Abstract:
A scroll compressor may be provided with a check valve interposed between a balance chamber and a discharge chamber. The check valve may allow balance chamber pressure to remain low during start-up of the compressor. Low balance chamber pressure may allow an orbiting scroll to remain unclamped to a fixed scroll thereby reducing torque required to initiate rotation of the orbiting scroll. After steady-state operational speed is reached the check valve may close and the scrolls may be clamped together to produce a desired steady-state operating condition for the compressor.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to refrigeration compressors. More particularly, the invention relates to a system for providing a scroll compressor with low starting torque. 
     Scroll compressors may be employed to compress refrigerant gas in cooling systems. In a particular application, a scroll compressor may be used in a distributed cooling system of a commercial aircraft. In that context, the scroll compressor may be required to start compression of refrigerant gas in high temperature conditions. For example, the aircraft may be positioned on the ground at a location with a high ambient temperature (e.g. air temperature of 110° F. or higher). In such a case, aircraft equipment bay temperature may be as high as 160° F. Consequently vapor pressure at an inlet side of an idle compressor may be as high as 200 to 250 psia. 
     A conventional scroll compressor may require application of high torque during start-up under these circumstances. In order to assure that high starting torque may be available; a conventional aircraft cooling system may be constructed with a high-torque motor for driving the compressor. A driving motor that is sized to provide high starting torque may be larger and heavier than a motor that may be sized only to accommodate steady state operational loads of the compressor. In that regard, the conventional compressor may be considered to need an oversized motor. A high-torque driving motor may also require a high capacity (i.e., oversized) inverter to provide a high level of AC current for the motor during compressor start-up. Oversized motors and inverters may add undesirable weight and cost to an aircraft. 
     As can be seen, there is a need for an aircraft cooling system in which a scroll compressor may be operated with a motor that may be sized in accordance with the compressor&#39;s steady state operational loads. Additionally there is a need for a scroll compressor which may be started with such a motor irrespective of ambient temperature in which the aircraft may be present. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a distributed cooling system for an aircraft may comprise: an evaporator-chiller; and a scroll compressor for compressing refrigerant from the evaporator-chiller, wherein the clamping force between an orbiting scroll and a fixed scroll of the compressor is produced by pressure in a balance chamber of the compressor, and wherein the pressure in the balance chamber is equalized with inlet pressure of the compressor at start-up so that starting torque of the compressor is reduced. 
     In another aspect of the present invention, a scroll compressor may comprise a check valve interposed between a balance chamber and a discharge chamber, wherein the check valve is adapted to permit flow of refrigerant gas from the balance chamber into the discharge chamber whenever gas pressure in the discharge chamber is less than gas pressure in the balance chamber. 
     In still another aspect of the present invention, a method for starting a scroll compressor may comprise: opening a gas flow passage between a balance chamber and a discharge chamber of the compressor; initiating rotation of an orbiting scroll while the gas flow passage is open; and closing the gas flow passage after the orbiting scroll is at its steady state operating speed. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a distributed cooling system in accordance with an embodiment of the invention; 
         FIG. 2  is cross-sectional view of a scroll compressor in accordance with an embodiment of the invention; 
         FIG. 3  is a collection of graph lines that illustrate dynamics of operation of the scroll compressor of  FIG. 2  in accordance with an embodiment of the invention; and 
         FIG. 4  is a flow chart of a method for starting a scroll compressor with a low starting torque in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
     Various inventive features are described below that can each be used independently of one another or in combination with other features. 
     The present invention generally provides a cooling system that uses a scroll compressor for compressing refrigerant wherein the scroll compressor is provided with an internal check valve that allows the compressor to start with a low starting torque. 
     Referring now to  FIG. 1 , a distributed cooling system  10  is shown in block diagram format. In an exemplary embodiment of the invention, the system  10  may comprises a plurality of cooled storage boxes  12  which may be used for storing food and beverage on a commercial aircraft (not shown). In the system  10 , heat from the boxes  12  may be extracted through a fluid-filled cooling circuit  14  and conveyed to an evaporator-chiller  16 . The evaporator-chiller  16  may extract heat from the cooling circuit  14 . Heated air may be removed from the aircraft though an exhaust passage  18 . 
     A refrigerant circuit  20  may interconnect the evaporator-chiller  16  to a compressor  22  at an inlet side  22 - 1 . In an exemplary embodiment of the invention, the compressor  22  may be a scroll compressor. The compressor  22  may be driven by an AC motor  24  which may be provided with electrical power through a dedicated inverter  26  which may be connected to a DC bus  28  of the aircraft. The compressor  22  may be interconnected, at an outlet side  22 - 2 , to the evaporator-chiller  16  through a condenser  30 . 
     Referring now to  FIG. 2 , the compressor  22  may be seen in a cross-sectional format. In an exemplary embodiment, the compressor  22  may comprise a fixed scroll  22 - 4  and an orbiting scroll  22 - 6 . In operation, the compressor  22  may employ an axial pressure balance system, wherein an intermediate pressure between the fixed scroll  22 - 4  and the orbiting scroll  22 - 6  may be fed into a balance chamber  22 - 8 . The balance chamber  22 - 8  may be adjacent the fixed scroll  22 - 4  (as shown in  FIG. 2 ) or, in an alternate embodiment (not shown), the balance chamber  22 - 8  may be adjacent the orbiting scroll  22 - 6 . 
     This intermediate pressure may be referred to as balance pressure. The balance pressure may be proportional to compressor inlet pressure. The proportionality may be a function of location of a bleed hole  22 - 10 . The balance pressure may create a clamping force that may counteract an axial separation force that may be proportional to compressor inlet pressure. The clamping force may keep the fixed scroll  22 - 4  and the orbiting scroll  22 - 6  in sealed contact with each other. This sealed contact may reduce leakage of refrigerant from a high pressure side to a low pressure side. 
     A check valve  22 - 12  may be positioned between the balance chamber  22 - 8  and a discharge chamber  22 - 14 . The check valve  22 - 12  may provide a gas flow passage for refrigerant gas from the balance chamber  22 - 8  into the discharge chamber  22 - 14  during start-up of the compressor  22 . It must be noted that at start-up, inlet pressure and outlet pressure are substantially equal. Thus the check valve  22 - 12  allows pressure in the balance chamber  22 - 8  to be substantially equal to outlet pressure. Consequently, a differential between balance pressure and outlet pressure may be substantially absent at initiation of start-up. Because of this virtual absence of pressure differential, the fixed scroll  22 - 4  and orbiting scroll  22 - 6  may move freely relative to one another. In other words, there may be virtually no torque needed to initially rotate the orbiting scroll  22 - 6 . 
     It may be seen that, even if inlet pressure is high, the check valve  22 - 12  may allow balance pressure to be no higher than outlet pressure. 
     Referring now to  FIG. 3 , a series of graph lines may illustrate dynamics of the compressor  22  during start-up.  FIG. 3  illustrates possible start-up operation of the compressor  22  on a hot day when initial inlet pressure, shown as graph line  32 , may be particularly high (e.g. about 210 psia). At time T 0  outlet pressure (graph line  34 ) may be equal to inlet pressure  32 . Also at T 0 , balance pressure (graph line  36 ) may be only slightly greater than inlet pressure  32  and/or outlet pressure  34 , because the check valve  22 - 12  of  FIG. 2  may allow equalization of outlet pressure  34  and balance pressure  36 . A slight difference between balance pressure  36  and outlet pressure  34  may result from a small pressure drop in the flow through the check valve  22 - 12 . This differential may kept relatively low by employing a high-flow check valve as the check valve  22 - 12 . 
     Because, at T 0 , there may be virtually no differential between balance pressure  36  and inlet pressure  32 , clamping force (graph line  38 ) may be low. As described above low clamping force may result in low starting torque requirement. 
     It may also be seen that as start-up progresses, inlet pressure  32  may drop and outlet pressure  34  may increase. In other words, a differential between balance pressure  36  and inlet pressure  32  may increase as start-up progresses. Within a short time period, at a time T 1 , (which in an exemplary embodiment may be about 20 seconds) the balance pressure  36  and the outlet pressure  34  may equalize. At that time, the check valve  22 - 12  may close, but the inlet pressure  32  at time T 1  may be lower than the inlet pressure at time T 0 . At time T 1 , the clamping force  38  may have increased to its normal operational level so the scrolls  22 - 4  and  22 - 6  may be sealed together. The compressor  22  may then be operational without undesirable leakage between the scrolls  22 - 4  and  22 - 6 . 
     It may be noted that at time T 1 , inlet pressure  32  is reduced and rotational speed of the motor  24  (of  FIG. 1 ) may have increased. Thus torque load on the motor  24  may be equivalent to steady state torque. In other words, start-up of the compressor  22  may be accomplished in accordance with the invention, without ever applying a torque load to the motor  24  that exceeds its maximum steady state torque load. 
     Referring now to  FIG. 4 , an exemplary method  400  may be employed to start a scroll compressor with starting torque that is no greater than steady-state operational torque. In a step  402 , orbiting of a scroll in the compressor may be initiated (e.g., the motor  24  may drive the orbiting scroll  22 - 6 ). In a step  404 , a gas flow passage in the compressor may be opened (e.g., the check valve  22 - 12  may be opened to allow refrigerant gas flow between the balance chamber  22 - 8  and the discharge chamber  22 - 14  of the scroll compressor  22 ). In a step  406  gas pressure at an outlet side of the gas flow passage may be increased (e.g., interaction between the orbiting scroll  22 - 6  and the fixed scroll  22 - 4  may increase pressure in the discharge chamber  22 - 14 ). In a step  408 , the gas flow passage may be closed (e.g., pressure in the discharge chamber  22 - 14  may exceed pressure in the balance chamber  22 - 8  so that the check valve  22 - 12  may close). In a step  410 , clamping force between the orbiting scroll and the fixed scroll may be increased (e.g., pressure in the discharge chamber  22 - 14  may continue to increase, thereby increasing axial loading between the orbiting scroll  22 - 6  and the fixed scroll  22 - 4 ). In a step  412 , steady state operation of the compressor may continue (e.g., with the scrolls  22 - 4  and  22 - 6  properly clamped together, the compressor  22  may compress refrigerant gas at its normal capacity). 
     It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.