Patent Publication Number: US-2018038042-A1

Title: Heat pump type laundry dryer and method for controlling the same

Description:
The present invention relates to a heat pump type laundry dryer and a method for controlling the heat-pump type laundry dryer. 
     Several techniques have been devised for drying the wet laundry. In particular, the ventless laundry dryers such as the heat-pump type laundry dryers and the condenser type laundry dryers are commonly known. In such laundry dryers, the clothes are generally dried by conveying dehumidified heated process air through the rotating laundry drum. The heat pump technology constitutes an energy-efficient method of drying the laundry. A heat pump type laundry dryer generally comprises a process air circuit and a refrigerant circuit which are arranged so as to mutually exchange heat. The process air circuit comprises a duct for circulating the process air through the laundry drum by means of a fan inside the duct. The refrigerant circuit includes an evaporator, a compressor, a condenser and an expansion valve or a capillary tube in serial connection for circulating the refrigerant. The evaporator and the condenser are arranged in heat-exchange with the process air inside the duct for dehumidifying and heating the process air. The heat absorbed by the evaporator is partly transferred back to the process air by means of the condenser. Thereby, the drying performance of the laundry dryer can be improved. 
     A problem with the drying process is that when the laundry is completely dried the risk that the laundry suddenly catches fire increases. Therefore, according to the IEC standards the temperature of the laundry must be reduced after termination of the drying process in order to reduce the fire risk. In particular, it is prescribed that the process air temperature must be reduced below 55° C. in a cooling process which is to be commenced after termination of the drying process. 
     In order to comply with the above-mentioned safety prescription, the process air circuit and the refrigeration circuit, in particular the process air fan inside the duct and the compressor motor must continue to run during such cooling process. However, this prolongs the operation time and also increases the overall energy consumption. In general, the duration of the cooling process may vary from case to case and cannot be precisely predicted. Therefore, the displayed estimated operation time generally differs from the actual operation time. Thereby, the consumer satisfaction may easily decrease. 
     An objective of the present invention is to provide a heat pump type laundry dryer and a method for controlling the heat pump laundry dryer which overcomes the aforementioned drawbacks of the prior art in a cost effective way and which enables an energy-efficient and safe operation. 
     This objective has been achieved by the heat pump type laundry dryer as defined in claim  1  and the method for controlling the heat pump type laundry dryer as defined in claim  5 . Further achievements have been attained by the subject-matters respectively defined in the dependent claims. 
     The heat pump type laundry dryer of the present invention comprises a fan for cooling the compressor and a sensor for sensing the process air temperature T_air, wherein the control unit is configured to set a first threshold temperature T_th 1  which is equal to or smaller than 55° C., to set a second threshold temperature T_th 2  which is less than T_th 1  and to initiate, prior to the termination of the drying process a cooling process in which the compressor cooling fan is turned ON when T_air reaches T_th 1  and the compressor cooling fan is turned OFF when T_air drops to T_th 2 . 
     With the present invention, the temperature of the laundry can be reduced below 55° C. prior to the termination of the drying process. Thereby, the need for conducting an additional cooling process after the termination of the drying process can be obviated. With the present invention, the total operation time of the laundry dryer can be prevented from prolonging. Thereby the energy consumption can be reduced and the energy efficiency can be improved. With the present invention the risk of fire can be eliminated or reduced as much as possible and thus the safety can be improved. Thereby, the consumer satisfaction can be increased. 
     In an embodiment, the cooling process is initiated when the user selects a predetermined drying program or after elapse of a predetermined duration whichever condition enters first. 
     In another embodiment, an auxiliary cooling process is initiated prior to the initiation of the cooling process, or after the initiation of the cooling process when T_air has not yet reached T_th 1 . In the auxiliary cooling process the compressor cooling fan is turned ON or OFF and/or the compressor motor is turned ON or OFF or driven at higher or lower rotational speeds in accordance with the refrigerant temperature T_rfg so as to maintain the latter within the limits of an upper threshold and a lower threshold. The measures taken in the auxiliary cooling process in respect of the compressor motor can also be similarly applied in the cooling process. 
     In another embodiment, the process air temperature T_air is sensed in the vicinity of an air filter of the process air circuit which is located on the downstream of the drum. 
    
    
     
       Additional advantages of the heat pump type laundry dryer according to the present invention and the method for controlling the heat pump type laundry dryer according to the present invention will become more apparent with the detailed description of the embodiments with reference to the accompanying drawings in which: 
         FIG. 1 —is a schematic view of a heat pump type laundry dryer according to an embodiment of the present invention; 
         FIG. 2 —is a flowchart showing the steps of a method for controlling the heat pump type laundry dryer according to an embodiment of the present invention; 
         FIG. 3 —is a flowchart showing the steps of a method for controlling the heat pump type laundry dryer according to an embodiment of the present invention. 
     
    
    
     The reference signs appearing on the drawings relate to the following technical features.
       1 . Laundry dryer     2 . Drum     3 . Process air circuit     4 . Refrigerant circuit     5 . Compressor     6 . Control unit     7 . Fan     8 . Sensor     9 . Sensor     10 . Duct     11 . Fan     12 . Condenser     13 . Expansion element     14 . Evaporator   T_air: Temperature of the process air   T_rfg: Temperature of the refrigerant   T_th 1 : First threshold process air temperature   T_th 2 : Second threshold process air temperature   T_th 3 : Third threshold process air temperature   T_th 4 : Fourth threshold process air temperature   A: Auxiliary cooling process   

     The heat-pump type laundry dryer ( 1 ) comprises a rotating drum ( 2 ) for receiving the laundry, a process air circuit ( 3 ) for conveying dehumidified heated process air through the drum ( 2 ), wherein the process air circuit ( 3 ) includes a duct ( 10 ) and a process air fan ( 11 ) inside the duct ( 10 ), a refrigerant circuit ( 4 ) for dehumidifying the process air and heating the dehumidified process air, wherein the refrigerant circuit ( 4 ) includes a refrigerant compressor ( 5 ), a condenser ( 12 ), an expansion element ( 13 ) such as a capillary tube or an expansion valve and an evaporator ( 14 ) and a control unit ( 6 ) for conducting the drying process of the laundry. 
     The heat-pump type laundry dryer ( 1 ) of the present invention further comprises: a fan ( 7 ) for cooling the compressor ( 5 ) and a sensor ( 8 ) for sensing the process air temperature T_air, wherein the control unit ( 6 ) is configured to set a first threshold temperature T_th 1  which is equal to or smaller than 55° C., to set a second threshold temperature T_th 2  which is less than T_th 1  and to initiate, prior to the termination of the drying process a cooling process in which the fan ( 7 ) is turned ON when T_air reaches T_th 1  and the fan ( 7 ) is turned OFF when T_air drops to T_th 2  ( FIGS. 1 to 3 ). 
     In an embodiment, the control unit ( 6 ) is further configured to initiate the cooling process when a predetermined drying program has been selected by the user via a user interface (not shown) or a predetermined duration has elapsed since the drying process was commenced. In a version of this embodiment, said drying program is a program which is specific for drying cotton laundry to a level of dryness that is suitable for ironing. In another version of this embodiment, said predetermined duration is shorter than the total operation time of the drying program. 
     In another embodiment, the heat-pump type laundry dryer ( 1 ) further comprises a sensor ( 9 ) for sensing the refrigerant temperature T_rfg. In this embodiment, the control unit ( 6 ) is further configured to set a third threshold temperature T_th 3 , to set a fourth threshold temperature T_th 4  which is equal to smaller than T_th 3  and to initiate an auxiliary cooling process (A) prior to the initiation of the cooling process or after the initiation of the cooling process when T_air has not yet reached T_th 1 , wherein in the auxiliary cooling process (A) the fan ( 7 ) is turned ON when T_rfg exceeds T_th 3  and the fan ( 7 ) is turned OFF when T_rfg drops below T_th 4 . 
     In another embodiment, the process air circuit ( 3 ) comprises an air filter (not shown) which is located immediately downstream of the drum ( 2 ). In this embodiment, the sensor ( 8 ) for sensing the process air temperature T_air is located in the vicinity of the air filter. 
     The method for controlling the heat-pump type laundry dryer ( 1 ) comprises: a step (S 1 ) of starting the drying process in which the laundry is dried by process air. 
     The control method the present invention further comprises: a step (S 2 ) of setting a first threshold temperature T_th 1  which is equal to or smaller than 55° C. and a second threshold temperature T_th 2  which is less than T_th 1 ; a step (S 2 ) of initiating a cooling process prior to the termination of the drying process, wherein the cooling process comprises: a step (S 3  to S 6 ) of sensing the process air temperature T_air, turning the compressor cooling fan ( 7 ) ON when the process air temperature T_air reaches T_th 1  and turning the compressor cooling fan ( 7 ) OFF when the process air temperature T_air drops to T_th 2 . 
     In another embodiment, in the initiating step (S 2 ) the cooling process is initiated when a predetermined drying program has been selected by the user or a predetermined duration has elapsed since the drying process was commenced. 
     In another embodiment, the control method further comprises a step of initiating (S 2 -S 3 ) an auxiliary cooling process (A) prior to the initiation of the cooling process, or after the initiation of the cooling process when T_air has not yet reached T_th 1 , wherein the auxiliary cooling process (A) comprises: a step (S 7 , S 9 ) of setting a third threshold temperature T_th 3  and a fourth threshold temperature T_th 4  which is equal to or smaller than T_th 3  and a step (S 7  to S 10 ) of sensing the refrigerant temperature T_rfg, turning the compressor cooling fan ( 7 ) ON when the refrigerant temperature T_rfg exceeds T_th 3  and turning the compressor cooling fan ( 7 ) OFF when the refrigerant temperature T_rfg drops below T_th 4  ( FIGS. 1 to 3 ).