Abstract:
A compressor system includes a first compressor, which has a first low side oil sump, in a first shell and a second compressor, which has a second low side oil sump, in a second shell. The first and second compressors are connected in series. There is an oil transfer conduit connected between the first low side sump of the first compressor and the second low side sump of the second compressor. The system also includes a normally open check valve in the oil transfer conduit. A method for effecting oil balance in a compressor system, the method includes establishing a first compressor in a first shell having a first low side oil sump and establishing a second compressor in a second shell having a second low side oil sump. The first and second compressors are connected in series. The method also includes positioning an oil transfer conduit between the first low side sump and the second low side sump and positioning a normally open check valve in the oil transfer conduit. Additionally, a bleed is provided to effect oil transfer via the oil transfer conduit when the normally open valve is closed

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation-in-part application under 35 U.S.C. §120 of U.S. patent application Ser. No. 11/952,366 filed Dec. 7, 2007, which, in turn, is a continuation of U.S. patent application Ser. No. 10/959,254 filed on Oct. 6, 2004, the entire contents of both of which are incorporated herein by reference and priority to both of which is hereby claimed. This application is also a continuation-in-part of application PCT/US05/34651, filed Sep. 27, 2005, which, in turn, claims priority to U.S. patent application Ser. No. 10/959,254 filed Oct. 6, 2004, the entire contents of both of which are incorporated herein by reference and priority to both of which is hereby claimed. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention of parent application Ser. No. 10/959,254 relates to an oil balance system for compressors connected in series. More particularly, that invention relates to apparatus and a method for an oil balance system in which each compressor is contained in a separate shell, and in which each oil sump for each compressor is a low side sump, i.e., the inlet to each compressor is open to its respective shell, and the outlet from each compressor is sealed to the compressor. 
         [0003]    My prior U.S. Pat. No. 5,839,886, the entire contents of which are incorporated herein by reference, relates to an oil balance system for primary and booster compressors connected in series for a heating/cooling or refrigeration system. The primary compressor has a low side sump, but the booster compressor has a high side sump (i.e., the inlet to the booster compressor is sealed to the compressor, and the outlet from the compressor is open to its shell. An open conduit extends between the oil sumps of the two compressors to transfer oil from the sump of the booster compressor to the sump of the primary compressor when the oil level in the booster compressor exceeds a normal operating level. 
         [0004]    My prior U.S. Pat. Nos. 5,927,088 and 6,276,148, the entire contents of both of which are incorporated herein by reference, relate to boosted air source heat pumps especially suitable for cold weather climates. In the systems of these patents, a booster compressor and a primary compressor are connected in series. 
         [0005]    Most compressors will entrain and pump out some oil, entrained in the refrigerant, during the normal course of operation. So, for a system of series connected compressors housed in separate casings, the pumped out oil will eventually return to the first compressor in the system, thus tending to raise the oil level in the sump of that compressor. As that oil level rises, this will likely cause the first compressor to pump oil to the inlet to the second compressor, so some oil will be delivered from that first compressor to the second compressor in the system, thus tending to prevent a dangerous loss of lubricant in the second compressor. Various compressor designs react differently in regard to this characteristic of pumping out oil entrained in the refrigerant, and it is known to make modifications to specific designs to enhance the tendency to pump out more oil as the level of oil rises. 
         [0006]    However, during the course of operation of a series connected compressor system, such as the heat pump systems of my U.S. Pat. Nos. 5,927,088 and 6,276,148, refrigerant/oil imbalances can occur due to such things as, e.g., defrosting requirements, extreme load changes, etc. These imbalances may lead to unbalancing the oil levels in the two compressors; and this may result in taxing the normal oil balancing tendencies beyond their normal capabilities. Accordingly, it may be desirable to incorporate a specific oil balance system in the series connected compressor system. 
         [0007]    In particular regard to the present continuation-in-part application, the advent of big bore, short stroke reciprocating compressors, such as the Benchmark compressors made by Bristol Compressors, makes it desirable to improve on the oil balance system disclosed in parent application Ser. No. 10/959,254, although the improved oil balance system of this invention is not limited to such compressors. 
       SUMMARY OF THE INVENTION 
       [0008]    In accordance with the invention of parent application Ser. No. 10/959,254, an oil balancing system is incorporated in a series connected compressor system, such as the heat pump system of my U.S. Pat. Nos. 5,927,088 and 6,276,148, wherein each compressor is housed in a hermetic casing and has a low side oil sump. An oil transfer conduit extends from the sump of the first compressor in the system (usually the booster compressor) to the sump of the second compressor (usually the primary compressor). When the first compressor is not operating and the second compressor is operating, the pressure within the casing of the first compressor is slightly higher than the pressure within the casing of the second compressor, so oil will, as desired, flow from the sump of the first compressor to the sump of the second compressor when the oil level in the first sump exceeds the height of the oil transfer conduit. However, when both compressors are operating, the pressure in the shell of the second compressor will be much higher than the pressure in the shell of the first compressor, which could cause undesirable oil and/or back-flow of compressed gas from the sump of the second compressor to the sump of the first compressor. Accordingly, and most importantly, the oil transfer conduit has a check valve which permits oil flow from the first compressor sump to the second compressor sump, but which prevents oil and/or gas flow from the second compressor sump to the first compressor sump. 
         [0009]    In accordance with the invention of this continuation-in-part application, an improved oil balance system is presented that is directed particularly to the prevention of the undesirable accumulation of oil in the sump of the second compressor when both of the compressors are operating. This is preferably accomplished by the incorporation of a bleed through the check valve or a bypass line around the check valve to achieve oil balance flow from the sump of the second compressor to the sump of the first compressor when both compressors are operating without experiencing unacceptable blowback of previously compressed refrigerant vapor from the second compressor casing to the first compressor casing 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic of the oil balance system of the invention of my parent application Ser. No. 10/959,254 and continuation application Ser. No. 11/95,2366. 
           [0011]      FIG. 2  is a sectional view of the oil balance check valve of  FIG. 1 . 
           [0012]      FIG. 3  is an enlarged sectional view similar to  FIG. 2  of a modified oil balance check valve in accordance with this continuation-in-part invention. 
           [0013]      FIG. 4  is a schematic of a modified oil balance system of this continuation-in-part invention. 
           [0014]      FIG. 5  is a schematic of another modified oil balance system of this continuation-in-part invention. 
           [0015]      FIG. 6  is a schematic of another modified oil balance system of this continuation-in-part invention. 
       
    
    
       [0016]    In  FIGS. 3-6 , parts which are the same as or similar to corresponding parts in  FIGS. 1 and 2  are numbered as in  FIGS. 1 and 2 . 
       DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0017]    The invention of my parent application and the invention of this continuation-in-part application are described in the context of a boosted air source heat pump as disclosed in my prior U.S. Pat. Nos. 5,927,088 and 6,276,148. However, it will be understood that these invention are applicable to any system of compressors in series where the compressors each have low side oil umps. 
         [0018]    Referring to  FIG. 1 , a booster compressor  10  is housed in a hermetically sealed casing  12 , and a primary compressor  14  is housed in a hermetically sealed casing  16 . The compressors are preferably reciprocating compressors, but rotary or other types of compressors may be used. Each compressor is a low side sump compressor. That is, the inlet to each compressor is open to the shell of the compressor, and the outlet from each compressor is sealed to the compressor. Each compressor/casing has an oil sump at the bottom of the casing, the normal level of which is shown in shown in  FIG. 1 . The oil in these sumps is used to lubricate the compressors in ways presently known in the art. 
         [0019]    An oil balance conduit  18  extends between the compressor shells at the lower parts thereof. Oil balance conduit  18  is positioned just slightly above the normal level of the sump oil in booster casing  12 . A normally open check valve  20  is positioned in oil balance conduit  16 . Check valve  20  permits oil flow from the sump of booster casing  12  to the sump of primary casing  16  when primary compressor  14  is on and booster compressor  10  is off or when both compressors are off, but prevents oil flow from the sump of primary casing  16  to the sump of booster casing  12  whenever both compressors are on. 
         [0020]    A conduit  22  is connected to the low side of a system (e.g., an evaporator in a heating or cooling system), to receive refrigerant from the system low side. A branch conduit  24  is connected to the inlet  26  to primary compressor casing  16  to deliver refrigerant to the interior volume of casing  16  and to primary compressor  14 . A check valve  28  in conduit  24  controls the direction of flow in conduit  24 . Check valve  28  is preferably normally open to minimize the pressure drop of the fluid flowing through check valve  28  to primary inlet  26 . Another branch conduit  30  connects conduit  22  to the inlet  32  to booster compressor casing  12  to deliver refrigerant to the interior volume of casing  12  and to booster compressor  10 . 
         [0021]    One end of a booster compressor discharge line  34  is sealed to booster compressor  10 , and the other end of discharge line  34  is connected to branch conduit  24  downstream of check valve  28 , whereby discharge line  34  delivers the discharge from booster compressor  10  to primary inlet  26  and to the interior volume of primary casing  16  and to primary compressor  14 . 
         [0022]    One end of a primary compressor discharge line  36  is sealed to primary compressor  14  and the other end of discharge line  34  is connected to the high side of the system (e.g., a condenser in a heating or cooling system). 
         [0023]    If the system includes an economizer, a conduit  38  would be connected to conduit  24  downstream of check valve  28 . 
         [0024]    Normally open check valve  20  may be maintained normally open in any chosen manner. Examples may be understood by reference to  FIG. 2  where valve  20  has a spherical chamber  40  in the segments  18 ′ and  18 ″ of oil balance line  18 . Chamber  40  is divided into upper and lower segments by a wall  42  which has peripheral flow passages  44 . A ball  46  is loaded against wall  42  either by the force of gravity, or by a light spring  48  or by magnets  50 . Regardless of the mechanism chosen, valve  20  is normally open to permit flow in line  18  from booster casing  10  to primary casing  16  when the pressure in the interior volume of primary casing  16  is essentially equal to or lower than the pressure in the interior volume of booster casing  12 . However, if the pressure in the interior of primary casing  16  is substantially higher than the pressure in the interior volume of booster casing  12 , ball  46  will be moved to engage a conical or spherical seat  52  to close the entrance from line  18 ′ to the upper segment of chamber  40 , thus blocking flow in oil balance line  18 . In the operation of this invention, check valve  20  must be open when primary compressor  14  is on and booster compressor  10  is off, and when both the primary compressor  14  and the booster compressor  10  are off; and check valve  20  must be closed when both the primary compressor and the booster compressor are on. 
         [0025]    Normally open check valve  28  may be held normally open in the same manner as valve  20  if it is also mounted vertically. However, if valve  28  is mounted horizontally, spring or magnetic loading will be required. 
         [0026]    When both primary compressor  14  and booster compressor  10  are off, the gas pressure in primary shell  16  and in booster shell  12  will be equal. Accordingly, oil flow in balance line  18  will be bi-directional depending on the oil heads in the sumps of the primary and booster shells. 
         [0027]    In the heating mode of operation, the booster compressor is off and only the primary compressor is operating at low heating load on the system. In this situation, normally open check valves  20  and  28  are open; and the pressure in booster shell  12  is slightly higher than the pressure in primary shell  16 . Therefore, if the oil level in the sump of booster shell  12  is higher than its intended normal level, which means that the oil level in the sump of primary shell  16  is lower than normal, oil will flow via balance line  18  from the sump of booster shell  12  to the sump of primary shell  16  to restore normal oil levels in both sumps. Also, if the oil level in the sump of primary shell  16  is very high, which means that the oil level in the sump of booster shell  12  is low, and the pressure drop between the sump of booster shell  12  and the sump of primary shell  16  is low enough, oil can flow via balance line  18  from the sump of primary shell  16  to the sump of booster shell  12 . 
         [0028]    At higher heating loads on the system, both the booster compressor and the primary compressor will be operating. In that situation, the pressure in the primary shell will be higher than the pressure in the booster shell, because the discharge from booster compressor  10  will be delivered via line  34  to casing  16 , check valve  28  will be closed, and system low side will be connected via conduits  22  and  30  to the inlet  32  to booster shell  12 . Accordingly, normally open check valve  20  will be closed, thus preventing back-flow of compressed gas (which would go from the discharge of booster compressor  10  to primary shell  16  and then back to booster shell  12  via balance line  18  if check valve  20  were open). However, the closure of check valve  20  also prevents oil balance flow via line  18 , which can lead to oil imbalance in the sumps of the compressors, particularly creating a concern about low oil level in the sump of primary shell  16 . 
         [0029]    Some oil becomes entrained in the circulating refrigerant during the operation of the system. When both booster compressor  10  and primary compressor  16  are on, all oil entrained in the refrigerant is delivered to the shell  12  of booster compressor  10 , where it tends to separate out and fall into the sump of booster shell  12 . If the oil accumulates in the sump of booster shell  12  above the predetermined normal level, operation of the booster compressor will tend to agitate the oil to create a mist that will be entrained in the refrigerant discharged from booster compressor  10 . This entrained oil will be delivered to the interior of primary shell  16 , where it will tend to drop out from the gas due to differences in gas and oil velocities upon entering into the interior of primary shell  16 . This separated oil will fall into the sump of primary shell  16  to replenish the level of oil in this sump. 
         [0030]    Since this concern about low oil level in the sump of primary shell  16  occurs only when both the booster and primary compressors are operating, other steps can be taken to address the potential problem in addition to relying on the mist and precipitation action described in the preceding paragraph. One solution is to program the system to turn off the booster compressor for a short time (on the order of 2-4 minutes). As described above for the operational state where the primary compressor is on and the booster is off, this will result in opening normally open valve  20 , and any oil built up above normal level in the sump of booster shell  12  will be transferred to the sump of primary shell  16  via transfer line  18 . 
         [0031]    Also, during defrost cycling and cooling operation, the booster compressor is off, and only the primary compressor is operating. Thus, normally open check valve  20  will be open, and oil balance transfer can take place from the sump of booster shell  12  to the sump of primary shell  16 . 
         [0032]    Turning now to the subject matter of this continuation-in-part application, there are operating conditions and circumstances, such as, for example, too frequent defrosting, or restarting after an extended power outage, whereby excess oil may have previously accumulated in the sump of the primary compressor. If both compressors are subsequently required to operate, it will be desirable to transfer oil via oil balance line  18  from the sump of the primary compressor casing  16  to the sump of the booster compressor casing  12  to achieve and maintain oil balance between the sumps of the two compressors. In accordance with the invention of my parent application, oil transfer via balance line  18  is prevented when both compressors are operating because check valve  20  is closed when both compressors are operating. However, in accordance with the invention of this continuation-in-part application, the closure of check valve  20  is bypassed to permit oil transfer via balance line  18  from the sump of primary compressor casing  16  to the sump of booster compressor casing  12  to achieve oil balance between both sumps when both compressors are operating, without encountering unacceptable back-flow of compressed gas from primary shell  16  to booster shell  12 . 
         [0033]    Referring to  FIG. 3 , the first, and preferred, embodiment for bypassing the closed state of check valve  20  is shown. In  FIG. 3 , normally open flow control valve  20  is shown in its closed position, where ball  46  is seated in its conical seat  52 . However, a small bypass bleed channel  100  is formed in conical seat  52 , as by machining, forging or other suitable techniques, to establish a bleed channel connection from the upper interior part of chamber  40  of valve  20  to line  18 ′, and hence to the sump of booster compressor casing  12 . Accordingly when, both booster compressor  10  and primary compressor  14  are operating, which causes normally open valve  20  to be moved to its closed position because of the higher pressure in the sump of primary compressor casing  16  than the pressure in the sump of booster compressor casing  12 , bleed channel  100  establishes a bypass path for the flow of oil past what would otherwise be a closed valve  20 . Bearing in mind that the pressure in the sump of primary compressor casing  16  is higher than the pressure in the sump of booster compressor casing  12  when both compressors are operating, an accumulation of oil above the normal level in the sump of primary casing  16  will result in oil flow from the sump of primary compressor casing  16  to the sump of booster compressor casing  12  via oil transfer line  18  and segment  18 ″ to the interior of chamber  40  of valve  20 , and then via bleed channel  100  to oil transfer line segment  18 ′ and to the sump of booster compressor shell  12  to balance the oil levels in the sumps of the two compressor casings. Since bleed channel  100  is relatively small compared to the size of oil balance line  18  (on the order of ½ of 1% of its flow area), bleed channel  100  permits this bypass flow of oil past the otherwise closed valve  20  without permitting an unacceptable amount of back-flow of compressed gas from primary shell  16  to booster shell  12 . Bleed channel  100  is self cleaning because any flow impeding debris will immediately be removed every time valve  20  opens. Any probability of total flow blockage is essentially eliminated by use of a channel instead of a very small unfiltered orifice. 
         [0034]    Referring now to  FIG. 4 , another embodiment is shown for bypassing closed valve  20 . In this embodiment, a solenoid operated valve  102  is positioned in a bypass line  104  around valve  20  of  FIG. 2 , bypass line  104  being connected between conduit  18  and branch  18 ′. When both compressors are off, or when only primary compressor  14  is on, and valve  20  is in its normally open state, solenoid valve  102  is closed. However, when both compressors are operating and valve  20  is closed, a system controller is programmed to open solenoid valve  102  is opened on a time schedule to permit excess oil in the sump of primary casing  16  to flow from the sump of primary compressor casing  16  to the sump of booster compressor casing  12 . The oil flow is from the sump of primary casing  16  to oil balance conduit  18  to bypass line  104  to conduit segment  18 ′ to the sump of booster casing  12 . The flow volume of bypass line  104  is large enough to allow high flow rates and is not susceptible to blocking. Solenoid  102  is opened only at predetermined times, and then only for short periods of time, such as upon termination of a defrost cycle when booster compressor operation is called for along with primary compressor operation. Alternatively an oil level sensor on the primary casing could be used to open solenoid valve  102  when both compressors are operating and the oil level in the primary sump rises above a predetermined level. Another example of when solenoid valve  102  might be open would be if the booster compressor is a scroll compressor and the primary compressor is a reciprocating compressor, and if the normal entrained oil pumping rate of the booster is higher than that of the primary. When both compressors are operating, the oil level will rise in the sump of the primary compressor until its entrained oil pumping rate matches what is coming to it from the booster. A relatively minor problem resulting from this situation would be excessive power consumption of the primary compressor as its running parts become submerged in oil. A far worse problem would be an impact on primary compressor reliability and oil starvation of the booster compressor as it loses oil to the primary compressor. Programmed opening of solenoid valve  102  to permit oil transfer from the sump of the primary compressor to the sump of the booster compressor will prevent these problems. 
         [0035]    Referring now to  FIG. 6 , another embodiment is shown for bypassing closed valve  20 . In this embodiment valve  20  of  FIG. 2  is bypassed by a small fixed orifice  108  in bypass line  104  connected around valve  20  from conduit  18  to conduit branch  18 ′. The small fixed orifice  108  permits oil flow from the sump of primary casing  16  to the sump of booster casing  12  when both compressors are on, valve  20  is closed, and oil accumulates over the normal oil level in primary casing  16 . The oil flow is from the sump of primary casing  16  to oil balance conduit  18  to bypass line  104  through fixed orifice  108  to branch conduit  18 ′ to the sump of booster casing  12 . As with bypass line  100 , bypass line  104 , and capillary tube  106 , the flow volume through small fixed orifice  108  is small enough to prevent an unacceptable back-flow of compressed gas from primary casing  16  to booster casing  12 . 
         [0036]    If the capillary of the embodiment of  FIG. 5  or the fixed orifice of the embodiment of  FIG. 6  is used, a strainer should be positioned upstream (in the direction of bypass flow) of the orifice or the capillary to avoid blocking of the bypass line with debris. 
         [0037]    It should be noted that in the embodiments of this continuation-in-part application the positions of conduits  22 ,  24 , and  30  have been modified (relative to  FIG. 1 ), as seen in  FIGS. 4-6 , to reflect current practice. This modification is intended to cause a majority of the oil circulating in the system to be returned to the sump of booster compressor casing  12 . It should also be noted that for each of the embodiments of  FIGS. 3-6 , which are directed to the situation where both compressors are operating and normally open valve  20  is closed, oil transfer between the sumps of the booster and primary compressors via oil balance conduit  18  will be as described for  FIGS. 1 and 2  when both compressors are off or when only the primary compressor is on, and valve  20  is in its normally open condition. 
         [0038]    While a preferred embodiment of the present invention has been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.