Device for sensing and controlling frost formation on a refrigerator evaporator

Abstract: A method for sensing frost formation on a usual evaporator (1, 20) of a refrigerator, for example of static type, comprises measuring the variation in an electrical signal (V A ) generated by at least one electrically powered conductor (8) associated with the evaporator (1, 20) and, on the basis of this variation, activating a usual heating element (13) associated with the evaporator (1, 20) for its defrosting. This method is implemented by a device comprising at least one electrical conductor (8) associated with the evaporator (1, 20), activation means (30) for powering said conductor (8) and control means (32) for an electrical signal (V A ) generated by said conductor, said signal (V A ) varying in accordance with the frost present on the evaporator (1, 20), said control means (32) comparing said signal (V A ) with a reference signal (V R ) and acting on the heating element on the basis of this comparison.

Description: This invention relates to a device for sensing and controlling frost formation on the evaporator of a refrigerator, of the type specified in the pre-characterising portion of appended claim 1. A device of the above type is known from US-A- 4347709. As is well known, frost forms on the evaporator during the use of the refrigerator. This is due to the condensation of moisture on the evaporator (which has a temperature considerably lower than any other part of the refrigerator), this moisture being present in the compartment by having penetrated into it following the opening of its door. The frost is also due to condensation of the moisture which is released naturally by the food placed in the refrigerator for its preservation. Said frost deposits on the evaporator and reduces heat transfer between this latter and the environment or compartment with which it is associated. This results in lesser elimination of moisture from the compartment air, and a consequent worsening of the preservation conditions for the food placed in the refrigerator. To free the evaporator from the layer of frost which forms on it, there is associated with it a usual electrical resistance element (or other equivalent heating element) which when current flows through it melts the frost and converts it into water. This is then removed from the evaporator via a suitable channel. Said defrosting is effected in various ways. For example, the evaporator may be defrosted automatically after a fixed period of operation of the usual refrigeration circuit of the refrigerator (or of the known compressor of this circuit). However this method of defrosting does not take account of whether it is really necessary to remove frost from the evaporator at that time, hence defrosting may take place when there is no real need for it (with obvious disadvantages in terms of refrigerator operation). A device for sensing the presence of frost on the evaporator and using capacitive sensing means is known from the above mentioned US-A- 4347709, these sensing means either activating or not activating the heating element associated with the evaporator (while at the same time interrupting the operation of the refrigeration circuit of which this latter forms part). This device is based on the use of the evaporator itself as one plate of the capacitor. Due to the earth connection of the evaporator through the compressor, and due to the related instability of the signal caused by the variation vs. time of the earth potential, said device does not allow accurate measurement of the frost thickness on the evaporator, and is able to sense only whether a layer is present on a given portion of it. Consequently, operation of the heating element associated with the evaporator, based on the sensing of frost on it, may sometimes be unnecessary, so negatively affecting the performance of the refrigerator. An object of the present invention is to provide a device for sensing the presence of frost on the evaporator by which this latter is defrosted only if it is covered with a layer of frost to the extent of changing its heat transfer characteristics. A further object is to provide a device of the aforesaid type which is of low implementation cost and which enables the evaporator defrosting process to be optimized. A further object of the invention is to provide a device for evaluating the thickness of frost on an evaporator which is of simple construction, of reliable use and operation, and is able to activate the heating element associated with the evaporator only when on the surface of this latter there is a quantity of frost present such as to change its heat transfer characteristics. These and further objects which will be apparent to the expert of the art are attained by a device according to the appended claim 1. The present invention will be more apparent from the accompanying drawing, which is provided by way of non-limiting example and in which: Figure 1 is a schematic perspective view of a finned evaporator to which the device of the invention is applied; Figure 2 is a schematic cross-section through a first embodiment of the device of the invention applied to the evaporator of Figure 1; Figure 3 is a schematic cross-section through a second embodiment of the invention applied to the evaporator of Figure 1; Figures 4 and 5 are diagrams relative to the use of a device according to the invention, the horizontal axes representing the frost thickness on the evaporator in mm and the vertical axes representing capacitance in farads multiplied by 10 -9 (Figure 4) and by 10 -11 (Figure 5); Figure 6 is a block diagram of the electrical circuit associated with the device according to the invention; and Figure 7 is a more detailed diagram of the circuit of Figure 6. With reference to Figures 1 and 2, a known finned evaporator is indicated overall by 1 and comprises parallel fins 2 with opposing surfaces 2A and 2B. On the surface 2A of a first of two adjacent fins and on the facing surface 2B of the second there are arranged two flat elements 4 and 5 connected to electrical lines 6 and 7. These elements define the plates of a capacitor 8. Each element is associated with the respective fin in any known manner. Between the elements and the corresponding fins there are interposed flat electrically insulating elements 10 and 11. These insulating elements have high thermal conductivity and are formed for example of mica, alumina or similar materials. These elements are associated with the faces 4A and 5A of the elements 4 and 5, while the surfaces 4B and 5B of these latter face each other. The electrical lines 6 and 7 terminate in an operating and control circuit 12 for powering the capacitor 8 (defining a capacitive frost sensor) and for powering (or not powering) a usual defrosting resistance element (or other equivalent heating element) associated with the evaporator 1. The circuit 12 is therefore able to alter the state of the resistance element 13 (to cause it to operate or remain at rest by powering it or not). As described hereinafter, the circuit 12 evaluates the capacitance of the capacitor 8, said capacitance varying with the deposition of frost on the evaporator 1. In this respect, the frost replaces the air which forms the dielectric of the capacitor 8 between the faces 4B and 5B of its elements or plates 4, 5 during the initial stage of use of the refrigerator or following defrosting of the evaporator. As stated, between the flat elements or plates 4, 5 and the fins 2 of the evaporator 1 there are electrically insulating elements 10 and 11 of high thermal conductivity. If it is desired to avoid the use of elements 10 and 11 of such characteristics (their presence implying certain problems for example of a constructional nature), the capacitor 8 can be formed as shown in Figure 3. In this, parts corresponding to those of Figures 1 and 2 are indicated by the same reference numerals. In Figure 3, the plates 4 and 5 of the capacitor 8 are not located on adjacent consecutive fins 2 but on two alternate fins, between which there is a fin without flat elements. The plates are positioned on those faces of the corresponding fins which face the intermediate fin. In the case under examination, the intermediate fin forms part of the dielectric of the capacitor 8 and its presence results in a reduction in the capacitance bf the electrical capacitor. Figure 6, in which parts corresponding to those of the already described figures are indicated by the same reference numerals, shows the electrical operating and control circuit 12 for the heating element or resistance element 13 and for powering the capacitor 8. The capacitor is connected to a usual oscillator circuit 30 which powers it and which is itself connected to a frequency/voltage converted member 31 connected to a unit 32 for controlling the defrosting or heating element 13. More specifically, with reference to Figure 7 (in which parts corresponding to those of the already described figures are indicated by the same reference numerals), the capacitor 8 is connected in parallel with a second capacitor 36 for increasing the total capacitance measurable by the circuit 12. The converter 31 is also connected to earth at 38 and to a power line (not shown) by which power also reaches the capacitor 8. The unit 32 is connected to this converter and comprises a comparator 39 with its inverting input 40 connected to the converter 31, its non-inverting input 41 connected to a current divider 42 and its output 43 to the base 44 of a transistor 45 via a resistor 46. The transistor emitter 47 is connected to earth at 48 and the collector 50 is connected to a relay 51 (powered by said power line) operating on a movable contactor 52 for closing fixed contacts 54 in the power line 55 to the resistance element 13. The method of the invention will now be described in relation to the use of the circuit 12 and of the device illustrated in the aforesaid figures. During the use of the refrigerator, moisture deposits on the evaporator 1 and forms frost. It also deposits between the plates of the capacitor 8 (whatever form this takes) and changes its capacitance. In this respect, the frost (similar to ice) has a dielectric value much higher than the air present between the plates 2 of the capacitor 8 at the commencement of use of the refrigerator or after its evaporator has been defrosted. Hence as there is a known relationship between the dielectric and the capacitor capacitance, this latter increases considerably as the layer of frost on the evaporator increases and the frost deposits between the capacitor plates. This can be seen in Figures 4 and 5, from which it can be seen that the curve C, representing the relationship between the thickness of frost on the evaporator and the capacitance of the capacitor 8, rises considerably for thicknesses exceeding 2 millimetres. However the capacitance change is already measurable for thicknesses less than just one millimetre (see Figure 5). The capacitance variation is measured continuously by the circuit 12 in the illustrated example. Specifically, a signal V A from the capacitor 8 is fed to the oscillator (which at the same time feeds the capacitor) 30. This latter feeds a signal of determined frequency to the converter 31. This converts the frequency signal into a voltage signal V B and applies it to the inverting input 40 of the comparator 39. The comparator compares the signal V B with a reference signal V R defined (adjustably) by the voltage divider 42 and corresponding to a layer of frost on the evaporator 1 of an extent which does not (or only negligibly) affect the heat transfer characteristics of the evaporator. When the signal V B exceeds the signal V R , the comparator generates a signal V C which reaches the base 44 of the transistor 45, which becomes saturated. In this manner the relay 51 is connected to earth and can act on the contactor 52 to close its contacts 54. In this manner the resistance element 13 becomes powered and can begin to defrost the evaporator. This powering lasts for a predetermined time sufficient to obtain the required defrosting. This can be achieved by using a relay with sufficiently long operating times or by connecting the power line of the relay 51 to a known timer circuit which cuts off the power after a determined time period. Advantageously, the unit 32 forms part of a member which controls the refrigerator operation, preferably of microprocessor type. Consequently, when the signal Vc reaches the transistor 45 (or another equivalent static switch), said member halts the refrigerator operation by cutting off electrical power to the compressor of the usual refrigeration circuit of the refrigerator. Alternatively, the unit 32 can be provided with an electrical branch 50 (shown dashed in Figure 6) which connects it to the refrigerator control member and along which the signal V C reaching the transistor 45 is fed. Particular embodiments of the invention have been described. However different embodiments can be provided (for example using other conductive means in place of the capacitor or in which the signal originating from these means is evaluated discretely at determined time intervals) and are to be considered as falling within the scope of the present document. In addition the conductive means (capacitors or the like) can' be used for determining when the evaporator defrosting is complete. This is achieved via the means used for evaluating the frost formation. In this case said means generally evaluate the presence of frost on the evaporator and act on the heating element in the sense of activating it (to defrost the evaporator) or to halt its operation (to hence halt defrosting when said means sense the absence of frost on said evaporator). In all cases the alteration in the state of the heating element (ie for the purpose of its activation or deactivation) is effected on the basis of the measurement of frost on the evaporator by the use of said conductive means. This measurement can be discrete (ie when the frost has been measured the conductive means can be deactivated by the control means) or continuous (said conductive means then sensing the variation in the frost layer on the evaporator, to halt evaporator defrosting when the layer on it has been reduced to the extent of not affecting heat transfer between the evaporator and the environment in which it is contained).

Claims: A device for sensing and controlling frost formation on the evaporator (1) of a refrigerator, comprising a heating element (13) associated with the evaporator for its defrosting, electrically powered means (8) for generating an electrical signal (V A ) which varies with the presence of frost on the evaporator, activation means for powering said electrically powered means (8), and control and comparison means (12) for comparing said signal (V A ) with a predetermined electrical reference signal (V R ) corresponding to a predetermined extent of frost on the evaporator, said control and comparison means (12) acting on the heating element (13) on the basis of this comparison, and said electrically powered means (8) comprising at least two flat elements (4, 5) which are supported by the evaporator, electrically connected to the control and comparison means (12) and electrically insulated from the evaporator, characterised in that the flat elements (4, 5) define the plates of a capacitor (8). A device as claimed in claim 1, characterised in that the two flat elements (4, 5) are associated with adjacent consecutive fins (2) of the evaporator (1), said flat elements (4, 5) being associated with opposing faces (2A, 2B) of said adjacent consecutive fins (2). A device as claimed in claim 2, characterised in that electrically insulated elements (10, 11) of high thermal conductivity are interposed between the flat elements (4, 5) and the fins (2) which support them. A device as claimed in claim 1, characterised in that the two flat elements (4, 5) are disposed on alternate fins (2) of the evaporator (1), between said flat elements (4, 5) there being disposed a further intermediate fin (2). A device as claimed in claim 1, characterised in that the control and comparison means (12) comprise at least one control unit (32) comprising at least one comparison member (39) to which the reference signal (V R ) is fed, first switch means (45) connected into the line which powers means (51) for operating second switch means (52) connected into the line (55) powering the heating element (13). A device as claimed in claim 5, characterised in that the first switch means are at least one static switch (45). A device as claimed in claim 5, characterised in that the operating means are a relay (51). A device as claimed in claim 5, characterised in that the second switch means are a movable contactor (52). Use of a device according to any of the preceding claims for sensing and controlling frost formation on the evaporator of a refrigerator.

Inventors: TURETTA DANIELE, TURETTA, DANIELE, Turetta, Daniele, c/o Whirlpool Europe S.r.l.
Applicants: WHIRLPOOL EUROP, WHIRLPOOL EUROPE B.V.
Application year: 1993
Main classification: F25D21
IPC codes: F25D21