JP2024008249A - ground freezing system - Google Patents

Abstract

To increase cooling capacity of a ground freezing system.SOLUTION: A ground freezing system that freezes the ground includes a freezing pipe 101 buried in the ground, at least one pair of flat-shaped refrigerant pipes 201 for cooling circulation that are placed facing each other within the freezing pipe 101, a heat conduction member 301 which, inside the refrigerant pipes 201, has a refrigerant piping side surface 301a that is in contact with or comes closer to surfaces facing each other, and a freezing pipe side surface 301b that is in contact with or comes closer to the inner surface of the freezing pipe 101, and a heat transfer fluid filled in the freezing pipe 101.SELECTED DRAWING: Figure 2

Description

The present invention relates to a technique for freezing the ground when excavating the ground.

A ground freezing method that freezes the ground is sometimes used for applications such as the starting and reaching parts of shield tunneling machines, connecting shafts between tunnels, underground connections in tunnels and shafts, and underground widening of tunnels. The ground freezing method used for such applications has a very large scale of freezing due to the large scale and deep underground structures, and the freezing period from the start of freezing is a long period ranging from several months to several years. It is necessary to maintain frozen soil.

It is known that the strength of frozen soil is temperature dependent, and as the temperature decreases, the strength increases. Pressure resistance as well as water-stop performance is expected even in the frozen ground used in the above applications, so in order to ensure the design strength, the design size (thickness x width x height) is usually -10°C or less. Therefore, it is required to maintain it for a long period of time.

The ground freezing method basically involves burying a freezing pipe in the ground and flowing a low-temperature refrigerant through the freezing pipe to cool the ground around the pipe and freeze the ground.

There are two methods for cooling the cryotube: a brine method and a low-temperature liquefied gas method. One of the brine methods is to cool the ground by cooling an antifreeze solution (brine) such as an aqueous calcium chloride solution to about -30°C in a ground-based refrigerator and circulating it through a freezing tube. On the other hand, in the low-temperature liquefied gas method, liquid nitrogen transported by tank truck is poured directly into a freezing tube, the heat of vaporization cools the ground, and the vaporized nitrogen gas is released into the atmosphere. The low-temperature liquefied gas method is usually applied to small-scale, short-term construction work with an amount of frozen ground of 200 ^m3 or less, or soil sampling in soil surveys.

The mainstream of ground freezing methods used in tunnel construction and other projects is to use brine, and as it circulates through freezing pipes buried in the ground, the secondary refrigerant (brine), whose temperature has been raised by freezing the surrounding ground, freezes. It is cooled by the machine's evaporator.

In such a system, a primary refrigerant (such as refrigerant R404a) in a freezer is vaporized by exchanging heat with a secondary refrigerant, and the vaporized primary refrigerant is liquefied by exchanging heat with water in a condenser. The amount of heat supplied from the primary refrigerant to the water in the condenser is radiated in the cooling tower.

In the conventional brine method described above, the brine is cooled to a low temperature of about -30°C using an above-ground refrigerator. When using the ground freezing method for applications such as the starting and reaching parts of shield tunneling machines, connecting shafts between tunnels, and underground connections in tunnels and shafts, if the freezing scale is extremely large, a large amount of brine may be frozen at a low temperature. The problem is that a large amount of energy is required for cooling.

In addition, brine is a fluid with a high viscosity that is about 10 times higher than the viscosity coefficient of water, so in order to efficiently absorb underground heat with brine, the diameter of the frozen pipe should be increased and the brine inside the frozen pipe should be It must be circulated at a high flow rate. Therefore, since a pipe material with a large diameter is required, boring costs and pipe material costs rise. At the same time, a high-output brine circulation pump is required, and the loss of the brine circulation pump and the pump drive energy become too large, which poses an economical problem.

Here, the freezing tubes used in conventional brine systems often have a double-tube structure, consisting of an outer tube through which the sending brine from the refrigerator flows, and an inner tube that absorbs heat from the ground and returns. . The pipe material for such frozen pipes needs to be buried underground from several meters to about 100 meters, and for example, steel pipes such as gas pipes are used. From the steel pipe manufacturing plant to the construction site using the ground freezing method, the steel pipes are transported by truck in a fixed length of 5.5 m, and are welded and bonded directly above the borehole at the site, where they are buried underground.

If the diameter of the frozen pipe is large, truck transportation costs and pipe material lifting crane costs will rise, and the labor involved in welding and joining will be large, which poses economic problems. Furthermore, if the frozen pipe is large, the process of boring and burying the frozen pipe will take a long time, which will increase the construction cost.

Furthermore, if there is a defect in the welded joint of the frozen pipe and brine leaks into the ground, the ground in the leaked area will no longer freeze, causing water leakage and insufficient strength of the frozen soil in the leaked area, making it difficult to carry out construction work. There is a risk of becoming

On the other hand, with the conventional low-temperature liquefied gas method, if liquefied carbon dioxide is injected into the ground and the soil around the pipe material is frozen by the heat of vaporization, the connection between the starting part of the shield excavator, the reaching part, and the tunnel Applications such as underground connections in shafts, tunnels and shafts require large amounts of liquefied carbon dioxide to maintain frozen soil for long periods of time.

When the ground surrounding the area where liquefied carbon dioxide gas is injected begins to freeze, it becomes difficult for the liquefied carbon dioxide gas to reach areas far away from the frozen ground, making it impossible to form frozen ground below -10°C. A problem exists.

In addition, in the method of freezing the ground using a double-pipe structure that utilizes the extremely low boiling point of liquid nitrogen, the nitrogen gas is released into the atmosphere, making liquid nitrogen so-called ``disposable,'' and if the scale of freezing is large. Since a large amount of liquid nitrogen is consumed (disposable), there is an economical problem. In addition, if large amounts of nitrogen are released into the ground or into the air, the oxygen concentration at the construction site may drop.

Therefore, as a ground freezing system that has good thermal efficiency of the refrigerant and allows the ground to be frozen and excavated without releasing the gas phase refrigerant underground or into the air, for example, A flat-shaped refrigerant pipe for refrigerant circulation is arranged, and the space between the freezing pipe and the refrigerant pipe is filled with a heat transfer fluid such as tap water. A device that circulates a refrigerant is known (for example, see Patent Document 1).

The technology for circulating a refrigerant such as liquefied carbon dioxide through the microchannels in the flat-shaped refrigerant piping described above is based on the conventional method of ground freezing using sensible heat that circulates an aqueous solution of calcium chloride (brine) as a refrigerant. Compared to other technologies, it uses latent heat of vaporization, so the circulation flow rate can be reduced, and the refrigerant temperature can also be lowered to around -45°C, compared to around -30°C for brine.

The flat-shaped refrigerant piping for refrigerant circulation, which contains microchannels, can withstand high internal pressure due to the circulation of refrigerants such as liquefied carbon dioxide, and has a uniform cross-section for frozen pipe lengths of several tens of meters. , manufactured using extrusion technology such as aluminum.

This refrigerant piping has extremely low bending rigidity in the pipe axis direction, so it is difficult to form a borehole directly in the ground and insert it. Therefore, a steel freezing pipe is inserted into the borehole, and multiple refrigerant pipes are inserted inside it. It becomes.

Japanese Patent Application Publication No. 2016-118024

In order to increase the cooling capacity in the above-mentioned ground freezing system, for example, it is possible to increase the number of refrigerant pipes placed inside the freezing pipe, but it is not possible to significantly increase the number of refrigerant pipes with the limited inner diameter of the freezing pipe. This is not easy, and it has not always been easy to significantly increase cooling capacity even by increasing the number of refrigerant pipes.

The present invention has been made in view of the above points, and an object of the present invention is to easily increase the cooling capacity of a ground freezing system.

In order to achieve the above objectives,
The present invention
A ground freezing system that freezes the ground,
Frozen pipes buried in the ground,
at least one pair of flat-shaped refrigerant pipes for cooling circulation disposed oppositely within the freezing tube; and side surfaces of the refrigerant piping that are in contact with or close to mutually opposing surfaces of the refrigerant piping, and contacting or close to the inner surface of the freezing tube. a thermally conductive member having adjacent cryotube sides;
a heat transfer fluid filled in the freezing tube;
It is characterized by having the following.

As a result, the inner surface of the refrigerant pipe and the area on the inner surface of the freezing tube where the refrigerant pipe is not located are separated by a heat conductive member made of a material with high thermal conductivity such as copper or aluminum. By providing and connecting the refrigerant pipes in such a manner, the contribution of the inner surface of the refrigerant pipe to the cooling effect can be increased, and the cooling capacity can be easily increased.

According to the present invention, the cooling capacity of the ground freezing system can be easily increased.

It is a block diagram showing a schematic structure of a ground freezing system. FIG. 3 is a cross-sectional view showing a state in which refrigerant piping is provided within the freezing tube. FIG. 3 is a perspective view showing a detailed configuration of refrigerant piping. It is an explanatory view showing a process of inserting a refrigerant pipe into a freezing pipe. It is a graph showing an example of cooling capacity of a ground freezing system. FIG. 7 is a cross-sectional view showing a modification of the heat conductive member. It is an explanatory diagram showing an example of use of a ground freezing system.

Hereinafter, embodiments of the present invention will be described in detail based on the drawings.

(Schematic configuration of ground freezing system)
In the ground freezing system, as shown in FIG. 1, a freezing pipe 101 (casing) is buried in order to freeze the ground G. A flat refrigerant pipe 201 for refrigerant circulation is arranged in the freezing tube 101 . The secondary refrigerant circulating in the refrigerant pipe 201 is liquefied carbon dioxide, and the liquefied carbon dioxide supplied from the ground exchanges heat with the ground G, so that the ground G is frozen by the latent heat of vaporization and sensible heat of the liquefied carbon dioxide. It has become.

The ground freezing system includes a cooling device 110 that cools liquefied carbon dioxide and supplies it to the freezing pipe 101, and a refrigerant circulation pump 114. The cooling device 110 includes a liquefier 111, a condenser 112, and a cooling tower 113. The partially gasified liquefied carbon dioxide that exchanged heat with the ground G while circulating in the refrigerant pipe 201 exchanges heat with the primary refrigerant in the liquefier 111, becomes low-temperature liquefied carbon dioxide, and returns to the refrigerant pipe. 201 and circulated. The primary refrigerant of the cooling device 110 is, for example, refrigerant R717 (ammonia), and the secondary refrigerant, liquefied carbon dioxide, is evaporated and vaporized by the heat supplied from the ground G, and exchanges heat with water in the condenser 112. It cools down and liquefies. The water, which has been heated by the heat of vaporization of the primary refrigerant (such as refrigerant R717) in the condenser 112, is cooled in the cooling tower 113.

(Configuration inside freezing tube 101)
As shown in FIG. 2, a pair of refrigerant pipes 201 are provided in the freezing tube 101 so as to be parallel to each other and facing each other in a cross section. The refrigerant pipe 201 is made of aluminum, for example, and has a microchannel structure in which a plurality of minute refrigerant channels 202 are provided, as shown in FIG. 3, through which the refrigerant flows. A heat conductive member 301 is provided between the pair of refrigerant pipes 201 . More specifically, the heat conduction member 301 has a rectangular outer shape surrounded by a refrigerant pipe side surface 301a that contacts or is close to the inner surface 201a of the refrigerant pipe 201, and a freezing tube side surface 301b that contacts or is close to the inner surface of the freezing tube 101. It has a cross-sectional shape. The thermally conductive member 301 is made of aluminum, for example, but is not limited to this, and may also be made of a material with excellent thermal conductivity such as copper, duralumin, tungsten, steel, gold, or silver. The internal space of the freezing tube 101 is filled with a heat transfer fluid such as water. When the refrigerant circulates through the refrigerant pipe 201, that is, when freezing starts, the filled water changes from liquid water to solid ice that easily conducts heat, thereby transmitting the cold heat of the refrigerant pipe 201.

By the way, the specific heat of ice relative to copper and aluminum is about 4.8 times (vs. copper) and 2.3 times (vs. aluminum), respectively, and the thermal conductivity of ice is approximately 1/170 (vs. copper) and 1/1, respectively. 100 (relative to aluminum), and the thermal properties of ice in the freezing tube 101 are replaced by the thermal properties of the high thermal conductivity of the heat conducting member 301.

(Installation of refrigerant piping 201 and heat conduction member 301)
The refrigerant piping 201 and the heat conduction member 301 can be installed in the freezing pipe 101, for example, as follows.

When the shape is flat and the material is aluminum, as shown above, it is relatively easy to bend and stretch. The refrigerant piping 201 of the corresponding length is rolled up at a factory or the like, transported to the site, and stretched into a straight line while sandwiching the heat conductive member 301. If necessary, the refrigerant piping 201 is bundled with a binding member, etc. The cryotube 101 can be inserted into the cryotube 101 embedded in the cryotube.

The refrigerant pipe 201 must be a continuous body because a liquid refrigerant flows therethrough, but the heat conductive member 301 only needs to be tightly sandwiched between the refrigerant pipes 201, so it does not need to be a continuous body and can be It suffices if it is divided into lengths that are easy to transport and insert into the freezing pipe 101, and connected discontinuously to the length of the refrigerant pipe 201.

(Cooling effect of ground freezing system)
When the heat conductive member 301 is provided sandwiched between a pair of refrigerant pipes 201 as described above, for example, as shown in FIG. The same level of cooling effect could be obtained. That is, compared to the case where the heat conduction member 301 is not provided between the pair of refrigerant pipes 201, the cooling rate becomes faster, and the number of days required for the ground to freeze and drop to a predetermined temperature can be shortened. . Here, FIG. 5 shows the temperature change at the center point between the frozen tubes when the frozen tubes with a diameter of 100 mm are arranged in rows at a pitch of 800 mm on sandy soil, as determined by unsteady heat conduction analysis.

As described above, the inner surface 201a of the refrigerant pipe 201 and the area on the inner surface of the freezing tube 101 where (the outer surface side of) the refrigerant pipe 201 is not located are provided with the heat conductive member 301 interposed therebetween. By connecting the inner surfaces 201a of the refrigerant pipes 201, the contribution to the cooling effect can be easily increased. Here, in order to increase the cooling effect as described above, even if the refrigerant pipe 201 and the heat conduction member 301 and the inner surface of the freezing tube 101 do not necessarily come into close contact with each other, the heat conduction member 301 itself must If the heat conduction effect is large, the cooling effect can be easily increased.

(Modified example)
The cross-sectional shape of the heat conductive member 301 is not limited to the solid rectangular shape as described above, but can be formed into various cross-sectional shapes as shown in FIG. 6, for example, to reduce weight and materials. That is, as shown in FIG. 6A, for example, even if the refrigerant pipe side surface 301a and the freezing tube side surface 301b of the heat conduction member 301 have a cross-sectional shape connected through a somewhat thin heat conduction part 301c. good. Further, as shown in FIG. 6(b), the refrigerant pipe side surface 301a and the freezing tube side surface 301b of the heat conduction member 301 are connected via a square heat conduction part 301c having a cavity 301d. Good too. When such a hollow portion 301d is provided, it is possible to easily secure a space for accommodating a heating wire for defrosting. Further, as shown in FIG. 6(c), the refrigerant pipe side surface 301a and the freezing tube side surface 301b may be directly connected at a corner to form a cross-sectional shape to form a cavity 301d. Moreover, in these cross-sectional shapes, as shown in FIG. 6(d), the opposing side of the cryotube side surface 301b may be formed into an arcuate surface 301e. Moreover, in these cross-sectional shapes, as shown in FIG. 6(e), the refrigerant pipe side surface 301a may have a cross-sectional shape in which a recess 301f into which the refrigerant pipe 201 fits is formed. In this case, assembly workability can be easily improved.

In addition, the heat conductive member 301 is not limited to the so-called shaped material such as aluminum as described above, but also small-diameter round chips produced during the manufacturing of punching metal, scraps of aluminum material, and recycled solidified materials. Alternatively, the gap may be filled with water or the like. Such a heat conductive member 301 is effective in cases where it is not necessary to remove the frozen pipe 101 from the ground after the freezing work is completed, and where there is no need to use only water as a conventional filler. .

(Example of use of ground freezing system)
The ground freezing system as described above is not limited to freezing the ground by embedding the freezing pipe 101 in a vertical hole, but can also be used, for example, for tunnel excavation using the shield construction method, as shown in FIG. That is, by extending the freezing tube 101 in a conical shape around the shield machine 401 from an already excavated part, it is possible to easily proceed with excavation while freezing the excavation tip.

In other words, it is not limited to vertical freezing pipes, but any freezing pipe that has this ground freezing system, such as the radial freezing pipe shown in Figure 7 or the horizontal freezing pipe not shown, is not limited to its installation direction. can be applied not only to straight lines but also to curved lines.

101 Freezing tube 110 Cooling device 111 Liquefier 112 Condenser 113 Cooling tower 114 Refrigerant circulation pump 201 Refrigerant piping 201a Inner surface 202 Micro refrigerant channel 301 Heat conduction member 301a Refrigerant piping side 301b Freezing tube side 301c Heat conduction part 301d Cavity part 301e Arc-shaped surface 301f recess 401 shield machine

Claims (7)

A ground freezing system that freezes the ground,
Frozen pipes buried in the ground,
at least one pair of flat-shaped refrigerant pipes for cooling circulation disposed oppositely within the freezing tube; and side surfaces of the refrigerant piping that are in contact with or close to mutually opposing surfaces of the refrigerant piping, and contacting or close to the inner surface of the freezing tube. a thermally conductive member having adjacent cryotube sides;
a heat transfer fluid filled in the freezing tube;
A ground freezing system characterized by: The ground freezing system according to claim 1,
A ground freezing system characterized in that the heat conductive member has a cross-sectional shape having a rectangular outer shape surrounded by a side surface of the refrigerant pipe and a side surface of the freezing tube. The ground freezing system according to claim 1,
A ground freezing system, wherein the heat conduction member has a cross-sectional shape in which the refrigerant pipe side surface and the freezing tube side surface are connected via a heat conduction part. The ground freezing system according to claim 3,
A ground freezing system characterized in that the heat conductive member has a cross-sectional shape having a cavity inside the heat conductive part. The ground freezing system according to claim 1,
A ground freezing system characterized in that a side surface of the freezing tube of the heat conductive member has an arcuate surface corresponding to an inner surface of the freezing tube. The ground freezing system according to claim 1,
A ground freezing system characterized in that a side surface of the refrigerant pipe of the heat conductive member has a cross-sectional shape in which a recess into which the refrigerant pipe fits is formed. The ground freezing system according to claim 1,
The refrigerant pipe has a microchannel structure in which a plurality of micro refrigerant channels are formed,
A ground freezing system characterized in that the refrigerant flowing through the micro refrigerant flow path is liquefied carbon dioxide.