MXPA96001441A - Intensification of evaporation and deca transfer - Google Patents

Abstract

The present invention relates to a heat exchanger for evaporating water in a stream of air through a humid air passage, characterized in that it comprises at least three separate parallel blades defining between them said humid air passage and a passage of air. dry air, a plurality of wettable wicks attached in face-to-face contact with one of said sheets, in a separate parallel arrangement in said humid air passage, and extending in a generally normal direction to an air stream therethrough when the exchanger is in use, and at least one fan for establishing said air stream in said humid air passage and an additional stream of air through said passage of dry air in a heat exchange relationship with said first stream.

Description

INTENSIFICATION OF EVAPORATION AND HEAT TRANSFER This invention applies to both the evaporation and the heat transfer through the surface of a heat exchanger that occurs in a heat exchanger where there is an air flow with low Reynolds number and therefore the air flow tends to be laminar, and the invention also refers to a humidifier, a heat exchanger, a water evaporation method in j ...}. an air stream in an evaporative cooler, and a method of heat transfer in a heat exchanger.

BACKGROUND OF THE INVENTION In the prior art it was attempted that the mass and energy transients would occur continuously of extended surfaces, for example large air passages in a heat exchanger. However, a characteristic of the heat exchanger along a surface is that The thickness of a gas barrier layer constitutes an obstruction for the transfer of mass or energy, although the; Heat exchangers of the prior art have frequently used large passages defined by walls of constant transverse shape, for example tubes, and frequently operate with low Reynolds numbers where the barrier layer can develop a very significant thickness, which requires that the heat transfer takes place through a thickness of the air or other gases or vapors, although said air or other gases or vapors are very resistant to heat transfer. Consequently, excessive heat exchanger areas have been used for heat transfer, for example from a wet channel to a dry channel, and very small cross-sectional tubes have been used in large numbers to create an exchange of heat. heat that has a very large area of heat exchange surface to obtain a low outlet temperature of the cooled air below its wet bulb temperature. It is known that the need to use a large excess of material was due to the requirement of mass and heat transfer that took place not only through thin solid boundary walls of an air passage but also through laminar layers of gas inside. of that passage, and the water in a passage attached. Reference can be made to page &B of the textbook entitled "Engineering Thermodynaics Work and Heat Transient," Roger and Mayhew (1957), where the following statement can be found: "... once it has been established When the flow is completely (in a tube), the fluid may not have normal speed components to the wall anywhere in the cross-section, the successive velocity profiles could otherwise not be identical, there is no divergence of the current lines in distance from the wall ..., and the heat flow in the radial direction must be entirely by conduction ". Gases are notorious insulators against driving. It has been established that the use of water passing through an absorbent pad in one direction and cooling by evaporation the air passing through the pad in transverse flow is only able to obtain air cooling by reducing the temperatures reaching the wet bulb temperature. The wicks are old and well known in the technique of evaporative air conditioning, and it has been found that by using wicks (which can be vertical, lateral or preferably inclined) it is possible to build a device that is susceptible to obtain the temperature of initial moist bulb and towards the condensation point for the relevant temperature. The main objects of this invention are to provide improved evaporation of water in an air stream, and to provide a heat exchanger having a heat and mass transfer higher than that of the prior art known to the applicant in another way, and an additional object is >; Provide an efficient cooler that uses water evaporation. This invention utilizes air passages wherein the laminar flow is interrupted by wet wicks in a manner sufficient for low Reynolds number conditions to develop sufficient turbulence to effect the periodic restoration of the process of moisture evaporation from the wicks. It should be noted that the evaporation process is closely related to the process of heat transfer, both processes involving a similar molecular movement within a passage- In accordance with the aforementioned relation between air flow and heat flow in an address at right angles thereof, additional study was conducted by applicants of the evaporation behavior as the air passes over a wet surface which has indicated that the main evaporation and therefore the main cooling occurs at the end upstream of a passage of elongated moist air, and that the velocity of the transfer of energy and mass decreases as the air continues to transfer over the wet surface. Evaporation also intensifies (although to a lesser degree) at the trailing edge of a wet surface. Theoretical studies have also confirmed that this phenomenon is due to the thickness of the air barrier layer as it passes over an internal surface of a tube, where its thickness of displacement causes the center line speed to accelerate until a profile is reached of fully developed speed. This defines an entry transition profile. High wall shear stresses are what allow the transfer of mass and energy to occur. The Reynolds analogy is valid since the mechanisms of evaporation (mass transfer) and energy transfer depend both on the similar molecular movement within the barrier layer. The mass transfer rate during passage of the air over a wet wall of constant transverse shape i (/ depends on the local value of the mass transfer coefficient, which is progressively reduced from the input zone in a downstream direction to a fixed fully developed value, this affects the inclination of the moisture curve against distance, and the gradient of .15 concentration will be reduced with respect to the displaced distance, as the flow is humidified. The graphs appended hereto compare the distance that the air moved from its entrance area and the humidity, with the tubes of large and small diameter with constant transverse shape, and the corresponding temperature changes.

BRIEF SUMMARY OF THE INVENTION In one embodiment of the invention, cooling is carried out in multiple stages, passing air over a series of separate wet evaporation pads or wicks and with the interruption of the air flow by means of said wick pads thereby providing a periodic restoration of the evaporation. In addition, in the invention, the improved cooling associated with improved evaporation is also associated with a heat exchanger, wherein the same interruption imparts an improved transfer of detectable heat. Optimal evaporation conditions can be achieved and the heat transfer conditions can also be greatly increased. In some embodiments of the invention the heat transfer will take place through a very thin wall of impermeable material (eg plastics), which divides the wet and dry parts of the heat exchanger. The optimal stay between the wet pads needs to be determined in conjunction with the number of variables that include the additional flow resistance induced by the interruptions, and thus may vary with the objectives of the application. For example, if the target is a very compact evaporator or heat exchanger, the flow interruption can be very frequent for high mass / energy transfer rates with the disadvantage of a high flow resistance. An objective of the reduced operating cost request may extend the distance between interruptions to obtain a good transfer to internal flow resistance. Applications have been found that involve successful evaporation and heat transfer which is often an optimal ratio of wicks of wet pads to the distances between them along the flow, one part of the wet pads to two parts between them , and three parts between them respectively for optimum evaporation and optimal heat transfer.

BRIEF DESCRIPTION OF THE PREFERRED MODALITIES The embodiments of this invention are described hereinafter with some detail in reference to and are illustrated in the accompanying drawings in which: Figure 1 is an illustration of a humidifier with a series of discrete moistened wicks attached to a surface of a thin-walled substrate that may not necessarily be porous; Figure 2 shows an extreme elevation in section of Figure 1 drawn to a larger scale and illustrating the manner in which air will pass over the wet wicks; Figure 2 however, shows several layers of a heat exchanger complex; Figure 3a is a diagrammatic representation of two surfaces that define a passage of air flow? separated from one another and indicating how a barrier layer will be constituted to retain its shape after the initial entry of air into the passage has been completed; & Figure 3b is a graph showing an expectation of heat transfer against the distance along the air passage of Figure 3a, and an area marked "area A"; Figure 4a shows the effect of interruption of the barrier layer, in this example by a series of wet wicks that are separated adjacent to each other on opposite sides of the barriers of an air flow passage; Fig. 4b shows diagrammatically the heat transfer against the distance along the tube of the air flow in the arrangement of Fig. 4b; and Figure 5 shows a counterflow heat exchanger with separate wet blades. Figures 6 and 7 are graphs illustrating the very rapid evaporation asymptote in a small diameter tube of 1 mm and 6 mm or the separation between the parallel surfaces. Figures a to 4b are indicative of how the principles of this invention can be incorporated, although it will be clear that other configurations can be used. In the embodiment itrated in FIGS. 1 and 2, a substrate 10 comprising a panel of thin plastic material (eg, thin-walled dense polyethylene film) has a plurality of separate porous wettable wicks adhered face to face 11 and these run the function of repeatedly interrupting the barrier layer of the air flow, which would otherwise be consistent on the substrate 10. As the wettable wicks 11 are encountered, the air becomes turbulent thereby disturbing the layer of air. barrier, and as the next downstream strip is found, it is cooled more rapidly by the mass transfer than it would be if it will pass over a continuous wide pad. In Figure 1 a fan 9 is shown diagrammatically to itrate the source of air flow. The total amount of heat that can be transferred is compared in Figures 3a, 3b and 4a, 4b. In Figures 3a, 3b the amount of heat that is transferred is asymptotic along a minimum heat transfer level, as the air flow proceeds downstream from an inlet, in a passageway between two waterproof solid films 16, and in Figure 3b, "area A" is an integral of the heat transfer along the tube, so that area A is representative of the total heat transfer. Drawn to the same scale in Figures 4a and 4b, the wicks 11 are shown to repeatedly interrupt the barrier flow which is designated as lβ so that maximum evaporation may occur on the wicks, particularly at their entry and exit edges, and Figure 4b shows how there is a repeated restoration of evaporation. Area B will be observed much larger than area A, and therefore indicates a much higher degree of heat transfer, in other words, for the same amount of heat transfer, a much smaller and more economical heat exchanger . The graph of figure 6 itrates the asymptote of very rapid evaporation in a tube of small diameter of one millimeter or the separation between parallel surfaces, the evaporation that is not obvious takes place after the air passes through &; millimeters from its entry point. The graph of Figure 7 shows, in comparison, that the evaporation continues beyond a distance of 350 mm from the point of entry into a tube having a diameter of 6 millimeters. The cooling effect by transferring heat through the substrate 10 is similarly more effective if the substrates of a stack are more widely separated, for example up to 6 mm. These effects are used to have an advantage in the humidifier of Figure 1 (for direct evaporative cooling), and the heat exchangers in Figures 2 and 5 (for indirect evaporative cooling). In indirect evaporative cooling, the second advantage of increasing heat transfer is of particular importance. In Figure 1, the flow of warm dry ambient air is converted by periodically restored evaporation from wet strips in a humid cold air stream 12, and an array of substrates each with wettable strips 11 can provide a cooling pad excellent for a. simple evaporative cooler. However, there is also the advantage in disturbing the flow of dry air in a heat exchanger, and as shown in Figure 2 there is a passage of moist air 13 which separates the dry air passages 14 through the substrate films. 10. The wet wicks 11 disturb the barrier layer and cause some turbulence in the wet passages 13, while the projections 20 will have an effect in some way different in the dry passages 14, although, l? they will increase the heat transfer. The illustrations of FIGS. 1 and 2 show a layer of wet strips that improve the evaporative efficiency, and for example an evaporative cooler can be of simplified construction if the separate wet wicks replace the conventional wood chip. However, the invention also extends to a heat exchanger 25, shown in Figure 5. The embodiment of Figure 5 also uses a plurality of separate wicks 11 on film substrates 10, and for the For purposes of disclosure, the ends 22 of the wicks 11 project outwardly beyond the ends of a stack 23 of substrates, and a pump 24 spills water onto the ends of the projection wick 22, by means of a pair. of perforated sprinkler tubes 26. The wicks 11 are horizontal or inclined, not vertical as in the prior art, and this increases the transport of water along the wicks.

A consideration of the above embodiment immediately indicates to the reader that the invention is extremely simple although it can be implemented in many ways. For example, the wicks 11 are not necessarily necessarily adhered to the substrates although they may otherwise be transported by the substrates 10, for example fastened at separate intervals, and if the mass transfer is taken at maximum efficiency, the heat transfer It will also be done more efficiently.

Claims (7)

NOVELTY OF THE INVENTION CLAIMS

1. - A cleaner for evaporating water in a flow of air flowing in a first direction, comprising a fan for establishing said air stream, a substrate, a plurality of wettable wicks carried by said substrate in a separate parallel arrangement and each one extending in a direction generally normal to the flow to the air stream, and means of humidification to moisten said wicks.

2. A humidifier in accordance with claim 1, further characterized in that said wettable wick is joined in a rare contact face to said substrate.

3. A heat exchanger comprising a humidifier in accordance with claim 1, further characterized in that it comprises two separate sheets, said substrate being one of said sheets, said sheets defining a passage of moist air and confining said air stream, and a third sheet parallel to the first of said sheets and separated from the substrate defining a passage of dry air and confining an additional air stream.

4. A heat exchanger according to claim 3, further characterized in that it comprises a plurality of parallel sheets defining a stack of alternating dry and wet air passages.

5. A heat exchanger according to claim 3, further characterized in that the ends of the wettable strands project outwardly between said sheets of the wet passage, and further comprises means of application of water placed to moisten the ends of the wick. projection.

6. A heat exchanger according to claim 3, further characterized in that the humid and dry air currents flow in opposite directions.

7. A heat exchanger according to claim 3, further characterized in that said sheets defining the dry air passage comprise projections entering said dry air passage sufficiently to cause some turbulence of said air stream. & . - A heat exchanger according to claim 3, further characterized in that said sheets and said wicks are not vertical. 9. A heat exchanger according to claim 3, further characterized in that said sheets and said wicks are generally horizontal. 10. An air humidification method comprises humidifying each of a plurality of separate parallel wicks located between a pair of leaves defining a humid air flow passage, and urging a stream of air through said flow passage. of humid air to be interrupted repeatedly by said moistened wicks. 11. An air cooling method in an evaporative cooler comprising moistening each of a plurality of separate strands located between two sheets defining a humid air flow passage, urging a first stream of air through said passage of wet flow in one direction to be repeatedly interrupted by successive moistened and humidifying wicks thereto, and to drive a second stream of air through a dry passage defined by said sheet and said third sheet separated therefrom to effect this mode a sensible heat exchange between said air currents. 12. A method according to claim 11, further characterized in that it comprises effecting the exchange of sensible heat by driving the second stream of air on a surface of said sheet, the opposite surface of which has the parallel wicks adhered to the same in a face-to-face relationship. 13. A humidifier substantially as previously described herein with reference to and illustrated in the appended representations. 14. A heat exchanger substantially as described herein with reference to and illustrated in the appended representations. 15. An air cooling method substantially co-or as described herein with reference to and illustrated in the appended representations. INTENSIFICATION OF EVAPORATION AND HEAT TRANSFER SUMMARY OF THE INVENTION An evaporative air cooler evaporates water in multiple stages by passing air through a series of separate parallel moistened wicks that repeatedly interrupt the flow of air; this increases both the evaporation rate and the heat transfer rate; the humidification of the wicks 11 is achieved at their ends 22, and the wicks are inclined or horizontal, not vertical, so that the passage of water along the wicks is not prevented by gravity. RML / cpm *