AU2010201392A1 - Method and Means for Operating Evaporative Coolers - Google Patents

Description

F. F. SEELEY NOMINEES PTY LTD COMPLETE SPECIFICATION Invention Title: Method and Means for Operating Evaporative Coolers The invention is described in the following statement: Method and Means for Operating Evaporative Coolers Field of the Invention The present invention has been divided from application 2006206035, the contents of which are included herein by reference, and relates to methods of operating evaporative coolers and more particularly to improvements in watering systems for evaporative heat exchangers wherein adjacent wet and dry airflow channels are in counter current airflow heat exchange relationship. This invention can be applied to self-contained air conditioning units suitable for supplying cooled air to an enclosed space, and to self-contained conditioning units suitable for supplying cooled water for use in heat exchange units forming part of a system for the cooling of enclosed spaces. Description of the Prior Art Throughout this description and the claims which follow, unless the context requires otherwise, the word "comprise', or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or. steps. The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that that prior art forms part of the common general knowledge in Australia. The use of evaporative air coolers for the cooling of enclosed spaces is well known in the art. These coolers are typically constructed with outer walls containing a wettable, permeable media, which is kept wet with water pumped from an internal reservoir. Air from outside the building is drawn through the wetted media by means of a fan located within the evaporative cooler, and delivered either directly into the enclosed space or through a system of ducting to the enclosed space. 1 As air passes through the wetted media, a phenomenon known as adiabatic saturation takes place. Moisture from the surfaces of the wetted pad evaporates into the air passing through in accordance with the humidity of the air, or its ability to take up additional water vapour. This evaporation causes an exchange of energy wherein the energy required for liquid water to evaporate to a vapour is derived from the water within the wetted pad, thereby cooling the water. The warm air entering the pad is then cooled by heat exchange to the cool water surface. The limit to which air can be cooled by this phenomenon is known as the Wet Bulb Temperature as defined in any reference work on psychrometrics. The air delivered by an evaporative cooler is cooled to a temperature which is always greater than the Wet Bulb Temperature, to a degree determined by the efficiency of the design of the evaporative cooler. The air delivered is also always more humid than the air entering the cooler. This limitation in achievable temperature and the addition of moisture to the air severely limits the degree of cooling available by this method, as well as limiting the use of this means of cooling to relatively hot, dry climates. In a typically hot, dry location, such as Adelaide, Australia, the design condition for evaporative cooling is 38 0 C Dry Bulb Temperature, 21"C Wet Bulb Temperature. Under these design conditions, a typical evaporative air cooler will deliver air at around 23.5 0 C, but which has been substantially humidified. This air is much less amenable to providing comfort conditions within the enclosed space than, say, a refrigeratively cooled air conditioning system, which might deliver air at 15'C, and to which no additional moisture has been added. There is also known, in the prior art methods, that air can be cooled to temperatures below the Wet Bulb Temperature of the incoming air while still using only the evaporation of water as the mechanism of cooling. These methods typically pre cool the incoming air without the addition of moisture by means of dry heat exchange, prior to the air coming in contact with the moist surfaces for evaporation. The pre-cooling of air without addition of moisture reduces both the Dry Bulb and Wet Bulb temperatures of the air as can be observed on any psychrometric chart. When the air is then brought into contact with the wetted surfaces, it will be cooled to a temperature which approaches the now depressed Wet Bulb Temperature rather than the original Wet Bulb Temperature. If this process is taken to the limit, it is possible to produce cooled air which approaches the Dew Point of the incoming air, without the addition of moisture. This process of indirect evaporative cooling of air is well known. SU 979796 by Maisotsenko discloses a configuration wherein a main stream of air is passed along a dry duct, simultaneously passing an auxiliary air stream counter currently along a moist duct which is in heat-exchange relation with the dry duct. The auxiliary stream is obtained by subdividing the total stream into main and auxiliary streams. This configuration is further developed by Maisotsenko in US 4,977,753 wherein the wet and dry ducts are divided into two separate sections which allows for pre cooling of the dry airstreams prior to their entry into the wet duct thereby resulting in enhanced cooling efficiency. A practical implementation and method of construction of the configuration of US 4,977,753 is disclosed in US 5,301,518 by Morozov et al. US 5,301,518 discloses a construction consisting of alternating dry ducts, which may be constructed from a variety of materials, and wet ducts constructed from capillary porous material. The airflow configuration is arranged such that the air streams in the dry and wet ducts are in counter flow as in previous disclosures. Furthermore, the configuration divides the heat exchanger into two separate stages for the purpose of achieving the requisite temperature reduction while relieving the high pressure drop inherent in the narrow air passages required for adequate heat transfer. Wetting of the porous 3 material of the wet ducts is achieved by vertical wicking from a water reservoir beneath the heat exchanger. The disclosure of US 5,301,518 has been demonstrated in practical working machines, which produce air cooled to temperatures approaching the Dew Point without the addition of moisture to the air. However, the construction suffers a number of deficiencies. Resistance to air flow is high as a result of the narrow air passages needed for effective heat transfer. Heat transfer between the wet and dry air passages is inefficient due to the air boundary layers at both sides of the medium between the passages, requiring large surface areas for effective transfer of heat. The heat exchanger height is limited by the ability of the porous wet duct material to wick vertically, which in practical terms is about 200 mm. The available delivered airflow for a given size of heat exchanger is therefore low, resulting in an unacceptably large and costly construction for practical airflows. There are also considerable practical difficulties with the construction and operation of such an indirect evaporative cooler. Manifolding of air streams to the respective wet and dry ducts requires individual separation of the ducts with laborious and expensive sealing systems. When used with normal potable water supplies, water evaporated from the wet duct leaves behind salts, which cannot be easily removed, eventually clogging the heat exchanger. It is also well known that heat exchange and wet surface evaporation rates from flat, plane surfaces can be greatly enhanced by arranging adjacent surfaces in the form of corrugations set at different angles for each adjacent sheet. This principle was disclosed by Bredberg in US 3,262,682 and Norback in US 3,395,903 for the construction of evaporative media for use in evaporative air coolers and cooling towers. The interaction of air streams within the adjacent corrugations in this construction of wetted media results in intense evaporation from the wet surfaces and intense heat transfer from the cold surfaces formed as a result of that

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evaporation. A compact, high efficiency evaporative media can be constructed with minimal pressure loss from airflow. As disclosed in parent application 2006206035, the intensity of evaporation and heat exchange demonstrated in corrugated evaporative media can greatly enhance the performance of an indirect evaporative cooler if applied to the airflow configuration needed for indirect cooling when such media is adapted to that environment. A difficulty associated with indirect evaporative coolers is maintenance ofjust the right amount of moisture in the wettable media which heretofore has been placed within the wettable media by wicking. This wetting requirement comes about due to the temperature gradient through the wet passage necessary for operation of evaporative coolers. The wetted surfaces at the delivery end of the core must be close to the Dew Point of the incoming air if the delivered air temperature is to approach the Dew Point, whereas the wetted surface temperature at the entry end of the core must approach the temperature of the incoming hot dry air if evaporation and heat transfer are to occur. Thus there must be a temperature gradient in the wetted surfaces through the core from the delivery end to the entry end. This gradient has been achieved in prior art arrangements by wicking water from a reservoir to the point where it is to evaporate. Any surplus of water over this requirement to evaporate and keep the surfaces wet will degrade thermal performance and it will no longer be possible to approach the Dew Point in delivered air temperature. If the wetted surfaces were to be flood irrigated as is the practice with direct evaporative cooling, it would only be possible for the delivered air temperature to approach the Wet Bulb temperature of the incoming air. This temperature can be considerably above the Dew Point depending on incoming air psychrometrics.

Summary of the Invention The present invention provides a method of operating an evaporative cooler which includes a heat exchange core wherein adjacent wet and dry airflow channels are in counter current airflow heat exchange relationship with water being supplied to the wet channels in a descending flow pattern, said method comprising supplying water to the wet channels over a plurality of segments from an air entry end to an air outlet end of said core during operation of said cooler and circulating water through each segment relatively separately from adjacent segments such that an appropriate temperature gradient is established from an air inlet end to an air outlet end of the core by maintaining different circulating water temperatures in each segment. Further in accordance with the present invention there is provided an evaporative cooler having a heat exchange core with adjacent wet and dry airflow channels in counter current flow with heat exchange therebetween; said core being segmented between an air entry end and an air outlet end and having water supplying means for supplying water to the wet channels in the respective segments during operation of the cooler such that in use water is circulated through each segment relatively separately from adjacent segments to effect different circulating water temperatures in each segment. Preferably, the method and apparatus of this invention includes separate water supplies to each segment. In a further modification of a preferred embodiment of the present invention the separate water supplies are interconnected to facilitate flow of water between each of the water supplies. 6 Brief Description of the Drawings Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows schematic views of airflow paths and a water distribution method of a prior art indirect evaporative cooler; Figure 2 is a schematic showing an embodiment of a water distribution system in accordance with the present invention where the heat exchange core is divided into segments; and Figure 3 is an isometric view of an assembled indirect evaporative cooler core detailing water and airflow systems in accordance with the embodiment of Figure 2. Description of Embodiments In Figure 1, a prior art indirect evaporative cooler construction is shown. Hot, dry air 10 enters the dry air passage 12, proceeding past the dry air passage boundary 14. When the construction has been operating for at least a short period, the dry air passage boundary 14 will be cooler than the dry air entering the passage 12. Heat exchange will occur and the dry air will be progressively cooled as it proceeds down the dry air passage. The incoming hot dry air 10 has been cooled considerably when it leaves the dry air passage 14 at 15. A flow resistance device 28 is installed in the airflow path thereby causing an increase in air pressure at 15. This increase in pressure causes some of the now cool, dry air to turn at 26, and proceed through the wet air passage 16. The wet air passage contains a wetted media 18, kept moist by the wicking of water 7 from a water reservoir 22. Since the air has not yet had any change in its moisture content, evaporation takes place from the wetted media 18 thereby humidifying the air and cooling the water within the wetted media by the same mechanism described above for evaporative media. As the air continues its flow down the wet passage, heat from the adjacent dry passage 12 will tend to raise the temperature of the now moistened air 26, thereby increasing its ability to evaporate moisture further. Further evaporation and heating takes place until the air 26 reaches a barrier in its path at 20, causing it to flow to exhaust 21. Air which flows through the flow resistance 28 becomes the delivered air 24. This air has been cooled without the addition of moisture. In the limit of low airflows and good heat exchange, the temperature of delivered air 24 can approach the Dew Point of the incoming air. Figure 2 shows an arrangement in accordance with an embodiment of an aspect of the present invention for wetting of the wettable media in the wet passages in a segmented manner. The arrangement of Figure 2 divides the core 94 into a number of segments 62 (shown as five segments in Figure 2, but a lesser or greater number of segments could be used). Each segment has its own pumping means 60, its own water reservoir 66 and its own water distribution system 68. Each segment 62 of core 94 with its corrugated construction, tends to pass water from its associated water distributor 68, through the core 94 to respective water reservoir 66 with little mixing of water from adjacent segments. Since, in operation, all segments are circulating water simultaneously, any tendency of the circulating water in a segment to pass through to an adjacent segment is approximately balanced by an equal and opposite tendency for water to come back from that adjacent segment. Thus, for each segment water is circulated relatively independently of each of the adjacent segments. The circulating water temperature in each of the segments can therefore be different, thus providing the temperature gradient necessary to thermal performance of the indirect evaporative cooler, and thus allow the delivered air temperature to approach the Dew Point. This arrangement for water supply to the core has several advantages over the prior art, including removal of the restriction on core height due to the wicking capability of the wettable media; water flow surplus to the requirement for evaporation flushes away any salt concentration due to evaporation and water quality can be easily monitored for salt concentration and diluted before critical concentrations are reached. This arrangement would approach the ideal wetting condition of wicking if there were many segments. Thermal performance is compromised if there are too few segments. In practice it has been found that dividing the core into 4-6 segments gives thermal performance approaching a wicking system with a considerably more robust and enduring core for practical applications. In practical examples, it has been found that water descending through the core does not entirely remain in separated segments as in the ideal case. There is, in practice, some drift of water between the segments resulting in the accumulation of water in some segment water reservoirs, and a deficiency of water in other segment reservoirs. This practical difficulty is overcome by the provision of a bypass conduit 70 between the reservoirs, where the bypass conduit 70 is connected to each of the segment water reservoirs via an opening 72. Should the surplus/deficiency problem of water descending through the core arise, water level variations in the reservoirs 66 will equalise through the conduit 70 until a steady state of flow between the reservoirs is established. This arrangement also allows for water filling or addition at one reservoir only, by allowing water levels to again equalise according to the steady state requirements of the individual segments.

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Figure 3 shows the complete core 94 with the water distribution system 68 and the entry airflow system 104 in place. Each water distributor is located within a space 101 kept separate from the water distributor space of adjacent segments by barriers 100. The sealed spaces 101 and barriers 100 are necessary to prevent airflow exiting from the wet passages of the core thereby causing air in the wet passages to travel all the way along the wet passages. A similar sealing system is necessary to separate each water reservoir 66 from adjacent water reservoirs. Each water reservoir 66 is sealed to the core by barriers 102 thus preventing any air from leaving the wet passages through the water reservoirs. Immediately after the entry end of the core, the wet passage space is left open at 106. The opening 106 allows the now moist, warm air flowing in the wet passages to exhaust from the core 94. In the preferred embodiment, an exhaust opening 106 is provided at both the top and bottom of the core although only the top opening is shown in Figure 3. However, if provision of an opening 106 at the bottom of the core is impracticable, satisfactory performance can still be achieved with only the opening 106 at the top with some degradation of thermal performance. The ratio of delivered air to exhaust air is adjusted by means of a flow restriction 108 in the delivered air stream. Closing flow restriction 108 increases the pressure in chamber 109 at the delivery end of the core 94, thereby increasing the flow of air back through the wet air passages toward exhaust opening 106. It will be understood by those skilled in the art that the present invention is not limited to the above described embodiments and numerous variations and modifications may be made thereto in accordance with the invention as defined by the claims.

Claims (11)

1. A method of operating an evaporative cooler which includes a heat exchange core wherein adjacent wet and dry airflow channels are in counter current airflow heat exchange relationship with water being supplied to the wet channels in a descending flow pattern, said method comprising supplying water to the wet channels via a plurality of segments from an air entry end to an air outlet end of said core during operation of said cooler and circulating water through each segment relatively separately from adjacent segments such that an appropriate temperature gradient is established from an air inlet end to an air outlet end of the core by maintaining different circulating water temperatures in each segment.

2. A method as claimed in claim 1, including separately supplying water to each segment.

3. A method as claimed in claim 2, including sourcing the water supplied to each segment from respective water sources.

4. A method as claimed in claim 3, including interconnecting the respective water sources to facilitate flow of water therebetween.

5. An evaporative cooler having a heat exchange core with adjacent wet and dry airflow channels in counter current flow with heat exchange therebetween; said core being segmented between an air entry end and an air outlet end and having water supplying means for supplying water to the wet channels in the respective segments during operation of the cooler such that in use water is circulated through each segment relatively separately from 1 adjacent segments to effect different circulating water temperatures in each segment.

6. An evaporative cooler as claimed in claim 5, wherein the water supplying means includes a plurality of water distributors spaced apart above the core with at least one water distributor for each segment.

7. An evaporative cooler as claimed in claim 6, wherein the at least one water distributor for each segment is connected to a respective water reservoir.

8. An evaporative cooler as claimed in claim 7, wherein the reservoirs of all segments are interconnected to enable equalisation of water levels between reservoirs.

9. A method of operating an evaporative cooler substantially as hereinbefore described with reference to Figure 2 or 3 of the accompanying drawings.

10. An evaporative cooler substantially as hereinbefore described with reference to Figure 2 or 3 of the accompanying drawings.

11. An evaporative cooler as claimed in claim 13 substantially as hereinbefore described.