Method of control of compressors with more than two capacity states

Abstract

A method of operating a reciprocating compressor for a vapour compression system is disclosed. The reciprocating compressor comprises at least two cylinders and at least two unloaders, each unloader can be operated in an idle mode or in an active mode and therefore the reciprocating compressor can run in more than two capacity states. The capacity states alternates periodically between states in such a way that a substantially continuous range of effective capacities can be obtained while the individual cylinders are evenly loaded.

Claims

1. A method of operating a reciprocating compressor defining two or more capacity levels, the reciprocating compressor comprising a plurality of cylinders and a plurality of unloaders, each unloader having settings for operating at least one of the cylinders in an idle mode or in an active mode, wherein at least some of the capacity levels of the reciprocating compressor are obtainable by two or more combinations of unloader settings, the method comprising the steps of: selecting, by a controller, an effective capacity to be delivered by the reciprocating compressor, selecting, by the controller, a first sequence of capacity levels of the reciprocating compressor for providing the selected effective capacity, for each capacity level in the first sequence, selecting, by the controller, a second sequence of unloader settings for obtaining the capacity level, at least one of said second sequences comprising two or more different combinations of the plurality of unloaders being set to an idle mode or active mode to obtain the capacity level, defining, by the controller, a final sequence of setting combinations of the unloaders, based on the first sequence of capacity levels and each of the second sequences of setting combinations of the unloaders, in such a manner that: the capacity levels provided by the setting combinations of the final sequence provides the capacity levels of the first sequence of capacity levels, and for each of the capacity levels of the first sequence of capacity levels, the final sequence provides the setting combinations of the corresponding second sequence of setting combinations of the unloaders, and operating, by the controller, the reciprocating compressor in accordance with the final sequence; and wherein the controller selects the second sequence of unloader settings and defines the final sequence of setting combinations of the unloaders so that a load is evenly distributed across the plurality of cylinders during the operating of the reciprocating compressor; and wherein, during the final sequence, a frequency at which the capacity levels change is greater than a frequency at which individual unloaders shift between the idle and active modes.

2. The method according to claim 1, wherein the final sequence defines a cyclic switching pattern for the unloaders.

3. The method according to claim 1, wherein the final sequence is defined in such a manner that when switching from one setting combination of the unloaders to another, only one of the unloaders is switched between a position providing an idle mode of the corresponding cylinder(s) and a position providing an active mode of the corresponding cylinder(s).

4. The method according to claim 1, wherein the first sequence defines a cyclic switching pattern.

5. The method according to claim 1, wherein the first sequence comprises alternating between a first capacity level of the reciprocating compressor and a second capacity level of the reciprocating compressor.

6. The method according to claim 5, wherein the first capacity level is lower than the effective capacity to be delivered by the reciprocating compressor, and the second capacity level is higher than the effective capacity to be delivered by the reciprocating compressor.

7. The method according to claim 1, wherein the first sequence defines dwelling times of each of the capacity levels, the dwelling times being selected in such a manner that the selected effective capacity of the reciprocating compressor is obtained.

8. The method according to claim 1, wherein the step of operating the reciprocating compressor in accordance with the final sequence comprises alternating between the setting combinations of the unloaders at high frequency, thereby obtaining a substantially continuous range of effective capacities being obtainable.

9. The method according to claim 1, wherein the step of operating the reciprocating compressor in accordance with the final sequence comprises operating the unloaders by forcing at least one suction valve to remain open.

10. The method according to claim 1, wherein the step of operating the reciprocating compressor in accordance with the final sequence comprises operating the unloaders by blocking a flow of gas into at least one cylinder.

11. The method according to claim 2, wherein the final sequence is defined in such a manner that when switching from one setting combination of the unloaders to another, only one of the unloaders is switched between a position providing an idle mode of the corresponding cylinder(s) and a position providing an active mode of the corresponding cylinder(s).

12. The method according to claim 2, wherein the first sequence defines a cyclic switching pattern.

13. The method according to claim 3, wherein the first sequence defines a cyclic switching pattern.

14. The method according to claim 2, wherein the first sequence comprises alternating between a first capacity level of the reciprocating compressor and a second capacity level of the reciprocating compressor.

15. The method according to claim 3, wherein the first sequence comprises alternating between a first capacity level of the reciprocating compressor and a second capacity level of the reciprocating compressor.

16. The method according to claim 4, wherein the first sequence comprises alternating between a first capacity level of the reciprocating compressor and a second capacity level of the reciprocating compressor.

17. The method according to claim 2, wherein the first sequence defines dwelling times of each of the capacity levels, the dwelling times being selected in such a manner that the selected effective capacity of the reciprocating compressor is obtained.

18. The method according to claim 3, wherein the first sequence defines dwelling times of each of the capacity levels, the dwelling times being selected in such a manner that the selected effective capacity of the reciprocating compressor is obtained.

19. The method according to claim 4, wherein the first sequence defines dwelling times of each of the capacity levels, the dwelling times being selected in such a manner that the selected effective capacity of the reciprocating compressor is obtained.

20. The method according to claim 5, wherein the first sequence defines dwelling times of each of the capacity levels, the dwelling times being selected in such a manner that the selected effective capacity of the reciprocating compressor is obtained.

21. The method according to claim 1, wherein the defining, by the controller, the final sequence comprises merging the first sequence of capacity levels with the second sequence of unloader settings in such a manner that the first sequence is followed, and each time a particular capacity level of the first sequence of capacity levels is needed, the corresponding second sequence of setting combinations of the unloaders for the particular capacity level is followed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described in further detail with reference to the accompanying drawings in which

(2) FIG. 1 is a configuration of a refrigeration system, on which the method of this invention can be applied, and

(3) FIGS. 2a and 2b illustrate a method by which unloaders may be incorporated into a reciprocating compressor.

DETAILED DESCRIPTION

(4) FIG. 1 shows a common configuration of a refrigeration system, including one or more compressors 1, a heat rejecting heat exchanger 2, a receiver 3, one or more consumers 4 and a control system 5. The one or more consumers 4 can, e.g., be display cases or cold rooms and incorporate one or more evaporators 7 and one or more expansion devices 6. The one or more compressors 1 extract gaseous refrigerant from the one or more consumers 4 and delivers this refrigerant at a higher pressure and temperature to the heat rejecting heat exchanger 2, in which the refrigerant is condensed into liquid. The liquid refrigerant then flows into the receiver 3, from which it re-enters the one or more consumers 4 through the one or more expansion devices 6, after which the refrigerant is evaporated in the one or more evaporators 7. The control system 5 monitors one or more parameters of the system and determines, among other things, the required capacity of each of the one or more compressors 1, in order to maintain optimal values of the one or more parameters. These one or more parameters may include, among others, the refrigerant pressure inside the one or more evaporators 7, a suitable temperature inside the one or more consumers 4 and the air humidity inside the one or more consumers 4.

(5) FIGS. 2a and 2b show two sketches of a cylinder head assembly 10 comprising an unloader 23. The gaseous refrigerant enters the cylinder head 13 through a first bore hole 20 through the compressor housing 11, and through the valve plate 12 that leads to the suction plenum 14. From the suction plenum 14 the refrigerant passes through the suction valve 15 into the cylinder 16 and, after being compressed, it flows out of the cylinder 16 into the discharge plenum 18, through the discharge valve 17. Finally, the refrigerant leaves the cylinder head assembly 10 through a second bore hole 19 through the valve plate 12 and through the compressor housing 11.

(6) The unloader 23 mechanism consists of a plunger 21 and an actuator 22. This plunger 21 can be retracted by the actuator 22, as shown in FIG. 2a, thereby allowing refrigerant to flow into the suction plenum 14. This corresponds to the active mode of the unloader 23. The plunger 21 can also be extended by the actuator 22, as shown in FIG. 2b, such that it blocks the flow of refrigerant into the suction plenum 14, and therefore into the cylinder 16. This corresponds to the idle mode of the unloader 23. Retraction and extension of the plunger 21 by the actuator 22 is typically controlled by an electrical signal from a controller 5.

(7) Returning to the example of table 1, effective capacities between zero and one third of the maximum capacity can, e.g., be obtained by periodically changing the capacity state in the order 0-1-0-2-0-4, and repeating this sequence. In this case, the first sequence of capacity levels could be regarded as {0; ; 0; ; 0; }. The second sequence of setting combinations of the unloaders, or capacity states, corresponding to the zero capacity level could be regarded as {0; 0; 0}. The second sequence of setting combinations of the unloaders, or capacity states, corresponding to the capacity level could be regarded as {1; 2; 4}. The first sequence of capacity levels is merged with the second sequences of setting combinations of the unloaders in such a manner that the first sequence is followed, and each time a zero capacity level is needed, the second sequence corresponding to the zero capacity level is followed, and each time a capacity level is needed, the second sequence corresponding to the capacity level is followed. Thereby the final sequence of setting combinations of the unloaders, {0; 1; 0; 2; 0; 4} is obtained.

(8) Effective capacities between one third and two third of the maximum capacity can, e.g., be obtained by periodically changing the capacity state in the order 1-3-2-6-4-5, and repeating this sequence. In this case, the first sequence of capacity levels could be regarded as {; ; ; ; ; }. The second sequence of setting combinations of the unloaders, or capacity states, corresponding to the capacity level could be regarded as {1; 2; 4}. The second sequence of setting combinations of the unloaders, or capacity states, corresponding to the capacity level could be regarded as {3; 6; 5}. Merging the first sequence of capacity levels with the second sequences of setting combinations of the unloaders, in the manner described above, results in the final sequence of setting combinations of the unloaders, {1; 3; 2; 6; 4; 5}.

(9) Effective capacities between two third of the maximum capacity and maximum capacity can, e.g., be obtained by periodically changing the capacity state in the order 3-7-5-7-6-7, and repeating this sequence. In this case, the first sequence of capacity levels could be regarded as {; 1; ; 1; ; 1}. The second sequence of setting combinations of the unloaders, or capacity states, corresponding to the capacity level could be regarded as {3; 5; 6}. The second sequence of setting combinations of the unloaders, or capacity states, corresponding to the maximum capacity level could be regarded as {7; 7; 7}. Merging the first sequence of capacity levels with the second sequences of setting combinations of the unloaders, in the manner described above, results in the final sequence of setting combinations of the unloaders, {3; 7; 5; 7; 6; 7}.

(10) It is noted that other sequences that yield the same effective capacity ranges are also possible. It is also noted that all final sequences presented in the paragraphs above share the feature that each individual state transition changes the operating mode of only one unloader. Finally, it is noted that, for all final sequences presented in the paragraphs above, the distribution between the idle mode and the active mode is the same across all unloaders, and therefore the same across all cylinders.

(11) By varying the period of time in which a compressor is operated in a particular capacity state, any effective capacity can be obtained. Returning to the example of table 1, eight ninth of the maximum capacity can be obtained by alternating between capacity states corresponding to two third of the maximum capacity and the capacity state corresponding to the maximum capacity, while the contribution to the effective capacity of the capacity states that correspond to two third of the maximum capacity is half of the contribution to the effective capacity of the capacity state that correspond to full capacity. In other words, that the compressor is operated twice as long at its maximum capacity than it is operated at two third of its maximum capacity. An example of such a sequence is illustrated in table 2.

(12) TABLE-US-00002 TABLE 2 Capacity state 3 7 5 7 6 7 Duration in 5 10 7 14 6 12 second Capacity level 2/3 3/3 2/3 3/3 2/3 3/3

(13) It is noted that, in this example, even more evenly distributed load across the cylinders can be achieved when the duration in which the compressor operates in states 3, 5 and 6 are equalized. E.g. when the compressors operate for five second in states 3, 5 and 6, and for three times ten second in state 7.

(14) It is also noted that, in this example, the compressor capacity changes six times during this sequence while each individual unloader, and therefore each individual cylinder, only changes operating mode twice. Since compressor manufacturers often pose minimum limits to the time between changing the operating mode of individual unloaders, this feature implies that the frequency at which the compressor capacity changes can be significantly higher than the maximum frequency at which individual unloaders may be operated. This helps to reduce pressure variations resulting from capacity changes.

(15) Some compressors only allow part of the cylinders to operate both in idle mode as well as in active mode while the remaining cylinders can only be operated in active mode. Table 3 shows an example of a four cylinder compressor with two unloaders, in which each unloader operates on a single cylinder. Such a compressor is capable of operating on half of its maximum capacity, at three quarters of its maximum capacity or at its maximum capacity. When operated at three quarters of it maximum capacity, a controller for such a compressor can achieve evenly distributed load across those cylinders that can be operated in idle mode or in active mode by changing capacity state in the order 1-2, and repeating this sequence. Any effective capacity between half of the maximum capacity and three quarters of the maximum capacity can be achieved by changing the capacity state in the order 0-1-0-2, and repeating this sequence. Any effective capacity between three quarters of the maximum capacity and the maximum capacity can be achieved by changing the capacity state in the order 1-3-2-3, and repeating this sequence. Thus, a substantially continuous range from half of the maximum capacity to the maximum capacity can be achieved.

(16) TABLE-US-00003 TABLE 3 Capacity state 0 1 2 3 Unloader1 Idle Idle Active Active Unloader2 Idle Active Idle Active Capacity level 2/4 3/4 3/4 4/4

(17) While the present disclosure has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this disclosure may be made without departing from the spirit and scope of the present disclosure.