EP0680594B1

EP0680594B1 - Heat exchanger device and method of transferring heat

Info

Publication number: EP0680594B1
Application number: EP94905894A
Authority: EP
Inventors: Klaus Lorenz; Lars Broman
Original assignee: SOLVIS Solarsysteme GmbH
Current assignee: SOLVIS Solarsysteme GmbH
Priority date: 1993-01-23
Filing date: 1994-01-24
Publication date: 2000-09-27
Anticipated expiration: 2014-01-24
Also published as: SE9300209D0; DE69426016T2; ATE196686T1; AU5981994A; EP0680594A1; DE69426016D1; WO1994017355A1; SE9300209L

Abstract

A problem in connection with fluid operated solar collector systems is in a simple and non-expensive way to be able to separate the circulatory system of the solar collector from a fluid based energy storage system without any considerable part of the supplied energy being lost. This problem is solved by a heat exchanger (2, 2a, 2b) according to the invention which is provided between the systems. This includes at least two concentric layers (12-15) of helically wound tubes (12a-15e) in a space that is surrounded by mainly cylindric surfaces (10-11). A method according to the invention for transfer of heat between a first fluid that circulates through a tube system (2, 2a, 2b) and a second fluid that surrounds the tube system consists in that the first fluid is pumped through the tube system containing capillary tubes so that the pressure drop across the tube system is at least 100 times larger than the pressure drop across the tube system in the second fluid which is self-circulating.

Description

The present invention refers to heat exchangers and in particular to heat exchangers in low temperature systems where heat is to be transferred from a fluid in a circulatory system to a fluid in another system. An example of use of such systems is between a solar collector circuit and an accumulator tank.
Heat exchangers are known in the art, for example of US 4,619,317, US 3,556,199, US 3,448,792, SU 958 830. US 4,619,317 relates to a heat exchanger comprising a support frame and a feed line, a heater for the forward flow, heat exchange tubes, a heater for the return flow and a discharge line for a heat transfer medium. The heat exchange tubes are capillary tubes which are fixed interlocking one another by means of connecting elements, and the ends of the capillary tubes lead into the forward flow heater and the return flow heater of the heat transfer medium. The heat exchange capillaries particularly have an external diameter of from 0,1 to 10 mm and wall thicknesses of from 40 % to 5 % of the external diameter of the heat exchange capillaries. The clear spacing between the capillary windings are 1 to 5 times the external diameter of the heat exchange capillaries. Large and small heat exchanger packs or blocks of any desired capacity can be produced. The individual helices of the heat exchange capillaries always have the same pressure drop being provided with the same internal diameter and having the same length. In order to protect the helices of the heat exchange capillaries during the free or forced flow of the heat exchange medium around them from local deflections, flexural vibrations or the like due to the turbulence which necessarily occurs the heat exchange capillaries are arranged with a mutual interlock and connected to one another by connecting elements. These are for example straight wires or even capillaries mounted between the support frames.
The US 3,556,199 relates to a free convection cooling method and apparatus. To lower the temperature of a circulating heated transport fluid a heat exchanger submerged in a coolant bath being constructed and arranged to obtain the greatest possible temperature-density differential between the circulating transport fluid and environmental coolant productive of strong natural convection currents. In the operation of the system built up by a plurality of elongated parallel plates held in a separated relation at their ends by closure inserts and longitudinally extending insert spacers, heated fluid from a source is pumped to an inlet manifold. The actual heat source is formed by an engine or electronic package or the like with respect to which the heat transport fluid is passed in heat absorbing relation. The liquid of the bath the heat exchanger being provided in has a temperature lower than that at which transport fluid is delivered to heat exchanger. The temperature differential is utilised to cool or to remove excess heat from the transport fluid. Nowhere in the heat exchanger are conditions created in which transport fluid of maximum temperature encounters coolant of minimum temperature. Hot spots or isolated locations of maximum temperature differential which may produce localised heavy convection currents with little or no convection flow in other portions of the heat exchanger, are avoided.
The US 3,448,792 relates to a formal convection condenser and a method of use. A rapid heat transfer from a heat exchange fluid to a cooler fluid is effected by deposition of a condenser in the cooler fluid. The heat condenser comprises at least two coils, a smaller coil being positioned within a larger coil. The coils are held in a spaced relationship to each other and an inner and an outer shielding by spacers. The spacers are providing a vertical gap between the coils and shield means. The condenser apparatus is positioned in a liquid container or tank containing a liquid such as water. The apparatus is preferably positioned near the bottom of the container. By the open end shrouded design of the heat exchanger a chimney effect is provided thereby excellerating the flow of water across the heat exchange coils and thereby more rapidly extracting the heat therefrom as described. Preferably the heat exchange fluid is superheated steam of a temperature of about 212 to about 900 degrees Fahrenheit or more.
The superheated steam is gained for example by passing water at a temperature of about 70 degrees Fahrenheit through a heat storage composition maintained at a temperature of about 900 degrees Fahrenheit. Afterwards the superheated steam is passed into the condenser being submerged in a tank of water. The water having an original temperature of about 70 degrees Fahrenheit afterwards is rapidly heated at a temperature of about 140 degrees Fahrenheit. Thereby strong convection currents are generated by the heat differential. This causes a rapid circulation of water throughout the tank of water being heated.
A problem in connection with fluid operated solar collector systems is in a simple and non-expensive way to be able to separate the circulatory system of the solar collector from a fluid operated energy storage system without any considerable part of the supplied energy being lost. This problem is solved by a heat exchanger according to the invention as claimed, the heat exchanger being provided between the systems.
The heat exchanger structure according to the invention is particularly intended to optimize the heat transfer for use in which

the fluid is pumped on the primary side, i.e. that a relatively large pressure drop is accepted and is utilized for a good heat transfer,
the fluid on the secondary side flows through the heat exchanger by means of self-circulation, i.e. that the structure is optimized in order to obtain a large heat transfer coefficient at the outside despite an extremely low pressure drop,
a relatively high temperature difference with these low flows on both sides of the heat exchanger is desired.

The invention will now be described in closer detail with reference to the drawing, on which

Figure 1: diagrammatically illustrates an energy storage tank with two alternative locations of a heat exchanger according to the invention,
Figure 2: diagrammatically illustrates production of a capillary tube intended to be included in a heat exchanger according to the invention, and
Figures 3 and 4: diagrammatically illustrate the structural design of the heat exchanger.

Figure 1 shows a typical mode of application for the heat exchanger according to the invention in the solar heating technique. The contents of an accumulator tank 1 is heated by heat exchange from the solar collector fluid. In order to minimize the costs of installation, pump and control equipment, the heat exchanger should either be in the tank (2a) built in or connected for self-circulation outside the tank (2b). At the alternative with built in heat exchanger 2a the solar collector fluid is supplied at 3a and is returned at 4a while self- circulation 5a, 6a occurs in the tank 1 through the heat exchanger 2a. At the other alternative the solar collector fluid circulates the path 3b, 2b, 4b while the fluid in the tank 1 is supplied to the heat exchanger 2b at 5b and is passed through tubes 7 back to the tank at 6b.
In contrast to existing systems the heat exchanger according to the invention performs a temperature raise of the cold water at the bottom of the tank from for example 30 °C up to 60 to 70 °C at through flow. This is performed at a logarithmic temperature difference between primary and secondary side of only about 5 degrees. Such a heat exchanger promotes the stratification of the tank, enables low values of flows in the solar collector circuit and totally leads to a system with higher performance. This can be accomplished by lower material consumption in the form of thinner tubes in the solar circuit and much less material in the heat exchanger which makes the system much more cost-effective.
The heat exchanger is constructed from capillary tubes connected in parallel. By using a number of slender tubes with thin walls with an inner diameter of less than 3 mm and an outer diameter of less than 5 mm a substantially laminar flow in the tubes is obtained, whereby the the heat transfer between the fluid that is pumped through the tubes and the surrounding fluid is considerably improved with respect to tubes of thicker dimensions. The heat exchanger will be particularly well dimensioned at an inner diameter of 1 - 2 mm, preferably 1.5 mm and a wall thickness less than 0.5 mm, preferably about 0.25 mm. The length of each tube should be one metre or more and the number of tubes depends on the power that is to be transmitted. The sizes are based on tubes of copper and pumping of water with antifreeze in the primary circuit. The capillary tubes can be arranged in various ways in order to obtain a good heat exchange, whereby in itself prior known arrangements may be used.
Preferably the capillary tubes 8 on the outside are provided with a sparsely wound on wire 9, see figure 2. The wire 9, which preferably is constituted by a smooth wire of copper, in the completed heat exchanger serves the function that it partly creates a defined distance between concentric helices of capillary tubes provided with the wire, and partly enhances the heat transfer on the outside by using flange effect and eddy forming of the wire.
The capillary tubes 8 can also be wound by a wire of other metal or by a wire consisting of several entwined thinner wires. Normally the wound on wire has only to be fixed to the capillary tube at the ends thereof but it is also possible to fix the wire at evenly spaced intervals or along the entire length thereof. Suitable means for this may be soldering, bonding, immersing in liquid tin or the like. As the heat exchanger is constructed from helically wound capillary tubes all or only a part of the capillary tubes can be wound with wire. It is also possible to use capillary tubes without a wound on wire for a heat exchanger according to the invention, whereby other means may be arranged to keep appropriate distance between the tube helices.
In order to produce a heat exchanger according to the invention a number of capillary tubes are first cut to mainly equal length. All capillary tubes will be connected in parallel and they have to be approximately of equal length in order to obtain the same pressure drop and by this the same temperature drop at flowing through at the inside.
The capillary tubes then are formed to helices with various diameters in such a way that the pitch angle for each helix is alike. Thereby is achieved that the various helices obtain mainly the same length. For the various diameters a different number of capillaries is provided in respective helix in such a way that the number capillaries is generally proportional to the diameter.
In the embodiment according to Figure 3 and 4 the helix 12 with the smallest diameter contains two capillary tubes 12a and 12b wound in helix with the diameter 20 mm whereby the number of turns per tube becomes about 40 and the length of the helix about 400 mm. The next helix 13 with the diameter 30 mm contains three capillary tubes (13a-13c) forming about 27 turns each in order to attain the same length. The third turn 14 with 40 mm diameter contains four capillary tubes (14a-14d), which each attain about 20 turns and the fourth helix 15 with 40 mm diameter contains five capillary tubes (15a-15e), which attains about 16 turns for same length. If the ring shaped space between the central cylinder shaped body 11 and the outer tube 10, which is filled up by the helices 12-15 has the diameters 15 and 55 mm the 14 capillary tubes in the example will fill up this space equally and the heat transfer from the primary flow to the secondary becomes equal over the entire tube slot. The external diameter of the capillary tube was 2,8 mm and the wire 1 mm.
The surrounding tube 10 extends longer than the very heat exchanger part 2 with the capillary tubes to improve the self circulation. The tube 10 shall also be able to accommodate couplings (not shown) between the capillary tubes and the external solar collector circuit, which can be realized in an arbitrary way which allows mainly uniform tube lengths and minor obstruction in the self circulatory circuit.
Coupling to the ends of the capillary tubes can preferably be performed by following method: All tubes are brought together into a cover, after which the cover is filled with solder. Thereafter the cover is cut off so that all tube openings appear in the section surface, which then simply can be coupled to inflow and drain respectively. This procedure has proved to be a cost-effective method to join the tubes, without which a heat exchanger with many capillary tubes according to the invention could not have been produced without problems.
The tubes 12a-15e contained in the helices 12-15 are shown in Figure 4 for the sake of simplicity as circles to symbolize the approximate distribution of the tubes in an imagined cross section somewhere in the heat exchanger part 2 in Figure 3. By a correct sectional figure each cutaway tube end should obtain an elongated curved elliptical shape along the helix.
As previously mentioned all capillary tubes in the heat exchanger shall have essentially the same length. Since the pressure drop increases somewhat with decreasing diameter of the tube helix this can be compensated by giving the tubes somewhat smaller length with decreasing helix diameter.
In order to allow self circulation in the secondary circuit the heat exchanger 2 needs also to be dimensioned such, that the secondary fluid flow is not obstructed too much by the package of helically wound capillary tubes. This is achieved thereby that the fluid volume surrounding the capillary tubes in the heat exchanger part 2 relates to the fluid volume inside the capillary tubes as at least 2:1 and preferably more than 5:1. There is of course also an upper limit above which the heat transfer is deteriorated.
The flow direction of the fluid in the self circulatory circuit is in the main perpendicular to the capillary tubes, whereby the specific heat transfer capability is enhanced. In the heat exchanger a combination of heat transfer from a tube in undisturbed fluid and superponated active flow is utilized and is determined by the density difference between cold and hot water at inlet and outlet respectively secondary side of the heat exchanger as well as the total height of the tube 10.
In an appropriately dimensioned heat exchanger according to the invention the pressure drop on the self-circulatory side can be within the interval 30 - 100 Pa. The internal pressure loss by pumping should be at least about 100 times larger, preferably about 1000 times larger or more, and can be within the interval of about 10 - 100 kPa.

Claims

Heat exchanger device for transfer of heat between a first fluid that circulates through a tube system to a second fluid that surrounds the tube system, wherein the tube system (2) includes at least two concentric layers (12-15) of helically wound tubes (12a-15e) in a space that is surrounded by mainly cylindric surfaces (10-11),
wherein

the tubes (12a-15e) are of a capillary type with small internal diameter of less than 3 mm,

the first fluid is intended to be pumped (3a-4a, 3b-4b) through the tube system (2,2a,2b) and

the space is intended to be arranged in or in connection to a tank (1) so that the second fluid can flow (5a-6a, 5b-6b) through the heat exchanger device by self-circulation.
Device according to claim 1,
characterized in
that the cylindrical surface (10), which surrounds the tube system (2, 2a, 2b) is extended to improve the self-circulation.
Device according to claim 1 or 2,
characterized in
that each layer of helically wound tubes (12-15) contains at least two tubes (12a-15e), that all tubes (12a-15e) are of essentially the same length and that the number of tubes in respective layer in the main is proportional to the diameter of the helix.
Device according to any of claims 1-3,
characterized in
that the first fluid is pumped through the tube system containing capillary tubes so that the pressure drop across the tube system amounts to about 10-100 kPa and that the other fluid is self-circulating so that the pressure drop in this will be in the order of at the highest 100 Pa.
Device accoding to any of claims 1 - 4,
characterized in
that the outer diameter of the capillary tubes (12a-15e) is less than 5 mm.
Device according to any of claims 1 - 5,
characterized in
that the fluid volume in the space that surrounds the tube system (2, 2a, 2b) is at least the double and preferably five times as large as the total fluid volume inside the tubes.
Device according to any of claims 1 - 6,
characterized in
that the tube system (2, 2a, 2b) is joined together by all capillary tubes at one end of the tube system being brought together in a cover, that the cover is filled by solder, that the cover is cut off so that all tube openings appear at the cut surface, which then simply can be connected to inflow and drain respectively.
Device according to any of claims 1-7,
characterized in
that in each layer of helically wound tubes (12-15) at least one consists of a tube that is surrounded by a flange and that the flange consists of a wire (9) wound about the tube (8).
Method of transferring heat between a first fluid circulating through a tube system and a second fluid that surrounds the tube system,
wherein

the first fluid is pumped through a capillary tube system with small internal diameter of less than 3 mm, with a pressure drop across the tube system of at least 100 times larger than the pressure drop across the tube system in the second fluid, which is self-circulating.
Method according to claim 9,
characterized in
that the pressure drop across the tube system in the first fluid amounts to 10-100 kPa.