EP3759405B1 - Refrigeration device and method for operating a refrigeration device - Google Patents

Description

The present invention relates to a refrigeration appliance with a defrost heater, in particular a household refrigeration appliance in a no-frost design, in which an evaporator to be defrosted by the defrost heater is housed in an evaporator chamber that is separate from a storage chamber and the storage chamber is cooled by an air flow provided by a fan is circulated between the evaporator chamber and the storage chamber.

Moisture that is released by the refrigerated goods stored in the storage chamber or that enters the storage chamber from the outside when a door of the storage chamber is opened is deposited on the evaporator as frost over time. The increasing thickness of the frost layer over time hinders both the air circulation and the heat exchange between the air circulating in contact with the evaporator and a refrigerant evaporating into the evaporator, so that as the thickness of the frost layer increases, an ever higher performance of the fan is required, to maintain air circulation between the evaporator chamber and the storage chamber, and an ever lower evaporator temperature is required to cool the circulating air down to a desired temperature for the storage compartment. Both increase the energy consumption of the refrigerator.

In order to enable energy-efficient operation of the refrigeration device, the layer of frost must be removed from time to time. A simple solution is to operate the defrost heater at regular intervals. However, this results in the defrost heater being switched on unnecessarily if little or no frost has formed within the fixed time interval. In such a case, the energy required for defrosting unnecessarily affects the energy efficiency of the refrigerator.

In order to achieve needs-based and therefore energy-efficient defrosting, various sensors have been proposed. In US 5,522,232 A a frost sensor is described, in which a first temperature sensor is in a hermetically sealed chamber and a second temperature sensor is in one with its surroundings via slots communicating chamber and the temperature difference between the sensors disappears when the slots are closed by frost.

The US 2014150477 A1 discloses a refrigeration device in which frost formation on an evaporator occurs by observing fan behavior in terms of current or speed.

EP0 713 065 B1 describes the use of a capacitive sensor to detect ice on an evaporator. Optical and acoustic methods for detecting ripeness are also known. What all of these methods have in common is that additional, sometimes complex sensors are required for ripeness detection.

The DE 2922633 A1 describes detecting a frosted evaporator by detecting a pressure change in a pressure generated by a fan.

The WO 2017131426 A1 discloses a refrigeration device with a differential pressure sensor for determining a frosted evaporator.

The US 3643457 A discloses a refrigeration device with an air speed sensor for determining a frosted evaporator.

The object of the present invention is to create a refrigeration device and an operating method for it that enable demand-based control of the defrost heater using simple and cost-effective means.

The task is solved by a refrigeration device according to claim 1.

The representative variable responsible for the pressure drop is an electrical variable of the fan. Electrical variables can be recorded without the need for an additional sensor in the area around the evaporator; The measurement data required to determine the representative size can be connected to a motor via suitable circuits

Fan and in particular on a power supply of the fan can be tapped.

In particular, the electrical power of the fan can be used as a representative variable. Since the operating voltage of the fan is fixed and unchangeable in the simplest case, measuring the current drawn by the fan is equivalent to determining the power. In the event that the operating voltage of the fan is variable, the quotient of electrical power and operating voltage can be determined in an equivalent manner as a representative variable.

If the fan supplies a tachometer signal, the speed of the fan can also be used as a representative variable linked to the performance.

The control unit should be set up to monitor the ratio of the representative size when the storage chamber door is closed and to interrupt monitoring when the door is open. There can be several reasons for such an interruption, for example the operation of the fan can be linked to the position of the door in order to prevent warm, moist air from entering the storage chamber when the door is open by switching off the fan when the door is open and their moisture can be separated immediately on the evaporator. If the fan is switched off with the door open, no meaningful measurement values for the power and speed of the fan are available for the frost thickness.

A second reason is that the performance of the fan is not only determined by the flow resistance of the evaporator, but also by that of the storage chamber. Therefore, due to the removal or addition of refrigerated goods in the storage chamber when the door is open, the flow resistance can change suddenly without this being due to a change in the amount of frost on the evaporator.

In the simplest case, especially if the door is not opened for a long time, the limit value can be the sum of the representative size immediately after the evaporator has defrosted and a predetermined deviation. By recording the representative size immediately after defrosting, one can a priori unknown flow resistance of the storage chamber must be taken into account; As soon as the representative size has increased by the specified deviation, it can be assumed that the frost layer on the evaporator has become thick enough to make defrosting necessary.

Since a large part of the moisture that forms as frost on the evaporator gets into the storage chamber when the door is opened, the case that the limit value is exceeded without the door having been opened since the previous defrosting is quite rare in practice. The door is usually opened one or more times between two defrosting processes. In order to take into account the changing flow resistance of the storage chamber due to the addition or removal of refrigerated goods, the control unit should be set up to update the limit value after the door is closed

The control unit is set up to record the representative size before and after the door is closed and to change the limit value based on the difference between these two recorded sizes. In this way, changes in the representative size that occur when the door is open due to the removal or addition of refrigerated goods can be prevented from affecting the control of the defrost heater.

Furthermore, the control unit should be set up to record the representative variable before a compressor is switched off, so that if the door is opened while the compressor is switched off, a meaningful measured value for the representative variable is available.

The task is further solved by a method according to claim 7.

Fig. 1:

einen schematischen Schnitt in Tiefenrichtung durch ein erfindungsgemäßes Haushaltskältegerät;

Fig. 2:

den Zusammenhang zwischen Druckverlust und Lamellenabstand in einem Lammellenverdampfer;

Fig. 3:

Kennlinien eines Ventilators;

Fig. 4:

eine exemplarische zeitliche Entwicklung des Druckabfalls am Verdampfer des Kältegeräts aus Figur 1; und

Fig. 5:

ein Flussdiagramm eines von einer Steuereinheit des Kältegeräts der Figur 1 ausgeführten Arbeitsverfahrens.

Further features and advantages of the invention result from the following descriptions of exemplary embodiments with reference to the attached figures. Show it:

Fig. 1:

a schematic section in the depth direction through a household refrigeration appliance according to the invention;

Fig. 2:

the relationship between pressure loss and fin spacing in a fin evaporator;

Fig. 3:

Characteristic curves of a fan;

Fig. 4:

an exemplary temporal development of the pressure drop at the evaporator of the refrigeration device Figure 1 ; and

Fig. 5:

a flowchart of a control unit of the refrigeration device Figure 1 carried out work procedure.

Shows as an example of a refrigeration device according to the invention Fig. 1 a No-Frost combination device in a schematic section in the depth direction. In a body 1 of the refrigeration appliance, two cavities are delimited by an inner container 2 which is preferably deep-drawn in one piece from plastic. One of the cavities is a storage chamber, here a normal refrigerator compartment 3. The other cavity is divided by a vertical partition 4 into a second storage chamber, here a freezer compartment 5, and an evaporator chamber 6. Both storage chambers 3, 5 are each closed by a door 20. The invention described below is of course also applicable to refrigeration appliances with a single or more than two storage chambers.

The evaporator chamber 6 contains a finned evaporator 7 with parallel to the cutting plane of the Fig. 1 arranged slats. A defrost heater 10 for defrosting the finned evaporator 7 is accommodated in a free space 8 of the evaporator chamber 6 located below the finned evaporator 7. A compressor 19 for driving the refrigerant flow through the finned evaporator 7 is housed in a machine room separated from the body 1 at the level of the freezer compartment 5.

The free space 8 here forms an inlet volume on an upstream side of the finned evaporator 7, which communicates with the freezer compartment 5 via an inlet gap 11.

The vertical intermediate wall 4 contains a distribution chamber 12, which communicates with a second, here downstream, free space 14 of the evaporator chamber 6 above the evaporator 7 via an opening at which a fan 13 is arranged. A first outlet 15 of the distribution chamber 12 opens into the freezer compartment 5 close to the ceiling. Another outlet is formed by a line 16 extending in a wall of the body 1 to the normal refrigerator compartment 3. A flap controlled by a thermostat can be provided in this line 16, which allows the cold air supply to the normal refrigerator compartment 3 to be stopped if there is only a need for cooling in the freezer compartment 5. If there is a need for cooling in the normal refrigerator compartment 5 and the flap is therefore open, the cold air circulated by the fan 13 is distributed to both storage chambers 3, 5.

Alternatively, a structure can also be considered in which a fan in the evaporator pumps cooled air into the freezer compartment, air from the freezer compartment enters the normal refrigerator compartment via a gap or other passage, and air is sucked in from the normal refrigerator compartment into the evaporator.

wobei L die Länge des Verdampfers 7 in Flussrichtung der Luftströmung, H die quer zur Flussrichtung in der Ebene einer der Lamellen gemessene Höhe des Verdampfers, d die freie Spaltbreite zwischen zwei Lamellen, n die Zahl der Lamellen, µ die dynamische Viskosität der Luft und V̇ den Volumenstrom bezeichnet. Der Druckverlust Δp ist umgekehrt proportional zur dritten Potenz der freien Spaltbreite d und reagiert damit empfindlich auf die Dicke der Reifschicht auf den Lamellen.Moisture, which is absorbed by the air as it circulates through the storage chambers 3, 5, is deposited on the fins of the evaporator 7 and thus reduces the free gap width between the fins. This gap width has a strong influence on the pressure loss of the circulating air. The pressure loss Δp at the evaporator 7 can be estimated using the following formula: $$\mathit{\Delta p}=\frac{12\mathit{\mu L}}{d^3\left(n+1\right)H}\dot{v}$$

where L is the length of the evaporator 7 in the flow direction of the air flow, H is the height of the evaporator measured transversely to the flow direction in the plane of one of the fins, d is the free gap width between two fins, n is the number of fins, µ is the dynamic viscosity of the air and V̇ denotes the volume flow. The pressure loss Δp is inversely proportional to the cube of the free gap width d and is therefore sensitive to the thickness of the frost layer on the slats.

In order to measure the pressure loss Δp, a differential pressure sensor 21 can be connected to the two free spaces 8, 14. Since the pressure in the storage chambers 3, 5 (at least under stationary operating conditions, when the doors 20 have been closed long enough ago) does not differ significantly from atmospheric pressure, an absolute pressure sensor can alternatively also be provided on one of the two free spaces 8, 14. A pressure sensor is not required if the pressure loss Δp is estimated based on electrical operating variables of the fan 13, as described below.

The diagram of the Fig. 2 illustrates the pressure loss Δp against which the fan 13 works as a function of the gap width d. For a just defrosted, ice-free evaporator 7, a free gap width d of 5 mm between the fins is assumed and for the circulation through the storage chambers 3, 5 a contribution to the pressure loss Δp of 15 Nm/m ² is assumed, which is independent of the gap width. A solid curve shows the development of the absolute pressure loss Δp as the gap width d decreases due to frost growth; a dashed curve shows the percentage change in the pressure loss Δp compared to the condition at d = 5 mm. Halving the gap width d leads to a clearly measurable change in the pressure loss Δp.

The electric motor of the fan 13 can react differently to the change in pressure loss Δp depending on the design or operating point, for example by running more slowly or by increasing power consumption. Fig. 3 shows exemplary characteristics for the power P, the efficiency n and the pressure loss Δp of the fan 13 as a function of the volume flow V̇ . The operating point of the fan 13 should be in the vicinity of a maximum of efficiency n, shown as a solid curve. In this in the diagram of the Fig. 3 In the area highlighted by hatching, both the pressure loss Δp, shown as a dashed curve, and the power P, shown as a dash-dotted curve, are clear functions of the volume flow V̇ , so that a measured power P of the fan 13 clearly indicates the pressure loss Δp on the evaporator 7 and thus the thickness of the frost layer can be concluded. If the operating voltage of the fan 13 is fixed, knowledge of the current drawn by the fan 13 is sufficient to be able to estimate the thickness of the layer of frost on the fins of the evaporator 7.

The practical application of this idea is based on the Figures 4 and 5 explained. Figure 4 shows schematically a development of the pressure drop over time at the evaporator 7 of the refrigeration device Figure 1 ; Fig. 5 shows a flowchart from a control unit 18 of the refrigeration device Fig. 1 carried out work procedure.

The free gap width in the evaporator 7 is maximum when all frost adhering to the fins of the evaporator 7 has been removed by the operation of the defrost heater 10. In this case, the pressure loss Δp, against which the fan 13 has to work, is essentially determined by a flow resistance of the storage chambers 3, 5. However, this is not known a priori, since refrigerated goods placed in the storage chambers 3, 5 depend on their quantity and Arrangement can hinder the flow of air to varying degrees.

The description of the process therefore begins in Fig. 5 This means that in step S1 the defrost heater 10 is switched off after the evaporator 7 has completely defrosted. This point in time corresponds to the point in time t = 0 in the diagram Figure 4 .

In step S2, the control unit 18 starts the compressor 19 to resume cooling the evaporator 7. At the same time, preferably somewhat delayed after the start of cooling of the evaporator 7, the fan 13 is also switched on in step S3. When this has reached a stationary speed after a few seconds, the current intensity l absorbed by the fan 13 is recorded as a variable representative of the pressure loss Δp (S4). The measured value l ₀ obtained is saved in step S5. Its amount will generally vary from one defrosting process to another since it depends on the distribution of the goods to be cooled in the chambers 3, 5.

A limit value l _end of the current strength is then set, if this is exceeded it is assumed that so much frost has accumulated on the evaporator 7 that defrosting is necessary. In order to ensure that defrosting takes place with the same thickness of the frost layer even when the storage chambers 3, 5 are loaded differently with refrigerated goods, the limit value l _end is calculated in step S6 as the sum of the previously stored initial value l ₀ and a predetermined difference value D.

In step 7, the current intensity l is recorded again and compared with the limit value l _end in step S8. As long as the limit value l _end has not yet been reached, the method branches to step S9 to check whether the door 20 of one of the storage chambers 3, 5 is open.

If the doors 20 are closed, it is next checked in step S10 whether the compressor 19 is switched off - because the cooling requirement in both storage compartments 3, 5 is satisfied. The fan 13 is then also switched off, so that no meaningful measurement of the current intensity I can be obtained. In this case, the checks in steps S9, S10 are repeated until either the need for cold in at least one of the storage areas 3, 5 leads to the compressor 19 and subsequently also the fan 13 being switched on again and the process to step 7 returns, or a user opens one of the doors 20.

In the latter case, the last measured value l _t of the current intensity (which can be a value that has no longer been updated since the compressor 19 was switched off) is saved in step S11. The fan 13 is switched off S12 in order to prevent moist ambient air, which enters the storage chamber 3 or 5 through the open door 20, from being pumped from there immediately to the evaporator 7 and contributing to the formation of frost there. This phase corresponds, for example, to time t ₁ in the diagram of Fig. 4 . Since frost has deposited again on the fins of the evaporator 7 since time t = 0, the current drawn by the fan 13 has increased from l ₀ to l ₁ . The remaining difference to the limit value of the current intensity l _end is calculated (S13) and saved as a new difference value D. The processing unit then waits until the time t ₂ in Fig. 4 the door 20 is closed again and then returned to step S3.

During the time period [t ₁ , t ₂ ] in which the door 20 was open, the user has invited fresh refrigerated goods into the chambers 3, 5, whereby the pressure loss Δp increases significantly, so that when step S5 is repeated , a significantly higher current l ₂ than was measured before the door was opened.

This new measured value is in turn saved in step S5, and the limit value l _end is recalculated in step S6 on the basis of the difference value D updated in the previous step S13.

From the time t ₂ when the door is closed, the fan 13 runs again and the layer of frost in the evaporator 7 continues to increase in thickness. As in Fig. 4 shown, the current intensity can exceed the limit value that was valid in the time interval [0, t ₁ ] without this triggering a start of the defrost heater 10. At time t ₃ , the door 20 is opened again, which the control unit 18 recognizes in step S9, the most recent current measurement value obtained in the meantime is saved in step S11 and based on the stored value, the difference value D is updated again in step S13.

At time t ₄ the door is closed again 20 so that the process returns to step S3. This time, the storage chambers 3, 5 have been largely emptied, so that the refrigerated goods contained therein hardly contribute to the pressure loss Δp and the current intensity l ₄ now measured is significantly lower than before the door was opened. The limit value l _end is updated again in step S6. Since the few refrigerated goods still contained only release a small amount of moisture, the increase in frost in the evaporator 7 is also reduced, which is reflected in a slower increase in the current intensity l from t ₄ compared to the time interval [t ₂ , t ₃ ].

At time t ₅ it is determined that the current limit value l _end has been exceeded. The method now branches to step S15, in which the heater 10 is switched on. At the same time, unless this has already happened before, compressor 19 and fan 13 are switched off. The difference value D is reset to a predetermined value D ₀ corresponding to an evaporator 7 that is completely free of frost. If it is determined in step S17 that the defrosting process has been completed and there is a need for cold again in one of the storage chambers 3, 5, the method returns to step S2.

With the method described above it can be ensured that, despite changing loading of the storage chambers 3, 5, defrosting is triggered as required at a given thickness of the frost layer in the evaporator 7, without the installation of sensors in the storage chambers 3, 5 or the evaporator chamber 6 is required.

REFERENCE MARKS

1

Corpus

2

inner container

3

Normal refrigerator compartment

4

partition wall

5

freezer

6

Evaporator chamber

7

(finned) evaporator

8th

free space

9

Bottom edge

10

Defrost heater

11

Entry gap

12

distribution chamber

13

fan

14

free space

15

outlet

16

Line

17

flap

18

Control unit

19

compressor

20

door

21

Differential pressure sensor

Claims (7)

1. Refrigeration appliance, in particular household refrigeration appliance, having an evaporator (7), a ventilator (13) for driving a flow of air through the evaporator (7), a storage chamber (3, 5) cooled by the flow of air, a defrost heater (10) for defrosting the evaporator (7) and a control unit (18) for controlling the operation of the defrost heater (10), wherein the control unit (18) is designed to compare a variable (I) which is representative of a drop in pressure on the evaporator with a limit value (I_end) and to start the defrost heater (10) when the limit value is exceeded, characterised in that the representative variable is an electrical variable of the ventilator (13), and the limit value is the sum of the representative variable immediately after defrosting the evaporator (7) and a predetermined deviation (D), and the control unit (18) is designed to detect the representative variable before and after closing a door (20) of the storage chamber (3, 5) and to change the limit value using the difference between these two detected variables in order to take into account a changing flow resistance of the storage chamber (3, 5).
2. Refrigeration appliance according to claim 1, characterised in that the representative variable is derived from the output and operating voltage of the ventilator (13).
3. Refrigeration appliance according to claim 1, characterised in that the representative variable is the electrical output or operating current strength (I) of the ventilator (13) with a given operating voltage.
4. Refrigeration appliance according to one of the preceding claims, characterised in that the representative variable is derived from the rotational speed of the ventilator (13).
5. Refrigeration appliance according to one of the preceding claims, characterised in that the control unit (18) is designed to monitor the representative variable (I) when the door (20) of the storage chamber (3, 5) is closed and to interrupt the monitoring when the door (20) is open.
6. Refrigeration appliance according to one of the preceding claims, characterised in that the control unit (18) is designed to detect the representative variable (I) before switching off a compressor (19).
7. Method for operating a refrigeration appliance, which comprises an evaporator (7), a ventilator (13) for driving a flow of air through the evaporator (7), a storage chamber (3, 5) cooled by the flow of air and a defrost heater (10) for defrosting the evaporator (7), having the steps:

   a) detecting a variable (I) (S4) which is representative of a drop in pressure (Δp) on the evaporator (7), wherein the representative variable is an electrical variable of the ventilator (13),

   b) defining (S6) a limit value (I_end) (S6), wherein the limit value is the sum of the representative variable immediately after defrosting the evaporator (7) and a predetermined deviation (D),

   c) detecting the representative variable (I) before and after closing a door (20) of the storage chamber (3, 5) (S7) S4),

   d) changing the limit value on the basis of the difference between these two detected variables (S6), in order to take into account a changing flow resistance of the storage chamber (3, 5),

   e) detecting the variable (I) (S4) which is representative of a drop in pressure (Δp) on the evaporator (7),

   f) comparing the detected variable with the limit value (I_end) (S8) and

   g) starting the defrost heater (10) when the limit value (I_end) (S 15) is exceeded.

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