Laundry drying apparatus with heater unit having adjustable temperature thresholds

Abstract

A laundry drying apparatus, in particular dryer or washing machine having drying function, has: a cabinet, a control unit (4) adapted to control a laundry drying cycle according to at least one drying program, a drum (18) rotatably arranged within the cabinet for receiving laundry to be dried, a drying air channel (20) adapted at least to guide drying air into the drum (18), and a heater unit (6) arranged at or in the drying air channel (20) and being adapted to heat the drying air. The heater unit (6) has two or more heating levels. A temperature sensor unit is adapted to detect the temperature of the drying air and to provide at least one temperature signal to the control unit (4). The control unit (4) is adapted to control the heater unit (6) to heat the drying air at a selected one of the two or more heating levels. The control unit (4) is adapted to select or determine and apply during the drying cycle one of two or more temperature threshold sets, wherein the two or more temperature threshold sets are used by the control unit (4) for determining the heating level to be applied by the heater unit (6) in dependency of the at least one temperature signal from the temperature sensor unit. The control unit (4) is adapted to select or determine one of the two or more temperature threshold sets in dependency of one or more operating parameters of the apparatus, wherein the temperature thresholds of the temperature threshold sets are mutually different or are not identical. During execution of the laundry drying cycle the control unit (4) is adapted to select or determine one of the two or more temperature threshold sets in dependency of one or more of the following operation parameters: a) the time or temporal progress of the laundry drying cycle, b) one or more operation parameters of or for the heater unit (6), c) an electrical or mechanical parameter of the drum (18) motor, d) a temporal gradient of the drying air temperature during execution of the laundry drying cycle, and e) an ambient temperature indicative for the temperature outside the apparatus cabinet or a temperature indicative of an apparatus component temperature.

Claims

1. Laundry drying apparatus, in particular dryer or washing machine having drying function, comprising: a cabinet, a control unit adapted to control a laundry drying cycle according to at least one drying program, a drum rotatably arranged within the cabinet for receiving laundry to be dried, a drying air channel adapted at least to guide drying air into the drum, a heater unit arranged at or in the drying air channel and being adapted to heat the drying air, wherein the heater unit has two or more heating levels, and a temperature sensor unit adapted to detect the temperature of the drying air and to provide at least one temperature signal to the control unit, wherein the control unit is adapted to control the heater unit to heat the drying air at a selected one of the two or more heating levels, wherein the control unit is adapted to select or determine and apply during the drying cycle one of two or more temperature threshold sets (T.sub.MIN1 . . . 3, T.sub.MAX1 . . . 3), wherein the two or more temperature threshold sets are used by the control unit for determining the heating level to be applied by the heater unit in dependency of the at least one temperature signal from the temperature sensor unit, wherein the control unit is adapted to select or determine one of the two or more temperature threshold sets (T.sub.MIN1 . . . 3/T.sub.MAX1 . . . 3) in dependency of one or more operating parameters (t.sub.OFF.sub._.sub.ON, .sub.ON.sub._.sub.OFF, E.sub.OFF.sub._.sub.ON) of the apparatus, and wherein the temperature thresholds of the temperature threshold sets (T.sub.MIN1 . . . 3/T.sub.MAX1 . . . 3) are mutually different or are not identical, wherein during execution of the laundry drying cycle the control unit is adapted to select or determine one of the two or more temperature threshold sets (T.sub.MIN1 . . . 3/T.sub.MAX1 . . . 3) in dependency of one or more of the following operation parameters: a) the time or temporal progress of the laundry drying cycle, b) one or more operation parameters of or for the heater unit, c) an electrical or mechanical parameter of the drum motor, d) a temporal gradient of the drying air temperature during execution of the laundry drying cycle, and e) an ambient temperature indicative for the temperature outside the apparatus cabinet or a temperature indicative of an apparatus component temperature.

2. Laundry drying apparatus according to claim 1, wherein the b) one or more operation parameters of the heater unit include one or more of: the number of times (n_OFF) the control unit reduces the heating level of the heater unit or the number of times (n_ON) the control unit increases the heating level of the heater unit or the number of periods in which the control unit controls a reduction and increase of the heating level of the heater unit, the time or duration of a period (t.sub.ON.sub._.sub.OFF) in which the control unit increases the heating level of the heater unit to the subsequent reduction of the heating level, the time or duration of a period (t.sub.OFF.sub._.sub.ON) in which the control unit decreases the heating level of the heater unit to the subsequent increase of the heating level, the time or duration of a period in which the control unit controls a reduction and the subsequent increase of the heating level of the heater unit, the heating energy supplied to the heater unit during a period in which the control unit controls a reduction and the subsequent increase of the heating level of the heater unit, and the power supplied to the heater unit averaged over predetermined time periods.

3. Laundry drying apparatus according to claim 1, wherein the c) electrical or mechanical parameter of the drum motor is one or more of: the motor current, the phase voltage, the consumed motor power, the phase of the voltage applied to the motor, and the torque generated by the motor.

4. Laundry drying apparatus according to claim 1, wherein the temporal gradient of the drying air temperature is determined using the one or more signals of the temperature sensor unit.

5. Laundry drying apparatus according to claim 1, wherein the heater unit has at least three heating levels comprising a non-heating level and a first and a second heating level, wherein the heating power at the first and second heating level is above 0 W and wherein the heating power of the second heating level is higher than the heating power of the first heating level, or wherein the heater unit is an electrical resistivity heater unit having at least two resistivity heater elements wherein a first heating level with a first heating power is provided by powering a first one of the resistivity heater elements and a second heating level with a second heating power is provided by powering a second one of the resistivity heater elements or the first and second resistivity heater elements.

6. Laundry drying apparatus according to claim 1, wherein the laundry drying apparatus is a condensate-type drying apparatus having a closed drying air loop comprising the drying air channel and the drum as a portion of the drying air loop, and wherein a condensation unit is arranged in the drying air channel upstream the heater unit.

7. Laundry drying apparatus according to claim 1, wherein for controlling the heater unit the temperature sensor unit comprises at least one temperature sensor (10a-c) arranged to detect the drying air temperature at one or more of the following positions: downstream the heater unit and upstream the drum air inlet, downstream the drum air outlet, within a tub surrounding the drum, and at the drum.

8. Laundry drying apparatus according to claim 1, wherein the control unit determining or selecting of the one of the two or more temperature threshold sets (T.sub.MIN1 . . . 3/T.sub.MAX1 . . . 3) in dependency of one or more operating parameters of the apparatus comprises: a storing element of the control unit stores at least two different predetermined temperature threshold sets (T.sub.MIN1 . . . 3/T.sub.MAX1 . . . 3) and the control unit is adapted to select one of these predetermined temperature threshold sets (T.sub.MIN1 . . . 3/T.sub.MAX1 . . . 3) in dependency of the one or more operating parameters, a storing element of the control unit stores at least three different predetermined temperature thresholds and, in dependency of the one or more operating parameters, the control unit is adapted to select one of these predetermined temperature thresholds as the first lower temperature threshold (T.sub.MIN1 . . . 3) and one as the second higher temperature threshold (T.sub.MAX1 . . . 3), or the control unit is adapted to determine one of the two or more temperature threshold sets (T.sub.MIN1 . . . 3/T.sub.MAX1 . . . 3) by calculating at least one of the temperature thresholds for the next predetermined temperature threshold set to be applied on the basis of the one or more operating parameters.

9. Laundry drying apparatus according to claim 1, wherein each one of the temperature threshold set (T.sub.MIN1 . . . 3/T.sub.MAX1 . . . 3) comprises a first higher temperature threshold (T.sub.MAX1 . . . 3) used by the control unit for determining a drying air temperature state at which the heating power of the heater unit has to be set to a lower heating power, and a second lower temperature threshold (T.sub.MIN1 . . . 3) used by the control unit for determining a drying air temperature state at which the heating power of the heater unit has to be set to a higher heating power.

10. Laundry drying apparatus according to claim 1, wherein during execution of the laundry drying cycle the control unit is adapted to select the one of the two or more temperature threshold sets (T.sub.MIN1 . . . 3/T.sub.MAX1 . . . 3) independent of a detected laundry humidity or a current state of the laundry.

11. Laundry drying apparatus according to claim 1, wherein during the drying cycle the control unit is adapted to control the heater unit using a selected one of two or more heater control subroutines by applying the selected or determined one of the temperature threshold sets (T.sub.MIN1 . . . 3/T.sub.MAX1 . . . 3), and wherein the control unit is adapted to select one of the two or more heater control subroutines in dependency of one or more of the following operation parameters: a) the time or temporal progress of the laundry drying cycle, b) one or more operation parameters of or for the heater unit, c) an electrical or mechanical parameter of the drum motor, d) a temporal gradient of the drying air temperature during execution of the laundry drying cycle, e) an ambient temperature indicative for the temperature outside the apparatus cabinet or a temperature indicative of an apparatus component temperature, f) a laundry parameter indicating the state or condition of the laundry received in the drum, and g) a user selection of one of two or more different drying programs input by the user via an input unit of the apparatus.

12. Laundry drying apparatus according to claim 11, wherein the f) laundry parameter is one or more of the following: the type of laundry, the starting humidity or targeted final humidity of the laundry, the humidity of the laundry, the laundry load, and the temperature of the laundry.

13. Laundry drying apparatus according to claim 11, wherein when a first heater control subroutine is executed by the control unit, the control unit is adapted to control the heater unit to change between a first and a second heating power level, and when a second heater control subroutine is executed by the control unit, the control unit is adapted to control the heater unit to change between a first and a fourth or between a third and a fourth heating power level.

14. Laundry drying apparatus according to claim 11, wherein, when the control unit changes the selected or determined temperature threshold set to be applied, it concurrently changes the heater control subroutine to be applied, or wherein the control unit changes the selected or determined temperature threshold set independent or temporally independent of a change of the selected heater control subroutine.

15. Laundry drying apparatus according to claim 1, wherein the operation parameters a) to e) for selecting or determining the one of the two or more temperature threshold sets (T.sub.MIN1 . . . 3/T.sub.MAX1 . . . 3) or the operation parameters a) to g) of claim 11 for selecting the one of the two or more heater control subroutines are in turn determined on the basis of one or more of the following parameters: k) a drying program selector position; l) a drying program options selected by a user; m) the laundry type; n) the laundry load; o) the temperature of ambient where the drying apparatus is placed; p) a temperature of the laundry drying apparatus; q) the laundry weight; r) the laundry humidity; s) an electrical or mechanical parameter of a motor rotating the drum; and t) the drum inertia.

16. Laundry drying apparatus according to claim 1, wherein, subsequent to a first period of applying a first temperature threshold set (T.sub.MIN1/T.sub.MAX1) and/or a first heater control subroutine or subsequent to a second period of applying a second temperature threshold set (T.sub.MIN2/T.sub.MAX2) and/or a second heater control subroutine, for determining or selecting a second or third time the temperature threshold set (T.sub.MIN3/T.sub.MAX3) to be used for heater unit control and/or for determining a second or third heater control subroutine, the respective operation parameters are determined on the basis of one or more of the following parameters: the cycle time; the number (n_OFF) of the heater unit switching OFF or ON detected in a first or second period of the drying cycle; the time (t.sub.ON.sub._.sub.OFF) elapsed between the heater unit switching ON(OFF) and subsequent switching OFF(ON) detected in a first or second period of the drying cycle; the time (t.sub.Start.sub._.sub.OFF) elapsed from the first switching ON of the heater unit and the first switching OFF thereof in the first or second part of the drying cycle; the gradient of drying air temperature detected in a first or second part of the drying cycle; the rate of heating power supplied to drying air detected in a time slot during a first or second period of the drying cycle; the rate of heating power supplied to drying air detected in a time slot between two subsequent operations (switching) of the heater unit during a first or second part of the drying cycle; and when being dependent on claim 15, the operation parameters k) to t) as applicable and as detected or determined in a first or second period of the drying cycle.

17. Laundry drying apparatus according to claim 1, wherein the control unit is adapted to select or determine, after having changed within the running drying cycle from applying a first temperature threshold set to a second temperature threshold set, a next temperature threshold set on one or more of the following operation parameters: one or more of the operation parameters a) to e), f) a laundry parameter indicating the state or condition of the laundry received in the drum, and g) a user selection of one of two or more different drying programs input by the user via an input unit of the apparatus.

18. Method of controlling the temperature during a drying cycle in a laundry drying apparatus, in particular in an apparatus according to claim 1, in a dryer or a washing machine having drying function, the apparatus comprising: a cabinet, a drum rotatably arranged within the cabinet for receiving laundry to be dried, a heater unit for heating the drying air, wherein the heater unit has two or more heating levels for heating the drying air at a selected one of the two or more heating levels, and a temperature sensor unit for detecting the temperature of the drying air, wherein the method comprises the steps of: selecting or determining one of two or more temperature threshold sets (T.sub.MIN1 . . . 3, T.sub.MAX1 . . . 3), determining the heating level to be applied by the heater unit in dependency of the at least one selected or determined temperature threshold set, and operating the heater unit at the determined heating level; wherein the temperature thresholds of the temperature threshold sets (T.sub.MIN1 . . . 3/T.sub.MAX1 . . . 3) are mutually different or are not identical, and wherein the one of the two or more temperature threshold sets (T.sub.MIN1 . . . 3/T.sub.MAX1 . . . 3) is selected or determined in dependency of one or more of the following operating parameters (t.sub.OFF.sub._.sub.ON, .sub.ON.sub._.sub.OFF, E.sub.OFF.sub._.sub.ON) of the apparatus: a) the time or temporal progress of the laundry drying cycle, b) one or more operation parameters of or for the heater unit, c) an electrical or mechanical parameter of a drum motor adapted to rotate the drum, d) a temporal gradient of the drying air temperature during execution of the laundry drying cycle, and e) an ambient temperature indicative for the temperature outside the apparatus cabinet or a temperature indicative of an apparatus component temperature.

19. Method of claim 18, wherein the selecting or determining one of two or more temperature threshold sets is made permanently, repeatedly, periodically, in predefined intervals, or upon a change of one or more of the apparatus operation parameters.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Reference is made in detail to preferred embodiments of the invention, examples of which are illustrated in the accompanying figures, which show:

(2) FIG. 1 a schematic view of a laundry dryer,

(3) FIGS. 2a-b graphs illustrating a first embodiment of operating a laundry dryer an example of which is shown in FIG. 1,

(4) FIGS. 3a-b graphs illustrating a second embodiment of operating a laundry dryer an example of which is shown in FIG. 1,

(5) FIGS. 4a-b graphs illustrating a third embodiment of operating a laundry dryer an example of which is shown in FIG. 1, and

(6) FIGS. 5a-c graphs illustrating a fourth embodiment of operating a laundry dryer an example of which is shown in FIG. 1.

(7) The following figures are not drawn to scale and are provided for illustrative purposes.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

(8) FIG. 1 shows schematically depicted a laundry dryer 2 comprising a housing 3 or cabinet enclosing below described exemplary components of the dryer 2. The process air or drying air flow A within the dryer 2 is guided through a laundry storing compartment 17 of the dryer 2, i.e. through a compartment for receiving articles to be treated, e.g. a drum 18. The articles to be treated are laundry 19, clothes, shoes or the like. The process air flow is indicated by arrows A in FIG. 1 and is driven by a process air blower 8. A process air channel 20 guides the process air flow A outside the drum 18 and comprises different sections, including the section forming a rear channel 20a in which the process air blower 8 is arranged. The air conveyed by blower 8 is guided upward in a rising channel 20b to the backside of the drum 18. A heater unit 6 is arranged downstream the blower 8 to heat the process air before it enters the drum 18. The air exiting the drum 18 through the drum outlet (which is the loading opening 36 of the drum 18) is filtered by a fluff filter 22 arranged close to the drum outlet in or at the channel 20. The optional fluff filter 22 is arranged in a front channel 20c forming another section of channel 20 which is arranged behind and adjacent the front cover of the dryer 2. A condensation unit or condenser 12 for dehumidifying the process air is arranged downstream the drum 18. The condensate formed at the condenser 12 is collected and guided to the condensate collector 14. For example an air-cooled condenser 12 may be provided.

(9) The condensate collector 30 is connected via a drain conduit 30, a drain pump 16 and a drawer pipe 38 to an extractable condensate drawer 40. I.e. the collected condensate can be pumped from the collector 30 to the drawer 40 which is arranged at an upper portion of the dryer 2 from where it can be comfortably withdrawn and emptied by a user. The dryer 2 comprises a control unit 4 for controlling and monitoring the overall operation of the dryer 2. As shown in FIG. 1, the control unit 4 also controls the heater unit 6. Additionally, the control unit 4 is able to control other parts of the dryer 2.

(10) For example the heater unit 6 may be an electrical resistivity heater unit which comprises at least two resistivity heater elements. In this case a first heating level with a first heating power is provided by powering a first one of the resistivity heater elements and a second heating level with a second heating power is provided by powering a second one of the resistivity heater elements and/or the first and second resistivity heater elements. As an example for a multi-level heater unit 6, the heater unit 6 may comprise two heater elements R1, R2 having different heating power, such that four heating levels may be achieved, namely 0 (zero=all heater elements switched-off), R1, R2, R1+R2.

(11) At least one temperature sensor 10a-c is provided to detect the temperature of the process air A. As shown in the example of FIG. 1, a temperature sensor 10a is arranged at a drum inlet to detect the temperature of the air before or when entering the drum (T.sub.DRY.sub._.sub.IN). Additionally or alternatively temperature sensor(s) 10b-c may be arranged at different positions, e.g. at a drum outlet to detect the temperature of the air exiting the drum 18 (sensor 10b) or within the drum 18 (sensor 10c). Preferably the temperature sensor(s) 10a-b is arranged just upstream or downstream the drum 18.

(12) The heater unit 6 is driven through control unit 4 by detecting the process air temperature and activating the heater unit 6or at least a heater elementwhen the detected process air temperature drops below a minimum temperature threshold (T.sub.MIN), and deactivating the heater unit 6or at least a heater elementwhen the detected process air temperature gets higher than a maximum temperature threshold (T.sub.MAX). Thus, a so-called Hysteresis-control is applied.

(13) Hysteresis values for controlling process air temperature may be changed during a drying cycle. Such change may involve changing just one of the two hysteresis values (T.sub.MIN/T.sub.MAX) or both values.

(14) An operation parameter that determines a change of at least one of the process air temperature hysteresis thresholds so as to change the threshold set from a first set to a second set of thresholds, may be one or more of the following group I: cycle time; umber (n_OFF) of heating elements switching OFF or ON; time (t.sub.OFF.sub._.sub.ON) elapsed between heating elements switching OFF (ON) and subsequent switching ON (OFF)compare FIGS. 2a-b; rate of heating power supplied to drying air over a drying cycle time slot, or between two subsequent operations (switching) of the heater unit 6; electrical or mechanical parameter of a motor rotating the drum (current, voltage, power, phase supplied to the motor, torque supplied by the motor); and/or gradient of air temperature over a drying cycle time slot.

(15) One or more of the operation parameters is detected during a drying operation. When the detected value of the operation parameter reaches a predetermined threshold, the process air temperature hysteresis threshold set is changed.

(16) When process air temperature hysteresis threshold(s) are changed, a heating power control logic for supplying heating power to the drying air may change at the same time or afterwards. Such change is operated to adapt the operation of the heater unit 6 to the new thresholds while avoiding too frequent activations of the heating device or a too slow ramp for reaching the new temperature thresholds.

(17) In particular, a change in the heating power control logic may be operated even before the process air temperature hysteresis threshold(s) change, i.e. independently from the change of the change of hysteresis threshold changes. This provides the advantage of adapting the energy consumption to a desired amount or regulating the drying cycle time.

(18) The heating power control logic controlling the heater unit 6 for supplying heating power to process air can be changed among a set of heating power control logics based on one or more of the following group II: a parameter of group I, and/or a parameter indicating laundry status/condition within the drum (humidity, weight, temperature);

(19) Heating power control logic means the way of driving and regulating the heater unit 6 such that the heating power supplied to drying air by the heater unit 6 is comprised between a minimum and a maximum value (T.sub.MIN, T.sub.MAX). Such values can be selected between groups of values that the heater unit 6 can provide. For example the heater unit 6 as described above in form of an electric resistivity heater having two branches may be controlled using a first logic in which both branches are switched ON and OFF, or using a second logic in which a first branch is always ON and only the second branch is switched ON and OFF. The number of control logics available depends not only from the heating power amounts that the heater unit 6 can provide but also from the way in which such heating power amounts are supplied to drying air. The set, i.e. the number, of heating power control logics selectable by a dryer control unit during a laundry drying cycle may be determined based on a parameter that may be selected among one or more of parameters indicated in Group III and IV below. For example, when a user selects a drying cycle, a specific group of heating power control logics may be assigned to that cycle by the control unit 4. The control unit 4 will change the heater unit 6 control from one control logic to another one based on comparison between the detected value of one or more operation parameters listed in Group II and specific thresholds assigned to the control unit 4. Assignment of such thresholds is described below.

(20) It has to be noted that the change of at least one threshold of process air temperature hysteresis thresholds and the change of heating power control logic can be operated not only on the base of the same parameter or composition of parameters, but also based on different parameters or composition of parameters.

(21) The threshold determined for the parameter, or group of parameters, causing a change of at least one of the process air temperature hysteresis thresholds may be the same, or not the same, as the threshold determined for the parameter, or group of parameters, causing a change of the heating power control logic. In this way, the change of at least one of the process air temperature hysteresis thresholds and the change of the heating power control logic does not necessarily happen at the same time even in case they are modified based on the same parameter.

(22) A first set of process air temperature hysteresis thresholds and/or heating power control logics to be used in a first part of a drying cycle may be determined based on one or more of the parameters indicated in following group III: drying program selector position; drying program options selected by a user; laundry type; laundry load; ambient temperature where the laundry drying apparatus is placed; temperature of laundry drying machine; laundry weight; laundry humidity; and electrical and/or mechanical parameter(s) of a motor rotating the drum.

(23) A second or third set of process air temperature hysteresis thresholds and/or heating power control logics to be used in a second or third part of a drying cycle, subsequent to the first or second part of the same drying cycle, may be determined based on one or more of the following parameters indicated in following group IV: parameters of group III detected in a first or second part of the drying cycle; cycle time; number (n_OFF) of heating elements switching OFF or ON detected in a first or second part of the drying cycle; time (t.sub.ON.sub._.sub.OFF) elapsed between heating elements switching ON(OFF) and subsequent switching OFF(ON) detected in a first or second part of the drying cycle; time (t.sub.Start.sub._.sub.OFF) elapsed from the first switching ON of the heating elements and the first switching OFF thereof in the first or second part of the drying cycle. gradient of drying air temperature detected in a first or second part of the drying cycle; and rate of heating power supplied to drying air detected in a time slot during of a first or second part of the drying cycle, or between two subsequent operations (switching) of the heater unit 6 during a first or second part of the drying cycle.

(24) The second process air temperature hysteresis thresholds set may be lower or higher than the first set.

(25) For example, the third set may be lower than the first set but higher than the second set. In this way a too low threshold set change operated as first change may be compensated by means of a second change that will put the threshold set between the first set and the too low second set.

(26) With the above described drying apparatus a drying cycle may be updated by means of a second threshold change, which determines whether the first change was appropriate or not in terms of activating/deactivating operations of the heater unit 6 (to reduce or minimize hardware stress), drying air temperature, drying speed/efficiency, energy consumption and overall drying time.

(27) The following graphs relate to a laundry drying apparatus 2 as described above having a temperature sensor 10a for detecting drying air temperature at the drum inlet, and a heater unit in the form of an electrical resistivity heater 6 having at least two independently controllable heater elements or branches. The following FIGS. 2 to 5 show exemplary graphs to illustrate some of the above described embodiments. Further sensors and/or algorithm may be provided for detecting or calculating the parameter(s) causing the change of one or both thresholds of the drying air temperature.

(28) The graph shown in FIG. 2a depicts the temperature of drying air entering the drum (T.sub.DRY.sub._.sub.IN) in dependency of elapsed time t of a drying cycle. For the same time span as shown in FIG. 2a, FIG. 2b depicts the time elapsed between switching OFF at least one branch of the heater unit 6 and switching ON at least one branch of the heater unit 6 (t.sub.OFF.sub._.sub.ON), wherein n_ON indicates the number of times the heater unit 6 or a branch thereof has been switched-on. In each graph two thresholds are indicated for each threshold set (T.sub.MIN1 . . . 3/t.sub.MAX1 . . . 3). Generally, the terms switching OFF and switching ON of the heater unit 6 includes the meaning of reducing to the next lower heating power level (OFF) and/or increasing to the next higher heating power level (ON).

(29) As described above, the (selected) threshold set is changed during the drying cycle (periods I to III). As shown in FIG. 2b, during a first period I of the drying cycle a first threshold set T.sub.MIN1/T.sub.MAX1 is selected for controlling the heater unit 6, i.e. the heater unit 6 (or at least a branch thereof) is switched OFF when the maximum temperature threshold T.sub.MAX1 is reached and switched ON when reaching the minimum temperature threshold T.sub.MIN1.

(30) As shown in FIG. 2b, when a minimum threshold value t.sub.OFF.sub._.sub.ON.sub._.sub.THR1 between switching the heater unit 6 OFF and ON is reached or exceeded, a second threshold set T.sub.MIN2/T.sub.MAX2 is selected for controlling the heater unit 6 during a second period II of the drying cycle. The values of the second threshold set are lower than the values of the first threshold set. In particular, the second maximum threshold T.sub.MAX2 is lower than the first maximum threshold T.sub.MAX1 and the second minimum threshold T.sub.MIN2 is lower than the first minimum threshold T.sub.MIN1.

(31) When a maximum threshold value t.sub.OFF.sub._.sub.ON.sub._.sub.THR2 between switching the heater unit 6 OFF and ON is reached or exceeded, a third threshold set T.sub.MIN3/T.sub.MAX3 is selected for controlling the heater unit 6 during a third period III of the drying cycle. The minimum and maximum temperature values of the third threshold set are higher than the respective temperature values of the second threshold set.

(32) This embodiment provides that the time or period between switching the heater unit 6 ON and OFF is maintained in a (time) frame which minimizes hardware stress and provides efficient heating.

(33) FIGS. 3a and 3b show graphs for illustrating a further embodiment where the respective temperature threshold sets T.sub.MIN1 . . . 3/T.sub.MAX1 . . . 3 are selected based on the temperature gradient .sub.ON.sub._.sub.OFF in the time between switching ON of at least one branch of a heater unit 6 and subsequently switching OFF of at least one branch of the heater unit 6. As described above, FIG. 3a shows the course of the process air temperature T.sub.DRY.sub._.sub.IN entering the drum 18 over time t. A threshold set T.sub.MIN1 . . . 3/T.sub.MAX1 . . . 3 with two threshold values is indicated for each drying cycle period I to III.

(34) In this embodiment, the detected operation parameter on which the selection of the threshold values depends is the temperature gradient .sub.ON.sub._.sub.OFF. As shown in FIG. 3b, when a minimum temperature gradient .sub.ON.sub._.sub.OFF.sub._.sub.THR1 is reached or exceeded, a second set of temperature thresholds T.sub.MIN2/T.sub.MAX2 is selected for controlling the operation of the heater unit 6. Similar to above FIGS. 2a-b, the minimum and maximum values of the second threshold set T.sub.MIN2/T.sub.MAX2 are lower than the respective values of the first threshold set T.sub.MIN1/T.sub.MAX1.

(35) When a maximum temperature gradient .sub.ON.sub._.sub.OFF.sub._.sub.THR2 is reached or exceeded, a third set of temperature thresholds T.sub.MIN3/T.sub.MAX3 is selected for controlling the operation of the heater unit 6. The minimum and maximum values of the third threshold set T.sub.MIN3/T.sub.MAX3 are lower than the respective values of the first threshold set T.sub.MIN1/T.sub.MAX1 and higher than the respective values of the second threshold set T.sub.MIN2/T.sub.MAX2.

(36) FIGS. 4a and 4b show graphs for illustrating a further embodiment where the respective threshold sets are selected based on the power E.sub.OFF.sub._.sub.ON [Watt/h] supplied to the heater unit 6 between switching OFF of at least one branch of the heater unit 6 and subsequently switching ON at least one branch of the heater unit 6. In this graph it is supposed that at least one branch of the heater unit 6 remains ON, i.e. active, when a switching OFF action is operated on the heater unit 6. As described above, FIG. 4a shows the course of the process air temperature T.sub.DRY.sub._.sub.IN entering the drum 18 over time t. A threshold set T.sub.MIN1 . . . 3/T.sub.MAX1 . . . 3 with two threshold values is indicated for each drying cycle period I to III.

(37) In this embodiment, the detected operation parameter on which the selection of the threshold values depends is the power E.sub.OFF.sub._.sub.ON. As shown in FIG. 4b, when a minimum power E.sub.OFF.sub._.sub.ON.sub._.sub.THR1 is reached or exceeded, a second set of temperature thresholds T.sub.MIN2/T.sub.MAX2 is selected for controlling the operation of the heater unit 6. The minimum and maximum values of the second threshold set T.sub.MIN2/T.sub.MAX2 are lower than the respective values of the first threshold set T.sub.MIN1/T.sub.MAX1.

(38) When a maximum power E.sub.OFF.sub._.sub.ON.sub._.sub.THR2 is reached or exceeded, a third set of temperature thresholds T.sub.MIN3/T.sub.MAX3 is selected for controlling the operation of the heater unit 6. The minimum and maximum value of the third threshold set T.sub.MIN3/T.sub.MAX3 is lower than the respective value of the first threshold set T.sub.MIN1/T.sub.MAX1 and higher than the respective value of the second threshold set T.sub.MIN2/T.sub.MAX2.

(39) This embodiment prevents that a first branch of the heater unit 6which remains switched ON while the second branch is switched OFFis overstressed. Thereby the service life of the heater unit 6 is increased. Additionally to changing the applied temperature threshold set, above described heating power control logic for the heater unit 6 may be changed in all above and below described embodiments to provide a more energy efficient operation of the heater unit 6.

(40) FIGS. 5a, 5b and 5c show graphs for illustrating a further embodiment where respective threshold sets are selected based on power E.sub.OFF.sub._.sub.ON and the temperature gradient .sub.ON.sub._.sub.OFF. Power E.sub.OFF.sub._.sub.ON and temperature gradient .sub.ON.sub._.sub.OFF are defined as described above.

(41) In contrast to above embodiments of FIGS. 2 to 4, FIGS. 5a-c illustrate an embodiment where each temperature threshold (T.sub.MAX and T.sub.MIN) of the drying air temperature T.sub.DRY.sub._.sub.IN can be changed independently from one another by using two different operation parameters (here: E.sub.ON.sub._.sub.OFF and .sub.ON.sub._.sub.OFF as an example). A predetermined threshold (T.sub.MIN or T.sub.MAX) is assigned to each of said operation parameters to determine when the process air temperature threshold to which the respective operation parameter is linked to has to be changed. In this example, reaching or exceeding a minimum power E.sub.ON.sub._.sub.OFF.sub._.sub.THR.sub._.sub.MIN (FIG. 5b) causes a change (here: reduction) of the lower temperature threshold value from T.sub.MIN1 to T.sub.MIN2, while the upper temperature threshold T.sub.MAX1 is maintained (period II). Reaching or exceeding a maximum temperature gradient .sub.ON.sub._.sub.OFF.sub._.sub.THR.sub._.sub.MAX (FIG. 5c) causes a change (here: reduction) of the upper temperature threshold T.sub.MAX1 to T.sub.MAX2, while the lower temperature threshold T.sub.MIN2 is maintained (period III).

REFERENCE NUMERAL LIST

(42) 2 laundry dryer 3 housing/cabinet 4 control unit 6 heater unit 8 blower 10a-c temperature sensor 12 condenser/condensation unit 14 condensate collector 16 drain pump 17 laundry storing compartment 18 drum 19 laundry 20 process air channel 20a rear channel 20b rising channel 20c front channel 22 fluff filter element 30 drain conduit 36 loading opening 38 drawer pipe 40 condensate drawer A process air flow