Compressor system for a refrigeration system

Abstract

A compressor system for a refrigeration system, comprising a plurality of compressors (1), one of which has a speed-independent continuous capacity control (6), and the compressor system being configured to meet the refrigeration system's power requirements by controlling the first compressor (1) as the lead compressor and switching the other compressors (1) on and off as secondary compressors, is intended to increase the service life of the system's compressors (1). For this purpose, each of the other compressors (1) has a speed-independent continuous capacity control (6), and the compressor system is further configured to alternatively meet the refrigeration system's power requirements by controlling one of the other compressors (1) as the lead compressor and switching the first and, if necessary, the remaining compressors (1) on and off as secondary compressors.

Description

[0001] The invention relates to a compressor system for a refrigeration system, comprising a plurality of compressors, a first compressor of which has a speed-independent continuous capacity control, wherein the compressor system is configured to meet the refrigeration system's capacity requirements by controlling the first compressor as the lead compressor and switching the other compressors on and off as follow-up compressors. Such a compressor system is known from US 2019 / 063805 A1. This patent further relates to methods for controlling and retrofitting such a compressor system.

[0002] A refrigeration cycle is a system used to cool a device to a desired temperature, such as a food freezer. A refrigerant circulating in this closed loop undergoes several changes of state: The gaseous refrigerant is first compressed by a compressor. In the subsequent heat exchanger, it condenses, releasing heat. The liquid refrigerant then expands due to the pressure change, passing through a throttling device, such as an expansion valve or a capillary tube. In the downstream second heat exchanger (evaporator), the refrigerant evaporates at a low temperature, absorbing heat (evaporative cooling). The cycle can then begin again. This process must be maintained externally by supplying mechanical work (drive power) to the compressor.

[0003] In larger refrigeration systems, such as those used in cold storage facilities or server farms, compressor systems with multiple compressors are typically employed to provide the required capacity. Usually, one compressor is used to cover the base load, and its speed is continuously controlled by a frequency converter. This compressor acts as the lead compressor in the system. The remaining load is typically covered by additional compressors through switching on and off. These secondary compressors operate as follow-up compressors, maintaining the base load.

[0004] A major disadvantage of this solution is the high number of operating hours and the resulting wear and tear on the mechanical components of the lead compressor. This means that the service life of the lead compressor is often many times shorter than that of the subsequent compressors.

[0005] It is therefore an object of the invention to provide a compressor system and method of the type mentioned at the outset which increase the service life of the compressors of the system.

[0006] This problem is solved according to the invention by each of the further compressors having a speed-independent continuous power control, and by the compressor system being further designed to alternatively provide the power requirement of the refrigeration system by controlling one of the further compressors as a lead compressor and switching on and off the first and, if necessary, the remaining further compressors as follow-up compressors.

[0007] The invention is based on the premise that an improvement in service life could be achieved, in particular, by reducing the load on the primary compressor. To this end, it would be desirable to reduce the operating hours of the primary compressor by switching it off periodically. However, this would require a second compressor designed to continuously adjust its output for the necessary power control. Using frequency converters known in the prior art for this purpose is quite complex, as a frequency converter is a relatively intricate component. Therefore, alternative power control methods should be employed, namely speed-independent power control systems, which are technically simpler to implement.This makes it possible to use additional compressors alternately as lead compressors, thus equalizing the operating hours and reducing wear.

[0008] The entire compressor system is designed symmetrically such that each compressor in the system has the aforementioned speed-independent continuous power control. This allows each compressor to be used alternately as the lead compressor, and the symmetry makes it possible to optimally balance the operating hours of each compressor.

[0009] In an advantageous embodiment, continuous power control independent of rotational speed is achieved by controlling the valve opening times, particularly based on pulse-width modulated control of the valve opening times. Such systems can continuously regulate the power using fast-switching solenoid valves without the need for a frequency converter.

[0010] In a further advantageous embodiment of the system, each compressor additionally features mechanically staged capacity control. If the respective compressors are designed as reciprocating compressors, the mechanically staged capacity control is advantageously based on cylinder bank deactivation. Such staged control makes it possible to operate each compressor independently of the continuous control at only 50% capacity (with four cylinders) or 33% and 66% (with six cylinders), with comparatively little technical effort. This makes it possible to avoid power surges when switching on and off other compressors, even with smaller control ranges of the lead compressor (the control factor (CF) is reduced).

[0011] The compressor system is advantageously designed to synchronize compressor operating times by alternately using each of the speed-independent, continuous-capacity compressors as the lead compressor. In other words, each compressor cyclically assumes the role of the lead compressor, thus equalizing operating times. The activation of the follower compressors also occurs in accordance with base load changes; ideally, the compressor with the shortest operating time is always activated as the follower compressor.

[0012] The above-mentioned task is further solved by a method for controlling a compressor system described above, in which the operating times of the compressors are equalized by alternating the use of each of the compressors equipped with a speed-independent continuous power control as a guide compressor.

[0013] In a further advantageous embodiment of the method, the cycle times of the power controllers of the cylinder banks of the respective compressor are synchronized. The described combination of power control with a pulse-width modulated signal and the option of cylinder bank shutdown also makes it possible to apply the pulse-width modulated signal alternately to only individual cylinder banks, i.e., to combine cylinder shutdown with pulse-width modulated control. This makes it possible to synchronize the cycle times, i.e., the times during which the respective cylinder bank is active due to the control pulses, and thus also to compensate for wear between the cylinder banks within a compressor.

[0014] Advantageously, within a specified time interval, the operating hours of each compressor or the cycle times of the power controllers of the cylinder banks are kept constant within a deviation of 20%, preferably 10%. For example, the control objective could be to align the operating hours of all compressors within a weekly cycle to within 10%.

[0015] The problem is further solved by a method for retrofitting a compressor system comprising a plurality of compressors, one of which is a first compressor with continuous capacity control, wherein the compressor system is designed to provide the power requirement of the refrigeration system by controlling the first compressor as the lead compressor and switching the other compressors on and off as follow-up compressors, wherein at least one of the other compressors is equipped with continuous capacity control, the respective continuous capacity control being designed as a speed-independent capacity control, and wherein the compressor system is further designed to alternatively provide the power requirement of the refrigeration system by controlling one of the other compressors as the lead compressor and switching the first and, if necessary, the other compressors on and off as follow-up compressors.

[0016] The advantages achieved with the invention lie particularly in the fact that the design of the compressors in a system with valve-controlled, speed-independent power regulation makes it possible to equalize the operating hours of the compressors, and thus also to ensure uniform stress on all mechanical components. This increases the service life of the compressors. Furthermore, an optimal CF factor is achieved at every operating point, in every season. The uniform operation also has the advantage of reducing switching cycles. The best possible partial load is achieved, regardless of which primary compressor is currently active. An identical design for each compressor is also advantageous with regard to performance testing and maintenance.

[0017] Exemplary embodiments of the invention are explained in more detail with reference to the drawings. These show the FIGeinen Kältekreislauf mit einem Kompressorssystem mit Mehr Kompressoren.

[0018] Figure 1 schematically shows a refrigeration cycle K. The refrigeration cycle K is described below starting with a compressor system consisting, for example, of three compressors 1 connected in parallel. The refrigerant compressed in the compressors 1 is fed into a condenser 2, where it is cooled and liquefied. From there, it flows via a refrigerant receiver 3 and an injection valve 4 into an evaporator 5. Here, the refrigerant expands and absorbs heat, thus achieving the desired cooling effect. From the evaporator 5, the now gaseous refrigerant flows back into the compressor 1, is compressed there, and the cycle begins again.

[0019] The refrigeration circuit K is part of a refrigeration system (not shown in detail), for example, in a server farm or in cold storage facilities for food. It comprises a variety of other components, the description of which is not relevant to the function of the invention presented here and which are therefore omitted, such as oil separators, safety valves, etc. The refrigeration circuit K can also be even more complex, for example, with multiple pressure stages.

[0020] Depending on the cooling system's capacity requirements, it is necessary to regulate the output of the parallel compressors 1 in the compressor system. This is achieved by continuously controlling one compressor 1 as the master compressor, for example, within a capacity range of 10% to 100% of its maximum capacity. If the cooling system demands a higher capacity, for example, due to increased heat input into the room to be cooled, or during summer operation, additional compressors 1 are activated as follow-up compressors. However, these follow-up compressors 1 operate at fixed capacity levels, so that fine-tuning of the capacity remains the responsibility of the master compressor.

[0021] To equalize the operating hours and thus the wear of the compressors 1 and to increase their service life, the compressor system in the exemplary embodiment is designed symmetrically, with each compressor 1 equipped with two power controllers 6 controlled by a central control unit 7. In the exemplary embodiment, the compressors 1 are designed as four-cylinder compressors, and each power controller 6 in each compressor 1 acts on one of the two cylinder banks per compressor 1 and is a stepless, speed-independent power controller. For this purpose, corresponding solenoid valves are provided in the cylinder head, which are controlled by a pulse-width modulated signal, thus enabling stepless power control.

[0022] The power controllers 6 thus combined the function of a mechanical power control, which allows the power of each compressor 1 to be regulated in stages by means of cylinder bank deactivation (e.g., with a four-cylinder compressor at 50% and 100% power), with a stepless, speed-independent power control. According to the exemplary embodiment, this combination allows, for example, the pulse-width modulated signal to act on only one cylinder bank. For instance, for power control to 10%, a cycle time of 6 seconds per minute could be provided for both cylinder banks, or a cycle time of 12 seconds per minute for only one cylinder bank. For cylinder deactivation when used as a follow-up compressor, no pulse-width modulated control is used, and the cylinder banks are simply switched completely on or off.

[0023] Compressor 1, acting as the lead compressor, is – as shown above – practically in continuous operation, while the remaining compressors 1 are only activated as needed. The compressor system shown in the FIG nevertheless allows the operating times of the compressors 1 to be synchronized, since the identical design of each compressor 1 enables each compressor 1 to be used both as a lead compressor and as a follow-up compressor.

[0024] For this purpose, a corresponding control algorithm is provided in the controller 7, which independently determines, based on various criteria, which compressor 1 assumes the role of the lead compressor and which compressor(s) 1 are switched on as follow-up compressors. This can occur, for example, cyclically, whereby the follow-up compressors can also be exchanged cyclically, or based on the actual recording of operating times. The aim of the control is always to align the operating times of the compressors 1, i.e., those compressors 1 with the fewest operating hours are generally put into operation. In further embodiments, additional compressors 1 could also be operated as lead compressors and continuously controlled, up to and including complete continuous control of all compressors 1. In this case, however, sufficient stability of the control system would have to be ensured.

[0025] The additional mechanical power controllers 6 make it possible to optimize the CF factor (Control Factor), i.e., the ratio of the adjustable power range of the continuously controlled compressor 1 and the power of the next switchable compressor 1, since another compressor 1 can initially be switched on with only partial power.

[0026] The described design of the compressor system makes it possible to align the operating hours of compressor 1 and increase their service life, while simultaneously ensuring high control accuracy across the entire capacity range. This is particularly advantageous for transcritical refrigeration systems, which exhibit a significant difference in power demand between winter and summer operation. Reference symbol list

[0027] 1 Compressor 2 Condenser 3 Refrigerant receiver 4 Injection valve 5 Evaporator 6 Capacity regulator 7 Control unit

Claims

1. Compressor system for a refrigeration system, comprising a plurality of compressors (1), of which a first compressor (1) has a speed-independent continuous power control (6), wherein the compressor system is designed to provide a power requirement of the refrigeration system by controlling the first compressor (1) as the lead compressor and switching on and off the further compressors (1) as follow-up compressors, characterized by the fact that Each of the additional compressors (1) has a speed-independent continuous power control (6), and the compressor system is further designed to alternatively provide the power requirement of the refrigeration system by controlling one of the additional compressors (1) as a lead compressor and switching on and off the first and, if necessary, the remaining additional compressors (1) as follow-up compressors.

2. Compressor system according to the preceding claim, wherein the respective speed-independent continuous power control (6) is achieved by controlling the valve opening times.

3. Compressor system according to the preceding claim, wherein the respective speed-independent continuous power control (6) is based on pulse width modulated control of the valve opening times.

4. Compressor system according to one of the preceding claims, wherein each compressor (1) additionally has a mechanical stepped power control (6).

5. Compressor system according to the preceding claim, wherein the respective compressors (1) are designed as reciprocating compressors, and the mechanical staged power control (6) is based on cylinder bank shutdown.

6. Compressor system according to one of the preceding claims, which is designed to align the operating times of the compressors (1) by alternating the use of each of the compressors (1) equipped with a speed-independent continuous power control (6) as a lead compressor.

7. Method for controlling a compressor system according to one of the preceding claims, wherein the operating times of the compressors (1) are equalized by alternating use of each of the compressors (1) equipped with a speed-independent continuous power control (6) as a guide compressor.

8. Method for controlling a compressor system according to the preceding method claim, wherein the cycle times of the power controllers (6) of the cylinder banks of the respective compressor (1) are aligned.

9. Method according to one of the preceding method claims, wherein within a predetermined time interval the operating hours of each compressor (1) and / or the cycle times of the power controllers (6) of the cylinder banks of the respective compressor (1) are kept equal within a deviation of 20%, preferably 10%.

10. Method for retrofitting a compressor system comprising a plurality of compressors (1), one of which a first compressor (1) has a speed-independent continuous capacity control (6), wherein the compressor system is configured to provide a power requirement of the refrigeration system by controlling the first compressor (1) as a lead compressor and switching on and off the further compressors (1) as follow-up compressors, wherein each of the further compressors (1) is equipped with a speed-independent continuous capacity control (6), and wherein the compressor system is further configured to alternatively provide the power requirement of the refrigeration system by controlling one of the further compressors (1) as a lead compressor and switching on and off the first and, if applicable, the remaining further compressors (1) as follow-up compressors.

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