EP2417405B1

EP2417405B1 - Refrigerating circuit and method for controlling the oil distribution within the same

Info

Publication number: EP2417405B1
Application number: EP09776516.8A
Authority: EP
Inventors: Markus Hafkemeyer; Jan Siegert
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2009-04-06
Filing date: 2009-04-06
Publication date: 2020-03-18
Anticipated expiration: 2029-04-06
Also published as: WO2010115435A1; EP2417405A1; CN102388278A; CN102388278B

Description

The invention relates to a refrigerating circuit as well as to a method for controlling the oil distribution within a multi-compressor unit of a refrigerating circuit.
JP 2000 337 726 A discloses an air conditioning system with a plurality of outdoor machines respectively including a compressor. The outdoor machines are connected in parallel with an indoor machine. The compressors are fluidly connected with each other through uniform oil pipes which can transport residual oil in each compressor. Discharge bypass valves and a capillary tube are provided on a discharge of the compressor of each outdoor machine. The bypass pipings are connected with a pressure container of each compressor, respectively.
In a conventional multi-compressor unit of a refrigerating circuit that comprises two or more oil lubricated compressors of the same or different size working in parallel the oil level in the individual compressors cannot be kept constant. Once started some compressors lose oil that others accumulate by numerous reasons. This effect is even stronger if the rotational speed of one or more of these compressors changes.
If a compressor of the multi-compressor unit runs out of oil, this compressor is likely to be damaged. The same happens, when the oil accumulated in one compressor exceeds a certain level. One conventional approach to ensure the oil level in the compressors being in an acceptable range is to apply an oil distribution program in which the compressor of the multi-compressor unit having an unacceptable oil level if switched out of operation and supplied with the necessary amount of oil or the excess oil is sucked off. This approach however significantly reduces the efficiency of the multi-compressor unit and the refrigerating circuit, since quite often one compressor cannot be operated in the refrigeration mode for a certain time interval.
Accordingly it would be beneficial to provide a refrigerating circuit and a method for controlling the oil distribution within a multi-compressor unit of a refrigerating circuit that provide an improved efficiency and that avoid switching off compressors from operation to ensure an acceptable oil level range.
A refrigerating circuit according to exemplary embodiments of the invention comprises - in flow direction - a multi-compressor unit, a condenser/gas cooler, a receiver, at least one evaporator having a respective expansion device arranged before it, and conduits circulating a refrigerant containing oil therethrough, wherein the multi-compressor unit comprises a first compressor the rotational speed of which can be controlled and at least one further compressor running at a constant rotational speed, wherein the suction sides and the pressure sides of the compressors are connected in parallel, wherein an oil balance line is provided between the oil sumps of the compressors, said oil balance line connecting the oil sumps of the compressors at substantially the same positions of height, and wherein a solenoid valve allowing oil flow in either direction is arranged in the oil balance line between the first compressor and the at least one further compressor for controlling the oil distribution between the oil sumps of the compressors during operation of the compressors of the multi-compressor unit. Means are provided for measuring a pressure difference between the first compressor and one of the further compressors, and wherein the solenoid valve is controllable so as to close if the pressure difference exceeds a predetermined threshold value, thereby blocking an oil flow between the first compressor and the further compressors, The solenoid valve is controllable so as to close if the rotational speed of the first compressor exceeds a predetermined upper threshold value or falls below a predetermined lower threshold value, thereby blocking an oil flow between the first compressor and the further compressors.
A method for controlling the oil distribution within a multi-compressor unit of a refrigerating circuit, the multi-compressor unit comprising a first compressor the rotational speed of which is controllable and at least one further compressor running at constant speed, wherein the suction sides and the pressure sides of the compressors are connected in parallel, comprising the following steps carried out while all compressors of the multi-compressor unit are running:

(a) allowing an oil flow between the oil sumps of the compressors, and
(b) blocking an oil flow between the first compressor and the further compressors, if a sensed pressure difference between the first compressor and one of the further compressors exceeds a predetermined threshold value, and/or if the rotational speed of the compressor exceeds an upper threshold value or falls below a lower threshold value, especially by closing a solenoid valve arranged in the oil balance line between the first compressor and the at least one further compressor, said oil balance line connecting the oil sumps of the compressors at substantially the same positions of height.

Exemplary embodiments of the invention will be described in more detail with reference to the enclosed figure, which shows a schematic diagram of a refrigeration circuit applying a multi-compressor unit according to an embodiment of the invention.
The refrigeration circuit 2 comprises in flow direction a multi-compressor unit having a speed controlled compressor 4, a first constant speed compressor 6 and a second constant speed compressor 8, a condenser/gas cooler 12, a receiver/collecting container 14, three evaporators 18, 22 and 26 having a respective expansion valve 16, 20 and 24 arranged before it and conduits/piping circulating a refrigerant containing oil therethrough. The operation of the refrigeration circuit 2 is known to a skilled person and does not need to be explained further.
The conduit portion connecting the outputs of the evaporators 18, 22 and 26 to the input sides of the compressors 4, 6 and 8 is called suction line 28 hereinafter. The suction line 28 branches off into three separate parallel lines leading to the input sides of the compressors 4, 6 and 8. The suction sides of the compressors 4, 6 and 8 are therefore connected in parallel.
The compressors 4, 6 and 8 can be reciprocating compressors. The speed controlled compressor 4 can be a VSD-controlled compressor.
The conduit portion between the output sides of the compressors 4, 6 and 8 and the entrance of the condenser/gas cooler 12 is called pressure line 10 hereinafter. The pressure line portions from the output sides of the compressors 4, 6 and 8 join before the condenser/gas cooler 12. Hence, the pressure side of the compressors 4, 6 and 8 are also connected in parallel. The refrigerant flow direction through these elements of the refrigerating circuit 2 is schematically depicted by an arrow having the reference numeral 38.
The condenser/gas cooler 12 works as a condenser liquefying the refrigerant, if the refrigeration circuit 2 is operated in a sub-critical mode. The condenser/gas cooler 12 works as a gas cooler not liquefying but only cooling the gaseous refrigerant, if the refrigeration circuit 2 is operated in a transcritical mode. The refrigerant circulating in the refrigeration circuit can be of any conventional kind, however, it is particularly suitable for transcritical operation. Thus, also CO2 can be used as refrigerant.
All of the compressors of the multi-compressor unit, namely the VSD-controlled compressor 4, the first constant speed compressor 6 and the second constant speed compressor 8 comprise an oil sump, and an oil balance line 30 attaches to the same positions of height of the oil sump of the VSD-controlled compressor 4 and the constant speed compressor 6 and connects the oil sumps of these compressors 4 and 6. This oil balance line 30 also extends to the second constant speed compressor 8, and is connected to the oil sump level of the second constant speed compressor 8 attaching to the same position of height of its oil sump level. This oil balance line to the second constant speed compressor 8 has the reference numeral 34. The oil sump levels are provided with a sight glass for allowing monitoring of the oil sump levels within the oil sump of the compressors 4, 6 and 8. In the oil balance line 30 between the VSD-controlled compressor 4 and the first constant speed compressor 6 a solenoid valve 32 is provided that allows oil flow in either direction and that is capable for controlling the oil distribution between the oil sumps of the VSD-controlled compressor 4 on the one hand and the constant speed compressor 6 and 8 on the other hand during operation of the compressors 4, 6 and 8 of the multi-compressor unit. The oil flow direction within the oil balance line 30 is depicted by an arrow having the reference numeral 36.
In one particular embodiment of operation, the oil sump levels of the compressors 4, 6 and 8 are monitored and the solenoid valve 32 is closed if the oil sump level of one of the compressors 4, 6 and 8 exceeds a predetermined upper threshold value or falls below a predetermined lower threshold value, thereby blocking an undesired oil flow between the speed controlled compressor 4 and the constant speed compressors 6 and 8.
In another particular embodiment of operation, the oil sump levels of the compressors 4, 6 and 8 are monitored and the solenoid valve 32 is closed if the oil sump levels difference between the speed controlled compressor 4 and one of the further constant speed compressors 6 and 8 exceeds a predetermined upper threshold value, thereby blocking an undesired oil flow between the speed controlled compressor 4 and the constant speed compressors 6 and 8.
The pressure difference between the speed controlled compressor 4 and the constant speed compressors 6 and 8 is monitored and the solenoid valve 32 is closed if this pressure difference exceeds a predetermined threshold value, thereby blocking an undesired oil flow between the speed controlled compressor 4 and the constant speed compressors 6 and 8.
Also, the rotational speed of the speed controlled compressor 4 is monitored and the solenoid valve 32 is closed if the rotational speed of the speed controlled compressor 4 exceeds a predetermined upper threshold value or falls below a predetermined lower threshold value, thereby blocking an undesired oil flow between the speed controlled compressor 4 and the constant speed compressors 6 and 8.
These control features can employ means for sensing the required values (not shown in the Figure), for example means for monitoring the oil sump levels, means for measuring a pressure difference or means for measuring the rotational speed, and the solenoid valve is controlled so as to close if the sensed values are no more in an acceptable range and/or so as to close if the sensed values are in an acceptable range again.
When the rotational speed is used for as control feature, it is not necessary to provide a certain device. It is possible to just refer to the known frequency controlled by the VSD. Since every other compressor runs at the same, constant and of course known frequency of the local power supply and the frequency of the VSD output is given the difference can be used to calculate the difference of rotational speed.
These control features provide a reliable oil distribution between the oil sumps of the compressors, and only require a minimum equipment of sensors and control.
The solenoid valve 32 can be closed and opened up at predetermined intervals for blocking, and respectively, allowing oil flow between the speed controlled compressor 4 and the constant speed compressors 6 and 8.
By closing and opening up the solenoid valve 32 at predetermined intervals, the extent of the oil distribution between the speed controlled compressor 4 and the constant speed compressors 6 and 8 can be limited in terms of time, and the opening and closing intervals can be selected according to the specification and the expected load or performance of the refrigerating circuit. By this embodiment a reliable oil distribution can be attained nearly without any sensor equipment.
As described above, the solenoid valve closes and avoids an oil distribution between the speed controlled compressor and the other constant speed compressors.
It is possible to provide a unit controller (not shown in the Figure) that controls the condenser/gas cooler 12, the compressors 4, 6 and 8 and the solenoid valve 32. The control of this unit controller can be carried out by a control algorithm for the solenoid valve 32 being integrated into the unit controller or by a separate impulse generator.
As described above, the oil distribution between the compressors can be effected during normal operation of the compressors of the multi-compressor unit. It is neither necessary to switch off a compressor in order to run an oil distribution program nor is it necessary to provide an additional oil separator. The oil distribution between the oil sumps of the compressors can be controlled during operation of all the compressors running which avoids switching off one or the other compressor for a certain time interval and which significantly improves the efficiency of the refrigerating circuit. Moreover, the oil levels in the oil sumps of all the compressors can reliably be kept in an acceptable range.
As described above, the oil balance line attaches to positions of height at the oil sumps of the compressors that corresponds to the necessary oil level of the compressors in operation. Alternatively, as an example not being part of the present invention, the oil balance line can attach to other positions of height within an acceptable oil level range between a position of minimum required oil level and a position of maximum required oil level or even lower.
However, it is required that the oil balance line always attaches to substantially the same positions of height of the oil sumps present in the multi-compressor unit, wherein a variation of a few millimeters is acceptable.
According to exemplary embodiments, as described above, the suction pressure of the compressors of the multi-compressor unit lies at substantially the same suction pressure level for all compressors. The refrigerating circuit according to exemplary embodiments, as described above, does not use a pressure difference for regulating the oil level, and the equipment necessary for such oil regulation due to pressure differences can be avoided, which saves further costs and effort.
While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition many modifications may be made to adopt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention include all embodiments falling within the scope of the claims.

List of Reference Numerals

2: refrigeration circuit
4: VSD controlled compressor
6: constant speed compressor
8: constant speed compressor
10: pressure line
12: condenser/gas cooler
14: receiver
16: first expansion device
18: first evaporator
20: second expansion device
22: second evaporator
24: third expansion device
26: third evaporator
28: suction line
30: oil balance line
32: solenoid valve
34: oil balance line to additional compressor
36: oil flow direction
38: refrigerant flow direction

Claims

Refrigerating circuit, comprising - in flow direction - a multi-compressor unit (4, 6, 8), a condenser/gas cooler (12), a receiver (14), at least one evaporator (18, 22, 26) having a respective expansion device (16, 20, 24) arranged before it, and conduits circulating a refrigerant containing oil therethrough,
wherein the multi-compressor unit (4, 6, 8) comprises a first compressor (4) the rotational speed of which can be controlled and at least one further compressor (6, 8) running at a constant rotational speed,
wherein the suction sides and the pressure sides of the compressors (4, 6, 8) are connected in parallel,
wherein an oil balance line (30) is provided between the oil sumps of the compressors (4, 6, 8), said oil balance line (30) connecting the oil sumps of the compressors (4, 6, 8) at substantially the same positions of height,
wherein a solenoid valve (32) allowing oil flow in either direction is arranged in the oil balance line (30) between the first compressor (4) and the at least one further compressor (6, 8) for controlling the oil distribution between the oil sumps of the compressors (4, 6, 8) during operation of the compressors (4, 6, 8) of the multi-compressor unit (4, 6, 8), and
characterized in that means are provided for measuring a pressure difference between the first compressor (4) and one of the further compressors (6, 8), and wherein the solenoid valve (32) is controllable so as to close if the pressure difference exceeds a predetermined threshold value, thereby blocking an oil flow between the first compressor (4) and the further compressors (6, 8); or
in that the solenoid valve (32) is controllable so as to close if the rotational speed of the first compressor (4) exceeds a predetermined upper threshold value or falls below a predetermined lower threshold value, thereby blocking an oil flow between the first compressor (4) and the further compressors (6, 8).
Refrigerating circuit according to claim 1, wherein the compressors (4, 6, 8) of the multi-compressor unit operate substantially at the same suction pressure level.
Refrigerating circuit according to claim 1 or 2, wherein means to monitor the oil sump levels are arranged in the oil sumps of the compressors (4, 6, 8), and wherein the solenoid valve (32) is controllable so as to close if the oil sump level of one of the compressors (4, 6, 8) exceeds a predetermined upper threshold value, thereby blocking an oil flow between the first compressor (4) and the further compressors (6, 8).
Refrigerating circuit according to any of the preceding claims, wherein means to monitor the oil sump levels are arranged in the oil sumps of the compressors (4, 6, 8), and wherein the solenoid valve (32) is controllable so as to close if the oil sump level of one of the compressors (4, 6, 8) falls below a predetermined lower threshold value, thereby blocking an oil flow between the first compressor (4) and the further compressors (6, 8).
Refrigerating circuit according to any of the preceding claims, wherein means to monitor the oil sump levels are arranged in the oil sumps of the compressors (4, 6, 8), and wherein the solenoid valve (32) is controllable so as to close if the oil sump level difference between the first compressor (4) and one of the further compressors (6, 8) exceeds a predetermined upper threshold value, thereby blocking an oil flow between the first compressor (4) and the further compressors (6, 8).
Refrigerating circuit according to claim 1 or 2, wherein the solenoid valve (32) is controllable so as to close and open up at predetermined intervals for blocking and, respectively, allowing oil flow between the first compressor (4) and the further compressors (6, 8).
Refrigerating circuit according to any of the preceding claims, further comprising a unit controller for controlling the condenser/gas cooler (12), the compressors (4, 6, 8) and the solenoid valve (32).
Refrigerating circuit according to claim 7, wherein the control algorithm for the solenoid valve (32) is integrated into the unit controller.
Refrigerating circuit according to any of claims 1 to 7, wherein the control of the solenoid valve (32) is carried out by a separate impulse generator.
Method for controlling the oil distribution within a multi-compressor unit of a refrigerating circuit, the multi-compressor unit (4, 6, 8) comprising a first compressor (4) the rotational speed of which is controllable and at least one further compressor (6, 8) running at constant speed, wherein the suction sides and the pressure sides of the compressors (4, 6, 8) are connected in parallel, the multi-compressor unit (4, 6, 8) further comprising an oil balance line (30) extending between the first compressor (4) and the at least one further compressor (6, 8), and allowing an oil flow between the oil sumps of the compressors (4, 6, 8);
characterized in that, while all compressors (4, 6, 8) of the multi-compressor unit are running,
the oil flow between the first compressor (4) and the further compressors (6, 8) is blocked, when a sensed pressure difference between the first compressor (4) and one of the further compressors (6, 8) exceeds a predetermined threshold value, and/or when the rotational speed of the compressor (4) exceeds an upper threshold value or falls below a lower threshold value, especially by closing a solenoid valve (32) arranged in the oil balance line (30), said oil balance line (30) connecting the oil sumps of the compressors (4, 6, 8) at substantially the same positions of height.