Background Art In general, the multiple tank cryogenic reservoir is used for the purpose of storing a plurality of liquefied gases which are used simultaneously to produce a mixed gas or used separately at the same place. To achieve the same purpose, a plurality of single cryogenic reservoirs, of the number corresponding to the kinds of liquefied gases to be used, may be used. However, with respect to space-saving and convenience of operation, the multiple tank cryogenic reservoir is more advantageous.

For this reason, conventionally, various kinds of multiple tank cryogenic reservoirs have been proposed (e. g., Jpn. Pat. Appln. KOKAI Publication No. 7-151300

and Utility Model Registration No. 3003616).

Generally, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-151300, the conventional multiple tank cryogenic reservoir is of a type for storing liquefied gases of about the same temperature in the inner tanks. Therefore, since the amount of heat transferred between the inner tanks is small, detrimental effects of the heat transfer have not become evident. Accordingly, techniques for avoiding the detrimental effects have not been fully developed.

In recent years, there has been an increasing demand for multiple tank cryogenic reservoirs for storing liquefied gases having greatly different temperatures. As an example of the multiple tank cryogenic reservoirs of this type, there is a reservoir for use in equipment for supplying a mixed gas containing carbon dioxide as a component, e. g., a welding mixed gas or foodpackage-filling mixed gas.

As in the case of carbon dioxide, if the solid- liquid equilibria line (or solid-gas equilibria line) in an equilibrium diagram is close to storing conditions of the liquefied gas (the storage pressure and the storage temperature), the liquefied gas can be easily frozen (converted to dry ice) as it is cooled. Therefore, from the viewpoint of prevention of accidents, it was difficult to store carbon dioxide in the conventional multiple tank cryogenic reservoir

together with another cryogenic liquefied gas.

For example, liquefied argon gas, a component of a mixed gas, is preserved in cryogenic temperature (-186°C), whereas liquefied carbon dioxide is stored at about-30°C. Thus, there is a very large difference in temperature. Therefore, heat is transferred from the inner tank storing the liquefied carbon dioxide to the inner tank storing the liquefied argon gas, resulting in that the liquefied carbon dioxide is cooled and tends to partially freeze. If the liquefied carbon dioxide partially freezes, the inner tank or an appended pipe may be clogged. This may result in unexpected increase in pressure or expansion, interference with the operation of the apparatus, or an adverse effect on the durability of the apparatus.

To cope with the above problems, a method is considered, in which the thermal insulation between the inner tanks is enhanced to prevent liquefied carbon dioxide from freezing. To enhance the thermal insulation, generally, the interior of the outer vessel is filled with a granular heat-insulating material such as pearlite for interrupting radiation of heat, and air is evacuated. In this case, if the thermal insulation is to be enhanced by, for example, increasing the degree of vacuum, the manufacturing cost and the apparatus size will be increased. As a more simple method, the distance between the inner tanks may be

lengthened. However, since this method enlarges the apparatus, it is contradictory. to the object of employing the double reservoir, i. e., saving in space.

Therefore, it is not considered to be an effective measure.

Further, to ensure safety, it is necessary to prepare a measure in preparation for the case where the degree of vacuum in the outer vessel is lowered. In the method of increasing the degree of vacuum, if the degree of vacuum is lowered, the problems will be more serious. The method of increasing the distance between inner tanks is not practical.

Disclosure of Invention An object of the present invention is to provide a multiple tank cryogenic reservoir comprising a plurality of inner tanks, which can store liquefied gases of different temperatures without increasing the distance between the inner tanks, accordingly without increasing the size of the outer vessel. Another object of the present invention is to provide a mixed gas supplying apparatus comprising the multiple tank cryogenic reservoir as mentioned above.

A multiple tank cryogenic reservoir of the present invention comprises: a plurality of inner tanks respectively storing liquefied gases of different temperatures; an outer vessel containing the inner tanks and

forming a vacuum for a purpose of heat insulation between an inner surface thereof and outer surfaces of the inner tanks; and a partition plate interposed between adjacent inner tanks, fixed to the inner surface of the outer vessel and made of a heat conductive material.

With the multiple tank cryogenic reservoir of the present invention, heat flows to the partition plate from the wall material constituting the outer vessel.

Therefore, heat is not carried away from the liquefied gas of a higher temperature stored in one of the inner tanks to the liquefied gas of a lower temperature stored in another. Consequently, the former liquefied gas is prevented from being cooled and frozen.

With the multiple tank cryogenic reservoir of the present invention, even when liquefied gases of different temperatures are stored in the inner tanks, it is unnecessary to lengthen the distance between the inner tanks. Therefore, the size of the outer vessel can be small.

The partition plate is, for example, a metal plate having an opening through which pipes are passed.

It is preferable that the space between the inner surface of the outer vessel and the outer surfaces of the inner tanks be filled with a granular heat- insulating material made of an inorganic material.

In a case where the inner tanks of the multiple

tank cryogenic reservoir of the present invention respectively store liquefied carbon dioxide and liquefied gas having a temperature lower than that of the liquefied carbon dioxide, the inner tank of the liquefied carbon dioxide is placed under the other inner tank.

When the multiple tank cryogenic reservoir of the present invention is applied to a mixed gas supplying apparatus, the apparatus comprises: a plurality of inner tanks respectively storing liquefied gases of different temperatures; an outer vessel containing the inner tanks and forming a vacuum for a purpose of heat insulation between an inner surface thereof and outer surfaces of the inner tanks; a partition plate interposed between adjacent inner tanks, fixed to the inner surface of the outer vessel and made of a heat conductive material; evaporators, corresponding to the respective inner tanks and arranged outside the outer vessel, for receiving and evaporating the liquefied gases supplied from the inner tanks; and a mixer for mixing gases generated by the evaporators, thereby producing a mixed gas.

Brief Description of Drawings FIG. 1A is a schematic diagram showing a state of heat flows in the conventional multiple tank cryogenic

reservoir; FIG. 1B is a schematic diagram showing a state of heat flows in a multiple tank cryogenic reservoir according to the present invention; FIG. 1C is a schematic diagram showing a state of heat flows in a multiple tank cryogenic reservoir according to the present invention; FIG. 2 is a diagram showing a multiple tank cryogenic reservoir according to the present invention; FIG. 3 is a diagram showing a partition plate constituting a part of the multiple tank cryogenic reservoir according to the present invention; and FIG. 4 is a diagram showing a mixed gas supplying apparatus according to the present invention.

Best Mode for Carrying Out the Invention First, a state of heat flows, produced in a multiple tank cryogenic reservoir of the present invention, will be described with reference to FIGS. 1A to 1C.

FIG. 1A shows a state of heat flows in the conventional multiple tank cryogenic reservoir. It is assumed that an inner tank 3 on the lower side stores liquefied gas having a relatively high temperature, and an inner tank 2 on the higher side stores liquefied gas having a relatively low temperature. Air in the space between an outer vessel 1 and the inner tanks 2 and 3 is evacuated for the purpose of heat insulation.

However, a little heat is transferred from the outer vessel 1 to the inner tanks 2 and 3 via a granular heat-insulating material (not shown) stuffed in the space to interrupt convection and radiation heat.

In addition to this heat transfer, if there is no partition plate between the two inner tanks 2 and 3 as shown in FIG. 1A, heat is transferred from the inner tank 3 on the high temperature side (hereinafter referred to as the second inner tank) to the inner tank 2 on the low temperature side (hereinafter referred to as the first inner tank) (the arrow A1).

In contrast, as shown in FIG. 1B, if a partition plate 4 made of a heat conductive material is provided between the two inner tanks 2 and 3 and fixed to the inner surface of the outer vessel 1, heat flows from the wall material constituting the outer vessel 1 to the interior of the outer vessel 1 through the partition plate 4 (the arrow A2). The temperature at a position P1 where the partition plate 4 is located is higher than that in the case shown in FIG. 1 due to the heat flow. Accordingly, the difference in temperature between the position P1 and the second inner tank 3 is reduced. As a result, the heat flow from the second inner tank 3 is reduced. If the aforementioned difference in temperature is zero, no heat flows from the second inner tank 3. In this case, therefore, only heat flow from the partition plate 4 to the first inner

tank 2 is generated (the arrow A3). FIG. 1B shows this state. If the temperature at the position Pi is further increased, heat flows from the partition plate 4 to both the first and second inner tanks 2 and 3, as shown in FIG. 1C (the arrows A3 and A4).

If the degree of vacuum in the outer vessel 1 is lowered, since heat flows from the wall material of the outer vessel 1 to the interior of the outer vessel 1 via the partition plate 4, the second inner tank 3 is prevented from being excessively cooled.

More specifically, if the vacuum condition inside the outer vessel 1 is spoiled for some reason, the heat conductivity in the space inside the outer vessel 1 is about 10 times that in the case of the vacuum condition (for example, 0.1 Torr or less). Therefore, in the case where no partition plate 4 is provided as shown in FIG. 1A, the heat flow from the second inner tank 3 to the first inner tank 2 (the arrow Al) is so increased that it cannot be compensated for by the heat flow from the outer vessel 1 to the second inner tank 3.

For this reason, it is highly possible that the liquefied gas in the second inner tank 3 will be frozen. On the other hand, in the case where the partition plate 4 exists between the two inner tanks 2 and 3 and the heat flows as shown in FIG. 1C are generated, even if the degree of vacuum is lowered, heat is supplied to the first inner tank 2 via the

partition plate 4. Therefore, even if the temperature of the position P1 is lowered, the condition inside the outer vessel 1 merely approaches the condition shown in FIG. 1B, and there is no possibility that the second inner tank 3 is excessively cooled.

In the structure descried above, the partition plate 4 made of the heat conductive material is interposed between the inner tanks 2 and 3 adjacent to each other. With this structure, the problem involved in the heat flow from the second inner tank 3 to the first inner tank 2 can be solved without complicating the structure or adding supplementary equipment.

Moreover, since it is unnecessary to lengthen the distance between the inner tanks, the size of the outer vessel need not be increased.

For example, the partition plate 4 is a metal plate and fixed to the inner surface of the outer vessel 1. The increase in manufacturing cost required to add the partition plate 4 is very small. Generally, the multiple tank cryogenic reservoir has pipes inside the outer vessel 1 to keep the heat insulating property and make the structure simple. In this case, an opening to allow passage of the pipes is formed in the partition plate 4. If a gap is formed between the inner periphery of the opening and the pipes, heat conduction via a contact therebetween can be prevented.

If a granular heat-insulating material made of

an inorganic material (for example, pearlite having a grain size of about 1.2 to 0.15 mm) is used to block heat radiation inside the outer vessel 1, the interior of the outer vessel 1 can be filled with the granular heat-insulating material through the opening.

Alternatively, the partition plate 4 may have another opening exclusive for introducing the granular heat- insulating material.

The multiple tank cryogenic reservoir of the present invention is particularly effective in a case where it stores liquefied carbon dioxide along with a liquefied gas of a temperature lower than that.

As described before, since the solid-liquid equilibria line of liquefied carbon dioxide is close to storing conditions (the storage pressure and the storage temperature), the liquefied gas can be easily frozen as it is cooled. According to the multiple tank cryogenic reservoir of the present invention, liquefied carbon dioxide can be effectively prevented from being frozen.

In this case, it is preferable that the inner tank storing liquefied carbon dioxide be located on the lower side for the following reason. Generally, pipes are drawn from a portion near the bottom of the outer vessel to the outside. Therefore, if the inner tank storing liquefied carbon dioxide is located on the lower side, a pipe for extracting the liquefied carbon dioxide can be easily drawn out of the outer vessel

without passing by the other inner tank which stores liquefied gas of a lower temperature. As a result, it is possible to eliminate a possibility of the liquefied carbon dioxide being frozen in the pipe.

(Embodiment 1) FIG. 2 is a diagram showing a multiple tank cryogenic reservoir according to the present invention.

The two inner tanks 2 and 3 are arranged above and below in the vertical direction within the outer vessel 1. The first inner tank 2, located on the upper side, stores liquefied argon gas (about-186°C).

The second inner tank 3, located on the lower side, stores liquefied carbon dioxide (about-30°C). The air in the outer vessel 1 is evacuated for the purpose of heat insulation. Evacuation is carried out by exhausting air from the outer vessel 1 through an evacuating pipe (not shown). The degree of vacuum of the inside is measured by a vacuum gage (not shown) and regulated to a predetermined pressure (for example, 0.1 Torr or lower). Further, the space between the outer vessel 1 and the first and second inner tanks 2 and 3 is filled with the granular heat-insulating material (not shown) made of pearlite in order to block radiation heat. The granular heat-insulating material is introduced through a filling port 5 formed in a top portion of the outer vessel 1. The outer vessel 1 is fixed to the floor surface by supporting legs 11.

Filling pipes 6a and 6b for filling the tanks with liquefied gases (or guiding the liquefied gases to a pressure regulating circuit), pipes 7a and 7b for extracting the liquefied gases from the tanks, and gas introducing pipes 8a and 8b for introducing gases to top portions of the inner tanks 2 and 3, are connected to the first and second inner tanks 2 and 3, respectively. Vent pipes (not shown) communicating with the atmosphere are connected to the top portions of the respective inner tanks. Level indicator pipes (not shown) connected to a level indicator are connected to top and bottom portions of each inner tank. The inner tanks 2 and 3 are supported by supporting members 9a and 9b, having heat insulating properties, to the inner circumferential surface of the outer vessel 1.

The pipes connected to the first inner tank 2 and the pipes connected to the second inner tank 3 are placed on opposite sides of the central axis within the outer vessel 1. Thus, an adverse influence of heat transfer between the pipes of the tanks is prevented.

If the second inner tank 3 storing the liquefied carbon dioxide has a less volume than that of the first inner tank 2 because of the composition of the mixed gas to be produced, the second inner tank 3 may have a smaller outer diameter corresponding to the volume. In this case, the heat transfer between the second inner tank 3

and the pipes connected to the first inner tank 2 can be reduced accordingly.

In this embodiment, the temperature Tf of the liquefied argon gas stored in the first inner tank 2 is about-186°C and the temperature Th of the liquefied carbon dioxide stored in the second inner tank 3 is about-30°C. In the following description, the average of these temperatures is referred to as Tav.

In this embodiment, the partitioning wall 4 made of a metal plate is arranged at an intermediate position between the second inner tank 3 and the first inner tank 2 adjacent to each other. The partition plate 4 is fixed to the inner surface of the wall material constituting the outer vessel 1, so that heat is transferred to the interior of the outer vessel 1 through the partition plate 4.

FIG. 3 is a plan view of the partition plate 4.

An opening 4a is formed in proximity to a peripheral portion of the partition plate 4. The filling pipe 6a, the pipe 7a and the gas introducing pipe 8a are passed through the opening 4a. Besides the opening 4a, three openings 4b are provided at three positions spaced 90° apart in the circumferential direction of the partition plate 4 to allow passage of the granular heat-insulating material. Further, a gap between the inner periphery of the opening 4a and the pipes (6a, 7a and 8a) is maintained to allow passage of the heat

insulating material. Therefore, the granular heat- insulating material can be supplied from the upper side of the partitioning wall 4 to the lower side through the gap and the openings 4b, so that the space around the second inner tank 3 on the lower side can be filled with the granular heat-insulating material.

A supporting ring 10 made of metal is welded to the inner circumferential surface of the outer vessel 1. The partition plate 4 is fixed on the supporting ring 10 and supported such that it is substantially inscribed in the inner circumferential surface. With this structure of supporting the partition plate 4, heat is uniformly conducted through the overall periphery of the partition plate 4 via the supporting ring 10. The partition plate 4 may be simply placed on the supporting ring 10, but preferably fixed to the ring by bolts.

The distance between the top of the second inner tank 3 on the high temperature side and the partition plate 4 and the distance between the bottom of the first inner tank 2 on the low temperature side and the partition plate 4 are determined by weighing the merit that the heat flow from the outer vessel 1 through the partition plate 4 prevents the second inner tank 3 from being cooled with the demerit that the first inner tank 2 is heated by the heat flow. If there is a sufficient distance between the partitioning wall 4

and the bottom of the first inner tank 2 (for example, the same distance as that between the inner circumferential surface of the outer vessel 1 and the outer circumferential surface of the first inner tank 2), the amount of heat transferred between the partitioning wall 4 and the bottom of the first inner tank 2 can reach an insignificant level. The same applies to the distance between the partition plate 4 and the top of the second inner tank 3.

On the other hand, if the partition plate 4 is closer to the bottom of the first inner tank 2 on the low temperature side, the temperature Tc at the central portion of the partition plate 4 is gradually lowered, and the detriment of the heat flow from the partition plate 4 to the first inner tank 2 is increased. When Tc becomes lower than Tav (the average of the temperature Tf in the first inner tank 2 and the temperature Th in the second inner tank 3), the detriment of cooling due to the heat flow from the top of the second inner tank 3 on the high temperature side to the partition plate 4 becomes greater. In contrast, if the partition plate 4 is closer to the top of the second inner tank 3 on the high temperature side, the amount of heat transferred between the partitioning wall 4 and the top of the second inner tank 3 is increased. If Tc is higher than Th, no detriment due to cooling may occur. Therefore,

if the top of the second inner tank 3 and the bottom of the first inner tank 2 are arranged close to each other, it is preferable that the position of the partition plate 4 be determined to satisfy the condition"Th < Tc < Th + 50°C", depending on the heat conductivity, shape and thickness of the partition plate 4.

In the above description, deterioration of the degree of vacuum is not take into consideration.

To ensure safety in a case where the degree of vacuum is deteriorated, the partition plate is arranged as follows. If there is a distance between the partitioning wall 4 and the inner tanks 2 and 3 which corresponds to the distance between the inner circumferential surface of the outer vessel 1 and the outer circumferential surface of the first inner tank 2, the influence of heat flowing through the partitioning wall is little in the normal case (when the degree of vacuum is maintained). However, when the degree of vacuum is lowered, since convection occurs within the outer vessel 1, the heat flowing to the inner tanks through the partitioning wall 4 is increased. Therefore, the existence of the partitioning wall 4 is very effective. In this state, the value of Tc is considerably lowered.

In preparation for the case where the degree of vacuum is deteriorated, it is preferable that the

position of the partition plate 4 be determined to satisfy the condition"Th + 30°C < Tc < Th + 50°C", when the degree of vacuum is maintained.

In general, a metal plate is used as the partition plate 4. The amount of heat flowing to the inner tanks through the partition plate 4 can be adjusted by selecting the material or thickness of the metal plate.

In the above embodiment, to reduce the detriment that the first inner tank 2 is heated by the heat flowing through the partitioning wall 4, it is only necessary to reduce the thickness of the metal plate.

(Embodiment 2) FIG. 4 is a schematic diagram showing a mixed gas supplying apparatus according to the present invention.

The mixed gas supplying apparatus comprises a multiple tank cryogenic reservoir 40, evaporators 15 and 16, a mixer 19, etc. The multiple tank cryogenic reservoir 40 is the same as that shown in FIG. 2, comprising the outer vessel 1, the first inner tank 2, the second inner tank 3, the partitioning wall 4 and various supplementary equipment.

Liquefied argon gas is stored in the first inner tank 2 on the upper side, and liquefied carbon dioxide is stored in the second inner tank 3 on the lower side.

The liquefied argon gas is withdrawn through the pipe 7a from the first inner tank 2, supplied to the evaporator 15 via a valve 20 and evaporated therein.

The evaporated gas is introduced into the mixer 19 via a pressure reducing valve 17. On the other hand, liquefied carbon dioxide is withdrawn through the pipe 7b from the second inner tank 3, supplied to the evaporator 16 via a valve 21 and evaporated therein.

The evaporated gas is introduced into the mixer 19 via a pressure reducing valve 18.

A pressure regulating circuit for the first inner tank 2 comprises a valve 31, a pressurizing evaporator 32 and a pressure regulator 33. The liquefied argon gas is withdrawn from the bottom portion of the first inner tank 2 through the filling pipe 6a, supplied to the pressurizing evaporator 32 via the valve 31, and evaporated therein. Thereafter, the evaporated gas is returned to the top of the first inner tank 2 via the pressure regulator 33 and the gas introducing pipe 8a.

The pressure of the first inner tank 2 is regulated by the pressure regulator 33.

Similarly, a pressure regulating circuit for the second inner tank 3 comprises a valve 36, a pressurizing evaporator 37 and a pressure regulator 38. The liquefied carbon dioxide is withdrawn from the bottom portion of the second inner tank 3 through the filling pipe 6b, supplied to the pressurizing evaporator 37 via the valve 36, and evaporated therein. Thereafter, the evaporated gas is returned to the top of the second inner tank 3

via the pressure regulator 38 and the gas introducing pipe 8b. The pressure of the first inner tank 2 is regulated by the pressure regulator 33.

In the above embodiment, as shown in FIG. 3, a metal plate is used as the partition plate 4.

However, a metal net, a punching metal or an expanded metal may be used as the partition plate 4. If a material having a high rate of an opening area is used, the heat flowing through the partition plate 4 is reduced. Therefore, the amount of heat flow can be regulated in accordance with the rate of the opening area. At the same time, a granular heat-insulating material can be stuffed through the opening. It is particularly preferable that a punching metal net or a punching metal be used, since heat conductance is relatively uniform in this case.

Further, in the above embodiment, the present invention is applied to a multiple tank cryogenic reservoir comprising two inner tanks. However, the present invention can also be applied to a multiple tank cryogenic reservoir comprising three or more inner tanks. In this case, a partition plate may be arranged only at a position between adjacent inner tanks where the heat transfer due to the difference in temperature between the liquefied gases stored therein is particularly significant.

Furthermore, in the embodiment described above,

liquefied carbon dioxide is stored along with liquefied argon gas. However, also'in the case where liquefied carbon dioxide is stored along with liquid oxygen, or liquid nitrogen, instead of liquefied argon gas, the effect of the present invention can be obtained in the same manner.

In FIG. 2, various supplementary equipment is omitted from the multiple tank cryogenic reservoir.

However, if necessary, various valves, pressure regulators, evaporators, a mixer, a buffer tank, an analyzer, etc. may be provided, as shown in FIG. 4.