Search Images Maps Play YouTube News Gmail Drive More »
Sign in
Screen reader users: click this link for accessible mode. Accessible mode has the same essential features but works better with your reader.

Patents

  1. Advanced Patent Search
Publication numberUS2134835 A
Publication typeGrant
Publication dateNov 1, 1938
Filing dateOct 9, 1937
Priority dateOct 9, 1937
Publication numberUS 2134835 A, US 2134835A, US-A-2134835, US2134835 A, US2134835A
InventorsNordberg Bruno V E
Original AssigneeNordberg Manufacturing Co
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Compressor unloader
US 2134835 A
Abstract  available in
Images(3)
Previous page
Next page
Claims  available in
Description  (OCR text may contain errors)

MW, L mm B, v. E. mommmramz 2 COMPRESSOR UNLOADER Filed Oct. 9, 1957 5 Sheets-Sheet l F \N M l I i 242 Ma /f2;

' o 46 41 4'7 24h 49 15h A L L H r 7 "48 A 28 v l'mvenfor 5 fb/wmovifno/zbw attorneys Nov. 1, 1938. B, v. E.-NORDBERG COMPRESSOR UNLOADER Filed Oct. 9, 1937 3 Sheets-Sheet 2 awn Gttornegs 1938. E. v. NORDBERG COMPRESSOR UNLOADER Filed 001:. 9, 1937 3 Sheets-Sheet 3 (Ittornegs Patented Nov. 1', 1938 v I UNITED STATES PATENT OFFICE 2,134,835 COMPBESSOR UNLOADEB Bruno 'V. E. NordbergpMilwaukee, Wls., assignmto Nordberg Manufacturing Company, Milwaukee, Win, a corporation ofWlsconsln Application October 9, 1937, Serial No. 168,266 14 Claims. (01. 230-31) This invention relates to compressors and par- Fig. 4 is a horizontal section through the masticularly to unloading means therefor. ter mechanism on the line 4-4 of Fig. 3.

In a prior application, Serial No. 49,624, filed Fig. 5 is a sectional view, somewhat diagram- November 13, 1935, I describe and claim an unmatic in character showing the regulating mech- 3 loading mechanism applicable particularly to a'nism with the servo motor and controlling valve 5 compressors having automatic valves. Accordfor setting the regulating mechanism. ing to that invention a master mechanism delivers Fig. 6 is a. section on a reduced scale taken on timed hydraulic impulses to motors which conthe line 6--6 of Fig. 4. trol the closing movement of corresponding inlet Referring first to Fig. l the bed of the compres- 10 valves, the effect of the valve control being to sor is indicated at 6, and the frame of the com- 10 vary progressively the effective displacement I pressor at 8. The crank shaft 9 turns in bearings stroke of the compressor. The master mechal l and I2, and reciprocates'a piston in the cylnism was self-contained. It was driven by the inder l3. The crank, connecting rod, cross-head, crank shaft of the compressor, and not only genpiston rod and piston are not visible in the drawerated the timed hydraulic impulses, but also ings, as the illustrated compressor is of the en- 15 developed in an accumulator hydraulic pressure, closed type. Being conventional these parts are which under control of a follow up valve respontoo well known to require illustration. sive to receive pressure, actuated the means to The compressor may be driven by any suitable vary the duration of the hydraulic impulses. means, but a Synchronous motor of the fly-Wheel The present invention involves similar printyp s p red a s co n o y indicated ciples and is subordinate to the prior application. at I. It differs from the prior device chiefly in the The intake or suction l4 leads to the inlet means'used to generate and time the hydraulic valves. There are four such valves, two for the impulses. In the prior device there was a disad nd. ind at d at H in Fig. and tw r placement plunger for each valve motor and the the crank end similarly indicated at live. All four 25 commencement of the impulse was timed by varyinlet valves are identical in construction. The ing the closure of a spill back valve. According discharge v lv wh h m y be f y i a l to the present invention there is a displacement y are not visible in the drawings but discharge plunger for each valve motor, but no spill back. to connection 15 which leads to receiver [1. The Instead, the timing of the plunger movements is p p It d liv rs h mp ss d r to h p int so varied relatively to the compressor crank shaft, o 1 so that the impulses commence at different com- I P e e a both inlet and discharge Va ves b pressor crank-angles. of the "reed or "feather type claimed in the As is the case in my prior application, I prefer to Nordhelg Patent. 1,570,392. a a y 1925,

5 use valve controlling motors which are biased to and Such inlet Valves are chosen illustration no-load position and move to loading position in Each inlet Valve is Provided With an unloader response to the impulse. Thus the c n motor whose function is toholdthe valve open for ment of the impulse marks the closure of the invariable periods through the initial Portion of the let valve and consequently the commencement of discharge Stroke the I p Working 40 the effective working (displacement) stroke. This Space- Eaflh P Of motors as th S of p e 40 has practical operative advantages fully develinlet Valves) is cohtllllled y a 9 responding oped in my prior application. impulse generator and timer in the ousing l9.

The invention will be described as applied to a The drive Shaft 1| Of this master medhanism is single cyllnder-double-acting compressor. Such a driven from Crank Shaft 9 at tihli Same angular cbmpressor has two working spaces it mm; velocity through sprocket chain 22. Receiver valves timed 180 of crank angle apart. Pressure is transmittal! y P pe 23 t the pressure I th drawings:-- responsive element "of a timing mechanism in Figure 1 is a plan view of a compressor of the housing There are head end aJld Crank type stated, showing the invention applied. L end impulse medhahisms in hqllsillg d these Fig. 2 is a fragmentary section, transverse to are connected by pipes 24h and 240 with the unthe axis of the cylinder, through'the center line of loader motors of the head end inlet valves and one of the inlet valves and its associated unloadof the crank end inlet valves, respectively. ing motor. Referring now to Fig. 2 which shows a head Fig. 3 is a vertical sectionthrough the master end inlet valve, the construction of all the inlet mechanism, on the line 3-4 of Fig. 6. valves can be explained. 5\5

The cylinder i3 is cored to produce a jacket, portions of which appear at 25 and an inlet passage 26 leading to the inlet valves from the inlet connection l4. Seated on rib 21 surrounding inlet port 26 and sealed thereto by gasket 29 is the grid-like valve seat member 3|. This is held down by the inverted V-shaped Yokes 32 engaged by studs threaded in bonnet i571. (and not visible in Fig. 2), a construction for which no novelty is here claimed. The guard grid 33 underlies the seat and is rigidly spaced therefrom, as shown, the bars of the grid overlying the slot ports in the seat and confining the valve leaf assemblies, each of which comprises a long narrow valve plate 34 and associated bow-spring 35.

To unseat the reed valve elements 34 during the unloading operation, there is provided a yoke 36 having two toothed comb elements 31. The teeth of the elements 31 project through the slot ports in seat 3| and in their inner position toward the axis of the cylinder hold all the reed valves 34 unseated. The yoke 36 is carried on a reciprocable stem 36 guided in a hub 39 integral with the bonnet I5h.

Fixed on the upper end of stem 36 is a rectangular yoke 4| whose upper end in the retracted (loading) position engages a combined stop and spring seat 42. The spring seat is mounted on the outer neck-like end of a sleeve 43 which is threaded into bonnet I5h, and is centered and held by the apertured cap 44. The compression spring 45 reacts between seat 42 and yoke 4| and urges the stem 36 in a direction to unseat the valves.

The yoke 4| embraces a piston 46 provided with packing grooves and working in a cylinder 41 freely encircled by the yoke and positioned in bonnet |5h by means of the opposite lugs 46 which seat in a counterbore. The sleeve 43 acts to force the lugs downward and clamp the cylinder in place. Inward motion of stem 36 is limited by collision of the outer end of yoke 4| with the outer end of cylinder 41.

The impulse pipe 2471. (in the case of the head-end valves) communicates by means of a nipple 49 and port 5| with the working space in cylinder 41 below piston 46. The spring 45 holds the unloader in unloading position except when an impulse is directed through pipe 24h to the space below piston 46. In other words, the inlet valves are normally held open and are allowed to close for chosen periods by the development of hydraulic pressures for such periods in the cylinder 41.

The inlet valve mechanism is thus the same as that described in my prior application in somewhat greater detail.

The mechanism which develops the timed pressure impulses for the head end and crank end inlet valves is shown in Figs. 3-6 and will now be described. Oil will be described as the hydraulic fluid because it is preferred, but without limiting implication.

The shaft 2| of the impulse unit has been described as driven at the same angular speed as the compressor crank shaft by chain 22 (Fig. 1). This chain runs on sprocket 62 (Fig. 4). Shaft 2| and a parallel cam shaft 53, carry identical small sun pinions 64 and 65 (Fig. 5). These do not mesh directly but are connected by larger meshing planetgears 66. 61. A pair of bell cranks 66 and a pair of radius links 56 connected by spacer links 6| carry the .idler gears 56, 51, the pins 62 and 63 on which the gears are respectively Journaled serving also to hinge the parts 56 and 6| and also 6| and 59 together, as shown.

Obviously angular displacement of bell crank 56 on shaft 2| causes angular displacement of gear 55 relatively to gear 54. More specifically, the gears turn in opposite directions, but shifting of bell crank 58 clockwise (as to Fig. 5) displaces shaft 53 clockwise relatively to shaft 2|.

Thus shifting of bell crank 58 in opposite directions causes shaft 53 to be advanced or retarded relatively to the rotation of shaft 2|, and is used to change the timing of two cams 64 and 65 (which are fast on shaft 53 and drive the unloader impulse plungers 66 and 61) with respect to crank shaft 9 of the compressor.

Two cams are used because the compressor has two working spaces, and the cams are apart because the working spaces are the opposite ends of this same double acting cylinder.

Adjustment of bell crank 58 is in response to receiver pressure and is effected by a doubleacting piston 66 in cylinder 69. Piston rod 1| is connected by link 12 with one arm of bell crank 56. A governor piston 13 is subject to receiver pressure arriving via pipe 23 and port 14, and acts upward on lever 15 fulcrumed at 16 and loaded at 11 (see Fig. 3). Lever 15 is connected by two articulated links 18a, 181), with one end of combining lever 8|. The purpose of two links 16a and 1817 will be later discussed, but normally they act as a single link and for present purposes will be so considered. Link 12 and consequently bell crank 58 is connected by link 19 with the other end of combining lever 8|. Lever 6| is pinned intermediate its ends to a balanced piston distributing valve 62 of the inside admission and outside exhaust type, clearly shown in Fig. 5. and controlling distribution to the two ends of cylinder 69. This valve has the minimum practical lap on the cylinder ports on both inlet and exhaust edges so as to afford a sensitive and responsive control of piston 66, and yet ensure that the piston 66 will be locked by the lap of the valve against yielding to the substantial force reactions transmitted through bell crank 56. This is a well known follow up valve mechanism.

Aspiston 13 moves up or down under changes in receiver pressure, piston 66 moves down or up (i. e. reversely) and for every position of piston 13 there is a definite corresponding position of piston 66. Thus cams 64 and 65 are angularly advanced or retarded relatively to compressor crank shaft 9 in response to the fall and rise of receiver pressure respectively.

In the embodiment illustrated the crank shaft 9 turns under". the direction of rotation being indicated by an arrow on Fig. 1, and the arrangement is such that as receiver pressure falls. the cams are advanced. Such advancement has the effect of increasing the loading of the compressor because the impulse plungers act earlier in the compressor displacement stroke and thus permit the inlet valves to close earlier.

Each of the plungers 66, 61 is urged toward shaft 53 by a corresponding coil compression spring 63 or 64 of substantial strength. The plungers 66 and 61 work in cylinders, 35 and 66, whose ends are connected respectively with the tubes 24c and 24h which lead to the crank end and head end unloader motors, as already explained.

Each plunger 66, 61 in its retracted position (see plunger 61, Fig. 3) overtravels an inlet port "5 81, 88 (see 88, Fig. 3) through which oil enters the cylinder from reservoir 89. Reservoir 89 is in communication with oil tank 9| in which shaft 53 runs, by way of port 92 and passage 93. A strainer 94 carried by closure plug 95, is mounted in passage 93 between tank 9| and port 92.

The displacement of each plunger 66, 61 is greater than the aggregate displacement of the related pistons 46 (there being two pistons 46 driven by each plunger), so the unloaders are shifted promptly upon movement of the plunger. The excess liquid is displaced through loaded relief check valves 96, 91 and through passage 98 to an accumulator chamber 99 beneath plunger lfll which is loaded by spring I02. A pipe I03 leads from accumulator 99 to the chamber in which valve 82 works, and supplies the pressure fiuid (oil) to operate piston 88, and adjust the position of cam shaft 53. Hence the device is wholly self-contained.

Piston IUI in its extreme upper position exposes relief port I04 which delivers through passage lll5 to the air separating cup I 86 with bafiie Hi1. From cup I06 oil overflows to cup I08 from which it may pass through opening 92 to chamber 89.

The parts are so timed that with the cams til, 65 in their most advanced setting, plungers 86, 61 will cause retraction of the valve holding combs at the start of the displacement strokes so that the inlet valves close as the stroke starts. Retardation of the cams progressively, causes retardation of the action of the plungers so that the combs are retracted progressively later. In the fully retarded position, the inlet valves are not permitted to close at all.

Since the displacement of the plungers 66, 6'! exceeds the displacement of the unloader pistons and since the plungers overtravel the inlet ports 81, 88, the plungers exert suction on their back strokes (under the urge of springs 83 and M), and help to restore the unloader pistons and hence" the combs to that position in which they obstruct the closing movement of the inlet valves.

Reference has been made to the use of two links Illa and 1829 between pressure responsive lever 15 and combining lever 8!. This is really an optional feature permitting the application of means to unload the compressor completely when it stops and maintain it unloaded for a substantial interval after it starts, so that it may come to full speed under no load.

To effect this result it is necessary to shift links 18a, 18b to the dotted line position (Fig. 3)

, as an incident to stopping, and to restore them slowly to full line position as an incident to starting, so slowly that the compressor attains full speed before it is loaded in any material degree. Various means might be used for this purpose, but a convenient means and one which broadly considered has been used to unload compressors during starting, comprises a pressure motor responsive to pressure in a pressure oiling system. Pressure in such system fades rapidly when the compressor stops and is built up gradually as the compressor starts.

Cylinder ill has a reciprocating piston ill? connected by rod l 13 with the hinge bet'ween links 18a, "Mb. An annular stop H4 limits outward motion of piston and a coil compression spring M5 urges the piston H2 inward. The working space behind piston H2 is connectedby pipe lit with the oil pressure line leading from oil pump H1. A check valve ilB loaded by a light spring i l9 opens for rapid flow from the working space,

} 3 while a choke l2! limits return flow to a. slow rate.

Consequently, when the compressor stops and lubricating oil pressure drops, piston H2 moves rather rapidly inward, raising lever 8i and cansing piston 68 to shift to unload the compressor completely. When the compressor starts piston III will be slowly forced outward and after a suitable time interval, determined by the size of choke I2I, the links 18a, 18b resume the normal position shown in full lines in Fig. 3. In this p0 sition the degree of loading depends on the pressure under piston 13.

Failure of lubrication tends to protect the compressor by unloading it.

The device, like that of my copending application, generates its own hydraulic impulses and hence is wholly self-contained. It can be applied to existing compressors in a very simple manner.

While a particular type of inlet valve has been illustrated, this is merely exemplary, for the invention can be applied to any valve whose movements can be arrested for unloading purposes. Any number of working spaces can be controlled by simple duplication of impulse mechanisms one for and timed to suit the particular space which it serves.

Various other modifications are possible within the scope of the invention.

What is claimed is:

l. The combination of a compressor having successive operating cycles, each comprising a suction stroke and a displacement stroke; liquid displacing means for generating successive hydraulic pressure impulses, one for each cycle, said means being capable oi developing impulses of uniform duration and at least as long as the duration of the displacement stroke; means for varying the timing of said impulses with reference to the timing of the displacement stroke, whereby the impulses start at variable points in the displacement stroltes; and means subject to suchimpulses for unloading the compressor during the displacement stroke until such impulse is initiated and for thereafter loading the com pressor throughout the remainder of such stroke.

2. The combination of a compressor having a cylinder, a coacting piston, and an inlet valve; unloading means having two positions, in one of which it renders the inlet valve inoperative and in the other of which it permits the inlet valve to operate; means biasing said unloading means to one of said positions; a pressure motor adapted when under pressure'to shift said unloading means to the other of said positions; an impulse piston driven by the compressor and connected to deliver pressure impulses to said motor; and means interposed in the drive between said compressor and said impulse piston and adjustable to vary the timing of the impulse piston re1atively to the strokes of said compressor piston.

3. The combination of a compressor having. a cylinder, 21. coasting piston, and an inlet valve; unloading means having two positions, in one of which it renders the inlet valve inoperative and in the other of which it permits the inlet valve tooperate; means biasing said unloading means. to one of said positions; a pressure motor adapted when under pressure to shift said unloading means to the other of said positions; an impulse piston driven by the compressor and connected to deliver pressure impulses to said motor; means interposed in the drive between said compressor and said impulse piston and adjustable to vary the timing of the impulse piston relatively to the strokes of said compressor piston; and means responsive to the pressure against which the compressor discharges for adjusting the last named means.

4. The combination of a compressor having a cylinder, a coacting piston, and an inlet valve; unloading means having two positions in one of which it renders the inlet valve inoperative and in the other of which it permits the inlet valve to operate; means biasing said unloading means to one of said positions; a pressure motor adapted when under pressure to shift said unloading means to the other of said positions; an impulse piston adapted to be driven by the compressor and connected to deliver pressure impulses to said motor; and a driving connection interposed between the compressor and said impulse piston, including a gear train of the sun and planet type having a portion displaceable to vary progressively the timing of said impulse piston with relation to the piston of the compressor.

5. The combination of a compressor having a cylinder, a coacting piston, and an inlet valve; shiftable means adapted to obstruct the closing movement of the inlet valve; means biasing said shifting means toward obstructing position; an hydraulic motor serving when subjected to pressure to withdraw said obstructing means; an hydraulic impulse piston adapted to be actuated in timed relation to the compressor piston and having a displacement which exceeds the displacement of the hydraulic motor; means for establishing an hydraulic pressure transmitting connection between the impulse piston and the hydraulic motor; a loaded relief valve permitting flow of excess hydraulic liquid from said connection; and adjustable driving connections interposed between said compressor and said hydraulic impulse piston and adjustable to vary the timed relation between the impulse piston and the piston of the compressor.

6. The combination of a compressor having a cylinder, a coacting piston, and an inlet valve;

shiitable means adapted to obstruct the closing movement of the inlet valve; means biasing said shifting means toward obstructing position; an hydraulic motor serving when subjected to pressure to withdraw said obstructing means; an hydraulic impulse piston adapted to be actuated in timed relation to the compressor piston and having a displacement which exceeds the displacement of the hydraulic motor; means for establishing an hydraulic pressure transmitting connection between the impulse piston and the hydraulic motor; a loaded relief valve permitting flow of excess hydraulic liquid from said connection; adjustable driving connections interposed between said compressor and said hydraulic impulse piston and adjustable to vary the timed relation between the impulse piston and the piston of the compressor; an accumulator fed by said loaded relief valve; a fluid pressure servomotor supplied with motive fluid by said accumulator and connected to adjust said driving means; and a pressure operated valve mechanism responsive to the pressures against which the compressor discharges and serving to control said servo-motor.

7. The combination of a compressor having a cylinder and a coacting piston; unloading means for said compressor; an unloader piston for actuating said unloading means; an impulsepiston adapted to be actuated in timed relation to the compressor piston and having a displacement which exceeds the displacement oi the unloader 2,1s4,sss

efiect of said pressure responsive means and the position of said servo-motor, whereby the servomotor is caused to assume different positions corresponding to the various pressure against which the compressor discharges.

8. The combination defined in claim 7 in which the adjustable driving connection between the compressor and the impulse piston includes a sun and planet gear train whose adjustment effects the timing of the impulse piston with respect to the compressor piston.

9. The combination of a compressor having successive operating cycles; liquid displacing means for developing liquid pressure impulses of uniform duration in timed relation to the opcrating cycles of the compressor; an adjustable drive train between thecomprcssor and said liquid displacing means for modifying the time relation between the compressor and said displacing means; and an unloading means for the compressor operated by said impulses and serving to unload the compressor to a degree which varies with the timing of the impulses relatively to the compressor.

10. The combination of a compressor having successive operating cycles; liquid displacing means for developing liquid pressure impulses of uniform duration in timed relation to the operating cycles of the compressor; an adjustable drive connection between the compressor and said liquid displacing means for modifying the timing of the liquid pressure impulses with reference to the operating cycle of the compressor; and means responsive to the pressure against which the compressor discharges and operated by pressure fluid derived from said impulses to adjust said adjustable drive.

11. The combination of a compressor of the expansible chamber type adapted to perform successive operating cycles; an unloading device having two positions in one of which it renders the compressor inoperative and in the other of which it renders it operative; means biasing said unloader'toward one of said positions; a pressure motor serving when energized to shift said unloader to the other of said positions; an impulse piston operable in timed relation to the cycles of the compressor; means establishing a fluid pressure transmitting connection between said impulse piston and said compressor motor; and a drive train between said impulse piston and said compressor, said drive train including an adjustable element for varying the timing of the impulse piston relatively to the cycles of the compressor.

12. The combination of a compressor of the expansible chamber type having an inlet valve and adapted to perform successive operating cycles, each comprising a suction stroke in which said inlet valve is normally open and a discharge stroke in which said inlet valve is normally closed; an unloading device having two positions in one,

of which it holds the inlet valve open and in the other of which it permits the inlet valve to operate normally; means biasing said unloader piston toward the first of said positions; a pressure motor serving when energized to shift said'unloader to the second of said positions; an impulse piston operable in timed relation to the cycles of the compressor; means establishing a fluid pressure transmitting connection between said impulse piston and said pressure motor; and an adjustable drive between said compressor and said impulse piston and adjustable to cause the impulse generated by said piston to overlap the whole or any desired part of the discharge stroke of said compressor.

13. The combination of a compressor of the expansible chamber type having an inlet valve and adapted to perform successive operating cycles, each comprising a suction stroke in which the inlet valve is normally open and a discharge stroke in which the inlet valve is normally closed; an unloading device having two positions in one of which it obstructs the'closing movement of the inlet valve and in the other of which it permits the inlet valve to operate normally; means biasing said-unloader to the first of said positions; a pressure motor serving when subject to pressure to shift said unloader to the second of said positions; an impulse piston operable in timed relation to the cycles of said compressor and having a displacement which exceeds the displacement of said pressure motor; means estab-. lishing a fluid pressure transmitting connection between the said impulse piston and the compressor motor; a pressure accumulator; a relief valve for delivering excess liquid from said con- '0ver Z ne entire displacement stroke;

nection to said accumulator; driving connections between said compressor and said impulse piston, said drive connection including adjustable means for varying the timing of the impulse piston with reference to the compressor cycle, whereby the impulses may be caused to coincide with the whole or any desired portion of the discharge stroke of the compressor; a servo-motor connected to adjust said drive; means responsive to the pressurelagainst which said compressor discharges; and a controlling valve for said servomotor subject to conjoint control by said pressure responsive means and by the adjusting means for said drive connection.

14. The combination of a compressor having successive operating cycles each comprising a suction stroke and a displacement stroke; liquid displacing means for developing successive hydraulic pressure impulses, one for each cycle, said means being capable of developing such impulses control means for varying the point in the displacement stroke at which such impulse is initiated, between a no load position in which it occurs at the end of the displacement stroke and a full load position in which it occurs substantially at the beginning of the displacement stroke; means for unloading the compressor during the displacement stroke until said impulse is initiated and for thereafter loading the compressor through the remainder of the stroke; and means rendered efiective by stopping the compressor, to move said control means to no load position and for inhibiting its return to a loading position for a time interval after the compressor is started.

BRUNO V. E. NORDBERG

Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US2470380 *Apr 20, 1945May 17, 1949Nordberg Manufacturing CoVariable-capacity controller for compressors
US2657850 *Nov 21, 1949Nov 3, 1953Schneider And Cie SocAutomatic regulator for compressors
US2865557 *May 27, 1955Dec 23, 1958Worthington CorpCompressor unloader
US8157538Jul 22, 2008Apr 17, 2012Emerson Climate Technologies, Inc.Capacity modulation system for compressor and method
US8308455Jan 27, 2010Nov 13, 2012Emerson Climate Technologies, Inc.Unloader system and method for a compressor
US8807961Mar 21, 2012Aug 19, 2014Emerson Climate Technologies, Inc.Capacity modulation system for compressor and method
USRE44636Jun 15, 2005Dec 10, 2013Emerson Climate Technologies, Inc.Compressor capacity modulation
Classifications
U.S. Classification417/281, 417/298, 92/140, 417/286
International ClassificationF04B49/22, F04B7/00, F04B7/04, F04B49/24, F04B49/02
Cooperative ClassificationF04B49/022, F04B7/04, F04B49/243
European ClassificationF04B49/24B, F04B7/04, F04B49/02C