[0001] This invention relates to a method for sensing and controlling frost formation on
the evaporator of a refrigerator, for example of static type, said refrigerator comprising
a compartment provided with its own door and containing the evaporator, a usual heating
element associated with this latter for its defrosting, and usual means, such as a
compressor, defining a known refrigeration circuit incorporating said evaporator.
The invention also relates to a device for implementing the aforesaid method.
[0002] 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.
[0003] 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.
[0004] 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). Methods and devices for
sensing the presence of frost on the evaporator are known (such as those using optical
sensing means), these then, on the basis of this, 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). These
methods and devices however have a level of implementation and/or construction cost
which negatively affects the overall cost of the refrigerator. In addition, said methods
and devices do not allow accurate measurement of the frost thickness on the evaporator,
and are 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.
[0005] An object of the present invention is to provide a method 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 method of the aforesaid type which is of low implementation
cost and which enables the evaporator defrosting process to be optimized.
[0006] 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 method of the aforesaid type, characterised
by measuring an electrical signal generated by electrically powered conductive means
associated with the evaporator, said signal being compared with a predetermined electrical
reference signal corresponding to an extent of frost on the evaporator such as to
only negligibly modify its heat transfer characteristics, the state of the heating
element being altered on the basis of this comparison.
[0007] This method is implemented by a device applied to a refrigerator of tile aforesaid
type, characterised by comprising conductive means associated with the evaporator,
activation means for powering said conductive means, and control and comparison means
for an electrical signal generated by said electrically powered conductive means,
said signal varying in accordance with the frost present on the evaporator, said control
means comparing said signal with a predetermined signal corresponding to an extent
of frost on the evaporator such as to only negligibly modify its heat transfer characteristics,
said means acting on the heating element on the basis of this comparison.
[0008] 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;
Figure 4 is a schematic cross-section through a third embodiment of the invention
applied to the evaporator of Figure 1;
Figure 5 shows a fourth embodiment of the invention, applied to a known flat evaporator
defined by superposed plates, and commonly known as a roll-bond evaporator;
Figure 6 is a schematic section on the line 6-6 of Figure 5;
Figures 7 and 8 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 7) and
by 10⁻¹¹ (Figure 8);
Figure 9 is a block diagram of the electrical circuit associated with the device according
to the invention; and
Figure 10 is a more detailed diagram of the circuit of Figure 9.
[0009] 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.
[0010] 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.
[0011] 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).
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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
of the electrical capacitor.
[0016] Figure 4 shows a capacitor (again indicated overall by 8) comprising several fins
2, each used as a plate of the capacitor. In this figure, in which parts corresponding
to those of the already described figures are indicated by the same reference numerals,
the fins are all connected to an electrical line 7 via a common support 15. At the
same time, a second support 16 carries flat elements 17 projecting perpendicularly
from it and defining the second plates cooperating with the fins 2. In this manner
a multiple flat capacitor 8 is defined, formed from several capacitors connected in
parallel.
[0017] Figures 5 and 6, in which parts corresponding to those of the already described figures
are indicated by the same reference numerals, show a usual flat evaporator having
opposing faces 21 and 22 and usual internal channels 23.
[0018] A flat element 25 is positioned in correspondence with at least a part 24 of said
face 21. It is secured to and spaced from this face by elements 27 formed of any known
electrically insulating material. The evaporator 20 is connected to the power line
7 and the flat element 25 to the line 6. The part 24 and the element 25 define the
capacitor 8.
[0019] The usual defrosting resistance element 13 is positioned in correspondence with the
evaporator face 22.
[0020] Figure 9, 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.
[0021] 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.
[0022] More specifically, with reference to Figure 10 (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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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 7 and 8, 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
8).
[0027] The capacitance variation is measured continuously by the circuit 12 in the illustrated
example.
[0028] 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.
[0029] 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.
[0030] 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.
[0031] Advantageously, the unit 32 forms part of a member which controls the refrigerator
operation, preferably of microprocessor type.
[0032] Consequently, when the signal V
C 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 9) which connects it
to the refrigerator control member and along which the signal V
C reaching the transistor 45 is fed.
[0033] 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.
[0034] 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.
[0035] 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).
[0036] 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).
1. A method for sensing and controlling frost formation on the evaporator of a refrigerator,
for example of static type, said refrigerator comprising a compartment provided with
its own door and containing the evaporator, a usual heating element associated with
this latter for its defrosting, and usual means, such as a compressor, defining a
known refrigeration circuit incorporating said evaporator, said method being characterised
by measuring an electrical signal (VA) generated by electrically powered conductive means (8) associated with the evaporator
(1), said signal being compared with a predetermined electrical reference signal (VA) corresponding to an extent of frost on the evaporator (1) such as to only negligibly
modify its heat transfer characteristics, the state of the heating element being altered
on the basis of this comparison.
2. A method as claimed in claim 1, characterised in that the comparison is effected continuously
during refrigerator operation.
3. A method as claimed in claim 1, characterised in that the comparison is effected discretely
during refrigerator operation.
4. A method as claimed in claim 1, characterised in that the reference signal is adjustable.
5. A method as claimed in claim 1, characterised in that the refrigerator operation,
ie the operation of the compressor of its refrigeration circuit, is halted on the
basis of the effected comparison.
6. A method as claimed in claim 1, characterised in that the alteration in the state
of the heating element (13) involves switching it from a deactivated state to an activated
state in which electric signal passes through it to defrost the evaporator.
7. A method as claimed in claim 1, characterised in that the alteration in the state
of the heating element (13) involves switching it from an activated state to a deactivated
state to hence halt defrosting of the evaporator.
8. A device for implementing the method claimed in claim 1, characterised by comprising
conductive means (8) associated with the evaporator (1), activation means (30) for
powering said conductive means (8), and control and comparison means (32) for an electrical
signal (VA) generated by said electrically powered conductive means (8), said signal (VA) varying in accordance with the frost present on the evaporator (1), said control
means (32) comparing said signal (VA) with a predetermined reference signal (VR) corresponding to an extent of frost on the evaporator (1) such as to only negligibly
modify its heat transfer characteristics, said means (32) acting on the heating element
(13) on the basis of this comparison.
9. A device as claimed in claim 8, characterised in that the conductive means are at
least one capacitive sensor (8).
10. A device as claimed in claim 9, characterised in that the capacitive sensor (8) comprises
at least two flat elements (4, 5) 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).
11. A device as claimed in claim 10, characterised in that electrically insulating elements
(10, 11) of high thermal conductivity are interposed between the flat elements (4,
5) and the fins (2) which support them.
12. A device as claimed in claim 9, characterised in that the capacitive sensor (8) comprises
at least two flat elements (4, 5) disposed on alternate fins (2) of the evaporator,
between said flat elements (4, 5) there being disposed a further intermediate fin
(2).
13. A device as claimed in claim 9, characterised in that the capacitive sensor comprises
at least one capacitor (8) a plate of which is defined by a fin (2) of the evaporator,
and a flat element (17) positioned in correspondence with but in front of said plate
(2), said fin (2) and flat element (17) being connected to different electrical power
lines (6, 7).
14. A device as claimed in claim 9, characterised in that the capacitive sensor comprises
at least one capacitor defined by at least one portion (24) of a flat evaporator (20)
and a flat element (25) opposite said evaporator and maintained spaced from it by
insulating elements (27).
15. A device as claimed in claim 8, characterised in that the control and comparison means
comprise at least one control unit (32) comprising at least one comparison member
(39) to which the reference signal (VR) 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.
16. A device as claimed in claim 15, characterised in that the first switch means are
at least one static switch (45).
17. A device as claimed in claim 15, characterised in that the operating means are a relay
(51).
18. A device as claimed in claim 15, characterised in that the second switch means are
a movable contactor (52).
19. A device as claimed in claim 15, characterised in that the control and comparison
means control the operation of the refrigerator, and in particular of the compressor
of this latter.