WO1988004017A1

WO1988004017A1 - Procedure for drawing heat from freezing liquid

Info

Publication number: WO1988004017A1
Application number: PCT/FI1986/000138
Authority: WO
Inventors: Risto Saari
Original assignee: Inventio Oy
Priority date: 1986-11-27
Filing date: 1986-11-27
Publication date: 1988-06-02

Abstract

Procedure for drawing thermal energy from freezing liquid, e.g. from lake or sea water, by evaporating said liquid in a crystallizer (10) where prevails the pressure of the triple point, whereby part of the liquid is converted into vapour and part of it freezes to crystals. The vapour is condensed in the evaporator (13) of a heat pump (15) in heat exchange with the heat transfer fluid, e.g. Freon, of the heat pump after the pressure and temperature of said vapour have been increased to be above the freezing point, this being done by pumping it to said higher pressure with a vapour jet ejector (12) in which as operating vapour serves vapour that has been generated in a supercooler (14) provided for the heat transfer fluid of the heat pump and which is high-pressure vapour compared with the pressure of the triple point, and in this way the vapour can be made to condense to liquid and freezing over of the heat exchange surfaces of the heat pump's evaporator is avoided. The invention is also applicable in crystallizing and concentrating other substances, and in desalinating sea water.

Description

Procedure for drawing heat from freezing liquid

Although the interest displayed towards heat pumps has increased in recent time, awkward restrictions are imposed on the use of heat pumps in the Nordic climate by nature, exactly during the coldest season when heat would be most needed. Conventional heat pumps utilizing water bodies for heat source are then only able to operate with partial capacity, as otherwise the heat exchangers might freeze.

In the art also certain heat pump designs are known which allow the water to freeze. The simplest problem solution of this kind is that in which the water freezes to form a solid crust on the surface of the heat pump's evaporator. When the ice crust has grown to sufficient thickness, warm heat transfer fluid is conducted into the evaporator, whereby the ice crust is detached. The ice is detached by heating different parts of the evaporator in alternation, implying that at any time part of the evaporator's heat surface will be passive as regards operation of the heat pump. Moreover, the heat transfer is substantially impaired by the forming of the ice crust.

Another problem solution has also been presented, which consists of evaporation of the liquid serving as heat source and of its simultaneous crystallizing in a tank under low pressure, or a crystallizer. If the heat source is water, the pressure in the crystallizer is about 611 Pa and the temperature is 0°C, which corresponds to the triple point of water, that is the point where liquid, vapour and ice are in equilibrium with each other. As ice crystals are formed, the released latent heat goes to the vapour, which is condensed to form an ice layer on the surface of the heat pump's evaporator. This must be removed from time to time, either by conducting at predetermined intervals warm vapour into the joint vapour space of the crystallizer and the frozen-over evaporator or by subdividing the evaporator into a plurality of sectors, into each of which in turn is conducted warm vapour to melt the ice. As a consequence of formation of said ice layer, the area of the evaporator's heat surface in these heat pump designs is necessarily a multiple of that which is required if no ice forms an the heat exchange surfaces and the vapour condenses to liquid instead.

The salient characteristic feature of the invention now under discussion is that freezing of vapour onto the heat exchange surfaces is prevented by increasing the pressure of the vapour generated in the crystallizer with the aid of a vapour jet, before it flows to the evaporator of the heat pump. Hereby the condensation temperature of the vapour rises slightly above the freezing point and the vapour condenses to liquid, which runs off the heat exchange surfaces. In this way the drawbacks, mentioned above, of procedures of prior art can be avoided. Good heat transfer is maintained all the time, as a result of which the heat exchanger can be constructed with relatively smaller size. Furthermore, the whole heat exchanger is kept active, and periodic regulating measures implied by ice thawing are unnecessary and can be left out.

Certain embodiments of the invention are described in greater detail with the aid of the drawings attached hereto. Fig. 1 is a flow chart illustrating the application of the present procedure as a heat pump system. Fig. 2 is likewise a flow chart, and this chart describes the compression of the vapour in a plurality of stages and the removal of unccndensable gases from the system. Fig. 3 shows a way in which the heat pump can be operated on partial load, and the application thereof e.g. in sea water desalination. Fig. 4 depicts, in vertical section, one structural design of the crystallizer and Fig. 5, likewise in vertical section, a way in which the crystallizer, the vapour jet ejector and the evaporator can be combined to constitute a functional entity.

in the embodiment of Fig. 1, the compressor 15, condenser 16 and evaporator 13, plus the throttling valve 23, constitute a conven tional heat pump using e.g. Freαn for its heat transfer fluid. The condenser 16 gives off the heat produced by the heat pump, to an external fluid flowing in the pipeline 31/32, e.g. to water circulating in an area heating network. In addition to these conventional components, there has been associated with the flow circuit of the heat transfer fluid, a super-cooler 14, in which the condensed heat transfer fluid is cooled before it flows through the valve into the evaporator.

The liquid serving as heat source, e.g. sea water, releases heat in the crystallizer 10, in which prevails a pressure equivalent to or lower than the triple point. While part of the liquid evaporates, part of it freezes at the same time, releasing its latent heat to the vapour that is being produced. For instance, when water evaporates, at the same time the 7.5-fold quantity of ice is formed, compared with that of vapour. Ice crystal and water flew from the crystallizer through the pipe 18 to a crystal separator 11, where the cryastal rise to the surface and depart through a channel 34 from the process. The liquid, containing no crystals, flows through the pump 17 and pipe 19 back to the crystallizer. Ihe departing ice crystal and the generated vapour are replaced with additional liquid flowing to the process by the pipe 33. The vapour generated in thec crystallizer is at a temperature equal to the freezing point of the liquid or lower. If it were condensed as it is, it would freeze to ice. This can be prevented with the aid of the vapour jet ejector 12, which draws (20) the vapour from the crystallizer and delivers (22) it at higher pressure into the evaporator 13, where it condenses to liquid at a temperature higher than the freezing point. The operating vapour for the ejector is produced in the supercooler 14, whence it flows (21) to the ejector 12. The condensate departs (28) from the evaporator 13 with the aid of the pump 29, and further (24) out tram the process. Part of the condensate is supplied (30) to the supercooler (14), where it evaporates to became operating vapour for the ejector, in heat exchange with the heat transfer fluid that is being supercooled. If, for instance, the heat source is water and the heat transfer fluid is Freαn 12, the temperature of the generated vapour may be -1°C, the evaporation temperature of the Freαn, -3°C and the condensation temperature, 80°C. If the Freαn is supercooled in the supercooler 14 to 57°C, it beccmes possible to produce therein water vapour at 55°C. This vapour is able to raise in the ejector 12 the pressure of a four-fold vapour quantity so that it will condense in the evaporator 13 at +2°C.

If a larger heat pump is concerned, for instance a plant of a few MW, the heat exchanger of the evaporator tends to became so big that the flow losses of the condensing water vapour cool it again close to the freezing point. It is also noted that under such circumstances the removal from the heat exchanger of uncondensable gases, such as air, is exceedingly difficult or outright impossible to accαnplish. Ihe design of Fig. 2 presents a solution to this problem. Here, the evaporator 13 has been subdivided into two parts 13' and 13". In the evaporator 13' is condensed the greater part, e.g. 80%, of the vapour arriving (22) from the ejector 12. As it flows through the evaporator 13', the vapour may cool down close to the freezing point. Ihe pressure-boosting ejector 35 draws (38) the cooled residual vapour, in our exemplary case 20%, and transfers it under higher pressure and at higher temperature to the evaporator 13". When condensing there, the vapour now can afford to cool down, whereby the uncondensable gases can also be removed together with a reasonable vapour quantity with the aid of the air removal ejector 36, which transports (40) them to an air separator 37. The vapour jet ejectors 35 and 36 derive their operating vapour from the supercooler 14 in like manner as the ejector 12. They may equally have a vapour generator of their own, in heat exchange with the Freon. There may also be several pressure-boosting ejectors in succession, depending on the size of the installation and on the vapour temperature. Ihe air separator may be any kind of air separator operating at subatmospherlc pressure, known in the art, to which the pump 42 supplies the heat-releasing liquid and from which the pump 43, e.g. a water ring pump, removes the uncon densable gases which have been separated there.

The load of the heat pump may vary within wide limits, but on the other hand the vapour jet ejector is rather inflexible as regards load variations. In Fig. 3 is shown a problem solution teaching hew the ejector may operate at full load while the load of the heat pump varies. If the area heating system 31/32 operates at less than full load, part of the heat transfer fluid may be directed (48,49) to bypass the condenser 16, directly to the supercooler 14. Bus unit, as well as the ejector 12, may theh operate at full capacity in these circumstances as well. However, the vapour flew coming from the ejector cannot be wholly condensed in the condenser 13; instead, part of the vapour is conducted (47) to the crystal melter 45 and only part of the vapour goes further (53) to the evaporator 13. When the installation operates on full load, the ice crystals separated in the crystal separator 11 depart (52,54) from the process. When it operates an partial load, part of the ice crystals is conducted (50,44) to the crystal melter 45, where they melt to became liquid and are removed with the pump 46. With this arrangement, the crystallizer and vapcur ejector, and the supercooler 14, operate on full load all the time even though the compressor 15 and condenser 16 operate on partial load.

In the embodiment just described, pure liquid is obtained from the vapour that is condensing in the crystal melter 45, and from the melting crystals. The invention may in fact be applied in this manner towards the producing of pure liquid, for instance to accomplish sea water desalination. In that case the condenser 16 is totally omitted and to the crystal melter 45 is connected a crystal washing step prior to their melting with condensate passing in countercurrent to the direction of flow of the crystals. The procedure of the invention may also be applied in this manner to accomplish crystallizing and concentration of other liquids, in which connection either crystals (51) or pure liquid (46) may be produced, the latter by melting said crystals with vapour. The crystallizing and vapourizing liquid has a great tendency to form solid ice in the crystallizer, and this would be likely to plug the flow passages and to step the process. This eventually occurs if a placid liquid surface develops, or if liquid and vapour meet en a hard, cold surface. On the other hand, efficient evaporation implies an extensive contact surface between liquid and vapour. Fig. 4 shows a way in which this can be achieved. The heat-releasing liquid flaws into the crystallizer (10) upwards from below, along flow passages 59, all of these passages terminating an the same level (57). Ihe liquid discharges into the crystallizer space over the top rims 57 of these passages and flows thence further downward in a film along the surface 58. In this way an extensive liquid surface is produced which keeps moving all the time. It is also essential that all surfaces below the liquid entrance height 57 are constantly flushed with liquid and that no dry spots can develep on which a solid ice crust would form. This is however not feasible in the upper part of the crystallizer's jacket 54, above the liquid inflow height 57; this jacket 54 must be slightly heated, at least in the vicinity of the liquid inflow height 57, to a temperature higher than the freezing point of the liquid, for instance with liquid flowing in passages 55 encircling said jacket. In the passages 56 remaining between the flowing liquid films 58, the vapour flows upward, and at their lower end a mixture of liquid and crystals flows downwards towards the crystal separator 11.

When the invention is applied to draw heat from freezing water, it is well known that the specific volume of the vapour is very large. In Fig. 5 is shown, in vertical section, a design by which the largest components of the installation can be placed so that construction of big vapour pipes is avoided. Ihe crystallizer 20 has been constructed around the top end of the vertical ejector 12 so that the mixing cone 61 of the ejector lies, partly at least, inside the crystallizer. The evaporator 13 has similarly been constructed around the lower end of the ejector 12 so that the diffusor cone of the ejector lies partly within the evaporator 13. The crystal separator 11 has been placed at barometric height below the lower margin of the crystallizer 10, and the ice sludge rising to the surface in the crystal separator is transported to the natural water body with the aid of a scraper 60 or in another way known in the art, for instance with a screw pump. The mixture of liquid and crystal flews freely down in the pipe 18 from the lower part of the crystallizer, and the pump 17 returns the liquid alone to the crystallizer 10.

in the foregoing, the invention has been described mainly in its application in connection with a heat pump using water for heat source. It may naturally equally be used in evaporating and crystallizing other liquids, and in concentrating solutions. It is also not intended to confine the embodάiients of the invention to the applications and structural designs presented by way of example in the foregoing.

In all embodiments, in the foregoing, application of the procedure has been described in connection with a compressor heat pump, in which a compressor 15 urges the heat transfer fluid from the evaporator 13 to the condenser 16. Ihe procedure may obviously equally be applied in connection with an absorption heat pump, in which case, for Instance in the embodiment of Fig. 1, all components are unchanged except that the compressor 15 is replaced, in a way known in the art, with the absorber and cooker of an absorption heat pump.

Claims

1. A procedure for drawing heat from freezing liquid by evaporating this liquid in crystallizer (10) wherein prevails a vapour pressure equivalent to or lower than the freezing point of the liquid (the triple point) and wherein part of the liquid at the same time crystallizer and by condensing the vapour in heat exchange with the heat transfer fluid of a heat pump, characterized in that the vapour is compressed to a temperature higher than the freezing point with the aid of warmer vapour produced in a supercooler (14) for the heat transfer fluid of the heat pump by conducting this vapour into an ejector (12) which draws cold vapour from the crystallizer (10) and from which the warmed-up vapour is conducted to the evaporator (13) of the heat pump, where the vapour condenses to liquid.

2. Procedure for drawing heat from freezing liquid according to claim 1, characterized in that part of the vapour that has flown to the evaporator (13') of the heat pump is drawn further therefrom with an ejector (35) and once again compressed to higher temperature, at which it is condensed in a next evaporator (13"), and this is repeated once or several times until from the last evaporator the uncondensable gases are drawn off-with an ejector (36).

3. Procedure for drawing heat from freezing liquid according to claim 1 or 2, characterized in that the heat-releasing liquid flews into the crystallizer (10) from one or several flow passages (59) and discharges into the crystallizer from all passages at the same height (57), the walls (58) therebelow in the crystallizer space all being continuously flushed by the liquid and the walls (54) of the crystallizer thereabove being partly at least heated to be warmer than the freezing point of the liquid evaporating in thecrystallizer.

4. Procedure for drawing heat from freezing liquid according to claim 1, 2 or 3, characterized in that the mixture of liquid and crystalproduced in the crystallizer (10) flows to a crystal separator (11) located at barometric height below the crystallizer, where the crystal are separated from the liquid, and to the crystallizer is returned to the liquid containing no crystals.

5. Procedure for drawing heat from freezing liquid according to claim 1, characterized in that the vapour discharges from the ejector (12) is either partly or totally condensed by conducting it to a crystal melter (45) to which is conducted at least part of the crystals produced in the crystallizer (10), which then melt to became pure liquid.

6. Means for applying a procedure as defined above in any one of claims 1 to 5, characterized in that the mixing cone (61) of the vapour jet ejector (12) has at least partly been built into the crystallizer (10), and the diffusor cone (62) of said ejector (12) has at least partly been built into the evaporator (13).