The application generally relates to screw compressors used in vapor compression systems and more specifically to a vapor compression system utilizing a variable capacity screw compressor.
In positive-displacement compressors, capacity control may be obtained by both speed modulation and suction throttling to reduce the volume of vapor or gas drawn into a compressor. Capacity control for a compressor can provide continuous modulation from 100% capacity to less than 10% capacity, good part-load efficiency, unloaded starting, and unchanged reliability. In some positive-displacement compressors, capacity can also be controlled by a slide valve employed within the compressor. The slide valve can be operated to remove a portion of the vapor from the compression chamber of the compressor, thereby controlling the capacity of the compressor. Besides the slide valve, other mechanical devices, such as slot valves and lift valves, may be employed in positive-displacement compressors to control capacity. Adjustments to capacity control valves or variable displacement mechanisms can meet the demands of the system. In a refrigeration system, capacity can be regulated based upon a temperature setpoint for the space being cooled. In other systems in which the compressor is processing gas, capacity may be regulated to fully load the torque generator or prime mover (turbine or engine drive) for the compressor. However, all of the currently available methods are expensive and add to the initial cost of investment in the equipment.
In chiller applications where economy is desired both in the initial cost of the system and in operation of the system, a variable volume ratio application is desired. The volume, or compression ratio Vr in a screw compressor, is the ratio of the volume of a groove at the start of compression to the volume of the same groove when the discharge port begins to open. Hence, the size and shape of the discharge port is a factor in determining the volume ratio of a screw compressor.
For maximum efficiency, the pressure generated within the grooves during compression should exactly equal the pressure in the discharge line when the volume begins to open to it. If this is not the case, either overcompression or undercompression occurs, both resulting in internal losses. Furthermore, overcompression can harm the compressor. Such losses increase power consumption and noise, while reducing efficiency. Thus, volume ratio selection desirably should be made according to operating conditions when such an adjustment is available.
If the operating conditions of the system seldom change, it is possible to specify a fixed-volume ratio compressor that will provide good efficiency. Because overcompression can damage a compressor, when designing such a compressor, it is designed so that it does not frequently operate in an overcompression mode, if at all. As a result, such a compressor is designed to run at maximum compression under the most severe operating conditions, meaning that such a compressor runs in undercompression modes at all other operating conditions, so that inefficiency may result over extended periods of operation. What is needed is a system that permits adjustments to the volume ratio that changes the volume ratio depending on the conditions that the compressor experiences. This will allow the compressor volume to be adjusted to change the volume, and hence the volume ratio, as operating conditions change, allowing the compressor to operate at maximum efficiency.
A screw compressor for use in a refrigeration system is provided. The screw compressor includes a motor connected to a power source. A control panel controls operation of the compressor, including the motor and power source. The screw compressor has a variable volume capability. The screw compressor comprises a pair of meshing helical lobed rotors rotating within a fixed housing that are driven by a drive shaft connected to the motor. The housing encloses the rotors or screws, which operate in a working chamber within the housing. The working chamber has a length which varies based on the position of the rotors with respect to one another. The chamber has a maximum length when lobes of the rotor are not aligned with one another. The chamber has a minimum length when the rotors are in meshing alignment with one another.
Refrigerant gas enters the compressor from the suction or low pressure side of the refrigerant circuit through an inlet port when the rotors are arranged in the chamber to maximum length. The space between the lobes of the rotors, the interlobe region, is filled with refrigerant and the inlet port is closed. The refrigerant is compressed between the rotors in the interlobe region as they rotate, compressing the refrigerant gas and raising its pressure. As the highly compressed gas is ejected from the interlobe region, it is expelled into a volume in fluid communication with a discharge port, which ejects the high pressure gas into the refrigeration circuit.
The volume associated with the discharge port, referred to as the discharge port volume, can be varied. The housing adjacent the discharge port volume includes a penetration. This penetration in turn houses a movable member that is accessible from the exterior of the housing. The movable member can be adjusted from the exterior of the housing to open or close one or more apertures, that is, at least one aperture, or any portion of these apertures, to create or eliminate a path between the volume associated with the discharge port and the working chamber. When the movable member is adjusted to open the one or more apertures or a portion of an aperture, the refrigerant, at some point during the compression between the rotors, can follow a path through the one or more apertures to the discharge port without being fully compressed by the rotors. To close this path, the movable member is adjusted to fully close the one or more apertures, so that the refrigerant is compressed fully between the rotors as they rotate. The effect of adjusting the movable member to fully open the path, to fully close the path or to partially open the path by placing the movable member at some point intermediate a fully open position and a fully closed position is to change the compression volume at the discharge point, thereby affecting the volume ratio.
An advantage of a screw compressor of fixed capacity having a volume adjustment mechanism is that a machine can be manufactured and the volume ratio readily can be adjusted to maximize efficiency based on the climate of the area in which it is used without disassembly of the compressor after shipment.
Another advantage of a screw compressor having a volume adjustment mechanism is that a machine can be procured based on a maximum volume ratio for the most severe conditions, but the volume ratio can be adjusted based on seasonal variations by using the volume adjustment feature without disassembly of the compressor so that undercompression can be significantly reduced or completely avoided when conditions are not severe.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.
Referring to
Referring to
The compression ratio is provided as
Vr=ε1/κ
where
V is the volume ratio
ε is compression ratio and
κ is a refrigerant constant. For refrigerant R-134A, κ is 1.8, but will vary when other refrigerants are used.
In operation, when member 76 is in its first position in which member 76 completely covers apertures 74, refrigerant gas enters the compressor through inlet 62 and fills interlobe region 64. Because member 76 completely covers apertures 74, there is no flow path for the refrigerant gas except into interlobe region 64 where it is compressed, so the refrigerant gas is fully compressed in interlobe region and achieves its maximum compression ratio. The volume ratio is the ratio of the suction volume to the discharge volume. In this first position, the suction volume is the volume of the interlobal region before compression. The discharge volume is the sum of the volume of the interlobal region after compression+the volume in the discharge end. The volume in the discharge end is at a minimum value when member 76 completely covers apertures 74, so that both volume ratio and compression ratio are at a maximum, which is the desired operating condition when extreme environmental conditions are experienced.
In operation, when member 76 is in its second position in which apertures 74 are not blocked, refrigerant gas enters the compressor through inlet 62 and fills interlobe region 64. Because member 76 does not completely cover apertures 74, there is a flow path for the refrigerant gas as it is compressed within interlobe region 64. As the rotors turn, a portion of refrigerant gas is discharged through this flow path before it can be fully compressed in interlobe region. In this circumstance, the refrigerant gas does not achieve its maximum compression ratio. In this second position, the suction volume is the volume of the interlobal region before compression. The discharge volume is the sum of the volume of the interlobal region after compression+the volume in the discharge end+the volume of the additional flow path through apertures 74 and penetration 70 (which is bounded by member 76). As can be seen, positioning of member 76 can decrease the volume ratio from a maximum wherein the apertures are fully blocked to a minimum wherein member 76 is fully withdrawn. The compression ratio is also reduced, which is desired when environmental conditions are not severe. As used herein, severe operating conditions refer to the environmental conditions for which the compressor is designed to run at maximum compression, without overcompression.
The difference between a first position of member 76 and a second position of member 76 is the volume change that occurs as member 76 is moved within penetration 70. The volume change can be further increased by moving member 76 further outward to increase the volume within penetration 70. Besides Vr control, there is also another advantage that will be offered by such a mechanism. This advantage also is affected by the characteristics of the discharge port area as a plenum of fixed volume that gas flows into and out of at some rate. The volume may be favorable or unfavorable for sound generation depending upon pressure, temperature, and frequency of the gas moving through the plenum. There can be an infinite number of resonances that can occur given a wide range of operating speed of the screw, types of gases being compressed, as well as the pressure and temperatures of the gases. Changing the volume by adjusting the position of member 76 within penetration 70 may attenuate certain frequencies, thereby reducing noise or vibration and terminating these effects before they can achieve a resonance that excite discharge piping or components. This type of termination is similar to the phenomenon seen in a Helmholtz resonator.
As a practical matter, in extremely hot environmental conditions as occur in summer conditions, the screw compressor operates most efficiently when it is operating producing the highest refrigerant pressures. This condition is achieved when member 76 is in a first position completely blocking apertures 74 in housing 50 so that there is no alternate flow path for the flow of refrigerant and all apertures are blocked by member 76, the volume ratio being at a maximum. However, when the environmental conditions are not as extreme, for example in the winter, member 76 can be adjusted so that apertures 74 are not blocked and the flow path for the refrigerant from interlobe region 64, through apertures 74 into penetration 70 and into the discharge volume at outlet end 54 is maximized. The volume ratio will be reduced and the system will operate more efficiently at part load conditions, providing energy savings. Importantly, the adjustment from full load at position one to part load at position two, or any part load condition desired between position one and position two can be accomplished by adjusting member 76 without having to shut down or otherwise disassemble screw compressor 38. With the appropriate tool, member 76 can be adjusted inwardly or outwardly to achieve the desired volume ratio to match the environmental conditions. Furthermore, this adjustment can readily be made as often as the environmental conditions change. Thus, member 76 can be adjusted as required to an intermediate position between a first position and a second position during autumn and spring seasons.
Another advantage of this invention is that the manufacturer can provide the same screw compressor design (in terms of tonnage capacity) and provide for efficient operation by adjusting the position of member 76 within penetration 70 based on the temperatures experienced in a wide range of climates. Thus, the same compressor can be shipped to, for example, to subarctic climates or subtropics climates, and the volume ratio can readily be adjusted to match the climactic conditions by varying the position of member 76 within penetration 70 between its first position and its second position.
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
This patent application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/451,992 filed on Mar. 11, 2011, and entitled “STATIONARY VOLUME RATIO ADJUSTMENT MECHANISM”, the disclosure of which is hereby incorporated by reference herein in its entirety and made part of the present U.S. utility patent application for all purposes.
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Number | Date | Country | |
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Number | Date | Country | |
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61451992 | Mar 2011 | US |