Refrigeration – Storage of solidified or liquified gas – With conservation of cryogen by reduction of vapor to liquid...
Reexamination Certificate
2002-05-31
2003-10-21
Capossela, Ronald (Department: 3744)
Refrigeration
Storage of solidified or liquified gas
With conservation of cryogen by reduction of vapor to liquid...
C062S048100
Reexamination Certificate
active
06634178
ABSTRACT:
The invention relates to a method to regulate the pressure inside a cryogenic tank containing cold gas when a main stream for a consumer is withdrawn, whereby heated gas in the form of a heating gas secondary stream, which is regulated as a function of the tank pressure, is fed through the cryogenic tank via a pipe.
Moreover, the invention relates to a device to regulate the pressure inside a cryogenic tank when at least one main stream for a consumer is withdrawn, said device having a thermally insulated reservoir to hold a cold gas that is connected to a withdrawal line that serves to withdraw a gas stream, having a device to heat up the gas stream, having a pipe that runs through the reservoir, and having a pressure-control means with which the gas stream in the form of a heating gas secondary stream, which is regulated as a function of the tank pressure, is fed through the cryogenic tank via the pipe.
A cryogenic tank is a heat-insulated reservoir that holds liquid, deep-frozen gases. The term “cold gas” as used here and below also refers to liquefied gases. Such cryogenic tanks are employed, for example, in automotive technology as fuel tanks. They can contain liquid hydrogen, liquid natural gas, liquid nitrogen or the like. Hydrogen, in particular, is an especially environmentally friendly fuel since only water vapor is formed when it is burned. This is why great importance is ascribed to hydrogen when it comes to future-oriented automobile concepts. In order to be liquefied, hydrogen is cooled down to a temperature below −253° C. [−423.4° F.] and then stored in liquid form at a slight excess pressure in such a cryogenic tank. The pressure in the cryogenic tank drops when gas or liquefied gas is withdrawn. However, in many applications such as, for instance, the above-mentioned use as a fuel tank, the pressure in the tank has to be kept as constant as possible.
For purposes of regulating the pressure when a main stream for a consumer is withdrawn from a cryogenic tank, DE-A 196 45 492 proposes a method and a device of the above-mentioned type in the form of a so-called “intrinsic gas convector” whereby, in order to maintain or increase the pressure, heated gas in the form of a “heating gas secondary stream” is fed through a pipe that runs through the reservoir. The heat that is introduced into the cryogenic tank in this process promotes the evaporation of liquefied gas, thus increasing the tank pressure. The pressure drop in the reservoir caused by the withdrawal of the main stream can thus be equalized once again by a controlled circulation of the hot secondary stream. In the prior-art device, the heating gas secondary stream is conveyed in a closed circulation system and the throughput is regulated by a control valve as a function of the pressure in the tank. Even though the throughput of the requisite secondary stream is at a certain ratio relative to the throughput of the main stream conveyed to the consumer, it can vary over a wide range, depending on the design and operating conditions of the cryogenic tank and on the requirements of the consumer. The problem also arises that, depending on the quantity consumed, the main stream has to be throttled to different degrees, although the pressure drop between the reservoir and the consumer should remain as constant as possible. With the prior-art device and with the prior-art regulation methods, these requirements can only be met with highly complicated control technology and equipment.
The invention has the objective of creating a simple and safe operating method to regulate the main stream and the secondary stream while also providing a cost-efficient device for this purpose.
As far as the method is concerned, this objective is achieved according to the invention on the basis of the prior-art method in that the secondary stream and the main stream are fed to a pressure-control means equipped with a first adjustable throttle for the secondary stream and with a second adjustable throttle for the main stream, whereby the throttles are coupled to each other in such a way that they can be opened and closed in opposite directions as a function of the pressure in the tank.
The main stream is conveyed to a consumer. The secondary stream in the form of a heating gas stream flows through the cryogenic tank in a pipe, thus serving to raise the pressure in the tank by introducing heat. In order to regulate the throughputs of the main stream and of the secondary stream, each stream is provided with an adjustable throttle by means of which the minimum free flow cross section for the main stream and for the secondary stream can be adjusted. The two throttles are coupled to each other in such a manner that an enlargement in the flow cross section of one throttle is associated with a reduction in the flow cross section of the other throttle and vice-versa. The secondary stream is adjusted by means of the first throttle as a function of the pressure in the tank. Since the first throttle is coupled to the second throttle for the main stream, any change in the free flow cross section of the first throttle simultaneously influences the free flow cross section for the main stream. In this manner, the method according to the invention allows a very simple regulation of the main and secondary streams while the pressure in the tank is kept largely constant by a single pressure-control means. If the pressure in the tank is too low, the throttle for the secondary stream is opened or opened further and, simultaneously, the throttle for the main stream is cut back. By increasing the throughput for the secondary stream, a greater quantity of heat is introduced into the cryogenic tank, thus causing the pressure to rise. This pressure rise, in turn, causes the first throttle to close, thus correspondingly reducing the secondary stream while, at the same time, enlarging the flow cross section for the main stream.
It turned out to be particularly simple to employ an approach in which a primary gas stream from which the main stream and the secondary stream are branched off is withdrawn from the cryogenic tank. The main stream and the secondary stream are branched off from a shared primary gas stream. As a consequence, only one withdrawal line is needed. The throughputs for the main stream and for the secondary stream are regulated by the pressure-control means which, seen in the direction of flow of the primary gas stream, is arranged downstream from the branch-off site.
It has proven to be advantageous to employ a three-way pressure regulator as the pressure-control means and to feed the main stream to a first gas inlet and the secondary stream to a second gas inlet of the three-way pressure regulator. Accordingly, the three-way pressure regulator is fitted with two gas inlets and one gas outlet in the direction of the consumer, whereby the main stream and the secondary stream are conveyed to the three-way pressure regulator separately from each other. If there is a pressure gradient between the pressure tank and the three-way pressure regulator, this approach makes it possible to convey both gas streams in the direction of the pressure gradient so that auxiliary means such as, for example, blowers or pumps, are not needed in order to generate the gas flow. Such a pressure gradient can be created, for instance, by heating up the cold or liquefied gas contained in the cryogenic tank. The heating causes the gas density to decrease so that the volume and the tank pressure increase correspondingly.
Advantageously, the secondary stream is fed through the cryogenic tank, it is subsequently re-heated and then conveyed to the second gas inlet of the pressure-control means. The secondary stream cooled off in the cryogenic tank can be heated up, for example, in a heat exchanger that is kept at room temperature without the need for additional heating material. Heating up the tank contents by means of the secondary stream generates a pressure gradient so that the secondary stream flows in the direction of the pressure-control means without a
Koser Heinz
Michel Friedel
Schmitt Rainer
Capossela Ronald
Connolly Bove Lodge & Hutz
Messer Griesheim GmbH
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