Heat exchange – Flow passages for two confined fluids – Interdigitated plural first and plural second fluid passages
Patent
1986-02-25
1990-05-08
Davis, Jr., Albert W.
Heat exchange
Flow passages for two confined fluids
Interdigitated plural first and plural second fluid passages
165154, 165 70, 165903, 165167, F28F 1302, F28F 110
Patent
active
049230034
DESCRIPTION:
BRIEF SUMMARY
When, in the case of a heat exchanger, it is desired to achieve a high heat exchange per unit of volume, there are essentially three factors which are of importance, assuming that the nature of the two heat-exchange media involved, the volumetric flows thereof, and their input temperatures are known factors. The factors primarily affecting the exchange of heat in a heat exchanger are firstly the effective area of the medium contacting surfaces of the thermally conductive partition walls separating the two media; secondly the length of the paths along which the heat must be conducted within respective media, towards and away from said partition walls, and within said walls; and those percentages of the total temperature difference which lie along said path-lengths.
Conventional tube heat-exchangers or plate heat-exchangers, which completely dominate present day markets, operate with a turbulent flow of the heat-exchange media. Thus, in the passage, channel or like duct through which a medium flows there is found a central zone of turbulent flow within which the temperature is relatively constant and uniform over the whole cross-section of the zone, while adjacent the defining walls of said flow passage, these walls also forming partition walls which mutually separate the two heat-exchange media, there are found thin boundary layers of substantially laminar flow. Since the thermal conductivity of the material in the walls of the flow passage is much greater than that of the medium and the temperature difference within the central turbulent zone is low, the major part of the total temperature difference will occur over the laminar boundary layer. Consequently, the majority of those steps taken in turbulent-flow heat exchangers to increase the heat-exchange efficiency thereof are concentrated on maintaining a thin laminar-flow boundary layer and the assurance of good turbulence in said central zone. To this end, various kinds of "flow interrupters" are arranged in the flow passages
Conventional tube and plate type heat exchangers functioning in accordance with the aforementioned turbulent flow principle are encumbered with a number of serious disadvantages. Since the central turbulent zone in the flow passages takes up a large part of the total volume, that area of the heat-transferring partition wall which comes into contact with the heat-exchange media is relatively small, when calculated per unit of volume. When viewed from a purely theoretical aspect, it is possible to provide a larger contact surface area between the two heat-exchange media and the walls separating said media, by miniaturizing the flow passages, irrespective of whether these passages are formed by tubes of circular cross-section or some other cross-section, or are formed by the voids between mutually opposing planar plates. The extent to which such miniturization need be carried out in order to be really effective, however, leads to unacceptable pressure drops, and also presents significant disadvantages in the form of manufacturing problems and high manufacturing costs. In addition, existing problems of providing an effective seal between the two media, already of a serious nature, are worsened. Present day tube and plate type heat exchangers are highly susceptible to corrosion and are unable to withstand pressure to any great extent, due to the relatively thin walls between the heat-exchange media. Additional hereto, such heat exchangers have a large number of locations which require sealing, thereby creating risk of leakage between the media. These fundamental weaknesses have normally lead to the use of stainless steel and readily brazed or soldered, non-corroding copper alloys, while the use of aluminium alloys has generally been ignored, despite that the bulk price of aluminium alloys is lower than that of copper alloys.
Practical designs of heat exchangers of the kind in which the heat-exchange media have a laminar flow across the whole cross-section of the flow passages, i.e. with no central turbulent zone, are little known on pres
REFERENCES:
patent: 4368779 (1983-01-01), Rojey et al.
patent: 4445569 (1984-05-01), Sabol et al.
Davis Jr. Albert W.
Hypeco AB
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