Patent Description:
In a multiple-compressor system, such as a refrigeration system, one challenge is to maintain sufficient oil level in each of the compressors whether the compressor is running or not. Designing a system capable of moving equal amounts of oil to different compressors is difficult due to variations in the individual compressors and piping configurations to those compressors. A particular example of the state of the art with respect to suction gas distribution in a parallel compressor assembly is represented by WIPO patent publication <CIT> (Device For Suction Gas Distribution In A Parallel Compressor Assembly, And Parallel Compressor Assembly), which shows a distribution device for suction gas in systems with two or more compressors. A particular example of oil management in systems having multiple compressors is disclosed in <CIT> (Suction Line Flow Stream Separator For Parallel Compressor Arrangements).

Additionally, oil distribution systems for multiple-compressor arrangements are disclosed in <CIT>; <CIT>; and <CIT>, each of which is assigned to the assignee of the present application.

For example, when distributing oil from one compressor to another in in a refrigeration system having multiple compressors, the amount of oil distributed is at least partly dependent on the oil available to be drawn into the opening of an oil-supplying compressor such that the oil can then be distributed to one or more downstream oil-receiving compressors in the refrigeration system. It is also dependent on the oil sump pressures in the compressors. <CIT> discloses a method of operating a refrigerant system using a plurality of compressors providing that the method includes separating oil entrained in the refrigerant and returning more of the oil to a lead compressor regardless of whether the lead compressor is operating. <CIT> discloses an air-conditioner including a variable speed compressor and a constant speed compressor, the constant speed compressor being adapted to operate if necessary, in addition to the variable speed compressor when a room is air conditioned by the air conditioner. Suction tubes of both compressors are connected by a bypass tube in which an electromagnetic valve is arranged. According to this prior art after predetermined times the capacity and load of the compressors are checked and eventually the valve is opened for a predetermined time.

Embodiments of the invention provide an advancement over the state of the art with respect to oil distribution in multiple-compressor systems. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.

In one aspect, embodiments of the invention provide a method of operating a multiple-compressor refrigeration system, according to claim <NUM>. This method includes the steps of supplying, via a common supply line, refrigerant gas and oil to a plurality of compressors, and attaching an oil flow conduit between adjacent compressors of the plurality of compressors. The oil flow conduit is configured to move oil from a compressor with a relatively higher pressure to a compressor with a relatively lower pressure. The method further includes controlling the pressure for each of the plurality of compressors by regulating a speed at which each of the plurality of compressors operates in order to maintain a pressure differential between the adjacent compressors to facilitate the flow of oil from the compressor with the relatively higher pressure to the compressor with the relatively lower pressure.

In a particular embodiment, controlling the pressure for each of the plurality of compressors by regulating a speed at which each of the plurality of compressors operates includes attaching each of the plurality of compressors to a refrigeration system controller. Also, supplying refrigerant gas and oil to a plurality of compressors coupled in series may include supplying refrigerant gas and oil to a lead compressor and to one or more non-lead compressors located downstream of the lead compressor. In certain embodiments, at least one of the one or more non-lead compressors has a greater pumping capacity than the lead
compressor. Further, in some embodiments, a first non-lead compressor is located immediately downstream of the lead compressor, the first non-lead compressor having a greater pumping capacity than the lead compressor, and wherein a second non-lead compressor is located immediately downstream of the first non-lead compressor, the second non-lead compressor having a greater pumping capacity than the first non-lead compressor.

Additionally, each of the plurality of compressors may have the same pumping capacity. Also, supplying refrigerant gas and oil to a plurality of compressors may require supplying refrigerant gas and oil to a plurality of compressors via a corresponding plurality of inlet supply lines. In some embodiments, at least one of the plurality of inlet supply lines includes a flow restriction located on an interior portion of the at least one of the plurality of inlet supply lines. In other embodiments, at least one of the plurality of inlet supply lines includes a portion that protrudes into the common supply line to restrict a flow of refrigerant gas and oil into the at least one of the plurality of inlet supply lines. The plurality of inlet supply lines may be arranged to supply more oil to a lead compressor than is supplied to one or more non-lead compressors located downstream of the lead compressor.

In further embodiments of the method, each of the plurality of inlet supply lines is sized to create a pressure differential between adjacent compressors of the plurality of compressors, and the sizes of the plurality of inlet supply lines are configured to produce a higher pressure in the lead compressor than in the non-lead compressors.

In another aspect, embodiments of the invention provide a refrigeration system with a plurality of compressors connected in series with each other according to claim <NUM>. Each compressor has an oil sump located in a gravitational bottom of the compressor, and one or more oil flow conduits coupled between adjacent compressors of the plurality of compressors. The oil flow conduits are configured to facilitate a distribution of oil from an upstream compressor to a downstream compressor. A common supply line supplies refrigerant and oil to each of the plurality of compressors. The plurality of compressors includes a lead compressor and one or more non-lead compressors. The common supply line is configured to return more oil to the lead compressor than to the one or more non-lead compressors. A controller is coupled to each of the plurality of compressors. The controller regulates the speed of each of the plurality of compressors to control oil sump pressures for each of the plurality of compressors such that the lead compressor has a higher oil sump pressure than any of the one or more non-lead compressors in order to facilitate oil distribution from the lead compressor to the one or more non-lead compressors.

In a particular embodiment, the plurality of compressors includes the lead compressor being located upstream from a first non-lead compressor. The controller is configured to operate the lead compressor at a slower speed than the first non-lead compressor in order to maintain the higher pressure in the lead compressor. In some embodiments, the lead compressor and first non-lead compressor has the same pumping capacity. In other embodiments, the first non-lead compressor has a greater pumping capacity than the lead compressor. Furthermore, a second non-lead compressor may be coupled to, and downstream of, the first non-lead compressor.

In certain embodiments, the plurality of compressors comprises a second non-lead compressor coupled to, and downstream of, the first non-lead compressor, and wherein the controller is configured to operate the lead compressor at a slower speed that the first non-lead compressor in order to maintain the higher pressure in the lead compressor. Further, each of the plurality of compressors may have an inlet supply line to supply refrigerant and gas from the common supply line. In a particular embodiment, at least one inlet supply line includes one of: a portion that protrudes into the common supply line to restrict a flow of refrigerant gas and oil into the at least one inlet supply line; and a flow restriction located on an interior portion of the at least one inlet supply line.

In a further embodiment, each of the inlet supply lines is sized to create a pressure differential between adjacent compressors of the plurality of compressors, and the sizes of the inlet supply lines are configured to produce a higher pressure in the lead compressor than in the non-lead compressors.

The following detailed description describes embodiments of the invention as applied in a multi-compressor refrigeration system. However, one of ordinary skill in the art will recognize that the invention is not necessarily limited to refrigeration systems. Embodiments of the invention may also find use in other systems where multiple compressors are used to supply a flow of compressed gas. It should also be noted that, for the sake of convenience, certain embodiments of the invention may be described hereinbelow with respect to their application in systems having multiple scroll compressors for compressing refrigerant. While particular advantages and configurations are shown for scroll compressors, Applicants submit that the scope of the invention is not necessarily limited to scroll compressors, but may find use in a variety of multiple-compressor systems using compressor types other than scroll compressors.

In the context of this application, the terms "upstream" and "downstream" are used to refer to various compressors in relation to the flow of oil between the compressors. For example, in the embodiments of refrigeration systems described hereinbelow, the lead compressor receives most of the oil in the circulated refrigerant. As such, in the embodiments presented, the lead compressor is the most upstream of the compressors. Oil flows downstream from the lead compressor to the nearest, or adjacent, non-lead compressor. If the system has a third compressor, oil flows downstream, from the aforementioned non-lead compressor nearest the lead compressor, to the next non-lead compressor.

In multiple-compressor systems in which oil is distributed between compressors, the system may rely on pressure differentials between the compressors to move the oil, for example from a lead compressor to one or more non-lead compressors. In certain system arrangements, the pressure differential provides the driving force for the movement of oil between compressors. There are several factors which may affect the pressure in a given compressor. For example, the size of the inlet piping feeding oil to the compressor affects pressure. Additionally, as will be explained below, pressure can also be affected by the pumping capacity of the compressors.

<FIG> provides a schematic illustration of an exemplary multiple-compressor refrigeration system <NUM> having a plurality of N compressors <NUM>. The N compressors <NUM> of refrigeration system <NUM> are connected in a parallel circuit having inlet flow line <NUM> that supplies a flow of refrigerant to the N compressors <NUM>, and outlet flow line <NUM> that carries compressed refrigerant away from the N compressors <NUM>. In certain embodiments, the flow of refrigerant carries oil entrained within the flow, the oil used to lubricate moving parts of the compressor <NUM>. As shown, the outlet flow line <NUM> supplies a condenser <NUM>. In a particular embodiment, the condenser <NUM> includes a fluid flow heat exchanger <NUM> (e.g. air or a liquid coolant) which provides a flow across the condenser <NUM> to cool and thereby condense the compressed, high-pressure refrigerant.

An evaporation unit <NUM> to provide cooling is also arranged in fluid series downstream of the condenser <NUM>. In an alternate embodiment, the condenser <NUM> may feed multiple evaporation units arranged in parallel. In the embodiment of <FIG>, the evaporation unit <NUM> includes a shut off liquid valve <NUM>, which, in some embodiments, is controlled by the refrigeration system controller <NUM> to allow for operation of the evaporation unit <NUM> to produce cooling when necessitated by a demand load on the refrigeration system <NUM>, or to preclude operation of the evaporation unit <NUM> when there is no such demand. As shown in <FIG>, the refrigeration system controller <NUM> may also be directly connected to each of the N compressors <NUM>. The refrigeration system controller <NUM> may be configured to independently control each of the N compressors <NUM>. Specifically, the refrigeration system controller <NUM> may be configured to turn compressors <NUM> on or off, or independently control the speed of each compressor <NUM> during operation.

The evaporation unit <NUM> also includes an expansion valve <NUM> that may be responsive to, or in part controlled by, a downstream pressure of the evaporation unit <NUM>, sensed at location <NUM>. The expansion valve <NUM> is configured to control the discharge of refrigerant into the evaporation unit <NUM>, wherein due to the evaporation, heat is absorbed to evaporate the refrigerant to a gaseous state thereby creating a cooling/refrigeration effect at the evaporation unit <NUM>. The evaporation unit <NUM> returns the expanded refrigerant in a gaseous state along the inlet flow line <NUM> to the bank of N compressors <NUM>.

<FIG> illustrates an alternate embodiment of a refrigeration system <NUM> in which compressor speed is controlled by refrigeration system controller <NUM> in such a manner as to create sequentially-decreasing pressures from one compressor to the next in order to facilitate the distribution of oil from compressors with relatively higher oil sump pressures to those with relatively lower oil sump pressures. <FIG> is a schematic view of the refrigeration system <NUM> which includes lead compressor <NUM> connected in series with non-lead compressor <NUM>. In this embodiment, the two compressors <NUM>, <NUM> have the same pumping capacity. For the sake of simplicity, the embodiment of <FIG> shows only two compressors, however, it should be understood that the scope of the invention allows for the use of more than two compressors in accordance with the general system arrangement shown in <FIG>.

Refrigerant gas is supplied to the two compressors <NUM>, <NUM> via a common supply line <NUM>. Oil entrained in the refrigerant gas is also returned to the two compressors <NUM>, <NUM>. A first inlet supply line <NUM> carries refrigerant and oil to the lead compressor <NUM>, while a second inlet supply line <NUM> carries refrigerant and oil to the non-lead compressor <NUM>. In the embodiment of <FIG>, first inlet supply line <NUM> and the second inlet supply line <NUM> intersect the common supply line <NUM> at a gravitational bottom of the common supply line <NUM>, where the common supply line <NUM> runs horizontally. In alternate embodiment, the inlet supply lines may intersect with the common supply line <NUM> at locations other than the gravitational bottom.

Oil is distributed from the lead compressor <NUM> to the non-lead compressor <NUM> via oil flow conduit <NUM>. The oil flow conduit <NUM> is attached to a lower portion of each of the two compressors <NUM>, <NUM>, for example to a fitting attached to the compressor housings proximate the oil sump of each compressor <NUM>, <NUM>. In some of the embodiments disclosed herein, the systems are designed to return more oil to the lead compressor <NUM> than to any of the non-lead compressors.

In the embodiment of <FIG>, the first inlet supply line <NUM> for the lead compressor <NUM> is larger than the second inlet supply line <NUM> for the non-lead compressor <NUM>. In this case, the larger size of the first inlet supply line <NUM> allows for a greater pressure of refrigerant gas and oil into the lead compressor <NUM> than to non-lead compressor <NUM>. Successively smaller input supply lines could be attached to the non-lead compressors located downstream from the lead compressor <NUM>. These smaller input supply lines result in decreasing pressures in each successive compressor. Other piping means could be used to achieve a similar effect with respect to the pressure in the various compressors. For example, the system <NUM> could have multiple compressors with inlet supply lines of the same size where the flow into the inlet supply lines is limited by restrictions placed inside of the inlet supply lines. The greater the restriction, the lower the pressure in the respective compressor. As will be shown below, flows into the inlet supply lines can also be restricted by having the inlet supply line protrude into the interior of the common supply line <NUM>.

As stated above, the pressure in the two compressors <NUM>, <NUM> are also affected by the pumping capacity of the compressors. Thus, in particular embodiments of the invention, the refrigeration system controller <NUM> (shown in <FIG>) controls the compressor speed to maintain the desired pressure for controlling the flow of oil from the lead compressor <NUM> to the non-lead compressor <NUM>. Increasing the speed of the non-lead compressor <NUM> increases its pressure drop thereby lowering the pressure in that compressor and increasing the amount of oil it receives from the lead compressor <NUM>. Alternatively, decreasing the speed of the lead compressor <NUM> reduces its pressure drop thereby raising the pressure in that compressor and increasing the amount of oil it supplies to the non-lead compressor <NUM>.

When used in conjunction with inlet supply line arrangements such as shown in <FIG>, there is some added flexibility with respect to the aforementioned control of compressor speed to maintain the desired pressure differential to facilitate a flow of oil from the lead compressor <NUM>. For example, with the inlet supply line arrangement of <FIG>, it may be possible to operate the lead compressor <NUM> at the same speed, or even a higher speed, than the non-lead compressor <NUM> while maintaining the desired pressure differential.

<FIG> illustrates an alternate embodiment of a refrigeration system <NUM> in which compressor speed is controlled in such a manner as to create sequentially-decreasing pressures from one compressor to the next in order to facilitate the distribution of oil from compressors with relatively higher oil sump pressures to those with relatively lower oil sump pressures. The refrigeration system <NUM> has a lead compressor <NUM> coupled in series with a non-lead compressor <NUM>. A common supply line <NUM> provides refrigerant gas and oil to the lead compressor <NUM> via a first inlet supply line <NUM>, and to the non-lead compressor <NUM> via second inlet supply line <NUM>. Oil is distributed from the lead compressor <NUM> to the non-lead compressor <NUM> via oil flow conduit <NUM>. The oil flow conduit <NUM> is attached to a lower portion of each of the two compressors <NUM>, <NUM>, for example to a fitting attached to the compressor housings proximate the oil sump of each compressor <NUM>, <NUM>.

In the embodiment of <FIG>, the first inlet supply lines <NUM>, <NUM> are of equal size. However, the second inlet supply line <NUM> has an optional portion <NUM> (shown in broken lines) that protrudes into an interior portion of the common supply line <NUM>. This protruding portion <NUM>, which is oriented perpendicular to the flow of refrigerant and oil, reduces the flow of both into the non-lead compressor <NUM> reducing the pressure in that compressor. The larger capacity and lower oil sump pressure of non-lead compressor <NUM> facilitates a flow of oil from the lead compressor to non-lead compressor <NUM> through oil distribution line <NUM>, which is connected to lower portions of the lead and non-lead compressors <NUM>, <NUM>. Alternatively, instead of optional protruding portion <NUM>, an optional restriction <NUM> could be used in an interior portion of second inlet supply line <NUM> to effectively reduce the inner diameter of the second inlet supply line <NUM>, thereby reducing the flow of refrigerant and oil into non-lead compressor <NUM>, which results in a lower pressure in this compressor.

However, with or without the restriction <NUM> or the protruding portion <NUM> of the second inlet supply line <NUM>, the flow of oil from the lead compressor <NUM> to the non-lead compressor <NUM> can be controlled by regulating the speed, or pump capacity, of the two compressors <NUM>, <NUM>. In the configuration of <FIG>, the non-lead compressor <NUM> compressor is shown as larger than lead compressor <NUM>, indicating its greater pumping capacity than the lead compressor <NUM>. For example, if the two compressors <NUM>, <NUM> are scroll compressors, non-lead compressor <NUM> would have larger scroll compressor bodies, i.e., designed to compress more refrigerant in a given time period than the compressor bodies of the lead compressor <NUM>. The non-lead compressor <NUM> would also have a larger drive unit, and a larger compressor housing than the lead compressor <NUM>. Therefore, all things being equal, the larger-capacity non-lead compressor <NUM> would normally have a lower oil sump pressure than the lead compressor <NUM>. Thus, the non-lead compressor <NUM> could be run at the same speed, or even a lower speed, than the lead compressor <NUM> while still maintaining a pressure differential sufficient to drive oil from the lead compressor <NUM> to the non-lead compressor <NUM>. The use of portion <NUM> of second inlet supply line <NUM> allows even more flexibility in controlling the speeds of the two compressors <NUM>, <NUM> while still maintaining a pressure differential sufficient for oil distribution from the lead compressor <NUM> to the non-lead compressor <NUM>.

<FIG> is a schematic diagram showing a multiple-compressor refrigeration system <NUM>, according to an embodiment of the invention. Refrigeration system <NUM> includes lead compressor <NUM> connected in series with first non-lead compressor <NUM>, which is, in turn, connected in series to second non-lead compressor <NUM>. In the arrangements shown in <FIG>, as in the embodiments described above, the series connections indicates that, regardless of the number of compressors in the system, oil can only flow from an upstream compressor to the next adjacent compressor immediately downstream.

In the embodiment of <FIG>, in which the multi-compressor system <NUM> includes second non-lead compressor <NUM> downstream from the first non-lead compressor <NUM>, the first non-lead compressor <NUM>, having a pressure higher than that of the second non-lead compressor <NUM>, distributes oil downstream to second non-lead compressor <NUM> through a second oil flow conduit <NUM>. The first non-lead compressor <NUM> receives oil from the lead compressor <NUM> via first oil flow conduit <NUM>. This process can be repeated for however many compressors make up the multi-compressor system <NUM>, where the cascading pressure differential between the compressors allows oil to flow from the lead compressor <NUM> downstream to non-lead compressors. Thus, the multiple series-connected compressors distribute oil from the upstream-most compressor sequentially downstream to compressors with progressively lower oil sump pressures. While this design does require adjacent compressors to be running, it will be shown below that there are several different ways of achieving this cascading effect.

The refrigeration system <NUM> further includes a suction header arrangement <NUM> that includes a common supply line <NUM>, a first inlet supply line <NUM> coupling the lead compressor <NUM> to the common supply line <NUM>, a second inlet supply line <NUM> coupling the first non-lead compressor <NUM> to the common supply line <NUM>, and a third inlet supply line <NUM> coupling the second non-lead compressor <NUM> to the common supply line <NUM>.

In the embodiment shown, lead inlet supply line <NUM>, the first inlet supply line <NUM>, and the second inlet supply line <NUM> intersect the common supply line <NUM> at a gravitational bottom of the common supply line <NUM> where the common supply line <NUM> runs horizontally. A number of suction header arrangements <NUM> may be used to help achieve the cascading pressures described above. Optional first and second protruding portions <NUM>, <NUM> (shown in broken lines) of the second and third inlet supply lines <NUM>, <NUM>, respectively, restrict the flow of refrigerant and oil into those inlet supply lines <NUM>, <NUM>. Alternatively, optional first and second restrictions <NUM>, <NUM> (shown in broken lines) of the second and third inlet supply lines <NUM>, <NUM>, respectively, restrict the flow of refrigerant and oil into those inlet supply lines <NUM>, <NUM>. As can be seen from <FIG>, the second protruding portion <NUM> protrudes farther into the common supply line <NUM> than the first protruding portion <NUM>. As a result, the second non-lead compressor <NUM> will have a lower sump pressure than the first non-lead compressor <NUM>, which will have lower sump pressure than lead compressor <NUM>. Similarly, it can be seen that the second restriction <NUM> restricts flow more than first restriction <NUM>, which achieves the same result as the first and second protruding portions <NUM>, <NUM> in terms of refrigerant and oil flow to the three compressors <NUM>, <NUM>, <NUM>.

Therefore, in the embodiment of <FIG>, while all three compressors <NUM>, <NUM>, <NUM> receive a flow of refrigerant gas, having oil entrained therein, from a common supply line <NUM>, the common supply line <NUM> delivers more lubricating oil to the lead compressor <NUM>, via the first inlet supply line <NUM>. However, with or without the optional first and second restrictions <NUM>, <NUM> or the optional first and second protruding portions <NUM>, <NUM>, the flow of oil from the lead compressor <NUM> to the first non-lead compressor <NUM>, and from that compressor to the second non-lead compressor <NUM>, can be controlled by regulating the speed, or pump capacity, of the three compressors <NUM>, <NUM>, <NUM>.

As explained above, the refrigeration system controller <NUM> (shown in <FIG>) can run the first non-lead compressor <NUM> at a faster speed than the lead compressor <NUM> to create a desired pressure differential between those two compressors <NUM>, <NUM>. Further, the refrigeration system controller <NUM> can run the second non-lead compressor <NUM> at a faster speed than the first non-lead compressor <NUM> to create a desired pressure differential between those two compressors <NUM>, <NUM>. Thus, by controlling the compressor speeds and, therefore, the pressures in those compressors <NUM>, <NUM>, <NUM>, oil can be distributed among the compressors <NUM>, <NUM>, <NUM>. However, as in the embodiments above, the arrangement of the inlet supply lines provides flexibility with respect to how compressor speed can be controlled while maintaining the desired pressure differentials.

Claim 1:
A method of operating a multiple-compressor refrigeration system (<NUM>, <NUM>, <NUM>, <NUM>) comprising the steps of:
supplying, via a common supply line (<NUM>, <NUM>, <NUM>), refrigerant gas and oil to a plurality of compressors (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>);
attaching an oil flow conduit (<NUM>, <NUM>, <NUM>, <NUM>) between adjacent compressors (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) of the plurality of compressors (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>), such that the plurality of compressors (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) are coupled in series by said oil flow conduit so that, regardless of the number of compressors in the system, oil can only flow from an upstream compressor to the next adjacent compressor immediately downstream, the oil flow conduit (<NUM>, <NUM>, <NUM>, <NUM>) configured to move oil from a compressor (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) with a relatively higher pressure to a compressor (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) with a relatively lower pressure;
the method being characterised by controlling the pressure for each of the plurality of compressors (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) by regulating a speed at which each of the plurality of compressors (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) operates in order to maintain a pressure differential between the adjacent compressors (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) to facilitate the flow of oil from the compressor (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) with the relatively higher pressure to the compressor (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) with the relatively lower pressure.