US3913667A

US3913667A - Heat exchanger

Info

Publication number: US3913667A
Application number: US399513A
Authority: US
Inventors: Jean-Luc Ch Meylan; William H Frost; John G Meier
Original assignee: Battelle Memorial Institute Inc
Current assignee: Battelle Memorial Institute Inc
Priority date: 1972-09-22
Filing date: 1973-09-21
Publication date: 1975-10-21
Anticipated expiration: 1992-10-21
Also published as: FR2200492A1; FR2200492B1; CH557014A; GB1404414A; DE2347883A1

Abstract

A heat exchanger for the cooling of water by an airstream comprises a plurality of generally parallel hollow wall members forming vertical passages to be traversed by the cooling air, each wall member consisting of an envelope of thin plastic sheet material provided at the top with an inlet for hot water which is allowed to flow down on the inner shell surfaces in a continuous film of liquid. The envelope opens at the bottom into a pool of water and is sealed against the atmosphere to enable the maintenance therein of air under pressure inflating it against a reinforcing external grid of tensioned cables or the like.

Description

United States Patent Meylan et al.

[4 1 Oct. 21, 1975 HEAT EXCHANGER [75] Inventors: Jean-Luc Ch. Meylan, Geneva;

William H. Frost, Grand-Laney, Geneva; John G. Meier, Geneva, all of Switzerland [73] Assignee: Battelle Memorial Institute,

Carouge, Geneva, Switzerland [22] Filed: Sept. 21, 1973 [21] Appl. No.: 399,513

[30] Foreign Application Priority Data Sept. 22, 1972 Switzerland 13899/72 [52] US. Cl 165/115; 261/.Ol1 [51] Int. Cl. F28d 9/00 [58] Field of Search ..261/.011, 104, 107, 112; 165/1l1,115,122,125, 129

[56] References Cited UNITED STATES PATENTS 3,371,709 3/1968 Rosenblad 165/115 3,374,994 3/1968 Greor 3,412,778 11/1968 Witt et al 165/115 X 3,811,661 5/1972 Procter 261/107 FOREIGN PATENTS OR APPLICATIONS 571,510 8/1945 United Kingdom 261/112 Primary ExaminerCharles J. Myhre Assistant ExaminerTheophil W. Streule, Jr. Attorney, Agent, or Firml(arl F. Ross; Herbert Dubno [57] ABSTRACT A heat exchanger for the cooling of water by an airstream comprises a plurality of generally parallel hollow wall members forming vertical passages to be traversed by the cooling air, each wall member consisting of an envelope of thin plastic sheet material provided at the top with an inlet for hot water which is allowed to flow down on the inner shell surfaces in a continuous film of liquid. The envelope opens at the bottom into a pool of water and is sealed against the atmosphere to enable the maintenance therein of air under pressure inflating it against a reinforcing external grid of tensioned cables or the like.

l0 Claims, 6 Drawing Figures US. Patent Oct. 21, 1975 Sheet 1 on 3,913,667

\\\ 1 ww mx \&

FIG.

U.S. Patent Oct.21, 1975 Sheet20f3 3,913,667

FIG. 2

HEAT EXCHANGER FIELD OF THE INVENTION Our present invention relates to a heat exchanger designed to establish thermal contact between two fluids of different temperatures, at least one of these fluids being a liquid. In its more specific aspects it concerns a heat exchanger designed to cool large quantities of hot water, such as the effluents of electric power plants or nuclear reactors, by thermal contact with the atmospheric air.

BACKGROUND OF THE INVENTION In modern industry, and especially in the field of generating electric power, large quantities of heat are produced which must be dissipated at low cost and without detrimentally affecting the environment, i.e., with a minimum rise in temperature of an ambient medium into which this waste heat is discharged. In the past, rivers and lakes were rather extensively used for getting rid of hot effluents; recent ecological studies, however, have shown the necessity for imposing strict limitations upon this method of disposal.

An alternative way of dissipating waste heat is the utilization of atmospheric air as a coolant. This may be conventionally accomplished with the aid of cooling towers of either wet or dry" type through which the air is forced to circulate by natural draft and/or with the aid of blowers: In a wet tower the hot waste water is dispersed into the rising air stream for vaporization thereby; this is a relatively efficient way of cooling, resulting in only a minor rise in air temperature, but has the drawback of introducing additional moisture into the atmosphere which may lead to the formation of rather dense localized fog that can interfere with nearby road or air traffic. In a dry tower the water circulates through finned tubes which must have a large effective surface area and which entail considerably higher costs than wet towers of equivalent capacity; also, the rise in air temperature is substantially higher in that instance.

OBJECTS OF THE INVENTION The general object of our present invention, therefore, is to provide an improved heat exchanger for the purpose set forth which avoids the disadvantage of earlier systems of this type.

A more particular object is to provide means in such a heat exchanger for intensifying the thermal contact between two fluids, such as water and air, while keeping them physically separated from each other.

SUMMARY OF THE INVENTION These objects are realized, in accordance with our present invention, by the provision of a plurality of spacedly juxtaposed hollow wall members defining passages between them for the circulation of one of the thermally interacting fluids, such as an airstream. Each wall member includes a generally horizontal elevated support and an envelope of plastic sheet material suspended therefrom, this envelope comprising a pair of confronting sheets separated by an airspace. A channel on the elevated support receives the other (liquid) fluid, such as hot water, via a supply conduit and discharges it through gaps, leading to the aforementioned airspace, onto the inner surfaces of the sheets for distribution along upper edge portions thereof from which this fluid descends in a continuous film along the sheet surfaces into a collector at the bottom of the envelope. Thus, the liquid of the film interacts thermally over virtually the entire envelope area with the externally circulating air or other fluid.

Advantageously, each envelope is closed against the atmosphere at its top and sides while dipping at its bottom into a pool of liquid held in a basin which constitutes the aforementioned collector, the liquid level in that basin reaching above the envelope bottom so as to merge with the internally descending liquid films and to complete the seal. The interior of the envelope can then be inflated by air or some inert gas under pressure, preferably against an external reinforcing grid such as an orthogonal array of wires, cables or similar flexible elements under tension. These reinforcing grids may also be used as bracing points for spacers which extend between adjacent wall members to keep them separated by a predetermined distance.

A particularly advantageous system incorporating a heat exchanger of this description comprises an annular array of such wall members extending substantially radially with reference to a vertical axis. This arrangement ensures that the liquid descending on both inner sheet surfaces of each wall member is in contact with a thermally coacting external fluid such as an airstream traversing the intervening passages. The array of wall members may form part of a cooling tower including a flue centered on the axis thereof, this flue opening at its base into the air passages between the wall members.

The gaps extending from the supply channel of each wall member toward the inner sheet surfaces of its envelope may be constituted by series of apertures or by throughgoing longitudinal slots. They may be at least partly blocked by strips or threads of capillary material, such as porous polymers or textiles fabrics, which help distribute the liquid over the upper sheet edges and thus over the entire inner sheet surfaces.

BRIEF DESCRIPTION OF THE DRAWING The above and other features of our invention will now be described in detail with reference to the accompanying drawing in which:

FIG. 1 is a transverse sectional elevation of part of a heat exchanger according to our invention, including a pair of spacedly juxtaposed wall members;

FIG. 2 is a fragmentary view of a wall member as shown in FIG. 1 but drawn to a relatively reduced scale;

FIG. 3 is a somewhat diagrammatic top view of an annular array of wall members, drawn to a still smaller scale, as seen on the line III III of FIG. 4;

FIG. 4 is a sectional elevational view of a cooling tower embodying our invention, taken on the line IV IV of FIG. 3; and

FIGS. 5 and 6 are detail views showing modifications of the top of the wall members illustrated in FIGS. 1 and 2.

SPECIFIC DESCRIPTION In FIGS. 1 and 2 we have shown part of a heat exchanger designed to facilitate thermal interaction be tween a flow of hot water and a stream of atmospheric air. The heat exchanger comprises a multiplicity of wall members 12, only two of which have been shown in FIG. 1. Each wall member 12 comprises an overhead support in the form of a rigid tube 2 which encloses a supply channel 2b for hot water introduced, for example, by a pump as illustrated in FIG. 4. The tube 2 is suspended from a ceiling 23, which may form part of the base of a cooling tower 24 as shown in FIGS. 3 and 4, with the aid of two sets of vertical rods or cables 4a which are anchored under tension to respective sockets 5 rising from a foundation 3 on the bottom of a basin 16. Elements 4a form part of a pair of reinforcing grids each comprising a set of cables 4b which are also held under tension with the aid of nonillustrated springs, weights, jacks or the like. The

grids

4a, 4b bear upon the outer surfaces of a pair of sheets 1 of plastic material forming part of a flat upright envelope which in FIGS. 1 and 2 is shown draped about the tube 2 and which is also closed at its narrow sides as indicated at 1a. The open bottom of this envelope dips into a pool of water W in basin 16 whose level N is well above the lower edge lb of the envelope; thus, the interior of the envelope is completely sealed against the atmosphere. This interior is filled by a volume of air under pressure which results in a depression of the water level N within the envelope with reference to the outer level N.

Tube 2 is formed with two longitudinal rows of apertures 2a which open onto the inner surfaces of sheets 1 so as to distribute the incoming hot water over the upper edge portions of these surfaces. The distribution may be facilitated by wicks 22 extending from the apertures 2a into contact with the sheets. Thus, the water from channel 2b descends in a pair of continuous films 6 along the confronting inner sheet surfaces into, the pool W from which it is constantly removed at the same rate, e.g., by a pump 17 as shown in FIG. 4, to maintain the level N substantiallyconstant. In running down the sheets 1, the water is in intense heat-exchanging contact with an external airstream traversing the passage 25 between adjoining wall members 12, the width of this passage being maintained by a set of spacing bars 7 (only one shown in FIG. 1) engaging the reinforcing

grids

4a, 4b on the two wall members. The spacing of the tensioned

elements

4a, 4b of these grids is sufficient to give the air almost complete access to the outer sheet surfaces.

As illustrated in FIG. 5, the continuous tube 2 may be replaced by a profile 2' of inverted-U shape separated by narrow slots 2a from a plate 2'c to define a supply channel 2'b. A capillary liner 20, advantageously consisting of a porous polymer such as silicone sponge, surrounds the channel 2'b and overlies the slots 2a to help distribute the water over the inner surfaces of a pair of plastic sheets 1 so as to form again two continuous liquid films 6' descending along these surfaces. In this instance, the sheets 1' are not integral with each other but are interconnected in airtight fashion by the profile 2. The narrow sides of the envelope are formed by webs la which should be rigid enough to support the plates 2c.

In FIG. 6 we show another modification in which a U-shaped profile 2" constitutes a trough forming part of an upwardly open channel 2"b communicating via clearances 2"a with the inner surfaces of a pair of plastic sheets 1 these clearances being occupied by capillary layers 21 which may be of the same material (e.g., silicone sponge) as liner 20 in FIG. 2. The water in channel 2"b, overflowing the trough 2", penetrates the layers 21 and descends along the inner surfaces of sheets 1 in a pair of continuous films 6". The narrow sides of the envelope formed by sheets 1" are closed by,

webs 1"a.

The envelopes of FIGS. 5 and 6 are also sufficiently fluidtightto be filled with air or some neutral gas under pressure to inflate the sheets 1 or 1" againstexternal reinforcing grids which have not been illustrated in these Figures. I

FIGS. 3 and 4 show the overall construction of a heat exchanger according to our invention as embodied in. 1 cooling tower 24. This tower comprises a flue 10 centered on a vertical axis 0 which is also the center of an annular array of wall members 12 separated by passages 25 as described above. Air flows radially inwardly at the base of the tower, either exclusively on account 1 of the updraft created in the flue or with the assistance of nonillustrated blowers which could be disposed between two annular treatment zonesZ, and Z alongside a set of water separators 1 1. Outer zone Z is peripher ally divided into a multiplicity of sectoral compartments, aligned with respective radial passages 25, by piers 26 supporting the roof 23 which isintegral with the wall of flue 10. Hot water, arriving through an inlet 14, is delivered by pump 15 to a circular conduit or manifold 13 communicating with all the tubes 2 (or their equivalents as shown in FIGS. 5 and 6) of the multiplicity of wall members 12 which define the inner treatment zone Z The passing airstream cools the incoming water descending,,as described above, within the envelopes of wall members 12 and collecting at the bottom thereof in the annular basin 16 for extraction by the pump 17. The latter feeds another annular manifold 8 serving a multiplicity of outwardly radiating nozzles 9 which, within the compartments of zone 2,, disperse the water into the oncoming airstream. The water particles collect in an annular trough 18 at the bottom of these compartments, the cooled effluent being discharged via a conduit 19. Some of these particles are entrained by the air flow and are removed therefrom by the separator 1 1 which directs them into the trough 18. Thus, the air rising within flue 10 after lowering the temperature of the water has a humidity not much different from that of the ambient atmosphere.

A variety of hydrophilic compositions are available for the plastic sheet material of the envelopes 1, 1', 1

of the wall members 12. Cellulose acetate, for example,

is suitable even though it has a rather low flow resistance. The sheets could also consist of a relativelyhydrophobic substrate, such as polyethyleneterephtha-= late, with a surface rendered hydrophilic by radiochemical grafting of, for example, acrylic acid as is well known per se. It is also possible to coat the inner surfi face of a hydrophobic substrate with a layer of a relatively cheap hydrophilic material such as, for example, a

cotton gauze The thickness of these sheets may be a fraction of a millimeter, e.g., approximately 0.2 mm. 7

The width of the passages 25 (as determined by the spacers 7) may be a multiple of the width of the airspace within each envelope, i.e., the distance separating the confronting sheet, surfaces; the ratio of the widths may be on the order of 5:1. In practice, these widths may be on the order of centimeters, e.g., 5 cm for the passage 25 and 1 cm for the wall member 12. The height of the latter member may be on the order of meters, e.g., up to about 10 m, with a flue 111 ranging in height between approximately and m. Such I a heat exchanger may have a cooling capacity of about Q(Kcallhr) rate of heat dissipation ll ll Re(m=/hr) volumetric output (i.e. size of sheet in in times kinematic viscosity in m/hr) AT(C) temperature difference between fluid inlet and outlet At,,.(C) mean logarithmic temperature difference between the two fluids K(watts/m C) overall heat-transfer coefficient for airstream.

These numerical values, taken with the air passing in counterflow to the water and with the two flows shielded from the ambient atmosphere, are listed in the table below:

channel on said support, the latter being provided with gaps leading from said channel to said airspace and terminating adjacent the inner surfaces of said sheets for distributing said first fluid substantially uniformly along upper edge portions of said sheets and letting said first fluid descend in a continuous film along said inner surfaces;

collector means for said first fluid at the bottom of said envelope; and

circulation means for driving said second fluid through passages between said wall members in contact with the outer surfaces of their sheets for thermal interaction with the films on the inner surfaces thereof.

2. A heat exchanger as defined in claim 1 wherein said envelope is sealed against the atmosphere at its top and sides, said collector means comprising a basin filled with said first fluid to a level above the bottom of said envelope, said wall members being filled with a gas Test Q re AT AT K a (Kcal/hr) m hr (C) ("c") (watts/m (watts/m water air water air water air C) C)air It will be noted that the measured values for the overall heat-transfer coefficient K are very close to the theoretical values a for the transfer on the air side. This indicates that the heat exchange between the fluids is essentially determined by the heat transfer between the plastic sheet and the air. The value of K in the neighborhood of 18 watts/m C corresponds to the requirements of the contemplated use in a cooling tower.

Aside from the possible effect of the irrigation of the airstream in zone Z the heat exchanger according to our invention operates without any increase in the humidity of the ambient air. The wall members 12 can be manufactured at a cost substantially lower-than that of finned tubes of like capacity as conventionally used in dry cooling towers. The presence of the tensioned

grids

4a, 4b allows the spacing of the sheets to be made substantially uniform throughout each wall member,

with only a slight undulation of the sheet profiles,

owing to the gas pressure maintained in these envelopes. This gas pressure is constant for all points of the envelope and independent of the air flow through passages 25.

We claim:

1. A heat exchanger for establishing thermal contact between a first and a second fluid of different temperatures, at least said first fluid being a liquid, comprising:

a plurality of spacedly juxtaposed hollow wall members each including a generally horizontal elevated support and an envelope of plastic sheet material suspended from said support, said envelope comprising a pair of confronting sheets separated by an airspace;

conduit means for said first fluid terminating in a under pressure maintaining their envelopes in an inflated state.

3. A heat exchanger as defined in claim 2 wherein said wall members are further provided with a reinforcing grid along the outer surfaces of said sheets.

4. A heat exchanger as defined in claim 3 wherein said grid consists of intersecting flexible elements under tension.

5. A heat exchanger as defined in claim 3, further comprising spacing means between adjacent wall members bearing upon the grids thereof.

6. A heat exchanger as defined in claim 1 wherein said wall members form an annular array and extend substantially radially with reference to a vertical axis.

7. A heatexchanger as defined in claim 6 wherein said second fluid is air, further comprising a flue centered on said vertical axis, said flue opening at its base onto the passages between said wall members.

8. A heat exchanger as defined in claim 1 wherein said first fluid is water, said channel being provided with capillary means at least partly blocking said gaps for helping distribute the water over said upper edge portions.

9. A heat exchanger as defined in claim 1 wherein said sheets have a thickness of a fraction of a millimeter, said wall members and said passages have widths on the order of centimeters, and said wall members have a height on the order of meters.

10. A heat exchanger as defined in claim 9 wherein the width of said passages is several times that of said wall members.

Claims

1. A heat exchanger for establishing thermal contact between a first and a second fluid of different temperatures, at least said first fluid being a liquid, comprising: a plurality of spacedly juxtaposed hollow wall members each including a generally horizontal elevated support and an envelope of plastic sheet material suspended from said support, said envelope comprising a pair of confronting sheets separated by an airspace; conduit means for said first fluid terminating in a channel on said support, the latter being provided with gaps leading from said channel to said airspace and terminating adjacent the inner surfaces of said sheets for distributing said first fluid substantially uniformly along upper edge portions of said sheets and letting said first fluid descend in a continuous film along said inner surfaces; collector means for said first fluid at the bottom of said envelope; and circulation means for driving said second fluid through passages between said wall members in contact with the outer surfaces of their sheets for thermal interaction with the films on the inner surfaces thereof.

2. A heat exchanger as defined in claim 1 wherein said envelope is sealed against the atmosphere at its top and sides, said collector means comprising a basin filled with said first fluid to a level above the bottom of said envelope, said wall members being filled with a gas under pressure maintaining their envelopes in an inflated state.

7. A heat exchanger as defined in claim 6 wherein said second fluid is air, further comprising a flue centered on said vertical axis, said flue opening at its base onto the passages between said wall members.