1. Field of the Invention
The present invention relates to a system for stabilizing the fluid flow in a variable capacity compressor.
2. Description of the Prior Art
Conventional compressors may be operated over a broad range of loads. As the load varies, it is necessary to stabilize the fluid flow in the compressor. A low flow rate may cause instability of the flow, thereby allowing the formation of rotating stall pockets or cells in the diffuser which create noise, cause vibration, and lower compressor efficiency. This condition is known as incipient surge or stall. If the flow becomes even more unstable, complete reversals of fluid flow in the diffuser may occur. This phenomenon, known as surge, is characterized by fluid alternately surging backward and forward through the compressor. Surge creates noise, causes vibration, lowers compressor efficiency, creates heat, and can cause damage to the compressor. Varying the geometry of the diffuser by altering the diffuser width is a recognized method of stabilizing the fluid flow.
Several devices for varying the diffuser width are disclosed in the prior art. For example, U.S. Pat. No. 4,527,949 to Kirtland discloses a device in which hydraulic fluid is delivered under pressure into a first expandable chamber to move a movable diffuser surface toward a fixed diffuser surface and thereby decrease the width of the diffuser. Providing fluid under pressure in a second expandable chamber causes movement of the movable diffuser surface away from the fixed diffuser surface and thereby increases the width of the diffuser
Another type of hydraulic system uses spring pressure to urge the movable diffuser surface in the direction of the fixed diffuser surface. In this system a low pressure area is produced behind the movable diffuser surface to counteract the force of the spring and to move the movable diffuser surface in a direction away from the fixed diffuser surface. A system of this type is shown in U.S. Pat. No. 4,416,583 to Burns.
Such hydraulic systems often use a piston and cylinder arrangement having a single large diameter piston. In the inventor's opinion, the length to diameter ratio of the piston is usually very low due to the small amount of axial space available inside the compressor housing. The low length to diameter ratio increases the possibility of jamming of the piston in the cylinder. To achieve a reliable length to diameter ratio, it would be necessary to increase the axial length of the compressor.
Such hydraulic systems may use multiple pistons instead of a single piston. It is the inventor's experience that an almost perfectly matched set of pistons is necessary to allow the pistons to move in unison. If the pistons are not matched closely enough, jamming can occur.
In hydraulic systems, uneven frictional force on the piston seals can cause the movable diffuser surface to become misaligned and jam. The piston seals themselves are also a source of concern, because if a seal fails, it may render the diffuser system totally inoperative. Seal leakage allows excessive oil to enter the compressor discharge gas, thereby affecting performance of the heat transfer surfaces. Seal leakage may also hamper proper operation of the compressor by bleeding pressure away from the hydraulic system. Hydraulic systems generally do not allow for a mechanical override. A breakdown in the hydraulic system therefore prevents the movement of the movable diffuser surface by other means. In addition, in a hydraulic system the movable diffuser surface needs to be rather stiff to reduce the possibility of warping. Warping of the movable diffuser surface can cause jamming. This stiffness requirement translates into increased weight and inertia.
Pneumatic systems are also in the prior art. A pneumatic, or refrigerant gas, actuated system operates in substantially the same manner as the hydraulic system. In this system the liquid of the hydraulic system is replaced by a gas from the compressor discharge. The pneumatic piston is designed similarly to the piston of the hydraulic system and therefore suffers from the jamming caused by the low length to diameter ratio. Pneumatic systems also have reliability problems for many of the same reasons stated above in regard to the hydraulic system.
Some mechanical systems can be found in the prior art. An example of a mechanical system is shown in U.S. Pat. No. 3,032,259 to Jassniker. The mechanical systems used in the past require a great deal of space along the centerline of the compressor and therefore increase the axial length of the compressor. Known mechanical systems do not achieve a desirable degree of compactness.
In summary, while a number of different systems have been used in the past for varying the diffuser width, those systems present a number of problems.