EP0163579A1 - Method and apparatus for freezing the soil - Google Patents

Abstract

A refrigerant liquid flows through congelation probes (S1, S2, . . . ). The temperature of the liquid supplied to each probe is regulated as a function of the rate of congelation of the ground around the various probes and/or is progressively increased as the congelation progresses.

Description

The present invention relates to the freezing technique of soils. It relates firstly to a method of freezing soil of the type in which a cooling liquid is cooled by heat exchange with a cryogenic fluid, then this liquid is circulated in a series of probes sunk into the ground.

We know that consolidation of soils by freezing allows the opening of public works sites in damp and unstable soils. It is performed by injecting a coolant into probes introduced from place to place in the ground. This cooling freezes the soil gradually until it forms a continuous wall when the freezing zones of each probe have joined their neighbors.

It is known to inject either a cooled liquid or a cryogenic liquid such as liquid nitrogen into the probes.

Direct injection of liquid nitrogen has several drawbacks, in particular the difficulty of controlling the heat exchange coefficients with the soil: by yielding cold, the nitrogen vaporizes and the exchange coefficients between the probe and the nitrogen first pure liquid, then mixtures of liquid and gas in variable proportion, then cold gas alone, are very different. This results in a high degree of heterogeneity in the thickness of frozen soil around the probe and a loss of time and energy so that the less frozen zones meet to form the consolidated wall, while the most frozen zones are unnecessarily sub-cooled and oversized.

The injection of a cooled liquid does not have these drawbacks, but its effectiveness depends on the cooling method.

The cooling of a circulating liquid by a refrigeration unit makes it possible to inject the liquid at -40 ° C in the best case, more generally at -20 ° C or -30 ° C. These freezing conditions lead to a prohibitive frozen wall formation time, of the order of several weeks for a wall 1 m thick. This duration is generally incompatible with the duration of construction sites in cities.

To allow the liquid to circulate in the probes at a much lower temperature, for example -80 ° C or even -120 ° C, freezing methods of the type indicated above have also been proposed.

Such a method overcomes the aforementioned drawbacks but remains currently expensive for the following reasons: on the one hand, to accelerate freezing, it is necessary to cool the soil more than what is strictly necessary for its consolidation. On the other hand, the soil is always heterogeneous, and the consolidation of the frozen wall is governed by the weakest point, that is to say where the freezing advances the slowest. We are then forced to extend, sometimes in considerable proportions, the frozen areas as quickly as possible.

The object of the invention is to make it possible to considerably reduce the excess of cold and, consequently, to make the process much more economical, without thereby significantly increasing the duration of freezing.

To this end, the subject of the invention is a process for freezing soil of the aforementioned type, characterized in that the temperature of the coolant is varied, during the phase of freezing the soil, as a function of the progression of freezing.

In a first embodiment, the temperature of the liquid circulating in at least one of the probes is gradually increased, preferably in successive stages.

In a second embodiment of the invention, which can be combined with the first, the temperature of the liquid circulating in each probe is adapted to the rate of freezing of the soil around this probe, this temperature being set to a value the higher the higher the freezing speed.

The invention also relates to a soil freezing installation intended for the implementation of such a method. This installation, of the type comprising a heat exchanger supplied on the one hand with cryogenic fluid, on the other hand with a coolant, a series of freezing probes, and means for circulating the liquid in each probe, is characterized by the fact that it comprises means for varying the set temperature of the heat exchanger, and / or the fact that it comprises at least two independent heat exchangers having different set temperatures.

• - la figure 1 est un diagramme qui illustre un premier mode de mise en oeuvre de l'invention;
• - la figure 2 est un diagramme qui illustre l'avantage apporté par le procédé illustré à la figure 1 ;
• - la figure 3 est un schéma d'une installation correspondant à un second mode de mise en oeuvre de l'invention ; et
• - la figure 4 illustre schématiquement une variante.

Some examples of implementation of the invention will now be described with reference to the accompanying drawings, in which:

• - Figure 1 is a diagram which illustrates a first embodiment of the invention;
• - Figure 2 is a diagram which illustrates the advantage provided by the method illustrated in Figure 1;
• - Figure 3 is a diagram of an installation corresponding to a second embodiment of the invention; and
• - Figure 4 schematically illustrates a variant.

In each of the examples below, the invention relates to the formation in a sandy and moist soil of a frozen wall in the shelter of which certain work must be carried out. To do this, a series of freezing probes Sl, S2, etc., which are schematically illustrated in FIG. 3, are driven into the ground, and a coolant having an inlet temperature is circulated in each of them. determined. The liquid chosen must have a sufficiently low freezing point, and methanol is an appropriate liquid, which will be referred to below.

As shown in FIG. 3, this liquid circulates in a closed circuit between the probe and a heat exchanger El, E2, ..., called "cold plant", which comprises on the one hand passages for this liquid and on the other part of the passages for a cryogenic fluid, in particular liquid nitrogen. The rate of admission of liquid nitrogen into these latter passages is controlled by a valve 1 controlled by a temperature sensor 2 which detects the temperature of the coolant leaving the exchanger. Nitrogen passages can _T ar example, as illustrated in Figure 3, be constituted by a bushing 3 calender by a coil 4 for circulating the coolant against the flow of nitrogen. These elements have only been shown in FIG. 3 for the exchanger E1, for the sake of clarity of the drawing, but it is understood that if the installation includes several exchangers, like that of FIG. 3, all of these exchangers have a similar constitution.

Cooling liquid, leaving at a cold set temperature of an exchanger, is injected into the bottom of each probe connected to the latter by a central tube 5 thereof and rises between this tube and the cylindrical casing 6 of the probe to return to the exchanger. Between the inlet and the outlet of the probe, the liquid exchanges heat with the surrounding soil, through the casing 6.

According to the mode of implementation illustrated in FIG. 1, the temperature of the coolant injected into the freezing probes is modulated over time, gradually increasing this temperature from a minimum temperature at the start of freezing to a final temperature for keeping the already frozen wall cold. In the example illustrated, this increase takes place in successive stages.

As a numerical example (example I), we will assume that we want to consolidate by freezing in 100 hours a wall 1 m thick in moist sandy soil, to a depth of 20 m and a length of 50 m . To do this, fifty probes S1, S2, ..., S50 are inserted into the ground, spaced 1 m apart. Methanol is circulated between the probes mounted in parallel and a single heat exchanger cooled with liquid nitrogen such as the exchanger El described above. The temperature sensor 2 is equipped with an adjustment device which makes it possible to regulate at will the temperature of methanol between -80 ° C. (lower limit tolerable for this body) and -10 ° C.

The freezing is started by circulating the methanol with a set temperature at the outlet of the exchanger (and therefore upon injection into the probes) of -80 ° C. This set temperature is maintained for 50 hours. The temperature of the soil in the vicinity of the probes is then established at -70 ° C. and the frozen radius around the probes is 38 cm (ie a diameter of 76 cm).

At this point, the methanol setpoint temperature is set at -65 ° C. This temperature is maintained for 20 hours. The temperature of the. soil in the vicinity of the probes is established at -57 ° C. During this period of time, the progression of the freezing front of the wall is practically not slowed down, because it is governed by the temperature gradient in the vicinity of the freezing isotherm (0 ° C) and not by the temperature of the probe. A frozen diameter of 84 cm is thus obtained after 70 hours of freezing.

After 70 hours of freezing, the set temperature of methanol is fixed at -50 ° C. This set temperature is maintained for 15 hours. The temperature of the soil in the vicinity of the probes is established at -44 ° C. After 85 hours, the frozen diameter around the probes is 88 cm.

The set temperature of methanol is then fixed at -40 ° C. It is kept for 10 hours. The temperature of the soil in the vicinity of the probes is established at -35 ° C. After 95 hours of freezing, the frozen soil diameter around the probes is 90 cm.

The set temperature of methanol is then established at -35 ° C. This set temperature will be kept for the entire period of keeping the wall frozen. The temperature of the soil around the probes will equilibrate to -30 ° C. Freezing with a diameter of 100 cm will be obtained after approximately 100 hours.

It should be noted that the above indications correspond to a homogeneous soil and to an isolated probe; in fact, each freezing probe reacts with its neighbors, which, for a spacing of 1 m between probes, leads to a frozen wall of variable thickness: 1 m in front of the probes, about 80 cm midway between the probes.

It will also be understood that, as a variant, the different methanol set temperatures can no longer be obtained by means of a single exchanger with adjustable set temperature, but by means of several heat exchangers having different set temperatures but fixed, these exchangers can be selectively connected to the probes by a set of suitable valves. In addition, if the exchangers available do not allow individual cooling capacity to be supplied (proportional to the product of the methanol flow rate by the temperature difference between the inlet and the outlet of the exchanger) necessary. several exchangers in parallel set to the same temperature can be used for each set temperature.

FIG. 2 illustrates the advantage of the method described above. It represents the variation of the soil temperature T as a function of the radius R, counted from the outer wall of a supposedly isolated probe, this at the end of the freezing, that is to say when the frozen radius Rc becomes close to the half-distance separating the probes (approximately 0.5 m in the example above).

The lower curve A1 corresponds to the case where the probe would have been permanently supplied with methanol at -80 ° C., according to the prior art. This curve rises from -70 ° C for R = 0 to 0 ° C for R = Rc, then from 0 ° C to room temperature Ta. The upper curve A2 corresponds to the process according to the invention described above; it rises from -30 ° C for R = 0 to 0 ° C for R = Rc, then continues to grow from 0 ° C to Ta while remaining above the curve Al. The hatched area between the two curves Al and A2 is a representation of the economy of frigories achieved.

According to the mode of implementation represented in FIG. 3, the temperature of methanol is no longer regulated over time but in space, by adapting this temperature, for each probe, to the freezing speed of the soil around this probe, in order to avoid excessively sub-cooling the parts of the soil which freeze the fastest. Indeed, in reality, if a soil is generally relatively homogeneous in the radius of 50 to 60 cm which surrounds a probe, it is not the same from one probe to another.

For this, several heat exchangers El, E2, etc. are used, five in number in the example illustrated, having independently adjustable set temperatures and which can each be connected to all the probes. The rate of cooling of the soil at the start of freezing is measured, and methanol is sent to each probe at a temperature which is all the cooler as the soil concerned by this probe cools faster.

The determination of the freezing speed, which will make it possible to fix a set temperature for each probe and each heat exchanger can be done for example as follows.

• (a) - La mesure de la différence de température entre l'entrée et sa sortie du méthanol dans chaque sonde est une mesure caractéristique du flux de chaleur absorbé par le sol, pour un débit donné. Si cette différence est plus élevée pour une sonde particulière, il faut élever la température d'injection du méthanol dans cette sonde, car le sol absorbe beaucoup de froid.
• (b) - On peut également disposer parallèlement à la ligne des sondes une ligne de capteurs de température C1, C2, ..., par exemple comme représenté à la figure 4, où un capteur de température est disposé dans le sol, près de la surface, entre-les paires de sondes successives, à égale distance des deux sondes de chaque paire. De la même façon que précédemment, on fixe alors la température- d'injection du méthanol dans les sondes les plus voisines de ces capteurs en fonction de la vitesse de refroidissement du sol qu'ils montreront.

_. We can first perform global cooling measurements for each probe:

• (a) - The measurement of the temperature difference between the entry and exit of methanol in each probe is a characteristic measurement of the heat flux absorbed by the soil, for a given flow rate. If this difference is higher for a particular probe, the injection temperature of methanol in this probe must be raised, because the soil absorbs a lot of cold.
• (b) - It is also possible to arrange, parallel to the line of probes, a line of temperature sensors C1, C2, etc., for example as shown in FIG. 4, where a temperature sensor is placed in the ground, near the surface, between the pairs of successive probes, equidistant from the two probes of each pair. In the same way as above, the injection temperature of methanol is then fixed in the probes closest to these sensors as a function of the rate of cooling of the soil that they will show.

However, in practice, it frequently happens that, over the length of the wall to be frozen, the soil is heterogeneous not only horizontally, but also vertically, at least in certain areas. There may therefore exist, on the height of certain probes, regions which freeze quickly and others which freeze slowly. By subsequently, the above global measurement means risk leading to excessive slowing down of the cooling of a probe which would generally freeze quickly (this would appear, for example, from a large temperature difference between the incoming methanol and the outgoing methanol) but, in fact, very quickly over one portion of its length and very slowly over another.

• . (c) en début de refroidissement, mesurer la vitesse de refroidissement en chacun de ces points ; ou
• (d) un certain temps après le début de la congélation, injecter temporairement, par exemple pendant 10 à 30 minutes, du méthanol plus chaud que le sol, et mesurer la vitesse de remontée de la température aux différents points de mesure. En effet, cette vitesse de remontée varie dans le même sens que la vitesse de congélation du sol.

To avoid this risk, the measurement can be refined by placing several temperature sensors 7 along the length of the probes, on their outer wall, these sensors being adapted to measure the temperature of the ground in the immediate vicinity of the probes. There are two ways to do this:

• . (c) at the start of cooling, measure the cooling rate at each of these points; or
• (d) a certain time after the start of freezing, temporarily inject, for example for 10 to 30 minutes, methanol hotter than the ground, and measure the rate of temperature rise at the various measurement points. Indeed, this ascent rate varies in the same direction as the freezing speed of the soil.

If this procedure makes it possible to detect vertical heterogeneity of the soil, we will use the smallest temperature variation to determine the injection temperature of methanol in the corresponding probe (s).

Example II below illustrates the implementation of the invention from methods (a) and (d) above. The basic data are the same as before: it involves freezing in 1 hour a wall 1 m thick in moist sandy soil, to a depth of 20 m and a length of 50 m. There are fifty probes S1, S2, ..., S50 spaced 1 m apart, and cooled methanol is circulated there. Five independent heat exchangers E1 to E5 supplied with liquid nitrogen are used according to the diagram in FIG. 3. By means of a suitable set of pipes and valves (not shown), any probe can be supplied from any what exchanger. Each probe is provided with temperature sensors 8 and 9 measuring the temperature of methanol at its inlet and its outlet, respectively. Against the external wall of each probe, thermocouples 7 were placed to measure the temperature at 2 m, 10 m and 18 m deep.

After starting the injection into all the methanol probes at -80 ° C, wait 5 hours until the initial transient effects have passed. At this point, the temperature difference ΔT between the inlet and the outlet for methanol is observed on the probes.

The temperature of the external surface of the probes is not very variable at this time, between -70 ° C and -72 ° C for all the probes.

By changing the set temperature of exchangers E1 to E5, methanol is injected into the probes at -50 ° C for 20 min. The rate of rise of the external temperatures of the probes is measured. We note at 18 m deep, on probes S46 to S50, a rise in temperature three times slower than on the same probes at 10 m and 2 m deep; no heterogeneity is noted on the other sqndes.

The cold methanol injection is then restored by setting the set temperatures as follows.

As can be seen, despite the results of the overall measurement, the probes S46 to S50 have been treated as slow-freezing probes to take account of the slow freezing observed in their deepest part

In addition, certain groups of probes are supplied with two exchangers connected in parallel. This makes it possible to supply a methanol flow rate of the same order to all the probes. We also note that, in order not to complicate the installation too much, we supply the groups of sensors S1 to S4 and S 41 to S45 at the same temperature although, strictly speaking, the sensors of these two groups absorb different heat fluxes.

It follows from the foregoing that each probe is supplied with a cooling power that is lower the more the soil surrounding this probe freezes faster.

Example III below illustrates procedure (b) indicated above.

Using the same basic data as in the previous examples, a line of twenty-five temperature sensors C1, C2, ..., C25 is available 40 cm from the line of freezing probes. two freezing probes, as shown in Figure 4, each sensor being equidistant from two probes. The temperature sensor C1 is close to the freezing probes S1 and S2, the temperature sensor C2 is close to the freezing probes S3 and S4, etc.

We start by injecting methanol at -80 ° C into all freezing probes for 24 hours. After 24 hours, the following temperatures are observed on the temperature sensors.

From this moment, the probes are supplied at different temperatures, as follows:

The remarks made above concerning the use of the exchangers, alone or in parallel, remain valid in this example.

It should be noted that it is entirely possible to combine the various control methods described above, and in particular to vary the methanol injection temperature both over time and in the space. In this case, after having fixed the different methanol injection temperatures in the different groups of probes, a series of increasingly hot stages is defined for each group, distributed over the total duration of the freezing so as to supply end of freezing all the probes at the single set temperature which will be kept during the period of maintenance of the frozen wall.

Example IV below thus combines the lessons of examples I and III above and describes in table form the freezing procedure during the 100 hours allowed to obtain a wall 1 m thick.

Claims (13)

1. Soil freezing process, of the type in which a coolant is cooled. By heat exchange with a cryogenic fluid, then this liquid is circulated in a series of probes (S1, S2, ...) embedded in the soil, characterized in that the temperature of the liquid is varied, during the freezing phase of the soil, as a function of the progress of freezing. 2. Method according to claim 1, characterized in that the temperature of the liquid circulating in at least one of the probes (S1, S2, etc.) is gradually increased, preferably in successive stages. 3. Method according to any one of claims 1 and 2, characterized in that the temperature of the liquid circulating in each probe is adjusted (Sl, S2, ...) to the freezing speed of the soil around this probe, this temperature being set to a value the higher the higher the freezing speed. 4. Method according to claim 3, characterized in that, to determine the freezing speed around each probe (Sl, S2, ...), the temperature difference between the liquid entering the probe and the liquid which is measured comes out. 5. Method according to claim 3, characterized in that, at the start of cooling, the ground cooling speed is measured at several levels of each probe (SI, S2, ...), and the slowest of these speeds is retained . 6. Method according to claim 3, characterized in that, to determine the freezing speed around each probe (S1, S2, ...), is temporarily injected into each probe, some time after the beginning of freezing, liquid hotter than the ground in the vicinity of the probe, the speed of temperature rise is measured at different levels of the probe, and the slowest of these speeds is retained. 7. Method according to claim 6, characterized in that, to determine the freezing speed around each probe (S1, S2, ...), the temperature of the soil is measured at a predetermined distance from all the probes. 8. A method according to any one of claims 1 to 7, characterized in that sends in each probe (S1, S2, ...) a flow of refrigerant of the same order of magnitude. 9. Installation for the implementation of a method according to any one of claims 1 to 8, of the type comprising a heat exchanger (E1 to E5) supplied on the one hand with cryogenic fluid, on the other hand with a coolant, a series of freezing probes (S1, S2, ...), and means (5) for circulating the liquid in each probe, characterized in that it comprises means for varying the set temperature of the heat exchanger. 10. Installation for the implementation of a method according to any one of claims 1 to 8, of the type comprising a heat exchanger (E1 to E5) supplied on the one hand with cryogenic fluid, on the other hand with a coolant, a series of freezing probes (S1, S2, ...), and means (5) for circulating the liquid in each probe, characterized in that it comprises at least two independent heat exchangers having different set temperatures. 11. Installation according to claim 10, characterized in that each heat exchanger (El to E5) comprises means for varying its set temperature. 12. Installation according to any one of claims 9 to 11, characterized in that each probe (S1, S2, ...) comprises means (7) for measuring the temperature at different levels of its outer wall. 13. Installation according to any one of claims 9 to 12, characterized in that it comprises a series of temperature sensors (C1, C2, ...) arranged parallel to the series of probes (S1, S2, .. .), each temperature sensor being equidistant from two freezing probes.

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