EP0003927B1 - Method for supplying or extracting heat to or from a condensable working fluid and a device working according to this method - Google Patents

Description

The present invention relates to a method and a device for exchanging heat between fluids, one of these, which will hereinafter be called condensable working fluid, being maintained under conditions such that the heat exchanges cause vaporization or condensation of part of its mass.

More particularly, the present invention relates to a device usable in a clalage pumping installation.

Heat pumps have been known for a long time, but it seems difficult to say that their energetic yields and their prices reach the optimal level.

One of the reasons for this is that there is no ideal working fluid for the conditions where it is more especially envisaged to use them, that is to say at temperatures of the same order as ambient temperature. Freons are well suited in particular because of their condensation temperatures under moderate pressures, but they are expensive, must not be brought into contact with grease or spread in the atmosphere. Water at ordinary temperatures requires installations of prohibitive volume due to the corresponding low pressures. Ammonia has suitable pressures in the field of use considered, as well as good heat exchange properties and is inexpensive, but it is corrosive and toxic.

On the other hand, the existing installations are relatively large because of the mediocre usual exchange coefficients which lead to bulky exchangers, hence significant masses of working fluid, which is all the more annoying as this fluid is either expensive like freon or dangerous like ammonia.

One way to improve existing heat pumps is obviously to improve their thermodynamic efficiency, so as to obtain better heat transfers for an equal mass of working fluid.

A step in this direction is represented by patent CH-A-305,668, in which the vaporization of the working fluid is carried out in stages, at decreasing pressure and temperature, the vapor phase of the working fluid being extracted at each stage and sent to a corresponding stage of a compressor, where it joins the vapor coming from a lower pressure stage and which has already been compressed in another stage of the compressor. However, this concept did not spread, perhaps because it used freon or water as the working fluid. In addition, the improvement in thermodynamic efficiency is limited because the vapor which finally leaves the compressor is far from the saturation conditions.

Document FR-A-2,352,247 presents a more sophisticated solution to the problem of improving thermodynamic efficiency. Indeed, in each of the modules where at each stage the vaporization of the working fluid takes place in stages, the recompressed vapor coming from the neighboring module at lower pressure through a compressor stage is brought back into contact with the liquid phase, and is therefore brought back to the saturation conditions, so that on an entropy diagram, for example, the zigzag path representing the successive states of the vapor phase always remains close to the vaporization curve. This gives a variable energy gain depending on the nature of the working fluid and the operating conditions, but particularly advantageous in the case of ammonia. This advantage is further considerably increased by the fact that, in this document, the condensation of the working fluid, in the high temperature part of the heat pump, is also carried out in stages, in modules similar to those where the vaporization.

However, in this document, the exchangers are not the subject of a particular study and it appears from the description that there has been above all for the purpose of the exchangers constituted by receptacles partially filled with the liquid phase of the working, at free level, of the tubes through which the fluids with which heat exchange takes place, being arranged either in the liquid phase or in the overhanging vapor phase.

Such an arrangement entails the obligation to have relatively large masses of working fluid, which, as said above, is to be avoided, especially when the latter is ammonia. Furthermore, it does not guarantee that the exchange coefficients are optimal.

In addition, if one examines the pressure losses undergone by the fluid when it traverses the modules in the direction of decreasing pressures, one notes that most of the pressure difference between two consecutive modules corresponds to the pressure loss due to the "flashing" of the liquid when it enters the exchange compartment. This flashing is accompanied by non-recoverable energy losses.

In the work which led to the present invention, particular attention was paid to heat exchanges in exchangers and to the corresponding pressure drops.

It is generally known that when one wants to carry out heat exchanges in tubes, it is necessarily necessary to bring into play the pressure drop of the fluid passing through them and that, the greater this pressure drop, the greater is heat flow. This correlation (analogy of REYNOLDS) takes on a character especially in cases where boiling or condensation occurs in tubes; according to the orders of magnitude of the various parameters (specific flow, titer) involved, the physical phenomena which then occur give rise to a wide variety of operating regimes both from the hydrodynamic point of view (stratified, transient, annular flows) and from the thermokinetic point of view (evaporation, nucleated boiling regimes, etc.).

Among these regimes, some are of particular interest, because they make it possible to achieve very high heat fluxes, for example greater than 50 kW / m ² , with a small temperature difference between fluid and wall of the tube, for example 1 ° C. .

These regimes are obtained when the "mass titre", the ratio between the vapor flow rate and the total fluid flow rate, measured by weight, is between a value of the order of approximately 0.03 and a value of the order of About 0.97, that is to say as soon as the fluid is clearly two-phase, with a significant pressure drop, for example of the order of 0.3 bar per meter for tubes with a diameter of 20 mm (cf. 5th Int. Heat Transfer Conf. 1974 TOKYO, Handbook of heat transfer, ROSENOW).

Such conditions are compatible with the process of the aforementioned patents; in fact, a temperature difference of 5 ° between two successive modules gives rise, if ammonia is used, to a pressure difference which can be of the order of magnitude of 3 bars, which corresponds to a tube of 10 meters long with a pressure drop of 0.3 bar / meter.

The mass titer values which allow the above regimes can be obtained quite easily; in fact, in an exchanger which receives liquid which leaves a module and is substantially at equilibrium conditions with steam, it suffices to supply the liquid with little heat so that it begins to boil, ie so that the mass titer becomes greater than zero, and exceeds the minimum value. It suffices to calculate the flow rate as a function of the heat flow so that at the outlet of the exchanger, it has not been entirely transformed into the vapor phase, that is to say that the mass titer is less than 0, 97, and that it enters the lower pressure module a liquid-vapor mixture. Likewise, with regard to a module where the condensable fluid gives off heat, it suffices to provide that before the entry of the exchanger, a sufficient quantity of vapor is added to the liquid so that it is not completely condensed than in the vicinity of the exchanger outlet.

Thus, almost all, or all, of the surface of the exchanger will work under optimal heat flow conditions, on the side of the condensable fluid.

In general, it is not possible to place oneself under similar conditions on the other side of the wall of the exchanger, since the fluid which brings or removes heat is not normally under the conditions where it is condensable, but the problem is much less annoying insofar as it is not an expensive, dangerous or corrosive fluid, or in very limited quantity, but for example geothermal water or industrial cooling, or heater.

It is questionable why exchangers operating according to the above principles, and which are already known, are not more widespread. There are several reasons for this: the study of two-phase systems undergoing transformation is extremely complex, and the data necessary for the calculation are not available in all cases; the advantage of a high exchange coefficient is much less in a boiler, for example, where the temperature difference from one side to the other of a wall is several hundred degrees centigrade, than in a heat pump, where it is only a few degrees; whereas, in most cases, an increase in pressure drops is costly in energy, unlike the devices to which the present invention applies, where the pressure difference on either side of an exchanger is imposed, therefore free.

The aim of the present invention is a method for supplying or removing heat from a condensable working fluid, passing from a first enclosure where it is in saturation conditions to a second enclosure where it is also in saturation conditions but at a lower pressure and temperature, the heat being supplied or removed to the working fluid, as it passes through the tubes of an exchanger around which circulates a heat transfer fluid, heat supply or extractor, this process having, according to the invention, the particularity that at least part of said condensable working fluid is passed through the tubes of said exchanger under conditions such that it is in the two-phase state with a mass titer of approximately 0.03 to 0, 97 approximately over a significant part of its path, in the exchanger, and that it undergoes, during the passage of the tubes, a continuous pressure drop which corresponds to the major part of the pressure difference between the first and second speakers.

Although we have spoken above of the tubes of an exchanger, it is clear that the invention also applies to the case of a single tube exchanger.

Along its path in the exchanger, the condensable working fluid can be either two-phase with a mass titer between 0.03 and 0.97, or in an essentially gaseous state, the mass titer being greater than 0.97 or equal to 1, which is essentially liquid, the mass titer being less than 0.03 or zero. Where the fluid is essentially gaseous, the pressure drop is significant, although less than that observed when the fluid is two-phase, and the heat exchange coefficient is very low. This situation should therefore be avoided or minimized as much as possible.

Where the fluid is essentially liquid, the pressure drop is low, but the heat exchange coefficient is also very low. This situation is less unfavorable than when the fluid is essentially gaseous, but it corresponds to almost useless lengths of tube, hence excess investment and increase in size.

Given the importance of the pressure drop in the two-phase heat exchanger, it is clear that, unless special provisions are made, this is where the greatest pressure drop occurs between the two enclosures, unless contrary special provisions have not been provided, such as a throttling of the pipe upstream or downstream of the exchanger.

Such a measurement is, of course, to be avoided for the loss of energy that it causes, and because it limits the difference in exploitable pressure in the exchanger. However, for fine adjustment of the speed, it is possible to provide, upstream or downstream, a small and adjustable pressure drop.

Two-phase systems in an exchanger exist without any need at certain points of exchangers or boilers, but so far they have not been systematically exploited in stage heat pumps, where the fluid passes under the effect of its pressure, from one module to another, and where the liquid is in equilibrium with its vapor in each module.

In such a device, to obtain the desired operating mode, there must be a judicious correlation between the various parameters, namely the heat flow received or given up by the condensable working fluid, the geometry of the exchanger, the pressure drop in the exchanger and the flow. However, it is clear that we are not entirely in control of these parameters, the former representing external constraints, and the latter affecting the operation of the entire installation, so that any modification of these has repercussions, chain, on all floors. This makes it difficult to remedy disturbances or to start up.

It appeared that the most effective way of regulating the operation of such an installation was to act on the flow rate of the condensable working fluid which passes through the tubes of the exchanger. This implies that, unlike the prior art, the entire flow rate between two stages, which is fixed by other considerations, does not constantly pass through the tubes of the same exchanger, or the same tubes of the exchanger; in other words, that part of the flow which is transferred from one enclosure to the other does not pass through the exchanger and is deflected either in another exchanger or in a duct where it simply suffers a pressure drop, corresponding to the pressure difference between the two speakers.

The invention therefore also provides a heat exchange method in which heat supplied or removed by a heat transfer fluid is used to pass a condensable working fluid from the liquid state to the vapor state or vice versa, method in which the two fluids are made to circulate in a series of modules each comprising an enclosure forming a liquid-gas separator where the liquid and vapor phases of the condensable working fluid are substantially in equilibrium, said enclosures being at stepped pressures and temperatures, the vapor being extracted from each enclosure, compressed and sent to the neighboring module in the direction of increasing pressures, while the liquid phase of the condensable working fluid passes from one module to another in the direction of decreasing pressures, at least one part of this passage being done according to the method described above, this method having the particularity that, to maintain the desired conditions inside the exchange eur, only a fraction of the condensable working fluid which passes from one enclosure to the other is passed through it, this fraction being sent to the exchanger in the liquid phase when heat is supplied by the heat transfer fluid, and in the two-phase state when heat is removed by the heat transfer fluid, another fraction of the working fluid condensable in liquid phase going from one enclosure to another in the direction of decreasing pressures, without passing through said exchanger, and the distribution of the condensable working fluid is varied between these two fractions.

Of course, such a method applies to the case of the aforementioned document FR-A-2 352 247, in which the steam leaving a compression stage is sent to the neighboring module at higher pressure and temperature in order to be returned to it. contact with the liquid phase of the condensable working fluid.

According to an advantageous embodiment of the invention, when desuperheating, at the level of a module, the vapor flow rate of the condensable working fluid coming from the neighboring stage at lower pressure and temperature and having passed through the stage of the compressor of this last module, which can be operated by bringing into full contact as in the process of the document above, or in another way, to operate this desuperheating, at least part of the flow is sent into this steam flow condensable working fluid in the liquid phase which has not passed through the exchanger.

Furthermore, according to another advantageous method which can be combined with the previous one, at least part of the flow rate of the condensable working fluid in the liquid phase which has not passed through the exchanger is sent to an ejector where it is made to suck at least part of the flow rate of the condensable working fluid in the vapor phase coming from the neighboring module at lower pressure and temperature and having passed through the corresponding compressor stage in order to overpress and desuperheat said vapor. Thus, the energy due to the expansion of the liquid phase between two modules is not lost, but is used to relieve the compressor.

• Fig. 1 représente le schéma d'un module "réchauffé" et
• Fig. 2 représente le schéma d'un module "refroidi".

A device for implementing the method according to the invention will now be described with the aid of the figures among which:

• Fig. 1 represents the diagram of a “reheated” module and
• Fig. 2 shows the diagram of a "cooled" module.

The device to which the figures relate, and which is described by way of nonlimiting example, is a heat pump operating with ammonia as the condensable working fluid, the heat transfer fluid being either lukewarm water of geothermal origin. or from industrial installations providing heat, either water intended for heating or drying in industrial installations extracting heat.

The heat pump includes a number of "modules", which are traversed by ammonia.

• - un échangeur 1 dans les tubes duquel circule l'ammoniac, utilisé comme fluide de travail condensable, le fluide caloporteur étant de l'eau apporteuse ou extractrice de chaleur circulant autour des tubes,
• - une enceinte 2 formant un séparateur liquide-gaz de type cyclone, susceptible de réaliser une mise en contact des phases liquide et vapeur du fluide de travail condensable et un brassage énergique de ces phases de façon à obtenir une mise rapide en équilibre.
• - un étage de compresseur 3, opérant sur la phase vapeur,
• - un éjecteur 4, où la phase liquide à pression élevée voit son énergie potentielle transformée en partie en énergie cinétique qui lui sert à entraîner la phase gazeuse.

Each module normally includes:

• an exchanger 1 in the tubes of which the ammonia circulates, used as condensable working fluid, the heat-transfer fluid being water supplying or extracting heat circulating around the tubes,
• - An enclosure 2 forming a liquid-gas separator of the cyclone type, capable of bringing the liquid and vapor phases of the condensable working fluid into contact and vigorous mixing of these phases so as to obtain rapid equilibration.
• - a compressor stage 3, operating on the vapor phase,
• - An ejector 4, where the liquid phase at high pressure sees its potential energy partly transformed into kinetic energy which it uses to drive the gas phase.

Some modules are not complete; thus, the higher pressure terminal module does not include an exchanger, compressor stage or ejector, while, in the lower pressure terminal module, the separator is replaced by a simple container.

The installation includes two types of modules; some called "reheats" are the subject of FIG. 1, others, called "cooled", that of FIG. 2.

In the figures, the higher pressure and temperature modules are located on the right, and the lower pressure and temperature modules on the left. We denote by n a heated module, by n - 1 and n + 1 the neighboring modules, in the order of increasing pressures, and by p - 1, p, p + 1 of the cooled modules, in the same order.

In a module n, the ammonia compartment of the exchanger 1 is connected, for its supply, to the module n + 1 by a pipe 5 arranged to supply it essentially in the liquid phase with the condensable working fluid from the separator 2a of this module, that is to say opening into the lower part thereof. The water compartment is supplied with hot water by a pipe 6, coming either from the source, or from a stage n + 1 at higher pressure and temperature.

The water is then sent to stage n - 1, where to the evacuation by a pipe 7.

In the exchanger 1, the ammonia receives heat and vaporizes at least in part, so that it leaves a two-phase fluid, or only in the vapor phase, which is sent to the separator 2 via a pipe 8. This pipe can be omitted, the exchanger tubes then opening directly into the separator. As has been said above, it is not advantageous for the total vaporization to take place in the exchanger at a significant distance from the outlet, whereas it can be tolerated that the outgoing fluid is two-phase.

A fraction of the condensable working fluid in the liquid phase leaving the separator 2a of stage n + 1 is sent by another pipe 9 into the ejector 4, without passing through the exchanger, and acquires a high speed there by passing through an ejection nozzle, then it is in contact with the vapor coming from the compressor stage 3b of the module n - 1 via line 10. The ejection occurs with partial vaporization and it leaves the ejector a high-speed two-phase fluid, the vapor of which comes both from line 10 and from the vaporization of the condensable working fluid in the liquid phase supplied by line 9. This mixture is brought by line 11 into the separator 2 in which it joins the two-phase fluid coming from line 8.

From the top of the separator, the gas phase of the condensable working fluid is extracted and sent to the compressor stage 3, via line 12, while, from the bottom of the separator 2, the liquid phase of the condensable working fluid is sent to module n - 1 as indicated above.

Valves, not shown, distribute the flow of liquid leaving separator 2a of stage n + 1 between conduits 5 and 9 so as to maintain the end of the vaporization zone in exchanger 1 in the vicinity of exit.

• le fluide extracteur de chaleur circule dans l'échangeur en sens inverse d'un module réchauffé, c'est-à-dire qu'il arrive par le conduit 7 à partir du module p - 1, et s'en va par le conduit 6 vers le module p + 1.

A cooled module 1, as shown in FIG. 2, differs from a heated module only by the following points:

• the heat extracting fluid circulates in the exchanger in the opposite direction of a heated module, that is to say that it arrives via the conduit 7 from the module p - 1, and leaves via the conduit 6 to module p + 1.

The exchanger 1 must be supplied with condensable working fluid in the two-phase state from the separator 2a of the stage p + 1 and, for this, an additional pipe 13 starts from the top of the separator and joins the pipe 5 to 1. entry of the exchanger 1. This pipe 13 is equipped a valve 14; in fact, the quantity of vapor condensed in the exchanger is proportional to the heat flux which is removed by the water.

The other parts of the module are the same as those of a heated module; however, it should be noted that the pipe 8, which connects the exchanger 1 to the separator 2, should normally no longer be crossed except by liquid.

In modules of one type or another, the valves controlling the distribution of the flow rate between lines 5 and 9 are controlled by a parameter linked to a speed inside the exchanger. A simple regulation mode is linked to the quantity of liquid present in the separator.

On the other hand, it is not necessary for the entire vapor phase of the condensable working fluid coming from the stage of the compressor 3b to be brought into contact with the tolerance of the liquid phase of the condensable working fluid, a portion or even all of this vapor phase can go to compressor stage 3 without passing through the ejector or the separator, provided that it is brought into contact with a sufficient quantity of liquid phase to be brought back to saturation conditions , that is to say desuperheated. This desuperheating can be carried out in the compressor.

The terminal modules present particular problems: at the level of the "first" module, that is to say that at the lowest pressure and temperature, the separator is in fact replaced by a simple reservoir, the condensable working fluid of which does not only comes out as vapor. A liquid phase shipment that has not passed through the exchanger may cause it to become blocked. In this case, for the regulation intended to obtain the suitable speed in the exchanger, it is preferable to provide that an adjustable quantity of the working fluid condensable in liquid phase is withdrawn from the reservoir to be re-injected upstream (in the liquid flow direction), from the nearest exchanger, using a pump to overcome the pressure difference between modules.

Likewise, in the "last" module, at the highest pressure and temperature, the desuperheating of the vapor of the condensable working fluid from the last stage of the compressor cannot be effected using the liquid phase of the condensable work from an upper module, so that the last exchanger risks going out of optimal regime. It is preferable to provide, to avoid this drawback, that the liquid phase, withdrawn downstream of the last exchanger, is re-injected upstream of the latter, in quantity adjustable using a pump to compensate for the difference in pressure.

Furthermore, the present invention does not preclude the presence, in the series of modules, of "adiabatic" modules, that is to say without exchanger.

Claims (10)

1. Process for supplying or removing heat from a condensable working fluid passing from a first enclosure (2a) where it is under saturation conditions to a second enclosure (2) where it is also under saturation condtions but at a lower pressure and temperature, the heat being supplied or removed from the said condensable working fluid while it goes through the tube of an exchanger (1) around which flows a heat transfer fluid, a contributor or extractor of heat, characterised in that at least part of the said condensable working fluid is caused to pass through the tubes of the said exchanger (1) under conditions such that it is in a diphasic state with a mass ratio of about 0.03 to about 0.97 over a considerable part of its path in the exchanger, and such that it undergoes, in the course of its flow through the tubes, a continuous loss of head which corresponds to the major part of the pressure difference between the first and second enclosure.

2. Process of heat exchange in which heat brought or removed by a heat transfer fluid is used to change a condensable working fluid from the liquid state to the vapour state or vice versa, process in which the two fluids are caused to flow in a series of modules (n, p) each comprising an enclosure (2, 2a) forming a liquid-gas separator where the liquid and vapour phases of the condensable working fluid are substantially in equilibrium, the said enclosures being stepped pressures and pressures, the vapour being extracted from each enclosure, compressed and convoyed to the neighbouring module (N + 1, p + 1) in the direction of increasing pressures while the liquid phase of the condensable working fluid passes from a module (n.p.) to the other (n - 1, p - 1) in the direction of decreasing pressures at least a part of this passage occurring according to the process of claim 1, characterised by, for maintaining the desired conditions inside the exchanger (1), causing to pass through the latter only a fraction of the flow of the condensable working fluid which goes from one enclosure (2a) to the other enclosure (2) this fraction being conveyed to the exchanger (1) in the liquid phase when the heat is brought by the heat transfer fluid, and in the diphasic state when the heat is removed by the heat transfer fluid, another fraction of the condensable working fluid in the liquid phase going from one enclosure (2a) to the other (2) in the direction of decreasing pressures without passing via the said exchanger (1), and by varying the division of the condensable working fluid into these two fractions.

3. Process according to claim 2, and comprising desuperheating, at the level of a module, the vapour flow coming from the neighbouring module at less elevated pressure and temperature and having gone through the compressor stage (3b) of the latter module, characterised in that, for performing this desuperheating, at least a part of the liquid which has not passed through the exchanger is conveyed into this vapour flow.

4. Process according to one of claims 2 and 3 characterised in that at least a part of the liquid flow has not passed through the exchanger (1) and is conveyed into an ejector (4) where is aspirated from it at least a part of the flow of condensable working fluid in vapour phase coming from the neighbouring module at less elevated pressure and temperature and having gone through the corresponding compressor stage (3b) so as to supercompress and desuperheat the said vapour.

5. Heat exchanae device carrvina out the process according to claim 2 and comprising a series of central modules each comprising an exchanger (1), a compressor stage (3) and an enclosure (2) for bringing into contact the liquid and vapour phases of the condensable working fluid; terminal modules which may be of different structure; and conduits (10, 12, 5, 8) permitting to cause the vapour phase to pass from one module to another in the direction in increasing pressures through the compressor stages (3), and the liquid phase in the opposite direction through the exchanger (1), characterised in that it further comprises a conduit (9) permitting to cause the liquid phase to pass from one enclosure (2a) for bringing the phases into contact to another enclosure (2) for bringing into contact in the direction of decreasing pressures without passing via the exchanger (1) and means for controlling the ratio between the fraction of the liquid flow which passes through the exchanger and the fraction of the liquid flow which passes into the said conduit.

6. Device according to claim 5 characterised in that the said conduit (9) is supplied essentially with the liquid phase, while the condensable working fluid which enters the exchanger (1) is in the liquid phase when heat is brought by the heat transfer fluid and in the diphasic state when heat is removed by the heat transfer fluid.

7. Device according to claim 5 or 6, characterised in that the said conduit (9) brings the said fraction of liquid into an ejector (4) arranged so that it entrains therein gaseous phase exiting the compressor stage (3b) of the neighbouring module, at less elevated temperature and pressure and in that the mixture of liquid and vapour is conveyed into the enclosure (2) forming a liquid-gas separator.

8. Device according to any one of claims 5 to 7, characterised in that it comprises means for drawing off the liquid phase from the first enclosure (2) for bringing the liquid and vapour phases into contact in the direction of increasing temperatures, and for reinjecting liquid thus drawn off upstream of the closest exchanger (1).

9. Device according to one of claims 5 to 8, characterised in that it comprises means for drawing off liquid phase downstream of the exchanger (1) from the penultimate module in the direction of increasing temperatures and pressures, and for reinjecting the liquid thus drawn off upstream of the said exchanger.

10. Device according to one of claims 5 to 9, characterised in that it comprises means for causing the vapour phase to pass directly from a stage (3b) of the compressor to the next (3) in the order of increasing pressures, and means for injecting the liquid phase directly into the compressor in order to desuperheat this vapour phase.

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