Variable fan speed control in HVAC systems and processes

Abstract

A method for controlling one or more condenser fans (8) in an HVAC system, comprising: sensing, with a first device (202), the ambient air temperature; determining, with a second device (204), an instantaneous load on a variable-load compressor (1), wherein the variable-load compressor (1) is a variable-speed compressor (1); determining, with a control unit (206), a desired condenser fan capacity suitable for controlling at least one condenser fan (8) based on the sensed ambient air temperature and the determined instantaneous load on the variable-load compressor (1); and outputting, with the control unit (206), the desired condenser fan capacity suitable for controlling the at least one condenser fan (8).and controlling the at least one condenser fan (8) based on the output of the desired condenser fan capacity, so that the performance of the HVAC system is controlled via the power consumed by the variable load compressor (1) and the at least one condenser fan (8).

Description

Area

[0001] The following disclosure relates to heating, ventilation, and air conditioning (HVAC) systems and methods, and in particular to a variable fan speed control used in such HVAC systems and methods. Generally, systems and methods are described that are designed to control the fan speed in order to control the power consumption, for example, by the HVAC compressor(s) and the HVAC fan(s). The control is based on various operating conditions of the compressor load and the ambient air temperature. background

[0002] Condenser fans are used in HVAC units and systems, such as in water chillers, to extract heat from a refrigerant flowing through the system. This heat can be extracted from the refrigerant and transferred to another fluid, such as air or water, or to the surrounding environment. Condenser fan control can be used to minimize the final pressure on the compressor(s), for example, by maximizing the number of running fans in units / systems that use fixed-speed fans. Such control can maximize the efficiency of a chiller, for example, at full load. Summary

[0003] Under many operating conditions, a chiller cannot be operated at full load. For chillers with one or more variable-speed fans and / or fans with multiple or discrete stages, and with one or more variable-speed compressors, a method is needed to determine the optimal fan speed based on the chiller's operating conditions. This allows for improvements to the fan control in HVAC units or systems. Generally, systems and methods for variable fan speed control are described. Performance optimization, for example in a chiller, can be achieved using the following methods / systems under various idle conditions, i.e., conditions not at full load or at partial load.The following systems and procedures are based on the observation that, for example, reducing the fan speed can save energy under certain conditions in relation to the costs associated with a corresponding increase in the final pressure on the compressor.

[0004] In general, the control can be functionally based on an algorithm implemented in software and a control program, suitable for controlling the fan(s) of a unit, such as a refrigeration unit or a heat pump operating in cooling mode. For example, such a unit could be an air-cooled water chiller with at least one variable-speed compressor, and at least one variable-speed fan, such as those used in HVAC systems, could employ the control methods and systems described below.

[0005] The control system is based on various operating conditions of the compressor load and the ambient air temperature.

[0006] In one embodiment, a method for controlling the condenser fan speed in an HVAC system involves sensing the ambient air temperature with a sensor and then determining the instantaneous load on a variable-speed compressor. A control unit can determine a target differential pressure between a condenser and an evaporator, based on the sensed ambient air temperature and the determined instantaneous load on the variable-speed compressor. The control unit can then output a condenser fan speed to determine a fan capacity suitable for achieving the specified target differential pressure, thus controlling the performance of the HVAC system via the relative power consumed by the variable-speed compressor and the fans.At least one variable-speed fan and / or a fan with multiple fan stages or discrete stages can be based on the output of the condenser fan capacity, for example, based on a fan speed.

[0007] In one embodiment, an HVAC system may include a unit, such as an air-cooled water chiller. The system includes at least one variable-speed fan and / or fans with a plurality of fan stages or discrete stages, a variable-speed compressor, a condenser, an evaporator, an ambient air temperature sensing device, and a device for determining instantaneous loads on the variable-speed compressor. The system includes a control unit to determine a target differential pressure between the condenser and the evaporator based on the sensing ambient air temperature and the determined instantaneous load on the variable-speed compressor. The control unit may also determine a condenser fan speed output to achieve a fan capacity suitable for reaching the determined target differential pressure.The control unit can operate at least one fan based on the output of a capacitor velocity to achieve a fan capacity, so that the performance of the HVAC system is controlled via a relative power input from the variable compressor and the fans.

[0008] It should be noted that the use of a target differential pressure is merely an example of a control parameter derived from a compressor load and is not intended to be restrictive. It should also be noted that the compressor load input(s), such as the compressor speed, and the ambient temperature input could be used to directly output the fan speed.

[0009] Other properties and aspects of the systems, processes and control concepts become apparent by considering the following detailed description and accompanying drawings. Brief description of the drawings

[0010] Reference is now made to the drawings in which the same reference symbols consistently represent corresponding parts.

[0011] Fig. Figure 1 is a schematic view of an HVAC system that can use variable fan speed control.

[0012] Fig. Figure 2 is a block diagram of a system with variable fan speed control.

[0013] Fig. Figure 3 is a flowchart of a procedure for variable fan speed control.

[0014] Fig. Figure 4 shows an exemplary blower control map using differential pressure setpoints based on compressor speed and ambient temperature. Detailed description

[0015] Improvements can be made to the fan control in HVAC units and / or systems. Generally, methods and systems are described for controlling the fan speed of an HVAC unit and / or system that has variable speed and / or fans with multiple fan stages or discrete stages and one or more variable-speed compressors. Variable fan speed control is based on a compressor load, such as compressor speed, and the ambient temperature.

[0016] Regarding the basic structure of an HVAC system, it shows Fig. 1 A schematic view of a refrigeration unit in an HVAC system, for example to control a fan or blower with variable speed. Fig. Figure 1 shows an embodiment of an air-cooled refrigeration machine which includes a compressor 1 , an evaporator 3 , a capacitor 4with air current 5 and blower 8 and a control unit 2 and control panel 7 It is worth noting that the compressor 1 a variable speed compressor and the blowers 8 They can have variable speed and / or be blowers with multiple or discrete fan speeds. The capacitor 4 and its air current 5 The embodiment shown is an example of an air-cooled condenser, although it should be noted that the specific condenser shown 4 The 5-turn combination is purely an example. The chiller can be considered a single unit within the HVAC system and can, for example, be enclosed by a frame. 6 It should be acknowledged that the special configuration, which is in Fig. Figure 1 is purely exemplary, as other designs, structures, and specific configurations of refrigeration machines can be used. For example, the refrigeration machine in Fig. 1. A known refrigeration machine with "W"-shaped windings; however, it should be noted that other winding types can be used, for example, a multitude of "V"-shaped windings or more than one circuit, employing a multitude of compressors, evaporators, and condensers. In general, the methods and systems for controlling the fan speed can be used in any type of air-cooled refrigeration unit with variable speed and / or with fans having a multitude of fan stages or discrete stages and one or more variable-speed compressors.

[0017] Fig. Figure 2 is a block diagram of a variable fan speed control. 200, which includes devices for sensing, determining, or otherwise ascertaining the necessary inputs for a control unit in order to determine the appropriate output to control the fan speed. In one embodiment, a device 202 used to measure ambient air temperature. The device 202 It can be any suitable sensor that can measure the ambient air temperature, for example, outside temperature or outdoor temperature, and transmit the ambient temperature measurement to the control unit. 206 It can communicate. It is commendable that the device 202 a sensor located near the air-side windings (e.g. 5 in Fig. 1) is attached. A device 204 It is used to determine the instantaneous load on the compressor of the circuit. It is worth noting that the device 204Any suitable sensor, detector, or measuring device capable of determining the instantaneous load on the compressor, some examples of which are given below, may be used. In some embodiments, the instantaneous load is based on a compressor running speed, which in some examples may be expressed as a percentage of a rated speed for a given compressor frame size, e.g., relative to the configured limit of the compressor's full speed. The device 204 For example, it should transmit the percentage of the compressor's rated speed to the control unit. 206 communicate. The devices 202 , 204 The measurements taken can be updated periodically as needed and / or desired, for example automatically, during / after a change in operation or altered conditions of the unit, and / or manually. It is worth noting that the control unit 206can use the latest available sensor data unless otherwise specified.

[0018] The control unit 206 can be found, for example, in the control unit 2 and control panel 7 , such as in Fig. 1. It is worth noting that it is known that the control unit 2 , as in the air-cooled refrigeration unit in Fig. 1 shown, generally includes a processor (not shown), memory (not shown) and optionally a clock (not shown) and an input / output (I / O) interface (not shown), and the control unit 2 It can be configured as input data from various components within an HVAC system, such as those in Fig. 1 and Fig. The two components shown can receive and also send command signals as output to various components within the system.

[0019] The control unit206 uses the measured ambient temperature and the determined compressor load to calculate a target differential pressure ∆P diff between the evaporator and condenser (e.g., evaporator) 3 and capacitor 4 in Fig. 1) to determine. The target differential pressure can be a function of the ambient temperature and compressor load, which can be used in an algorithm implemented in a control program to vary the fan speed as required and / or desired for the appropriate fan capacity, e.g., by controlling the fan(s) with variable speed. 208 .

[0020] It is worth noting that the target differential pressure can be added to the compressor's suction pressure to obtain a target compressor outlet pressure. Since the fans can vary their speed to achieve a target compressor outlet pressure, the unit's capacity, e.g., the chiller, can also change, resulting in a new compressor speed. This new compressor speed can then, in turn, alter the target differential pressure output by the compressor. Controlling the chilled water temperature then brings the chiller to the appropriate chiller capacity.

[0021] The fan control can determine the optimal fan speed to minimize the overall power consumption for a given unit capacity, for example, by using the compressor load and ambient temperature to determine the target differential pressure. This target differential pressure can then be used to determine the appropriate fan capacity, for example, based on the resulting fan speed, which minimizes the overall power consumption for that unit capacity, such as the relative power drawn by the compressor and the fan.

[0022] It is worth noting that the compressor speed in rpm is controlled by the control unit, e.g. 206The output parameter can be issued as a command, taking into account, for example, the control circuit for the water temperature of a water chiller, and the control unit may or would be known. In the methods and systems described here, the differential pressure can be the output parameter of the control unit, and the compressor load (e.g., speed in rpm or relative value of rpm) can be the input parameter used to determine the output parameter.

[0023] It should be noted that the use of a target differential pressure is merely one example of a control parameter determined from a compressor load and is not intended to be restrictive. It should also be noted that the compressor load input(s), such as the compressor speed, and the ambient temperature input could be used to directly output a fan speed, rather than first using them to determine the target differential pressure.

[0024] For example, in other situations where a target differential pressure is not used as the control parameter, it would be advantageous to use a conventional velocity sensor to determine the compressor load, e.g., the compressor speed, and to use a lookup table containing corrected compressor speeds and ambient temperatures. It is also advantageous to consider, depending on the compressor unloading method, the use of a mechanical unloading position sensor or sensors to determine the compressor load, e.g., the compressor speed. In other embodiments, the compressor load, e.g., compressor speed, could also be estimated indirectly, for example, by a change in the evaporator water temperature, determined by the control unit using, for example, one or more temperature sensors.

[0025] In some embodiments, the control unit 206 Use a high-pressure avoidance control to regulate the fan capacity in multiple stages. For example, as the condensation temperature approaches the condenser pressure limit, fan capacity can be added in discrete, fixed-speed fan stages in systems that use a variable-speed fan and / or a fan with multiple fan stages or discrete stages.

[0026] Fig. 3 is a flowchart of an embodiment of a process 300 a variable fan speed control. The procedure 300 for controlling variable-speed condenser fans in an HVAC system, features detection with a sensor that measures ambient air temperature 302and then determining an instantaneous load on a variable-speed compressor 304 A control unit can be used to determine a target differential pressure between a condenser and an evaporator. 306 , which can be based on the measured ambient air temperature and the determined instantaneous load on the variable-speed compressor. The control unit can output a condenser fan speed suitable for achieving a given fan capacity and the specified target differential pressure. 308 At least one variable-speed blower can be controlled. 310, based on the output of the condenser fan speed to achieve the fan capacity. It should be noted that the fans can be fans with multiple fan speeds or discrete stages, or a combination of one or more variable-speed fans and one or more fans with multiple fan speeds or discrete stages.

[0027] When using a system with such a control or procedure, the differential pressure target can be varied based on a function of the ambient temperature and compressor speed to optimize the fan speed, which can save the system power relative to potential costs due to increases in compressor discharge pressure. Historically, the differential pressure target was set to a fixed PSI value for a given, or maintained, ambient temperature, so the differential pressure target would not change even if the ambient temperature was lower than the given ambient temperature.Under partial and full load operating conditions, the controller can optimize the fan speed to achieve efficiencies in the circuit, allowing for a balance between condenser fan output (which can increase to keep the system differential pressure relatively low) and compressor output (which can increase as the system differential pressure rises).

[0028] Fig.Figure 4 shows an example blower control map using differential pressure setpoints based on compressor speed and ambient temperature. As shown, the delta pressure setpoint can increase as a function of ambient temperature, as well as for different speeds; see the graph curves, for example, for speeds 1 to 4. In some examples, the delta pressure setpoint can be as low as, for example, 35 PSID (Pounds per Square Inch Differential) and as high as approximately 190 PSID. It is worth noting that the delta pressure setpoint can vary depending on the general operating / load conditions of the unit and can also vary depending on the oil flow requirement through the unit / system. Part-load efficiency of the circuit can be optimized by adjusting the delta pressure for different operating conditions of varying ambient temperature and compressor speed.

[0029] With regard to the following aspects, it should be noted that each of aspects 1–6 can be combined with each of aspects 7–21, each of aspects 7–14 can be combined with each of aspects 15–21, and each of aspects 15–17 can be combined with each of aspects 18–21. Aspect 1. A method for controlling condenser fans in an HVAC system, comprising: Detect, with a device for detecting the ambient temperature, the ambient temperature; Determine, using a device for determining instantaneous compressor loads, an instantaneous load on a variable-load compressor; Determine, with a control unit, a desired condenser fan capacity that is suitable to control at least one condenser fan based on the detected ambient air temperature and the determined instantaneous load on the variable load compressor; Output, with the control unit, the desired condenser fan capacity, which is suitable to control at least one condenser fan; and Controlling the at least one fan based on the output of the desired condenser fan capacity, so that the performance of the HVAC system is controlled via the power consumed by the variable load compressor and the at least one condenser fan. Aspect 2. The procedure according to Aspect 1, wherein the step of determining the instantaneous load includes determining the instantaneous load on a variable-speed compressor. Aspect 3. The method according to Aspect 1 or 2, wherein the step of determining the instantaneous load comprises determining the instantaneous load by the instantaneous compressor load determination device which includes a velocity sensor. Aspect 4. The method according to any of aspects 1 to 3, wherein the step of determining the instantaneous load comprises determining the instantaneous load by the device for determining instantaneous compressor loads which has a mechanical relief position sensor. Aspect 5. The method according to any of aspects 1 to 4, wherein the step of determining the instantaneous load comprises determining the instantaneous load by estimating a change in the evaporator water temperature, as determined by the instantaneous compressor load determination device which includes a temperature sensor. Aspect 6. The method according to any of aspects 1 to 5, wherein the step of determining a desired condenser fan capacity includes determining the speed of the at least one condenser fan, and the step of outputting the desired condenser fan capacity includes outputting the speed of the at least one condenser fan. Aspect 7. An HVAC system, including: a variable load compressor; an evaporator that is fluidically connected to the variable load compressor; a capacitor that is fluid-mechanically connected to the variable load compressor, wherein the capacitor has at least one capacitor fan; a device for measuring the ambient air temperature; a device for determining instantaneous loads on a variable-load compressor; and a control unit that is operatively connected to the device for detecting the ambient air temperature, to the device for determining instantaneous loads on the variable load compressor and to the condenser which has at least one condenser fan, wherein the control unit is configured to determine a desired condenser fan capacity suitable for controlling at least one condenser fan based on the detected ambient air temperature and the determined instantaneous load on the variable load compressor, wherein the control unit is configured to operate the at least one condenser fan based on the desired condenser fan capacity, so that the performance of the HVAC system is controlled via the power consumed by the variable load compressor and the at least one condenser fan. Aspect 8. The system according to aspect 7, where the variable load compressor is a variable speed compressor. Aspect 9. The system according to aspect 7 or 8, wherein the device for determining instantaneous compressor loads includes a speed sensor. Aspect 10. The system according to one of aspects 7 to 9, wherein the device for determining instantaneous compressor loads has a mechanical relief position sensor. Aspect 11. The system according to any of aspects 7 to 10, wherein the device for determining instantaneous compressor loads has at least one temperature sensor configured to determine the instantaneous load by estimating a change in the evaporator water temperature. Aspect 12. The system according to one of the aspects of claims 7 to 11, wherein the control unit is configured to determine the speed of the at least one condenser fan when determining the desired condenser fan capacity, and is configured to output the speed of the at least one condenser fan in order to operate the at least one condenser fan based on the desired condenser fan capacity. Aspect 13. The system according to one of aspects 7 to 12, wherein the control unit has a high-pressure avoidance control of the at least one blower under the condition that the condensation temperature approaches the condenser pressure limit. Aspect 14. The system according to one of aspects 7 to 13, wherein the system is an air-cooled water refrigeration machine with at least one variable-speed compressor and at least one variable-speed fan. Aspect 15. A method for controlling condenser fans in an HVAC system, comprising: Detect, with a device for detecting ambient air temperatures, the ambient air temperature; Determine, using a device for determining instantaneous compressor loads, an instantaneous load on a variable-speed compressor; Determine, using a control unit, a target differential pressure between a condenser and an evaporator based on the detected ambient air temperature and the determined instantaneous load on a variable speed compressor; Output, with the control unit, a condenser fan speed suitable for achieving the specified target differential pressure; and Controlling at least one fan based on the output of the condenser fan speed to determine a fan capacity suitable to control at least one condenser fan, so that the performance of the HVAC system is controlled via the power consumed by the variable speed compressor and the fans. Aspect 16. The method according to Aspect 15, wherein the step of determining the instantaneous load comprises determining the instantaneous load by the device for determining instantaneous compressor loads which has a mechanical unloading position sensor. Aspect 17. The method according to Aspect 15 or 16, wherein the step of determining the instantaneous load comprises determining the instantaneous load by estimating a change in the evaporator water temperature, as determined by the instantaneous compressor load determination device which includes a temperature sensor. Aspect 18. An HVAC system, including: a variable speed compressor; an evaporator that is fluidically connected to the variable-speed compressor; a capacitor that is fluid-mechanically connected to the variable-speed compressor, wherein the capacitor has at least one capacitor fan; a device for measuring the ambient air temperature; a device for determining instantaneous loads on the variable-speed compressor; and a control unit that is operatively connected to the device for detecting the ambient air temperature, to the device for determining instantaneous loads on the variable-speed compressor, and to the condenser, which has at least one condenser fan, wherein the control unit is designed to determine a target differential pressure between the condenser and the evaporator based on the detected ambient air temperature and the determined instantaneous load on the variable-speed compressor, wherein the control unit is configured to determine an output of the condenser blower capacity suitable for achieving the specified target differential pressure, and The control unit is designed to operate at least one condenser fan based on the output of the condenser fan capacity, so that the performance of the HVAC system is controlled via the power consumed by the variable speed compressor and the at least one condenser fan. Aspect 19. The system according to aspect 18, wherein the device for determining instantaneous compressor loads has a mechanical relief position sensor. Aspect 20. The system according to aspect 18 or 19, wherein the device for determining instantaneous compressor loads has at least one temperature sensor configured to determine the instantaneous load by estimating a change in the evaporator water temperature. Aspect 21. The system according to one of aspects 18 to 20, wherein the control unit is configured to determine the speed of the at least one condenser fan when determining the desired condenser fan capacity, and is configured to output the speed of the at least one condenser fan in order to operate the at least one condenser fan based on the desired condenser fan capacity.

[0030] With regard to the preceding description, it should be understood that changes can be made to the details without deviating from the scope of the present invention. It is intended that the description and the illustrated embodiments are to be considered only as examples, the true scope and spirit of the invention being indicated by the broad scope of the claims.

Claims

[1] A method for controlling condenser fans in an HVAC system, comprising: Detect, with a device for detecting the ambient temperature, the ambient temperature; Determine, using a device for determining instantaneous compressor loads, an instantaneous load on a variable-load compressor; Determine, with a control unit, a desired condenser fan capacity that is suitable to control at least one condenser fan based on the detected ambient air temperature and the determined instantaneous load on the variable load compressor; Output, with the control unit, the desired condenser fan capacity, which is suitable to control at least one condenser fan; and Controlling the at least one fan based on the output of the desired condenser fan capacity, so that the performance of the HVAC system is controlled via the power consumed by the variable load compressor and the at least one condenser fan. [2] The method according to claim 1, wherein the step of determining the instantaneous load comprises determining the instantaneous load on a variable-speed compressor. [3] The method according to claim 1, wherein the step of determining the instantaneous load comprises determining the instantaneous load by the instantaneous compressor load determination device which includes a speed sensor. [4] The method according to claim 1, wherein the step of determining the instantaneous load comprises determining the instantaneous load by the device for determining instantaneous compressor loads, which has a mechanical unloading position sensor. [5] The method according to claim 1, wherein the step of determining the instantaneous load comprises determining the instantaneous load by estimating a change in the evaporator water temperature, which is determined by the instantaneous compressor load determination device which includes a temperature sensor. [6] The method according to claim 1, wherein the step of determining a desired condenser blower capacity comprises determining the speed of the at least one condenser blower, and the step of outputting the desired condenser blower capacity comprises outputting the speed of the at least one condenser blower. [7] An HVAC system, comprising: a variable load compressor; an evaporator that is fluidically connected to the variable load compressor; a capacitor that is fluid-mechanically connected to the variable load compressor, wherein the capacitor has at least one capacitor fan; a device for measuring the ambient air temperature; a device for determining instantaneous loads on a variable-load compressor; and a control unit that is operatively connected to the device for detecting the ambient air temperature, to the device for determining instantaneous loads on the variable load compressor and to the condenser which has at least one condenser fan, wherein the control unit is configured to determine a desired condenser fan capacity suitable for controlling at least one condenser fan based on the detected ambient air temperature and the determined instantaneous load on the variable load compressor, wherein the control unit is configured to operate the at least one condenser fan based on the desired condenser fan capacity, so that the performance of the HVAC system is controlled via the power consumed by the variable load compressor and the at least one condenser fan. [8] The system according to claim 7, wherein the variable load compressor is a variable speed compressor. [9] The system according to claim 7, wherein the device for determining instantaneous compressor loads includes a speed sensor. [10] The system according to claim 7, wherein the device for determining instantaneous compressor loads comprises a mechanical unloading position sensor. [11] The system according to claim 7, wherein the device for determining instantaneous compressor loads has at least one temperature sensor configured to determine the instantaneous load by estimating a change in the evaporator water temperature. [12] The system according to claim 7, wherein the control unit is configured to determine the speed of the at least one condenser fan when determining the desired condenser fan capacity, and is configured to output the speed of the at least one condenser fan in order to operate the at least one condenser fan based on the desired condenser fan capacity. [13] The system according to claim 7, wherein the control unit has a high-pressure avoidance control of the at least one blower under the condition that the condensation temperature approaches the condenser pressure limit. [14] The system according to claim 7, wherein the system is an air-cooled water refrigeration machine with at least one variable-speed compressor and at least one variable-speed blower. [15] A method for controlling condenser fans in an HVAC system, comprising: Detect, with a device for detecting ambient air temperatures, the ambient air temperature; Determine, using a device for determining instantaneous compressor loads, an instantaneous load on a variable-speed compressor; Determine, using a control unit, a target differential pressure between a condenser and an evaporator based on the detected ambient air temperature and the determined instantaneous load on a variable speed compressor; Output, with the control unit, a condenser fan speed suitable for achieving the specified target differential pressure; and Controlling at least one fan based on the output of the condenser fan speed to determine a fan capacity suitable to control at least one condenser fan, so that the performance of the HVAC system is controlled via the power consumed by the variable speed compressor and the fans. [16] The method according to claim 15, wherein the step of determining the instantaneous load comprises determining the instantaneous load by the device for determining instantaneous compressor loads, which has a mechanical unloading position sensor. [17] The method according to claim 15, wherein the step of determining the instantaneous load comprises determining the instantaneous load by estimating a change in the evaporator water temperature, as determined by the instantaneous compressor load determination device which includes a temperature sensor. [18] An HVAC system comprising: a variable speed compressor; an evaporator that is fluidically connected to the variable-speed compressor; a capacitor that is fluid-mechanically connected to the variable-speed compressor, wherein the capacitor has at least one capacitor fan; a device for measuring the ambient air temperature; a device for determining instantaneous loads on the variable-speed compressor; and a control unit that is operatively connected to the device for detecting the ambient air temperature, to the device for determining instantaneous loads on the variable-speed compressor, and to the condenser, which has at least one condenser fan, wherein the control unit is designed to determine a target differential pressure between the condenser and the evaporator based on the detected ambient air temperature and the determined instantaneous load on the variable-speed compressor, wherein the control unit is configured to determine an output of the condenser blower capacity suitable for achieving the specified target differential pressure, and The control unit is designed to operate at least one condenser fan based on the output of the condenser fan capacity, so that the performance of the HVAC system is controlled via the power consumed by the variable speed compressor and the at least one condenser fan. [19] The system according to claim 18, wherein the device for determining instantaneous compressor loads comprises a mechanical unloading position sensor. [20] The system according to claim 18, wherein the device for determining instantaneous compressor loads has at least one temperature sensor configured to determine the instantaneous load by estimating a change in the evaporator water temperature. [21] The system according to claim 18, wherein the control unit is configured to determine the speed of the at least one condenser fan when determining the desired condenser fan capacity, and is configured to output the speed of the at least one condenser fan in order to operate the at least one condenser fan based on the desired condenser fan capacity.