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CompressorCompressor description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20080008602, Compressor. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0001]National Stage application of International Application No. PCT/EP2005/000279 filed Jan. 13, 2005, which claims priority to British Application No. GB 0400986.6 filed Jan. 16, 2004. BACKGROUND OF THE INVENTION [0002]This invention relates to a rotary liquefied natural gas boil-off compressor. The invention also relates to a method of compressing boiled-off natural gas. [0003]Liquefied natural gas is required to be stored in thermally-insulated tanks. Notwithstanding the thermal insulation, there is always a discernible inflow of heat from the surroundings which causes the liquefied natural gas to boil at a modest rate. The resulting boiled-off liquefied natural gas may be compressed and reliquefied or may be used as a fuel. Use of the boiled-off natural gas as a fuel usually requires its compression. For example, it has been proposed to use the boil-off gas from a shipboard liquefied natural gas storage tank to fuel a gas turbine forming part of the ship's propulsion system. Such a gas turbine, typically requires the boiled-off natural gas to be compressed to a pressure in the order of 20 to 40 bar. In another example, the natural gas is employed together with diesel fuel in an engine employing both fuels. In this example, the natural gas may be compressed to a pressure in the range of 5 to 7 bar. [0004]Conventional boil-off compressors employ six compression stages in series if a pressure as high as 40 bar needs to be achieved. The compression of the gas in each stage generates heat. Accordingly the natural gas is cooled between each pair of successive stages by indirect heat exchange with water. Such machines typically require quite large motors and have a substantial power consumption. BRIEF SUMMARY OF THE INVENTION [0005]It is an aim of the present invention to provide a liquefied natural gas boil-off compressor which has a reduced size and power consumption. [0006]According to the present invention there is provided a rotary liquefied natural gas boil-off compressor having at least two compression stages in series, a gas passage passing through the series of compression stages, the gas passage extending through and being in heat exchange relationship with at least one cooling means between the or each pair of compression stages, characterised in that the cooling means or at least one of the cooling means is a cryogenic cooling means and in that there is valve means for controlling flow of cryogenic coolant into the cryogenic cooling means in response to the inlet temperature, or a related parameter, of the next compression stage downstream of the cryogenic cooling means so as, in use, to maintain said inlet temperature at a chosen sub-ambient temperature or between chosen sub-ambient temperature limits. [0007]The invention also provides a method of operating a rotary liquefied natural gas boil-off compressor having at least two compression stages in series and a gas passage passing through the series of compression stages, the method comprising cooling the compressed boiled-off natural gas by means of a cryogenic coolant downstream of one of the compression stages and upstream of another, monitoring the inlet temperature, or a related parameter, of the compressed natural gas at the inlet to the other compression stage, and adjusting the flow rate of cryogenic coolant so as to maintain said inlet temperature at a chosen sub-ambient temperature or between chosen sub-ambient temperature limits. [0008]By use of a cryogenic coolant in accordance with the invention, particularly between each pair of successive compression stages, it is possible to increase the ratio of the outlet pressure to the inlet pressure of each such stage, and thereby typically reduce the number of compression stages required to achieve a particular pressure. For example, it is possible by means of the compressor and method according to the invention to raise the pressure of boiled-off liquid natural gas from 1 bar to approximately 40 bar using only four stages of compression, whereas a comparable conventional compressor employing non-cryogenic cooling, typically water cooling, requires six stages to reach such a high pressure. As a result, the invention makes it possible in these circumstances to achieve the same increase of pressure with a smaller machine using fewer compression stages and a lower power consumption. [0009]The or each cryogenic cooling means may be either an indirect cooling means, e.g. separate passes of a heat exchanger, or a direct cooling means, e.g. the gas passage may extend through a chamber into which a cryogenic liquid is introduced, for example, in the form of a spray. It is preferred to have a cryogenic cooling means intermediate each pair of compression stages. If there are three or more compression stages it is preferred that at least one of the cryogenic cooling means is an indirect cryogenic cooling means and at least one other is a direct cooling means. In one preferred arrangement a cryogenic liquid is only partially vaporised in an indirect cryogenic cooling means and there is a passage placing the inlet of a direct cryogenic cooling means in communication with the outlet of the indirect cryogenic cooling means. [0010]There may also be a direct or indirect cryogenic cooling means downstream of the final compression stage. If indirect, the cryogenic cooling means may have an outlet communicating with an inlet to a direct cryogenic cooling means upstream thereof. [0011]The source of the cryogenic coolant is preferably the same liquefied natural gas storage tank or array of storage tanks from which the boil-off gas is evolved. Such tanks are conventionally equipped with so-called stripping pumps which may be employed to supply the cryogenic liquid to the cryogenic coolant means. Alternatively, a dedicated cryogenic coolant supply pump may be used. [0012]There may be a cryogenic cooling means upstream of the first compression stage. Such a cryogenic cooling means will not normally be operated as the boiled-off natural gas is usually at a cryogenic temperature, but may be required when the liquefied natural gas storage tank is nearly empty, and the boil-off gas is therefore typically received at an undesirably high temperature, a condition that typically occurs after an ocean going LNG tanker has discharged its load of LNG to a shore-based terminal. The upstream cooling means may also be employed at start up when the piping is warm. [0013]In order to supplement the rate at which natural gas is compressed, the compressor according to the invention may have an intermediate inlet communicating with a forcing liquefied natural gas vaporiser. [0014]The forcing vaporiser and the cryogenic cooling means may if desired share a common pump for the supply of the cryogenic liquid. [0015]The inlet temperature of each stage of the compressor is preferably maintained at a temperature in the range of minus 50 to minus 140.degree. C. By this means, it is possible to achieve a pressure ratio across each stage in the range of 2.15:1 to 3:1, and typically in the range 2.5:1 to 3:1. It is particularly desirable to avoid the presence of any droplets of liquid in the natural gas entering any stage of the compressor. Accordingly, if any direct cryogenic cooling means is employed, the resulting cooled gas may be passed through an apparatus for disengaging particles of liquid therefrom. [0016]Compressors and methods of their use according to the invention will now be described by way of example with reference to the accompanying drawings, in which: BRIEF DESCRIPTION OF THE DRAWINGS [0017]FIGS. 1 to 5 of the accompanying drawings which are all schematic flow diagrams, and [0018]FIG. 6 shows a modification to any of the direct cooling stages of the compressors shown in FIGS. 2, 4 and 5. DETAILED DESCRIPTION OF THE INVENTION [0019]Referring to FIG. 1 of the drawings, there is shown an LNG storage tank 2. For the purposes of ease of illustration, various pipes and valves associated with the tank 2, for example, its fill pipe and its LNG discharge pipe, are not shown in FIG. 1 and the other drawings. The configuration and operation of such LNG tanks is however well known in the art. The tank 2 is typically located on board an ocean-going tanker (not shown). The tank 2 is shown containing a volume 4 of LNG. There is an ullage space 6 above the surface of the volume 4 of LNG in the tank 2. The tank 2 is vacuum-insulated or has another form of thermal insulation associated therewith so as to keep down the rate of flow of heat from the ambient environment into the liquid 4. Since LNG boils at a cryogenic temperature, notwithstanding the thermal insulation of the tank 2, there is continuous adsorption of heat by the LNG from its surroundings and hence continuous evaporation of the LNG into the ullage space 6. As a result there is a continuous flow of vaporised gas out of the tank 2 into a passage 8. The passage 8 provides a flow of gas to a plural stage, centrifugal, natural gas compressor 10. Apart from the arrangements for cooling the gas downstream of each stage of the compressor 10, and an optional arrangement for cooling the gas upstream of the first stage thereof, it is essentially conventional but is made of materials suitable for use at cryogenic temperatures. Unlike, say, a nitrogen compressor, the natural gas compressor 10 is built to be explosion- proof. Because many features of the natural gas compressor 10 are conventional they are not illustrated in FIG. 1 of the drawings. Thus, for example, the rotary devices within the individual compression stages are not shown. Continue reading about Compressor... Full patent description for Compressor Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Compressor patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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