(2) Description of the Related Art [0002] Reciprocating compressors are well known in the refrigeration industry. These may be used in a variety of applications involving cooling, heating, or combinations. The interaction of the refrigerant and lubricant (hereafter "oil") influences a number of aspects of operation. The engineering of refrigeration systems and their components may reflect a careful balancing of issues such as capacity, thermodynamic efficiency, manufacturing cost, service life, and maintenance cost.

[0003] One particular area of concern regarding refrigerant-oil interaction involves start-up transient behavior. Especially during periods of non-operation, additional refrigerant may tend to accumulate in the compressor crankcase. The additional refrigerant may be wholly or partially mixed with the oil in the crankcase. At startup, if the crankcase is vented to the suction plenum, the crankcase may experience a pressure decrease. This pressure decrease may cause outgassing of refrigerant from the refrigerant-oil mixture. The outgassing may produce a foaming effect which may be exacerbated by mechanical agitation. The foaming may, in turn, increase the crankcase pressure relative to the suction plenum pressure. The pressure increase (or spike) may lead to increased bearing loading and, thereby, wear. If the effective vent area between the crankcase and the suction plenum is too small, the pressure spike and associated loading will be too high. If the effective venting area is too great, too much oil will become entrained in the refrigerant flow, thereby impeding efficiency and/or adversely affecting compressor lubrication. If it thus known to select the effective venting area so as to achieve a desired balance of pressure/loading control on the one hand and oil loss on the other hand.

[0004] Nevertheless, there remains room for improvement in the art.

SUMMARY OF THE INVENTION [0005] Accordingly, one aspect of the invention involves a compressor having a crankshaft, at least one cylinder, and at least one piston. Each such piston is positioned for reciprocal motion at least partially within an associated such cylinder and is coupled to the crankshaft so as to drive the reciprocal motion. The compressor has an inlet, a suction plenum, an outlet, a sump, and a valve between the sump and suction plenum.

[0006] In various implementations, the valve may be pressure-actuated. The valve may have a first threshold relief pressure and a second threshold reset pressure of like sign. There may be first and second such cylinders. An oil pump may draw oil from an accumulation in the sump. A refrigerant flow may pass from the inlet to the outlet. The refrigerant may be selected from the group consisting of HFCs and HCFCs.

[0007] Another aspect of the invention involves a compressor having a crankshaft, at least one cylinder, and at least one piston. Each such piston is positioned for reciprocal motion at least partially within an associated such cylinder and is coupled to the crankshaft so as to drive the reciprocal motion. The compressor has an inlet, a suction plenum, an outlet, and means, including a valve, for limiting sump-to-suction plenum lubricant loss while limiting a bearing load.

[0008] In various implementations, the valve may be a single pressure-actuated mechanical valve.

[0009] Another aspect of the invention involves a method for operating a reciprocating-type compressor. The compressor is started. An overpressure of a sump pressure in excess of a suction plenum pressure is permitted to reach substantially a first threshold amount. A valve is opened to at least partially relieve the overpressure.

[0010] In various implementations, the valve may be reclosed when the overpressure is relieved to substantially a second threshold amount. The opening and reclosing may be mechanically pressure-actuated and may be repeated for a number of cycles. The first threshold amount may be 50-150 psi in excess of the suction plenum pressure. The second threshold amount may be 5-70 psi less than the first threshold amount.

[0011] Another aspect of the invention is a method of remanufacturing or reengineering a compressor or compressor configuration. An open aperture between a sump and a suction plenum is replaced with a valve.

[0012] In various implementations, the replacing may include positioning the valve at the aperture. The valve may be a pressure-actuated valve. The valve may be selected so as to provide a desired discharge of refrigerant from a sump oil accumulation while providing a desired control of sump pressure. The discharge may be quicker and/or the sump pressure may be held to a lower peak in the remanufactured or reengineered compressor or compressor configuration relative to a baseline. The selecting may include testing with a number of such valves having different pressure responsive characteristics or varying a pressure response characteristic of a single such valve or test valve.

[0013] The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims .

BRIEF DESCRIPTION OF THE DRAWINGS [0014] FIG. 1 is a longitudinal, partially sectional, view of a compressor according to principles of the invention.

[0015] FIG. 2 is a view of the compressor of FIG. 1 in an alternate condition.

[0016] FIG. 3 is a plot of plenum-to-sump pressure difference for the compressor of FIGS. 1 and 2 and two hypothetical prior art compressors.

[0017] Like reference numbers and designations in the various drawings indicate like elements . UETAILED DESCRIPTION [0018] FIG. 1 shows a compressor 20 for compressing a refrigerant flowing along a flow path between an inlet 22 and an outlet 24. The exemplary compressor includes a housing assembly 26. A crankshaft 28 is mounted within the housing 26 and held by a number of bearing systems 30 for rotation about a central axis 500. An electric motor has a stator 32 held stationary within the housing and a rotor 34 secured to the crankshaft 28 to drive the crankshaft 28 in at least a first direction about the axis 500. The housing 26 defines first and second cylinders 40 and 42 in which associated first and second pistons 44 and 46 are positioned for reciprocal movement along cylinder axes 502 and 504. The pistons 44 and 46 are coupled via wrist pins 48 to the distal ends of connecting rods 50 and 52 whose proximal ends engage first and second crank portions 54 and 56 of the crankshaft 28.

[0019] In the exemplary embodiment, the first and second pistons 44 and 46 form one of three piston banks angularly offset about the axis 500, each bank having a first piston coupled by an associated connecting rod to the first crank 54 and a second piston coupled by an associated connecting rod to the second crank 56. Each bank has a cylinder head 60 above the headspaces of the associated pistons and carrying valves (not shown) placing the headspaces in selective communication with the inlet 22 on the one hand and the outlet 24 on the other hand. Communication with the outlet 24 is via a suction plenum 66 which generally surrounds the cylinders and the motor. A crankcase wall 72 separates the suction plenum 66 from a crankcase volume or sump 74 containing the crank portions 54 and 56 of the crankshaft 28.

[0020] The sump 74 contains an oil accumulation 76 having an upper surface 78. An oil pickup tube 80 extends from a first end connected to a screen unit 82 immersed in the oil accumulation 76 to a second end connected to an oil pump 84 mounted at a forward end of the crankshaft 28. In operation, the oil pump 84 draws oil from the oil accumulation 76 through the screen unit 82 and oil pickup tube 80. The oil pump 84 then pumps the oil through passageways (not shown) to various locations in the compressor. Such passageways may include one or more passageways through the crankshaft 28 to lubricate the cranks 54 and 56 and the bearing systems 30. In the exemplary embodiment, the oil pump 84 includes a forwardmost of such bearing systems. In the exemplary embodiment, a main bearing-engaging portion 86 of the crankshaft may be located generally between the motor and the crank portions. The oil pumped to this portion 86 may represent a major portion of the total oil flow through the oil pump 84 and may return to the sump via a return conduit 88. In the exemplary embodiment, a pressure regulator 90 is positioned in the return conduit 88 so as to maintain a desired oil pressure condition and associated flows within the oil passageways. A pressure relief valve (not shown) may couple the suction plenum to the outlet to protect the compressor (e.g., against blockages elsewhere in the refrigeration system) . Such pressure relief valves are discussed in Underwriters Laboratories Inc. (UL) Standard for Safety for Hermetic Refrigerant Motor-Compressors UL 984, Seventh Edition, May 31, 1996, ISBN 1-55589-983-2.

[0021] An aperture 100 extends through the wall 72 to provide communication between the sump 74 and suction plenum 66. As thus described, the compressor may be similar to a prior art compressor having an aperture in a similar location. In the prior art, such apertures serve to vent the sump to the suction plenum. The prior art apertures may be dimensioned so as to provide a balance of venting and oil retention as discussed above. [0022] According to the present invention, a valve 102 is positioned (e.g., in the aperture 100) to control communication between the suction plenum 66 and sump 74. The exemplary valve 102 is a normally-closed pressure relief valve. One aspect of the exemplary valve 102 is to remain closed until a pressure difference of the sump pressure above the suction plenum pressure reaches or exceeds a first threshold whereupon the valve 102 opens. For further pressure increases, the valve 102 may remain open. A second aspect is that, as the difference decreases, the valve 102 will remain open yet below the first threshold until the difference decreases to a second threshold difference. The second threshold may, advantageously, also be a positive difference of sump pressure above suction plenum pressure. For further pressure decreases, the valve 102 may remain closed.

[0023] FIG. 2 shows the compressor 20 in an exemplary second condition wherein the compressor has been off for a period of time sufficient for refrigerant to accumulate in the sump. In the exemplary second condition, the additional refrigerant is fully mixed with the oil accumulation 76, having substantially elevated the surface 78 above its FIG. 1 level. In alternative second conditions, there may be a substantial refrigerant-only second phase (e.g., floating atop a saturated refrigerant-oil mixture) .

[0024] FIG. 3 shows a plot 400 of pressure difference (axis 402) against time (axis 404) . Over a first interval until a time 406, the pressure increases until it reaches a first threshold pressure difference 408, whereupon the valve 102 opens (relieves) and the pressure difference drops. During this first interval, the permitted pressure difference largely controls outgassing. The drop continues over a second time interval until a time 410 when the pressure difference has dropped to a second threshold pressure difference 412, whereupon the valve 102 again closes (resets) . During this second interval, outgassing and foaming may substantially increase. The closing, however, helps retain the foamed oil in the sump. The pressure difference then again builds until a time 414 when the pressure has again reached the first threshold 408 and the valve reopens. The pressure difference may continue to cycle between the thresholds 408 and 412. The interval duration for cycles between the two thresholds may increase. Eventually, the pressure difference may cease to reach the first threshold 408, peaking at a value intermediate the thresholds, and plunge below the second threshold 412, decaying to a steady state of zero. The decay to essentially zero may occur over a brief interval such as 1-5 minutes.

[0025] By way of contrast, FIG. 3 further shows a plot 440 of the pressure difference for an otherwise similar prior art compressor with a relatively large vent opening. The pressure may peak at a value 442 and then decay to zero. This whole process may occur over a somewhat shorter total time than with the plot 400. FIG. 3 further shows a plot 460 for an otherwise similar prior art compressor having a relatively small vent opening. In this example, the pressure builds to a value 462 and may remain for a brief interval before decaying to zero. This whole process may occur over a somewhat longer total time than with the plot 400.

[0026] The compressor 20 may be formed as a modification of an existing compressor configuration or a modification of an actual compressor. In the latter situation, the existing vent aperture may be enlarged and threaded or otherwise modified so as to permit installation of the valve 102. The values of the first and second thresholds may be selected based upon experimental and/or theoretical factors and data. For example, the first threshold 408 may be chosen in view of the bearing load capability of the particular compressor configuration and construction. A low bearing load capability would generally require a similarly low first threshold 408. The second threshold 412 may be selected to achieve desired oil retention in the sump. This may be influenced by various compressor configuration factors, by the particular oil and refrigerant combination, by ambient conditions at startup and steady state operation, and by any initial sump conditions if not otherwise considered. The frequency characteristics of the valve cycles may be influenced both essentially independently of and dependently of the thresholds. Independent factors may include the valve effective cross-sectional area for flow and other valve time response characteristics. A desirable frequency could experimentally be determined to achieve a desirably quick discharge of the refrigerant from the oil with desired oil retention. The particular frequency characteristic and the valve characteristics appropriate to achieve such frequency characteristic may similarly be influenced by the oil and refrigerant combination, ambient conditions, and other startup conditions .

[0027] Exemplary refrigerants are HFCs and CFCs. Exemplary oils are polyol esters (POEs), which may be particularly suitable for refrigerants such as R404A, or mineral oil or alkylbenzene which may be particularly useful with HCFCs such as R22.

[0028] As noted above, exemplary threshold values will be influenced by factors relating to the compressor configuration, desired operating conditions, the types of refrigerant and oil, and the like. For example, for some implementations, the first threshold 408 may be somewhere in the range of 50-150 psi, more narrowly, 90-120 psi. Similarly, the second threshold 412 may be 5-120 psi, more narrowly, 50-100 psi. An exemplary difference between these thresholds may be 5-70 psi, more narrowly, 20-40 psi. Although specific threshold pressures are identified, these should be regarded as characteristic or illustrative. Also, real world performance factors may prevent instantaneous response (e.g., even in a single embodiment, the valve opening/closing may- occur over ranges of pressures and such pressures may vary based upon various conditions) . Other simplified characteristics, which are useful for discussion, need not be limiting.

[0029] The foregoing principles may be used to reengineer a compressor configuration or remanufacture a compressor from a baseline configuration or compressor to a reengineered or remanufactured configuration or compressor. The remanufacturing or reengineering may add the valve 102 in place of a previously open aperture and may include enlarging the aperture, threading the aperture, and/or otherwise modifying the aperture. The valve, or its parameters, may be chosen to improve one or more of speed of refrigerant discharge from the oil accumulation, the degree of oil retention, and control of sump oil pressure relative to the baseline values. This may include testing on actual hardware, theoretical calculations, simulations, or combinations thereof.

[0030] One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, when applied as a reengineering of an existing compressor configuration or a remanufacturing of an existing compressor for an existing application, details of the existing compressor and/or application may influence details of the implementation. Accordingly, other embodiments are within the scope of the following claims .