Abstract:
In a method of compressing a gas, a compression which is simple to realize is achieved in that, in a first step, a foam ( 21 ) is formed from the gas and a liquid, in which foam ( 21 ) the sonic velocity is markedly lower than in the gas and in the liquid taken by themselves, in that, in a second step, the foam is directed at supersonic velocity through a nozzle ( 19, 20 ) and the gas located in the foam is thereby compressed, and in that, in a third step, the compressed gas and the liquid are separated from one another behind the nozzle ( 19, 20 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of compression technology. It concerns a method of compressing a gas and a compression apparatus for carrying out the method. 
     2. Discussion of Background 
     In the prior art (see, for example, the publication U.S. Pat. No. 5,083,429) it has already variously been proposed to compress a flowing gaseous medium by first of all accelerating it to supersonic velocity in a suitable device (compression tube) and then decelerating it again with the generation of shock waves and subsequent increase in pressure. The heat produced during the compression may be dissipated, for example, by spraying water into the corresponding tube section. A disadvantage with this type of compression is that the sonic velocity of the gaseous medium (e.g. air) is generally relatively high and that some outlay is therefore required in order to bring the gas flow to supersonic velocity. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of the invention is to provide a novel method and a novel apparatus for the compression of a gaseous medium, which method and apparatus can work at a markedly reduced sonic velocity and can therefore be realized with reduced outlay. 
     The object is achieved in a method of the type mentioned at the beginning in that, in a first step, a foam is formed from the gas and a liquid, in which foam the sonic velocity is markedly lower than in the gas and in the liquid taken by themselves, in that, in a second step, the foam is directed at supersonic velocity through a nozzle and the gas located in the foam is thereby compressed, and in that, in a third step, the compressed gas and the liquid are separated from one another behind the nozzle. The essence of the invention consists in using a foam-like gas/liquid system for the compression, this gas/liquid system being distinguished by a sonic velocity which is markedly reduced compared with the individual components. In this way, it is possible with reduced outlay to achieve the supersonic velocity required for the compression operation. At the same time, the heat produced during the compression can be dissipated in a simple manner via the liquid, which is to be separated off again subsequently. 
     A preferred embodiment of the method according to the invention is distinguished by the fact that an essentially static foam is produced, that the nozzle is moved at supersonic velocity through the foam, and that the movement of the nozzle is executed as a circular movement about an axis of rotation. This way of conducting the method proves to be especially favorable for realizing the method in terms of equipment. 
     A development of this embodiment which is preferred on account of its simplicity is distinguished by the fact that the foam is collected behind the nozzle in a collecting container moving along with the nozzle, and that the centrifugal force arising in the collecting container during the rotation is used for the separation of gas and liquid. 
     Another preferred embodiment of the method according to the invention is distinguished by the fact that, to produce the foam, the gas is introduced into a volume of the liquid in a distributed manner, and that the gas is introduced from below through a porous base into a layer of the liquid above the base. In this way, a fine-pored foam, which is especially suitable for the compression according to the invention, can be produced over a large area without moving parts. 
     The compression apparatus according to the invention for carrying out the method according to the invention comprises a container for the foam produced, which container is connected to first means for producing the foam, as well as at least one nozzle, which can be moved relative to the foam in such a way that the foam passes at supersonic velocity through the nozzle, as well as second means for the separation of the foam into gas and liquid, which second means are arranged behind the nozzle. 
     A first preferred embodiment of the apparatus according to the invention is distinguished by the fact that the first means comprise a porous base, which closes off the container at the bottom and to which gas can be admitted over the surface area from the underside, that the at least one nozzle is arranged inside the container on an arm so as to be rotatable about a central axis of rotation and essentially tangentially to the circle of rotation, that the arm is driven by a motor, that the second means in each case comprise a collecting container, which is attached behind the nozzle, is connected to the nozzle and is in each case arranged on the end of the arm, and that in each case third means for the separate discharge of the gaseous and liquid components separating during the rotation are provided in the collecting container. 
     A preferred development of this embodiment is distinguished by the fact that the arm is in each case of tubular design, that the third means comprise a first inner tube running in each case inside the arm, and that the liquid is drawn off through the first inner tube and the gas is drawn off in the intermediate space between the first inner tube and the arm. 
     Further embodiments follow from the dependent claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIG. 1 shows a diagram of the sonic velocity in an air/water system plotted against the ratio ε of the air volume V air  to the total volume V of the air/water mixture; 
     FIG. 2 shows a plan view of a preferred exemplary embodiment of a compression apparatus having two nozzles on two arms; and 
     FIG. 3 shows the compression apparatus according to FIG. 2 in partly sectioned side view. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, an essential feature of the present invention is the use of a gas/liquid system for the compression of the gas itself. In two-phase gas/liquid flows, the sonic velocity is often much lower than the sonic velocity of the pure gas or of the pure liquid. Thus, for example, the sonic velocities are less than 40 m/s in an air/water system if the volumetric ratio ε of air to the mixture as a whole is between 0.1 and 0.9 (FIG.  1 ). This means that supersonic velocity can be produced relatively simply and that such a mixture can be compressed to a high degree by a reduction in the cross section of flow. 
     A special form of such a gas/liquid or air/water mixture is foam. Foam is distinguished by high volumetric proportions of gas or air (ε≈0.9). Foam is defined as a dispersion of gas in a liquid which contains one or more surface-active substances. The liquid is mainly present in the form of thin films as an envelope of the bubbles present in the foam. The size (diameter) of the bubbles varies between a few micrometers (fine-pored foam) and several millimeters (coarse-pored foam). The surface-active substances are soluble in the liquid and reduce its surface tension, so that the formation of stable bubbles is made possible. In the exemplary air/water mixture, foam may be formed, for example, by means of a 1.5% butyl/glycol/water solution and air. The mixture may range from a 1 to 5% solution with a 1.5% being preferred. 
     The method according to the invention may now be carried out by means of a compression apparatus, of which a preferred exemplary embodiment is reproduced in FIGS. 2 and 3. The compression apparatus  10  shown comprises a container  11 , in which the desired foam  21  is produced. The container  11  is closed off at the bottom by a porous base  23 , above which there is always a layer of the liquid  22  used (in particular water plus surface-active substances) during the operation of the compression apparatus  10 . Arranged below the porous base  23  is a feed space  24 , which can be filled via a feed  25  with the gas used (in particular air), which is to be compressed. The gas passes in the form of small bubbles from the feed space  24  through the porous base  23 —which may also be designed as a perforated plate or the like—penetrates into the liquid  22  above it and produces the foam  21  when passing through the liquid  22 , the foam  21  filling the container  11  above the liquid  22  to a more or less considerable degree. 
     In the region of the foam  21 , a system is arranged inside the container  11  so as to be rotatable about a central axis of rotation  12 , and this system is moved at a circumferential velocity which is higher than the sonic velocity of the foam  21 , by means of a motor  26  (or a drive having the same effect), catches the foam  21  at this circumferential velocity and causes it to flow through a reduction in cross section. To this end, two tangentially directed nozzles  19 ,  20  are provided on two opposite arms  15 ,  16 , through which nozzles  19 ,  20  the foam  21 , which is static relative to the nozzles  19 ,  20  rotating about the axis of rotation  12 , flows and passes into collecting containers  17 ,  18  at the rear. It goes without saying that, instead of the two nozzles  19 ,  20  shown in the example, only one nozzle or more than two nozzles may also be used. 
     In the collecting containers  17 ,  18 , an inner tube  29 ,  30  running concentrically inside the tubular arm  15 ,  16  ends in each case in front of the container wall. The radial inner tubes  29 ,  30  are run to an axial inner tube  14 , lying in the axis of rotation  12 , and are attached to this inner tube  14 . The tubular arms  15 ,  16  connect the collecting containers  17 ,  18  to an axial outer tube  13 , which concentrically surrounds the axial inner tube  14 . The axial tubes  13 ,  14  serve as a shaft. The axial tubes  13 ,  14  and the arms  15 ,  16  fastened to them are rotated by the motor  26  arranged under the container  11 . The axial tubes  13 ,  14  are closed at the bottom. They are accessible from outside at the top through suitable outlets  27 ,  28 . 
     The compression apparatus  10  shown in FIGS. 2 and 3 now functions as follows: the two nozzles  19 ,  20 —driven by the motor  26 —rotate together with the associated collecting container  17 ,  18  counter-clockwise (rotation arrows in FIG. 2) in the container filled with the foam  21 . In the case of the exemplary and preferred air/water mixture, the velocity of rotation is about 100 m/s, i.e. the nozzles  19 ,  20  move relative to the foam  21  at supersonic velocity. Such a velocity can be achieved, for example, if the rotational frequency of the motor  26  is 50 Hz and the nozzles  19 ,  20  are at a distance of about 0.3 m from the axis of rotation  12 . 
     Compression of the 2-phase mixture occurs in the nozzles  19 ,  20 . The liquid (the water) is centrifuged radially outward in the collecting contains  17 ,  18  behind the nozzles  19 ,  20  on account of the centrifugal force and is transported via the radial inner tubes  29 ,  30  and the axial inner tube  14  to the outlet  28 . The liquid discharging at the outlet  28 —if need be after heat extraction—may be fed back again into the container  11  for the formation of foam. The gas (air) remaining behind during the centrifuging is directed in the intermediate space between the arms  15 ,  16  and the radial inner tubes  29 ,  30  to the axial outer tube  13  and may be extracted (in compressed form) at the outlet  27 . 
     As already mentioned above, the base  23  of the container  11  consists of a porous material or a perforated plate. There is always a liquid layer  22  on the base  23 . The gas (air) flows through the base  23  and forms bubbles when passing through the liquid layer  22 . A fresh foam  21  is thus always obtained. 
     At initial volumetric ratios of ε=0.9 (in the case of the air/water mixture), the mass ratio of water to air is 85.9, i.e. the heat released during the compression of the air is absorbed by the water without an appreciable temperature increase occurring. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.