EP4619688A1

EP4619688A1 - Method and system for controlling a flow of fluid in a mixing loop

Info

Publication number: EP4619688A1
Application number: EP23806236.8A
Authority: EP
Inventors: Jan Carøe Aarestrup; Carsten Skovmose Kallesøe
Original assignee: Grundfos Holdings AS
Current assignee: Grundfos Holdings AS
Priority date: 2022-11-14
Filing date: 2023-11-14
Publication date: 2025-09-24
Also published as: WO2024105026A1

Abstract

Preferred embodiments of the invention relate to a method of controlling a flow of fluid in a mixing loop of a system of mixing loops comprising a number of mixing loops each comprising a thermal load and each being fluidicly connected in parallel to a fluid source, in each mixing loop pressurization of said fluid is provided at least partly by a pump being a variable speed pump, and a bypass of said flow of fluid is recirculated in the mixing loop by a 3-way valve comprising a first inlet port fluidicly connected to said fluid source to receive a supply of fluid, a second inlet port fluidicly connected to receive said bypass and an outlet port fluidicly connected to an inlet side of said pump. Preferred embodiments of the invention also relates to a system of mixing loops.

Description

80070PC01 1 METHOD AND SYSTEM FOR CONTROLLING A FLOW OF FLUID IN A MIXING LOOP FIELD OF THE INVENTION Preferred embodiments of the invention relate to a method of controlling a flow of fluid in a mixing loop of a system of mixing loops comprising a number of mixing loops each comprising a thermal load and each being fluidicly connected in parallel to a fluid source, in each mixing loop pressurization of said fluid is provided at least partly by a pump being a variable speed pump, and a bypass of said flow of fluid is recirculated in the mixing loop by a 3-way valve comprising a first inlet port fluidicly connected to said fluid source to receive a supply of fluid, a second inlet port fluidicly connected to receive said bypass and an outlet port fluidicly connected to an inlet side of said pump. Preferred embodiments of the invention also relates to a system of mixing loops. BACKGROUND OF THE INVENTION Mixing loops are widely used in many heating and cooling systems. Such mixing loops typically feed warm or cool water to a load by use of a pump. Such loads are typically designed as a heat exchanger, where a heat exchange takes place between the fluid in the heat exchanger and the surroundings. If the temperature of the fluid is higher than the temperature of surroundings, heat is transported to the surroundings (heating) and if the temperature of the fluid is lower than the surroundings, heat is transported to the fluid (cooling). Typical examples on such heat exchangers are radiators, water based floor heating, heating or cooling towers, water to air heat exchangers e.g. used in air handling units or fan coils. The fluid is typically heated or cooled by a heat source such as a boiler, a chiller, a heat pump, etc. (cooling is considered to be a heat source with negative heat addition). The fluid from the heat source is forwarded in a supply line to the mixing loop and through the load. After the fluid has passed through the load, the temperature of the fluid has changed (either increased or decreased). A fraction of the fluid leaving the load, often referred to as bypass fluid, is mixed into the fluid coming from the heat source and the remaining fluid is fed back to the heat source through a return line. Thus, the fluid flowing into the heat source is mix of fluid coming from the heat source and a fluid already having flown through the load. The division of the fluid is typically carried out by an adjustable 3-way valve, 80070PC01 2 where an opening degree of the valve sets the fraction, typically being a number between 0% and 100%, of fluid being bypassed. Accordingly, by controlling the 3- way valve, the temperature of the fluid flowing towards the load may be controlled without adjusting the temperature of the fluid leaving the heat source, at least as long at the amount of bypass fluid may provide a desired temperature of the fluid flowing towards the loads. While such a mixing loop is well proven and can be operated in a satisfying manner when used as a stand-alone installation, technical problems arise when mixing loops are arranged in parallel and connected to the same heat source via the same supply line feeding fluid from the heat source to the mixing loops, and connected to the same return line feeding fluid from the mixing loops back to the heat source. In such systems, each mixing loop is provided with a pump driving the flow in the mixing loop and sucking fluid from the heat source to the mixing loops and feeding the fluid back from the mixing loops to the heat source, whereby this supply and return often is considered to be pressure less. The pressure driving the flow in a mixing is typically carried out by a local pump, which is a pump arranged in the mixing loop. However, it has been found in connection with the present invention that the pressure losses at the heat source are significant, and the pressure difference between the supply and return line cannot be said to be pressure less. By assuming such pressure less supply and return and using simple proportional pressure or constant pressure control, leads to cross coupling between the mixing loops. That is, it has been found that an individual operation of each mixing loops are affected by other mixing loops connected to the same supply and return lines. This may be referred to as that the mixing loops are fluidicly cross coupled. In systems involving a number of parallel connected mixing loops, such a cross coupling is clearly a disadvantage since changes in one mixing loop cannot be made without changes needs to be imposed in other mixing loops to avoid for instance a temperature change in one mixing occurring due to imposed changes in another mixing loop. 80070PC01 3 Hence, an improved method of controlling flow in such mixing loops would be advantageous, and in particular a more efficient and/or reliable method of controlling would be advantageous. In order to operate mixing loops, focus is made on the forward flow temperature and the mixing loops are typically provided with a single sensor for measuring the forward temperature and the three-way valve is operated to provide a desired forward temperature. Further, the speed of the pump is set to operate at a constant speed. Although this is workable implementation, it would be beneficial to know e.g. an operating point, e.g. RPM or pressure increase provided, for the pumps used in the system or at least in the mixing loop. It may also be seen as beneficial to e.g. know the differential over the thermal load. Installation of sensors and/or determination of the operation of a pump, are often time consuming, costly and often involves installation of special designed fitting for e.g. the sensors. Hence, a more efficient and/or reliable method of installation of parallel connected mixing loops would be advantageous. OBJECT OF THE INVENTION It is an object of the present invention to provide an improved method of controlling flow mixing loops. It is a further object to provide a control method for mixing loops which mitigates or reduces fluidic cross coupling between parallel connected mixing loops. It is a further object of the present invention to provide an alternative to the prior art. It is a further object of the present invention to provide a more efficient and/or reliable method of installation of parallel connected mixing loops. SUMMARY OF THE INVENTION Thus, the above described object and several other objects are intended to be obtained in a first aspect of the invention by providing a method of controlling a flow of fluid in a mixing loop of a system of mixing loops comprising a number of 80070PC01 4 mixing loops each comprising a thermal load and each being fluidicly connected in parallel to a fluid source, in each mixing loop pressurization of said fluid is provided at least partly by a pump being a variable speed pump, preferably arranged either upstream or downstream of said thermal load, and a bypass of said flow of fluid is recirculated in the mixing loop by a 3-way valve comprising a first inlet port fluidicly connected to said fluid source to receive a supply of fluid, a second inlet port fluidicly connected to receive said bypass and an outlet port fluidicly connected to said thermal load to deliver said fluid to said thermal load. In preferred embodiments, “at least partly” as used in (and similar expressions) “in each mixing loop pressurization of said fluid is provided at least partly by a pump” preferably refers to “at least partly” such as “substantially fully”. In preferred embodiments, where the pump is arranged upstream the thermal load, an inlet of the pump may be fluidicly connected to the outlet port of the 3- way valve. In preferred embodiments, where the pump is arranged downstream of the load, an inlet of the pump may be fluidicly connected to an outlet of the load. In this regards, fluidicly connected may comprise a direct fluid connection or a fluid connection comprising one or more components to be passed by the fluid. The controlling in each of said mixing loops is based on fluid-wise measurements in at least fluid flowing through two of said ports of said 3-way valve and comprising: • controlling said pump to at least partly compensate a recirculating pressure being a pressure difference between said second inlet port and said outlet port, and • controlling said 3-way valve to provide a controlled temperature of fluid flowing towards a thermal load, In preferred embodiments, “at least partly” as used in (and similar expressions) “controlling said pump to at least partly compensate a recirculating pressure” preferably refers to “at least partly” such as “substantially fully”. 80070PC01 5 The fluid wise measurements are provided by one or more sensor(s) preferably comprised in said pump and/or said 3-way valve. Terms used herein are used in manner being ordinary to a skilled person. Some the used terms are elucidated here below: Pressure difference as used herein is used to reference either a pressure difference obtained by measurement of pressures, e.g. by use of a pressure sensors, or a pressure difference determined based fluid wise measurements. Fluid wise measurement as used herein is used to reference measurement of a fluid property such as pressure, temperature, volume flow or the like, to determine the magnitude of the fluid property. A mixing loop as used herein is used to reference of fluid circuit in which a fluid flows and where a fraction of the fluid recirculates within the fluid circuit. The fraction may typically take values between 0% and 100%. The fraction is typically also referred to as a bypass. 3-way valve as used herein preferably refers to a device having three ports for fluid flow. In preferred embodiments, the 3-way valve is used so that it has two inlet ports (A, B) and one outlet port (AB). The inflows typically mix inside the 3- way valve resulting in that the outflow through the outlet port is a mix of the two inflows. The flow through the inlets may preferably be controllable restricted by gradually opening and closing the ports (e.g. by increasing of reducing a flow area), so that the amount of inflow through the two inlet ports are controllable. In preferred embodiments, the opening and closing of the ports are interlocked so that reducing flow area of one inlet port, automatically increases flow area of the other inlet port. Recirculating pressure as used herein is used to reference the pressure difference between the pressure at the AB port and the B port of the 3-way valve also referred herein to as ΔpB-AB. 80070PC01 6 Fluidicly connected as used herein may refer to a direct fluid connection or a fluid connection comprising one or more components to be passed by the fluid. The invention relates in a second aspect to a system of mixing loops comprising a number of mixing loops each comprising a thermal load and each being fluidicly connected in parallel to a fluid source, each mixing loop comprises: • a pump being a variable speed pump for pressurization of the fluid in the mixing loop, said pump being arranged upstream or downstream of said thermal load, • a 3-way valve comprising a first inlet port fluidicly connected to said fluid source to receive a supply of fluid, a second inlet port fluidicly connected to receive said bypass and an outlet port fluidicly connected to said thermal load to deliver said fluid to said thermal load, wherein the 3-way valve is electronic actuatable to provide an amount-wise controllable bypass of said flow of fluid is recirculated in the mixing loop, wherein • one or more temperature sensors, pressure sensors, e.g. to measure differential pressure over the outlet port and the second inlet port and/or flow sensors are embedded in the 3-way valve. BRIEF DESCRIPTION OF THE FIGURES The present invention and in particular preferred embodiments thereof will now be described in more detail with reference to the accompanying figures. The figures show ways of implementing the present invention and are not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set. Figure 1 schematically illustrates preferred embodiment of a system of mixing loops comprising three mixing loops fluidicly connected in parallel to a fluid source; Figure 2 schematically illustrates a mixing loop in two different preferred embodiments and a preferred pump characteristics (upper left corner); 80070PC01 7 Figure 3 schematically illustrates a first control configuration according to a preferred embodiment; Figure 4 schematically illustrates a preferred embodiment of a sensor configuration which may be used in connection with the control configuration according to Fig. 3; Figure 5 schematically illustrates a second control configuration according to a preferred embodiment; Figure 6 schematically illustrates a preferred embodiment of a sensor configuration which may be used in connection with the control configuration according to Fig. 5; Figure 7 schematically illustrates a third control configuration according to a preferred embodiment; Figure 8 schematically illustrates a preferred embodiment of a sensor configuration which may be used in connection with the control configuration according to Fig. 7; Figure 9 schematically illustrates a fourth control configuration according to a preferred embodiment; Figure 10 schematically illustrate a preferred embodiment of a sensor configuration which may be used in connection with the control configuration according to Fig. 9; Figures 11A and 11B illustrate experiments carried out with the embodiment disclosed in Figs. 3 and 4; and Figures 12A and 12B illustrate experiments carried out the embodiment disclosed in Figs. 5 and 6. 80070PC01 8 DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Reference is made to Fig. 1 schematically illustrating a system of mixing loops comprising three mixing loops fluidicly connected in parallel to a fluid source according to a preferred embodiment. It noted that the invention is not limited to three mixing loops as a system comprising two or more mixing loops are considered within the scope of the present invention. Fig. 2A schematically illustrates one of the mixing loops 1 of Fig. 1 and as presented herein preferred embodiments of the invention relates to a method of controlling a flow of fluid in a mixing loop of a system of mixing loops comprising a number of mixing loops. In many preferred embodiments, the fluid is water but other types of fluids may be used. Each of the mixing loops 1 comprises a thermal load 4, such as a radiator or other heating device. However, it is noted that the thermal load 4 may also be a cooling device. As illustrated in Fig. 1 each of the mixing loops 1 is fluidicly connected in parallel to a fluid source 5. In each mixing loop 1 pressurization of the fluid is provided at least partly by pump 2 being a variable speed pump. It noted that each mixing loop 1 comprising a pump and that a pump in one mixing loop 1 can be controlled independently of pumps in other mixing loops. In Figs. 1 and 2A and 2B, dotted lines mark return flow whereas non-dotted lines mark forward flow. As illustrated in Fig. 2A (also in Fig. 1), some of the return flow is bypassed and in the mixing loop by a 3-way valve 3 whereby the bypassed fluid is mixed into forward flow, hence the labelling “mixing loop”. It is to be emphasised that in case the three way valve is closed for the bypassed fluid, no mixing occurs. The purpose of a bypassing fluid is to obtain a typically, preselected temperature in the flow towards the load. Typically, the fluid flowing towards the load could be set to have a lower or higher temperature than the temperature of the fluid from source and by mixing bypass fluid into the fluid from the source, the temperature of the fluid towards the load may be controlled by varying the amount of fluid being bypasses. In cooling applications, the fluid flowing towards the load will have a lower temperature than the temperature of the bypass fluid. In heating applications, the fluid flowing towards the loads will have a higher temperature 80070PC01 9 than the temperature of the bypass fluid. The 3-way valve 3 typically comprises an actuator which upon receipt of an electric signal sets the opening characteristics of the valve. In Fig. 2A, the three-way valve 3 comprises a first inlet port A fluidicly connected to fluid source 5 to receive a supply of fluid, a second inlet port B fluidicly connected to receive said bypass and an outlet port AB fluidicly connected to an inlet side of said pump 2. In the embodiment shown in Fig. 2A, the pump 2 is arranged upstream of the load 4. While preferred embodiments of controlling are detailed with a mixing loop configuration as illustrated in Fig. 2A, a mixing loop may alternatively be configured as illustrated in Fig. 2B, while controlling the flow as detailed herein is unaltered. In the mixing loop illustrated in Fig. 2B the pump 2 is arranged downstream of the load 4. Also in this embodiment, the three-way valve comprises three ports, where a first inlet port A is fluidicly connected to fluid source 5 to receive a supply of fluid, a second inlet port B is fluidicly connected to receive said bypass and an outlet port AB fluidic is fluidicly connected to the load 4. For both of the configurations (Fig. 2A and Fig. 2B) the 3-way valve is controlled to provide a controlled temperature of the fluid flowing towards the thermal load 4, such as the temperature of the fluid flowing out of outlet port AB – also referred to herein as TAB. In the configuration shown in Fig. 2A, the temperature of the fluid flowing through the pump 2 is essentially the same as the temperature of the fluid flowing out of port AB, and the temperature TAB may, thus, be determined by a temperature sensor located in the pump 2. However, for the configuration shown in Fig. 2B the temperature of the fluid flowing through the pump 2 cannot be said be essentially the same as TAB since the load has either added or subtracted heat from the fluid. In such embodiment, a temperature sensor, preferably placed at or in the 3-way valve 3, is used to determine TAB. The fluid source 5 comprises a heat source which either cools or heats the fluid to provide the fluid from the fluid source 5 with a desired temperature. As disclosed 80070PC01 10 herein, the heat source may be a boiler, a cooler or other devices configured to supply or extract heat from the fluid. A main flow passage, is provided between port A and AB. Port B is fluidicly connected to the main passage in a manner so the flow into the main passage is controllable e.g. by a variable cross sectional opening. The controlling in each of said mixing loops is based on fluid-wise measurements in at least fluid flowing through two of said ports of said 3-way valve 3. As will become apparent from the following, fluid-wise measurement may be a measurement of a volume flow, temperature or pressure. An aim of preferred embodiments of method according to the invention is to mitigate or at least reduce a change in flow in one mixing loops impacting on flow in other mixing loops, or said in another way, a local control at one mixing loop mitigates changes in the conditions on the supply line due to for example changing conditions in other mixing loops received fluid from the same supply line. Or said in yet another way, preferred embodiments utilizes a local control at one mixing loop that mitigates changes in the conditions on the supply line due to for example changing conditions in other mixing loops. Preferred embodiments of the invention comprises: • controlling said pump 2 to at least partly compensate a recirculating pressure ΔpB-AB, being a pressure difference between the second inlet port B and the outlet port AB, and • controlling the 3-way valve 3 to provide a controlled temperature of fluid flowing towards a thermal load 4. The pressures involved are schematically illustrated in Fig. 2, which also includes the pressure rise Δpp over the pump 2. Ideally, the 3-way 3 valve would be designed such that a change in the valve setting would not lead to a change in ΔpB-AB . However, this requires that the pressure drop over the fluid source 5 is zero, which is not the case. Therefore, there is a need to compensate by use of the pump 2 to keep the same pressure/flow in the mixing loop independently of 80070PC01 11 changes at the fluid source 5. This also means that if the opening in the 3-way valve leading bypass into the main flow passage is changed, the pressure provided by the pump is changed accordingly to the changed flow situation. As will become apparent from the following description, the pressure ΔpB-AB may be determined directly, but may also be determined by other parameters such as volume flow and/or temperature measurement. One advantage of preferred embodiments is that the fluid-wise measurements on which the controlling is based, may be provided by one or more sensor(s) comprised in the pump 2 and/or said 3-way valve 3. By comprised here typically means that the sensors are embedded in the pump 2 and/or the 3-way valve 3. This reduces the installation time needed and complexity of the installation as the sensors are readily available when the pump 2 and 3-way valve 3 are installed. In preferred embodiments the load 4 is supplied by pressure provided by the pump 2. Such a type of supply is typical in regards with for example radiators, floor heating/cooling, or fan coil systems, where local thermostats control the local air temperatures. In such as well as in other systems, the pump 4 is typically operated in proportional pressure mode. That is, the pump 4 is controlled according to a proportional curve as the one shown in the left hand side of Fig. 2. The proportional pressure curve is a preferred manner to accommodate pressure losses in the piping between a supply pump, typically located at or in the fluid source 5 and in the load 4, and thereby ensure near constant pressure over the load 4. In Fig. 2, the proportional curve is given as: ^^⁽ ^^ _{^^ ^^} ⁾ = ^^₁ ∙ ^^ _{^^ ^^} + ^^₀ where ^^₁ and ^^ _^^ are constants ^^ ^^. (1) It is noted that the invention is not limited to such proportional pressure control. However, preferred embodiments disclosed based on proportional pressure control eases the disclosure. Thus, in some preferred embodiments, the method further comprising controlling the pump 2 to provide a predefined pressure over the thermal load 4. In such embodiments, the pump 2 is preferably controlled so as to provide a pressure increase over the pump ∆ ^^ _^^: 80070PC01 12 ∆ ^^ _^^ = ∆ ^^ _^^ + ∆ ^^ _{^^− ^^ ^^} where subscript p refers to pump and ^^ refers to load (2) as also indicated in Fig. 2. With reference to equation 1 above, ∆ ^^ _^^ may be evaluated as: ∆ ^^ _^^ = ∆ ^^ _^^ + ∆ ^^ _{^^− ^^ ^^} = ^^₁ ∙ ^^ _{^^ ^^} + ^^₀ + ∆ ^^ _{^^− ^^ ^^} (3) While the pump 2 may essentially be located at any position in the mixing loop 1 to provide the recirculation of fluid, the pump 4 is in preferred embodiments arranged so that a delivery side of the pump 2 is fluidicly connected to an inlet of the thermal load 4. In other preferred embodiments (see e.g. fig. 2B) the pump 2 is arranged with its inlet fluidicly connected to an outlet of the thermal load 4. In order to allow for an easy configuration of the mixing loop 1 in which an amount of fluid is bypassed into port B of the 3-way valve 3, one or more such as each mixing loop may comprise a flow bifurcation 10. Such flow bifurcation may have a bifurcation inlet 11 and a first and a second bifurcation outlet 12, 13. The bifurcation inlet 11 is fluidicly connected to the thermal load 4, the first bifurcation outlet 12 is fluid connected to the second inlet port B and the second bifurcation outlet 13 is fluidicly connected to the fluid source 5. Such a flow bifurcation may be implemented by a T-fitting and is preferably devised and installed to avoid or at least minimize pressure losses in the fluid flowing through the flow bifurcation 10. In embodiments, where the pump 2 is located downstream of the load 4, the bifurcation inlet 11 is still fluidicly connected to the thermal load 4, albeit through the pump 2. In preferred embodiments, the invention aims at providing a certain temperature or a certain temperature range of the fluid flowing into the thermal load 4. This temperature is typically mainly dependent on the temperature of the fluid from the source 5, the temperature of the bypass fluid and the magnitudes of volume flows, where the 3-way valves can be used to regulate the magnitude of the flow volume of the bypass fluid. In such embodiments, the 3-way valve 3 and pump 2 in combination are controlled to provide a pump pressure ΔpP compensating at 80070PC01 13 least said recirculating pressure ΔpB-AB and a pressure difference (ΔpL) over the thermal load 4, and provide a predefined temperature in a fluid at said outlet port AB. Also in such embodiments, the pump pressure compensating the pressure difference ΔpL over the thermal load may be determined on the basis of a proportional pressure relation, relating a pressure difference ΔpL over and a volume flow qf through said thermal load. In many preferred embodiments, the volume flow qf is identical to the volume flow qAB, however one or more drains or sources may be present in the mixing loop 1. If this is the case, this could be taken into consideration in the various considerations as to pressures occurring the mixing loop 1. As detailed above, the fluid-wise measurement may comprise measurement of other fluid property(ties) than pressure. However, in some preferred embodiments the recirculating pressure ΔpB-AB is provided by the fluid-wise measurements comprising measuring a bypass pressure in said bypass of fluid, preferably measured at the second inlet port B. This pressure is in preferred embodiments measured by a sensor comprised in the 3-way valve. In preferred embodiments, the sensor is a differential pressure sensor embedded in the 3-way valve and configured to measure directly the pressure difference ΔpB-AB. In embodiments involving where the fluid-wise measurement comprising measurement of pressure, the controlling may comprise the following steps: • Providing a value representing a volume flow qAB out of the outlet port (AB) and providing a value representing an estimated pressure difference ΔpL over said thermal load based on the volume flow qAB out of said outlet port AB. As detailed above, this may be provided by eq. (1) written as: • With the pressure over the load determined, the summed pressure difference ΔpL+ ΔpB-AB is evaluated by summing the recirculating pressure and the estimated pressure difference ΔpL over the thermal load, as: ∆ ^^ _^^ = ^^₁ ∙ ^^ _{^^ ^^} + ^^₀ + ∆ ^^ _{^^− ^^ ^^} 80070PC01 14 which is to be compared with an actual pressure Δpp provided by the pump 2. The purpose of this comparison is to investigate whether or not the pump is over- or under-compensating the recirculating pressure ΔpB-AB. • In order to compare the actual pressure provided by the pump 2 and the summed pressure difference, the method provides an actual pressure difference Δpp provided by said pump (2). This may be carried out in numerous ways either be a direct measurements, which often is output by modern pumps or estimated based on the actual RPM and/or power consumption of the pump 2. • Next, the method typically involves the step of adjusting the speed of the pump 2 on the basis of an evaluated difference, if any, between the actual pressure difference and said summed pressure difference. While the above disclosed eq.4 has proven to be a good way to estimate the pressure over the load, other arithmetic correlations may be used, such as depending on ^^ _^ ^{^} _^ ^{^} _^^ where ^^ may be an integer larger than zero even a decimal figure larger than zero. Further in some situations, such as for a floor heating system the pressure difference over the load – in a floor heating system the load is the pipes through which the fluid flow – the may be set to be constant, e.g. It is noted that the actual pressure provided by the pump 2 typically is readily available by modern pumps as they are equipped with sensors or other functionality allow e.g. read-out of the actual pressure provided. Such a functionality may also be used in the other embodiments disclosed herein involving the actual pressure. Fig. 3 is a schematically illustration of the preferred embodiment outlined above. In the illustration of Fig. 3, the preferred embodiment is implemented in manner involving PI controls (proportional integral controls), although the invention is not limited to use of such PI controls. As illustrated in Fig. 3, the volume flow qAB is derived from the mixing loop (indicated by a box encircling inter alia a 3-way valve, a pump and a bifurcation). This volume flow qAB is used to estimate the pressure over the load by eq. 4 80070PC01 15 above. The bypass pressure ΔpB-AB is added to the estimated pressure over the load 4 and the actual pressure Δp provided by the pump 2 is subtracted. Based on this, a control signal, u in Fig. 3, is generated by the PI control and sent to the pump 2. The control signal represents a new RPM setting for the pump 2, if any. Fig. 3 also illustrates a preferred embodiment of controlling the setting of the 3- way valve. By setting is typically referred to a setting regulating the amount of bypass volume flow qB through the valve. As illustrated, this preferred embodiment also uses a PI control, and comprises the following steps: • Providing a value representing a target temperature of fluid flowing out the outlet port AB • Providing a value representing an actual temperature TAB of fluid flowing out of the outlet port AB • Subtracting the actual temperature from the target temperature (or vice versa) and determining a control signal Xp (such as an opening degrees – detailed below) by the PI control representing a change in setting of the 3-way valve 3, if any. In case the difference between the actual and target temperature is negligible or even zero, the PI controller will, in general, not alter the setting of the 3-way valve 3. While Fig. 3 illustrates that the control of the pump 2 and the control of the 3-way valve 3 are carried out in parallel, the invention is not limited to a parallel implementation. The control of the pump 2 and the control of the 3-way valve may be carried out in series. Further, the controls may be carried out continuously or sequentially. In embodiments involving sequential control, the frequencies at which the control of the pump 2 is executed and at which the control of the 3-way valve 3 is executed may be different. Fig. 4 schematically illustrates a preferred embodiment of a sensor configuration which may be used in connection with the controlling according to Fig. 3 Reference is made to Fig. 5 and in particular Fig. 6 which schematically illustrates a second control configuration according to a preferred embodiment (Fig. 6 schematically illustrates a sensor configuration). In the illustrated embodiment of 80070PC01 16 Fig. 6, the recirculating pressure ΔpB-AB is provided by fluid-wise measurements comprising • measuring a bypass temperature TB in the bypass fluid, preferably measured at the second inlet port B, • measuring a supply temperature TA in the supply, preferably measured at the first inlet port A, • measuring a forward temperature TAB, in fluid flowing out of the outlet port AB, • measuring the volume flow out of the outlet port AB. Alternatively to measuring the volume flow out of port AB, the volume flow may be estimated based on the operation state of the pump 2, since the volume flow through the pump 2 will be the same as out through the port AB. It worth noting, that in such embodiments, there is no need to measure the recirculating pressure ΔpB-AB by pressure measurements as this pressure can be estimated based on the volume flow out of the outlet port AB, as will become apparent from the following. Hydraulic conductivities, Kv, of a 3-way valve may in many instances be found based an opening degrees xp of the valve. xp typically takes values between 0 and 1, where 0 is fully closed and 1 is fully open. Typically, this is expressed as a function f(x) so that the volume flow qB can be calculated by the following equation: (A similar expression may be formulated for qa) Based on this expression, the recirculating pressure Δ ^^ _{^^− ^^ ^^} can be estimated, in the following manner. A control volume around the 3-way valve and mass and power conservations are considered. The conservations are described by the following two expressions: ^^ _{^^ ^^} = ^^ _^^ + ^^ _^^ , ^^ _{^^ ^^} ^^ _{^^ ^^} = ^^ _^^ ^^ _^^ + ^^ _^^ ^^ _^^ Considering forward flows, ^^ _^^ ≥ 0, and ^^-port opening degrees ^^ _^^( ^^ _^^) ≥ ^^ > 0 and the conservations, the estimated pressure Δ ^^ _{^^− ^^ ^^} is given by 80070PC01 17 In practical implementation, the recirculating pressure can be estimated by: And, the desired pressure increase over the pump can be estimated by: The numerical value of ^^ is typically empirical determined to avoid division by zero in eq. 6. Non-limiting examples on numerical values of ^^ are 0.1, such as 0.05, or selected in the interval of 0.15-0.02. While the temperatures of eq. (6) is measured, the opening degrees xp is typically known as it is a control parameter according to which the opening of the 3-way valve is set. According, in some preferred embodiments, the recirculating pressure ΔpB-AB is provided based on a relation relating the recirculating pressure with the bypass temperature TB, the supply temperature TA, a forward temperature TAB, a volume flow qAB out through said outlet port AB and a hydraulic value for the 3-way valve 3, such as a hydraulic conductivity, Kv, but other value(s) may be used to provide an estimated flow volume based on opening characteristics of the 3-way valve and pressure. Reference is made to Fig. 5 which schematically illustrates a control configuration based on the above disclosed control method involving measurement of temperatures. The method of Fig. 5 is drawn similar to Fig. 3 and is implemented by use of PI controls. The box labelled GB refers to a numerical implementation of equation 6 above. A filtering may be applied to the signals to smoothen the 80070PC01 18 signals (input to the box Gb). Hence, in some embodiments, the box Gb may also include such filtering. This may also be applied to the embodiment of Fig. 7, box Gc, and to the embodiment of Fig. 9, box Gd. The preferred embodiment of Fig. 5 comprises: • Providing a value representing a volume flow qAB out of said outlet port AB and providing a value representing an estimated pressure difference ΔpL over the thermal load based on said volume flow qAB out of said outlet port AB. As detailed above, this may be provided by eq. (1) written as: • Determining a summed pressure difference ΔpL + ΔpB-AB by summing said provided recirculating pressure and said estimated pressure difference over said thermal load. As detailed above, the recirculating pressure ΔpB-AB is determined by equation 5 based on the temperature measurements. • Providing a value representing an actual pressure difference Δp provided by said pump 2 and subtracting the summed pressure and the actual pressure. • If the actual pressure difference Δp over- or under-compensate the summed pressure difference, the PI control provides a signal u adjusting the speed of the pump 2 on the basis of a difference (if any) between the actual pressure difference and the summed pressure difference. Fig. 5 also illustrates, as Fig. 3, a preferred embodiment of controlling the setting of the 3-way valve 3. The method applied as is disclosed in regards to Fig. 3. However, contrary to Fig. 3, the opening degrees xp is input to the numerical implementation of equation 5. Fig. 6 schematically illustrates a preferred embodiment of a sensor configuration which may be used in connection with the controlling according to Fig. 5 With reference to Figs. 7 and 8 a further preferred embodiment will be detailed. As illustrated in Fig. 8, a preferred sensor configuration comprises temperature sensors arranged to measure the temperature at the first inlet port A, at the second inlet port B and at the outlet port AB of the 3-way valve 3. A flow sensor is provided to measure the volume flow qA into the first inlet port A. 80070PC01 19 A similar reasoning leading to eq. (4) above leads to: ^² 2 Where the term ^^_{^ ^^} ^^ _^^( ^^ _^^) is a hydraulic value for the 3-way valve 3, which value 2 2 saturated such that independent on the opening degree ^^ _^^ the term ^^ _{^^ ^^} ^^ _^^( ^^ _^^) > ^^. In this embodiment, the volume flow ^^ _{^^ ^^} out of the outlet port and through the pump is determined. This flow can be estimated from volume flow qA into the first inlet port A, and the determined temperatures including mass and energy conservation, leading to the following equation for the volume flow ^^ _{^^ ^^}: The determined value of qAB is used to determine ∆ ^^ _^^ by equation (4) above. By this, the flow in the control loop may be controlled in the following manner, as depicted in Fig. 7: • measuring the supply temperature TA in the fluid flowing into the first inlet port A; • measuring the bypass temperature TB in the fluid flowing into the second inlet port B; • measuring the forward temperature TAB in the fluid flowing in the fluid flowing out of said outlet port (AB) • measuring the volume flow qA into the inlet port A. With these fluid-wise measurements carried out, the control is carried out as depicted in Fig. 8, that is • providing a value representing the recirculating pressure (ΔpB-AB) by use of eq. (8) above • estimating the pressure difference over the thermal load by use of eq. (4) (or other arithmetic correlation between volume flow qAB and load pressure) and eq. (9), 80070PC01 20 • determining a summed pressure difference, ΔpL+ ΔpB-AB, by summing said provided recirculating pressure and said estimated pressure difference over said thermal load, • providing a value representing an actual pressure difference Δp provided by the pump 2, and • adjusting the speed of said pump 2 on the basis of a difference, if any, between said actual pressure difference and said summed pressure difference. In Fig. 7, the function GC refers to eqs. (4), (8) and (9). The control of the forward temperature TAB follows the method outlined above in connection with Fig. 3. With reference to Figs. 9 and 10 a further preferred embodiment will be detailed. As illustrated in Fig. 10, a preferred sensor configuration comprises a temperature sensor at the outlet port AB of the 3-way valve 3 to measure the temperature TAB of the flow out of the outlet port AB. A flow sensor is provided to measure the volume flow qA into the first inlet port A and a sensor is provided to measure the volume flow qAB out of the outlet port AB. Is noted that due to mass conservation, the volume flow qAB is the same as the flow through the pump 2 and may be provided by a flow sensor in the pump 2 or the flow may be estimated based on actual RPM and/or power consumption of the pump 2. By conservation of mass, the volume flow qB can be determined based on qA and qAB the equation: as: Eq. (10) ^² 2 Also here ^^_{^ ^^} ^^ _^^( ^^ _^^) is referred to as a hydraulic value for the 3-way valve 3 as detailed above including the limitation 0 < ^^. With these fluid-wise measurements carried out, the control is carried out as depicted in Fig. 10, that is 80070PC01 21 • providing a value representing the recirculating pressure (ΔpB-AB) by use of eq. (10) above • estimating the pressure difference over the thermal load by use of eq. (4) (or other arithmetic correlation between volume flow qAB and load pressure) and qAB, • determining a summed pressure difference, ΔpL+ ΔpB-AB, by summing said provided recirculating pressure and said estimated pressure difference over said thermal load, • providing a value representing an actual pressure difference Δpp provided by the pump 2, and adjusting the speed of said pump 2 on the basis of a difference, if any, between said actual pressure difference and said summed pressure difference. In Fig. 10, the function GC refers to eqs. (4) and (10). The control of the forward temperature TAB follows the method outlined above in connection with Fig. 3. As presented above, the various embodiments utilizes sensors preferably embodied in the pump and/or in the 3-way valve. Such sensors may advantageously be provided during manufacturing of the pump and the 3-way valve, whereby implementation of an embodiment of the invention is made relatively simple, as instead of installing sensors and electrically connecting the sensors to e.g. a controller, the sensors may only need to be electrically connected to the controller. Non-limiting examples on advantageous sensor arrangements are listed in the following table: 80070PC01 22 Legends used in table: p: pressure sensor T: temperature sensor q: volume flow sensor n: rotational speed sensor (or output from pump) used to provide ∆ ^^ _^^ P: power, typical electrical power, consumed by the pump (the power consumption is correlated with the flow and can be used to estimate the flow through the pump) It is noted that the above table may be used as a reference to which sensors are applied and that it may from a production-wise point of view be most cost effective to produce e.g. with temperature sensors at both inlets and at the outlet and only use the relevant sensors. For instance, in embodiment D no sensors needs to be present in the pump to carry out the control according to this embodiment. Embodiments of the method of controlling typically involves an electronic processor. Such a controller is typically electronically connected to the sensors providing the fluid-wise measurements and is configured, by suitable software instructions, to carry out the processing of the fluid-wise measurement based on relations (such as the equations disclosed herein) to provide a control signal to 80070PC01 23 the pump at least partly compensating the recirculating pressure, and provide a control signal to an actuator of the 3-way valve to control the 3-way valve to provide a controlled temperature of fluid flowing towards the thermal load. As detailed in connection with the preferred embodiments, PI control controls are used and such PI controls may also be executed by the controller by suitable software instructions. The software instructions may be downloadable to the controller or may be firmware. The electronic processor may in preferred embodiments be or form part of a computer whereby preferred embodiments of the method of controlling are computer implemented. RESULTS In the following some experimental results are presented, which experimental results are carried out by the different embodiments disclosed herein. The configuration of the mixing loops are as illustrated in Fig. 2A. The experiments carried out are carried out in a system configuration as shown in Fig. 1, that is a system having three parallel connected mixing loops. Each of the mixing loops is referred to as branch, numbered arbitrarily from left to right by 1^st branch, 2^nd branch and 3^rd branch. Figs. 11A and 11B present experiments carried out with the embodiment of the control method disclosed in Fig. 3 and the sensor configuration disclosed in Fig. 4. Figs. 12A and 12B present experiments carried out with the embodiment of the control method disclosed in Fig. 5 and the sensor configuration disclosed in Fig. 6. Both experiments are examples on the coupling between branches in the system of Fig. 1. In Figs. 11A and 12A, a step down in valve operation is performed in 2^nd branch and in Figs. 11B and 12B a step up in valve operation is performed in the 2^nd branch. A step down means that the volume flow qAB is decreased and a step up means that the volume flow qAB is increased. The experiments clearly shows that the branch pressure (that is the pressure over the load ΔpL – given in head in meters) is kept constant and at the same level as before the changes are 80070PC01 24 introduced (see upper figures labelled “branch pressure”). The spike occurring in inter alia in the branch pressure is due to that the control methods needs some time to react on the change. Further, the experiments also illustrates: • “branch flow” being the volume flows ^^ _^ ¹ _{^ ^^} ; ^^ _^ ² _{^ ^^} and ^^ _^ ³ _{^ ^^} where superscript refers to a branch (mixing loop);“pump pressure” being ∆ ^^ _^^1; ∆ ^^ _^^2 and ∆ ^^ _^^3 where the integer of the subscript refers to branch (mixing loop); • “temp” being the temperature ^^ _^ ¹ _{^ ^^} ; ^^ _^ ² _{^ ^^} and ^^ _^ ³ _{^ ^^} where superscript refers to branch (mixing loop); • “Xp” being the opening degree of the 3-way valve.

80070PC01 25 ITEMIZED LIST OF PREFERRED EMBODIMENTS: Item 1. A method of controlling a flow of fluid in a mixing loop of a system of mixing loops comprising a number of mixing loops each comprising a thermal load (4) and each being fluidicly connected in parallel to a fluid source (5), in each mixing loop (1) pressurization of said fluid is provided at least partly by a pump (2) being a variable speed pump arranged either upstream or downstream of said thermal load (4), and a bypass of said flow of fluid is recirculated in the mixing loop by a 3-way valve (3) comprising a first inlet port (A) fluidicly connected to said fluid source (5) to receive a supply of fluid, a second inlet port (B) fluidicly connected to receive said bypass and an outlet port (AB) fluidicly connected to said thermal load (4) to deliver said fluid to said thermal load (4), wherein said controlling in each of said mixing loops is based on fluid-wise measurements in at least fluid flowing through two of said ports of said 3-way valve (3) and comprising: • controlling said pump (2) to at least partly compensate a recirculating pressure (ΔpB-AB) being a pressure difference between said second inlet port (B) and said outlet port (AB), and • controlling said 3-way valve (3) to provide a controlled temperature of said fluid flowing towards a thermal load (4), wherein said fluid wise measurements are provided by one or more sensor(s) comprised in said pump (2) and/or said 3-way valve (3). Item 2. A method according to item 1, wherein said controlling further comprising controlling said pump (2) to provide a predefined pressure over said thermal load (4). Item 3. A method according to Item 1 or 2, wherein a delivery side of said pump (2) is fluidicly connected to an inlet of said thermal load (4) or an inlet of said pump (2) is fluidicly connected to an outlet of said thermal load (4). Item 4. A method according to any of the preceding items 1-3, wherein each mixing loop comprising a flow bifurcation (10) having a bifurcation inlet (11) and a first and a second bifurcation outlet (12, 13), where said bifurcation inlet (11) is fluidicly connected to said thermal load (4), the first bifurcation outlet (12) is fluid 80070PC01 26 connected to said second inlet port (B) and the second bifurcation outlet (13) is fluidicly connected to said fluid source (5). Item 5. A method according to any one of the preceding items, wherein said 3- way valve (3) and said pump (2) in combination are controlled to provide a pump pressure (Δp) compensating at least said recirculating pressure (ΔpB-AB) and a pressure difference (ΔpL) over said thermal load (4), and provide a predefined temperature in a fluid at said outlet port (AB). Item 6. A method according to item 5, wherein the pump pressure compensating the pressure difference (ΔpL) over said thermal load is determined on the basis of a proportional pressure relation, relating a pressure difference (ΔpL) over and a volume flow (qf) through said thermal load. Item 7. A method according to any one of the preceding items, wherein said recirculating pressure (ΔpB-AB) is provided by said fluid-wise measurements comprising measuring a bypass pressure in said bypass of fluid, preferably measured at said second inlet port (B) and measuring an outflow pressure in fluid flowing out of said outlet port (AB), preferably measured at said outlet port (AB), such as said fluid-wise measurements comprising measuring a differential pressure between said outlet port (AB) and said second inlet port (B). Item 8. A method according to item 7, wherein the controlling comprising • providing a value representative of a volume flow (qAB) out of said outlet port (AB) and providing an estimated pressure difference (ΔpL) over said thermal load based on said volume flow (qAB) out of said outlet port (AB), • determining a summed pressure difference (Δp) by summing said provided recirculating pressure (ΔpB-AB) and said estimated pressure difference (ΔpL) over said thermal load, • providing a value representing an actual pressure difference (Δpp) provided by said pump (2), and • adjusting the speed of said pump (2) on the basis of a difference, if any, between said actual pressure difference and said summed pressure difference. 80070PC01 27 Item 9. A method according to any one of the preceding items, wherein said recirculating pressure (ΔpB-AB) is provided by said fluid-wise measurements comprising measuring a bypass temperature (TB) in said bypass of fluid, preferably measured at the second inlet port (B), measuring a supply temperature (TA) in said supply, preferably measured at said first inlet port (A), measuring a forward temperature (TAB), in fluid flowing out of said outlet port (AB), and measuring or estimating the volume flow out of said outlet port (AB), or measuring or estimating the volume flow into said first inlet port (A). Item 10. A method according to item 9, wherein said recirculating pressure (ΔpB- AB) is provided based on a relation relating said recirculating pressure with said bypass temperature (TB), said supply temperature (TA), said forward temperature (TAB), a volume flow (qAB) out through said outlet port (AB) and a hydraulic value of said 3-way valve (3). Item 11. A method according to item 10, wherein the controlling comprising • providing a value representing a volume flow (qAB) out of said outlet port (AB) and providing a value representing an estimated pressure difference (ΔpL) over said thermal load based on said volume flow (qAB) out of said outlet port (AB), • determining a summed pressure difference (ΔpL+ ΔpB-AB) by summing said provided recirculating pressure and said estimated pressure difference over said thermal load, • providing a value representing an actual pressure difference (Δpp) provided by said pump (2), and • adjusting the speed of said pump (2) on the basis of a difference, if any, between said actual pressure difference and said summed pressure difference. Item 12. A method according to item 9, wherein said measuring of volume flow is measuring the volume flow into said first inlet port (A), and wherein said recirculating pressure (ΔpB-AB) is provided based on a relation relating said recirculating pressure with said bypass temperature (TB), said supply temperature (TA), said forward temperature (TAB), said volume flow (qA) into said first inlet port out through said outlet port (AB) and a hydraulic value of said 3-way valve (3). Item 13. A method according to item 12, wherein the controlling comprising 80070PC01 28 • providing a value representing the recirculating pressure (ΔpB-AB) based on said determined temperatures and said hydraulic value; • providing a value representing the pressure difference (ΔpL) over the thermal load based on said determined temperatures and said determined volume flow (qA) into said first inlet port (A), • determining a summed pressure difference (ΔpL+ ΔpB-AB) by summing said provided recirculating pressure and said estimated pressure difference over said thermal load, • providing a value representing an actual pressure difference (Δpp) provided by the pump (2), and • adjusting the speed of said pump (2) on the basis of a difference, if any, between said actual pressure difference and said summed pressure difference. Item 14. A method according to any one of the preceding items 1-8, wherein said recirculating pressure (ΔpB-AB) is provided by said fluid-wise measurements comprising measuring a volume flow (qAB) out of said outlet port (AB), a volume flow into said first inlet port (A) or a volume flow (qB) into said second inlet port (B). Item 15. A method according to item 14, wherein said recirculating pressure (ΔpB- AB) is provided based on a relation relating said recirculating pressure with said volume flow (qAB) out of said outlet port (AB) and said volume flow into said first inlet port (A) or said volume flow (qB) into said second inlet port (B) a hydraulic value of said 3-way valve (3). Item 16. A method according to item 14 or 15, wherein the controlling comprising • providing a value representing the recirculating pressure (ΔpB-AB) based on said determined volume flows and said hydraulic value; • providing a value representing the pressure difference (ΔpL) over the thermal load based on said determined volume flow(qAB) out of said outlet port (AB); • determining a summed pressure difference (ΔpL+ ΔpB-AB) by summing said provided recirculating pressure and said estimated pressure difference over said thermal load, • providing a value representing an actual pressure difference (Δp) provided by the pump (2), and 80070PC01 29 • adjusting the speed of said pump (2) on the basis of a difference, if any, between said actual pressure difference and said summed pressure difference. Item 17. A method according to any one of the preceding items, wherein the controlling further comprising: • providing a value representing an outflow temperature (TAB) of fluid flowing out of said outlet port (AB) and adjusting the amount of bypass, by use of said 3-way valve (3), if said outflow temperature differs from preselected outflow temperature, such as differs more than 2.0^OC, preferably more than 5.0^OC. Item 18. A system of mixing loops comprising a number of mixing loops each comprising a thermal load (4) and each being fluidicly connected in parallel to a fluid source (5), each mixing loop (1) comprises: • a pump (2) being a variable speed pump for pressurization of the fluid in the mixing loop, said pump (2) being arranged upstream or downstream of said thermal load (4), • a 3-way valve (3) comprising a first inlet port (A) fluidicly connected to said fluid source (5) to receive a supply of fluid, a second inlet port (B) fluidicly connected to receive said bypass and an outlet port (AB) fluidicly connected to said thermal load (4) to deliver said fluid to said thermal load (4), wherein the 3-way valve (3) is electronic actuatable to provide an amount-wise controllable bypass of said flow of fluid is recirculated in the mixing loop, wherein • one or more temperature sensors, pressure sensors, e.g. to measure differential pressure over the outlet port (AB) and the second inlet port (B) and/or flow sensors are embedded in the 3-way valve. Item 19. A system according to item 18, said system being configured by use of an electronic control unit to carry out the method according to any one of item 1- 17. Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms “comprising” or 80070PC01 30 “comprises” do not exclude other possible elements or steps. Also, the mentioning of references such as “a” or “an” etc. should not be construed as excluding a plurality.. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

80070PC01 31 List of reference numerals used: 1 Mixing loop 2 Pump 3 3-way valve 4 Thermal load 5 Fluid source 10 Bifurcation 11 Bifurcation inlet 12 First bifurcation outlet 13 Second bifurcation outlet A First inlet port B Second inlet port AB Outlet port TA Temperature of supply of fluid TB Temperature bypass of fluid ΔpL Pressure difference over thermal load qf Volume flow through thermal load Δp Pump pressure

Claims

80070PC01 32 CLAIMS 1. A method of controlling a flow of fluid in a mixing loop of a system of mixing loops comprising a number of mixing loops each comprising a thermal load (4) and each being fluidicly connected in parallel to a fluid source (5), in each mixing loop (1) pressurization of said fluid is provided at least partly by a pump (2) being a variable speed pump arranged either upstream or downstream of said thermal load (4), and a bypass of said flow of fluid is recirculated in the mixing loop by a 3-way valve (3) comprising a first inlet port (A) fluidicly connected to said fluid source (5) to receive a supply of fluid, a second inlet port (B) fluidicly connected to receive said bypass and an outlet port (AB) fluidicly connected to said thermal load (4) to deliver said fluid to said thermal load (4), wherein said controlling in each of said mixing loops is based on fluid-wise measurements in at least fluid flowing through two of said ports of said 3-way valve (3) and comprising: • controlling said pump (2) to at least partly compensate a recirculating pressure (ΔpB-AB) being a pressure difference between said second inlet port (B) and said outlet port (AB), and • controlling said 3-way valve (3) to provide a controlled temperature of said fluid flowing towards a thermal load (4), wherein said fluid wise measurements are provided by one or more sensor(s) comprised in said pump (2) and/or said 3-way valve (3). 2. A method according to claim 1, wherein said controlling further comprising controlling said pump (2) to provide a predefined pressure over said thermal load (4). 3. A method according to claim 1 or 2, wherein a delivery side of said pump (2) is fluidicly connected to an inlet of said thermal load (4) or an inlet of said pump (2) is fluidicly connected to an outlet of said thermal load (4). 4. A method according to any of the preceding claims, wherein each mixing loop comprising a flow bifurcation (10) having a bifurcation inlet (11) and a first and a second bifurcation outlet (12, 13), where said bifurcation inlet (11) is fluidicly connected to said thermal load (4), the first bifurcation outlet (12) is fluid 80070PC01 33 connected to said second inlet port (B) and the second bifurcation outlet (13) is fluidicly connected to said fluid source (5). 5. A method according to any one of the preceding claims, wherein said 3-way valve (3) and said pump (2) in combination are controlled to provide a pump pressure (Δp) compensating at least said recirculating pressure (ΔpB-AB) and a pressure difference (ΔpL) over said thermal load (4), and provide a predefined temperature in a fluid at said outlet port (AB). 6. A method according to claim 5, wherein the pump pressure compensating the pressure difference (ΔpL) over said thermal load is determined on the basis of a proportional pressure relation, relating a pressure difference (ΔpL) over and a volume flow (qf) through said thermal load. 7. A method according to any one of the preceding claims, wherein said recirculating pressure (ΔpB-AB) is provided by said fluid-wise measurements comprising measuring a bypass pressure in said bypass of fluid, preferably measured at said second inlet port (B) and measuring an outflow pressure in fluid flowing out of said outlet port (AB), preferably measured at said outlet port (AB), such as said fluid-wise measurements comprising measuring a differential pressure between said outlet port (AB) and said second inlet port (B). 8. A method according to claim 7, wherein the controlling comprising • providing a value representative of a volume flow (qAB) out of said outlet port (AB) and providing an estimated pressure difference (ΔpL) over said thermal load based on said volume flow (qAB) out of said outlet port (AB), • determining a summed pressure difference (Δp) by summing said provided recirculating pressure (ΔpB-AB) and said estimated pressure difference (ΔpL) over said thermal load, • providing a value representing an actual pressure difference (Δpp) provided by said pump (2), and • adjusting the speed of said pump (2) on the basis of a difference, if any, between said actual pressure difference and said summed pressure difference. 80070PC01 34 9. A method according to any one of the preceding claims, wherein said recirculating pressure (ΔpB-AB) is provided by said fluid-wise measurements comprising measuring a bypass temperature (TB) in said bypass of fluid, preferably measured at the second inlet port (B), measuring a supply temperature (TA) in said supply, preferably measured at said first inlet port (A), measuring a forward temperature (TAB), in fluid flowing out of said outlet port (AB), and measuring or estimating the volume flow out of said outlet port (AB), or measuring or estimating the volume flow into said first inlet port (A). 10. A method according to claim 9, wherein said recirculating pressure (ΔpB-AB) is provided based on a relation relating said recirculating pressure with said bypass temperature (TB), said supply temperature (TA), said forward temperature (TAB), a volume flow (qAB) out through said outlet port (AB) and a hydraulic value of said 3-way valve (3). 11. A method according to claim 10, wherein the controlling comprising • providing a value representing a volume flow (qAB) out of said outlet port (AB) and providing a value representing an estimated pressure difference (ΔpL) over said thermal load based on said volume flow (qAB) out of said outlet port (AB), • determining a summed pressure difference (ΔpL+ ΔpB-AB) by summing said provided recirculating pressure and said estimated pressure difference over said thermal load, • providing a value representing an actual pressure difference (Δpp) provided by said pump (2), and • adjusting the speed of said pump (2) on the basis of a difference, if any, between said actual pressure difference and said summed pressure difference. 12. A method according to claim 9, wherein said measuring of volume flow is measuring the volume flow into said first inlet port (A), and wherein said recirculating pressure (ΔpB-AB) is provided based on a relation relating said recirculating pressure with said bypass temperature (TB), said supply temperature (TA), said forward temperature (TAB), said volume flow (qA) into said first inlet port out through said outlet port (AB) and a hydraulic value of said 3-way valve (3). 13. A method according to claim 12, wherein the controlling comprising 80070PC01 35 • providing a value representing the recirculating pressure (ΔpB-AB) based on said determined temperatures and said hydraulic value; • providing a value representing the pressure difference (ΔpL) over the thermal load based on said determined temperatures and said determined volume flow (qA) into said first inlet port (A), • determining a summed pressure difference (ΔpL+ ΔpB-AB) by summing said provided recirculating pressure and said estimated pressure difference over said thermal load, • providing a value representing an actual pressure difference (Δpp) provided by the pump (2), and • adjusting the speed of said pump (2) on the basis of a difference, if any, between said actual pressure difference and said summed pressure difference. 14. A method according to any one of the preceding claims 1-8, wherein said recirculating pressure (ΔpB-AB) is provided by said fluid-wise measurements comprising measuring a volume flow (qAB) out of said outlet port (AB), a volume flow into said first inlet port (A) or a volume flow (qB) into said second inlet port (B). 15. A method according to claim 14, wherein said recirculating pressure (ΔpB-AB) is provided based on a relation relating said recirculating pressure with said volume flow (qAB) out of said outlet port (AB) and said volume flow into said first inlet port (A) or said volume flow (qB) into said second inlet port (B) a hydraulic value of said 3-way valve (3). 16. A method according to claim 14 or 15, wherein the controlling comprising • providing a value representing the recirculating pressure (ΔpB-AB) based on said determined volume flows and said hydraulic value; • providing a value representing the pressure difference (ΔpL) over the thermal load based on said determined volume flow(qAB) out of said outlet port (AB); • determining a summed pressure difference (ΔpL+ ΔpB-AB) by summing said provided recirculating pressure and said estimated pressure difference over said thermal load, • providing a value representing an actual pressure difference (Δp) provided by the pump (2), and 80070PC01 36 • adjusting the speed of said pump (2) on the basis of a difference, if any, between said actual pressure difference and said summed pressure difference. 17. A method according to any one of the preceding claims, wherein the controlling further comprising: • providing a value representing an outflow temperature (TAB) of fluid flowing out of said outlet port (AB) and adjusting the amount of bypass, by use of said 3-way valve (3), if said outflow temperature differs from preselected outflow temperature, such as differs more than 2.0^OC, preferably more than 5.0^OC. 18. A system of mixing loops comprising a number of mixing loops each comprising a thermal load (4) and each being fluidicly connected in parallel to a fluid source (5), each mixing loop (1) comprises: • a pump (2) being a variable speed pump for pressurization of the fluid in the mixing loop, said pump (2) being arranged upstream or downstream of said thermal load (4), • a 3-way valve (3) comprising a first inlet port (A) fluidicly connected to said fluid source (5) to receive a supply of fluid, a second inlet port (B) fluidicly connected to receive said bypass and an outlet port (AB) fluidicly connected to said thermal load (4) to deliver said fluid to said thermal load (4), wherein the 3-way valve (3) is electronic actuatable to provide an amount-wise controllable bypass of said flow of fluid is recirculated in the mixing loop, wherein • one or more temperature sensors, pressure sensors, e.g. to measure differential pressure over the outlet port (AB) and the second inlet port (B) and/or flow sensors are embedded in the 3-way valve. 19. A system according to claim 18, said system being configured by use of an electronic control unit to carry out the method according to any one of claims 1- 17.