WO2007111558A1

WO2007111558A1 - Heat transfer elements using thin liquid films

Info

Publication number: WO2007111558A1
Application number: PCT/SE2007/000290
Authority: WO
Inventors: Sten Olof Zeilon
Original assignee: Sten Olof Zeilon
Priority date: 2006-03-28
Filing date: 2007-03-26
Publication date: 2007-10-04
Also published as: SE0600693L; SE529736C2; EP2032930A1

Abstract

A device for heat exchange between a liquid (1) and the surroundings comprise elements (8), coupled in parallel and in series, with the liquid driven by gravity as a thin liquid film (11) between a stiffened, curved metal sheet (2) and an elastic film (3) tightly stretched over the convex surface of the metal sheet.

Description

Heat transfer elements using thin liquid films.

The present invention concerns a device for heat exchange between a liquid and the surroundings with the liquid arranged in form of a thin liquid film driven by gravity between two layers.

The invention concerns devices for liquid born distribution of heat or cold via radiation and heat transfer, such as radiators and convectors for room heating or cooling, for sun heat absorbers, for heat exchange liquid/air etc.

The invention is functionally an essential development of a method for heat exchange between liquid and air described in my Swedish patent 8008235- 7. According to the method a liquid is driven by gravity as thin liquid films between two membranes of plastic film kept together by adhesion forces in the liquid. Air is driven along the dry surface of the plastic film and thus heat exchanged against the liquid. The method has not found any practical use due to mechanical instability in the membranes in reference to the air flow, to leakage problems in connection with membrane support and liquid feed, to membrane support blocking free vertical air flow and to difficulties to achieve uniform liquid flow along the width of the membranes.

The object of the invention is an utterly thin, material saving heat transfer element using a liquid film between two layers, that is necessarily mechanical stable, that is easy to fabricate, mount and dismount, that provides the means for a very uniform distribution of the liquid film over the element surface and that in any number may easily be coupled in parallel and series.

Said object is met by a device further characterized in that comprises elements with a first layer in form of a curved, thin metal sheet, provided by curved stiffening members and stiffened along side edges by folded flanges, further comprises the other layer in form of a sheet of a thin elastic film fastened along the flanges and by tightening means stretched over the convex surface of the first layer and further comprises means for element overlapping along upper and lower edges and means for even liquid distribution along the width of the upper edge.

The invention is described in the following in reference to the figures. Figure 1 shows an element in perspective. Figure 2 shows in vertical section overlapping between two elements. Figure 3 shows a flange detail. Figure 4 shows a plan section of mounted elements. Figure 5 illustrates in perspective solar heat absorption and figure 6 in section and perspective heat exchange air/liquid. Figure 7 shows a device for cooling or heating premises and figure 8 in plan an arrangement of elements.

In reference to figure 1 the mechanically stiff first layer of an element 8 comprises a thin metal sheet 2. The sheet is stiffened by curving, by curved stiffening members 21 and 22 such as curved metal bars laminated to the sheet 2 and by folded flanges 23 along the side edges. A sheet 3 of a thin elastic film is stretched as a second layer over the convex surface of the first layer and fastened along the flanges 23. In order to achieve an easy assemblage and a very tight connection between the two layers 2 and 3, crucial to a proper function of the element 8, the sheet 3 is laminated along each side to sheet metal strips 24 along its sides cut into clips 241 and 242. The strips 24 are fastened to the flanges 23 by bending clips 241 and protruding about 4 mm over the folding line 25 of the flanges. With the element 8 being squeezed in a rigid framework 7 in reference to figures 3 and 4 the sheet 3 will very efficiently be tightened by a lever action of the strip 24 in respect to the folding line 25. The element is fixed in the framework by clips 242 and staples.

In order to facilitate overlapping between elements 8 along upper and lower edges a pocket 26 about 2.5 mm deep may be formed along the upper edge by an impressed recess of the sheet 2 being laminated to a mentioned curved metal bar 21 sized about 2 x20 mm and a back plate 27. By folding the lower corners of the elements with folds 81 one element 8'may penetrate in the pocket 26 of another 8" in reference to figure 2 and the elements thus be coupled in series for a by gravity driven liquid flow 1. Overlapping may further be facilitated by the upper edges of the sheet plates 2 and 27 being cut in an oblique angel. hi many applications of the invention the elements 8 are fixed in inclined position. Liquid 1 is fed into the curved, inclining pocket 26 and is by means of mentioned metal bar 21 and back plate 27 led to the sides of the pocket feeding a liquid film 11 as follows.

Under influence by capillary and gravity forces the liquid will flow as a thin liquid film 11 between the tightly adjoining two layers 2 and 3. The above described method of tightening the elastic film 3 over the convex surface of the metal sheet 2 ensures a very uniform liquid flow across the width of the element, crucial to a proper function of heat transfer. The flow 11 will flow in wave fronts. While the metal sheet is stiff the elasticity of the film 3 will allow the forming of these wave fronts.

Mass flow in the liquid film may vary within a wide range and in practice in order 0.4 - 1.01 / min, element. Water film speed is of order 0.1 m /s.

In reference to figure 3 capillary action is broken along the folding lines 25, what hinders liquid seeping out by the flanges 23. The element may be exemplified in a very handy size using for the metal sheet 2 a standard size 1030x760x0.28 mm of reused aluminium offset printing sheet. For an aggressive liquid 1 the metal sheet 2 may be coated in an appropriate way or stainless steel used as sheet material. As a very form, temperature and corrosion stable film 3 may be used a polyester or PEN film of thickness around 100 micron. The mean thickness of a water film 11 amounts to about 0.12 mm and the total thickness of the heat transferring element 8 thus to only 0.5 mm.

For mentioned lamination operations may be used a 0.1 mm adhesive transfer tape such as 3M 9473 VHB stable up to + 140°.

Heat transfer applications with the elements 8, coupled in any groups in parallel or in series, comprise the liquid 1 being drained from bottom elements to a vessel 51 and pumped by a pump 52 to top elements via heat transfer contact to a cooler or heater 5. Such applications are exemplified below, demonstrating the versatility of the invention.

Sun heat absorber.

In reference to figure 5 are radiation absorbing elements 8 with coated, concave metal surface 2 turned upwards mounted in a framework 7 in parallel rows and in each row overlapping in series. The framework 7 is inclined and south oriented and designed as a ladder to accommodate simple mounting of the elements from top and downwards. Glass or transparent plastic will cover the elements. When direct sun strikes the absorber water 1 is circulated as follows. Water heated for instance to +70° in the elements 8 is drained to a vessel 51 and pumped by a pump 52 in a heat exchanging loop 53 running from top to bottom in an accumulator tank 5 stratified in an upper hot water zone 56 (+70°) and lower cool water zone 55 (+20°). The water flow , expanding the hot water zone 56 downwards and cooled in the process to about +20°, is further fed to the top elements 8. Heating a flow of tap water includes a heat exchanging loop 54 running from bottom to top cooling the zones 55,56 with tap water being heated up to +70°.

As soon as sun is shaded, water circulation is stopped and the elements drained of water to vessel 51.

The absorber is cheap to build and yet very effective. It responds quickly to small bursts of sun due to a very low heat mass in the elements and the water circulation, it has a substantial absorber efficiency due to a low mean surface temperature , about +45°, and it has no problems with thermal expansion in the elements, water leakage in joints or freezing. A row of for instance 6 elements, coupled in series with an absorber surface of 6 xlx 0.75 = 4.5 sq.m will absorb around 2 200 Kcal/h, heating a flow 1 of 0.73 1/min from +20° to +70°. This small flow is evenly spread over the element surface in way described above.

For coating the sheets 2 may for upper, cooler elements be used a cheap, simple blackening and for lower hot elements any available absorbtion efficient and expensive coating.

Countercurrent heat exchange liquid / air. In reference to figure 6 are parallel rows of elements 8, each row comprising a number of overlapping elements in series, confined within an elongated air channel 7, acting as mentioned framework for the elements. A liquid 1 is circulated as follows. Liquid is pumped by a pump 52 from a vessel 51 through a heater or cooler 5 and evenly distributed by small part flows 1 'to the top elements 8, further flowing as described liquid films 11 along the rows of elements and drained back from bottom elements to the vessel 51. Countercurrent to the liquid flow is driven an air flow 9 in the spaces between the element rows. Heat transfer liquid film /air occurs directly through the thin membranes 2 and 3 of the elements. With as example an element width = 0.75 m , element row spacing= 20 mm and air velocity = 3 m/s mass flow of air per element row amounts to 2.7 cubm/min, in heat capacity corresponding to a mass flow of water per element row = 0.9 1/min. Reducing spacing to 10 mm will correspondingly reduce water flow = 0.451/min. Both these small mass flows of water are very evenly spread over the element surfaces.

The liquid 1 may evenly be distributed to the rows of elements in said part flows 1 'using small, flexible plastic tubes of equal length and pressure drop and squeezed into mentioned pockets 26 of top elements 8. With liquid flow stopped the heat exchanger is drained of liquid to the vessel 51 and cannot freeze.

Elements 8 may be combined in any numbers in parallel and series and spacing to meet any demands in efficient heat transfer between air and liquid, especially where heating/cooling involves a very wide temperature range and where one in the media flow direction highly elongated transfer surface is needed. To obtain a desired efficiency in any heat transfer operation requires a corresponding number of elements 8, which may be distributed either in shorter element rows and narrow spacing or longer element rows and wider spacing, whatever is practical in the case.

Heating or cooling of premises. hi reference to figure 7 any number of elements 8 or groups of elements are distributed in premises and coupled in parallel to flows of heating or cooling water 1. The water is drained from the elements via bottom channels 6 to a common vessel 51 and pumped back to the elements via a pump 52 and a heater or cooler 5. Condensate forming on the element surface in case of cooling is collected in the channel 6 and mixed into the flow of water 1. Heat transfer is a combination of radiation and air convection.

Figure 8 illustrates an installation of ceiling high elements in room corners.

For heating or cooling of premises the device has many advantages. It is a very low cost installation. Slender plastic tubing may be used for liquid distribution. Besides a small pressure needed for the liquid pumping the system is void of static pressures and offers few leakage problems. Element surface may easily be enlarged to accommodate low tempered heating systems. Due to low heat mass in elements and circulating liquid 1 it reacts quickly to changing heat demands, for instance for intermittently heated premises.

Claims

1. A device for heat exchange between a liquid (1) and the surroundings with the liquid arranged in form of a thin liquid film (11) driven by gravity between two layers (2,3) characterized in that it comprises elements (8) with a first layer (2) in form of a curved, thin metal sheet, provided with curved stiffening members (21,22) and stiffened along side edges by folded flanges (23), further comprises the second layer (3) in form of a sheet of a thin elastic film fastened along the flanges and by tightening means (24) stretched over the convex surface of the first layer and further comprises means (81,26) for element overlapping along the upper and lower edges and means ( 21,27) for even liquid distribution along the width of the upper edge.

2. A device according to claim 1 characterized in that said tightening means comprise sheet metal strips (24) laminated to the elastic film (3), the strips fastened to said flanges (23) protruding about 5 mm over the folding line (25) of the flanges and the film (3) stretched by a lever action of the flanges in respect to the folding line with the elements 8 being squeezed along the flanges into a rigid framework (7).

3. A device according to claims 1 and 2 characterized in that said means for element overlapping comprises a pocket (26) along the upper edge of the metal sheet (2) and the element (8) having bottom corners folded with folds (81).

4. A device according to claims 1-3 characterized in that said means for liquid distribution comprises one said stiffening member (21) and a back plate (27) extended towards the ends of said pocket (26).

5. A device according to claims 1 and 2 characterized in that in any group of said elements (8) the liquid (1) is drained from the bottom elements to a vessel (51) and further pumped via a pump (52) and a cooler or heater (5) back to the top elements.

6. A device for sun heat absorption according to claims 1-5 characterized in that it comprises said elements (8), coupled in parallel and in series, mounted with coated, concave metal surfaces turned upwards in an inclined, south oriented framework (7) and comprises cool water (1) fed to the top elements cooled in a cool bottom zone (55) of an accumulator tank (5), heated water drained from bottom elements to said vessel (51) and pumped to heat transfer (53) with a hot water zone (56) of the tank.

7. A device for countercurrent heat exchange between air/liquid according to claims 1 and 5 characterized in that it comprises parallel rows of elements (8) mounted in an air channel (7) for an upwards driven air flow (9).

8. A device for heating or cooling premises according to claims 1 and 5 characterized in that it comprises elements (8) drained to bottom channels (6) and further to said vessel (51).