Abstract:
Just before shutdown, or at least prior to a significant pressure equalization in a refrigeration system, an accumulator containing oil is isolated from the rest of the refrigeration system in such a way that oil is at a pressure that is higher than the pressure of the rest of the system. The oil in the accumulator is maintained in a state of higher pressure while the refrigeration system is shutdown with the aid of a spring-loaded piston. Preliminary to start up of the refrigeration system, the pressurized oil is placed in fluid communication with structure requiring lubrication which is thereby lubricated.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Some components of refrigeration compressors are supported by bearings. To achieve reliable operation for long periods of time, bearings, and other compressor components, require lubrication by a lubricant with adequate viscosity. In a refrigeration system, this is provided by the use of a suitable oil. After long periods of compressor non-operation, oil can completely drain from the bearings. If the compressor is started after such a period, the bearings, and or other components, will operate for some period of time with no lubricant, causing metal-to-metal contact between parts . This can result in wear, ultimately shortening the useful life of the compressor. Additionally, in some compressor refrigeration systems, the pressure differential between compressor compartments may be used to develop lubrication flows. In such systems, some time may be required after initial start up to develop pressure differences adequate for establishing lubrication flows. During this time, there is no delivery of oil to the bearings and other components, thereby resulting in their wear.  
           [0002]    One method of accomplishing lubrication, shortly before and/or during start up, is by the use of a positive displacement pump (with suitable piping) which is activated prior to start up, thereby drawing lubricant from an oil reservoir and delivering it to the bearings and other components. A positive displacement pump used for this purpose adds its own reliability risk as well as substantial cost. The pump can be of substantial size because it may be required to deliver a significant amount of oil to provide an adequate amount of lubrication to the bearings and other components of the compressor.  
         SUMMARY OF THE INVENTION  
         [0003]    Prior to shutdown, pressurized oil, or oil-rich oil-refrigerant solution is isolated from the rest of the refrigeration system in a dedicated accumulator. The isolated oil is at a pressure that is higher than the pressure existing in the bearing cavities and other components at the time of start up and is maintained at this higher pressure by applying a preload on a spring acting on a piston so as to form a spring-driven piston. The spring is preloaded by a pressure differential acting across the piston which is established prior to compressor shutdown. Accumulator pressure loss and accompanying loss of spring preload can occur due to the long term effects of leakage across the piston face and valves during long periods of compressor shutdown. To alleviate this problem, a small, positive displacement pump can be added to the system solely for preloading the spring by delivering pressurized oil to the side of the piston opposite the spring and thereby pressurizing the accumulator chamber acting against the spring.  
           [0004]    Preliminary to restarting the refrigeration system, the state of isolation of this pressurized oil is ended by placing the oil in fluid communication with the bearings and possibly other components to be lubricated. Flow of the oil results by virtue of its pressure being higher than the pressure at the bearings and other components as the oil is being expelled from the accumulator by the spring driven piston, thereby accomplishing pre-start lubrication.  
           [0005]    It is an object of this invention to provide lubrication shortly before and/or during start up using a pre-charged fluid reservoir.  
           [0006]    It is another objective of this invention is to provide and enhance lubrication of compressor components shortly before and/or during start up using a small, inexpensive pump in combination with a pre-charged fluid reservoir.  
           [0007]    It is a further object of this invention to provide a method and apparatus for lubrication delivery prior to and/or during start up that is compatible with the normal operation of the lubrication system. These objects, and others as will become apparent hereinafter, are accomplished by the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    For a fuller understanding of the present invention, reference should now be made to the following detailed description thereof taken in conjunction with the accompanying drawings wherein:  
         [0009]    [0009]FIG. 1 is a schematic representation of a refrigeration system employing a first embodiment of the present invention; and  
         [0010]    [0010]FIG. 2 is a schematic representation of a refrigeration system employing a second embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0011]    In FIG. 1, the numeral  10  generally designates a refrigeration system. Refrigeration system  10  includes a positive displacement compressor  12  which is illustrated as a screw compressor having screw rotors  12 - 1  and  12 - 2  which are supported at their ends by a plurality of suitable bearings  12 - 3 . Refrigeration system  10  includes a fluid circuit serially including screw compressor  12 , discharge line  14 , oil separator  15 , condenser  16 , expansion device  20 , evaporator  24 , and suction line  28 . Screw compressor  12  is driven by motor  13  under the control of microprocessor  90 .  
         [0012]    Compressor lubrication systems can vary somewhat in their layout and working function.  
         [0013]    Accumulator  40  is dedicated to pre-start lubrication and is divided into two chambers,  40 - 1  and  40 - 2 , respectively, by piston  42 . As pistons, diaphragms and bellows are equivalents, the piston  42  may be replaced by a diaphragm or bellows as where long time shutting down of the system  10  would result in leakage across piston  42 . Accumulator chamber  40 - 2  is connected to oil separator  15  via line  50  which contains one-way valve  51 . Oil separator  15  is at or close to discharge pressure during compressor operation. The valve  51  allows the flow of oil into chamber  40 - 2  from oil separator  15 , but not out of this chamber. The chamber  40 - 2  is connected by line  52  to line  73  for injecting oil via line  73  into, and thereby lubricating, the screw compressor components such as bearings  12 - 3  prior to starting compressor  12 . Line  72  connects oil separator  15  to line  73  for injecting oil into screw compressor  12  to provide lubrication to compressor  12  and components such as bearings  12 - 3  when it is operating. Solenoid valve  74  is located in the line  72  under the control of microprocessor  90  which can open or close the solenoid  74 - 1  of solenoid valve  74  to permit or prevent oil flow from oil separator  15  to compressor  12 . Accumulator chamber  40 - 2  is serially connected via line  52  which contains solenoid valve  53  and line  73  to bearings  12 - 3  and other components that require lubrication. Solenoid valve  53  is controlled by microprocessor  90  by opening and closing solenoid  53 - 1 . Spring  44  is located in chamber  40 - 1  and tends to bias piston  42  towards chamber  402 . The one-way valve  51  allows movement of oil into the chamber  40 - 2 , but not out of this chamber. Accumulator chamber  40 - 1  is connected to the suction side of system  10  via line  54  which contains optional one-way valve  55 . When used, valve  55  allows movement of refrigerant vapor out of chamber  40 - 1 , but not into the chamber and this assures the establishment of the greatest pressure differential across piston  42  experienced during operation. This is desired because under some operating conditions there is a very small pressure differential between suction and discharge and would be unsuitable to preload spring  40 . Alternatively, chamber  40 - 1  can be connected to some intermediate location in the system  10  where the pressure is below discharge pressure.  
         [0014]    In operation of refrigeration system  10 , gaseous refrigerant is drawn via suction line  28  into compressor  12  where it is compressed. The resultant, hot high pressure refrigerant gas is discharged from the compressor  12  and then supplied via discharge line  14  to oil separator  15  where a substantial amount of oil mist entrained in the hot, high pressure refrigerant gas is separated out and collected. Hot, high pressure gas then passes to condenser  16 . In condenser  16 , the gaseous refrigerant condenses as it gives up heat due to heat transfer via air, water or brine-cooled heat exchangers (not shown). The condensed refrigerant passes through expansion device  20  thereby undergoing a pressure drop and partially flashing as it passes into evaporator  24 . In evaporator  24 , the remaining liquid refrigerant evaporates due to heat transfer via air, water or brine-cooled heat exchangers (not shown). The gaseous refrigerant is then supplied via suction line  28  to compressor  12  to complete the cycle. During operation, as oil is separated from discharging gaseous refrigerant by oil separator  15 , it can pass to chamber  40 - 2  through oil flow line  50  and one-way valve  51  if it is at a higher pressure than the pressure which exists in chamber  40 - 2 .  
         [0015]    Solenoid valve  74  is kept open during normal compressor operation allowing oil from the oil separator  15  to be injected back into the compressor via lines  72  and  73  for lubrication of screw compressor components such as bearings  12 - 3 . During normal compressor operation, solenoid valve  53  is closed and the pressurized oil can be delivered from oil separator  15  to the chamber  40 - 2  if the pressure in the oil separator  15  is higher than the pressure in chamber  40 - 2 . Thus, the pressure in chamber  40 - 2  is at the highest discharge pressure seen by compressor  12  during its normal operating cycle. If oil from oil separator  15  is supplied to chamber  40 - 2 , that causes the piston  42  to be displaced into the chamber  40 - 1 . As piston  42  is displaced into chamber  401 , the pressure in chamber  40 - 1  tends to rise with the decreasing volume. When pressure in chamber  40 - 1  exceeds the pressure on the other side of one-way valve  55 , refrigerant vapor is exhausted from chamber  40 - 1 . Thus, the pressure in chamber  401  is the lowest suction pressure seen by compressor  12  during its normal operation. Movement of piston  42  into chamber  40 - 1  pre-loads the spring  44 .The spring  44  is compressed until the spring pre-load is balanced by the pressure forces acting on the piston  42  in chambers  40 - 1  and  40 - 2 . This pressure force is equal to the pressure differential across the piston face multiplied by the area of the piston. The spring pre-load is maximized as one-way valve  55  assures that chamber  40 - 1  is maintained at the lowest suction pressure and one-way valve  51  assures that chamber  40 - 2  is maintained at the highest discharge pressure seen by the compressor  12  during its operation.  
         [0016]    After the compressor  12  is shutdown, the solenoid valve  53  remains closed, while the one-way valve  55  prevents inflow of vapor into the chamber  40 - 1 , thus maintaining the same low pressure in chamber  40 - 1  as existed before the compressor shutdown. At the same time, the one-way valve  51  prevents outflow of oil from chamber  40 - 2 , thus maintaining the same high pressure in chamber  40 - 2  as before the compressor shutdown. This effectively makes accumulator  40  a pressure tight vessel with high pressure in chamber  40 - 2  and low pressure in chamber  40 - 1 .  
         [0017]    After the compressor shutdown, the solenoid valve  74  is closed to prevent short-circuiting of oil from chamber  40 - 2  into the oil separator  15 , thus assuring that the oil will be delivered into the compressor bearings  12 - 3  and not into the oil separator  15 . With the solenoid valve  74  closed, the solenoid valve  53  is opened at a predetermined time prior to the compressor start up. The opening of the solenoid valve  53  results in a drop in the pressure in chamber  40 - 2 . The drop in pressure in chamber  40 - 2  causes a reduction of the pressure differential across the piston  42 , this causes the preloaded spring  44  to displace the piston  42  towards chamber  40 - 2  expelling the oil out of chamber  40 - 2  into the line  52  and then line  73  to lubricate the screw compressor bearings  12 - 3  and other components.  
         [0018]    If the compressor application requires very long periods of shutdown, then a possibility exists that the chamber  40 - 2  can become de-pressurized and pressure in chamber  40 - 1  can increase due to the effects of long term leakage across the piston  42  and through one-way valves  55  and  51 , as well as solenoid valve  53 . This potential problem is resolved by placing a small, inexpensive, positive displacement pump  160  in line  150  of system  110  under the control of microprocessor  190 , as shown in FIG. 2. In FIG. 2 structure corresponding to structure in FIG. 1 has been number one hundred higher. The basic change in operation is that accumulator chamber  140 - 2 , although isolated by closed solenoid valve  153  and pump  160 , is not maintained pressurized during shutdown. Pump  160  will be activated by microprocessor  190  prior to the compressor start up to increase the pressure and amount of oil in the chamber  140 - 2  such that sufficient preloading of spring  140  takes place to provide a sufficient delivery of oil from chamber  140 - 2  via lines  152  and  173  to bearings  112 - 3  when solenoid valve  153  is opened. The oil supplied to chamber  140 - 2  will be delivered by pump  160  from the oil separator  115  via line  150  and pump  160  will normally be operated until the compressor  112  starts its normal operation. If the pump  160  is added to the circuit, there is no need to have one-way valves  55  and  51  in system  110  of FIG. 2. Otherwise, the structure of system  110  is the same as that of system  10  and accumulator  140  delivers pre-start lubrication in the same manner when solenoid  153  is opened. The advantages of using a small oil pump  160  in conjunction with an accumulator  140  is the reduction in size and cost of pump  160  as compared to a larger and more expensive pump that is used without the accumulator i.e. the current art. The reduction in size and cost is accomplished using the dedicated accumulator that can be pre-charged using a small, low flow rate pump prior to the start up. The smaller pump can be utilized in this case because the required flow rate to pre-charge the accumulator is much smaller than the required flow rate that the dedicated pump must deliver to lubricate the bearings upon the start up.  
         [0019]    Although preferred embodiments of the present invention have been illustrated and described, other changes will occur to those skilled in the art. For example, although a screw compressor has been specifically disclosed, the present invention may be employed with other positive displacement compressors. It is therefore, intended that the scope of the present invention is to be limited only by the scope of the appended claims.