US2173330A - Fluid compressing and exhausting device - Google Patents

Description

Sept. 19, 1939. T. D. GREGG 2,173,330

FLUID COMPRES SING AND EXHAUSTING DEVICE Filed June 25, 1936 .INVENTOR n ATTORNEY Patented Sept. 19, 1939 UNITED STATES PATENT OFFICE FLUID COMPRESSING AND EXHAUSTING DEVICE 14 Claims.

This invention relates to fluid-compressing and exhausting devices, and particularly to a device of the type in which a compressible fluid, such as steam and other vapors, may be compressed or exhausted by another fluid.

Such a device is especially applicable in the compression of water vapor formed under high vacua, which process is employed in water-vapor refrigeration systems widely used in air conditioning, and in the dehydration of foods and chemicals.

Heretofore, there has been used in those systems one or more nozzles of the convergent-divergent type, known as DeLaval nozzles which expand the driving fluid down to the vacuum maintained by suitable means in the source of fluid or evaporation system, in the case of refrigeration, causing the driving fluid to be projected, at velocities well above that of sound, into a mix- 20 ing-chamber. Here it mechanically entrains its load of vapor or other fluid and by its momentum compresses the combined stream in a converging tube, with decreasing speed until it reaches the narrowest part or throat. Beyond the throat, the tube gradually enlarges and the combined fluid is further compressed until it reaches the exit of the diverging diffuser tube where it reaches its maximum pressure and its speed is incidentally very greatly reduced. If higher pressures are required, another jet-compressor of the type just described may be employed to repeat the aforedescribed process upon the combined fluid as it leaves the first-stage compressor.

It will be shown more fully hereinafter that while the friction losses in a compressor of that type are large, the losses due to the shock of impact of the high velocity driving fluid upon the relatively stationary induced fluid are very much larger, so that the efficiency of such a device is relatively low.

One of the objects of the present invention is to reduce those shock losses by utilizing some of the kinetic energy of the driving medium to accelcrate the induced stream of fluid at the first point of its contact with the driving fluid to a velocity comparable with a velocity of the latter fluid itself at that point and incidentally in a direction as nearly as possible parallel to it.

This invention will be clearly understood from the following description when read in connection with the attached drawing of which Fig. 1 represents the well-known steam-jet type of compressor to which reference will be made in order to bring out clearly the improvement effected by the applicants invention; Fig. 2 is a longitudinal section of one form of compressor, designated Type A, embodying the applicants invention; Figs. 2a, 2b and show various forms of the aerofoil vanes that may be used in the compres- 5 sor-of Fig. 2; Fig. 3 is a view, in fragmentarry form of the aerofoil cone and the nozzle of Fig, 2; and Fig. 3a. serves to illustrate the method to be followed to obtain the correct proportions of orifice through which the vapor to be compressed passes into the mixing chamber.

In Fig. 1 a De Laval nozzle I extends into a mixing chamber 2 with which there is also connected an inflow pipe 3 by which there is introduced into the mixing chamber 2 the water-vapor or other fluid to be compressed by steam projected into the said chamber by the nozzle I. The steam, and the water vapor entrained thereby, are projected into the compression tube 4, and thence pass into the diffuser 5 which is con- 2 nected to the outflow pipe 6 by which the com pressed fluid is conveyed to a condensing chamber or to the apparatus in which it performs useful work. The velocity of the jet of steam is well above the velocity of sound and may be of the order of 4,000 feet per second, which is greatly in excess of the velocity of the water vapor from the input pipe 3.

The efficiency of such compressors is very low.

The principal causes of loss in such a compressor are as follows: I

Per cent Friction loss in nozzle 15 Shock and friction turbulence in mixing chamber 57 or more Friction in compression tube and diffuser 10 or less Total losses 82 approx. The efliciency, e, is therefore 18 or less Obviously, by far the greatest loss in such a device is that due to shock and turbulence in the mixing chamber, and it is the purpose of the applicant's invention to greatly reduce or substantially eliminate those losses and thus increase the efliciency of the device.

Such result is attained by giving to the induced stream of water-vapor a high velocity at the first point of contact with the steam of the driving jet from the DeLaval nozzle. This is accomplished by means of the arrangement shown in Fig. 2 in which two similar vanes have been placed equidistant from or symmetrically with respect to the longitudinal axis of the mixing chamher 2. A cross section of each of those vanes has the exact proportions of aero-foils, i. e., small model airplane wings, and are so arranged that the surface of the vane corresponding to the upper surface of the airplane wing will be nearer the axis of the mixing chamber.

The compressor shown in Fig. 2 in which the same numerals have been used to represent parts similar to those in Fig. 1, includes a DeLaval nozzle I adapted to direct a jet of steam into the mixing chamber 2 to which is also connected an input pipe 3 by which water-vapor is drawn into the mixing chamber. Suitably supported in the said chamber and symmetrically arranged with respect to the longitudinal axis of the chamber are two vanes, cross-sections of which are designated by 8 and 8'. These may have the form shown in Fig. 2a or each vane may be formed as shown in Fig. 2b; or the separate vanes may be merged into a single element as shown in Fig. 2c.

The cross-section, designated 8 or 8' in Fig. 2, represents a vertical cross-section through vanes of the type shown in Fig. 2a; or a section of the types of vanes shown in Figs. 2b or 20 lying in a plane that passes through the axis of rotation of vanes shown in the latter figures.

The cross sectional form of these vanes, as shown in Fig. 2, is that of a good aerofoil section, which means one that by wind tunnel tests possesses the characteristics desirable in an airplane wing, namely, very high lift and low drag coefiicients and also a high ratio of lift to profile drag. Those characteristics are found in modern gliders particularly'those having the proportions of the Joukowski profile. It is important that the sharp trailing edges 1, f of the aerofoils shall be in line with the inner surface of the compression tube 4 at the junction d as well shown in Fig. 3. The exact size of all other openings is important and must be calculated either by the laws of hydrodynamics and thermodynamics, as herein described, or by the laws of similitude, based upon the dimensions of tested models. It is desirable that the aerodynamic centers of the aerofoils shall lie in or close to the plane that passes transversely through the orifice of the nozzle. The area of the opening d, outside the trailing edges, must be just sufficient for the admission of the starting impulse necessary for the formation of the starting vortices hereinafter referred to. The shape of the lobes of the mixing chamber is similar geometrically to that of the aerofoils, and their relative proportions may be determined from rules hereinafter described.

Before setting forth the design formulae for a compressor of the type embodying the applicants invention, it seems desirable to state certain un derlying principles and assumptions, knowledge of which is essential to the application of those formulae to a given set of requirements.

In proportioning the openings between the exit of the nozzle I and the up-stream. ends of the vanes, indicated by 0 upon Fig. 2 it is assumed that for maximum efficiency all inflowing induced vapor passes through the openings 0 and none passes through the openings between the trailing edges 1 and the throat d of the mixing chamber, except at the moment of starting.

Referring to Fig. 2, it is obvious that in order for vapor to flow through the opening 0, the pressure PC within that part of the chamber enclosed by the opening 0 must be less than the pressure Pb maintained in the input pipe 3; and the smaller the ratio of absolute pressures,

down to or below the critical value hereinafter stated, the greater the velocity Vtb and the larger the weight, Wb, of flow of the water vapor to be compressed. In order to create and maintain the pressure difference Pb-Pc it is not enough to expand the driving steam. of the nozzle I down to the point where Pc'=Pc, where P0 is the pressure at the exit. A portion of the kinetic energy of the driving jet must be consumed in the work of maintaining the difference of pressure.

That portion equals that is, the kinetic energy of the induced stream at the point of maximum speed, and wherein W =weight in pounds per second of the induced vapor V =velocity in space at narrowest point, 0,

g=acceleration in ft. per sec. due to gravity A=reciprocal of the mechanical equivalent of The remainder of the kinetic energy of the driving steam does the work of compression and friction of the combined streams.

The difference in pressure is created and maintained in the following manner:

Assuming the jet in operation, with the openings at the trailing edges ,1 closed, a small amount of water vapor will be entrained by the jet and compressed by the driving steam to a pressure Pe' at the exit of the expansion tube 5. and a small pressure drop Pb-Po will be created at c. To avoid excessive shock losses in the nozzle, the exit area will be assumed to be such that the pressure at the exit will equal Po, and the velocity equal to Vc' (less than Vc) The kinetic energy W V 2g is just enough to do the work of compression and overcome the losses due to impact and friction.

Now assume that the openings between the trailing edge f and the throat d is opened. This introduces a new resistance, termed the starting resistance by Dr. L. Prandtl and O. J. Tietjens in the book entitled Applied Hydro and Aero Mechanics published by McGraw-I-Iill Co., the starting resistance being due to the creation of vorticity.

To overcome that resistance requires an increase in the kinetic energy of the jet of steam and of its velocity Vc. In order to increase the kinetic energy the exit aperture of the DeLaval nozzle is conceived to be enlarged and the pressure just Within the exit, at c, is diminished to PC. It will now be foundthat a factor, called circulation, created by the work of overcoming the starting resistance (the nature and function of which will be described later) causes the pressure in the narrowest part of the opening 0 to decrease and hence the weight and speed of flow of the induced vapor will correspondingly be increased. This larger flow tends to increase the work of compression, and to cause the impact and friction losses to be overcome, which requires an adjustment of the jet energy.

The reduction of pressure in the plane of caused by the circulation is mathematically related to the aerofoil as will be shown hereinafter. The latter may be designed according to the method of Joukowski, Kutta and Lanchester which is described in Applied Hydro-and Aero Mechanics by O. J. Tietjens, published by Mc- Graw-Hill, or a tested aerofoil section, having the required characteristic may be used. By following the method described on page 181 of said publication an aerfoil section may be designed giving the required reduction in pressure.

If the aerofoils be made of such length that the driving steam and the streams of induced vapor have a common speed where they pass the trailing edges 1, f, and the openings are just sufficient to admit the starting vortex, which may be determined experimentally, little or no vapor will flow through the opening or openings at that point. Practically all of the suction vapor from the pipe '3 will pass across the rounded front end of the attack of between 25 and 30 degrees.

aerofoils at c and will join the driving jet at high velocity. With equal to or less than the critical value (for gases, 0.528 and for dry steam, 0.577) the velocity will equal or be greater than that of sound and the shock and other losses will be a minimum. From 1 the orifice d to the exit of the compressor tube 4,

the design depends on the application of the laws of thermodynamics and the flow of fluids.

The theory of the property called circulation, described fully in the treatise by Tietjens referred to hereinbefore, is as follows:

By circulation is meant the tendency of the fluid to move around the vanes, which tendency has the effect of slowing up the flow of fluid past the outer surfaces of the vanes and increasing the pressure there and of increasing the speed of flow past the inner surfaces and reducing the pressure within the aerofoil enclosure or ring. Circulation does not mean circulatory motion of the fluid. Of the initial energy required by this phenomenon, approximately one half is required to overcome the starting resistance, that is, the setting up of vortices at the trailing edge I of the aerofoil; the remainder of the energy is required to maintain the tendency of the fluid to circulate around the vanes. The circulation remains constant as long as the velocity Vc of the flow remains unchanged, or as long as the angle of incidence of the inflowing vapor with the surface of the aerofoils remains unchanged. Change of either results in a new starting resistance and new starting vortices. The phenomenon has the attribute of inertia and hence is a physical reality and not merely a convenient mathematical or physical concept.

Suitable cross-sections of tested aerofoils may be found in a list of sections published by the National Advisory Committee on Aeronautics. The data concerning those sections is given in Technical Aerodynamics by Carl D. Wood (McGraw- Hill 1935), pages 259 to 261. The preferred cross section is that conforming to the Kutta-Joukowski profile with a keen trailing edge. From aerofoil wing data, an aerofoil section must be selected having a high coeflicient of lift at an angle of The angle of attack or is the angle between the velocity vector of the incoming suction fluid and. the geometric chord of the aerofoil section, as indicated in Fig. 5.

It is desirable that the incoming vapor from the input pipe shall have a direction of flow at the point of maximum speed corresponding to the angle of attack.

This is attained by proper proportioning of the space between the nose of the aerofoil and the wall of the enclosing chamber through which the fluid, to be compressed, enters the device. To proportion this, the line a is drawn from a corner of the nozzle to the edge of the aerofoil as shown on Fig. 3a, the line a being such that the area of the input duct at that point is smaller than the area through any other line from the said corner to the said edge. The line b is then drawn perpendicular to the line a. Then the chord c is drawn, and, the line (1 is erected perpendicular to ,c and tangent to the curve of the nose of the aerofoil. The-line e is then drawn through the tangential point and the intersection of the lines a and e at 0 establishes a center from which radii may be drawn, the lengths of which determine the shape of the curve of the wall of the chamber, and, thereby, the proportions of the input duct. Thus, by drawing the radius o-f we know the length of that part designated y between o and the edge of the aerofoil section. We also know the length of the line a, viz, r, and the length of that part of line a. between 0 and the point of intersection of a with the edge of the aerofoil designated 2/. Consequently we can determine the length of the radius o-j, designated :12 by the formula In like manner the length of other radii, as o-g, o-h, etc., may be determined and the locus of the points 1, g, h, etc., is the curve of the inner surface of the wall of the chamber forming one side of the input duct. The stream of fluid to be compressed passing through the duct tends to strike the driving jet at the angle 5 which is relatively small. The effect of circulation upon the stream to be compressed is to further reduce 1 and to tend to bring about parallelism between the driving and the driven streams.

The aerofoils, obviously require some means to support them Within the mixing chamber. In the form shown in Fig. 2a, in which the aerofoils resemble airplane wings, the ends of the aerofoils are in contact with the side walls of the chamber and are supported thereby. When the aerofoils are in the form of a half ring or a whole ring, as in Figs. 2b and 2c, suitable lugs, welded or otherwise fastened to the vanes and the Wall of the chamber, and suitably streamlined to reduce friction, would serve to support the aerofoils.

While the invention has been disclosed in the form, designated type A, in which the driving fluid comes through a nozzle coaxial with the mixing chamber, and the fluid to be compressed comes in through a duct formed by the aerofoil and the side of the chamber, it is possible to accomplish the same result by a form of the invention, designated type B, in which the fluid to be compressed enters the mixing chamber through a duct coaxial therewith and the driving fluid enters by a duct formed by the aerofoil and the side of the chamber.

It seems desirable to point out that whereas, in the assumed illustrative case, the device is spoken of a compressor (and the same term is employed in the claims) the device may be employed not only for the primary purpose of compressing one fluid by another but also for the purpose of exhausting a fluid from the chamber by means of another fluid, the term compressor being intended as generic to compressing and exhausting devices- The invention is, therefore, not limited to the forms and arrangements of parts shown since it is capable of embodiment in other forms and arrangements without departing from the spirit and scope of the appended claims.

What is claimed is:

1. In a device for compressing one fluid by another, the combination with a mixing chamber of an input pipe connected thereto by which the fluid to be compressed is introduced into said chamber, means to project a jet of fluid into said chamber, and guiding means within said chamber to bring together the fluid of said jet and the fluid to be compressed, the said guiding means having the cross-sectional form of an aerofoil with high lift coefiicient, and the curved surfaces of the said guiding means being substantially detached and separated from the side wall of the said chamber.

2. In a device for compressing one fluid by another, the combination with a mixing chamber of a conically shaped guiding member therein having a duct extending therethrough from the base to the apex thereof, a nozzle adapted to project a jet of fluid into said chamber, the axis of said nozzle substantially coinciding with the axis of the said guiding member, and a source of fluid to be compressed also connected to said chamber, a cross-section of the wall of said guiding member lying in a plane through the axis of said member having the proportions of an aerofoil with high llit coeflicient, the surface of said member closer to the axis of said member corresponding to the upper surface of said aerofoil.

3. In a device for compressing one fluid by another, the combination with a mixing chamber in which a stream of fluid is to be compressed by a jet of fluid, of a conically shaped guiding member having a duct extending between the base and apex thereof, through which said fluids pass, the axis of said member coinciding with the axis of said jet, and the cross-section of the wall of said member in a plane through said axis having the same proportions as an aerofoil, the inner edge of said cross-section corresponding to the upper edge of said aerofoil, the said guiding member being so positioned as to provide a substantially continuous space between its outer surface and the inner surface of said chamber.

4. In a device for compressing one fluid by another, the combination with a mixing chamber of a De Laval nozzle to project a jet of fluid into said mixing chamber, a source of low pressure vapor to be compressed by said jet in said mixing chamber, and a conically shaped guiding member having a duct therein extending from the base to the apex of said member and coaxial with said nozzle, a cross-section of the walls of said member lying in a plane through said axis, having the same proportions as an aerofoil, and the orifice of said duct at the apex of said member being of the same diameter as the outlet orifice of said mixing chamber, and coaxial therewith, the said guiding member being spaced from the walls of said mixing chamber.

5. In a device for compressing one fluid by another, the combination with a compression chamber of a nozzle to project a jet of fluid into said chamber, a source of fluid to be drawn into said chamber and compressed therein, and a guiding surface of an annulus to guide the stream of fluid from said source so that it will be substantially parallel to the fluid of said jet at the first point of contact, the cross-section of the said annulus in a plane through its axis having the same form as the Joukowski profile of an aerofoil.

6. In a device for compressing one fluid by another, the combination with a mixing chamber of a nozzle to project a jet of the driving fluid into said chamber at a velocity well above that of sound, a source of fluid to be drawn into said chamber to be compressed therein, and an aerofoil guide within said chamber proportioned to increase the velocity of the stream of fluid to be compressed to a value substantially equal to that of sound whereby shock losses due to impact will be substantially eliminated, the said guiding means being shaped to guide the stream of fluid from said source to its point of confluence with the driving stream and to increase the velocity of the fluid to be compressed and being supported in such manner as to provide a space between said guiding means and the wall of said chamber.

'7. In an arrangement for compressing one fluid by another, the combination with a mixing chamber in which a stream of vapor to be compressed is mixed with a jet of fluid to efiect such compression, a guiding member whose cross-section is substantially similar to an aerofoil having a high lift coeflicient over the surface of which the said vapor is drawn, a compression tube connected to the output of said mixing chamber, the inner surface of said tube being in line with the trailing edge of the said guiding member but separated therefrom, and a diffuser connected to the output of said compression tube.

8. In a device for compressing one fluid by another, the combination with a mixing chamber of a nozzle to project a jet of fluid into said chamber at a velocity well above that of sound, a source of fluid to be drawn into said chamber to be compressed therein, a guiding member, having the cross-sectional form of an aerofoil, positioned within said chamber so that its axis coincides with that of said nozzle, the said guiding member being supported in such manner as to provide a space between it and the wall of said chamber, and proportioned to increase the velocity of the stream of fluid to be compressed to a value approximating that of sound thereby minimizing shock losses due to impact.

9. A steam jet-compressor characterized by the use of an annular guiding means to bring into confluence the driving jet and the fluid to be compressed, the said guiding means having the crosssectional form of an aerofoil of high lift coefficient, the said means being coaxial with the jet and supported in such manner as to provide a substantially continuous space between said guiding means and the wall of the said compressor.

10. In an arrangement for compressing one fluid by another, the combination with a mixing chamber in which a stream of vapor to be compressed is mixed with a jet of fluid to effect such compression, a guiding member, whose cross-section is substantially similar to an aerofoil having a high lift coefficient and a keen trailing edge, over the surface of which member the said vapor is drawn, and a compression tube connected to the output of the said mixing chamber, the inner surface of the said tube being in line with but separated from the trailing edge of the said guiding member.

11. In an arrangement for the compression of one fluid by another, the combination with a mixing chamber of means to introduce therein a stream of vapor to be compressed, a nozzle inserted in said chamber to create a jet of fluid to effect such compression, and a guiding member, whose cross-section is substantially similar to an aerofoil having a high lift coeificient and a keen trailing edge, over the surface of which member the said vapor is drawn, the said guiding member being supported by but separated from the walls of the said mixing chamber.

12. In a device for compressing one fluid by another the combination with a mixing chamber of a nozzle to project a jet of fluid into the said chamber, a source of fluid to be drawn into the said chamber and to be compressed therein, and guiding means supported in said chamber through which the said jet is projected, the said means having the cross-sectional form of an aerofoil of high lift coeiiicient and sharp trailing edge when out by a plane normal to the surface of said guiding means and containing the axis of the said jet.

13. A steam jet compressor comprising a mixing chamber, having connected thereto means to project a jet of steam into said mixing chamber,

a source of water vapor connected to said chamber and a plurality of aerofoil guides positioned within said chamber so that the jet will be projected into the space between the said guides, the said guides being so positioned and formed that the stream of water vapor will be drawn into said space, the said guides being so positioned and supported as to provide a substantially continuous space between said guides and said mixing chamber.

14. In a devicefor compressing one fluid by another, the combination with a compression chamber of a nozzle to project a jet of fluid into said chamber, a source of fluid to be drawn into said chamber and compressed therein, and a plurality of guiding surfaces supported in said chamber, each having a crosssection similar to the Joukowski profile of an aerofoil, the said guiding surfaces being so proportioned as to increase the velocity of the fluid to be compressed so that it will approximate the velocity of the driving fluid at the moment of impact therewith.

TRESHAM DAlVEES GREGG.

Images