US1097259A - Preventing pulsations in centrifugal compressors. - Google Patents

Description

M. I. NUSM. PREVENTING PULSATIONAS IN CBNTRIF'GAL COMPRESSORS APYLIGATION FILED SEPT.11,1913.

Patented May 19, 1914.

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@TT I PATEN Fl MELACH I. NUSIIVI, OF LYNN, MASSACHUSETTS, ASSIGNOR TO GENERAL ELECTRIC COMPANY, A CORPORATION 0F NEW YORK,

noemen vSpeecation of Letters Patent.

Patented May ie, 1er/i.

Application filed September 11, 1913. Serial No. 789,259.

T0 all 'whom it maj/concern Be it known that I, Memoir l. Nostri, a citizen of the United States, residing at Lynn, county of Essex, State of Massachusetts, have invented certain new and useful Improvements in Preventing Pulsations in Centrifugal Compressors, otwhich the following is a specification.

This invention relates to centrifugal air compressors of that type in which a rapidly rotating impeller throws the air through curved or tangential passages in a stationary diffusing ring surrounding said impeller. The action of these passages is to convert the velocity of the air into pressure, and when properly designed such machines may as in .the present case operate on what is known as a rising characteristic. This means that .the pressure does not remain constant for all volumes of air flowing through the intake, but varies with the volume delivered. ln order to secure an efficient and stable operation, the pressure should be above a predetermined value, or so-called rated pressure. If it drops below this, the diusing ring causes the air column to fluctuate or pulsate. The pressure curve, plotted with varying volumetric output as abscissas, shows that it starts at a point slightly below the desired rated pressure and rises slowly as the volume increases until at about half load it becomes slightly in eX- cess of said rated pressure, when it suddenly jumps up to a higher pressure. Atter this it maintains a fairly even pressure with increasing volume for some time, and then `radually drops to almost the rated pressure a't vfull load. This sudden jump in pressure is called the critical or breakdown point, and is the source of great trouble when the compressor is trying to carry a light load, the pressure fluctuating` violently and noisil y and setting up dangerous vibrations and shocks in the machine.l and preventing the delivery of a steady blast or' air, which is what is desired 'from an air compressor,

My invention aims to overcome these very objectionable surgii'igs or pulsations and insure a quiet, smooth operation of the. compressor at all loads. l' accomplish this by adjusting a multistage compressor to draw in a volume in excess of the breakdown load and then providing means foi-'blowing olf g such portion of the air as is not needed at that point in the casing of the machine vwith the frame 22.

the object of my invention to locate the valve in such a position as to release air from the casing at the lowest possible pressure, whereby thcstability of the compressor is regained with the least possible waste of air, and a saving in power necessary for driving is effected. Thus the preferable position of the valve in relation to the stages is about one-third the length of the casing away from the intake. In a three or four stage machine, the valve would be in the first stage; in a nine sta-ge machine, at the third or fourth stage. rll'he blow-oft1 valve can be operated either by hand or automatically; in the latter case it is controlled by a device which is responsive preferably to the air pressure in the, intake pipe of the com' presser.

In the accompanying drawing which illustrates one embodiment of the invention, Figure l is a front elevation of a multistage centrifugal compressor with t'he valve in position. Fig. 2 is a fragmentary section of the compressor showinga blow-o valve. Fig. El is a eross-section of the same showing in addition an automatic regulator. Fig. 4 illustrates a curve which givesthe pressure chariuftcristics.

Referring to the drawings, l0 indicates a casing for' a` multistage centrifugal conipressor having an intake l1 and a discharge 12. Located in the casing is an impeller 1.3 secured to the shaft which is mounted in the bearings 15 and 1G. A chamber 17 is cast integral with the casing and is connected to the inner chamber 18 of the first stage ot the compressor' by -a passage lt). Controlling this passage is a valve 2t) which is normally held on its seat by the spring 2l. One end of the spring 21 abuts against the rigid frame 92 and on the other against the washer 23 which is tast on the spindle 24. For opening the valve 2O a lever 25 is provided, ulcrunied on the bracket 26 which is cast Thel lever 25 can be manually operated or connected to an automatic regulator for opening the valve under pulsating condition. When the valve 2O 1s lifted from its seat air from the casing enters the chamber 17 and is led oft1 through the pipe Q7 and can be used for any desired purpose.

Located in the intake pipe 11 is a tube 28 acting like a Venturi tube which causes the aiif passing through it to increase in velocity and decrease in pressure, thereby causing a difference in pressure from that in the intake. These pressures are led olf through the pipes 29 and 30 to the diaphragm 31 located in the casing 32. The high pressure when at or above the rated value acts against the diaphragm 31 and balances the low pressure and the spring 33. Secured to the diaphragm 31 is a spindle 34 the opposite end of which is secured to the bell crank 35 fulcrume-d at 36. The rod 37 connects the bell crank 35 and the lever 25. The illustration is diagrammatic and the proportions of the parts are obviously incorrect from a practical standpoint.

When the discharge volume drops so low that the discharge pressure falls to or near to the breakdown point, the difference in pressures on the opposite sides of the da# phragm 31 becomes less due to the lessened rate of flow through the intake. The spring 33 is therefore able to overbalance the higher pressure and forces the spindle 34 to the left lifting the valve 2O to afford an additional discharge of air from the low pressure stage of the machine. Whether accompli/shed by hand or automatically, the effect of this is to permit la reduction below the breakdown point of the volume of air passing out of the dischar e 12 while maintaining a volume entering t rough the intake 11 which is above the breakdown. The result of this is to stop the pulsations, because actual experience indicates that the breakdownpoint occurs for a certain minimum How of air at the inlet of the machine, and if the flow at: this point can be kept above said minimum no trouble Will occur.

Fig.- 4 illustrates a curve which gives .the characteristics of a centrifugal compressor having rated volume and pressure and a discharge pressure indicated by the irregular line which cuts the line of rated pressure. As the flow or volume of air is reduced below the rated value, the pressure at first rises slightly, and when the volume is further reduced to one-half or one-third the rated value, there is a' sudden dro in pressure which is due to the fact that the internal assages in the diffusin ring are not suited) for eiiicient actionat lig ter loads.

In accordancelwith the provisions of the patent statutes, I have described the principle of operation of my invention together with the apparatus which I now conslder to represent the best embodiment thereof; but I desire to have it understood that the a-ppaf ratus shown is only illustrative and that the invention can be carried out by other means.

'What I claim as rlew and desire to secure by Letters Patent of the United States, is

1. The method of reventing pulsations at light loads in a multi-stage centrifugal air compressor, which consists in running said machine at a rate of iiow above the required load, and blowing oil' the excess of air from a lower stage.

2. A multistage centrifugal air compressor having means for blowing off air from a lower stage.

3. A multistage centrifugal vair compressor having a blow-ofi'l opening in a lower stage of its casing and a valve controlling said opening.

4. Amultistage centrifugal air compressor having ya blow-0E opening in a'lower stage of its casing and a valve cont-rolling said opening, a spring normally holding said valve closed,` and means for opening said valve.

5. A multistage centrifugal air compressor having a blow-0H opening in a lower stage of its casing and a. valve controlling saidl opening, a spring normally holding said valve closed, and means for opening said valve automatically when the air pressure approaches the breakdown point.

6. A multistage centrifugal air compressor having a blow-off opening in a lower stage of its casing, a valve controlling said opening, a spring normally holding said 'valve closed, and means for opening said valve automatically when the air pressure approaches the breakdown point, comprising a device responsive to the flow of fluid in the intake of the machine, and operative connections between said device and the blow-off valve.

7. A multistagel centrifugal compressor having a'blow-oif opening associated with an intermediate pressure stagez a valve controlling t-he opening, and afdevice responsive -to changes inthe rate of How of the iluid passing ,through the compressor for opening the valve when the load on the compressor-`v has been reduced to or below the breakdown point.

In witness whereof, I have hereunto set my hand this sixth day of September 1913.

. MELACH I. NUSIM. lVitnesses p J A. MoMANUs,-Jr., FRANK G.. HATTIE.

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