Control of refrigerator with multiple evaporators

Abstract

Described is a refrigerator (10) comprising a freezer compartment (16) and a fresh food compartment (12). The refrigerator has a cooling system (30) comprising a variable speed compressor (32), and a condenser (34), and at least a first freezer evaporator (37) and a second fresh food evaporator (38).The freezer evaporator (37) and a second fresh food evaporator (38) are connected in series such that the outlet from the freezer evaporator (37) is connected to the inlet of the fresh food evaporator (38). A controller (50) is configured to control the cooling system (30) by switching the variable speed compressor (32) in cycles between an ON phase and an OFF phase, wherein the controller (50) is configured to during the ON phase control the variable speed compressor (32) to a high-speed state followed by a low-speed state where in the low-speed state the variable speed compressor (32) has a lower speed than in the high-speed state.

Description

[0001] Control of refrigerator with multiple evaporators

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to a refrigerator with at least two evaporators. In particular the present disclosure relates to a refrigerator comprising two evaporator in series connection and a variable-speed compressor.

[0004] BACKGROUND

[0005] Compressor cooling systems are typically used in today’s household refrigerators. Further, many of these cooling systems do not run continuously, but turn on and off in cycles. There is then a compressor on-phase followed by a compressor off-phase. This can also be employed for variable speed compressors.

[0006] DE 102017003523 describes a refrigerator with an evaporator including serial evaporator segments and a variable speed compressor. The compressor is controlled based on cooling requirement to run with varying ON time and / or varying speed.

[0007] There is a constant desire to improve the performance in a refrigerant system and to provide more efficient refrigeration system. Hence, there is a need for an improved refrigerator apparatus and to a cooling system used in a refrigerator for multiple evaporator cooling system.

[0008] SUMMARY

[0009] It is an object of the present invention to provide an improved refrigerator having at least two evaporators.

[0010] This object and / or others are obtained by the refrigerator as set out in the appended claims. As has been realized by the inventors, it is possible to provide an efficient control of a refrigerator with two compartments without using fans and / or an electro-valve. Instead, independent control of the temperatures of the Freezer and Fresh-food compartments of a refrigerator with two evaporators in series can be achieved by providing a control scheme for a variable-speed compressor.

[0011] In accordance with the invention is a refrigerator comprising a freezer compartment and a fresh food compartment is provided. The refrigerator has a cooling system comprising a variable speed compressor, and a condenser, and at least a first freezer evaporator and a second fresh food evaporator. The freezer evaporator and a second fresh food evaporator are connected in series such that the outlet from the freezer evaporator is connected to the inlet of the fresh food evaporator. A controller is configured to control the cooling system by switching the variable speed compressor in cycles between an ON phase and an OFF phase. The controller is configured to during the ON phase control the variable speed compressor to a high-speed state followed by a low-speed state where in the low-speed state the variable speed compressor has a lower speed than in the high-speed state. Hereby the cooling between the two compartment can be distributed to the respective refrigerator compartments without the use of electro valves or fans so that the complexity and cost for the cooling control can be reduced.

[0012] In accordance with one embodiment, the duration of each cycle with an ON Phase and an OFF phase is constant. Hereby an efficient implementation can be obtained.

[0013] In accordance with one embodiment, wherein the speed of the variable speed compressor is constant pre-determined speed in the high-speed state and the low-speed state, respectively. Hereby a simple and robust implementation can be obtained. In accordance with one embodiment, the controller is configured to offset the variable speed compressor speed from the constant speed in the high-speed state based on a trigger event. Hereby additional control possibilities can be provided.

[0014] In accordance with one embodiment, a fan is provided in at least one of the freezer compartment and the fresh food compartment and wherein the controller is configured to control the speed of the fan(s). Hereby improved control can be obtained when the refrigerator is provided with fans.

[0015] In accordance with one embodiment, the controller (50) is configured to control the speed of the fan(s) based on a weighted linear combination of the temperature errors of the fresh food compartment and the freezer compartment. Hereby an efficient control of the fan(s) can be obtained.

[0016] In accordance with one embodiment, the controller is configured to compute a cooling capacity set point for the compressor based on a weighted combination of the temperature errors of the fresh food compartment and the freezer compartment. Hereby an efficient control of the cooling capacity delivered by the compressor for the refrigerator can be obtained.

[0017] In accordance with one embodiment, the controller is configured to compute the relative duration of the high-speed and low speed phases based on a weighted combination of the temperature errors of the fresh food compartment and the freezer compartment. Hereby, the durations of the high-speed and low speed phases during the compressor ON time can be determined in an efficient manner. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The present invention will now be described in more detail by way of non-limiting examples and with reference to the accompanying drawings, in which:

[0019] - Fig. 1 illustrates a refrigerator,

[0020] - Fig. 2 illustrates a cooling system for a refrigerator,

[0021] - Fig. 3 illustrates cooling cycles for a variable speed compressor,

[0022] - Figs. 4 and 5 illustrate different cooling scenarios,

[0023] - Fig. 6 is a flow chart illustrating some steps for control of a variable speed compressor,

[0024] - Figs. 7 and 8 illustrate computation of different parameters,

[0025] - Figs. 9 and 10 illustrate computation of fan speeds, and

[0026] - Fig. 11 illustrates a controller.

[0027] DETAILED DESCRIPTION

[0028] The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. For example, like or similar components of different embodiments can be exchanged between different embodiments. Some components can be omitted from different embodiments. Like numbers refer to like elements throughout the description.

[0029] In Fig. 1, a typical household refrigerator 10 comprising a Fresh Food (FF) compartment 12 and a Freezer (FZ) compartment 16. The exemplary refrigerator 10 in Fig. 1 is of the type known as side by side. However, the cooling system as described herein can be employed in any type of refrigerator such as a French door type, top freezer type, bottom freezer type, or any other type of refrigerator. A door 14, shown in FIG. 1 as open, is mounted to the refrigerator body by hinges and serves to close the front of the fresh food compartment 12 as well as provide access to the interior of the fresh food compartment. A door 18, shown in Fig. 1 as open, also is mounted to the refrigerator body by hinges and serves to close the front of the freezer compartment 16 as well as provide access to the interior of the freezer compartment. The fresh food and freezer compartments can include a variety of shelves 20, closed drawers 22 and basket-like drawers 24 for storing articles of food and the like. Also, other configurations of the household refrigerator 10 can be envisaged depending on the type of refrigerator and to meet different needs. To cool the refrigerator a cooling system 30 is provided. The cooling system 30 can typically be a multiple evaporator cooling system. The evaporators can be arranged to cool different compartments. For example, one evaporator can cool a refrigerator compartment and another evaporator can cool a freezer compartment. Additional evaporators can be provided to cool other compartments.

[0030] An exemplary cooling system 30 will now be described in more detail in conjunction with Fig. 2. In Fig. 2 a cooling system 30 is illustrated. The cooling system 30 has a variable speed compressor (VSC) 32. The variable speed compressor drives a flow of refrigerant in a cooling circuit formed by a condenser 34 followed downstream by an expansion valve 36 of some kind such as a capillary tube. In the configuration illustrated in Fig. 2 the expansion valve 36 is followed downstream by two evaporators 37 and 38 arranged in series. The first evaporator 37 can be an evaporator arranged to cool a freezer compartment 16. The second evaporator 38 can be arranged to cool a fresh food compartment 12. In accordance with some embodiments, fans 43, 44 can be located in the freezer compartment 16 or the fresh food compartment 12 or in both compartments 12, 16. Further, air temperature sensors 41, 42 can be arranged in the freezer compartment 16 and in fresh food compartment 12. The cooling arrangement can be controlled by a controller 50 that can be operatively connected to different sensors and to the compressor 32 for controlling the speed of the compressor 32. As has been realized, the temperature balancing and the independent temperature control of the two compartments of a refrigerator with evaporators in series such as the configuration shown in Fig. 2 can be difficult especially in absence of an electrovalve able to divert the refrigerant flow either to the fresh food evaporator 38 or to the freezer evaporator 37 or to exclude one of the two evaporators from the circuit. One way to improve the performance could be to use evaporator fans to promote the heat exchange in the single compartment, allowing for a certain degree of controllability. However, in absence of an electrovalve and fans, it is difficult to achieve independent control of the two compartments. In order to balance the cooling capacity between the two compartments, typically, design compromises need to be used to enable proper operation, such as having a freezer colder than necessary, in this way reducing the efficiency of the system. In other cases, a heater could be located in the Fresh Food compartment (also called balancing heater) switched-on when necessary to keep the Fresh-food compartment warmer and avoid icing, while permitting to increase the compressor speed to cool down the freezer. As is clear, such methods are not energy efficient.

[0031] To provide a more efficient control of series connected evaporators where the outlet of a first evaporator is connected to the inlet of a second evaporator control of a variable-speed compressor that allows a better independent operation of the two evaporators for different cooled compartments can be provided. The control can be aiming at permitting some degree of independency in the temperature control of the two compartments 12, 16. The control can be used without evaporator fans in the Freezer compartment and Fresh Food compartment, but it is also possible to provide control of the variable speed compressor with evaporator fans.

[0032] To achieve an improved control the variable speed compressor can be operated cyclically alternating in ON-OFF phases. The length (time duration) of the on-phase can be varied during operation in response to varying operating conditions, typically compartment air temperature feedbacks and user settings. Typically, the more the compressor is in an ON phase, the more cooling capacity can be delivered to the refrigerator. At each cycle, during the compressor ON-time, the compressor speed is applied according to a speed pattern having two phases of different speed and time-duration, with the highspeed phase followed by a low-speed phase; in that order. The relative duration of the two phases is determined such as to distribute the cooling capacity between the compartments, and can be varied during operation as a consequence of varying operating conditions such as compartment air temperature feedbacks and user settings.

[0033] The overall cooling capacity delivered by the compressor in one cycle summing up the cooling delivered to two compartments is represented by the ON time for the compressor and the speed of the compressor during the ON time. This can bee seen as the area below the speed pattern curves shown in Figs. 3 - 5.

[0034] This control is based on that at the start-up of the compressor, when the compressor is operated at the highest speed (high-speed phase), the cold refrigerant reaches first the Freezer evaporator, while the Fresh-food evaporator, due to transport delay, is still crossed by relatively warmer refrigerant previously standing in the Freezer evaporator. In the subsequent low-speed phase, the lower compressor throughput makes the freezer evaporator to be filled with refrigerant which is warmer than in the previous phase. Therefore, to have more cooling capacity toward the Freezer compartment rather than the Fresh Food compartment, the relative length (time-duration) of the high-speed phase can be made longer than that of the low-speed phase. Conversely, to decrease the cooling capacity in the freezer the duration of the high-speed phase can be made shorter than that of the low-speed phase.

[0035] Fig. 3 shows the cyclic operation of the compressor with a two-speed pattern in accordance with the above in an exemplary embodiment. The time-duration of the cycles can typically be a constant parameter named CYCLE_LENGTH. Also, the values of high and low compressor speeds (named HIGH SPEED and LOW SPEED) can be set constant in normal operation, with LOW_SPEED<= HIGH_SPEED<=MAX_SPEED, where MAX SPEED is an upper bound for the compressor speed determined by datasheet or by noise-limiting consideration or some other limitation. The value of the LOW SPEED constant parameter is determined based on datasheet considerations or can be set higher to the minimum operating speed for the compressor, in order to allow for ON-OFF operation in all operating conditions.

[0036] During operation, the HIGH SPEED can be offset from its constant predetermined value. This can happen for example when user-selectable functionalities are triggered. For example, in a Superfreezing mode, the HIGH SPEED can be set to the Maximum speed for the compressor. The high speed can also be offset based on other trigger events. The area below the speed curve in one cycle corresponds to the overall cooling capacity delivered by the compressor in the two compartments. Thus, the total cooling is

[0037] K_TOT = HIGH_SPEED*HS_LENGTH+LOW_SPEED*LS_LENGTH

[0038] Where K TOT is the total cooling, HIGH SPEED is the compressor cooling power in high speed, HS LENGTH is the duration of the high-speed phase, LOW SPEED is the cooling power in the low-speed phase and LS_LENGTH is then duration of the low-speed phase.

[0039] Also, the normalized overall cooling capacity as

[0040] Q_TOT = K_TOT / (MAX_SPEED* CYCLE_LENGTH)

[0041] Where MAX_SPEED is the compressor cooling power at maximum speed and CYCLE LENGTH is the total length of the compressor cycle.

[0042] In other words, this is the ratio between the delivered cooling capacity in the cycle and the maximum cooling capacity deliverable by the compressor when running continuously at the maximum speed. Further, the normalized relative duration of the low-speed phase versus the high-speed phase can be defined as: Q_REL = LS_LENGTH / ( HS_LENGTH +LS_LENGTH)

[0043] Where LS LENGTH and HS LENGTH are the time durations of the low speed and the high-speed phase, respectively.

[0044] In Figs. 4 and 5 two situations in which the overall cooling capacity K_TOT is the same, but with a different allocation of cooling capacity between the Freezer compartment and the Fresh Food compartment. It should be noted that the area below the speed curves in one period is the same.

[0045] In the scenario depicted in Fig. 4, the larger relative length (time duration) of the low-speed phase corresponds to a much higher delivery of cooling capacity toward the Fresh-food compartment. Conversely, the scenario depicted in Fig. 5 represents a situation in which the cooling capacity is allocated more to the Freezer compartment.

[0046] The control algorithm is implemented in the appliance controller 50 (microprocessor), is enabled to collect sensor feedbacks, receive user setpoints for the compartments, and to command the speed of the compressor and the fans, if present.

[0047] In Fig. 6 a flow chart illustrating some steps by a controller 50 that can be performed when controlling the refrigerator cooling. First in a step 601, the relative length of the low and high-speed phases Q REL, are determined. This can be performed based on a feedback regulator that uses the weighted difference of the temperature-errors in the two compartments, with possibly time-varying weights W_REL_FZ for the freezer compartment and W REL FF for the Fresh Food compartment.

[0048] Next, in a step 603, the normalized overall cooling capacity request Q TOT is determined.

[0049] This can be done based on a feedback regulator that uses the weighted sum of the temperature-errors in the two compartments, using possibly time-varying weights W TOT FZ for the Freezer compartment and W TOT FF for the Fresh Food compartment.

[0050] Next, in a step 605, the duration of the compressor-ON time in each cycle is determined. Also, the duration of the high and low speed phases HS LENGTH and LS LENGTH are determined. The duration of the high and low speed phases can be determined from the values of Q TOT and Q REL computed in the previous steps.

[0051] Finally in a step 607, the compressor is operated in accordance with the determined parameter values for the duration of the high and low speed phases.

[0052] The regulators used in steps 601, and 603 can be of PI or PID type, observer-based regulators, adaptive controllers, or model predictive controllers, or a suitable feedback regulator of any kind.

[0053] For the determination of Q REL, a feedback control scheme depicted in Fig. 7 can be used. Here the compartment errors are determined by subtracting the compartment sensor feedbacks from the compartment setpoints, that can be either user selectable, constants, or determined by some program of functionality. The weights W REL FZ and W REL FF can be varied dynamically in order to give more importance to one error or to the other. In some conditions, one weight can be set to 0 in order to force the regulator to adjust the error of just one of the compartments. Typical values are W_REL_FZ=1 and W_REL_FF=0.5 to give more priority to the freezer regulation, since the Freezer compartment can be more affected by the change in the relative duration of the high and low speed phases.

[0054] For the determination of Q REL, the feedback control scheme illustrated in Fig. 8 can be used. The weights W TOT FZ and W TOT FF can be varied dynamically in order to give more importance to one error or to the other. In some conditions, one weight can be set to 0 in order to force the regulator to adjust the error of just one of the compartments. Typical values are W_TOT_FZ=0.5 and W_TOT_FF=0.5 to balance the action of the regulator.

[0055] Using the values of Q TOT and Q REL computed by the feedback regulators, the duration of the low-speed and high-speed phases can then be computed as:

[0056] Further, in case fans are provided in one or both the compartments, dedicated control loops can be used to improve the temperature regulation in the compartments.

[0057] In particular, the speed regulator for the Fresh food compartment can be operated to not only consider the error in Fresh food compartment, but can combine with Freezer compartment weights W_FAN_FZ and Fresh Food compartment weights W_FAN_FF the errors of both the fresh food compartment and the freezer compartment. Indeed, due to the thermodynamical setup of a serial -evaporators system, and increased evaporation in the Fresh food compartment due to higher fan speed, may affect with negative sign the evaporation in the Freezer evaporator, reducing its cooling capacity.

[0058] Thus, Fig. 9 shows an exemplary control scheme used to determine the speed of the Fresh Food Fan (if present), using combined temperature errors from the two compartments. Also Fig. 10 shows an exemplary control scheme used to determine the speed of the Freezer Fan (if present), using the temperature error from the compartment.

[0059] Further, the controller 50 can be implemented using suitable hardware and or software. An exemplary controller is depicted in Fig. 11. The hardware can comprise one or many processors 501 that can be arranged to execute software stored in a readable storage media 502. The processor(s) can be implemented by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared or distributed. Moreover, a processor or may include, without limitation, digital signal processor (DSP) hardware, ASIC hardware, read only memory (ROM), random access memory (RAM), and / or other storage media. The controller 50 is adapted to send and receive signals from other entities such as the temperature sensors (41, 42) and is some embodiments also different other internal and or external sensors or other units using an interface 503.

[0060] Using the refrigerator as described herein can achieve a temperature control for a two- compartment refrigerator (Freezer and Fresh-food) equipped with a variable-speed compressor, a condenser heat exchanger, and two evaporators in series, with the outlet of the freezer compartment evaporator connected to the inlet of the fresh food compartment evaporator. The refrigerator can be used both to static compartments as well as to compartments provided with additional evaporator or recirculation fans. Both the compartments can be provided with air temperature sensors. No ambient temperature sensor is required.

Claims

CLAIMS1. A refrigerator (10) comprising:- a freezer compartment (16),- a fresh food compartment (12),- a cooling system (30) comprising a variable speed compressor (32), and a condenser (34), and at least a first freezer evaporator (37) and a second fresh food evaporator (38), wherein the freezer evaporator (37) and a second fresh food evaporator (38) are connected in series such that the outlet from the freezer evaporator (37) is connected to the inlet of the fresh food evaporator (38), the refrigerator further comprising- a controller (50) configured to control the cooling system (30) by switching the variable speed compressor (32) in cycles between an ON phase and an OFF phase, wherein the controller (50) is configured to during the ON phase control the variable speed compressor (32) to a high speed state followed by a low-speed state where in the low-speed state the variable speed compressor (32) has a lower speed than in the high-speed state.

2. The refrigerator (10) according to claim 1, wherein the duration of each cycle with an ON Phase and an OFF phase is constant.

3. The refrigerator (10) according to claim 1 or 2, wherein the speed of the variable speed compressor is constant pre-determined speed in the high-speed state and the low-speed state, respectively.

4. The refrigerator (10 according to claim 3, wherein the controller is configured to offset the variable speed compressor speed from the constant speed in the high-speed state based on a trigger event.

5. The refrigerator (10) according to any one of claims 1 - 4, wherein a fan (43, 44) is provided in at least one of the freezer compartment (16) and the fresh food compartment (12) and wherein the controller (50) is configured to control the speed of the fan(s) (43, 44).

6. The refrigerator (10) according to claim 5, wherein the controller (50) is configured to control the speed of the fan(s) (43, 44) based on a weighted linear combination of the temperature errors of the fresh food compartment and the freezer compartment.

7. The refrigerator (10) according to any one of claims 1 - 6, wherein the controller is configured to compute a cooling capacity set point for the compressor based on a weighted combination of the temperature errors of the fresh food compartment and the freezer compartment.

8. The refrigerator (10) according to any one of claims 1 - 7, wherein the controller is configured to compute the relative duration of the high-speed and low speed phases based on a weighted combination of the temperature errors of the fresh food compartment and the freezer compartment.

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