US3272426A

US3272426A - Refrigerant compressor control

Info

Publication number: US3272426A
Application number: US404847A
Authority: US
Inventors: Sidney A Parker
Original assignee: Lennox Industries Inc
Current assignee: Lennox Industries Inc
Priority date: 1964-10-19
Filing date: 1964-10-19
Publication date: 1966-09-13
Anticipated expiration: 1983-09-13
Also published as: DE1501084A1; NL6513097A; GB1067395A

Description

REFRIGERANT COMPRESSOR CONTROL Filed 001:. 19, 1964 5 Sheets-Sheet 1 Sept. 13, 1966 s FLARKER 3,272,426

REFRIGERANT COMPRESSOR CONTROL Filed Oct. 19, 1964 3 Sheets-Sheet z Sept. 13, 1966 PARKER 3,272,426

REFRIGERANT COMPRESSOR CONTROL Filed Oct. 19, 1964 3 Sheets-Sheet 5 wriiwz/zm WOW United States Patent 3,272,426 REFRHGERANT COMPRESSUR CONTRGL Sidney A. Parker, Fort Worth, Tern, assignor to Lennox Industries Inc., a corporation of Iowa Filed Get. 19, 1964, Ser. No. 494,847 6 Ciaims. (Cl. 230--31) This invention relates to a hermetic refrigerant compressor of the shell-within-a-shell type and, more particularly, to a novel capacity control mechanism for use in such hermetic refrigerant compressor.

In co-pending application Serial No. 361,126, filed April 20, 1964, in the name of Sidney A. Parker, there is disclosed a hermetic refrigerant compressor of the shell-Within-a-shell type. The compressor is designed to operate at a relatively constant high speed. In use, the load on the refrigeration system in which the compressor is arranged may vary, resulting in an ineflicient operation of the compressor unless the capacity of the compressor can be varied to comply with system variation. This invention is concerned with the provision of a refrigerant capacity control mechanism in such compressor which will provide for effective capacity control and will permit higher limits of compressor operation without producing excessive pressure drop.

In a shell-within-a-shell compressor, the compressor mechanism is resiliently supported within a hermetically sealed outer casing. The compression mechanism comprises a compressor block having a plurality of radial cylinders formed therein. Slidably movable in each cylinder is a piston. The pistons are connected to a drive shaft which is driven by an electric drive motor affixed to the top of the drive shaft. The end of each cylinder is closed by a cylinder head. Within each cylinder head there are openings which communicate with passage means in the compressor block. The passage means communicates the associated cylinder with the discharge gas manifold formed between the compressor block and an annular sleeve disposed thereabout. It is a feature of the present invention that resilient seal means be provided on the cylinder head of a cylinder to be unloaded for providing positive sealing means between the relatively high pressure discharge gas and the relatively low pressure suction gas when the cylinder is unloaded.

An object of the present invention is to provide a hermetic refrigerant compressor with a novel capacity control mechanism particularly suited for use within a shell and shell-type compressor, the capacity control mechanism being constructed and arranged to discharge bypassed discharge gas into the enlarged suction gas compartment between the compressor mechanism and the outer casing to minimize temperature build up in the compressor and to permit higher limits of operations.

Another object of this invention is to provide an improved hermetic refrigerant compressor with a unique capacity control arrangement and to provide O-ring seal means on the cylinder head closing each cylinder for providing positive sealing means between discharge pressure and suction pressure when the cylinder is unloaded, thereby assuring optimum performance of the capacity control mechanism and of the compressor. Other objects and advantages of the present invention will become more apparent hereafter.

The attached drawing illustrates a preferred embodiment of my invention in which like numerals refer to like elements, and in which:

FIGURE 1 is a side elevational view partially in section and with parts broken away of a hermetic refrigerant compressor embodying the novel capacity control mechanism of the present invention;

FIGURE 2 is a cross-sectional view of the hermetic Patented Sept. 13, 1966 refrigerant compressor of FIGURE 1, taken generally along the line 2-2; and

FIGURE 3 is a development of a portion of the compressor block of the four cylinder hermetic refrigerant compressor shown in FIGURE 1, and illustrating the connection of refrigerant capacity control mechanism to two of the four cylinders of such compressor.

Referring to FIGURE 1, there is illustrated a compressor embodying the present invention. The compressor 11) comprises a gas-tight outer housing including an upper shell 12 and a lower shell 13 integrally joined to one another as, for example, by welding. To the bottom of the exterior surface of the lower shell 13- are welded a plurality of legs 14, by means of which legs the compressor may be supported in upright position within a condensing unit or air conditioning unit.

Resiliently supported within the outer housing or casing of the compressor 10 by spring means 16 is a compression mechanism 18 which includes a compressor block having a motor flange portion 20 and a cylinder crankcase portion 22 separated by a partition wall 24. An annular sleeve 26, preferably made from sheet metal, surrounds the crankcase portion 22 of the compressor block.

Formed integrally on the top of annular sleeve 26 is an outturned ring-like flange 27. An annular member 28 having a transversely disposed ring-like flange 29 is affixed to the lower shell 13 of the outer casing. The spring means 16 are operatively associated with the flanges 27 and 29 for resiliently supporting the compression mechanism within the outer casing. The arrangement for mounting the compression mechanism is not part of this invention and, therefore, need not be described further herein. Reference may be made to co-pending application Serial No. 395,001 of Sidney A. Parker, filed September 8, 1964, for a more complete explanation of the construction and operation of the resilient spring means 16.

Provided within the compressor block are a plurality of radially-oriented cylinders 30. Though a four-cylinder compressor is illustrated herein, it will be understood that the present invention may be used in a hermetic refrigerant compressor having two or more cylinders. Cylinder sleeves of liners 32 are provided in each of the cylinders 30 and a piston 34 is slidably mounted for reciprocation within each of the cylinder liners 32. Each piston 34 has mounted therein a wrist pin. 36 upon which is journalled one end of a connecting rod 38. The other end of each connecting rod 38 is aflixed to the eccentric portion 40 of drive shaft 42.

The drive shaft 42 is connected at its upper end to an electric drive motor 44 by suitable retaining means 46. The motor 44 comprises a stator 47 which is supported within the motor flange portion 211 of the compressor block. The stator 47 is inductively connected to a rotor 48. Rotor 48 is operatively afiixed to the upper portion of the crankshaft 42 by means of a key or by shrinking the rotor onto the drive shaft or crankshaft 42.

Enclosing the top of the motor 44 is an end cap 50 which is connected to the top of the portion 20 of the compressor block.

The shaft 42 is journaled within a lower bearing 52 which is mounted in the lower bearing head 54. The lower bearing head 54 is maintained in position by a suitable wedge lock spring or retaining ring 56 seated within an annular groove in the lower portion 22 of the compressor block. Also provided in the lower bearing head 54 is a thrust bearing 58 which has a central opening 59 defined therethrough. Located in the lower portion of the shaft 42 adjacent the counterweight portion 60 is a coaxially disposed hole which constitutes the eye of the impeller of the pump means which are defined within the crankshaft. The pump means form no part of the present invention and reference may be made to the co-pending application of Sidney A. Parker, Serial No. 361,126, filed April 24, 1964, for a more complete description of the compressor lubrication arrangement and the pump means.

The crankshaft 42 is journaled in the compressor block intermediate its ends by means of the

bearings

61 and 62.

Provided at the end of each cylinder 38 and closing the end of the cylinder cavity are valve assemblies 64. Such valve assemblies may comprise a discharge valve 'unit 65', a suction valve plate 66, and a suction valve or reed member 67. The valve assemblies 64- are operative in a known manner.

Each valve assembly 64 is held in place in the end of a cylinder by a cylinder head or cap 68. A Belleville spring 69 and a retaining ring 70 cooperate with head 68 to maintain the head in position closing the end of a cylinder.

The cylinder head 68 is provided with a pair of annular recessesone on each side of passageway 77 in the compressor block. Within each recess, there is an O-ring 73 and 74, respectively, for preventing undesirable pressure loss and assuring that the entire discharge from the cylinder passes to the capacity control mechanism.

Provided on the crankcase are a pair of annular sealing flanges 71 and 72. These flanges are each provided with recesses within which are disposed O-rings 75 and 76 for sealing between the sleeve 26 and the respective flanges 71 and 72. Defined between the crankcase and the annular sleeve or shell 26 is a discharge gas muflling space 78 into which discharge gases are passed from the cylinders 30 after compression. The discharge gases pass from the annular chamber 78 through a discharge pipe 79 into the expansion loop of the discharge line 80, which extends through the outer shell 12 of the compressor 18 for conducting the discharge gas from the compressor to the condenser of a refrigerating system in a known manner. The annular space 78 is provided with a plurality of cavities to impart a muffling effect to the discharge gases which are passed from the cylinders of the compressor to the conduit 80, as will be more fully explained hereafter.

Suction gas enters the outer shell or housing of the compressor via suction inlet 82 and flows into a first compartment defined between the outer casing and compression mechanism below flange 29 on member 28. The gas passes through a second compartment or annular space 83 defined between flange 29 on the annular member 28 and flange 27 on the annular sleeve member 26 into a third compartment defined between the outer shell and the top of the compression mechanism 18. Then the gas passes through an opening 51 in the top of the motor cap or end cap 50 down over the electric motor 44, cooling the motor. The suction gas passes from the motor compartment through openings 84 in the compressor block into the valve assemblies 64.

A heat shield 85 is provided concentrically about the annular sleeve 26 for preventing heat transfer from the relatively hot discharge gas in the discharge gas mufiiing chamber 78 to the relatively cold suction gas entering the compressor.

A plurality of terminals 81 (two of which are shown) are provided in the top of the housing portion 12 in order to conduct electrical current from a suitable source to the motor and to provide for connection of suitable motor protection while preserving the hermetic nature of the compressor.

The novel means for varying the capacity of the hermetic refrigerant compressor comprises a valve member 86 operatively connected to a cylinder to be selectively loaded and unloaded and a pilot control member 88 for regulating the valve member 86 in response to predetermined conditions. The valve member 86 communicates with the cylinder via openings 91 in the cylinder head 68, opening 77 in the compressor block, and the bore through inlet fitting 87. Valve member 86 is supported by fitting 87 entirely within the compressor. The pilot control member 88 extends through an opening in the lower shell member 13 and through an opening in the backing plate 98 so as to be supported from the outer shell of the compressor. The space between the exterior of member 88 and the openings in the wall of the outer casing is sealed to maintain the hermetic integrity of the compressor. The pilot control member 88 may be operated from a thermostat or may be responsive to a suitable suction pressure control.

Referring to FIGURE 2, it is seen that the pilot control member 88, which is of a conventional construction and commercially available, includes a valve member 89 which is operatively connected by means of the conduit 92 to the discharge gas manifold 78 and by means of the conduit 93 to the top of the valve member 86. A plurality of spaced openings 26 may be provided in flange 27 of sleeve 26 for communicating the second compartment between flanges 27 and 29 with the upper compartment formed between the outer casing, the compression mechanism and flange 27.

Turning now to FIGURE 3, there is better illustrated the operative relationship between the capacity control means and the various cylinders within the hermetic refrigerant compressor. In FIGURES 1 and 2, there is shown only one cylinder unloading means, whereas in FIGURE 3, there are shown two unloading means.

The discharge gas muffling chamber 78 is defined between annular sleeve 26 and the adjacent portions of the compressor block enclosed between flanges 71 and 72. Discharge gas from each cylinder, identified herein as 30A, 30B, 38C and 30D, respectively, normally passes into the discharge muffling chamber 78. The gases from

cylinders

30B and 30D are discharged into a passageway 78a defined within the discharge gas mufiling chamber 78. The gas flows through the enlarged compartments formed between the intermediate flange and flange 72 and the

ribs

94, 95 and 96, respectively. Adjacent compartments are communicated by the restrictions between the ribs 94, and 96 and the flange 72. The gas passes between

ribs

96 and 96 through opening 98 into upper passageway 78b defined within the discharge gas muffling chamber. The enlarged compartment defined adjacent cylinder 30A and formed in part by flange 72, rib 96 and wall 97 functions as a dead end resonator cavity to further muflle the discharge gas,

Within passageway 78b, there are also a plurality of enlarged compartments and restrictions therebetween. The restrictions are defined between cylinder 30B and flange 71, cylinder 30C and the intermediate wall on the compressor block and cylinder 30D and flange 7 1. The discharge gas is discharged from the muffiing chamber 78 through passage 102 in rib member 102 which communicates at its upper end with conduit 79 and discharge line 80. A dead end resonator cavity is defined between flanges 71 and 72, cylinder 30A, and wall 97.

In embodiment of the invention shown in FIGURE 3, the capacity control means are associated with two of the four cylinders of the compressor, thereby providing for compressor operation at 75 percent of capacity (one cylinder unloaded) or 50 percent of capacity (two cylinders unloaded). Each of the cylinder heads associated with a cylinder to be selectively unloaded is provided with the novel seal means shown in FIGURE 1. The remaining cylinder heads may be provided with a single O-ring seal as seen in the compressor disclosed in said Parker application Serial No. 361,126.

The valve member 86 has an inlet 103 operatively connected by the conduit 184 to the discharge opening 77 from one of the cylinders 30C. The valve member 86 is provided with two outlets 106 and 108. The outlet 106 communicates with the suction gas in the chamber or compartment defined between the compression mechanism and the outer casing above flange 27 on annular sleeve 26. The outlet 108, which is connected to fitting 115, communicates with the passage 78a of the discharge gas muffiing chamber 78.

Slidably supported within the valve member 86 are a pair of interconnected and simultaneously movable valve elements 109 and 1 which are adapted to engage the seats 111 and 112, respectively, to selectively port dis charge gas (1) from the inlet N3 to the outlet 106 to unload the cylinder C or (2) from the inlet 1% to the outlet 108 to maintain the cylinder loaded. The top of the valve member 86 is operatively connected to the pilot control member by means of a conduit 93. Spider 113 within valve member 86 supports an end of stem 114, which integrally connects the two valve members 109 and 1 11 When the pilot control member 88 is energized in response to a predetermined condition, as for example, a predetermined suction gas pressure, the conduit 93 is communicated to suction gas pressure through passage 99 on valve body 89 and the pressure within the conduit 1% passing through inlet 1113 is sufiicient to raise the valve elements 109 and 110 into the position shown in FIG- URE 3, thereby permitting the bypass of discharge gas into the annular suction gas compartment defined between the compression mechanism and the outer shell above flange 27 to permit unloading of cylinder 30C. When the pilot control member 88 is deenergized, conduit 93 is closed to suction gas pressure and exposed to discharge gas pressure through conduit 92. Valve stem 114 is moved downwardly. The valve 110 will move from its valve seat 112 and the valve 109 will be seated on seat 111. Discharge gas will enter the valve member 86 via inlet 103 and pass through the opening between valve 110 and its seat 112 and through the openings in spider 113 for return to the passage 78a in the discharge gas manifold 78. The cylinder 3 3C is loaded at this time.

As seen from the foregoing, if one of the four cylinders is unloaded, three cylinders remain loaded and the compressor will operate at 75 percent capacity. If two of the four cylinders are unloaded, the other two cylinders remain loaded and the compressor will operate at percent capacity. Though a pilot control is not shown for the second refrigerator capacity control mechanism 86 illustrated in FIGURE 3, it will be understood that a pilot control member similar to that described is utilized with the second refrigerant capacity control.

In the present arrangement, when the cylinder 30C is unloaded, the discharge gas passes directly into the compartment between the compression mechanism and the outer casing, thus mixing with the incoming suction gas prior to the time the suction gas enters the opening in the motor end cap for passage over the electric motor 44. By this construction, higher limits of operation are possible than if the bypassed discharge gas were mixed with the suction gas immediately prior to entrance into the suction and discharge valve assemblies. The discharge gas mixes with a relatively large volume of incoming suction gas and the temperature build up of the gas entering passage or opening 51 in end cap 50 is thereby minimized. This permits a higher limit of operation of the compressor.

Another feature of the present invention is the utilization of a pair of O-rings in recesses in the cylinder head 68 on each side of the passageway communicating the cylinder with the discharge gas muffing chamber for sealing the cylinder head with respect to the surrounding cylinder wall or cylinder liner wall. By utilization of the novel O-ring seal means on the cylinder head of a cylinder to be unloaded, a positive seal is provided to isolate the cylinder from the discharge mufiing chamber during unloading of such cylinder, thereby sealing the high pressure discharge gas from the low pressure suction gas.

It will be understood that if only a 25 percent reduction in capacity is desired in the presently disclosed four cylinder hermetic refrigerant compressor, then only one .ing.

refrigerant capacity control is necessary (FIGURES 1 and 2). Only one cylinder need be adapted for unload- If 50 percent as well as 25 percent reduction in capacity is desired, the construction shown in FIGURE 3 may be used.

The present invention provides a relatively simple refrigerant capacity control construction that can advantageously be utilized in a hermetic refrigerant compressor of the shell-within-a-shell type.

While I have described a preferred embodiment of my invention, it will be understood that my invention is not limited thereto since it may be otherwise embodied within the scope of the following claims.

I claim:

1. In a hermetic compressor, the combination of a sealed casing, a compressor block resiliently supported within said sealed casing, a plurality of cylinders in said compressor block, a piston reciprocatingly movable in each cylinder, a drive shaft journaled in said compressor block and operatively driven by an electric motor disposed in said sealed casing, the end of each cylinder being closed by a cylinder head, annular wall means surrounding said compressor block, said compressor block being spaced from said sealed casing so as to define a compartment for receiving suction gas, said annular wall means and said compressor block defining therebetween a discharge gas muffiing chamber, each cylinder communicating with said discharge mufiiing chamber via passage means in said compressor block, said capacity control mechanism for selectively unloading one cylinder comprising valve means within said sealed casing having an inlet and first and second outlets, said inlet communicating with the passage means from said one cylinder for receiving high pressure discharge gas therefrom, said first outlet communicating with said suction gas compartment, said second outlet communicating with said discharge gas mufliing chamber, valve elements in said valve means for selectively communicating said inlet either to said first outlet to unload said one cylinder or to said second outlet to load said one cylinder, and control means for actuating said valve elements in response to predetermined conditions.

2. In a hermetic compressor, the combination of a sealed casing, a com-pressor block resiliently supported Within said sealed casing, a plurality of cylinders in said compressor block, a piston reciprocatingly movable in each cylinder, a drive shaft journaled in said compressor block and operatively driven by an electric motor disposed in said sealed casing, the end of each cylinder being closed by a cylinder head, annular wall means surrounding said compressor block, said compressor block being spaced from said sealed casing so as to define a compartment for receiving suction gas, said annular wall means and said compressor block defining therebetween a discharge gas mufiiing chamber, each cylinder communicating with said discharge muffling chamber via passage means in said compressor block, and capac ity control mechanism for selectively unloading one cylinder comprising valve means within said. sealed casing having an inlet and first and second outlets, said inlet communicating with the passage means from said one cylinder for receiving high pressure discharge gas therefrom, said first outlet communicating with said suction gas compartment, said second outlet communicating with said discharge gas muffling chamber, valve elements in said valve means joined for simultaneous movement so as to selectively communicate said inlet to one or the other of said outlets, and control means responsive to predetermined conditions for actuating said valve elements to (1) communicate said inlet with said first outlet, thereby bypassing said one cylinder and discharging the discharge gas into said compartment for receiving suction gas and (2) communicate said inlet with said second outlet to return discharge gas to said discharge gas muffiing chamber and maintain said one cylinder loaded.

3. In a hermetic compressor, the combination of a sealed casing, a compressor block resiliently supported Within said sealed casing, a plurality of cylinders in said compressor block opening to the peripheral surface thereof, a piston reciprocatingly disposed in each cylinder, a drive shaft journaled in said compressor block and operatively driven by an electric motor disposed in said sealed casing, the end of each cylinder being closed by a cylinder head, annular wall means surrounding said compressor block, said compressor block being spaced from said sealed casing so as to define a compartment for receiving suction gas, said annular wall means and said compressor block defining therebetween a discharge gas muflling chamber, each cylinder communicating with said discharge mufiiing chamber via passage means in said compressor block, O-ring means dispose-d about the cylinder head of a cylinder to be unloaded for providing a positive seal between the discharge gas and the suction gas when the said cylinder is unloaded, and capacity control mechanism for selectively unloading a cylinder comprising valve means within said sealed casing having an inlet and first and second outlets, said inlet communicating with the passage means from one cylinder for receiving high pressure discharge gas therefrom, said first outlet communicating with said suction gas compartment, said second outlet communicating with said discharge gas muflling chamber, valve elements in said valve means for selectively communicating said inlet to either said first outlet or said second outlet, and con-trol means for actuating said valve elements in response to predetermined conditions.

4. In a hermetic compressor, the combination of a sealed casing, a compressor block resiliently supported within said sealed casing, a plurality of cylinders in said compressor block, a piston reciprocatingly disposed in each cylinder, a drive shaft journaled in said compressor block and operatively driven by an electric motor disposed in said sealed casing, the end of each cylinder being closed by a cylinder head, annular wall means surrounding said compressor block, said compressor block being spaced from said sealed casing so as to define a compartment for receiving suction gas, said annular wall means and said compressor block defining therebetween a discharge gas muflling chamber, each cylinder communicating with said discharge gas muffling chamber via passage means in said compressor block, O-ring means disposed about each said cylinder head of a cylinder to be unloaded, said O-ring means being disposed between said cylinder head and its associated cylinder on each side of said passage means for positively sealing the high pressure discharge gas from the low pressure suction gas when the said cylinder is unloaded, and capacity control mechanism for selectively unloading one cylinder comprising valve means within said sealed casing having an inlet and first and second outlets, said inlet communicating with the passage means from one cylinder for receiving high pressure discharge gas therefrom, said first outlet communicating with said suction gas compartment, said second outlet communicating with said discharge gas muffling chamber, valve elements in said valve means for selectively communicating said inlet to either said first outlet or said second outlet, and pilot control means for selectively actuating said valve elements in response to predetermined conditions, said pilot control means being operatively associated with said valve means and said discharge gas muffiing chamber for moving said valve elements to open said first outlet, and pilot control means for selectively actuaing said one cylinder and for moving said valve elements to close said first outlet and open said second outlet, thereby loading said one cylinder.

5. For use in "a hermetic compressor of the type comprising an outer sealed casing, andcompression mechanism resiliently supported in said sealed casing, said compression mechanism including a compressor block, a plurality of pistons reciprocable in cylinders in said block, means for driving said piston, and annular wall means disposed about said block, said compression mechanism being spaced from said sealed casing for providing a space receiving suction gas, said wall means cooperating with said block to define a discharge gas mufiiing chamber; cylinder unloading mechanism comprising valve means having an inlet and a pair of outlets, said inlet communicating with one cylinder for receiving high pressure discharge gas therefrom, valve members within said valve means for selectively controlling gas flow from one or the other of said outlets, there being a first outlet communicating with said space receiving suction gas and a second outlet communicating with said discharge gas mufiiing chamber, and control means for actuating said valve means in response to a predetermined condition to open said first outlet and close said second outlet, thereby unloading said one cylinder, and for actuating said valve means to close said first outlet and open said second outlet, thereby maintaining said one cylinder loaded.

6. In a hermetic compressor, the combination of a sealed casing, a compressor block resiliently supported within said sealed casing, a plurality of radially disposed cylinders in said compressor block, a piston reciprocat ingly movable in each cylinder, a drive shaft journaled in said compressor block and operatively driven by an electric motor disposed in said sealed casing, the end of each cylinder being closed by a cylinder head, annular wall means surrounding said compressor block, said compressor block being spaced from said sealed casing so as to define therebetween a compartment for receiving suction gas, said annular wall means and said compressor block defining therebetween a discharge gas mufiling chamber, each cylinder communicating with said discharge muffling chamber via passage means in said compressor block, and capacity control mechanism for selectively unloading one cylinder comprising valve means Within said sealed casing having an inlet and first and second outlets, said inlet communicating with the passage means from said one cylinder for receiving high pressure discharge gas therefrom, said first outlet communicating With said suction gas compartment, said second outlet communicating with said discharge gas muflling chamber, valve elements in said valve means for selectively communicating said inlet either to said first outlet .to unload said one cylinder or to said second outlet to load said one cylinder, and control means for actuating said valve means in response to predetermined conditions to selectively bypass discharge gas to said suction gas compartment to unload said one cylinder, the cylinder head closing said cylinder to be selectively unloaded having a pair of O-rings disposed thereabout, one at each side of said passage means in said compressor block for providing -a positive seal between discharge gas pressure and suction gas pressure.

References Cited by the Examiner UNITED STATES PATENTS 1,484,674 2/1924 Redfield 230 27 2,836,345 5/1958 Gerteis 230-31 3,171,588 3/1965 Ayling 23o 206 3,209,985 10/1965 Sable 2.30 31 MARK NEWMAN, Primary Examiner.

W. J. KRAUSS, Assistant Examiner.

Claims

1. IN A HERMETIC COMPRESSOR, THE COMBINATION OF A SEALED CASING, A COMPRESSOR BLOCK RESILIENTLY SUPPORTED WITHIN SAID SEALED CASING, A PLURALITY OF CYLINDERS IN SAID COMPRESSOR BLOCK, A PISTON RECIPROCATINGLY MOVABLE IN EACH CYLINDER, A DRIVE SHAFT JOURNALED IN SAID COMPRESSOR BLOCK AND OPERATIVELY DRIVEN BY AN ELECTRIC MOTOR DISPOSED IN SAID SEALED CASING, THE END OF EACH CYLINDER BEING CLOSED BY A CYLINDER HEAD, ANNULAR WALL MEANS SURROUNDING SAID COMPRESSOR BLOCK, SAID COMPRESSOR BLOCK BEING SPACED FROM SAID SEALED CASING SO AS TO DEFINE A COMPARTMENT FOR RECEIVING SUCTION GAS, SAID ANNULAR WALL MEANS AND SAID COMPRESSOR BLOCK DEFINING THEREBETWEEN A DISCHARGE GAS MUFFLING CHAMBER, EACH CYLINDER COMMUNICATING WITH SAID DISCHARGE MUFFLING CHAMBER VIA PASSAGE MEANS IN SAID COMPRESSOR BLOCK, SAID CAPACITY CONTROL MECHANISM FOR SELECTIVELY UNLOADING ONE CYLINDER COMPRISING VALVE MEANS WITHIN SAID SEALED CASING HAVING AN INLET AND FIRST AND SECOND OUTLETS, SAID INLET COMMUNICATING WITH THE PASSAGE MEANS FROM SAID ONE CYLINDER FOR RECEIVING HIGH PRESSURE DISCHARGE GAS THEREFROM, SAID FIRST OUTLETS COMMUNICATING WITH SAID SUCTION GAS COMPARTMENT, SAID SECOND OUTLER COMMUNICATING WITH SAID DISCHARGE GAS MUFFLING CHAMBER, VALVE ELEMENTS IN SAID VALVE MEANS FOR SELECTIVELY COMMUNICATING SAID INLET EITHER TO SAID FIRST OUTLET TO UNLOAD SAID ONE CYLINDER OR TO SAID SECOND OUTLET TO LOAD SAID ONE CYLINDER, AND CONTROL MEANS FOR ACTUATING SAID VALVE ELEMENTS IN RESPONSE TO PREDETERMINED CONDITIONS.