Gas and liquid contact apparatus – With external supply or removal of heat – Processes
Patent
1996-04-18
1998-02-17
Miles, Tim R.
Gas and liquid contact apparatus
With external supply or removal of heat
Processes
261104, 261153, B01F 304
Patent
active
057188483
DESCRIPTION:
BRIEF SUMMARY
This invention applies both to evaporation and heat transfer across a heat exchanger surface occurring in a heat exchanger wherein there is an air flow with low Reynolds number and hence the air flow tends to be laminar, and the invention also relates to a humidifier, a heat exchanger and a method of evaporation of water into an air stream in an evaporative cooler, and a method of heat transfer in a heat exchanger.
BACKGROUND OF THE INVENTION
In prior art the transfers of mass and energy are intended to occur continuously along extended surfaces, for example long air passages in a heat exchanger. However, a characteristic of heat exchange across a surface is that the thickness of a boundary layer of gas constitutes an obstruction to transfer of mass or energy, but prior art heat exchangers have frequently used long passages defined by walls of constant cross-sectional shape, for example, tubes, and frequently operate under low Reynolds numbers wherein the boundary layer can develop very significant thickness, requiring the heat transfer to take place through a thickness of air or other gases or vapours, but such air or other gases or vapours are very resistant to heat transfer. Consequently, use has been made of excessive heat exchanger areas for transferring of heat, for example from a wet channel to a dry channel, and very small cross-sectional area tubes have been used in large numbers to create a heat exchanger having a very large area of heat exchange surface to obtain a low temperature output of air cooled below its wet bulb temperature. It is known that the necessity to use a lot of the excess of material was due to the requirement for mass and heat transfer to take place not only through thin solid boundary walls of an air passage but also through laminar layers of gas within that passage, and water in an adjoining passage.
Reference may be made to Page 488 of the text book entitled, "Engineering Thermodynamics Work and Heat Transfer," Rogers and Mayhew (1957), wherein the following statement may be found: no velocity components normal to the wall anywhere in the cross-section, otherwise successive velocity profiles would not be identical. There is no divergence of the streamlines away from the wall . . . , and the heat flow in the radial direction must therefore be entirely by conduction."
It has been established that the use of water passing through an absorbent pad in one direction and cooling by evaporation air passing through the pad in cross flow is only cable of achieving air cooling down to temperatures approaching the wet bulb temperature. Wicks are old and well known in the art of evaporative air conditioning, and it has been found that by using wicks (which can be vertical, lateral or preferably sloping) it is possible to construct a device cable of getting below the initial wet bulb temperature and towards the dew point for the relevant temperature.
The main objects of this invention are to provide an improved evaporation of water into an air stream, and to provide a heat exchanger having a higher heat and mass transfer than prior art otherwise known to the Applicant, and a further object is to provide an efficient cooler using evaporation of water.
This invention utilises air passages wherein laminar flow is interrupted by wet wicks sufficiently so that even under the very low Reynolds number conditions, sufficient turbulence is developed to effect periodic restart of the process of evaporation of moisture from the wicks. It should be noted that the process of evaporation is closely allied to the process of heat transfer, both processes involving a somewhat similar molecular movement within a passage.
Consistent with the above recited relationship between flow of air and heat flow in a direction at right angles thereto, further study conducted by the Applicants of the behaviour of evaporation as air passes over a moist surface has indicated that the main evaporation and therefore the main cooling occurs at the upstream end of an elongate wet air passage, and that the rate of energy
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"Engineering Thermodynamics Work and Heat Transfer", p. 488, Rogers and Mayhew (1957).
F F Seeley Nominees Pty Ltd
Miles Tim R.
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