2010

Cryogenic Transport of High-Pressure-System Recharge Gas

Advantages include low pressure and high density during transport.

A method of relatively safe, compact, efficient recharging of a high-pressure room-temperature gas supply has been proposed. In this method, the gas would be liquefied at the source for transport as a cryogenic fluid at or slightly above atmospheric pressure. Upon reaching the destination, a simple heating/expansion process would be used to (1) convert the transported cryogenic fluid to the room-temperature, high-pressure gaseous form in which it is intended to be utilized and (2) transfer the resulting gas to the storage tank of the system to be recharged.

altIn conventional practice for recharging high-pressure-gas systems, gases are transported at room temperature in high-pressure tanks. For recharging a given system to a specified pressure, a transport tank must contain the recharge gas at a much higher pressure. At the destination, the transport tank is connected to the system storage tank to be recharged, and the pressures in the transport tank and the system storage tank are allowed to equalize. One major disadvantage of the conventional approach is that the high transport pressure poses a hazard. Another disadvantage is the waste of a significant amount of recharge gas. Because the transport tank is disconnected from the system storage tank when it is at the specified system recharge pressure, the transport tank still contains a significant amount of recharge gas (typically on the order of half of the amount transported) that cannot be used.

In the proposed method, the cryogenic fluid would be transported in a suitably thermally insulated tank that would be capable of withstanding the recharge pressure of the destination tank. The tank would be equipped with quick-disconnect fluid-transfer fittings and with a low-power electric heater (which would not be used during transport). In preparation for transport, a relief valve would be attached via one of the quick-disconnect fittings (see figure). During transport, the interior of the tank would be kept at a near-ambient pressure — far below the recharge pressure. As leakage of heat into the tank caused vaporization of the cryogenic fluid, the resulting gas would be vented through the relief valve, which would be set to maintain the pressure in the tank at the transport value. Inasmuch as the density of a cryogenic fluid at atmospheric pressure greatly exceeds that of the corresponding gas in a practical high-pressure tank at room temperature, a tank for transporting a given mass of gas according to the proposed method could be smaller (and, hence, less massive) than is a tank needed for transporting the same mass of gas according to the conventional method.

Upon arrival at the destination, the transport tank would be connected to the tank to be recharged via a transfer line that would include a second low-power electric heater. The relief valve would be disconnected and the line to the gas system opened, causing the pressure in the transport tank to rise to the system pressure. The transport tank and transfer-line electric heaters would be turned on, causing the contents of the tank to expand under high pressure and flow out through the transfer line. The transfer-line heater would further warm the flowing fluid to room temperature. The relative power levels of the electric heaters would be set to ensure that the fluid expelled from the tank by the tank heater could be delivered as room-temperature gas to the tank to be recharged. The transfer of gas would be complete once the remaining gas inside the transport tank had been heated to room temperature. By virtue of the difference between densities, at completion, the majority of the mass of the transported cryogenic fluid would have been converted to gas and transferred to the recharged tank.

This work was done by Eugene K. Ungar and Warren P. Ruemmele of Johnson Space Center and William Carl Bohannon of The Boeing Co. For more information, download the Technical Support Package (free white paper) at www.techbriefs.corn/tsp under the Physical Sciences category. MSC-24343-1

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