CONTROL SYSTEM AND METHOD FOR A COOLING SYSTEM

Description

FIELD OF THE INVENTION

This invention relates to a method for controlling a cooling system, a control system, a cooling system and a building management system.

BACKGROUND

Chillers and other cooling systems are used to maintain a temperature of a load, which may be a computing server or another temperature controlled environment. In some cases, such as after a power loss or the failure of another cooling system, there may be a need to increase a rate of cooling from a cooling system quickly. This should be managed in order to avoid over-cooling of the system, which may lead to freezing of an internal fluid or damage to another part of the system or to the load.

Further, there is a desire to reduce the power consumption of cooling systems.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a method for controlling a cooling system, the cooling system comprising: a mechanical cooling circuit comprising: a compressor arranged to increase a pressure of a working fluid; a condenser arranged to receive the working fluid from the compressor and to allow heat to leave the working fluid; an expander arranged to receive the working fluid from the evaporator and to allow the pressure of the working fluid to decrease; and an evaporator arranged to receive the working fluid from the expander and to allow heat to be transferred to the working fluid from a cooling load, the evaporator being arranged to supply the working fluid to the compressor; and a free cooling circuit comprising: an external heat exchanger arranged to allow heat transfer between a process fluid and an ambient fluid external to the cooling system; and an internal heat exchanger arranged to receive the process fluid from the external heat exchanger and to allow heat transfer to the process fluid from the cooling load; the method comprising: receiving a temperature setpoint value indicative of a required temperature of the cooling load; receiving an ambient temperature value indicative of a temperature of the ambient fluid; calculating a difference between the temperature setpoint value and the ambient temperature value; determining whether to activate the mechanical cooling circuit and/or the free cooling circuit based on the calculated difference; and outputting a control signal to activate the mechanical cooling circuit and/or the free cooling circuit based on the determination.

It will be understood that free cooling may utilise a temperature difference between an ambient temperature and a desired temperature of the load to remove heat from the load and that this may require less energy than a refrigeration cycle, which may also be referred to as mechanical cooling.

By comparing the outside temperature to the temperature setpoint, the system may determine whether a free cooling cycle may be usable and whether an energy saving and a sufficient rate of cooling may be achieved. A temperature setpoint may be used for determining whether to use free cooling, rather than a measured value, in order to allow a faster decision to be made as to whether to use free cooling. This may increase the speed of the control method, since no measurement step is required. Consequently, the load may be cooled faster.

The determining may comprise determining to activate the free cooling circuit based on a calculation that the temperature set point value is greater than the ambient temperature value. The method may further comprise comparing the calculated difference to a free cooling minimum difference, and the determining may comprise determining whether to activate the free cooling circuit and/or the mechanical cooling circuit based on the comparison. By activating the free cooling circuit based on a determination that the temperature set point value is greater than the ambient temperature value, the prospect of the free cooling circuit acting to heat the load may be reduced. Further, using a free cooling minimum difference may allow the free cooling circuit to be used without the mechanical cooling circuit being activated, as it may be determined that the free cooling circuit alone may provide a sufficient rate of cooling.

The determining may comprise determining to activate the free cooling circuit and not to activate the mechanical cooling circuit based on the calculated difference being greater than the free cooling minimum difference. In this way, the energy usage of the system may be reduced when the free cooling circuit alone may provide sufficient cooling. The risk of over-cooling of a process fluid or of the load may also be reduced.

The free cooling circuit may comprise: a fluid mover arranged to move the ambient fluid relative to the external heat exchanger, and/or a valve arranged to control a flow rate of the process fluid, and/or a pump arranged to circulate the process fluid around the free cooling circuit, and the method may further comprise: determining an activation level of the free cooling circuit based on the calculated difference, and outputting an activation level control signal to control, based on the determined activation level: a fan speed of the fan fluid mover, and/or a valve position of the valve, and/or a pumping speed of the pump.

In this way, the rate of cooling provided by free cooling circuit may be varied in order to avoid overcooling of the load and/or a process fluid. The rate of cooling may also be varied in order to provide a sufficiently fast rate of cooling to cool the load within a required time period.

The determining may comprise determining not to activate the free cooling circuit and to activate the mechanical cooling circuit based on the calculated difference being less than the free cooling minimum difference. In this way, the load may be cooled at a sufficient rate when an ambient temperature is too high to allow the free cooling circuit to operate efficiently. Further, inadvertent heating of the load by the free cooling circuit may be avoided.

The temperature set point value may be a desired temperature of a process fluid for cooling the cooling load. The temperature of the process fluid may be measured on exit from a load heat exchanger for transferring heat from the load to the process fluid. This may allow a single sensor to be used to determine the temperature of the load and the process fluid temperature.

The method may further comprise receiving a rapid start signal, and performing the receiving of the temperature setpoint value and the ambient temperature value and/or performing the calculating in response to receiving the rapid start signal. Generally, the method may be commenced based on receipt of a rapid start signal. This may allow the system to be started in a steady state mode when a high rate of cooling is not required, which may be more efficient and may reduce the chance of over-cooling. Generally, this may allow the system to operate as required in a greater range of circumstances.

Receiving the rapid start signal may comprise receiving a signal from a user input device specifying a rapid start procedure. This may allow the system to be started with a high rate of cooling in situations where the system would otherwise start in a steady state mode, such as after a manual restart. Receiving the rapid start signal may comprise receiving a signal from a building management system. This may allow the building management system to manage the temperature of the load more closely, such as in case of a failure of another chiller arranged to cool the load

The method may further comprise: receiving a load signal indicative of a load of the cooling system, comparing the load to a start mode exit threshold, and determining whether to commence a steady-state control mode based on the comparing. The load signal may contain information indicative of a process fluid temperature and the comparing may comprise comparing the process fluid temperature to a process fluid temperature setpoint. In this way, the rapid cooling mode may be stopped before over-cooling of the process fluid and/or the load occurs.

The steady state mode may be characterised by a feedback cycle that includes lower gain values. This may reduce a rate of cooling and may allow the rate of cooling to be adjusted based on the temperature of the load to a greater extent.

The method may further comprise: receiving a time value indicative of a time since commencement of the method; comparing the time value to a start mode time limit; and determining whether to commence a steady-state control mode based on the comparing. By using a time limit for the rapid restart mode, the rapid restart mode may be exited before damage to the system occurs, such as in cases where the rate of cooling of the load that would be expected based on system control parameters is not achieved.

According to a second aspect of the present invention, there is provided a control system arranged to carry out the method of the first aspect. The control system of the second aspect may include any or all of the optional features and properties described above with reference to the first aspect.

According to a third aspect of the present invention, there is provided a cooling system comprising: a mechanical cooling circuit comprising: a compressor arranged to increase a pressure of a working fluid; a condenser arranged to receive the working fluid from the compressor and to allow heat to leave the working fluid; an expander arranged to receive the working fluid from the evaporator and to allow the pressure of the working fluid to decrease; and an evaporator arranged to receive the working fluid from the expander and to allow heat to be transferred to the working fluid, the evaporator being arranged to supply the working fluid to the compressor; a free cooling circuit comprising: an external heat exchanger arranged to allow heat transfer between a process fluid and an ambient fluid external to the cooling system; and an internal heat exchanger arranged to receive the process fluid from the external heat exchanger and to allow heat transfer to the process fluid; and a control system according to the second aspect, the control system being arranged to control the mechanical cooling circuit and the free cooling circuit.

According to a fourth aspect of the present invention, there is provided a building management system comprising a plurality of cooling systems according to the third aspect.

The building management system may further comprising a building management control system, the building management system being arranged to: determine a failure of a first cooling system of the plurality; and output a start signal to a second cooling system of the plurality based on the determination of the failure, and the control system of the second cooling system may be arranged to carry out the method of the first aspect based on receiving the start signal. In this way, 1 cooling system may quickly increase a rate of cooling in response to an unexpected stopping of another cooling system. This may reduce any undesirable increase in temperature of the load following the failure of a cooling system.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, by reference to the following drawings, in which:

FIG. 1 shows a first cooling system according to the present invention;

FIG. 2 shows a second cooling system according to the present invention;

FIG. 3 shows a schematic system diagram of a control system according to the present invention;

FIG. 4 shows a schematic system diagram of a building management system according to the present invention;

FIG. 5 shows a flowchart illustrating a method according to the present invention;

FIG. 6 shows a second flowchart illustrating a second method according to the present invention; and

FIG. 7 shows a graph illustrating a variation of an activation level of a free cooling circuit with ambient temperature.

DETAILED DESCRIPTION

FIG. 1 shows a cooling system 100. The cooling system 100 comprises a free cooling circuit 110 and a mechanical cooling circuit 150. Overall, the cooling system 100 is arranged to transfer heat from a load 112 to the environment. The heat may be transferred to the environment via a free cooling external heat exchanger 118 when the environment is at a low temperature and may be transferred to the environment via a condenser 154 when the environment is at a higher temperature. In some cases, it may be transferred to the environment via both of the free cooling external heat exchanger 118 and the condenser 154.

The load 112 may be a computing server, or may be a temperature controlled environment. Heat may be transferred from the load 112 via a heat exchanger, which may be arranged to have a process fluid passing through it, and which may be in contact with the load or may have a load-side process fluid passing through it, which may be air that may be blown through the heat exchanger via a fan. Generally, the load 112 may be any source of heat that may be cooled by the cooling system 100.

The free cooling circuit 110 comprises a pump 114 that is arranged to circulate a process fluid around the free cooling circuit 110, a free cooling external heat exchanger 118 that is arranged to transfer heat from the process fluid to the ambient environment, and the load 112.

In order to allow the free cooling external heat exchanger 118 to be used selectively, the free cooling circuit 110 may also include a bypass valve 120. The bypass valve 120 may be arranged in parallel with the free cooling external heat exchanger 118 and may be opened to allow the process fluid to bypass the free cooling external heat exchanger 118. The bypass valve 120 may therefore be opened when free cooling is not required or is not possible and may be closed when free cooling is desired.

The cooling system 100 also includes an ambient fluid mover 122. The ambient fluid mover 122 may be a fan that is arranged to move ambient air relative to the free cooling external heat exchanger 118, or may be a pump that is arranged to move ambient water relative to the free cooling external heat exchanger 118. Generally, the ambient fluid mover 122 may increase the rate of heat exchange at the free cooling external heat exchanger 118 by moving ambient fluid relative to the free cooling external heat exchanger 118. The speed of the ambient fluid heat fluid mover 122 may be varied in order to vary the rate of heat exchange at the free cooling external heat exchanger 118, and accordingly to vary the rate of cooling at the load 112.

The free cooling circuit 110 also includes an evaporator 116 of the mechanical cooling circuit 150. As explained below, the evaporator 116 is arranged to transfer heat away from the process fluid of the free cooling circuit 110 when the mechanical cooling circuit 150 is activated. When the mechanical cooling circuit 150 is not activated, there may be a minimal transfer of heat, or no transfer of heat, at the evaporator 116.

The mechanical cooling circuit 150 includes a compressor 152 that is arranged to increase the pressure of a gaseous working fluid introduced into the compressor 152, a condenser 154 that is arranged to allow heat from the working fluid of the mechanical cooling circuit 150 to leave the working fluid and move to the environment. In this way, the high pressure gaseous working fluid received from the compressor 152 may be condensed and may become at least partially liquid. The mechanical cooling circuit 150 also includes a throttle valve 156 that is arranged to reduce a pressure in the working fluid passing through it that may be received from the condenser 154. The working fluid may then evaporate in the evaporator 116, where it may take in heat from the process fluid. In this way, the mechanical cooling circuit 150 may operate as a refrigeration circuit.

The cooling system 100 also comprises a controller 160. The controller 160 may be arranged to receive a temperature from an ambient temperature sensor 126, which may measure a temperature of an ambient fluid adjacent to the free cooling external heat exchanger 118, and which may be indicative of an environmental temperature. The controller 160 may also receive a temperature from a temperature sensor 124, which may sense a temperature of the process fluid of the free cooling circuit 110, and in particular may sense the temperature of the process fluid as the process fluid leaves the load 112. This may be referred to as a return temperature.

The controller 160 may control the pump 114 and the bypass valve 120 of the free cooling circuit 110, as well as the ambient fluid mover 122, in order to control a rate of cooling of the load due to the free cooling circuit 110. The controller 160 may also control the compressor 152 and the throttle valve 156 of the mechanical cooling circuit 150 in order to control the rate of mechanical cooling.

It will be also understood that the controller 160 may receive signals from a building management system and/or one or more user interfaces in order to control the cooling system 100. The controller 160 may also receive data from other parts of the cooling system 100. The manner of control of the cooling system 100 is described below further with reference to FIGS. 5 and 6.

FIG. 2 shows a second cooling circuit 200. The second cooling circuit 200 includes a free cooling circuit 220 and a mechanical cooling circuit 250. The cooling system 200 also includes a load cooling circuit 210. It will be understood that the free cooling circuit 220 and the mechanical cooling circuit 250 should still be considered as cooling a load 212, since the circuits 220, 250 cool the load 212 indirectly.

The load cooling circuit 210 includes a pump 214 arranged to transfer a process fluid, a load 212, a free cooling heat exchanger 216, and an evaporator 218. The load 212 may be substantially similar to the load 112, described above with reference to FIG. 1, and the evaporator 218 may be substantially similar to the evaporator 118 described above with reference to FIG. 1.

The pump 214 may be a variable speed pump that may circulate process fluid around the load cooling circuit 210 at a variable speed. The free cooling heat exchanger 216 may be arranged to transfer heat from the load cooling circuit 210 to the free cooling circuit 220 and in particular from a process fluid of the load cooling circuit 210 to a process fluid of the free cooling circuit 220.

The free cooling circuit 220 may also include a pump 222, which may be variable speed pump substantially similar to the pump 114 of FIG. 1, and a free cooling external heat exchanger 224, which may be similar to the free cooling heat exchanger 118 of FIG. 1. The free cooling circuit 220 also includes a free cooling valve 226, which is operable to control an introduction of fluid into the free cooling circuit 220 from an external fluid source 228 and/or to reduce a flow rate of the process fluid in the free cooling circuit 220. In this way, the temperature of the process fluid in the free cooling circuit 220 may be controlled.

The pump 222 of the free cooling circuit 220 may be activated selectively depending on whether free cooling is possible. The pump 222 and the free cooling valve 226 may therefore be controlled by a controller 260.

The cooling system 200 may also include an ambient fluid mover 228, which may be substantially similar to the ambient fluid mover 122 of FIG. 1.

The mechanical cooling circuit 250 may be substantially similar to the mechanical cooling circuit 130 of FIG. 1, and may include a compressor 252, a condenser 254, and a throttle valve 256, as well as the evaporator 218, which be substantially similar to respective parts described above with reference to FIG. 1.

The controller 260 may control the pumps 214, 222, the compressor 252, the throttle valve 256, the free cooling valve 226 and the ambient fluid mover 228. Each of these controllable elements may be selectively powered by the controller based on the required activation and the required activation level of the controllable element, which may be determined by the method set out below with reference to FIGS. 5 and 6.

The controller may receive data from a process fluid temperature sensor 224, which may sense the temperature of the process fluid in the load cooling circuit 210 and in particular may sense the temperature of the process fluid after the process fluid has received heat from the load 212. The controller may also receive data from an ambient temperature sensor 226, which may sense a temperature of an ambient fluid proximate the free cooling external heat exchanger 224.

The controller 260 may receive data from further sources and may also receive inputs from user input devices and/or a building management system.

FIG. 3 shows a control system 300 that schematically represents the different inputs and outputs that may the in communication with a controller 302 of a control system 300 of a cooling system, such as the cooling systems described above with reference to FIGS. 1 and 2.

The controller 302 may receive an ambient temperature from an ambient temperature sensor 304, which may be used to determine the efficacy of a free cooling system.

The controller 302 may also receive an input signal from a BMS controller 306. The BMS controller 306 may be a controller that controls multiple different cooling systems, which may also be referred to as chiller units, in order to maintain a required temperature at a load. The building management system controller 306 may therefore output a signal to activate a cooling system, based on a failure of another cooling system.

The controller 302 may also receive an input from a user input device 308. For example, the user input device 308 may be local user input device, such as an operator panel within a building where the control system operates, or a remote input device, which may control the cooling system over a network. The user input device may enable or disable a rapid restart process, which is described below, selectively.

The controller 302 may also receive a temperature signal from a process fluid temperature sensor 310. The process fluid temperature sensor 310 may be a return temperature sensor arranged to sense a return temperature of a process fluid. The return temperature may be indicative of a temperature of the load, and the return temperature sensor 310 may be positioned to sense the temperature of fluid that has passed through the load immediately previously.

The controller 302 may control various components of a cooling system, such as a pump 312, which may circulate a process fluid around a circuit within a cooling system. In some cases, the controller 302 may control more than one pump, where more than one process fluid may be used, such as where the system may include more than one fluid circuit.

The controller 302 may also control a free cooling valve 314, which may be valve arranged to introduce new fluid into a free cooling circuit in order to control the temperature of a process fluid of the free cooling circuit.

The controller 302 may control a bypass valve 316, which may be the bypass valve 120 of FIG. 1, which may allow fluid to bypass a free cooling heat exchanger, in order to prevent free cooling or to prevent heat transfer with an free cooling external heat exchanger when an ambient temperature is above a temperature setpoint.

The controller 302 may also control a throttle valve 322 and a compressor 318 in order to control an amount of mechanical cooling due to a refrigeration circuit. The compressor 318 may be activated when mechanical cooling is desired and may be deactivated when mechanical cooling is not necessary. The throttle valve 322 may be controlled to open by varying amounts to manage a level of cooling from the refrigeration circuit.

A fan 320 of a free cooling circuit, or another ambient fluid mover 320, may also be controlled by the controller 302 in order to control heat exchange between an external heat exchanger of a free cooling circuit and an ambient fluid.

In this way, a controller 302 may manage a rate of cooling of a load and may cool the load in an energy-efficient manner.

FIG. 4 shows a building management control system 400, which may control multiple cooling systems in order to cool a load. The building management control system 400 may be controlled by a building management controller 402, which may also be referred to as a BMS controller 402. The BMS controller 402 may control four separate cooling systems 406, 408, 410, 412. It will be understood that while four separate cooling systems 406, 408, 410, 412 are shown in FIG. 4, any number of cooling systems may be controlled by the BMS controller 402. For example, the BMS controller 402 may control two cooling systems, or may control five or more cooling systems.

The BMS controller 402 may be controlled by a user input panel 404 and may output data to the user input panel 404 to allow a user to observe the operating conditions of the cooling systems 406, 408, 410, 412 and to allow a user to control the cooling systems 406, 408, 410, 412.

The BMS controller 402 may, for example, maintain a temperature of a load by turning a first cooling system 406 on in response to a second cooling system 408 failing to cooling the load, such as due to a malfunction of the second system 408 or a power loss of the second cooling system 408.

In some cases, a BMS 400 may have more cooling systems than are required for cooling a load. This may allow some cooling systems to be redundant. For example, where three cooling systems are sufficient to cool a load, a BMS may have four cooling systems so that, in case of failure of one cooling system, another may be activated in order to maintain the required level of cooling.

FIG. 5 shows a flowchart illustrating a method 500 of controlling a cooling system, such as the cooling systems illustrated in FIGS. 1, 2 and 3. The method 500 may be referred to as a rapid start sequence 500.

At step 502, the controller receives an ambient temperature value. The ambient temperature may be measured by an external temperature sensor, which may transmit data to the controller including information indicative of the ambient temperature. The ambient temperature may be the temperature of a fluid proximate an external free cooling heat exchanger. The fluid may be air or may be water, such as in a lake or river.

At step 504, the controller receives a temperature setpoint value. The temperature setpoint value may be stored within the controller or in a building management system and so receiving the temperature setpoint may comprise retrieving the temperature setpoint from an internal memory or receiving the temperature setpoint from a remote controller. The temperature setpoint value is a desired temperature of an element of the cooling system or the load. For example, the temperature setpoint may be a desired temperature of a process fluid after receiving heat from the load or may be a desired temperature of the load itself. The temperature setpoint may also be used in a calculation for exiting the rapid start sequence 500 and/or may be used in a steady state control method for the cooling system.

At step 506, the controller calculates a difference between the ambient temperature value and the temperature setpoint. This difference may give an indication of an available free cooling, or a rate of heat transfer at an external free cooling heat exchanger.

At Step 508, it is determined whether to activate a free cooling circuit and/or a mechanical circuit of the cooling system. The determination may be made by comparing the calculated difference to a free cooling minimum difference. The free cooling minimum difference may be 0°C, which may represent the point at which the free cooling system will operate to cool the load, as opposed to heating the load. However, in order to operate efficiently, the free cooling minimum difference may be between 10°C and 20°C.

If the calculated difference is greater than the free cooling minimum difference, i.e. the setpoint temperature is greater than the ambient temperature by the free cooling minimum difference or more, the controller may determine to activate only the free cooling system and not to activate the mechanical cooling system.

Alternatively, where the calculated difference is less than the free cooling minimum difference, i.e. the setpoint temperature is not greater than the ambient temperature by the free cooling minimum difference, the controller may determine to activate the mechanical cooling circuit and not to activate the free cooling circuit. This may allow cooling where the ambient temperature is relatively warm by virtue of the refrigeration cycle of the mechanical cooling circuit.

In some cases, it may be determined to activate both of the free cooling circuit and the mechanical cooling circuit. This may allow a faster cooling, but may risk over-cooling of the process fluid and/or the load.

If it is determined at step 508 to activate the mechanical cooling circuit, the method may move to step 510 and a level of activation of the mechanical cooling circuit may be determined. The level of activation of the mechanical cooling circuit may be determined based on a user input or by previously stored system parameters, which may be retrieved.

If it is determined at step 508 to activate the free cooling circuit, the method may move to step 512 and a level of activation of the free cooling circuit may be determined. The level of activation of the free cooling circuit is also illustrated in FIG. 7, which is explained further below. The level of activation of the free cooling circuit may be determined based on the difference between the temperature setpoint and the ambient temperature. Where there is a greater difference between the temperature setpoint and the ambient temperature, a lower level of activation of the free cooling circuit may be determined. This may reduce the risk of over-cooling of the load and/or of the process fluid.

The level of activation may be considered as a proportion of the available free cooling that is being used. At a lower level of activation, an ambient temperature mover and/or a variable speed pump may be operated at a lower speed in order to provide a reduced rate of heat transfer. Alternatively or additionally, a bypass valve may be opened to a non-zero amount and/or a mixing valve may be opened to introduce a new fluid into the free cooling circuit.

At step 514, a control signal is output by the controller to cause the cooling system to operate according to the determinations at steps 508, 510, and/or 512.

In some cases, steps 510 and/or 512 may be omitted from the method 500 and the method may operate by activating the mechanical cooling circuit and/or the free cooling circuit at a fixed activation level based on a determination to activate the respective circuit.

FIG. 6 shows a flowchart illustrating a method 600 of operating a cooling system.

The method 600 may be commenced by receiving a user input at step 602, by a cooling system start sequence at step 604, or by receiving a command from a building management system at step 604.

The user input at step 602 may comprise a rapid restart enable command and an auto stop command being set to “AUTO”. Generally, a user may request a rapid restart, i.e. the method 500 of FIG. 5 manually in order to increase a rate of cooling of a load quickly.

The start sequence at step 604 may comprise a cooling system being started and, during the start, detecting that there was a previous loss of power. In the case of a previous loss of power, the cooling system may determine whether a rapid restart sequence is enabled and whether the loss of power was within a previous time window. For example, if it is determined that a loss of power occurred within a time threshold before the start sequence, which may be 10 minutes, then the rapid restart, i.e. the method of FIG. 5, may be initiated.

The command from a building management system at step 606 may be output to the cooling system from the building management system and may be received by the controller. The building management system may output the command at step 606 in response to a different cooling system ceasing to provide cooling to the load. This may be due to a loss of power of the different cooling system or any other mode of failure. In general, the building management system may request that the cooling system starts in a rapid restart mode at step 606.

At step 608, the cooling system may carry out the method 500 of FIG. 5.

At step 610, the controller of the cooling system may receive one or more system parameters that may determine whether to exit the method 500. The system parameters may be load parameters indicative of a load of the cooling system. For example, the controller may receive a process fluid temperature value indicative of the temperature of a process fluid after receiving heat from the load. Alternatively, the controller may receive a load temperature value indicative of the temperature of the load. The system parameters received at step 610 may also include a time since commencement of the method 500.

At step 612, the received system parameters may be compared to one or more exit conditions, such as being compared to exit threshold values, which may be referred to as start mode exit threshold values. For example, the process fluid temperature value may be compared to a process fluid temperature setpoint, and/or the load temperature value may be compared to a load temperature setpoint value. If it is determined that the system parameters are in accordance with exit conditions, such as being below an exit threshold, for example due to being within a predetermined range of the setpoint value, the method may move to step 614. Alternatively, if the exit conditions are not fulfilled by the system parameters, the method may return to step 608.

Step 612 may also comprise comparing a time since commencement of the method 500 to a time threshold, which may be referred to as a start mode time limit. If a time greater than the time threshold has elapsed since commencement of the method 500, the method may move to step 614. Alternatively, the method may return to step 608.

At step 614, the controller may enter a steady state mode. The steady state mode may be characterised relative to the rapid restart mode by a closer feedback loop, meaning that a maximum rate of cooling may be reduced. A combination of mechanical and free cooling may also be used in the steady state mode where it might not be used in the rapid restart mode.

FIG. 7 shows a graph 700 illustrating a variation of activation level 704 of a free cooling circuit based on ambient temperature 702 for a given temperature setpoint. Point 706 is a minimum free cooling temperature, which may be determined by subtracting a free cooling minimum temperature difference from a temperature setpoint.

When the ambient temperature 702 is above the free cooling minimum temperature, the system may operate with no free cooling. In this case, in region M, the system may operate with only mechanical cooling. Put alternatively, the system may operate with only mechanical cooling when the ambient temperature minus the temperature setpoint is greater than the free cooling minimum difference.

When the ambient temperature 702 is greater than the free cooling minimum temperature, in region F, the system may operate with free cooling. In region F, the system may operate with no mechanical cooling.

The activation level of the free cooling circuit may be reduced where the ambient temperature is further reduced relative to the free cooling minimum temperature. This may result in the system providing a consistent rate of cooling. The activation level may be reduced proportionally to the difference between the ambient temperature and the free cooling minimum temperature. As explained above, the activation level of the free cooling circuit may be representative of a pump speed, a valve position and an ambient fluid mover speed commanded by the controller.