US2433328A - Gas-compressing plant having a reciprocating compressor - Google Patents

Description

Dec. 30, 1947. E. s. L. BEALE 2,433,328

' GAS-COMPRESSING PLANT HAVING A RECIPROCATING COMPRESSOR Filed April 19, 1944 2 Sheets-Sheet l ,INVENTOR, luezyn/ SIZ.fleaZ9 BY p a. W W

ATTORNEKF Dec. 30, 1947. E. s. L. BEALE 2,433,323

GAS-COMPRESSING PLANT HAVING A RECIiROCATING COMPRESSOR Filed April 19, 1944 2 Sheets-Sheet 2 aw M w. W MZ. M M M 5 2 7 ha y 7 4 a 4 w v I w m w 1. 5 -3 i m J a 7 Q m J 2 m B m 4 y 9% m m Q a M 5 W J 4 63 3 4 5 my. 4 E--- 3 O #3 F0 W w considerably-above that of the receiver.

Patented Dec. 30, 1947 UNITED STATES PATENT OFFICE GAS-COMPRESSING PLANT HAVING A RECIPROCATING COMPRESSOR Great Britain Application April 19, 1944, Serial No. 531,839 In Great Britain January 4 1943 When a, high-speed reciprocating compressor is fitted with ,efilcient delivery valves, of the multiple-blade type for instance, the flow of gas through the valves has a strong tendency to be oscillatory in character. The frequency of the oscillations is primarily determined by the volume of the compressor cylinder and the shape of the valve passages, the combination behavin as a, Helmholtz resonator.

Owing to the inertia of the air (or other gas) in the valve passages, the rate of flow through the valve cannot immediately become equal to the piston displacement when the pressure in the cylinder becomes equal to the receiver pressure, so that the cylinder pressure may rise very This excess pressure is produced by the inertia of the air and is not dependent on the frictional resistance of the air flowing through the valve, and in fact the efiects to be described are much more pronounced when the frictional resistance to flow is small. The amplitude of the first pressure wave in the cylinder increases as the speed of the compressor increases, because the rate of pressure rise in the cylinder before the delivery valve opens is higher, so that the delay due to inertia in establishing the flow through the valve gives rise to a higher excess pressure in the cylinder.

This initial pressure wave starts the major cylinder vibration at its natural frequency as a Helmholtz resonator, and if the valve is very efficient with little friction, e. g. the valve described in British patent specification No. 526,671 and hereinafter termed a Schurter valve, it has been observed that these vibrations may persist without much reduction in amplitude till the end of the delivery period, i. e., till I. D. C. (inner dead centre) is reached. For a little time before and after I. D. C. the rate of piston displacement is nearly zero, so that the pressure variation in the cylinder, representing compression and expansion of the air in the clearance space, may actually produce a considerable backward flow of airthrough the valve, even before I, D. C. is reached, 1. e., while the piston displacement is still outwards. This will occur at a time when the rate of rise of cylinder pressure is a maximum.

It has been observed that the valve blades may 7 Claims., (Cl. 230-56) then close at a high velocity on to their seats, owing to the negative pressure difference acting across them during reverse flow, and the blades may be damaged by impact against the seat. This condition may be aggravated it the shape of the receiver is such as to have a natural frequency close to the cylinder frequency.

An object of this invention is to introduce a degree of damping to these cylinder vibrations sufilcient to reduce their amplitude to a harmless value by the time the I. D. C. is approached, but without seriously increasing the pumping loss.

Embodiments of the invention will be described by way of example and with reference to the accompanying diagrammatic drawings in which:

Fig. 1 is a longitudinal section of a known type of internal-combustion free-piston power-gas generator,

Fig. 2 is a section on the line 2-2 in Fig. 1, the piston synchronising mechanism being omitted,

Fig. 3 is a sectional side elevation of the left hand portion of the delivery valve of the compressor part of the gas generator, and taken on the line 3-4 of Fig, 5.

Fig. 4 is a sectional side elevation oi the right hand portion of the delivery valve of'the compressor part of the gas generator, and taken on the line 4-4 of Fig. 5.

Fig. 5 is a sectional end elevation of the same valve, taken generally on the line 55 of Fig. 4, and

Fig. 6 is a diagram of a reciprocating air compressor and air receiver.

The gas-generator has two single-acting compressor cylinders I01 and" I0: disposed respectively on the two ends of the cylindrical engine case I l which acts as a scavenge air-receiver and which contains the two-stroke power cylinder I! having scavenge ports I21 and exhaust ports I22. The two opposed power pistons I 31 and I3: are directly coupled respectively .to the compressor pistons I41 and I42. The delivery valves l5 of the compressor cylinders are in the end plates H1 and H2 of the engine case II. The compressor cylinders are also provided with inlet valves l8. The chambers I! at the outer end of the compressor cylinders form pneumatic energy accumulators, hereinafter termed cushions. The two free-piston assemblies I31, I41 and I32, I41 are constrained to move equally and oppositely by the synchronizing mechanism which is not shown, but which is of well known type.

For convenience in'describing the mode of operation, it will be assumed that the engine case M contains air at superatmospheric pressure and that a charge of air has been further compressed between the power pistons I31 and I32; fuel is introduced into the power cylinder through an injection nozzle I23 and ignited by the heat of compression. The free-piston assemblies are consequently driven outwards, and the compressor pistons I41 and I42 draw free air through the valves l8 into the inner ends of the compressor cylinders and at the same time compress air in the cushions l3. Towards the end of the out strokes of the free-piston assemblies first the piston l3: uncovers the exhaust ports l2: and thereafter the piston I31 uncovers the scavenge ports |21, so that air from the engine case scavenges and charges the power cylinder. The mixture of gaseous combustion products and excess of scavenge air discharged through the exhaust ports I22 is maintained at superatmospheric pressure and constitutes the power gas generated. It may be used in any gasoperated machine, e. g.. a turbine arranged to do useful work. After the free-piston assemblies have reached their outer dead points, the energy stored in the cushions l9 drives them inwardly, so that the compressor pistons deliver the air that was induced through the valves l8 through the delivery valves l into the engine case M and at the same time the power pistons compress the fresh charge in the power cylinder l2.

The delivery valves I5, as well as the inlet valves I8, are of the kind described in the Farmer patent application Serial No. 454,381, filed August 11, 1942, now Patent 2,408,056, issued September 24, 1946, and are shown in the present Figs. 3, 4 and 5. Each valve includes two identical'side valve seat members 2|, which may be pres ure die castings, and which are of rectangul r profile as viewed from the side as in Fig. 3. The outer faces of these members are substantially plane, so as to fit within the housings shown in Fig. '1, and they are provided with a projecting step 22 at the outlet or inlet end, serving as a locating flange. The inner face 23 of each valve seat member is inclined at a small angle, which is 5 in this example, to the outer face, the inner face approaching the general plane of the outer face in the direction of air flow, which is upwardly in Figs. 3, 4 and 5. A depression 24 constituting an air inlet port is formed over a large part of the inner face 23 leaving a seating border. (Fig. 3) at the top and ends. The depression 24 extends to the inlet edge of the member 2|, forming an inlet opening 28 (Fig. 5). Vertical ribs 21 are provided in the depression 24 with their edges flush with the seating 23 for supporting a closure lamina when the valve is closed. The upper end of the depression 24 merges into the, seating 25 by a curve 28 (Fig. 5) forming a convergent wall portion of the air passage. Near the inlet end and at each of the front and back ends of each member 2| is formed a boss 29 through which passes a bolt hole 30 disposed perpendicularly to the outer face.

The two side valve seat members 2| are placed face to face at a suitable distance apart, being tied together by'two screw bolts 3! WlliQh pass through the holes 30. Furthermore, two end plates 32, which may be pressure die castings, are arranged to join the ends of the side valve seat members 2|, being secured to the latter by countersunk screws 33 and dowels 34. These end plates have plane inner faces lying parallel to each other. The outer surfaces of the end plates are generally plane and provided with a step 35, matching the corresponding locating step 22 of the side valve seat members 2|.

Adjacent to the inner face 23 of each side valve seat member 2| is mounted on the tie bolts 3| a closure sub-assembly including two closure laminae 36A and 36B disposed respectively on the two sides of a stop late 31, which may also be a pressure die casting. The border of the stop plate 31 at its entry end is bevelled on each side at 38 at an angle equal to the angle of inclination of the inner face of the side valve seat members-in this example 5. The borders of the two laminae 36A and 383 at their entry ends lie respectively flat-on these two bevels 38. The stop plate has two holes 33. and each lamina has two holes '40 accommodating the tie bolts 3|. A

clamping strip 4| of sheet metal, bent into V- section with the sides splayed at an angle of 10, embraces the lower borders of the two laminae, being held in place by rivets' 42 each passing through both wings of the strip 4|, the laminae and the interposed sto plate 31. The strip 4| extends substantially throughout the width of the part 26 of the inlet port between the bosses 29, stopping short of the ends of the sub-assemblyto enable the bosses 29 on the side valve seat member 2| to bear directly on the lamina 36A, which therefore normally lies with its side and discharge borders resting on the seating 25 of the member 2|. The ends of the stop plate 31 bear against the inner faces of the end plate 32, while the ends of the laminae have only a small working clearance from the end plates.

An intermediate valve seat member 43, which may also be a pressure die casting, is placed next to the closure subassembly. In end elevation the member 43 is of wedge section, with its narrow end at the discharge end of the valve and its plane faces inclined to each other at an angle of 10. This member 43 includes two end portions 44 and a bridge portion 45 joining the narrower ends of the end portions 44; the broader ends of the end portions are provided with bosses 46 through which pass holes, 41 for the tie bolts 3|. The plane faces on each side of the end bridge portions 44 and 45 form valve seatings. A series of tapered webs 48 depend from the bridge portion, with their edges flush with these seatings, for supporting the laminae in their closed position. The under surface of the bridge portion is curved at 43 (Fig. 5) to form convergent wall portions of the air passages.

Beyond the first intermediate valve seat member is placed a second closure sub-assembly, and so on to any desired number, until the other side valve seat member 2| is reached, the number of closure sub-assemblies exceeding by one the number of intermediate valve seat members. The tie bolts 3| serve to clamp the several elements of the valve rigidly together.

In operation, when the pressure on the inlet side of the valve begins to exceed the pressure on the discharge side, the laminae are deflected away from the seatings towards the stop plates 31, so that a narrow gap is formed between the discharge border of each lamina and the adjacent seating. The reduced pressure occurring at these gaps owing to the high velocity of air flow through them is transmitted across the discharge edges of the laminae to the spaces between them and the stop plates. The resulting pressure difference on the two sides of each of the laminae causes them to be strained back close against the stop plates. The throat now formed in the air passages in the neighborhood of the discharge edges of the laminae maintains a reduced pressure here, so that the laminae are subjected to a large enough ressure difference to keep them in the fully open position, indicated for one lamina by the dotted line 36C in Fig. 5.

The engine case H has a volume of about. twice the total swept volume of the compressor cylinders. Baflies I61 and I62 are fitted in the engine case about one-quarter of the length from the end plates respectively, so that the volume of the chambers I11 and I12 enclosed between each baille and the adjacent set of delivery valves I is equal'to'the swept volume of one compressor cylinder. It is important to observe that, since the gas compressed in each compressor cylinder is discharged from the efficient delivery valves 15 directly to the chamber I11 or I12, which has a volume of the same order as the swept volume of the compressor cylinder, there is a substantial degree of coupling between the cylinder and the 'said chamber when acting as resonators, and furthermore that the gas is discharged from this chamber through. the baifle I61 or I 62 imposing a damping resistance.

The free area through the baffles must be chosen so as to give enough resistance to flow to damp out thevibrations, but not so great as to. add substantially to the back pressure in the compressor cylinders. With a'substantial volume between the valve and the bafile, a given resistance to how through the bafile will cause a smaller pumping loss at the valve, because part of the air delivered through the valve can be accommodated in this volume, and only part of the air has to pass immediately through the bailie. In order to obtain a given degree of damping of the cylinder vibrations it is necessary to use a baffle having a higher resistance (i. 6., smaller free area) if a substantial volume is enclosed between it and the delivery valve, and this high resistance enables a good damping of the receiver vibrations to be secured.

If the enclosed volume is too large, no amount of resistance in the bailie will have much efiect on the cylinder vibrations, except insofar as the receiver vibrations are reduced. However, by choosing an enclosed volume of the same order as the swept volume of the compressor, th degree of coupling between the compressor cylinder and the chamber accommodating the said enclosed volume can be made large enough to give effective damping to the cylinder vibrations, while at the same time providing a substantial volume into which the air can be delivered without having to pass through the baffle. In this way a given degree of damping can be produced with a smaller increase in pumping loss than with a bafile put right up against the delivery valve.

Another result of fitting the baffle at some distance from the delivery valve is the damping of standing waves in the receiver itself, One aspect of this has already been mentioned, namely that by using a substantial enclosed volume the resistance of the bafiie can be greatly increased and so made to match the impedance of the receiver to give an almost non-resonant system. Another feature is that where the receiver is relatively small, as in the present case, the baille can in this way be placed near to a velocity loop of the most important mode of vibration in the receiver, where the particle velocity is high; and the bafile is much more effective than when placed close to the end, which would normally be a velocity node. Thus the position of the baflles shown in Fig. 1 is also suitable for damping the longitudinal air vibration in the engine-case having a velocity-node at each end (at the end plate Hi and H2) and one at the centre, since the baffles will then be close to the two velocity'loops. It may be of importance to damp out this particular node of vibration in the engine case, because its frequency may be close to the major cylinder vibration frequency. a

, bafile is about one-eighth of one compressor piston area. The mean iston speed is about 35 ft. per sec., the nominal velocity through the free area of the baflles being about ft. per sec. The perforations consist of moderately small holes, say A inch diameter, so that the jets of air formed at the holes will carry away the kinetic energy from the neighborhood of the holes and so provide proper dissipation of the vibrational energy. Holes of too large diameter would tend simply to provide inertia like the neck of a resonator.

Accordingly the word baffle means, in the present specification, an arrangement providing a plurality of apertures of circular or other section and providing a ratio of a gregate aperture perimeter to aggregate aperture area which is many times the ratio of circumference to area of a single circular aperture the area of which would be equal to said aggregate aperture area.

Fig. 6 shows diagrammatically an example in which the receiver III is cylindrical and the length through efficient delivery valves H5 of the Schurter type. An important mode of vibration of the air in this receiver may have a velocity node at each closed end and another at the center close to the delivery valves. Two baffles H61 and I I6: are fitted at velocity loops at about halfway between the valve and each end, where they will be most effective.

In one such plant tested the volume of the receiver III was about 300 cu. in., which was only about twice the swept volume of the compressor cylinder H0, namely 150 cu. in. Thus with the baflles fitted half-way .along the receiver halves, the enclosed volume Ill between them and the valve H5 would be about equal to the swept volume. In preliminary experiments the baliles were fitted so as to enclose only about one-quarter of the receiver volume instead of one-half. This gave very efiective damping but rather too high a back pressure.

The free area through the bafiles must be chosen with considerable care. In the test plant cited the total free areas of two alternative pairs of baffles were 1.6 sq. in. and 2.4 sq. in. respectively, representing about one-tenth and oneflfth of the area of the receiver. The compressor ran at 1500 R. P. M. The mean rate of piston displacement was therefore 7500 cu. in. per second. This would give nominal velocities through the holes in the baliles, with an enclosed volume equal to one-half the receiver volume, of 195 and 130 ft. per second, respectively. With an enclosed volume of only one-quarter of the receiver volume, these velocities would be 1.5 times as great, namely 290 and 195 ft. per second, respectively.

The total free area through the baflles in these two cases was about 85% and 130% respectively of the aggregate throat area of the Schurter delivery valves. The frictional pressure drop through these valves was about equal to one velocity head calculated on the throat area, whereas the discharge co-efficient for the holes in the baflle would be only about 0.7, giving a pressure drop of about 2 velocity heads. Thus for the same steady rate of flow through each (i. o., no allowance made for compression in the enclosed volume), the pressure drop through the bailles would be respectively about 2.8 times and 1.2 times that through the valves. This large ratio between the resistances of the two types of baffle, namely, 2.3:1, explains the large difference in the damping effect. If allowances are made for the enclosed volume, these pressure drops would be reduced to and of these values.

In the ordinary way the air receivers fitted to commercial compressors have a volume equal to very many times the swept volume, so that the amplitude of the pressure waves in the receiver itself is negligible. In such cases the position of the damping baffles may be chosen entirely from the point of view of damping the cylinder vibrations with the minimum pumping loss,

I claim:

1. A reciprocating gas-compressor having a. compressor cylinder, a piston arranged to sweep said cylinder, a chamber having a volume of the same order as the volume swept by said piston, a delivery valve communicating between said cylinder and said chamber, and means for discharging gas from said chamber while imposing on the discharging stream a resistance suflicient to introduce damping of oscillations of the gas stream through said valve, said means including perforated bailie means so dividing said chamber that the part thereof at one side of said baifie means and into which said valve opens has a volume not less than one-half of the volume swept by said piston, said baiiie means serving to impose on the gas stream delivered through said valve resistance damping oscillations of the gas.

2. A reciprocating gas-compressor having a compressor cylinder, a chamber, a delivery valve communicating between said cylinder and said chamber, said valve having a plurality of convergent-divergent passages each containing a closure member in the form of a resilient blade the free edge of which is in the neighborhood of the throat of the passage, and perforated baille means so dividing said chamber that the part thereof at one side of said baifle means and into which said valve opens has a volume not less than one-half of the volume swept by said piston, said baiile means serving to impose on the gas stream delivered through said valve resistance damping oscillations of the gas.

3. A reciprocating gas-compressor having a compressor cylinder, a piston arranged to sweep said cylinder, a chamber, a delivery valve communicating between said cylinder and said chamber, said valve having a plurality of convergent-divergent passages each containing a closure member in the form of a resilient blade the free edge of which is in the neighborhood of the throat of the passage, and perforated baffie means so dividing said chamber that the part thereof at one side of said baffle means and into which said valve opens has a volume of the same order as the volume swept by said piston, said bafiie means serving to impose on the gas stream delivered through said valve resistance dampin oscillations of the gas.

4. A reciprocating gas-compressor having a compressor cylinder, a piston arranged to sweep said cylinder, a chamber having a volume of the same order as the volume swept by said piston, a delivery valve communicating between said cylinder and said chamber, a receiver communicating with said chamber, and said means including perforated baflle means so dividing said chamber that the part thereof at one side of said bafile means and into which said valve opens has a volume not less than one-half of the volume swept by said piston, said baflie means serving to impose on the gas stream delivered through said valve resistance damping oscillations of the gas.

5. An internal-combustion-operated free-piston machine including a power cylinder, two opposed pistons slidable in said power cylinder, compressor cylinders disposed respectively at two opposite ends of said power cylinder, two compressor pistons slidable insaid compressor cylinders and operatively connected to said power.

pistons respectively, a scavenge air receiver at least partly'surrounding said power cylinder, air delivery valves communicating between the inner ends of said compressor cylinders and the ends of said receiver, two perforated bafiles disposed transversely in said receiver so as to divide it into three chambers, and a scavenge port opening from the middle one of said three chambers to said power cylinder, said middle chamber having a volume not less than one-half of the volume swept by said piston, said baffles serving to impose on the gas stream delivered through said valve resistance damping oscillations of the gas.

6. An internal-combustion-operated machine including a power cylinder, a power piston therein, a. scavenge air receiver communicating with said cylinder by a scavenge port, a scavenge compressor having a cylinder, 9, piston arranged to sweep said compressor cylinder and operatively connected with said power piston, a delivery valve communicating between said compressor cylinder and said receiver, and a flow-restricting baffle dividing said receiver so as to form on one side of the bafile a chamber which has a volume of the same order as the swept volume of said compressor cylinder and into which said delivery valve opens, said scavenge port opening from the other side of said baffle.

7. An internal-combustion-operated free-piston machine including a power cylinder, two opposed pistons slidable in said power cylinder, two compressor cylinders disposed respectively at opposite ends of said power cylinder, two compressor pistons slidable in said compressor cylinders and operatively connected to said power pistons respectively, a scavenge air receiver at least partly surrounding said power cylinder, air delivery valves communicating between the inner ends of said compressor cylinders and the ends of said receiver, and two perforated baffles disposed transversely in said receiver at substantially one-quarter of the length of said receiver from each end respectively, a scavenge port opening from the middle one or sald'three cham- Number bers to said power cylinder. 2,102,121 EVELYN STEWART LANSDOWNE BEALE. 2,108,890 1,950,063 REFERENCES CITED 5 2 5 29 The following references are of record in the file of this patent: Numb UNITED STATES PATENTS 391276 Number Name Date 10 731, 1,615,133 Pescara Jan. 18, 1927 479,546 2,016,613 Pescara Oct. 8, 1935 Name 4 Date Janicke Dec. 14, 1937 Janicke Feb. 22, 1938 Pescara, Mar. 6; 1934 Pescara, Aug. 8, 1939 FOREIGN PATENTS Country Date Great Britain May 4, 1933 France May 30, 1932 Germany July 20, 1929

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