GB2058233A - Oil return system and method - Google Patents

Abstract

A refrigeration system (10) including an oil return system for a refrigerant compressor (12) for returning oil accumulated within a first portion (46) of the compressor (12) operating at refrigerant discharge pressure to a second portion (44) of the compressor (12) operating at substantially suction pressure. The oil return system includes a first conduit (48, 50) connecting the first portion (46) of the compressor (12) with the second portion (44) thereof, for delivering lubricating oil accumulated within the first portion (46) to the second portion (44). A normally closed valve (28) is interposed in the conduit (48, 50) for controlling flow of lubricating oil through the conduit. The magnitude of the pressure in the second portion (44) of the compressor (12) is monitored, with the normally closed valve (28) being opened when the sensed pressure decreases below a predetermined level for enabling the oil to flow through the conduit (48, 50) from the first portion (46) to the second portion (44). <IMAGE>

Description

SPECIFICATION Oil return system and method This invention reiates to refrigeration systems in general, and in particular to an oil return system for refrigeration compressors employed in refrigeration systems wherein the compressor is continuously driven regardless of the refrigeration load on the system.

There are many refrigeration systems wherein the compressor employed in the system runs continuously irrespective of the refrigeration load on the system. For example, in transportation refrigeration systems, such as those used on buses or similar vehicles, the compressor is directly connected to the vehicle's engine and will run continuously as long as the engine is operating. Operation of the compressor will thus continue irrespective of the refrigeration load on the refrigeration system.

In continuously operating refrigeration systems, there are various types of capacity control arrangements which may be employed for varying the refrigeration load handling capabilities of the system at varying load conditions. In one such capacity control arrangement, a valve is interposed in the refrigeration suction line upstream of the refrigeration compressor to control the flow of the refrigerant gas to the compressor. The suction gas control valve varies the refrigerant flow in accordance with the load on the refrigeration system; at relatively high loads, the valve opens to increase the refrigerant flow to the compressor, whereas at relatively low load conditions the valve closes to decrease the flow of the gas to the compressor.

During operation of the compressor, relatively small quantities of lubricating oil will bypass the piston rings and be discharged into a discharge gas manifold such as a discharge gas cavity formed in the cylinder head of the compressor.

During normal operating conditions, any lubricating oil discharged from the compressor cylinder will be drawn with the relatively high velocity, high mass flow refrigerant gas through the refrigeration system and return to the lubricating oil sump of the compressor. However, at relatively low mass flow conditions which occur generally at relatively low refrigeration loads, there is an insufficient mass of refrigerant gas to carry the generally heavier lubricating oil therewith.

Thus, the oil flowing from the cylinder into the discharge gas cavity will accumulate therewithin.

Continuous operation of the compressor for prolonged periods of time at relatively low mass flow conditions, such as those encountered under reduced refrigeration loads, will result in an accumulation of substantially all of the lubricating oil in the discharge cavity thereby rendering the compressor subject to lubricating oil starvation.

Oil starvation of the compressor may result in damage to bearings and other moving parts of the compressor requiring lubrication. In addition, if the system were to stop, the accumulated oil in the discharge cavity of the compressor may drain into the compressor's cylinders. Upon restarting, the incompressible slugs of oil in the cylinders may cause damage to the compressor valves, pistons, rods, and/or compressor gaskets. Thus, as is obvious, it is essential that the accumulation of the refrigerant oil at relatively low mass flow rates be eliminated.

In view of the above, the present invention relates to a refrigeration system including a refrigerant compressor having an oil return system for returning oil accumulated within a first portion of the compressor operating at refrigerant discharge pressure to a second portion of the compressor operating at substantially refrigerant suction pressure. Conduit means connects the first portion of the compressor with the second portion of the compressor for delivering lubricating oil accumulated within the first portion to the second portion. Normally closed valve means is interposed in the conduit means for controlling flow of lubricating oil through the conduit means.

The valve is opened in response to a sensed operating parameter of the refrigeration system with the parameter being indicative of the flow rate of the refrigerant gas to the compressor.

This invention will now be described by way of example, with reference to the accompanying drawing in which: Figure 1 schematically illustrates a refrigeration system of a type employing the present invention and includes a partial sectional view of a refrigerant compressor; and Figure 2 is a partial sectional view of a valve employed in the system disclosed in Figure 1.

Referring now to the drawing, there is disclosed a refrigeration system 10 including the invention herein disclosed. Refrigeration system 10 includes a refrigerant compressor 1 2 connected through a discharge line 14 to a condenser 1 6. High pressure refrigerant gas delivered from compressor 12 is transformed into a high pressure liquid refrigerant in condenser 1 6 by passing in heat transfer relation with a low temperature medium, as for example air. The high pressure liquid refrigerant is delivered from condenser 1 6 through conduit 18, expansion device 20, to a refrigerant evaporator 22.Expansion device 20 is illustrated as being a thermal expansion valve of a type well known to those skilled in the art; however, the valve may be replaced by other suitable expansion devices as for example capillary tubes. The pressure of the liquid refrigerant is reduced as it passes through expansion device 20 resulting in the generation of a relatively low pressure mixture of liquid and vaporous refrigerant. This mixture is passed through evaporator 22 wherein the mixture is totally vaporized by absorbing heat from the medium to be cooled, as for example air. The low pressure vaporous refrigerant passes through conduit 24, throttling valve 26 and thence returns to a suction manifold 41 of compressor 12. Valve 26 modulates the flow of refrigerant to the suction side of the compressor in accordance with the refrigeration load on system 10.Valve 26 operates to reduce the flow of refrigerant as the load on system 10 decreases and conversely increases the flow of refrigerant as the load on system 10 rises. Valve 26 may be controlled manually or automaticaliy via suitable means (not shown) responsive to changes in the refrigeration load on system 10. It should be understood, valve 26 may be replaced by suitable alternate means for controlling the mass flow of refrigerant through the system in accordance with the refrigeration load thereon. The system hereinabove described is a conventional mechanical refrigeration system of a type well known to those skilled in the art.

Compressor 12 generally includes one or more cylinders 36 having pistons 37 connected by connecting rods 39 to an eccentric portion of crankshaft 38. Rotation of crankshaft 38 causes reciprocating movement of pistons 37 within cylinders 36. The compressor further includes a cylinder block 30 defining the cylinders of compressor 12. One or more cylinder heads 32 are suitably attached to cylinder block 30. Each cylinder head 32 defines a suction chamber 44 and a discharge chamber 46. Each suction chamber 44 is in communication with suction manifold 41. Each discharge chamber 46 is in communication with discharge manifold 43.A valve plate 42 is interposed between cylinder head 32 and cylinder 36; valve plate 42 includes suitable suction and discharge valves (not shown) for controlling the flow of gas from suction chamber 44 to the cylinder and thence from the cylinder, subsequent to the compression of the gas therewithin, to discharge chamber 46. One or more valves 28, to be more fully described hereinafter, are provided with each valve having a first conduit 50 in communication with the suction chamber 44 and a second conduit 48 in communication with discharge chamber 46.

Cylinder block 30 defines an oil sump 43 in which a body of oil 40 is stored. The oil is employed for lubricating the various moving parts of compressor 12. A check valve 52 is provided between suction manifold 41 and oil sump 43.

Referring now to Figure 2, there is disclosed a detailed view of valve 28. Valve 28 includes a body 51 in which a bellows 56 is suitably mounted. A spring 58 provides a first force acting on one side of the bellows. A suitable adjusting screw 54 controls the force generated by the spring. Valve 28 further includes a U-shaped bracket member 60 suitably attached to the bellows 56 and movable therewith. A needle valve 62 is affixed to one leg of U-shaped bracket 60; bellows 56 thus controls movement of valve 62 relative to valve seat 64 provided at one end of conduit 48. When valve 62 is unseated with respect to seat 64, oil will flow through conduit 48 into chamber 61 and thence into conduit 50. The flow of oil will occur due to the pressure differential between chambers 46 and 44.Conduit 50 delivers refrigerant gas at suction pressure to a chamber 61 for generating a force on bellows 56 in opposition to the force generated by spring 58.

In operation, as noted previously, valve 26 regulates the flow of refrigerant directly in accordance with the refrigeration load on system 10. Valve 26 is particularly employed in refrigeration systems wherein the refrigeration compressor operates continuously irrespective of the refrigeration load on the system. During operation of the compressor, relatively small quantities of lubricating oil will bypass the piston rings and flow with the refrigerant gas discharged from the cylinders into discharge chambers 46. It has been found that as the flow rate of the refrigerant is reduced, the oil will stagnate within each discharge chamber 46. At higher flow rates, the high mass flow, high velocity refrigerant discharged into chamber 46 will carry the lubricating oil through the refrigeration system and thence return the bypassed lubricating oil to the suction side of the compressor.The oil is separated from the refrigerant and returned to the oil sump. However, at low mass flow conditions, such as those that occur at relatively low refrigeration loads, the reduced flow of refrigerant is not able to carry the bypassed lubricating oil through the system and therefore the oil accumulates within each discharge chamber 46.

As is evident, the accumulation of oil within the discharge chambers, if continued for a relatively prolonged period of time, may result in oil starvation of the compressor. In addition, if the compressor were to be stopped the accumulated oil in discharge chambers 46 may drain into cylinders 36. Upon restarting, the incompressible slugs of oil in the cylinders may cause damage to the valves, pistons and other components of the compressor. In view of the foregoing, it is necessary that the accumulation of oil be prevented.

To achieve the above desiderata, valve 28 is employed. Refrigerant gas at suction pressure is delivered from chamber 44 through conduit 50 into chamber 61 of valve 28. As the pressure and flow rate of the refrigerant decreases, due to a decreased refrigeration load on system 10, the pressure in chamber 61 will concurrently decrease. The force generated by spring 58 acting on the opposed side of bellows 56 will soon exceed the force generated within chamber 61 by the suction pressure of the refrigerant gas, causing U-shaped bracket 60 to move to the right as viewed in Figure 2. Movement of the bracket as described results in valve 62 moving away from and thereby opening valve seat 64 to permit flow through conduit 48.

Conduit 48 is in communication with discharge chamber 46. Thus, oil accumulated within discharge chamber 46 will flow through the now open conduit 48 into chamber 61. The oil contained within chamber 61 will flow, as it is at a relatively higher pressure than the refrigerant gas at suction pressure downwardly through conduit SO into suction chamber 44 and thence into suction manifold 41 through port 66. The lubricating oil will accumulate within suction manifold 41 until the pressure thereof opens normally closed check valve 52 thereby permitting the lubricating oil to pass from suction manifold 41 into lubricating oil sump 43.Thus, at low mass flow conditions, normally closed valve 62 is opened, communicating chamber 46 with chamber 44 for enabling lubricating oil to flow from chamber 46 to chamber 44. Valve 62 is opened through the sensed occurrence of a parameter in the system indicative of low mass flow conditions.

As the load on the system increases, the refrigerant gas pressure of the gas returning to manifold 41 and thence into suction chamber 44 will likewise increase due to the opening of valve 26. The subsequent increase in pressure within chamber 61 of valve 28 will cause U-shaped member 60 to move to the left as viewed in Figure 2, placing valve 62 in its seated position with respect to seat 64. The movement of valve 62 into its seated position, terminates flow of oil through conduit 48. Any oil thereafter accumulated within chamber 46 will move through the refrigeration system 10 due to the high mass flow, high velocity conditions of the refrigerant gas.

The arrangement herein disclosed provides a system which prevents oil starvation of a refrigerant compressor operating at low suction pressure, low mass flow conditions. The arrangement is directly sensitive to the occurrence of the low refrigerant mass flow conditions which heretofore have caused problems as explained above.

While a preferred embodiment of the present invention has been described and illustrated, the invention should not be limited thereto but may be otherwise embodied within the scope of the following claims.

Claims (12)

1. An oil return system for a refrigerant compressor for returning oil accumulated within a first portion of the compressor operating at refrigerant discharge pressure to a second portion of the compressor operating at substantially refrigerant suction pressure comprising conduit means connecting the first portion of the compressor with the second portion of the compressor for delivering lubricating oil accumulated within said first portion to said second portion; and normally closed valve means interposed in said conduit means for controlling the flow of lubricating oil through said conduit means including sensing means for sensing the magnitude of the pressure in said second portion of the compressor and operating means connected to said sensing means for opening said valve means when the sensed pressure decreases below a predetermined level for enabling the oil to flow through said conduit means from said first portion to said second portion.

2. A system in accordance with claim 1 wherein said second portion of said compressor is said lubricating oil sump.

3. A system in accordance with claim 1 wherein said second portion includes means defining a refrigerant suction manifold said system further including means defining an oil sump; means communicating said suction manifold and said oil sump for delivering said oil from said manifold to said sump; and normally closed valve means for controlling the flow of oil from said manifold to said sump, said valve means opening when the quantity of said oil delivered into said suction manifold exceeds a predetermined magnitude.

4. A system in accordance with claim 3 wherein said normally closed valve means is a check valve, with said check valve being opened in response to the pressure of oil accumulated in said suction manifold.

5. A refrigeration system including a refrigerant compressor having a first portion operating at refrigerant gas suction pressure and a second portion operating at refrigerant gas discharge pressure comprising first means for controlling the flow rate of refrigerant gas to said compressor; conduit means connecting the second portion of the compressor with the first portion of the compressor for delivering lubricating oil accumulated within said second portion to said first portion; and normally closed valve means interposed in said conduit means for controlling flow of oil through said conduit means including means to sense an operating parameter of said system indicative of the flow rate of said refrigerant as controlled by said first means and operating means connected to said sensing means for opening said valve means when the flow rate of said refrigerant has decreased below a predetermined level for enabling the oil to flow through said conduit means from said second portion to said first portion.

6. A system in accordance with claim 5 wherein said operating parameter is the suction pressure of the refrigerant gas entering said compressor.

7. A system in accordance with claims 5 or 6 wherein said first portion includes means defining a refrigerant suction manifold, said refrigeration compressor is further including means defining an oil sump; means communicating said suction manifold and said oil sump for delivering said oil from said manifold to said sump; and normally closed valve means for controlling the flow of oil from said manifold to said sump, said normally closed valve means opening when the quantity of said oil delivered into said suction manifold exceeds a predetermined magnitude.

8. A system in accordance with claim 7 wherein said normally closed valve means is a check valve.

9. A method of returning refrigerant compressor lubricating oil accumulated in a first portion of the compressor operating at refrigerant discharge pressure to a second portion of the compressor operating substantially at refrigerant suction pressure, the oil being accumulated in the first portion when the flow rate of the refrigerant to the compressor is reduced below a predetermined level comprising the steps of sensing an operating parameter indicative of the flow rate of refrigerant to the compressor; and communicating the first portion of the compressor with the second portion of the compressor when the sensed parameter indicates the flow rate of refrigerant has decreased below the predetermined level for enabling refrigerant accumulated in the first portion to flow to said second portion.

10. A method in accordance with claim 9 wherein the operating parameter is the suction pressure of the refrigerant and said sensing step comprises sensing the pressure of the refrigerant in the second portion of the compressor.

11. An oil return system substantially as described herein and with reference to the accompanying drawing.

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12. A method of returning refrigerant compressor lubricating oil substantially as described herein and with reference to the accompanying drawing.