COMPRESSOR SYSTEM FOR A REFRIGERATION UNIT

Description

The invention relates to a compressor system for a refrigeration unit, comprising a plurality of compressors, of which a first compressor has a rotation-speed-independent continuous capacity control, wherein the compressor system is adapted to provide a capacity requirement of the refrigeration unit by controlling the first compressor as lead compressor and switching on and off the other compressors as slave compressors. Such a compressor system is known from US 2019/063805A1 . It also relates to methods for controlling and retrofitting such a compressor system.

A refrigeration circuit is a system that is used to cool a device to a desired temperature, for example a freezer for food. A refrigerant that is moved in the closed circuit undergoes various changes of aggregate state one after the other: The gaseous refrigerant is first compressed by a compressor. It then condenses in the subsequent heat exchanger, releasing heat. The liquid refrigerant is then expanded due to the change in pressure via a throttle element, for example an expansion valve or a capillary tube. In the downstream second heat exchanger (evaporator), the refrigerant evaporates while absorbing heat at a low temperature (boiling cooling). The cycle can now start again. The process must be kept going from outside by supplying mechanical work (drive power) via the compressor.

In larger refrigeration units, e.g. for cold stores or server farms, compressor systems with several compressors are typically used in order to provide the required capacity. Normally, one compressor is used to cover the base load, which is usually regulated continuously by means of speed control via a frequency converter. This acts as the lead compressor in the compressor system. The remaining load is usually covered by other compressors by switching them on and off. These compressors operate as slave compressors in the base load principle.

A major disadvantage of this solution is the high number of operating hours and the resulting wear on the mechanical components of the lead compressor. As a result, the service life of the lead compressor is often many times shorter than that of the slave compressor.

It is therefore the object of the invention to provide a compressor system and method of the type mentioned above which increase the service life of the compressors in the system.

This object is solved in accordance with the invention in that each compressor has a rotation-speed-independent continuous capacity control on the basis of a pulse-width-modulated control of the valve opening times, and in that the compressor system is also adapted to alternatively provide the capacity requirement of the refrigeration unit by controlling one of the further compressors as the lead compressor and switching on and off the first and if applicable the remaining further compressors as slave compressors.

The invention is based on the consideration that an improvement in service life could be achieved in particular by reducing the load on the lead compressor. To this end, it would be desirable to reduce the operating hours of the lead compressor by switching it off temporarily. To do this, however, a second compressor would have to be adapted in such a way that it would then be able to continuously adjust its output for the required capacity control. However, using frequency converters known in the state of the art for this purpose involves a great deal of effort, as a frequency converter is a comparatively complex component. Therefore, other power control systems should be used, namely rotation-speed-independent power control systems, which are technically easier to implement. This makes it possible to use additional compressors as lead compressors on an alternating basis, thus equalizing the operating hours and reducing wear.

The entire compressor system is adapted symmetrically in such a way that each compressor in the system has the above-mentioned rotation-speed-independent continuous capacity control. This means that each compressor can be used alternately as a lead compressor and the symmetry makes it possible to optimally adjust the operating hours of each compressor.

Further, rotation-speed-independent continuous power control is achieved by controlling the valve opening times, in particular on the basis of pulse-width modulated control of the valve opening times. Such systems can control the power continuously by means of fast-switching solenoid valves without the use of a frequency converter.

In an advantageous design of the system, each compressor also has a mechanical stepped capacity control. In case the respective compressors are adapted as reciprocating piston compressors, the mechanical stepped capacity control is advantageously based on a cylinder bank cut-off. This type of stepped control makes it possible to operate the respective compressor at only 50% capacity (with four cylinders) or 33% and 66% (with six cylinders) independently of the continuous control, with comparatively little technical effort. This makes it possible to avoid capacity jumps when switching other compressors on and off, even with lower control ranges of the lead compressor (the control factor (CF) is reduced).

In particular, the compressor system is advantageously adapted to equalize the operating times of the compressors by alternating use of each of the compressors provided with a rotation-speed-independent continuous capacity control as a lead compressor. In other words, each compressor cyclically takes over the task of the lead compressor so that the operating times are equalized. In this case, the slave compressors are also used in the base load change when they are switched on, i.e. ideally the compressor that currently has the lowest operating time is always switched on as the slave compressor.

The above object is further solved by a method for controlling a compressor system described above, in which the operating times of the compressors are equalized by alternately using each of the compressors provided with a rotation-speed-independent continuous capacity control as a lead compressor.

In a further advantageous embodiment of the method, the cycle numbers of the power controllers of the cylinder banks of the respective compressor are equalized. The described combination of controlling the power control with a pulse-width modulated signal and the possibility of cylinder bank deactivation also makes it possible to allow the pulse-width modulated signal to act alternately only on individual cylinder banks, i.e. practically to combine the cylinder deactivation with the pulse-width modulated control. This also makes it possible to synchronize the cycle times, i.e. the times during which the respective cylinder bank is active due to the pulses of the activation, and thus also to compensate for the wear between the cylinder banks within a compressor.

Advantageously, the operating hours of each compressor and/or the cycle times of the capacity controllers of the cylinder banks are kept the same within a specified time interval within a deviation of 20%, preferably 10%. For example, the control target could be to equalize the operating hours of all compressors to within 10% of each other within a weekly cycle.

The problem is further solved by a method for retrofitting a compressor system comprising a plurality of compressors, of which a first compressor has a continuous capacity control, the compressor system being adapted to provide a capacity requirement of the refrigeration unit by controlling the first compressor as lead compressor and switching on and off the further compressors as slave compressors at least one of the further compressors being equipped with a continuous capacity control, wherein the respective continuous capacity control is adapted as rotation-speed-independent capacity control on the basis of a pulse-width-modulated control of the valve opening times, and in that the compressor system is further adapted to alternatively provide the capacity requirement of the refrigeration unit by controlling one of the further compressors as lead compressor and switching on and off the first and, if applicable, the remaining further compressors as slave compressors.

The advantages achieved with the invention lie in particular in the fact that the design of the compressors of a system with a valve-controlled, rotation-speed-independent capacity control makes it possible to achieve an equalization of the operating hours of the compressor and thus also a uniform load on all mechanical components. This increases the service life of the compressor. In addition, an optimum CF factor is achieved at any operating point, at any time of year. The even operation also has the advantage that switching cycles are reduced. The best possible partial load is achieved regardless of which lead compressor is currently active. The identical design of each compressor is also advantageous in terms of performance checks and maintenance.

Examples of embodiments of the invention are explained in more detail with reference to drawings. Therein the

FIGURE shows a refrigeration circuit with a compressor system with several compressors.

The FIGURE shows a schematic diagram of a refrigeration circuit K. The refrigeration circuit K is described below on the basis of a compressor system consisting of three compressors 1 connected in parallel, for example. The refrigerant compressed in the compressors 1 is fed into a condenser 2, in which the compressed refrigerant is cooled and liquefied. From there, it flows via a refrigerant collector 3 and an injection valve 4 into an evaporator 5, where the refrigerant is expanded and absorbs heat so that the desired cooling effect is achieved. From the evaporator 5, the now gaseous refrigerant flows back into the compressor 1, where it is compressed and the cycle begins again.

The refrigeration circuit K is part of a refrigeration unit not shown in detail, for example in a server farm or in cold stores for foodstuffs. It comprises a large number of other components whose description is not relevant to the function of the invention shown 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 several pressure stages.

Depending on the capacity requirement of the refrigeration unit, it is necessary to regulate the capacity of the parallel compressors 1 in the compressor system. This is achieved by continuously regulating one compressor 1 as the lead compressor, e.g. in a capacity range from 10% to 100% of its maximum capacity. If a higher capacity is required by the refrigeration unit, e.g. with a higher heat input into the room to be cooled, or in summer operation, further compressors 1 are added as slave compressors, whereby these compressors 1 only have fixed capacity levels, so that the fine control of the capacity is still performed by the lead compressor.

In order to equalize the operating hours and thus the wear of the compressors 1 and to increase the service life, the compressor system in the design example is symmetrical in that each compressor 1 is provided with two capacity regulators 6, which are controlled from a central control unit 7. In the design example, the compressors 1 are adapted as four-cylinder compressors and each of the capacity regulators 6 in each compressor 1 acts on one of the two cylinder banks per compressor 1 and is a stepless, rotation-speed-independent capacity regulator. For this purpose, corresponding solenoid valves are provided in the cylinder head, which are controlled by means of a pulse-width modulated signal and thus enable stepless capacity control.

The power controllers 6 thus combine the function of a mechanical power control, with which the power of the respective compressor 1 can be controlled in stages by means of cylinder bank deactivation, in the embodiment example with four-cylinder compressors e.g. with 50% and 100% power, with a stepless, rotation-speed-independent power control. In this way, the combination according to the embodiment example makes it possible, for example, to have the pulse-width modulated signal only act on one cylinder bank. For example, a cycle time of 6 seconds per minute could be provided for both cylinder banks for power control to 10%, or a cycle time of 12 seconds per minute for only one cylinder bank. For cylinder deactivation when used as a progressive compressor, there is then no pulse width modulated control and the cylinder banks are simply switched off or on completely.

As shown above, the compressor 1 acting as the lead compressor is practically permanently in operation, while the other compressors 1 are only switched on as required. The compressor system shown in the FIGURE nevertheless allows the operating times of the compressors 1 to be matched to one another, as the identical design of each compressor 1 means that each compressor 1 can be used both as a lead compressor and as a slave compressor.

For this purpose, a corresponding control algorithm is provided in the control unit 7, which independently determines which compressor 1 takes on the role of the lead compressor and which compressor or compressors 1 are switched on as slave compressors based on various criteria. This can be done cyclically, for example, whereby the slave compressors can also be exchanged cyclically, or on the basis of the actual recording of operating times. The aim of the control is always to match the operating times of the compressors 1 to each other, i.e. those compressors 1 that have the fewest operating hours tend to be put into operation. In further embodiments, additional compressors 1 could also be operated as lead compressors and controlled continuously, up to a complete continuous control of all compressors 1. However, care would have to be taken here to ensure appropriate stability of the control.

The additional mechanical capacity controllers 6 also make it possible to optimize the CF factor (control factor), i.e. the ratio of the adjustable capacity range of the continuously controlled compressor 1 and the capacity of the next compressor 1 that can be switched on, since another compressor 1 can initially be switched on with only partial capacity.

The design of the compressor system described above makes it possible to match the operating hours of the compressors 1 to each other and increase the service life, while at the same time ensuring a high level of control quality across the entire capacity range. This is particularly advantageous for transcritical refrigeration units, which have a greater difference in terms of power requirements in winter and summer operation.

LIST OF REFERENCE NUMERALS

• • 1 Compressor
  • 2 Condenser
  • 3 Refrigerant collector
  • 4 Injection valve
  • 5 Vaporizers
  • 6 Power regulator
  • 7 Control system