The invention relates generally to cryogenic fluid delivery systems, and, more particularly, to a cryogenic fluid delivery system that vaporizes a portion of a pumped cryogenic liquid stream and uses the vaporized cryogen to drive the system pump.
Cryogenic fluids, that is, fluids having a boiling point generally below xe2x88x92150xc2x0 F. at atmospheric pressure, are used in a variety of applications. For example, liquid natural gas (LNG) is an alternative fuel for vehicles that is growing in popularity. As another example, laboratories and industrial plants use nitrogen in both liquid and gas form for various processes.
Cryogenic fluids are typically stored as liquids that require pressurization and sometimes heating prior to usage. The liquid nitrogen stored by laboratories and industrial plants typically must be pressurized prior to use as a gas or liquid. In the case of LNG fueling stations, the LNG is typically dispensed to a vehicle in a saturated state with a pressure head that is sufficient to meet the demands of the vehicle""s engine. The saturated state of the LNG prevents the collapse of the pressure head while the vehicle is in motion. Alternatively, the LNG may be stored onboard a vehicle in an unconditioned state. The onboard LNG may then be pressurized and heated as it is provided to the vehicle engine.
Prior art cryogenic fluid delivery systems typically pressurize and transport the cryogen via pumps that are powered by electricity or mechanically with fuels such as gas or oil. As a result, these prior art systems have energy requirements that increase their cost of operation. In addition, the pumps of these systems introduce complexities which result in higher maintenance requirements and costs. The pumps are expensive and thus also increase the initial cost of the system.
Some prior art pumps are powered by a piston that is driven by pressurized gas or liquid. For example, U.S. Pat. No. 3,234,746 to Cope discloses a pump for transporting liquid carbon dioxide from a storage tank. The pump is powered by carbon dioxide vapor from the head space of the storage tank. The pump of the Cope ""746 patent features two pistons and corresponding cylinders with a common piston rod. Carbon dioxide vapor is provided to opposing sides of the driving cylinder in an alternating fashion so that the other piston is driven. As a result, the driven piston pumps the liquid carbon dioxide in the tank to a second tank or container. Carbon dioxide vapor exhaust from the driving cylinder is vented to the atmosphere.
While the pump of the Cope ""746 patent is inexpensive to operate, the transfer rate and discharge pressure that it may achieve is limited by the pressure that is available in the head space of the storage tank. In addition, the liquid carbon dioxide in the storage tank must be warmed for the pump to operate. Warming the liquid carbon dioxide, or any cryogenic liquid, reduces the hold time of the tank. The hold time of the tank is the length of time that the tank may hold the LNG without venting to relieve excessive pressure that builds as the LNG warms. The pump of the Cope ""746 patent also fails to provide a means for heating the liquid carbon dioxide as it is transferred.
Most prior art cryogenic fluid delivery systems use pumps that are of the centrifugal or xe2x80x9csingle-actingxe2x80x9d piston variety. Single-acting piston pumps have a single chamber in which an induction stroke of the piston is followed by a discharge stroke. A disadvantage of such pumps is that they have relatively low pump delivery rates which results in increased fueling times.
In response to the limitations in delivery rates of prior art pumps, the pump illustrated in U.S. Pat. No. 5,411,374 to Gram was developed. The Gram ""374 patent features a dual-acting piston arrangement that is similar to the pump of the Cope ""746 patent. The pump of the Gram ""374 patent, however, is powered by a hydraulic motor circuit which provides liquid to opposing sides of the driving piston in an alternating fashion. While the pump of the Gram ""374 overcomes the discharge pressure shortcomings of the pump of the Cope ""746 patent and the prior art, the hydraulic motor circuit increases production, operating and maintenance costs.
As stated previously, LNG is typically saturated and pressurized prior to introduction to a vehicle""s fuel tank. A common method of saturating the LNG is to heat it as it is stored in the delivery system storage tank. This is often accomplished by removing a quantity of the LNG from the tank, warming it (often with a heat exchanger) and returning it to the tank. Alternatively, the LNG may be heated to the desired saturation temperature and pressure through the introduction of warmed cryogenic gas into the tank.
Warming LNG in the delivery system tank, as described above with regard to the Cope ""746 patent, is undesirable as it reduces the hold time of the tank. Furthermore, refilling a tank when it contains saturated LNG requires specialized equipment and additional fill time. Warmed LNG also is less dense than cold LNG and thus reduces tank storage capacity. While these difficulties may be overcome by providing an interim transfer or conditioning tank, such a tanks have to be tailored in dimensions and capacities to specific use conditions. Such use conditions include the amount of fills and pressures expected. As a result, the variety of applications for such a delivery system are limited by the dimensions and capacities of the conditioning tank.
Another approach for saturating the LNG prior to delivery to the vehicle tank is to warm the liquid as it is transferred to the vehicle tank. Such an approach is known in the art as xe2x80x9cSaturation on the Flyxe2x80x9d and is illustrated in U.S. Pat. No. 5,787,940 to Bonn et al. wherein heating elements are provided to heat LNG as it is dispensed. A disadvantage of the system of the Bonn et al. ""940 patent, however, is that electricity is required to operate the heating elements. In addition, the system of the Bonn et al. ""940 patent employs a conventional pump and thus suffers from the initial system, operating and maintenance cost disadvantages described previously.
U.S. Pat. No. 5,687,776 to Forgash et al. and U.S. Pat. No. 5,771,946 to Kooy et al. also illustrate systems that dispense cryogenic fluid and perform saturation on the fly. The systems disclosed in these two patents use heat exchangers, and therefore ambient temperature, to warm the cryogen as it is transferred to vehicles. The systems, however, also use conventional pumps to dispense the cryogen.
Accordingly, it is an object of the present invention to provide a cryogenic fluid delivery system that uses a pump that is economical to produce, operate and maintain.
It is another object of the present invention to provide a cryogenic fluid delivery system that provides a high discharge pressure for rapid delivery of the cryogen.
It is still another object of the present invention to provide a cryogenic fluid delivery system that provides for economical saturation on the fly.
The cryogenic fluid delivery system of present invention includes a pump having a pumping cylinder that is divided by a pumping piston into first and second chambers, each of which includes an inlet and an outlet. First and second inlet check valves communicate with the inlets of the first and second pumping cylinder chambers, respectively. In addition, first and second outlet check valves communicate the outlets of the first and second pumping cylinder chambers, respectively. The check valves cooperate to permit cryogenic liquid to flow into the first pumping cylinder chamber and out of said second pumping cylinder chamber when the pumping piston moves in a first direction and out of said first pumping cylinder chamber and into the second pumping cylinder chamber when said pumping piston moves in a second direction that is opposite of the first direction. A portion of the cryogenic liquid pumped by the pumping piston travels to a heat exchanger where it is vaporized.
The pump also includes an actuating cylinder that is divided by an actuating piston into first and second chambers, each of which includes an inlet and an outlet. The actuating piston is joined to the pumping piston by a connecting rod. An automated control valve is positioned in circuit between the heat exchanger and the actuating cylinder inlets and introduces cryogenic vapor from the heat exchanger into the first and second actuating cylinder chambers in an alternating fashion thereby propelling the actuating piston in the first and second directions in a reciprocating fashion. As a result, the pumping piston is also moved in the first and second directions in a reciprocating fashion.
Cryogenic vapor exiting the actuating cylinder is directed to a gas and liquid mixer. The portion of the pumped cryogenic liquid that is not vaporized is also directed to the gas and liquid mixer where it is heated by the cryogenic vapor for the actuating cylinder. A pressure control circuit is positioned in the line running from the pumping cylinder outlets to the mixer. The pressure control circuit may be adjusted to increase the pressure within the line so that a greater portion of the pumped cryogenic liquid is vaporized and ultimately directed to said gas and liquid mixer so that greater heating of the cryogenic liquid occurs therein.