US2459709A

US2459709A - Gas turbine system embodying rotary positive displacement compressor apparatus

Info

Publication number: US2459709A
Application number: US500010A
Authority: US
Inventors: Lysholm Alf
Original assignee: JARVIS C MARBLE; LESLIE M MERRILL; PERCY H BATTEN
Current assignee: JARVIS C MARBLE; LESLIE M MERRILL; PERCY H BATTEN
Priority date: 1936-03-28
Filing date: 1943-08-25
Publication date: 1949-01-18
Anticipated expiration: 1966-01-18

Description

Jan. 18, 1949.

A.UYSHOLM GAS TURBINE SYSTEM EMBODYING ROTARY POSITIVE DISPLACEMENT COMPRESSOR APPARATUS Original Filed March 28, 1936 3 Sheets-Sheet 1 3 S Q N Q s Q if 3 Q g A a 82 R *1 e Q 1 a x m N Q I}: a 8

N a a w M zg Q Q New S R w k a; J

I lgiENTOR. BY

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C Moms! Jan. 18, 1949. A. LYSHOLM 2,459,709

GAS TURBINE SYSTEM EMBODYING ROTARY POSITIVE DISPLACEMENT COMPRESSOR APPARATUS Original Filed March 28, 1956 3 Sheets-Sheet 2 Jan. 18, 1949. LYSHOLM 2,459,709- 7 GAS TURBINE SYS EMBODYING ROTARY POSITIVE DISPLACEMENT COMPRESSORAPPARATUS Original Filed March 28, 1936 3 Sheets-Sheet 3 ATTORNEY 2,459,709 UNITED-STATES PATENT OFFICE GAS TURBINE SYSTEM EMBODYING RO- TARY POSITIVE DISPLACEMENT COM- PRESSOR APPARATUS I Alf Lysholm, Stockholm, Sweden, assignor, by

Patented Jan. 18, 1949 losses.

mesne assignments, to Jarvis 0. Marble, Leslie M. Merrill, and Percy H. Batten, as trustees Au ust 7 Claim.

In the case of .a gas turbine plant where the compressor is driven by a turbine which is mechanically independent of the turbine producing power for external use, the compressor capacity may be controlled within wide limits by changing the speed of operation of the compressor independently of the speed of operation of the use- 25, 1943, Serial No. 500,010

In Sweden March 28, 1935 ful power turbine, the latter, particularly in turbine. and if the useful power turbine cannot be operated at variable speed because of the character of the power output therefrom. In

the latter instance, the quantity of air delivered by the compressor can be controlled by throttling to compensate for varying loads. This kind of control is however undesirable because it introduces into the system unavoidable throttling It has also been proposed heretofore to admit suction air to different stages of rotary compressors in a manner such that the suction air is admitted to a pressure stage higher than the normal low pressure stage when the load on the plant, and consequently'the required output from the compressor, is reduced. With this method also, the emciency of the compressor drops materially from its normal efllciency when the compressor is operated under loads mate-- rially different from the normal load for which the compressor is designed. Control of the compressor apparatus in order to take care of variable loads has also been suggested, in which the work of compression is divided up among a number of independently operable compressors, one or more of which may be disconnected, depending upon the amount of change in the load imposed on the system. This last-mentioned method of control is eflective only to a certain extent, since by changing the number of compressors in operation it is possible to effect only a very coarse control of the quantity or pressure of the air delivered and in order to obtain the requisite nicety of control, resort must be had in addition to throttling, which involves the introduction of lthe undesirable and unavoidable throttling osses.

Amongst the major objects of the present inventionare: to provide improved compressor apparatus capable of operating with acceptable efficiency over a wide load range while at the same time operating at constant speed or with variations in speed which are relatively minor as compared with the degree of variation of the load; to provide improved compressor apparatus of the screw type in which variable output may be obtained at constant speed of operation without involving undue losses; to provide improved screw compressor apparatus in which variations in capacity of the apparatus are obtained at constant speed by means of novel bleeding 'arrangements which permit a desired quantity of air to be bled from the compression spaces of the compressor before compression commences; and to provide improved gas turbine system apparatus of the continuous combustion type embodying compressor apparatus and controls therefor enabling the system to be operated efflciently at variable load and withconstant or substantially constant speed of operation of the compressor apparatus constituting a part of the system. Y

For a better understanding of the nature of the present invention and the advantages to be derived from its use, reference may best be had to the ensuing portion of this specification in which is described several embodiments of apparatus for carrying the invention into eflect and to the accompanying drawings forming a part hereof, in which such embodiments are illustrated.

In the drawings:

Fig. 1 shows more or or less diagrammatically in longitudinal cross section a gas turbine system of the continuous combustion type comprising apparatus embodying the invention;

Fig, 2 is a section similar to Fig. 1 showing another arrangement of apparatus embodying the invention;

Fig. 3 is a transverse section through compressor apparatus having hand operated bleeding means embodying the invention;

Fig. 4 is a longitudinal section partly in elevation of another form of compressor apparatus embodying the invention, the section being taken on the line 4-4 of Fig. 5;

Fig. 5 is a sectiontaken on the line 5-4 of Pig.

Fig. 6 is a plan view of the compressor shown in Fig. 4; 1

Fig. 7 is a view similar to Fig. 6 but with a portion of the compressor casing broken away and in section on the line 1-1 of Fig. and

Fig. 8 is a fragmentary section similar to Fig. 5 of a modification of the compressor apparatus shown in Figs. 4 to 7.

Referring now more particularly to Fig. 1, it designates generally a double rotation radial flow gas turbine of known kind having shafts l2 and i4 upon which are mounted the armatures of electric generators l6 and i8. The turbine and generators are mounted in a common frame or casing-20 having bearings 22, 24, 26 and 28. Shaft i2 is arranged to drive a compressor 3 0 and shaft l4 drives a compressor 32.

Compressor 30 is of the screw type and, in the embodiment illustrated, comprises two

rotors

34 and 36 mounted in suitable bearings in housing 38. The rotors are preferably space packed. That is, clearance is provided between the rotors and the walls of the casing and between the intermeshing portions of the rotors. The rotor construction may advantageously be in" accordance with the principles disclosed in my co-pending application, Serial No. 44,935, filed October 14, 1935, now Patent No.'2,243,8 74. Because of the utilization of space packing, the rotors may be operated at the very high speed resulting from direct connection of the rotors to the turbine. This connection is made through the coupling 40 connecting shaft i2 with the shaft part 42 of rotor 34. The gear 44, fixed on the rotor shaft 42 meshes with gear 48 fixed to the shaft part 48 of the rotor 36 and provides drive for the latter rotor and maintains the peripherally spaced relation of the intermeshing parts of the two rotors. Rotor 36 is provided with a pressure equalizing piston 50 for equalizing axial pressure. If the shaft l2 of the turbine is assumed to rotate clockwise when viewed from the left, air which is drawn in through the inlet openings 52 and 54 is-compressed between the lobes of the rotors forming working spaces the volumes of which diminish towards the right hand side of the compressor. The compressed air enters the outlet 56 on that side of the compressor which lies in front of the plane of the drawing and which may be termed the pressure side of the compressor as distinguished from the suction side located behind the plane of the drawing. In order to prevent direct communication between the inlet and the outlet of the compressor, the end surfaces at the right of the rotors are partly closed by an end wall member 49 which on its front side is pro- -vided with an opening SI for the flow therethrough of the compressed air, The compressor casing 38 is provided with a lateral opening 58 providing a cylinder for a valve member such as the plunger 60 which when it is in the position indicated in the figure closes the opening 58; An annular space 62 communicating with an outlet 64 provides communication between the interior of the compressor casing and the atmosphere when the valve member 80 moves outwardly beyond the space 62. It will thus be evident that the opening afforded at 64, which may conveniently be termed a bleeder opening, may be if the bleeder valve is opened, compression in such spaces will commence only after the time when the thread or lobe limiting the of the valve is flush with the inner surface of the casing andpreferably the inner face of the valve is made to conform to the curvature of the casing. The reason for this is to avoid any large leakage space at this point, through which the air being compressed in one compression space may leak past the posterior limiting lobe, to the next succeeding space on the other side of such lobe.

The bleeder valve member is provided with a'stem 66 forming the armature of a solenoid 6d and is also encircled by a spring 10 located between the housing and an abutment plate 12. Spring 10 tends to move the valve member to open position and this tendency is opposed by the action of the solenoid winding, which when fully energized has sufficient strength to overcome the action of the spring and maintain the valve member in the closed position shown in the drawing. Solenoid 68 is energized by current supplied through wires 14 and 16 from the generator mains indicated generally at 18.

The outlet 56 of the compressor 30 is connected by means oLconduit to a combustion chamber 82 having a combustion space 84' surrounded by a jacket 86. Fuel such for example as oil is supplied by a pump, such as that indicated at 88, to the burner nozzles indicated at and- 92. A governor 94 operated from the shaft i4- actuates a fuel control member 96 to control the amount of fuel fed to the combustion chamber under different conditions of load. A pressure relief valve 9| .connected to the discharge pipe of the pump 88 permits return of fuel oil to the supply pipe of the pump through suction side in front ofthe plane of the drawing,

brought into full or partial communication with is connected by means of coupling 98 to shaft i4 and is similar in construction to the compressor 30 previously described and need not be described in detail. Compressor 32 has air inlet openings at Hill and H32 and is provided with a bleeder valve I04 urged toward open position by spring I06 and closed by the solenoid I08, the latter being energized by current derived through wires H0 and H2 connected into the electrical system 18. The outlet 4 of compressor 32 is connected by means of conduit H6 to the combustion chamber 82.

Insofar as air flow is concerned, it will be seen that compressors 30 and 32 are arranged in parallel, both delivering to the outer chamber of the combustion apparatus, part of the air entering the inner chamber 84 to supply the oxygen necessary for combustion of the fuel and part flowing through the jacket space around the inner chamber. Motive fluid consisting of the combustion gases from the inner chamber and the suitable factor which changes assavoo 5 Iacketing air is conducted through the twin pipes H8 and I2II to the turbine III for expansion in the turbine.

As indicated. the generators I8 and I8 feed into a common electrical system and, as is usual in cases of this kind, operate synchronously.

With normal load on the system, the requisite quantity of f el is pumped to the combustion chamber and with the motive fluid generated therein by combustion of the fuel with the air compressed in the compressors, the turbine generates the power required to operate the generators and also the power required to in addition drive the compressors. Under such normal load conditions, the bleeder valves of the two compressors are closed and the compressors operate under what may be termed full load compression.

When the load on the plant changes, the fuel supply is governed so that an increased quantity of fuel is supplied upon increased load and a, reduced quantity of fuel is supplied with decreasing load. Assuming constant normal full load, the quantity of air delivered by the compressor remains constant. Upondecrease in load, the closing eifect exerted by the sole'noids which hold the bleeder valves closed is decreased and these valves will open to an extent governed by the degree of the reduction in the load, so' as to open more or less the bleeder openings. Consequently. the greater the decrease in load on the plant, the greater will be the quantity of air which is discharged through the bleeder openings and the smaller will be the quantity of air which is compressed in the compressorsto the final pressure for delivery to the combustion chamber. When the bleeder valves are opened, the final pressure of the compressed air as well as the quantity of air compressed per unit of time will be reduced, because the ratio of the volume of the compression spaces at the commencement of the compression period to the volume thereof at the end of the compression period is constant. If a constant final pressure is desired regardless of variations in load or if the final pressure is desired to be varied with variations in load in a manner other than that effected with this apparatus, this may be accomplished by means to be hereinafter described and adapted to vary the ratio of compression space volume at the commencement and end of the compression periods.

The bleeder valve control need not necessarily be made dependent upon the electrical load on the plant but may be controlled by any other dicative of changes in the value of the load.

with and is incompressors is expanded in the turbine to pro duce the power required for driving the generator and the compressors.

The low pressure compressor I28 is provided with a bleeder opening 84 and a bleeder valve 88 controlling this opening, the construction being similar to that previously described with reference to Fig. 1. In this instance, however, the control of the bleeder valve is different from that previously described. In this embodiment the stem 88 of the bleeder valve is provided with a piston I88 working with a tight fit in cylinder I81 and acted on by a spring 18 tending to move the piston and bleeder valve to a position in which the bleeder passage is opened. The upper side of piston I88 is placed in communication with the compressed air conduit I42 by means of the pipe I44. A

The hydraulic coupling I28 is placed in communication with a governor indicated generally, at I48 by means of a pipe I48. The governor comprises two plungers I80 and I82 connected to each other and working in a cylinder I84. A piston [88 is rigidly connected to the plungers I58 and I82 and works in a cylinder I88. .4. spring I88 acts on one side of piston I88 while the other side of the piston is exposed to the pressure existing in conduit I42 and pipe I44 by means of the connection I82. Pipes I84 and I88 connect the interior of the governor with a liquid supply vessel I88. The vessel I88 is located at a lower level than that of the hydraulic coupling so that the working liquid may under certain conditions of operation flow by gravity from the couplingto the supply vessel. A gear pum-p I18 is located in the pipe I84, the suction side being connected to the supply vessel and the discharge side to the governor.

The pump feeds working liquid in the direction of the arrow I12. A pressure relief valve I14, on the discharge side of pump I18, permits rea turn of working liquid from the pump to the An example of such other control is illustrated in the plant shown in Fig. 2. In this embodiment. the gas. turbine Illa is indicated as being ofthe axial'flow type connected for direct drive of a generator I22 which delivers the power output of the system. Turbine ,I8a also drives directly a compressor I24 similar in type to the compressors previously described except for the omission of the bleeder valve arrangement shown on the compressor illustrated in Fig. 1. Compressor I 24 supply vessel through the return pipe I18 when the discharge end of pipe I84 is closed by the governor.

Operation of the system is as follows. Under full load operating conditions, the bleeder valve 88 is closed, as shown in the drawing, the strength of spring 18 being such that the spring is compressed by application of normal full load air pressure to the upper side of piston I88. Governor I48, under the assumed full load condition, is as shown in the drawing, with the pipes I84 and I48 connected and with the inlet end of pipe I88 closed. The working chamber of the hydraulic coupling is filled with working fluid and the two compressors are connected so that both are operated by the tu'rbine. If it is now assumed that the load on the generator is reduced, the fuel supply is correspondingly reduced through the action of the governor and proportionately lower amount of fuel as compared with air supplied to the combustion chamber. The reduced temperature of the motive fluid results in a reduction in the pressure prevailing in the combustion chamber and in the pipe I42 communicating therewith. This will be easily understood 'from the following calculation. The quantity of motive fluid flowing through a turbine can be expressed by the equation:

where F isthe flow of motive fluid in pounds per unit of time, P the initial absolute pressure of the motive fluid in pounds per square inch, 1: the specific volume of the motive fluid at the pressure P, and K a constant. The value of u may be derived from the well-known thermodynamic equation Pv=BT, in which T is the absolute temperature and B a constant. Substituting the value of v in the first equation,

I K K At the first moments after reduction of the fuel supply, the amount of motive fluid flowing through the turbine is approximately the same as before, the reduction in quantity due to the decreased fuel supply being of negligible order as compared with the quantity of air delivered by the compressor. Thus, if P and T represent initial pressure and initial temperature respectively at normal load, and P1 and T1 represent initial pres-' sure and initial temperature respectively shortly upon reduction of the fuel supply,.

For instance, if at normal full load P=80 pounds per square inch, T:1800 degrees F. absolute, and if the temperature due to the reduced fuel supply is decreased to Ti=l600, the pressure in the combustion chamber and in the pipe I42 will immediately be reduced to 11650 Pl-80 I Due to this reduction in the pressure the bleeder valve 60 commences to open under the influence of spring III. This opening or partial opening of the bleeder valve 60 permits air to flow to a greater or lesser extent through the bleeder opening 64 to atmosphere. Thus, a reduced quantity of air, corresponding to the reduced load and the reduced fuel supply, will be com: pressed and supplied to the combustion chamber. Reduction of the quantity of air compressed will tend to bring the temperature of the motive fluid back to about normal value, the quantity of motive fluid produced per unit of time now being reduced as compared with full load conditions and the pressure of the motive fluid being reduced also because of the reduced pressure at which the air is finally delivered from the compressor system because of the bleeding of some =75.4 pounds per square inch of the air. It will be seen that for every value of part load, there is a definite position of the bleeder valve 50 and consequently for each value of part load, there is a definite quantity of air discharged. from the bleeder opening.

As the load on the system falls below the normal load value and as a consequence the pressure in conduit I42 is reduced, the pressure acting on piston I56 of the governor will also be reduced. This piston'then commences to move toward the right, as viewed in the figure, under the influence of spring I60 and this movement is transmitted to the right from the position shown in the figure will have no efiect on the hydrauliccoupling but after the loadand the'pressure oi the motive fluid have fallen below a predetermined value of part load, for example, idling load, the plunger I50 will close the outlet of pipe I64 and simultaneously the plunger I52 will open the inlet of pipe I66. Movement ofrthe governor to this position closes the connection between pump I70 and the coupling, which is instead connected through pipes I08 and N0 to the supply vessel I68 to which the working fluid in the coupling will flow by gravity, in the direction indicated by the arrow H8. Draining oi the working fluid from the hydraulic coupling renders this coupling inoperative and the compressor i28 ceases to operate. At this time a spring loaded check valve I80 in the air conduit I36 connecting the two compressors is opened by the suction created by the compressor I24 and air is now compressed only in this compressor.

If the load increases from the value at which compressor i28 is renderedinoperative, the accompanying increase in the amount of fuel supplied will result in rising temperature and pressure of the motive fluid. The increased pressure of the motive fluid acting on piston I56 of the governor moves the plunger valves of the latter to the left and when a predetermined value of load is reached, the original connections are estab-.

' lished, that is, the pump is agam connected with the hydraulic coupling and the drain pipe I66 is closed. The pump refills the coupling with working liquid and the low pressure compressor I28 is brought back into operation. Reestablished operation of the low pressure compressor produces pressure in the connecting conduit I36 and as a consequence valve I80 closes. With this valve closed, the compressors again operate in series. Further increase in load causes the bleeder valve 60 to be brought more and more nearlyto closed position, progressively cutting down the amount of air bled from the low pressure compressor and, when normal full load has been reached, the

4 parts are again in the position shown in the draw- Whiieautomatic control of the bleeder valve or valves is advantageously employed, manual control may be resorted to and in Fig. 3 a manually controlled valve is illustrated. In. this figure, which shows a compressor in transverse section,

rotors I82 and I86 operate in the direction indicated by arrows I86 and I88 in casing I90. The

' casing I90 is provided with a bleeder opening I92 at the lower end of a cylinder I94 in which the bleeder valve I96 of the plunger type operates.

Bleeder valve I96 is adapted to seat against the casing in the closed position of the valve shown in the figures and is advantageously provided with an extension or plug portion I96a, the lower surface of which is curved to form a continuation of the cylindrical inner surface, of the casing. A space I98 surrounding the valve communicates with the bleeder outlet passage 200. A stem 202 is attached to the valveand is provided with a key 202a working in a suitable slot in the yoke 206 to prevent rotation of the bleeder valve. The

to the plungers I50 and I52. Initial movement upper portion of the stem is threaded at 206 through the hub of a hand wheel 208, the rotation of which on the threaded stem acts to lift the valve.

In the position of the apparatus shown, the bleeder valve is closed and compression of trapped air takes place in the compression space 2I0 between the threads or lobes 2 i 2 and 2 I4 of the rotor I82. Space 2I0 extends helically toward the suction side of the compressor to a point where a cooperating thread or lobe of the rotor 34 pro- Jects into the space so as to progressively reduce its volume upon rotation of the rotors in the directions indicated. The succeeding compression space 2l8, looking in the direction of rotation of the rotor, is still in communication with the suction side and remainsso until the edge 2|8 of the lobe 2l2 reaches the left hand limiting edge 220 of the bleeder opening. At this moment, compression commences in space 218. If the bleeder valve I" is then opened, air may escape through,

the bleeder opening without itshaving been appreciably compressed. From the foregoingit follows that, for instance, space 2 i ii is in communication with the bleeder valve I98 during the period from the instant the edge 2i! of the lobe 2 reaches the left hand limiting edge 220 of'the bleeder opening to the instant the edge 2l8 of the lobe 2 l2 reaches the right hand limiting edge 222 of the bleeder opening.

Advantageously the bleeder opening I92 is shaped so that communication of the compression space with the atmosphere takes place with minimum throttling losses. To this end, it is preferable to make the limiting

edges

220 and 222 of the bleeder opening parallel, or approximately parallel, to the helical edge 2l 8 of lobe H2 and consequently in the same relation with respect to the corresponding edges of theremaining lobes of the rotor. With the limiting edges of the bleeder opening parallel to the outer edges of the rotor lobes, maximum speed of opening and closing a: the bleeder opening with respect to the compression spaces, will be obtained.

'10 This slide isaxially movable as indicated in Fig. 4, through the medium of a control rod 236 which may be either manually operated or operated automatically in response to load variations on the system in which the compressor is incorporated, as has already been described in connection with Figs. 1 and 2'. k y

The end of the slide" at the inlet end of the compressor is preferably provided with

curved edges

228 and 240 adapted to-abut against correspondingly curved edges 242 and 244'when the slide is moved to the right hand end of its travel as seen in the drawings. At the outlet end of the compressor, the slide is advantageously provided with

curved edges

248 and 248; these latter edges determining the area for radial exhaust from the compression spaces and consequently I affecting the total exhaust area from these spaces and the discharge pressure of the compressed air. If we now assume the slide to be moved to the extreme right hand position with

edges

228 and 240 abutting respectively against

edges

242 and 244, compression will commence at the moment intended for normalfull load compression. This will be at the time when the edges of the rotor lobes, which cooperate to form a given compression space, pass the limiting edges of the inlet ports.

By making the lower surface of the bleeder valve curved to conform to the curvature of the inner surface of the casing, leakage back from the compression space 2lli to the space 2l8 is prevented when the edge 218 passes the bleeder opening I92 under full load operating conditions when the bleeder valve is closed.

As previously mentioned, the relation of the variation in final compression pressure to the variation in the quantity of air compressed at part loads can be varied as desired by changing the compression ratio which is affected under different conditions of load.

Apparatus for accomplishing this is shown in Figs. 4 to 7.

Referring now to these figures, the compressor comprises a casing 224 in which rotors 22! and 228 are mounted, these rotors being geared together as in the compressors previously described and being adapted to be driven by means of a power input shaft 220. In this embodiment, six

lobes are shown on one rotor and seven on the other in contrast to the three lobes shown for example in Fig. 3. It will be understood that the number of lobes per rotor may be varied as.desired within the scope of the present invention.

The casing 224 is provided with inlet openings 22! and .221 and an outlet 222. In this instance, it will be observed that the inlet ports in the casing provide for both radial and axial admission of air to the compression spaces and both radial and If it is desired to decrease the quantity of air delivered by the compressor, the control member or slide 234 is moved to the left from its extreme right hand position, to a position such for example as that shown in Fig. 7 and this movement opens up passages 254 and 2" between the end tion. commencement of compression in any given compression space will be delayed until the edges of'the lobes defining the posterior boundary of such space pass beneath the slide edges 23! and 240. 4

If for the moment we ,disregard the effect on the outlet opening of the movement of the slide to the left from its full load position, it will be evident that the compression ratio will be altered by such movement to the left since the volume of the compression space at the moment compression commences is decreased .as compared with full load adjustment, while the volume of the compression space at the moment when the exhaust port opens remains constant. This would result in decreased pressure at the moment of discharge from the compressor space.

Obviously however, movement of the control slide toward the left to a part load position will cause the area of the outlet opening to be decreased and, consequently, as the volume of the compression space at the instant compression commences is: decreased by movement of the slide, the volume when exhaust occurs is decreased and the final compression volume is also decreased. It will be evident that the change in the relation of the volume of the compression space at the moment when compression commences to the volume of the compression space at the moment when exhaust or discharge come mences can be varied as desired by suitably relating the contours of the edges of the slide which control the commencement ofcompression and the termination of compression in the various positions of the slide;

Fig. 8 shows a modification of the apparatus illustrated in Figs. 4 to 7. The construction is similar to that described above except the form of the control member or slide which in this instance is in the iorm of a cylinder 280 the lower portion of which is cut off and connected with a member 262 the inner surface of which is formed to the same radius as the radii of the rotors. As in the example previously described, the control member is axially movable by means of a control rod 284. As will be easily'understood from the drawing, the shape of the control mem ber 260 facilitates manufacturing of the apparatus as compared with the shape of the control member according to Figs. 4 to '7.

From the foregoing description, it will be evi dent that many specific different forms of structure may be employed within the scope Of the invention and it is to be understood that the invention embraces all that falls within the scope of the appended claims.

I claim:

I L-In a gas turbine system of the continuous combustion type, means for compressing air to be used as a constituent of motive fluid for expansion in the system comprising a rotary positive displacement compressor, means for bleeding the compression spaces of said compressor to retard the time of commencement of compression in said spaces until their volume has been reduced to less than their normal maximum volume, and means responsive to variations in the load on the system for controlling the flow ofair through said bleeding means.

2. A system as set forth. in claim 1 in which said compressor is of the type having inter-meshing screw wheel rotors and in which said bleeding means comprises valve means carried by the compressor casing to vent the compressor spaces intermediate the ends of the rotors and movable responsive to said control means to delay commencement of compression and to reduce the quantity of air compressed in each of said spaces by retarding the time of closing of the spaces.

3. A system as set forth in claim 1 in which said bleeding means is responsive to the pressure of the air delivered by the air-compressing means and is arranged to bleed the air in accordance with predetermined drop in load on the system.

4. A system as set forth in claim 1 in which the means for compressing air comprises a plurality of compressors arranged in series and in which the bleeding means is associated with the low-pressure compressor whereby to control the quantity of air compressed inthe system.

5. A system as set forth in claim 1 in which the means for compressing air includes a plurality of serially connected compressors with said bleeding means associated with the low-pressure compressor, coupling means for disconnecting I the low-pressure compressor and means responsive to variations in load on the system for disconnecting the low-pressure compressor when the load on the system drops below a predetermined value.

6. A system as set 'forth in claim 1 in which the means for compressing air includes a plurality oi serially connected compressors with said bleed= ing means associated with the low-pressure compressor, coupling means for disconnecting the low-pressure compressor, and means responsive to variations in load on the system for disconnect- LYSHOLM.

REFERENCES CITED The following'references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,584,602 Bradshaw et al. May 11, 1926 2,048,566 Rosch July 21, 1936 2,110,714 Place Mar. 8, 1938 FOREIGN PATENTS Number Country Date 418,463 Great Britain Sept. 14, 1934