Pressure Equalizer For Unloading A Compressor During Start-up

Abstract

A pressure equalization passage, controlled by a normally open valve, intercouples the discharge outlet and suction inlet of a motor-driven compressor anytime the compressor is inoperative. With an equalized pressure across and therefore no load on the compressor, the starting torque requirement for the motor will be minimized. Upon and responsive to compressor operation, the valve closes and interrupts the passage to segregate the discharge outlet and suction inlet from each other. This is achieved by a spring-biased axially movable nozzle in the discharge line from the compressor, which nozzle serves as a movable valve member. When the compressor operates, fluid flow in the nozzle creates a pressure differential resulting in a force which moves the nozzle in opposition to its spring bias and to an operating position effective to close a vent in the equalization passage. In response to termination of fluid flow, occurring when the compressor ceases operation, the nozzle returns to a home position under the influence of its spring bias and unblocks the vent. In one application of the invention where the compressor pumps fluid in a closed series flow path, the nozzle also functions as the nozzle section of a jet pump which entrains or syphons fluid, previously diverted from the flow path and used for compressor motor cooling, back into the discharge line.

Description

BACKGROUND OF THE INVENTION

This invention relates to a novel arrangement for equalizing the pressure across a compressor during start-up in order to minimize the starting torque requirement. It may be employed in a variety of different environments and with a variety of different compressors, pumps, or the like. For convenience, the invention will be described in a refrigeration system suitable for use in air conditioning equipment.

During operation of a compressor a substantial pressure differential exists between its suction inlet and discharge outlet, and this is particularly true with respect to compressors of the type usually incorporated in refrigeration systems. When the compressor becomes inoperative for any reason, for example when the system requirements are satisfied or when there is a temporary power failure in a motor-driven compressor, that pressure differential remains for a relatively long time period and presents a considerable load to the drive source if an attempt is made to immediately restart the compressor. Until the pressure is equalized across the compressor, a high starting torque will be required to resume compressor operation. When the drive source constitutes a motor, very high starting current will be needed to meet such a starting torque requirement. A relatively expensive motor is necessary to overcome the severe strain placed on it during start-up.

In order that a less expensive drive source may be used, devices have been developed to ensure that there is no high pressure differential in the compressor during the start-up time. However, they are usually complex in nature and are not always reliable in operation. Applicant's pressure equalizer is of unique construction, requires relatively few parts, and is most reliable in operation. Besides, in one application of the invention in a refrigeration system some of the parts serve dual roles; they perform one function in the pressure equalizer and another entirely different function in the refrigeration system. This manifests in manufacturing economies.

It is, thereof, an object of the invention to provide a new and improved pressure equalization arrangement for unloading a compressor during the start-up period.

Another object is to provide a pressure equalizing device of relatively simple and economical construction.

A further object of the invention is to provide a novel pressure equalization arrangement in a known fluid processing apparatus where some of the components required in the apparatus also serve as elements of the pressure equalizer.

SUMMARY OF THE INVENTION

The pressure equalizing arrangement of the invention is to be incorporated in fluid processing apparatus of the type in which fluid in a closed series flow path is pumped by a compressor from a relatively low pressure at its suction inlet to a relatively high pressure at its discharge outlet and thence to a discharge line. The arrangement comprises, in accordance with one of its aspects, a pressure equalization passage which normally intercouples the discharge outlet and suction inlet so that their pressures will be equalized when the compressor is inoperative. There are valve means, responsive to operation of the compressor, for creating a pressure differential along the discharge line and for utilizing the differential to interrupt the equalization passage and to segregate the discharge outlet and suction inlet from each other.

In the illustrated embodiment of the invention the equalization passage includes, and is controlled by, a normally open vent in the valve means. The pressure differential is developed in the discharge line by a spring-biased axially movable nozzle which is movable, in opposition to its spring bias and by a force resulting from that pressure differential, to an operating position effective to close the vent and interrupt the equalization passage.

In accordance with another facet of the invention, the compressor is included in a hermetic motor-compressor unit and some of the fluid is extracted from the flow path and circulated as a coolant in the unit's motor cavity and at a pressure less than the discharge pressure at the compressor's outlet. The spring-biased axially movable nozzle doubles as the nozzle section of an aspirating means which drops the pressure in the discharge line in order to entrain the coolant into the flow path.

DESCRIPTION OF THE DRAWING

The features of the invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood, however, by reference to the following description in conjunction with the accompanying drawing in which:

FIG. 1 is a schematic representation of fluid processing apparatus, specifically a refrigeration system, incorporating a pressure equalizer constructed in accordance with one embodiment of the invention and depicting its condition during operation of the compressor; and,

FIG. 2 discloses a portion of the system of FIG. 1 and illustrates the condition of the pressure equalizer after the compressor ceases operation.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

The disclosed refrigeration system includes a hermetic motor-compressor unit 10 having a rotary vane compressor 12 and a motor 14 for driving it. Unit 10 may take any of a variety of different well-known constructions. Moreover, compressor 12 need not be of the rotary vane type. For example, it may be of the reciprocating or centrifugal types. Compressor 12 is coupled in series with a closed flow path or circuit containing a device 15, a condenser 17, an expansion valve 18, and an evaporator 19. Device 15 in a sense is a combination valve and jet pump or venturi section and thus may be referred to as an aspirating-valve means.

In conventional manner, during operation of compressor 12 fluid in the form of refrigerant gas is received at the compressor's suction inlet 21 at relatively low pressure and temperature and is pumped and compressed by the compressor in order to discharge that gas at the discharge outlet 22 at relatively high pressure and temperature. The discharge line, which includes device 15, conveys the hot discharge gas to condenser 17 wherein heat is removed therefrom while retaining a pressure slightly less than that at discharge outlet 22. Sufficient heat is removed to condense the discharge gas so that the refrigerant leaves condenser 17 in its liquid state. The liquid refrigerant in passing through expansion device 18 reduces its pressure and temperature and emerges primarily as a liquid. As it then flows through evaporator 19, which is in heat exchange relation or contact with the medium to be cooled, heat is absorbed from the medium and the entirety of the refrigerant assumes its gaseous state for return to suction inlet 21 of the compressor.

Some of the fluid is extracted from the closed series flow path and circulated as a coolant or cooling medium in the motor cavity of motor-compressor unit 10. More particularly, some of the liquid refrigerant emerging from the output of condenser 17 is conveyed to a chamber or cavity 24 where it flows in heat transfer relation with the motor. If desired, a flow restriction device of some sort may be incorporated in the line from condenser 17 in order to better control the quantity of refrigerant used for cooling. The parts of motor 14 are bombarded by the liquid refrigerant, which vaporizes or flashes into gaseous form, the parts thus cooling by the latent heat of vaporization of the coolant. After circulating through cavity 24, the coolant in its gaseous state exits from the motor cavity at outlet 25.

Due to pressure drops in condenser 17 and motor cavity 24, the vaporized liquid coolant leaving the cavity will have a pressure less than the discharge pressure at compressor outlet 22. For this reason, aspirating-valve means 15 is needed to induce the coolant into the discharge line from compressor 12. More specifically device 15, which may also be considered a modified jet pump, has a nozzle or gas accelerating section 27 axially movable within a chamber 28 but spring-biased to a home position (shown in FIG. 2) by means of a coil spring 29 located within the chamber. The modified jet pump also includes a fixed throat or pinched section 31 and a fixed diffuser or gas decelerating section 32. The nozzle, throat and diffuser sections are all in series with the compressor's discharge line. As the high pressure discharge gas flows through jet pump 15, its pressure falls in nozzle section 27 where its velocity is increased, a low pressure region thus developing in throat section 31. Downstream the gas pressure increases to approximately its original pressure as the velocity decreases in the diffuser section 32.

In accordance with a feature of the invention, the pressure differential created along the discharge line by nozzle 27 results in a force which effects axial movement of the nozzle in the direction toward throat section 31 and in opposition to the spring bias exerted by spring 29. The position to which nozzle section 27 is moved in response to operation of compressor 12, and consequent high pressure gas flow from discharge outlet 22, may be called the nozzle's operating position and is illustrated in FIG. 1. The reason for making nozzle 27 axially movable, rather than fixed as in a conventional jet pump, will be made apparent hereinafter. The pressure in throat section 31 will be sufficiently low, with respect to the coolant's pressure at motor cavity output 25, to entrain or aspirate the coolant into the discharge line, outlet 25 being coupled to the throat section via conduit 34 and chamber 28.

Liquid cooling of the compressor motor in the manner described is advantageous over both suction gas and discharge gas cooling for several reasons. The heat dissipated from the motor is kept out of the compression cycle, thus reducing the discharge gas temperature which in turn results in an improvement in the compressor efficiency and capacity. The cooling of the motor parts by impingement of liquid droplets at condenser temperature and pressure causes the motor to be cooler than if cooled by gas. This permits the mechanical size and cost of the motor to be minimized.

In accordance with the invention, a pressure equalization passage normally intercouples discharge outlet 22 and suction inlet 21 so that their pressures will be equalized immediately upon termination of compressor operation. This passage includes vent 36 in device 15, conduit 38 and a portion of the suction line connected to inlet 21. When the compressor operates and nozzle 27 is in its operating position, as shown in FIG. 1, vent 36 (which communicates with chamber 28) is covered and blocked by the nozzle. Thus, the pressure equalization passage will be interrupted or closed at that time, permitting suction inlet 21 and discharge outlet 22 to be isolated from one another.

When motor 14 is deenergized compressor 12 ceases operation, the high pressure fluid flow through the discharge line terminates, and the pressure differential along nozzle 27 disappears. Under the influence of spring 29, the nozzle then returns to its home position shown in FIG. 2. Vent 36 thus opens and communicates, via chamber 28, with the output of nozzle 27, as a consequence of which a path is provided from outlet 22 back to inlet 21 to effect pressure equalization across the compressor. At the same time, since both conduit 34 and the output of diffuser 32 couple to chamber 28, the high pressures in the motor cavity and at the condenser input are vented back to the suction inlet. The pressure throughout the entire system therefore equalizes almost instantaneously. In the absence of any pressure across (and consequently load on) the compressor, only a minimum torque requirement need be satisfied to immediately resume compressor operation.

Upon restarting the compressor the fluid flow velocity increases, pressure begins to build up at discharge outlet 22, and nozzle 27 starts to shift (in response to the pressure differential that it generates) from its home position (FIG. 2) to its operating position, FIG. 1. A predetermined velocity level must be reached by the fluid flow, produced by the compressor, before that flow causes nozzle 27 to assume its operating position. It is to be particularly noted that device 15, with regard to its valve action, operates in response to the fluid flow developed by compressor 12. In other words, nozzle 27 initially responds to fluid flow (of appropriate velocity) in the discharge line and develops a pressure differential within itself and it then responds to that differential and moves to its operating position. With nozzle 27 in its operating position, vent 36 once again will be closed and the pressure equalization passage from outlet 22 to inlet 21 will be broken. Continued operation of the compressor retains nozzle 27 in its operating position and device 15 continues to function as a jet pump, aspirating the coolant back into the flow path.

The pressure equalization arrangement may be appropriately designed, such as in the selection of spring 29, so that a predetermined desired time delay will elapse between the initiation of compressor operation and the time at which nozzle 27 reaches its operating position. This delayed action ensures that the compressor will be unloaded at start-up and then the load will be applied gradually.

The duration of the time delay, before vent 36 fully closes, is also positively tied to motor speed. This follows since the pressure drop across nozzle 27, and thus the force tending to push it toward throat section 31, is a direct function of fluid flow through it, which in turn is a direct function of motor rpm. As the flow velocity builds up with motor speed during start-up, the pressure differential across the nozzle likewise increases. With a suitable choice of parameters and dimensions, the force necessary to shift the nozzle to its operating position will not be developed until motor 14 reaches its running speed. Hence, the flow velocity level that must be present in the discharge line before the nozzle actuates will be that level which is developed when substantially full motor speed is attained. In this way, the compressor is not fully loaded until the motor assumes its normal running speed.

Of course, while aspirating-valve means 15 is separate and apart from unit 10 in the described embodiment, this has only been done for convenience of illustration. Preferably, device 15 would be included in and made integral with the housing of unit 10.

Applicant has therefore provided a novel pressure equalizer wherein in the absence of compressor operation the discharge outlet and suction inlet are intercoupled to equalize their pressures. In addition to functioning as a jet pump, device 15 serves as a valve means which, in response to operation of the compressor and specifically in response to fluid flow produced by the compressor, creates (in nozzle 27) a pressure differential along the discharge line and then utilizes that differential to interrupt the equalization passage and to segregate the discharge outlet and suction inlet from one another.

While a particular embodiment of the invention has been shown and described, modifications may be made, and it is intended in the appended claims to cover all such modifications as may fall within the true spirit and scope of the invention.

Claims

I claim:

1. In fluid processing apparatus of the type in which fluid in a closed series flow path is pumped by a compressor from a relatively low pressure at its suction inlet to a relatively high pressure at its discharge outlet and thence to a discharge line, an arrangement for equalizing the pressure across the compressor during start-up comprising:

valve means including a spring-biased axially-movable nozzle in series with the discharge line and through which nozzle flows the entirety of the fluid from the discharge outlet,

said nozzle, in response to the fluid flow therethrough when the compressor is fully operative, creating a pressure differential within said nozzle resulting in a force sufficient to effect movement thereof in opposition to the spring bias from a home position to an operating position, said pressure differential holding said nozzle in its operating position so long as said compressor remains fully operative;

and a pressure equalization passage, controlled by said valve means, for intercoupling the discharge outlet and suction inlet when said nozzle is in its home position so that the pressures at the outlet and inlet will be equalized to unload the compressor during start-up and until it is fully operative,

said passage being interrupted by said valve means when said nozzle is in its operating position in order to segregate the discharge outlet and suction inlet from each other when the compressor is fully operative.

2. In fluid processing apparatus of the type in which fluid in a closed series flow path is pumped by a hermetic motor-compressor unit from a relatively low pressure at the compressor's suction inlet to a relatively high pressure at the compressor's discharge outlet, wherein some of the fluid is extracted from the flow path and circulated as a coolant in the unit's motor cavity and at a pressure less than the discharge pressure, and in which the coolant rejoins the flow path in the discharge line from the compressor, and arrangement for equalizing the pressure across the compressor during start-up comprising:

aspirating-valve means coupled to the motor cavity and to the discharge outlet and including a spring-biased axially movable nozzle section for dropping the pressure in the discharge line to entrain the coolant,

the pressure differential between the input and output of said nozzle section resulting in movement thereof, in opposition to the spring bias, from a home position to an operating position;

said aspirating-valve means including a vent which is closed by said nozzle section when in its operating position but is open and communicates with the output of said nozzle section when in its home position;

and a pressure equalizing conduit coupled between the suction inlet and said vent to intercouple, via said nozzle section and when the compressor is inoperative, the discharge outlet and suction inlet in order that their pressures may be equalized to unload the compressor and minimize the starting torque requirement for the motor.

3. A pressure equalizing arrangement according to claim 2 in which said nozzle section is axially movable within a chamber and wherein said vent communicates with said chamber.

4. A pressure equalizing arrangement according to claim 3 in which the motor cavity is coupled to said chamber to equalize the cavity's pressure to that in the compressor when the compressor is inoperative.

5. A pressure equalizing arrangement according to claim 3 in which said aspirating-valve means also includes fixed throat and diffuser sections communicating with said chamber to equalize the pressure on the output side of said diffuser to that in the compressor when the compressor is inoperative.

6. A pressure equalizing arrangement according to claim 3 in which the spring bias is effected by a coil spring in said chamber.