[0001] The present invention refers to a clothes tumble dryer and a method for controlling
the same.
[0002] An important feature in tumble driers is the ability of the drying process to be
stopped as soon as the moisture content of the washload has been brought down to the
desired value. In fact, if the drying process be interrupted too early, the resulting
moisture content in the washload would be too high and this would again entail a number
of obvious disadvantages. On the other hand, interrupting a drying process too late
practically means an unnecessarily high energy usage, while the resulting too low
a moisture content of the washload causes it to be subject to increased creasing,
reduced ironability, increased likelihood for the washload items to suffer shrinking
or undergo such other adverse effects as becoming statically charged.
[0003] Condenser-type tumble dryers are known in the art to comprise a closed-loop drying
air circuit adapted to cause heated drying air to circulate through a perforated rotating
drum, and a condenser adapted to remove the moisture from the hot moisture-laden drying
air exiting the drum upon having flown through the drying load contained therein,
as well as moisture sensors adapted to detect the moisture content of the air exiting
the drum. Thus, the drying process is stopped when the moisture content of the drying
air leaving the drum is sensed to have reached down to a pre-established value. However,
this method has been unable to provide really satisfactory results, since it has been
practically found that the actual moisture content of the drying load upon interruption
of the drying process is greatly affected also by such variables as the amount of
drying load items in the drum and the type and nature of the fabrics being handled.
[0004] Also known in the art are condenser-type tumble dryers, in which electrodes are arranged
in the drum for measuring the electrical resistance of the load, based on the fact
that such electrical resistance is a parameter that is in direct relation with, i.e.
dependent on the moisture content of the drying load items. Other prior-art solutions
call for the use of a drum that is comprised of two halves, wherein the conductivity
between such halves is then measured to control the drying process. This method, however,
is unable to ensure any reliable measurement of the moisture content of the drying
load items, mainly owing to conductivity values being affected by such factors as
water hardness.
[0005] It therefore is a main object of the present invention to provide a clothes tumble
drier of the above-mentioned kind, in which a desired moisture content of the drying
load can be automatically attained to a high degree of accuracy at the interruption
of the drying process, and this in a manner that is highly independent of factors
such as the amount of drying load, the type of fabrics being handles, the initial
moisture content of the drying load, as well as water hardness.
[0006] According to the present invention, this aim is reached in a clothes tumble dryer
having the characteristics as recited and defined in the appended claims 1 to 14.
[0007] According to the present invention, the above-mentioned aim is also reached in a
method for controlling a clothes tumble dryer as defined and recited in the appended
claims 15 to 26.
[0008] Anyway, features and advantages of the present invention may be more readily understood
from the description that is given below by way of a non-limiting example with reference
to the accompanying drawings, in which:
- Figure 1 is a schematic view of a clothes tumble dryer according to the present invention;
- Figure 2 a flow chart describing a method for controlling a clothes tumble dryer according
to the present invention;
- Figure 3 is a diagrammatical view of the course followed versus time by the relative
humidity of the drying air entering the condenser during a drying process, along with
the threshold values of relative humidity corresponding to a pre-established drying
degree;
- Figure 4 is a diagrammatical view of the course followed versus time by the temperature
of the drying air entering the condenser and the temperature of the drying air exiting
the condenser during a drying process;
- Figure 5 is a diagrammatical view versus time of the temperature difference pattern
between the drying air entering the condenser and the drying air exiting the condenser
during a drying process, along with the threshold values of such temperature difference
corresponding to a pre-established drying degree;
- Figure 6 is a diagrammatical view versus time of the pattern of the end-of-cycle coefficients
dH and dT, as well as the control quantity D in the case of an "Extra Dry" drying cycle;
- Figure 7 is a diagrammatical view versus time of the pattern of the end-of-cycle coefficients
dH and dT, as well as the control quantity D in the case of a "Dry Cottons" drying cycle;
- Figure 8 is a diagrammatical view versus time of the pattern of the end-of-cycle coefficients
dH and dT, as well as the control quantity D in the case of a "Slightly Damp" drying cycle;
- Figure 9 is a diagrammatical view versus time of the pattern of the end-of-cycle coefficients
dH and dT, as well as the control quantity D in the case of an "Iron Dry" drying cycle;
- Figure 10 is a diagrammatical view versus time of the pattern of the end-of-cycle
coefficients dH and dT, as well as the control quantity D in the case of an "Mangle Dry" drying cycle.
[0009] The clothes tumble dryer according to the present invention - as generally indicated
by the reference numeral 1 in the Figures - comprises a closed-loop drying air circuit
adapted to cause heated drying air to be circulated through a perforated rotating
drum 2 holding the washload to be dried, heating means 3 adapted to heat up said drying
air, an air-cooled condenser 4 adapted to remove the moisture from the hot moisture-laden
drying air exiting the drum after having flown through the washload to be dried, an
open-loop cooling air circuit adapted to cause a stream of cooling air taken in from
outside to circulate through and over the condenser 4 to cool it, and be eventually
exhausted again into the outside atmosphere.
[0010] The above-mentioned tumble dryer also calls for temperature sensor means 5, 6 to
be provided in correspondence to a drying-air condenser inlet 7, as well as in correspondence
to a drying-air condenser outlet 8, in order to detect a temperature difference value
ΔT between the drying air entering the condenser 4 and the drying air exiting the
same condenser 4; it further calls for humidity sensor means 5 to be provided in correspondence
to the drying-air condenser inlet 7, in order to detect a relative humidity value
RH
in of the drying air entering the condenser.
[0011] In addition, the above-mentioned tumble dryer comprises control means coupled to
both the humidity sensor means 5, so as to be able to receive the humidity values
RH
in, and the temperature sensor means 5, 6 to also receive the temperature difference
values ΔT during the operation of the tumble drier.
[0012] The tumble dryer is further provided with memory means for storing a threshold temperature-difference
value ΔT
TH between the drying air entering the condenser 4 and the drying air exiting the same
condenser 4, and a threshold humidity value RH
inTH for the drying air entering the condenser, which are indicative of a pre-established
dryness level, i.e. degree of drying of the items loaded in the drum in accordance
with the particular drying cycle selected by the user, wherein said threshold temperature-difference
value ΔT
TH and said threshold humidity value RH
inTH are initialized in response to said tumble dryer being started to operate according
to the particular drying cycle selected by the user.
[0013] The control means are adapted to compare the actual humidity value RH
in with the threshold humidity value RH
inTH, and are also adapted to compare the temperature difference value ΔT with the threshold
temperature difference value ΔT
TH, for automatically turning off said heating means 3 when at least the temperature
difference value ΔT reaches up to or beyond the threshold temperature difference value
ΔT
TH or at least the humidity value RH
in reaches down to or below the humidity threshold value RH
inTH.
[0014] Through the closed-loop drying air circuit, the hot and moisture-laden drying air
is caused to leave the perforated rotating drum 2 and is conveyed towards the air-cooled
condenser 4; then, the dehydrated drying air exiting the condenser 4 is sent back
into the drum 3, upon having been duly heated up again, so as to remove additional
moisture from the clothes being tumbled in the drum 2.
[0015] The tumble dryer comprises first blower means 11 adapted to cause the drying air
to circulate inside and through the closed-loop drying air circuit. A first manifold
arrangement 12 is provided to convey the drying air exiting the drum 2 into the drying-air
condenser inlet 7, and a second manifold arrangement 13 is provided to convey the
drying air exiting the drying-air condenser outlet 8 again into the drum 2.
[0016] Heating means 3 are provided downstream of the drying-air condenser outlet 8 to heat
up the dehydrated drying air due to be sent again into the drum 2.
[0017] The condenser 4 comprises a plurality of fluid passageways, along which the drying
air is able to flow in view of having the moisture condensed and removed therefrom,
and these fluid passageways are capable of being invested by, i.e. are exposed to
the flow of cooling air flowing through the open-loop cooling air circuit. In particular,
the condenser 4 comprises a first array of spaced-apart fluid passageways 14 for the
drying air, which are fluidly connected with the first manifold arrangement 12 at
the drying-air condenser inlet 7, as well as fluidly connected with the second manifold
arrangement 13 at the drying-air condenser outlet 8.
[0018] The condenser 4 further comprises a second array of spaced-apart fluid passageways
15 for the cooling air, which are arranged in an alternating relationship relative
to said first array of fluid passageways 14, so that between two such adjacent fluid
passageways 14 for the drying air there is provided a fluid passageway 15 for the
cooling air.
[0019] The cooling air that flows along the second array of fluid passageways 15 is adapted
to brush, i.e. move in contact with the fluid passageways 14 of the drying air, thereby
cooling it down by convection so as to cause water vapour in the hot moisture-laden
drying air to precipitate by condensation.
[0020] The open-loop cooling air circuit comprises second blower means 16 adapted to cause
the cooling air to circulate, an air intake aperture 17 and an air exhaust aperture
18 fluidly connected with the second array of fluid passageways 15, through which
the cooling air is taken in fron the outside ambient, conveyed to and through the
condenser, to be eventually sent back into the outside ambient again.
[0021] The drying-air condenser inlet 7 substantially represents the zone of the condenser
at which the drying air to be dehydrated, and flowing in from the first manifold arrangement,
enters the first array of fluid passageways 14, whereas the drying-air condenser outlet
8 represents the zone of the condenser at which the dehydrated drying air exits the
first array of fluid passageways 14 to flow into the second manifold arrangement 13.
[0022] Most obviously, these zones are not fluidly connected with the cooling air passageways,
since both the inlet and the outlet of such second array of fluid passageways 15 are
suitably separated from the drying-air condenser inlet 7 and the drying-air condenser
outlet 8, respectively.
[0023] In the particular embodiment, in fact, the inlet and the outlet of the second array
of fluid passageways 15 are provided in a perpendicular arrangement relative to the
drying-air condenser inlet 7 and the drying-air condenser outlet 8, respectively,
so that the flows of drying air and the flows of cooling air practically cross each
other.
[0024] The temperature and humidity sensor means 5, 6 comprise a relative humidity sensor,
which is also capable of delivering accompanying temperature measurement values, and
which is arranged in the vicinity of the drying-air condenser inlet 7, and a temperature
sensor, such as for instance a thermocouple, which is situated in the vicinity of
the drying-air condenser outlet 8. Both sensors are connected to the control means,
which are adapted to receive - during the operation of the dryer - the signals being
output by the sensors so as to monitor the pattern of the temperature T
in of the drying air entering the condenser, as well as the pattern of the temperature
T
out of the drying air exiting the same condenser, along with the relative humidity RH
in of the drying air entering the condenser.
[0025] This data acquisition process by said control means may occur either continuously
or at discrete intervals, by setting in this case a proper time interval to separate
successive data acquisition events during which said control means are able to acquire
the temperature values T
in, T
out, and the relative humidity values RH
in being detected and output by the sensors.
[0026] Extensive experimental tests have emphasized the fact that both the relative humidity
values RH
in and the temperature values T
in, T
out of the drying air being detected as the clothes undergo gradual drying in the machine,
fully correlate, i.e. can be brought into mutual relation with different final drying
degrees of the clothes themselves.
[0027] It has in this way been possible to work out appropriate threshold temperature and
relative humidity values of the drying air, which correspond to the various degrees
of final drying that are usually required for clothes handled in such tumble dryers,
in such a way as to ensure that reaching any of these threshold values causes the
air heating means to be de-energized and, possibly, the operation of the dryer itself
to be stopped by the control means.
In detail, in view of judging the final drying degree of the clothes, reference has
been made - according to existing standards - to residual moisture MR (moisture regain),
as assessed in terms of difference in weight of a load of clothes subjected to a drying
cycle, to a same load of clothes under standard conditions, i.e. left for 24 hours
in a controlled atmosphere at a temperature of 20°C and 60% of relative humidity.
[0028] Some among the most used drying cycles commonly available in tumble dryers have then
been selected, i.e.:
Drying Cycle |
Final MR |
Extra Dry (ED) |
-6% → 0% |
Dry Cottons (DC) |
-3% → 3% |
Slightly Damp (SD) |
4% → 8% |
Iron Dry (ID) |
8% → 16% |
Mangle Dry (MD) |
18% → 24% |
[0029] For example, in the case of the Extra Dry cycle, the drying load - as removed from
the drum at the end of the cycle, has a weight ranging from about 6% less than the
weight of the above-specified standard load to almost the same weight as the standard
load. On the contrary, in the case of the Iron Dry cycle, the drying load - as removed
from the drum at the end of the cycle - has a weight ranging from 8% to 16% more than
the weight of said standard load.
[0030] The tests performed in this connection have given rise to the graph appearing in
Figure 3 illustrating the trend versus time of the relative humidity value RH
in of the drying air as detected by the moisture sensor means 5 at the drying-air condenser
inlet 7, with the heating means 3 operating, i.e. energized. As emphasized by the
graph, the relative humidity RH
in tends to decrease as the drying degree of the clothes increases. Further indicated
in Figure 3 are the threshold values for the relative humidity RH
inTH[ED,DC,SD,ID,M] of the drying air entering the condenser, which have been found as
corresponding to the different degrees of drying of the clothes in the above-noted
cycles ED,DC,SD,ID,M.
[0031] In the description following hereinbelow, the term "relative humidity value RH
in" will be used to univocally mean the value of relative humidity of the drying air
as detected by the moisture sensor means 5 at the drying-air condenser inlet 7, whereas
the term "threshold relative-humidity value RH
inTH[ED,DC,SD,ID,M]" will be used to univocally mean the threshold value of relative humidity
of the drying air, which - as detected again by the moisture sensor means 5 at the
drying-air condenser inlet 7 - has been found to correspond to the different degrees
of drying of the clothes in the cycles ED,DC,SD,ID,M as indicated above.
[0032] RH
o is used to indicate the initial value of the relative humidity of the drying air
flowing into, i.e. entering the condenser 4 as detected by the moisture sensor means
5 upon starting of the drying cycle selected by the user and, therefore, upon the
heating means having been switched on.
[0033] Immediately after energization of the heating means 3, the value of RH
o can be noticed to be extremely variable to eventually go steadier at a relative-humidity
value of approx. 96 to 97% after a first stabilization transient. It has been found
that the initial value RH
o is the same for all drying cycles.
[0034] During this first transient phase, the control means of the machine do not acquire
any of the relative-humidity readings RH
in that they receive from the moisture sensor means 5, since the related data are not
representative of the real initial relative-humidity condition RH
o.
[0035] Thus, the control means are programmed to set themselves in a kind of observation
state, in which they practically determine whether successive readings flowing in
from the moisture sensor means 5 reveal a substantially constant value. In this connection,
it will be readily appreciated that the control means may also be programmed to simply
wait until a certain time interval of a duration corresponding to the aforementioned
transient phase elapses, before they start acquiring the data being output by the
sensor means. It has been experimentally found that such transient phase has a duration
of approx. two minutes.
[0036] As this most clearly appears from the graph in Figure 3, the relative humidity RH
in decreases as the clothes in the drum becomes increasingly dry, until it eventually
reaches a respective threshold value corresponding to a certain final drying degree
or state reached by the clothes themselves. For example, when the value of relative
humidity RH
in reaches down to at least approx. 68%, it has been found that degree of drying reached
by the clothes bneing handled corresponds to a degree of drying that is typical of
the Extra Dry (ED) cycle.
[0037] As a result, this relative humidity value of 68% turns out as being the threshold
relative-humidity value RH
inTH[ED] that can be associated to said Extra Dry cycle ED, and - as such - is thereforee
stored in the memory means for due reference. When the user then selects an Extra
Dry cycle ED via the proper controls on the control panel of the tumble dryer, the
control means automatically retrieve the threshold relative-humidity value RH
inTH[ED] associated to such drying cycle from the memory means and, at the same time,
enable the dryer to start operation by energizing the heating means 3, switching on
the blower means 11, 16 and enabling the drum 2 to be driven rotatably.
[0038] Then, the control means - upon waiting for the initial transient observation phase
to elapse - are enabled to start comparing the relative humidity value RH
in with the threshold relative-humidity value RH
inTH[ED] for the Extra Dry [ED] cycle. When the relative humidity value RH
in is eventually found to have reached down to or below said threshold value RH
inTH[ED], the control means will then consider the selected drying cycle as being concluded
and, as a result, cause the operation of the tumble dryer to stop by de-energizing
the heating means 3, switching off the blower means 11, 16 and stopping the rotation
of the drum 2. Since a cold air circulation phase may also be contemplated to take
place at the end of the actual drying process in order to cool down the clothes before
removing them from the drum 2, when the relative humidity value RH
in is found to have reached down to or below said threshold value RH
inTH [ED], the control means would in this case simply de-energize the heating means 3,
while waiting for this cool air circulation phase to be concluded before stopping
the operation of the tumble dryer.
[0039] A degree of drying of the clothes corresponding to the one that may be considered
typical for the Dry Cottons (DC) cycle, has on the contrary been found to exist when
the relative humidity value RH
in reaches down to at least approx. 87%. This value is therefore stored in the memory
means as the threshold relative-humidity value RH
inTH [DC] associated to said Dry Cottons [DC] drying cycle.
[0040] Summarized in the table below are the threshold relative-humidity values RH
inTH[ED,DC,SD,ID,M] for the different drying cycles:
Cycle |
Final MR |
RHin Threshold |
Extra Dry |
-6% → 0% |
68% |
Dry Cottons |
-3% → 3% |
87% |
Slightly Damp |
4% → 8% |
93% |
Iron Dry |
8% → 16% |
93.5% |
Mangle Dry |
18% → 24% |
94% |
[0041] The tests that have been carried out to this effect have furthermore produced the
graph represented in Figure 4, which illustrates the trend versus time of the drying
ait temperature T
in as detected by the temperature sensor means 5 at the drying-air condenser inlet 7,
as well as the trend versus time of the drying air temperature T
out as detected by the temperature sensor means 6 at the drying-air condenser outlet
8, during the operation of the heating means 3.
[0042] Emphasized in this graph is the fact that the drying cycle is basically comprised
of three distinct phases, actually. A first initial transient phase, which comes immediately
after the energization of the heating means 3, and in which both temperatures T
in, T
out and also the evaporation rate increase.
[0043] The second phase is a saturation one, in which the temperatures T
in, T
out remain almost constant, since the thermal heating energy causes the water contained
in the clothes being handled to evaporate, while the condenser is capable of dissipating
most of the latent condensation heat.
[0044] There follows a third, final phase, in which the temperature of the drying air at
the condenser inlet T
in tends to increase, owing to the clothes becoming increasingly dry, i.e. less and
less damp, and the temperature of the drying air at the condenser outlet T
out tends on the contrary to decrease, since the evaporation rate is decreasing and the
condenser is able to dissipate also the heat in the drying air, thereby cooling it
down.
[0045] It clearly appears that the temperature difference ΔT between the drying air temperature
at the condenser inlet T
in and the drying air temperature at the condenser outlet T
out is fully capable of being correlated to the actual residual moisture content in the
clothes to be dried, i.e. the greater the value of ΔT = T
in-T
out, the higher the degree of drying of the clothes.
[0046] Even in this case, the temperature values detected by the temperature sensor means
5, 6 following the energization of the heating means 3, are extremely variable and
are by no means indicative of the actual trend of the temperatures. Therefore, the
control means wait for this initial observation or transient phase to be concluded
before starting with the acquisition of said values.
[0047] The graph in Figure 5 illustrates the trend versus time of the value of the temperature
difference ΔT = T
in-T
out, as well as the threshold values of this temperature difference ΔT
TH [ED,DC,SD,ID,M] of the drying air between condenser inlet and condenser outlet, which
have been found as corresponding to the various degrees of drying due to be reached
by the clothes in the respective drying cycles ED, DC, SD, ID, M.
[0048] For example, it has been found that, when the temperature difference ΔT reaches a
value of at least 15°C, the degree of drying reached by the clothes in the drum corresponds
to the degree of drying due to be reached in the Extra Dry cycle ED.
[0049] A temperature difference value of 15°C turns therefore out as being the threshold
temperature-difference value ΔT
TH [ED] associable to the Extra Dry cycle ED and, as such, it is stored in the memory
means.
[0050] In the description following hereinbelow, the term "temperature difference value
ΔT" will be used to univocally mean the value of the difference between the temperature
of the drying air as detected by the temperature sensor means 5 at the drying-air
condenser inlet 7 and the temperature of the drying air as detected by the temperature
sensor means 6 at the drying-air condenser outlet 8, whereas the term "threshold temperature-difference
value ΔT
TH" will be used to univocally mean the threshold value of the difference between the
temperature of the drying air as detected by the temperature sensor means 5 at the
drying-air condenser inlet 7 and the temperature of the drying air as detected by
the temperature sensor means 6 at the drying-air condenser outlet 8 that has been
found to correspond to the different degrees of drying of the clothes in the cycles
ED,DC,SD,ID,M in the experimental tests performed to this effect.
[0051] When a user selects an Extra Dry drying cycle ED via the proper control devices on
the control panel of the tumble dryer, the control means of the machine start by automatically
retrieving the threshold temperature-difference value ΔT
TH [ED] corresponding to that cycle from the memory means, and start dryer operation
by energizing the heating elements 3, switching on the air blower means 11, 16, and
enabling the drum 2 to be driven rotatably. Then, upon allowing the initial observation
time to elapse, the control means are capable of comparing the value of the temperature
difference ΔT, as detected by the temperature sensor means 5, 6 with the thershold
temperature-difference value ΔT
TH [ED] of the Extra Dry cycle ED. As soon as the temperature difference value ΔT reaches
at least said threshold value ΔT
TH [ED], the control means consider the selected drying cyucle as being concluded and
stop dryer operation by de-energizing the heating means 3, switching off the air blower
means 11, 16, and stopping drum rotation. In the case that a further unheated air
circulation phase is provided to take place after the conclusion of the actual drying
process in order to allow the clothes to be cooled down before they are removed from
the drum, the control means simply cause the heating means 3 to be de-energized when
the temperature difference value ΔT reaches said threshold temperature-difference
value ΔT
TH [ED], while waiting for said cold air circulation phase to be concluded to turn off
the tumble dryer and fully stop its operation.
[0052] Summarized in the table below are the threshold temperature-difference values for
the different drying cycles:
Cycle |
Final MR |
ΔTTH Threshold |
Extra Dry |
-6% → 0% |
15 °C |
Dry Cottons |
-3% → 3% |
7.4 °C |
Slightly Damp |
4% → 8% |
5.9 °C |
Iron Dry |
8% → 16% |
5.4 °C |
Mangle Dry |
18% → 24% |
4.9 °C |
[0053] The threshold temperature-difference values ΔT
TH[ED,DC,SD,ID,M] are independent of the initial quantity of water contained in the
clothes to be dried. In fact, the transition from the intermediate saturation phase
to the final phase of the drying cycle occurs at a given value of residual moisture,
i.e. moisture that is still in the clothes being dried, wherein this value solely
depends on the design parameters of the condenser.
[0054] According to the present invention, the method for controlling the drying cycles
in a tumble dryer calls for the drying cycle selected by the user to be first of all
read and the threshold humidity value RH
inTH and the threshold temperature-difference value ΔT
TH to be then set accordingly, i.e. on the basis of the selected cycle. These threshold
values are retrieved from the memory means, in which they are properly stored.
[0055] Such reading of the cycle being selected and such setting of the corresponding values
are performed by the control means, which are connected to the control devices in
a control panel provided on the tumble dryer. The selected drying cycle is then immediately
started by these control means by starting the operation of the dryer in the way described
hereinbefore.
[0056] The method according to the present invention then calls for the relative humidity
value RH
in and the temperature value T
in of the drying air entering the condenser, as well as the temperature value T
out of the drying air exiting the condenser to be detected as the drying cycle goes on.
The control means acquire the data delivered by the humidity and temperature sensor
means 5, 6 so as to monitor the trend of the relative humidity RH
in and the temperature difference value ΔT as the drying cycle progresses.
The method provides for the so detected relative humidity value RH
in to be compared with the threshold humidity value RH
inTH that has been set in accordance with the drying cycle selected by the user, and for
the temperature difference value ΔT to be compared with the threshold temperature
difference value ΔT
TH that has again been set in accordance with the drying cycle selected by the user.
[0057] The method then provides for the heating means 3 to be switched off when at least
the temperature difference value ΔT reaches up to or beyond the threshold temperature-difference
value ΔT
TH or when at least said humidity value RH
in reached down to or below the threshold humidity value RH
inTH.
[0058] When one of these threshold values is reached, further to switching off the heating
means 3, the method may also provide for the tumble dryer to be fully stopped from
operating, under de-energization of the air-circulating blower means 11, 16 and the
motor means used to rotatably drive the drum.
[0059] The method preferably also provides for a transient observation phase to be included
immediately upon the heating means 3 having been so switchen on, in which the control
means just wait for the humidity and temperature data from the sensor means to become
steadier before they start acquiring said data.
[0060] The provision of two distinct threshold values, i.e. RH
inTH and ΔT
TH, for each drying cycle that can be selected by the user, ensures greater effectiveness
and accuracy in detecting the actual end-of-cycle condition, particularly in the case
of drying cycles, such as Slightly Damp, Iron Dry and Mangle Dry, requiring less drying,
i.e. a lower degree of dryness in the clothes, and having threshold values that lie
rather close by.
[0061] It has been found and demonstrated experimentally that the trend of both RH
in and ΔT does not depend on the weight of the drying load, i.e. the amount of clothes
to be dried.
[0062] The method for controlling the drying cycles in a clothes tumble dryer according
to the present invention is not limited by, i.e. does not necessarily require the
use of a condenser that is made and designed as has been described above by way of
example. In the case of a tumble dryer that makes use of a different type of condenser,
or a different condenser design, the trend of both temperature difference ΔT and relative
humidity RH
in will in fact be much the same as the trends that have been found in the course of
the above-described experimental tests considered herein, since the thermodynamics
at the basis of these trends is anyway the same, regardless of the condenser being
actually used.
[0063] The curves describing the trend versus time of humidity RH
in and temperature difference ΔT will most obviously have a different steepness, and
different will clearly be also the threshold values RH
inTH, ΔT
TH corresponding to the degrees of drying to be reached in the various drying cycles
that can be selected by the user.
[0064] Also the drying cycles that are made available to the user may of course be set and
fixed according to principles that differ from the aforedescribed ones based on residual
moisture as compared to a same drying load under standard conditions as a control
parameter.
[0065] It can however be most readily appreciated that - even in the presence of a different
condenser design - it is anyway possible for both the threshold relative-humidity
values RH
inTH of the drying air entering the condenser and the threshold temperature-difference
values ΔT
TH of the drying air entering and exiting the condenser attributable to the various
user-selectable drying cycles to be derived experimentally.
[0066] Such threshold values will then be made available to the control means via the memory
means as soon as the operation of the tumble dryer is started, in accordance with
the particvular drying cycle selected by the user.
[0067] In a preferred embodiment of the present invention, the control means are adapted
to numerically filter the signals they receive from the moisture and temperature sensor
means 5, 6 so as to eliminate all detected values that are found to significantly
depart from the main trend versus time of both the relative humidity values RH
in and temperature difference values ΔT being monitored by said control means during
the drying cycle, i.e. as the clothes in the drum undergo drying.
[0068] Basically, the control means are adapted to discard any distorted, altered or perturbed
value as may be detected by the moisture and temperature sensors, in such manner as
to solely retain the main dynamics of the evolution of relative humidity RH
in and temperature difference ΔT with time.
[0069] Such disturbances in detected values include perturbed measurements that do not correspond
to the actual humidity and temperature values and can occur due to such measurements
being affected by the dynamic, i.e. flow conditions of the drying air.
[0070] A further cause of disturbance in detected values may be due to the fact that, for
an even and effective drying of the clothes being tumbled in the drum to be ensured,
during the drying cycle there are provided reversal periods, in which the heating
means 3 are switched off and the drum is rotated in the reverse direction, i.e. counter-rotated
so as to allow the clothes held therein to be counter-tumbled and reshuffled. These
reversal periods last just a few seconds and are separated from each other by pre-determined
intervals of a few minutes each provided therebetween.
[0071] Most obviously, detections performed by the sensor means within these reversal periods
tend to result in the measurement of humidity values RH
in and temperature difference values ΔT that deviate to a rather considerable extent
from the trend of the corresponding values that are on the contrary detected when
the heating means 3 are switched on. In fact, as this can clearly be noticed in the
graphs appearing in Figures 3, 4 and 5, in corrispondence to, i.e. during said reversal
periods the temperature-difference curve ΔT shows minimum-value peaks, whereas the
relative-humidity curve RH
in shows maximum-value peaks.
[0072] The control means filter these humidity values RH
in and temperature difference values ΔT with the aid of a numeric low-pass filter.
[0073] In particular, the control means associate a first numeric low-pass filter to the
humidity values RH
in and a second numeric low-pass filter to the temperature difference values ΔT, while
both filters preferably comprise a numeric unit-gain low-pass filter of the first
order according to following formulas:
in which the coefficient
a represents the parameter of the filters, the detected relative-humidity values RH
in represent the input variables of the first filter at the different instants
n, and
yn represents the current value of the first filter at the different instants
n, whereas the detected temperature-difference values ΔT represent the input variables
of the second filter at the different instants
n and
un represents the current value of the second filter at the different instants
n.
[0074] It has been found experimentally that - for the application considered - the parameter
a of the filters can take a value ranging from 0.9 to 0.99.
[0075] In the particular embodiment being described, it has been opted to use the same value
of the parameter
a for all drying cycles selectable by the user, and for both the numeric filters, in
order to avoid weighing too heavily on the processing load of the control means.
[0076] However, in order to ensure a more specific filtering effect, the value of the parameter
a may vary in accordance with the various drying cycles that can be selected by the
user, while within a same cycle selected by the user, the value of the parameter
a may again differ in the two numeric low-pass filters.
[0077] The values of the parameter
a of the filters are stored in the memory means and are adapted to be retrieved by
the control means when the drying cycle selected by the user is started.
[0078] The control means acquire the relative-humidity values RH
in and the temperature-difference values ΔT detected by the corresponding sensor means,
and then calculate, i.e. work out the related numeric filters to obtain a corresponding
filtered relative-humidity value RH
infiltered and a corresponding filtered temperature-difference value ΔTfiltered giving
the curves indicated in Figures 3 and 5.
[0079] The control means are adapted to compare each such filtered relative-humidity value
RH
infiltered with the respective threshold relative-humidity value RH
inTH[ED,DC,SD,ID,M] corresponding to the actual drying cycle selected by the user, and
are furthermore adapted to compare each such filtered temperature-difference value
ΔTfiltered with the respective threshold temperature-difference value ΔT
TH[ED,DC,SD,ID,M] that corresponds to the actual drying cycle selected by the user.
[0080] In particular, the control means are adapted to determine a first end-of-cycle coefficient
dH and a second end-of-cycle coefficient
dT, which represent how close the filtered relative-humidity value RH
infiltered has got to the respective threshold value RH
inTH[ED,DC,SD,ID,M], and how close the filtered temperature-difference value ΔTfiltered
has got to the respective threshold value ΔT
TH[ED,DC,SD,ID,M], respectively. These coefficients are determined by comparing the
filtered values with the respective threshold values, according to following formulas:
[0081] These end-of-cycle coefficients
dH and
dT turn therefore out as being pure numbers that tend to unit as the the filtered values
RH
infiltered and Δtfiltered get closer to the respective threshold values RH
inTH[ED,DC,SD,ID,M] and ΔT
TH[ED,DC,SD,ID,M].
[0082] Thus, the control means are adapted to compose the end-of-cycle coefficients
dH and
dT so as to obtain a single control quantity D, on the basis of which they therefore
decide that the drying cycle is to be caused to stop. This control quantity D is obtained
by calculating the weighted average of the end-of-cycle coefficients
dH and
dT through a weight factor δ that determines the specific contribution of the individual
coefficients in accordance with the drying cycle selected, according to following
formula:
wherein the value of the weight factor δ depends on the selected cycle, so as to determine
which quantity, between relative humidity RH
in and temperature difference ΔT, is more significant in view of identifying the end-of-cycle
condition. The weight factor δ is variable between 0 and 100 and depends on the selected
cycle.
[0083] This is of particular value in the case of drying cycles as the Iron Dry and Mangle
Dry ones, in which it is the value of temperature difference ΔT that prevainlingly
determines said end-of-cycle condition, since the value of relative humidity RH
in is less reliable.
[0084] The value of the weight factor δ has been determined experimentally for the different
drying cycles provided for selection in a clothes tumble dryer, and is stored in the
memory means so as to be made available for retrieval by the control means when the
selected drying cycle is started.
[0085] The end-of-cycle condition occurs when the thus calculated control quantity D turns
out to be equal to or in excess of a stop parameter sD that is variable from 85 to
100 in accordance with the cycle selected by the user.
[0086] Even the value of this stop parameter sD has been determined experimentally for the
different drying cycles provided for selection in a clothes tumble dryer, and is stored
in the memory means so as to be made available for retrieval by the control means
when the selected drying cycle is started.
[0087] Summarized in the following table are the threshold values RH
inTH[ED,DC,SD,ID,M], ΔT
TH[ED,DC,SD,ID,M], the values of the end-of-cycle coefficients
dH and
dT, the values of the weight factor δ, and the values of the stop parameters sD for each
one of a number of drying cycles:
Cycle |
RHinTH |
ΔTTH |
δ |
sD |
Extra Dry |
68% |
15 °C |
50 |
100 |
Dry Cottons |
87% |
7.4 °C |
50 |
100 |
Slightly Damp |
93% |
5.9 °C |
50 |
100 |
Iron Dry |
93.5% |
5.4 °C |
60 |
95 |
Mangle Dry |
94% |
4.9 °C |
60 |
85 |
[0088] When such end-of-cycle condition is found to exist by the control means, these cause
the heating means 3 to be switched off and, if provided or required so, the air-circulating
blower means to be de-energized along with the motor means used to rotatably drive
the dryer's drum.
[0089] According to a preferred embodiment of the present invention, the method for controlling
the operation of a clothes tumble dryer includes an initialization phase, in which
the control means read the cycle that has been selected by the user and set the threshold
relative-humidity values RH
inTH[ED,DC,SD,ID,M], the threshold temperature-difference values ΔT
TH[ED,DC,SD,ID,M], the value of the parameter
a of the filters, the values of weight factor δ and the value of the stop parameter
sD, which correspond to the selected cycle, by retrieving the related values from
the memory means.
[0090] The control means cause then the heating means 3 to be switched on and the air-circulating
blower means 11, 16 to be energized along with the drum rotation driving means.
[0091] It is to be noted that in a further simplified embodiment of the present invention
the end-of-cycle coefficients
dH and
dT can be determined by comparing the current values of temperature difference ΔT and
relative humidity RH
in, non filtered, with the respective threshold values. The end-of-cycle coefficients
dH and
dT are then composed, in the manner above described, to obtain the single control quantity
D on the basis of which the control means switch off the heating means 3.
[0092] The control means are adapted to check the heating means 3 for the on/off state thereof
through a proper operation sensor.
[0093] The method further includes a so-called transient observation phase, in which the
control means just wait for the values of relative humidity RH
in and the values of temperature difference ΔT to stabilize.
[0094] In a preferred manner, within this transient observation phase there may be provided
at least a reversal period, in which the heating means 3 are switched off and the
drum 2 driven to rotate in an opposite direction, i.e. counter-rotated.
[0095] When the detected values of relative humidity RH
in and the detected values of temperature difference ΔT are detected to have become
steadier, the control means check the heating means 3 for the on/off state thereof.
[0096] In the case that the heating means 3 are in the off state, i.e. the tumble dryer
is performing a reversal period, the control means do not acquire said values of relative
humidity RH
in and said values of temperature difference ΔT , and wait for the heating means 3 to
switch in the on state thereof.
[0097] In the case that the heating means 3 are found in the on state thereof, the method
calls for an initialization phase to occur, in which the control means acquire the
value of relative humidity RH
in and the value of temperature difference ΔT detected by the sensor means 5,6 to initialize
the related numeric low-pass filters by calculating the corresponding filtered initial
value of relative humidity RH
infiltered and the corrisponding filtered initial value of temperature difference ΔTfiltered
[0098] As a selected drying cycle progresses, the control means are adapted to acquire the
relative-humidity values RH
in and the temperature-difference values ΔT detected by the sensor means, and to filter
these values with the aid of the numeric low-pass filters at pre-determined successive
time intervals; in the described embodiment there is provided a waiting time interval
between two successive acquisition events of approx. 5 seconds.
[0099] The control means wait for such waiting time interval to elapse and, when this runs
out, check the heating means 3 to see whether they are on or off.
[0100] In the case that the heating means 3 are found to be in the off state thereof, i.e.
the tumble dryer is going through a reversal period, the control means - in a loop-like
cyclic manner - wait again for the waiting time interval to elapse and, when this
runs out, check again the heating means 3 for the on/off state thereof.
[0101] If the heating means 3 are on the contrary found to be in the on state thereof, the
control means acquire the value of relative humidity RH
in and the value of temperature difference ΔT as detected by the respective sensor means,
and calculate the related numeric low-pass filters to determine the corresponding
filtered relative-humidity value RH
infiltered and the corresponding filtered temperature-difference value ΔTfiltered, in
the same manner as described hereinbefore.
[0102] Then, the control means check whether the end-of-cycle condition has eventually occurred
by calculating the values of the end-of-cycle coefficients
dH and
dT, the value of the control quantity
D, and comparing the resulting value of the latter with the value of the stop parameter
sD.
[0103] If the value of the control quantity
D turns out to be equal to or in excess of the value of the stop parameter sD, the
control means switch off the heating means 3 and, if possibly provided or required
so by the cycle, cause the operation of the dryer to stop.
[0104] If the value of the control quantity D is found to be lower than the value of the
stop parameter sD, the control means - in a loop-like cyclic manner - wait again for
the waiting time interval to elapse and, when this runs out, check the heating means
3 for the on/off state thereof in view of acquiring again the values of relative humidity
RH
in and temperature difference ΔT, updating the related numeric low-pass filters and
checking again whether the end-of-cycle condition has been reached.
1. Clothes tumble dryer comprising a closed-loop drying air circuit adapted to circulate
heated drying air through a perforated rotating drum (2) holding a load to be dried,
heating means (3) adapted to heat up said drying air, an air-cooled condenser (4)
adapted to remove moisture from the hot moisture-laden drying air exiting said drum,
an open-loop cooling air circuit adapted to circulate over and through the condenser
(4) a stream of cooling air taken in from the outside ambient to cool said condenser,
and to let out said stream of cooling air again into the outside ambient, characterized in that it further comprises temperature sensor means (5, 6) arranged at a condenser drying-air
inlet (7) and at a condenser drying-air outlet (8) in order to detect a temperature
difference value between the drying air entering the condenser (4) and the drying
air exiting the condenser (4), humidity sensor means (5) arranged at said condenser
drying-air inlet (7) in order to detect a humidity value of the drying air entering
the condenser, control means coupled to said humidity sensor means (5) to receive
said humidity value (RHin), and coupled to said temperature sensor means (5, 6) to receive said temperature
difference value (ΔT) during the operation of said tumble drier, memory means for
storing a threshold temperature-difference value (ΔTTH) between the drying air entering the condenser (4) and the drying air exiting the
condenser (4), and a threshold relative-humidity value (RHinTH) of the drying air entering the condenser, which are representative of a pre-determined
dryness degree of the load and depend on the particular drying cycle selected by the
user, said threshold temperature-difference value (ΔTTH) and said threshold relative-humidity value (RHinTH) being initialized in response to said tumble dryer being started operating in accordance
with a given drying cycle selected by the user, said control means being further adapted
to compare said relative humidity value (RHin) with said threshold relative-humidity (RHinTH) and said temperature difference value (ΔT) with said threshold temperature-difference
value (ΔTTH) for switching off said heating means (3) when at least said temperature difference
value (ΔT) reaches said threshold temperature-difference value (ΔTTH) or at least said relative humidity value (RHin) reaches said threshold relative-humidity value (RHinTH).
2. Clothes tumble dryer according to claim 1, wherein said control means comprise numeric
filter means to numerically filter said relative humidity value (RHin) acquired and said temperature difference value (ΔT) acquired, in order to eliminate
all detected values deviating to a significant extent from the main trend versus time
of said values of relative humidity (RHin) and temperature difference (ΔT) being monitored by said control means as the drying
process of the clothes goes on, and establish a corresponding filtered relative-humidity
value (RHinfiltered) and a corresponding filtered temperature-difference value (ΔTfiltered).
3. Clothes tumble dryer according to claim 2, wherein said numeric filter means comprise
a first numeric low-pass filter associated to the relative humidity values (RHin) and a second numeric low-pass filter associated to the temperature difference values
(ΔT).
4. Clothes tumble dryer according to claim 3, wherein said first numeric filter and said
second numeric filter comprise a filtering parameter (a) that depends on the particular drying cycle selected by the user.
5. Clothes tumble dryer according to claim 4, wherein said memory means are adapted to
store the values of said filtering parameter (a), said control means being in turn
adapted to retrieve said values when the drying cycle selected by the user is started.
6. Clothes tumble dryer according to claim 2, wherein said control means are adapted
to compare said filtered relative-humidity value (RHinfiltered) with said threshold relative-humidity value (RHinTH) corresponding to the drying cycle selected by the user, and are further adapted
to compare said filtered temperature-difference value (ΔTfiltered with said threshold
temperature-difference value (ΔTTH) corresponding to the drying cycle selected by the user.
7. Clothes tumble dryer according to any of the preceding claims, wherein said control
means are adapted to determine a first end-of-cycle coefficient (dH) and a second end-of-cycle coefficient (dT) that represent how close the filtered relative-humidity value (RHinfiltered) has got to the respective threshold value (RHinTH), and how close the filtered temperature-difference value (Δtfiltered) has got to
the respective threshold value (ΔTTH], respectively.
8. Clothes tumble dryer according to any of the preceding claims, wherein said control
means are adapted to calculate a weighted average of said end-of-cycle coefficients
(dH, dT) through a weight factor (δ) that depends on the drying cycle selected by the user.
9. Clothes tumble dryer according to any of the preceding claims, wherein said weight
factor (δ) depends on the selected drying cycle and said control means are adapted
to retrieve the value of said weight factor (δ) from said memory means when the selected
drying cycle is started.
10. Clothes tumble dryer according to any of the preceding claims, wherein said control
means are adapted to compare the value of said weighted average with a stop parameter
(sD) that depends on the particular drying cycle being selected, in order to switch
off the heating means (3) when the value of said weighted average reaches up to or
beyond the value of said stop parameter (sD), said control means being adapted to
retrieve the value of said stop parameter (sD) from said memory means when the selected
drying cycle is started.
11. Clothes tumble dryer according to any of the preceding claims, wherein reversal periods
are provided during the drying cycle, in which the heating means (3) are switched
off and the drum (2) is driven to rotate in the opposite direction, said control means
being adapted to solely acquire said humidity values (RHin) and said temperature difference values (ΔT) when said heating means (3) are switched
on.
12. Clothes tumble dryer according to any of the preceding claims, wherein said control
means are adapted to acquire said relative humidity values (RHin) and said temperature difference values (ΔT) in correspondence to acquisition events
that are separated from each other by a pre-set waiting time interval.
13. Clothes tumble dryer according to any of the preceding claims, wherein said numeric
filters comprise a numeric unit-gain low-pass filter of the first order.
14. Clothes tumble dryer according to any of the preceding claims, wherein the value of
said filtering parameter (a) ranges from 0.9 and 0.99.
15. Method for controlling a clothes tumble dryer including a closed-loop drying air circuit
adapted to circulate heated drying air through the clothes to be dried held in a perforated
rotating drum (2), heating means (3) adapted to heat up said drying air, an air-cooled
condenser (4) adapted to remove moisture from the hot moisture-laden drying air exiting
said drum, an open-loop cooling air circuit adapted to circulate over and through
the condenser (4) a stream of cooling air taken in from the outside ambient to cool
said condenser, and then let out again said stream of cooling air into the outside
ambient,
characterized in that it comprises the steps of:
- detecting a relative humidity value (RHin) of the drying air entering the condenser (4), and
- detecting a temperature difference value (ΔT) between the drying air entering the
condenser (4) and the drying exiting the condenser (4),
- retrieving, in response to said tumble dryer being started operating, a stored threshold
temperature-difference value (ΔTTH) between the drying air entering the condenser (4) and the drying air exiting the
condenser (4), and a stored threshold relative-humidity value (RHinTH) of the drying air entering the condenser, both values depending on a drying cycle
selected by the user and being representative of a pre-determined dryness degree of
the clothes being handled,
- acquiring, during the operation of said tumble dryer, said relative humidity value
(RHin) and said temperature difference value (ΔT) detected by the respective sensor means,
- comparing said relative humidity value (RHin) with said threshold relative-humidity value (RHinTH) and said temperature difference value (ΔT) with said threshold temperature-difference
value (ΔTTH) for switching off said heating means (3) when at least said temperature difference
value reaches said threshold temperature-difference value or at least said relative
humidity value reaches said threshold relative-humidity value.
16. Method according to claim 15, further comprising the steps of:
- numerically filtering said acquired relative-humidity values (RHin) and said acquired temperature-difference values (ΔT), in order to eliminate all
detected values deviating to a significant extent from the main trend versus time
of said values of relative humidity (RHin) and temperature difference (ΔT) being monitored by said control means as the drying
process of the clothes goes on, and establish a corresponding filtered relative-humidity
value (RHinfiltered) and a corresponding filtered temperature-difference value (Δtfiltered).
17. Method according to claim 16, wherein said comparison step comprises the steps of:
- comparing said filtered relative-humidity value (RHinfiltered) with said threshold relative-humidity value (RHinTH) corresponding to the drying cycle selected by the user, and
- comparing said filtered temperature-difference value (ΔTfiltered) with said threshold
temperature-difference value (ΔTTH) corresponding to the drying cycle selected by the user.
18. Method according to any of the preceding claims, further comprising the steps of:
- providing reversal periods during the drying cycle, in which the heating means (3)
are switched off and the drum is driven to rotate in the opposite direction,
- solely acquiring said relative humidity value (RHin) and said temperature difference value (ΔT) if said heating means (3) are found to
be switched on.
19. Method according to claim 16, wherein said numerical filtering step comprises the
steps of:
- numerically filtering said acquired relative-humidity value (RHin) through a first numeric low-pass filter, and
- numerically filtering said acquired temperature-difference value (ΔT) through a
second numeric low-pass filter.
20. Method according to claim 16, wherein said numerical filtering step further comprises
the steps of:
- retrieving, in response to said tumble dryer being started operating, a stored numerical
filtering parameter (a) that depends on the drying cycle selected by the user,
- numerically filtering in a differentiated manner said relative humidity value (RHin) and said temperature difference value (ΔT) on the basis of said numerical filtering
parameter (a).
21. Method according to any of the preceding claims, wherein said comparison further comprises
the steps of:
- determining a first end-of-cycle coefficient (dH) and a second end-of-cycle coefficient (dT) that are representative of how close the filtered relative-humidity value (RHinfiltered) has got to the respective threshold value (RHinTH], and how close the filtered temperature-difference value (ΔTfiltered) has got to
the respective threshold value (ΔTTH], respectively.
22. Method according to any of the preceding claims, wherein said comparison step further
comprises the steps of:
- calculating a weighted average of the end-of-cycle coefficients (dH, dT) through a weight factor (δ) that depends on the selected drying cycle and determines
the specific contribution of the individual coefficients in accordance with the selected
cycle, so as to obtain a single control quantity (D),
- retrieving said weight factor (δ) in accordance with the selected drying cycle.
23. Method according to any of the preceding claims, wherein said comparison step further
comprises the steps of:
- comparing the value of said control quantity (D) with a stop parameter (sD) that depends on the selected drying cycle,
- retrieving the value of said stop parameter (sD) in accordance with the selected
drying cycle,
- switching off the heating means (3) in the case that the value of the thus calculated
control quantity (D) is found to be equal to or in excess of the value of said stop parameter (sD).
24. Method according to any of the preceding claims, wherein said acquisition step comprises
the steps of:
- acquiring said relative humidity value (RHin) and said temperature difference value (ΔT) in correspondence to acquisition events
that are separated from each other by a pre-set time interval.
25. Method according to claim 2, comprising the steps of:
- checking, after the tumble dryer has started operating, said relative humidity values
(RHin) and said temperature difference values (ΔT) for becoming steadier,
- starting to acquire said relative humidity values (RHin) and said temperature difference values (ΔT) if these are found to have stabilized.
26. Method according to any of the preceding claims, wherein said numerical filtering
step comprises the steps of:
- numerically filtering the so acquired relative humidity values (RHin) through a first numeric unit-gain low-pass filter of the first order,
- numerically filtering the so acquired temperature difference values (ΔT) through
a second numeric unit-gain low-pass filter of the first order.