EP0503586A1

EP0503586A1 - Improvement in clothes drying machines

Info

Publication number: EP0503586A1
Application number: EP92104162A
Authority: EP
Inventors: Ermanno Zaina; Piero Durazzani
Original assignee: Zanussi Elettrodomestici SpA
Current assignee: Electrolux Zanussi Elettrodomestici SpA
Priority date: 1991-03-12
Filing date: 1992-03-11
Publication date: 1992-09-16
Also published as: ITPN910016A0; IT1250380B; ITPN910016A1

Abstract

Clothes drying machine comprising a rotating drum (4) adapted to contain the washload to be dried and crossed by a flow of air heated by at least an external heater (15), said air being circulated by at least a blower (13) and being exhausted after having subtracted moisture from the clothes to be dried.

During the continuous operation of the clothes drying machine, said flow of exhaust air is vented at a progressively increasing temperature and with correspondingly decreasing moisture contents.

In order to avoid loosing efficiency in exhausting hot and increasingly drier air, said clothes drying machine is adapted to monitor the characteristics of the air inside the rotating drum and to recover heat from the exhaust air.

Said heat recovery can be achieved either through a pre-heating of the fresh in-take air through heat-exchanger means arranged around the drying drum (4) or by mixing the hot exhaust air with the fresh in-take air.

Description

The present invention relates to a clothes drying machine provided with a rotating drum which contains the washload to be dried and in which a continuous flow of hot air is blown to remove the moisture from the washload.
It is well-known that, in order to enable clothes loaded in the drum of traditional-type clothes drying machines to be dried satisfactorily and to also prevent the same clothes from being heated to such an excessive extent that they may be damaged, clothes drying machines may be provided with sensing means that are capable of measuring the conductivity of the clothes to be dried and, as a consequence, the degree of residual moisture in the same clothes, said sensing means are connected in the electrical circuit of the drying machine, together with the drum driving motor, the fan motor and at least a heating element to heat up the drying air.
Said sensing means, which may be given different shapes and sizes, are arranged in the drum so as to ensure that they will be in contact with the clothes throughout the drying process and will generate an output of electric signals corresponding to the varying moisture degree of the clothes during the drying process.
Clothes drying machines of the type described in the US-A-3335501 are further provided with a moisture control device which may for instance consist of a traditional-type electronic comparator circuit connected with both said sensing means and the electrical circuit of the clothes drying machine.
Further types of control arrangements and devices are also known, which may be used to determine the degree of moisture and the state of drying of a washload or, anyway, to ensure the performance of a correct drying cycle.
In general, such control arrangements or devices are adapted to perform a comparison of the electrical output signals generated by the sensing means with predetermined reference electrical signals corresponding to definite conditions under which the clothes may be assumed to be effectively and adequately dried.
Upon reaching such a condition, the control arrangement trips so as to switch off said heating elements and said motor, thereby causing the clothes drying cycle to be terminated under possible extension of the process with a further period of drum rotation or ventilation under no-heat conditions as an anti-crease measure.
Furthermore, such control arrangements may in the same way be adapted to stop clothes drying at any time during an on-going process, based on the signals corning from temperature control thermostats, in the case of any fortuitous failure or irregular operating condition of the machine which is likely to bring about an over-heating of the clothes and, as a consequence, to cause the same clothes to be damaged.
Such hot-air clothes drying machines, however, have a serious drawback. As a matter of fact, if they are operated in a continuous way, as they usually are, a phenomenon takes place, as anyone skilled in the art would most easily understand, according to which, owing to the moisture contents of the clothes decreasing gradually, the air which is exhausted from the machine will turn out to be increasingly hotter and, more than that, increasingly drier, ie. containing a continuously lowering amount of moisture, as this is symbolically illustrated in the diagram shown in Figure 1. This leads to the undesired consequence that part of the overall amount of energy that is spent to raise the temperature of the air at the inlet of the drum is practically wasted since, for an equal amount of heat being introduced in the drum, ie. for an equal heat input rate, there is a gradual reduction in the moisture removal rate.
As an ultimate consequence, a significant part of the hot air blown into the drum is not actually contributing to the evaporation of moisture from the clothes and, therefore, is not really giving any valuable result.
This practically results in the fact that the energy consumption of the machine increases in a macroscopic and unforeseeable way according to the particular nature of the fabrics to be dried and the process parameters selected (ie. weight of washload, drum rotating rate or speed, drum dimensions, air flow paths, etc.).
It is the purpose of the present invention to eliminate all afore mentioned drawbacks and limitations by providing a clothes drying machine which comprises a rotating drum and is further provided with means for recovering the heat exhausted by the machine through the use of simple construction technologies.
This and further aims are reached according to the invention in a clothes drying machine comprising a rotating drum adapted to contain the damp clothes to be dried, said drum being crossed by the flow of drying air that is heated up by at least a heating element, and various types of control means adapted to cause said heating element to be switched on and off appropriately.
According to a particular feature of the present invention, the clothes drying machine being considered is characterized in that the heat contained in the exhaust air is partly recovered according to various methods, which will be described hereinafter, so as to be used to pre-heat fresh air taken in from the outside.
One of the afore cited heat recovery methods calls for the direct use of the exhaust air as a heat-transfer medium. As a matter of fact, such an exhaust air, which is still hot, albeit damp, can be mixed in variable and controllable proportions with the fresh, substantially dry air taken in from the outside, thereby succeeding in effectively utilizing a significant part of the heat generated by the machine for virtually several phases of the drying process.
A second heat recovery methods may consists in using the same clothes containing drum as a heat-exchanger means to pre-heat the fresh air taken in from the outside.
The invention will be further described by way of non-limiting example with reference to the accompanying drawings in which:

Figure 1 is a diagram showing typical curves for the evolution of both temperature and moisture contents of the exhaust air of a traditional-type clothes drying machine;
Figure 2 is a side view showing schematically the cross-section of the clothes drying machine according to the present invention;
Figure 3 is the cross-section view of a detail of the clothes drying machine shown in Figure 2;
Figure 4 is a view schematically illustrating the second afore cited method for recovering exhaust heat from a clothes drying machine.

Referring particularly to Figure 2, it can be seen that the clothes drying machine according to the present invention consists of an outer casing 3 enclosing a rotating drum 4, which is adapted to contain the damp clothes to be dried and is rotatably driven by means of a drive motor of a per sè known type (not shown).
The rear flange 7 of the rotating drum 4 is provided with throughout openings 9 communicating with an air inlet opening 31 provided in the back wall of said casing 3. A conduit 10 is arranged to communicate on one of its sides with the access opening 11, which is provided in the front flange 6 of the rotating drum 4, and on its other side with an air exhaust opening 32, which is provided on the rear side of the machine and is partially coinciding with said air intake opening 31 depending on the pre-selected control inputs as described hereinafter.
A lint filter 12, a fan 13 driven by a motor which is not shown, and at least a heating element 15 of a traditional type are arranged in the conduit 10. The drying air taken in from outside by the fan 13 through the external opening 31 is heated up by said heating element 15, and it is then blown through the damp clothes being tumbled in the drum, so that the same clothes are gradually dried under removal of their moisture contents by said hot air, which is ultimately exhausted outside the drying machine through the appropriate opening 32.
In order to ensure effective, proper drying of the clothes, it is a well-known practice to further provide the afore mentioned clothes drying machine with means adapted to control the drying process up to the attainment of the desired degree or level of drying.
When this condition is reached, the control arrangement 30 trips to cause the heating element 15 and the respective drive motors of the drum 4 and the fan 13 to automatically switch off in a per sè known manner, so that the drying process itself is terminated.
As in the afore cited instances, even in this case the considered control arrangement is capable to stop, in the same manner, the clothes drying process at any moment during an on-going cycle upon occurrence of any failure or irregular operating condition of the drying machine that may bring about an over-heating of the clothes to be dried and, as a consequence, cause them to suffer damages.
According to the present invention, such a control and monitoring arrangement 30 can further control two thermostats 33 and 34 which are respectively situated at the inlet 9 of the hot air into the drum and at the outlet 6 of the same air before it is reconveyed into the exhaust conduit 10.
Under such conditions it is therefore possible to easily detect both when the temperature difference between the inlet and the outlet decreases under a pre-determined level and when the moisture reduction rate in the clothes during a certain time interval lowers below a pre-determined level.
In these cases the control and monitoring arrangement 30 can therefore appropriately trip to de-energize heating of the recirculating air, while allowing the air circulation fan to keep operating.
When such an event occurs, the clothes drying effect inside the drum goes on owing to the hot air that still resides there, however without any further addition of thermal energy. And this operation mode can go on until a particular condition occurs, eg. a sensible decrease in the moisture contents of the exhaust air is detected or a certain pre-set period of time is elapsed.
It is quite apparent that during this mode of operation, drying goes in a spontaneous, self-sustaining way, albeit at a reduced rate, without any further addition of thermal energy from outside, and this fact contributes to a significant improvement in the overall thermal efficiency of the clothes drying machine.
However, since the above described operating conditions would inevitably cause the temperature prevailing inside the drum to drop very rapidly, with a resulting loss in drying efficiency, a useful improvement according to the present invention consists in letting the fan operate normally with the heating element 15 operating at either full or partial power input, while blowing inside the drum an air mixture made up partly by the hot exhaust air, as appropriately intercepted and recovered, and partly by fresh air sucked in from the outside of the machine.
The recovery of said hot exhaust air and the subsequent mixture with the fresh air flow can be accomplished by means of an arrangement 8 as illustrated in Figures 2 and 3, said arrangement being controlled by the control element 30 to which also the signals from the two thermostats 33 and 34 are sent.
The machine therefore operates as follows: upon starting the drying cycle by the user, the two thermostats 33 and 34 start signalling the temperatures to said control element 30.
When the difference between the values of the two temperatures being signalled by said thermostats decreases under a certain limit, or when the temperature of the exhaust air raises above a certain limit, or more simply after a certain pre-determined period of time (said control element 30 may be adapted to accomodate all these and further equivalent control options according to the state of the art), then said control element 30 trips to send an appropriate control signal to an actuator means (not shown) commanding the flow diverter 8 for starting air mixing.
By thereupon changing its setting or orientation (according to its particular design), said flow diverter 8 intercepts a part of the hot air flow coming from the conduit 10 as well as the flow of fresh air sucked in from the outside of the machine through the opening 31, and conveys the resulting mixed flow again toward the heating element 15 so as to go on with and keep up the drying cycle.
The characteristics of the mixed air obtained in the afore described way may of course be varied according to the setting of the flow diverter 8, which may be adjusted into different postures. It is in this way possible to achieve and perform appropriately differentiated drying cycles under accordingly more or less reduced energy consumption rates.
Since exhaust air in a traditional clothes drying cycles usually has the characteristics as depicted in Figure 1 as far as temperature and moisture contents are concerned, a useful improvement according to the present invention results from the fact that air mixing is performed by said flow diverter 8 according to proportions that are not fixed or constant, but variable in such a way as to bring about toward the end of the drying cycle, ie. when the moisture contents of the exhaust air decreases sharply, an air mixture that is richer in recovered air taken from the conduit 10.
An optimally proportioned mixture is in this way obtained, since its temperature is ensured by the high temperature of the exhaust air, while its low moisture contents is ensured by the low moisture contents of the same air.
The control of said variability in the air mixing proportions is of course ensured by said control element 30, preferably on the basis of the moisture contents of the air vented from the drum.
A further method for recovering the heat exhausted from a clothes drying machine is according the embodiment illustrated in Figure 4, which shows a clothes drying machine comprising a drum 4, a fresh air suction port 31, an outlet 32 for venting the hot and damp air through the conduit 10, a fan 13 for the forced circulation of the air.
The main feature of a clothes drying machine of the above cited type lies in that the air taken in through the suction port 31, instead of being directly blown to heat up over traditional-type heating elements and then conveyed into the drum of the machine, is directly sent into the hollow space 41 provided between the outer casing 3 of the machine and the drum 4.
In this way, the air is caused to flow in contact with the outer surface of the drum 4 which, owing to the fact that it is made of metal material and has therefore a very good thermal conductivity, as well as that it has a very large outer surface at a temperature which is very close to the average temperature prevailing inside the drum, turns out to act as a heat source which is very easy and simple to carry out and, above all, is very economical and reliable.
In practice, by letting the fresh suction air flow first into the outer casing of the machine and then over the outer surface of the drum in the above described way, a heat-exchanging arrangement is achieved between the air inside the drum and the fresh air taken in from outside, before the latter reaches the heating elements provided for direct heating up.
A further improvement of the above described heat-exchange process is achieved by conveying the fresh air being taken in from the outside of the machine into an appropriately provided heat-exchanger arrangement 40 which is provided in a very close connection with the outer surface of the drum in such a way as to wrap the largest possible area of the same outer surface of the drum, said air being constrained to circulate within the boundaries of said heat-exchanger arrangement without getting scattered over the entire volume of said hollow space 44.
Anyone skilled in the art will be able to readily recognize the distinct advantage ensuing from this particular embodiment which is capable of ensuring a higher average temperature at the inlet of the port 42.
After this pre-heating phase, the fresh air is then sucked in through a port 42 and, through a conduit 43, is conveyed to a traditional-type heating element arrangement 15 and finally into the drum and through the clothes contained therein, according to the usual construction.
It is quite apparent that the heat taken up by said air in the afore described pre-heating phase can be saved when generating the heat required to heat up the air to the desired temperature during the subsequently occurring ultimate heating-up phase by means of the heating element 15.
As a consequence, even this constructional and operational feature of the present invention leads to achieving an energy saving effect in a simple, effective and safe way, without burdening the machine with additional costs to any significant extent.
It might be objected that the heat which is in this way subtracted from the drum during said heat-exchange process with the air outside the drum itself, causes the air inside the drum to cool down by an equal amount, thereby making it necessary to perform a longer lasting drying process and ultimately counterbalancing any real benefit that may actually derive from said heat recovery.
However, such an objection is not really supported by any technically sound consideration, since the amount of heat that is in any case released from the outer surface of the drum 4, would be inevitably dispersed outside the machine, and irremediably wasted, if it were not for the afore described energy recovery process through the passage of the inlet air flow.
It should be also considered that the heat subtracted and recovered according to the present invention is actually subtracted from the heat of the air inside the drum, at least a part of which is normally sent directly toward the vent conduit 10 and cannot therefore be used any further to perform a clothes drying action. And it is actually this exhaust heat that is basically recovered and re-used.
The afore stated observations have on the other hand been widely confirmed by the results of comprehensive experimental work done in the laboratory that demonstrated the technical soundness and the energy-related efficiency of the afore described invention.

Claims

Clothes drying machine comprising a rotating drum (4) adapted to contain the washload to be dried and crossed by a flow of air taken in from the outside of the machine through an inlet port (31) and heated up by at least an external heating element (15), said air being circulated by at least a blower (13) and being exhausted through a conduit (10) after having subtracted moisture from the clothes to be dried, characterized in that part of the heat vented outside by the clothes drying machine is recovered to be re-used so as to pre-heat the fresh inlet air being taken in.
Clothes drying machine according to claim 1, characterized in that said fresh air being taken in from the outside of the machine is mixed with the exhaust air diverted from the vent conduit (10), before being conveyed toward the heating element (15) and the drum (4).
Clothes drying machine according to claim 2, characterized in that said mixing of the air occurs by the interposition of a common flow diverter means (8) between the air suction conduit, as defined outwardly by the air intake ports (31), and the vent conduit (10).
Clothes drying machine according to claim 3, characterized in that said flow diverter (8) is adjustable so as to enable air mixtures to be obtained with variable shares of air from the vent conduit (10) and fresh inlet air.
Clothes drying machine according to claim 4, characterized in that said flow diverter (8) is driven by a motor and is controlled by a control arrangement (30).
Clothes drying machine according to claim 5, characterized in that it comprises two thermostats (33) and (34) for the air entering the drum and the air leaving the drum, respectively, said two thermostats being connected with said control arrangement (30).
Clothes drying machine according to claim 6, characterized in that during the drying process said control arrangement (30) adjusts the setting of said flow diverter (8), and therefore the mixing conditions of the air being sent to the heating element (15), depending on the temperature values detected by said thermostats (33, 34).
Clothes drying machine according to claim 5, characterized in that during the drying process said control arrangement (30) adjusts the setting of said flow diverter (8), and therefore the mixing conditions of the air being sent to the heating element (15), depending on the moisture contents of the exhaust air.
Clothes drying machine according to claim 7 or 8, characterized in that during the drying process said air mixture being blown into the drum consists of a progressively increasing share of exhaust air recovered from said vent conduit (10) and a correspondingly decreasing share of fresh air taken in from the outside of the machine.
Clothes drying machine according to claim 1, characterized in that the fresh air being taken in through the inlet port (31) is first introduced in the hollow space (44) between the drum (4) and the outer casing (3) of the machine, and is then sucked in through a further port (42) leading to the heating element (15) through a conduit (43), before being blown into the drum (4).
Clothes drying machine according to claim 10, characterized in that said fresh air being taken in from the outside of the machine is conveyed into a heat-exchanging conduit (40) arranged in close relationship with the outer surface of the drum, before being sucked in by said intake port (42).