5) is coated with the negative zeta potential layer.

Description

Cooling tower and method for preventing development of contamination on cooling tower heat exchanger

The subject of the invention is a cooling tower and a method of preventing development of contamination on a cooling tower heat exchanger.

The invention belongs to the field of adiabatic water- cooling systems.

Adiabatic cooling systems have been used worldwide for over hundred years. They have a great advantage of offering a possibility to lower a water temperature with no significant input of electrical energy. A useful cooling effect is provided due to partial evaporation of circulating water, which determines also the biggest disadvantage of such systems: a necessity to provide treatment of circulating water to ensure system's operation (if not treated, concentration of salts increases leading to deterioration of heat exchange conditions in the device on the one hand and to physical damage of its subsystems for example by scaling or development of biological contamination on the other) . With regard to both closed type systems where cooling medium has no contact with air and open type systems where cooling medium has a direct contact with air, water treatment systems rely on certain dosage of chemicals or regular discharges of circulating water. These measures are neither economical nor environmentally friendly. The proposed solution is to use MNB (Micro Nano Bubbles) to saturate the make-up water with the bubbles of oxygen, ozone, carbon dioxide or a mixture thereof having diameters below 0.1mm in order to limit formation of deposits and improve heat exchange inside the device which, in connection with additional coating of heat exchanger having the same potential as MNB, guarantees significantly more favorable system operation parameters. Collapsing micro-nano bubbles generate ultrasonic waves which have a cleaning effect on all solid surfaces, including the sprinkled heat exchanger of the cooling tower. Collapsing of micro-nano bubbles results in emergence of OH* hydroxyl radicals being very strong oxidants, stronger than ozone and atomic oxygen, thus protecting the tower against biological contamination including very dangerous Legionella. Hydroxyl radicals destroy all microorganisms such as bacteria or viruses and have a positive effect on an existing scale as collapsing MNB s mechanically clean a heat exchanging surface by protecting it against build-up of carbonates and similar scale forming minerals as well as protecting from formation of biofilms, especially in the open circuit type systems. Additionally, presence of MNB' s in water changes a solubility of minerals and shifts a saturation point thereof, thus greatly benefiting evaporative heat exchange processes.

Water used in cooling towers as a circulating medium require a proper treatment to prevent a process of biofouling on the working components of cooling towers as well as a process of scaling due to increasing total dissolved solids concentration resulting from cyclical evaporation of water. For this purpose cooling tower systems should include water treatment station designed to control water parameters and prevent the above mentioned growths by dosing adequate quantities of chemicals. Usually chemicals are used in composition and dosage depending on make-up water quality which efficiently maintain process parameters of circulating water and auxiliary systems within normal levels .

K. Yamasaki, M. Katoka, K. Chuhjoh and S. Imazu in their patent application No. US 2007/0284316 present a concept of process water saturation by ozone MNB generator. The generator saturates the water in the collection basin of the cooling tower i.e. in the place where the water is collected after the adiabatic cooling process . Unlike the solution known from the state of art, the proposed solution provides the application of MNB just before the sprinkler nozzles which sprinkle the water over the heat exchanger so that in addition to strong oxidizing properties allowing to prevent a growth of algae and destroy micro-organisms they atomize the sprinkled water thus expanding the effective water/air heat exchange surface. This directly improves efficiency of water cooling process by as much as 5% resulting in the increase of system capacity or reduction of electrical power consumption by the whole system.

R.D. Vidic, S.M. Duda and J.E. Stout in the report „Biological Control in Cooling Water Systems Using Non-Chemical Treatment Devices" present one of methods relying on generating the bubbles and saturating the process water with them in the cooling tower with the aim to use implosion forces, associated temperatures and ultrasonic waves generated at the collapse of these bubbles to prevent a growth of micro-organisms. In the method described therein the bubbles are generated by forcing two water streams to collide. The collision of these two streams creates a vacuum region, which resulted in the formation of cavitation bubbles. The solution according to the present invention provides the generation of micro-nano bubbles in the generator using pressure method, whereby the bubble diameter does not exceed 0.1mm which guarantees higher level of the solution saturation with the bubbles. Additionally, the surface of heat exchanger is coated with the layer having the same potential as MNB, which practically eliminates a process of scale formation and biofouling.

M. Al.-Bloushi, J. Saththasivam, S. Jeong and others in their article „Effect of organic on chemical oxidation for biofouling control in pilot-scale seawater cooling towers" describe a method of applying microbubbles of ozone to the stream of make- up water added to refill the level of process water in the cooling tower as an effective method of make-up water disinfection preventing a growth of micro-organisms. This increased the oxidation-reduction potential level to +600 mV. In the solution according to the current invention the outer surface of the heat exchanger is covered with a structure providing its hydrophilicity and negative zeta potential thus intensifying a positive effect of MNB' s . Patent application No. CN106944400 (A) by ZHANG TIANZHU; ZHANG HUIJUAN; XUE XIAOLI; YANG WENHUA; WU NA; ZHAO YUEGANG; REN QIANG describes a method of cultural relics cleaning by spraying the water with dissolved MNB particles using a hand-held spray gun. In the present invention a mixture of water and MNB is fed directly to the sprinkling system of the cooling tower heat exchanger to prevent scaling and biofouling.

Patent application No. WO2017127636 (Al) Use of micro and nano bubbles in liquid processing AMAMCHARLA JAYENDRA [US]; LI BINGYI [US]; LIU ZHE [US] presents application of MNB to a liquid with a purpose to reduce its viscosity and eventually reduce pumping costs. This solution is especially well suited for a food industry (for example pumping of milk) thanks to a favorable influence of MNB on dissolving of milk additives while reducing pumping costs. In the present invention a mixture of water and MNB is fed directly to the sprinkling system of the cooling tower heat exchanger to prevent scaling and biofouling.

In the solution according to the present invention a part of water circulating in the cooling tower of either open or closed circuit type is saturated with micro-nano bubbles (MNB) and supplied directly to the heat exchanger's sprinkling system. Moreover, the open or closed circuit type heat exchanger is coated with a layer which has the same potential as the micro- nano bubbles. With this procedure, the water which has a negative potential resulting from the saturation process and the heat exchanger coated with the hydrophilic layer with negative potential repel each other when contacted so that the contact of heat exchange surface with substances causing scale and biofilm development is limited to a minimum. In case of saturating the process water with carbon dioxide, a solubility of this gas in process water is improved while reducing a risk of carbonates build-up on heat exchange surfaces. The place of process water saturation with micro-nano bubbles (MNB) is important, too. According to the proposed solution it occurs just before the nozzles sprinkling the water on the heat exchanger. Thanks to this, the number of bubbles in the water is maximized during the contact with the heat exchanger - while the sprayed mist falls down the number of bubbles decreases due to the contact with cooling tower elements. The bubbles collapse and generate an ultrasonic wave which efficiently reduces deposits, scale and biofilm growth on the heat exchange surface. It is hence important to maximize the bubbles density in the upper section where the heat exchanger is located. In the prior art the water is saturated with the bubbles in the collection basin, where part of the bubbles are released from the water due to pumping in the pipeline and contacting rotary elements of circulation pump. This results in significant decrease of MNB number in the key area of the heat exchanger.

Moreover, the saturation of the process water with MNB just before its distribution to the sprinkler nozzles results in atomizing the sprayed water and forming a mist which substantially improves heat exchange parameters. The gas used in the process is ozone, oxygen or carbon dioxide which efficiently blocks development of micro-organisms, bacteria, viruses and prevents a process of biofouling and scale formation on working elements of cooling towers due to implosion forces and ultrasonic waves generated at the collapse of these bubbles which the process water is saturated with. The substance of the invention is the cooling tower comprising a fan system, a heat exchanger, a heat exchanger sprinkling system, a drain with blow-down valve, make-up water intake system with a pipeline eguipped with a pump, where a gas preparation station is connected by a gas pipeline with micro- nano bubbles generator which is connected to a water source by a pipeline. The micro-nano bubbles generator is connected by the pipeline with the sprinkling system of the cooling tower and the heat exchanger is coated with negative zeta potential layer .

In a favorable embodiment the heat exchanger is of a fill spray and tray type.

In a favorable embodiment the make-up water supply pipeline connected with the circulating water pipeline is connected to the MNB generator.

In a favorable embodiment MNB generator is connected with water pipeline connecting MNB generator with water source.

The method of preventing development of contamination, especially biological and scale build-up on the cooling tower heat exchanger where the gas after being produced in the gas generator is supplied through the pipeline to the MNB generator where it is mixed with the make-up water supplied from the water source thus enriching the water with micro-nano bubbles. The water containing the micro-nano bubbles is supplied through the pipeline directly to the sprinkling system.

In a favorable embodiment the water source is the water coming from the circulating water system delivered to the MNB generation through the pipeline.

In a favorable embodiment the water containing the micro- nano bubbles is delivered by the sprinkling system (3) onto the heat exchanger (5) coated with the negative zeta potential layer . The invention in its favorable embodiment is depicted on the drawing Fig.l presenting the adiabatic water cooling system .

Exapmple 1

The cooling tower comprising the centrifugal fan system (17) with the electrical power of llkW providing the air flow up to 140,000 m³/h, the heat exchanger sprinkling system (3) featuring the set of PVC sprinkler nozzles, water drain (11) made of DN50 PVC pipe and equipped with electromagnetic valve (13), make-up water supply system featuring a DN40 PVC pipeline (10) equipped with a pump (16) supplying the water to the collection basin (19) of the cooling tower. The gas preparation station (1) consists of the air intake equipped with filtration layers being removable filters, centrifugal fan, activated carbon humidity absorption system and adsorption type oxygen separator connected with ozone generator. Gas preparation station (1) is connected by the gas pipeline (6) with the micro- nano bubbles generator (2) which is connected by the PVC pipeline (12) to the water source (4) being a 316L stainless steel tank with a volume of 6m³. The MNB generator (2) is connected by the PVC pipeline (7) with the cooling tower water sprinkling system (3) located above the heat exchanger (5) which is permanently coated with 0.1mm thick negative zeta potential silicon carbide layer. The heat exchanger (5) has been coated with negative zeta potential silicon carbide layer by laser- assisted cold spray (LACS) method. The medium to be cooled in the closed circuit heat exchanger of finned pipe type is a distillate produced by the water desalination system. The results of using a mixture of water with ozone based MNB's include reduction of chemicals consumption, reduction of water consumption and improvement of heat exchange on the cooling tower surface due to eliminating of scale and biofilm deposition. In the conventional variant of this favorable embodiment cost of used chemicals amounts to USD 2500 per year, whilst in the variant according to the invention the total operational cost is USD 200 per year only or over twelvefold less. This value has to be increased by the volume of process water discharged from the collection basin upon reaching a limit of salt concentration in the process water which in case of MNB technology can reach even over 50000 ppm i.e. 44000 ppm more than in case of a conventional technology. This generates another source of savings since in the yearly operation cycle the volume of discharged water will be reduced by 2m³ per hour for each 1900kW of cooling capacity. The water is discharged through the brine discharge pipeline (11) after opening the valve (13) . There is also a positive effect of heat exchange properties improvement due to prevention of scaling and biofouling: in the conventional technology the efficiency of heat exchange drops even by 30% due to deposition of scale and biofilms on the heat exchange surfaces, whilst in the solution according to the invention, the heat exchanger works with 100% of its nominal efficiency even after a year of operation due to a constant process of cleaning of the heat exchange surfaces by collapsing micro-nano bubbles.

Example 2

The method of preventing development of contaminations, especially biological and scale build-up on the cooling tower heat exchanger (5) according to the invention has been implemented by supplying the air to the gas preparation station (1) through the integral ambient air inlet. The air has been dehumidified with adsorption dehumidifier using the activated carbon and purified by the set of filters. The prepared gas has been supplied by the duct (6) to the MNB generator (2), where the micro-nano bubbles are formed in the circulating water supplied by the PVC pipeline (18) . Afterwards, the stream of process water saturated with MNB has been supplied by the PVC pipeline (7) directly to the sprinkling system (3) . The circulating water has been saturated to the level of 5 mg per each kilogram of water. The circulating water saturated with MNB has been distributed with the sprinklers (3) over the heat exchanger (5) coated with 0.1 mm thick hydrophilic layer. In the same time, the fan (17) has constantly sucked the air, thus enabling a counterflow contact of sprinkled water with air. As a result of adiabatic air humidification, a heat has been received from the heat exchanger (5) fed by the pipelines (9) : intake pipeline (9a) made of PVC equipped with the circulation pump (15) and return pipeline (9b) made of PVC, thus obtaining a useful cooling effect of distilled water produced by a multi effect desalination process . The distillate has been supplied to the heat exchanger by the pipeline (9) using the circulation pump (15) at the temperature of 46°C. The cooling capacity of the system in this configuration is 1900 kW and results in lowering the temperature of the distillate stream to 38°C. The volume rate of the distillate flow through the pipeline (9) has been approximately 205 m³ per hour. Due to evaporation of the circulating water from the cooling tower amounting up to 2,5m³ per hour, the same volume of process water has been added through the pipeline (10) and the circulation pump (16) . Thanks to application of MNB the cooling efficiency of the cooling tower has been increased by over 5%. This is caused by expansion of head exchange surface between flowing air and water sprayed from the nozzles allowing to increase the cooling power of the cooling tower from 1900kW to 1995kW.

Claims

1. The cooling tower comprising the fan system (17), the heat exchanger (5) the heat exchanger sprinkling system (3), the water discharge outlet (11) equipped with the valve

(13), the make-up water supply system with the pipeline (10) equipped with the pump (16), where the gas preparation station (1) is connected by the gas pipeline (6) with the micro-nano bubbles generator (2) which is connected by the pipeline (12) to the water source (4) , specific with that the micro-nano bubbles generator (2) is connected by the pipeline (7) with the cooling tower sprinkling system (3) and the heat exchanger (5) is coated with the negative zeta potential layer.

2. The cooling tower according to claim 1 specific with that the heat exchanger (5) is of a fill spray and tray type.

3. The cooling tower according claim 1, 2 specific with that to the MNB generator (2) the water pipeline (18) is connected which is connected with the water pipeline (8) . 4. The cooling tower according to claims 1, 2, 3 specific with that to the MNB generator (2) the pipeline (12), connecting the MNB generator (2) with the water source (4), is connected.

5. The method of preventing development of contaminations, especially biological and scale build-up on the cooling tower heat exchanger (5) where the gas is produced in the gas generator (1) and supplied through the pipeline (6) to the MNB generator (2) where it is mixed with the make-up water supplied from the water source (4) thus enriching the water with micro-nano bubbles, specific with that the water containing the micro-nano bubbles is supplied through the pipeline (7) directly to the sprinkling system (3) .

6. The method according to claim 5 specific with that the water source (4) is the water coming from the system of circulating water supplied to the MNB generator (2) through the pipeline (18) .

7. The method according to claim 5, 6 specific with that the water containing the micro-nano bubbles is distributed by the sprinkling system (3) onto the heat exchanger (5) coated with the negative zeta potential.