Patent Publication Number: US-2022228312-A1

Title: Lint filter clogging detection in a dryer appliance using compressor temperature and referigerant mass flow

Description:
FIELD OF THE INVENTION 
     The subject matter of the present disclosure relates generally to dryer appliance for laundry and more particularly to the detection of lint filter clogging in a dryer appliance. 
     BACKGROUND OF THE INVENTION 
     Generally, a dryer appliance provides for drying wet articles of laundry usually after a washing process. The articles may include e.g., clothing, linens, and other items. The wet articles are placed into a compartment or drum through which relatively dry, heated air is passed in order to capture and remove moisture (e.g., water) from the articles. Depending on the type of dryer, the moisture-laden air may be vented in order to remove moisture from the appliance. Alternatively, the air may be recirculated after being cooled, which causes the water vapor present to condense so that it may be removed. 
     The circulated air is usually filtered in order to remove lint, which is an accumulation of textile fibers and other materials that may be released from the laundry articles during the drying process. One or more such filters may be utilized in the dryer appliance. As the lint accumulates, the filter must be periodically cleaned. Some laundry articles e.g., may shed more lint during a drying cycle thereby loading the filter more quickly. The frequency of cleaning required depends upon several variables including the materials from which the laundry articles were created and the frequency of use of the appliance. 
     As the amount of lint in the filter increases, the pressure drop of air passing through the filter also increases while the needed flow of drying air through the appliance decreases. This pressure drop may increase gradually or may occur more quickly. For example, the pressure drop may increase over the course of several drying cycles if the user neglects to regularly clean the filter, or a particular high-lint-shedding laundry load may clog the filter during a drying cycle. In appliances having an auto-cleaning filter, residual lint may simply accumulate over time even though the filter is automatically cleaned or if the auto cleaning cycle is not entirely effective. 
     Regardless, the increased pressure drop is undesirable because the concomitant reduction in air flow leads to increased drying times and, therefore, lower energy efficiency. The reduced air flow can also lead to undesirable overheating of the inlet air to the drum. For a dryer that uses a heat pump cycle, the reduced air flow may lead to overheating of the compressor, which can also undesirably heat the space where the appliance is located such as a laundry room of the user. 
     Conventional systems for detecting whether a lint filter needs cleaning have shown limited effectiveness. Such are sometimes based primarily on temperature measurements and can lack sensitivity to gradual accumulations in the filter. 
     Accordingly, a drying appliance equipped to detect the clogging of one or more lint filters would be useful. Such an appliance equipped to take one or more corrective actions once clogging to the lint filter is detected would be particularly useful. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Additional aspects and advantages of the invention will be set forth in part in the following description, or may be apparent from the description, or may be learned through practice of the invention. 
     In a first exemplary aspect, the present invention provides a method of operating an appliance used for drying a load of articles placed into a compartment of the appliance, the appliance having at least one lint filter and a heat pump system that includes a compressor and refrigerant circuit. The method includes beginning a drying cycle for the load of articles; determining when a steady state condition has been reached during the drying cycle for the load of articles; ascertaining whether a frequency of compressor temperature response exceeds a predetermined threshold value of compressor temperature response, and, if so, then detecting if a refrigerant mass flow rate is below a predetermined threshold value of refrigerant mass flow rate; and undertaking a corrective action if the refrigerant mass flow rate is below a predetermined threshold value of refrigerant mass flow rate. 
     In another exemplary aspect, the present invention provides a laundry appliance. The appliance includes a cabinet and a drum located in the cabinet and defining a compartment for receipt of articles for drying during a drying cycle. The appliance also includes a lint filter and a heat pump system having a compressor within a refrigerant circuit. A controller is configured for beginning a drying cycle for the load of articles; determining when a steady state condition has been reached during the drying cycle for the load of articles; ascertaining whether a frequency of compressor temperature response exceeds a predetermined threshold value of compressor temperature response, and, if so, then detecting if a refrigerant mass flow rate is below a predetermined threshold value of refrigerant mass flow rate; and undertaking a corrective action if the refrigerant mass flow rate is below a predetermined threshold value of refrigerant mass flow rate. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which: 
         FIG. 1  provides a perspective view of a laundry appliance in accordance with exemplary embodiments of the present disclosure. 
         FIG. 2  provides a perspective view of the exemplary laundry appliance of  FIG. 1  with portions of a cabinet of the laundry appliance removed to reveal certain components of the laundry appliance. 
         FIG. 3  provides a schematic diagram of an exemplary heat pump laundry appliance and a conditioning system thereof in accordance with exemplary embodiments of the present disclosure. 
         FIG. 4  illustrates a plot of the volumetric air flow as a function of time during drying cycles of an exemplary appliance. 
         FIG. 5  illustrates a plot of refrigerant mass flow rate as a function of time during drying cycles of an exemplary appliance. 
         FIG. 6  illustrates plots of compressor shell temperature as a function of time for two different drying cycles of an exemplary appliance. 
         FIG. 7  is a diagram of an exemplary method of operating an exemplary appliance of the present invention. 
         FIGS. 8 and 9  depict plots of temperature and relative humidity as function of time for a drying cycle of an appliance. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
       FIGS. 1 and 2  provide perspective views of a laundry appliance  10  according to exemplary embodiments of the present disclosure. Laundry appliance  10  is a dryer appliance for drying a load of articles in the illustrated embodiments and may also, in additional embodiments, include features for washing articles. For example, laundry appliance  10  may also, or instead, be a combination laundry appliance. In particular,  FIG. 1  provides a perspective view of dryer appliance  10  and  FIG. 2  provides another perspective view of dryer appliance  10  with a portion of a housing or cabinet  12  of dryer appliance  10  removed in order to show certain components of dryer appliance  10 . 
     As depicted, dryer appliance  10  defines a vertical direction V, a lateral direction L, and a transverse direction T, each of which is mutually perpendicular such that an orthogonal coordinate system is defined. While described in the context of a specific embodiment of dryer appliance  10 , using the teachings disclosed herein it will be understood that dryer appliance  10  is provided by way of example only. Other laundry appliances having different appearances and different features may also be utilized with the present subject matter as well. For instance, in some embodiments, laundry appliance  10  can be a combination washing machine/dryer appliance or a condensing laundry drying appliance. 
     Cabinet  12  includes a front panel  14 , a rear panel  16 , a pair of side panels  18  and  20  spaced apart from each other by front and rear panels  14  and  16  along the lateral direction L, a bottom panel  22 , and a top cover  24 . Cabinet  12  defines an interior volume  29 . A drum, or container  26  is mounted for rotation about a substantially horizontal axis within the interior volume  29  of cabinet  12 . Drum  26  defines a compartment or chamber  25  for receipt of articles for tumbling and/or drying. Drum  26  extends between a front portion  37  and a back portion  38 , e.g., along the transverse direction T. Drum  26  also includes a back or rear wall  34 , e.g., at back portion  38  of drum  26 . A supply duct  41  may be mounted to rear wall  34 . Supply duct  41  receives heated air that has been heated by a conditioning system  40  and provides the heated air to drum  26  via one or more holes defined in rear wall  34 . 
     As used herein, the terms “clothing” or “articles” includes but need not be limited to fabrics, textiles, garments, linens, papers, or other items from which the extraction of moisture is desirable. Furthermore, the term “load” or “laundry load” refers to the combination of clothing or articles that may be washed together in a washing machine or dried together in a dryer appliance (e.g., clothes dryer) and may include a mixture of different or similar articles of clothing of different or similar types and kinds of fabrics, textiles, garments and linens within a particular laundering process. 
     In some embodiments, a motor  31  is provided to rotate drum  26  about the horizontal axis, e.g., via a pulley and a belt (not pictured). Drum  26  is generally cylindrical in shape. Drum  26  has an outer cylindrical wall  28  and a front flange or wall  30  that defines an opening  32  of drum  26 , e.g., at front portion  37  of drum  26 , for loading and unloading of articles into and out of chamber  25  of drum  26 . Drum  26  includes a plurality of lifters or baffles  27  that extend into chamber  25  to lift articles therein and then allow such articles to tumble back to a bottom of drum  26  as drum  26  rotates. Baffles  27  may be mounted to drum  26  such that baffles  27  rotate with drum  26  during operation of dryer appliance  10 . 
     Rear wall  34  of drum  26  is rotatably supported within cabinet  12  by a suitable bearing. Rear wall  34  can be fixed or can be rotatable. Rear wall  34  may include, for instance, a plurality of holes that receive hot air that has been heated by a conditioning system  40 , e.g., a heat pump or refrigerant-based conditioning system as will be described further below. Moisture laden, heated air is drawn from drum  26  by an air handler, such as a blower fan  48 , which generates a negative air pressure within drum  26 . The moisture laden heated air passes through a duct  44  enclosing screen filter  46 , which traps lint particles. Other filters or placements of filter  46  may also be utilized in the scope of the invention and claims that follow. 
     As the air passes from blower fan  48 , it enters a duct  50  and then is passed into conditioning system  40 . In some embodiments, dryer appliance  10  is a heat pump dryer appliance and thus conditioning system  40  may be or include a heat pump system or sealed refrigerant circuit  80 , as described in more detail below with reference to  FIG. 3 . Heated air (with a lower moisture content than was received from drum  26 ), exits conditioning system  40  and returns to drum  26  by duct  41 . After the clothing articles have been dried, they are removed from the drum  26  via opening  32 . A door  33  provides for closing or accessing drum  26  through opening  32 . 
     In some embodiments, one or more selector inputs  70 , such as knobs, buttons, touchscreen interfaces, etc., may be provided or mounted on a cabinet  12  (e.g., on a backsplash  71 ) and are communicatively coupled with (e.g., electrically coupled or coupled through a wireless network band) at least one processing device or controller  56 . Controller  56  may also be communicatively coupled with various operational components of dryer appliance  10 , such as motor  31 , blower  48 , components of conditioning system  40 , and various sensors (e.g., temperature, relative humidity, and weight) as will be further described. In turn, signals generated in controller  56  direct operation of motor  31 , blower  48 , or conditioning system  40  in response user inputs to selector inputs  70 . As used herein, “processing device” or “controller” may refer to one or more microprocessors, microcontroller, ASICS, or semiconductor devices and is not restricted necessarily to a single element. The controller  56  may be programmed to operate dryer appliance  10  by executing instructions stored in memory (e.g., non-transitory media). The controller  56  may include, or be associated with, one or more memory elements such as RAM, ROM, or electrically erasable, programmable read only memory (EEPROM). For example, the instructions may be software or any set of instructions that when executed by the processing device, cause the processing device to perform operations. It should be noted that controller  56  as disclosed herein is capable of and may be operable to perform any methods or associated method steps as disclosed herein. For example, in some embodiments, methods disclosed herein may be embodied in programming instructions stored in the memory and executed by the controller  56 . 
       FIG. 3  provides a schematic view of laundry appliance  10  and depicts an air conditioning system  40  in more detail. For this exemplary embodiment, laundry appliance  10  is a heat pump dryer appliance and thus conditioning system  40  includes a sealed heat pump system  80 . In additional embodiments, the conditioning system  40  illustrated in  FIG. 3  and described herein may also be provided in, for example, a combination washing machine/dryer appliance. In other embodiments, the present invention is not limited to laundry appliance having a sealed system and may be used e.g., with a system that vents moisture laden air out of appliance  10 . 
     Continuing with  FIG. 3 , sealed system  80  includes various operational components, which can be encased or located within a machinery compartment of dryer appliance  10 . Generally, the operational components are operable to execute a vapor compression cycle for heating and cooling process air passing through conditioning system  40 . The operational components of sealed system  80  include an evaporator  82 , a compressor  84 , a condenser  86 , and one or more expansion devices  88  connected in series along a refrigerant circuit or line  90 . A cooling fan  89  may be provided to remove excess heat from the compressor  84 . Alternatively, or in addition thereto, an auxiliary condenser may be provided to supplement condenser  86 . In the illustrated embodiments, the expansion device  88  is an expansion valve, such as an electronic expansion valve. Refrigerant line  90  is charged with a working fluid, which in this example is a refrigerant. Sealed system  80  depicted in  FIG. 3  is provided by way of example only. Thus, it is within the scope of the present subject matter for other configurations of the sealed system to be used as well. For example, in some embodiments, the expansion device  88  may also, or instead, include a capillary tube. As will be understood by those skilled in the art, sealed system  80  may include additional components, e.g., at least one additional evaporator, compressor, expansion device, and/or condenser. As an example, sealed system  80  may include two (2) evaporators. 
     In some embodiments, the sealed system  80  may optionally include one or more sensors for measuring characteristics and operating conditions of the sealed system  80 . For example, the sealed system  80  may include a suction line temperature sensor  94 , e.g., upstream of the compressor  84 . As another example, the sealed system  80  may include an evaporator inlet temperature sensor  96  positioned at an inlet of the evaporator  82  and configured to measure a temperature of the refrigerant at the inlet of the evaporator  82 . Sealed system  80  can include a temperature sensor  91  for measuring a temperature of compressor  84  or, more particularly, for measuring the shell temperature of compressor  84 . Sealed system  80  may also include a refrigerant flow rate sensor  83  for measuring the mass flow rate of refrigerant in circuit  90 . Sensor  83  may be placed e.g., immediately upstream or downstream of compressor  84 . Other locations may be used as well. 
     In performing a drying and/or tumbling cycle, one or more laundry articles LA may be placed within the chamber  25  of drum  26 . Hot dry air DA is supplied to chamber  25  via duct  41 . The hot dry air DA enters chamber  25  of drum via a drum inlet  52  defined by drum  26 , e.g., the plurality of holes defined in rear wall  34  of drum  26  as shown in  FIG. 2 . The hot dry air DA provided to chamber  25  causes moisture (e.g., water) within laundry articles LA to evaporate. Accordingly, the air within chamber  25  increases in water content and exits chamber  25  as warm moisture laden air MLA. The warm moisture laden air MLA exits chamber  25  through a drum outlet  54  defined by drum  26  and flows into duct  44 . 
     After exiting chamber  25  of drum  26 , the warm moisture laden air MLA flows downstream to conditioning system  40 . Blower fan  48  moves the warm moisture laden air MLA, as well as the air more generally, through a process air flow path  58  defined by drum  26 , conditioning system  40 , and the duct system  60 . Thus, generally, blower fan  48  is operable to move air through or along the process air flow path  58 . Duct system  60  includes all ducts that provide fluid communication (e.g., airflow communication) between drum outlet  54  and conditioning system  40  and between conditioning system  40  and drum inlet  52 . Although blower fan  48  is shown positioned between drum  26  and conditioning system  40  along duct  44 , it will be appreciated that blower fan  48  can be positioned in other suitable positions or locations along duct system  60 . 
     As further depicted in  FIG. 3 , the warm moisture laden air MLA flows into or across evaporator  82  of the conditioning system  40 . As the moisture laden air MLA passes across evaporator  82 , the temperature of the air is reduced through heat exchange with refrigerant that is vaporized within, for example, coils or tubing of evaporator  82 . This vaporization process absorbs both the sensible and the latent heat from the moisture laden air MLA—thereby reducing its temperature. As a result, moisture in the air is condensed and such condensate (e.g., water) may be drained from conditioning system  40 , e.g., using a drain line  92 , which is also depicted in  FIG. 2 . 
     Air passing over evaporator  82  becomes cooler than when it exited drum  26  at drum outlet  54 . As shown in  FIG. 3 , cool air CA (cool relative to hot dry air DA and moisture laden air MLA) flowing downstream of evaporator  82  is subsequently caused to flow across condenser  86 , e.g., across coils or tubing thereof, which condenses refrigerant therein. The refrigerant enters condenser  86  in a gaseous state at a relatively high temperature compared to the cool air CA from evaporator  82 . As a result, heat energy is transferred to the cool air CA at the condenser  86 , thereby elevating its temperature and providing warm dry air DA for resupply to drum  26  of dryer appliance  10  through inlet  52 . The warm dry air DA passes over and around laundry articles LA within the chamber  25  of the drum  26 , such that warm moisture laden air MLA is generated, as mentioned above. Because the air is recycled through drum  26  and conditioning system  40 , dryer appliance  10  can have a much greater efficiency than traditional clothes dryers where all or most of the warm, moisture laden air MLA is exhausted to the environment. 
     In some embodiments, conditioning system  40  of dryer appliance  10  optionally includes an electric heater  102  positioned to provide heat to process air flowing along the process air flow path  58 , e.g., as shown in  FIG. 3 . Electrical heater  102  can receive electrical power (e.g., from a power source) and can generate heat based at least in part on the received electrical power. The generated heat can be imparted to the process air flowing along the process air flow path  58 . 
     With respect to sealed system  80 , compressor  84  pressurizes refrigerant (i.e., increases the pressure of the refrigerant) passing therethrough and generally motivates refrigerant through the sealed refrigerant circuit or refrigerant line  90  of conditioning system  40 . Compressor  84  may be communicatively coupled with controller  56  (communication lines not shown in  FIG. 3 ). Refrigerant is supplied from the evaporator  82  to compressor  84  in a low pressure gas phase. The pressurization of the refrigerant within compressor  84  increases the temperature of the refrigerant. The compressed refrigerant is fed from compressor  84  to condenser  86  through refrigerant line  90 . As the relatively cool air CA from evaporator  82  flows across condenser  86 , the refrigerant is cooled and its temperature is lowered as heat is transferred to the air for supply to chamber  25  of drum  26 . 
     Upon exiting condenser  86 , the refrigerant is fed through refrigerant line  90  to expansion valve  88 . Expansion valve  88  lowers the pressure of the refrigerant and controls the amount of refrigerant that is allowed to enter the evaporator  82 . The flow of liquid refrigerant into evaporator  82  is limited by expansion valve  88  in order to keep the pressure low and allow expansion of the refrigerant back into the gas phase in evaporator  82 . The evaporation of the refrigerant in evaporator  82  converts the refrigerant from its liquid-dominated phase to a gas phase while cooling and drying the moisture laden air MLA received from chamber  25  of drum  26 . The process is repeated as air is circulated along process air flow path  58  while the refrigerant is cycled through sealed system  80 , as described above. Although dryer appliance  10  is depicted and described herein as a heat pump dryer appliance, in at least some embodiments, dryer appliance  10  can be a combination washer/dryer appliance as previously stated. 
     For this exemplary embodiment, the electronic expansion valve  88  can be operable to adjust a pressure of the refrigerant flowing along sealed system  80 . For example, controller  56  may be configured to cause the electronic expansion valve  88  to adjust the pressure of the refrigerant flowing along the sealed system  80 . For instance, the electronic expansion valve  88  can be moved from a first position to a second position which is a closed position or an intermediate position (e.g., not fully open or fully closed) which is closer to the closed position than the first position. This can increase the pressure on the high side of sealed system  80  and decrease the pressure on the low side of sealed system  80 . Accordingly, the temperature of the refrigerant increases on the high side of sealed system  80  and the temperature of the refrigerant decreases on the low side of sealed system  80 . That is, adjustment of the electronic expansion valve can drive higher temperatures in condenser  86  and can lower the temperature of the evaporator  82 . 
     Further, adjustment of the electronic expansion valve  88  can maintain a constant superheat in the sealed system  80  and in particular a constant level of superheat into the compressor  84 , such as to avoid liquid refrigerant reaching the compressor  84 . For example, the controller  56  may be configured to automatically adjust the electronic expansion valve  88  to maintain a constant degree of superheat into the compressor  84 . As the degree of superheat in the sealed system  80  decreases, e.g., when the remaining moisture content in the laundry articles LA is below a certain level or threshold, the electronic expansion valve  88  may be closed (or partially closed, e.g., moved to an intermediate position which is closer to the closed position than a prior position) to restrict the flow of refrigerant in the sealed system  80 . Thus, in some embodiments, the degree of superheat in the sealed system  80  and therefore the dryness of the laundry articles LA may be determined based on the position of the electronic expansion valve  88 . For example, the laundry appliance  10  may include a position sensor or other expansion valve position tracking system which may be used to determine the position of the electronic expansion valve  88  and thereby determine or detect dryness of the laundry articles LA based on the position of the electronic expansion valve  88 . 
     As shown, appliance  10  may include one or more lint filters  46  and  110  to collect lint during drying operations. By way of example, lint filter  46  is readily accessible by a user of the appliance. As such, lint filter  46  should be manually cleaned by removal of the filter, pulling or wiping away accumulated lint, and then replacing the filter  46  for subsequent drying cycles. Alternatively, or in addition to lint filter  46 , appliance  10  may include one or more of an auto-cleaning lint filter  110  that is automatically cleaned at certain times as part of the operation of appliance  10 . Each of these filters  46  and  110  is placed into the path  58  of air flow through appliance  10  and includes a screen, mesh, other material to capture lint in the air flow. The location of lint filters in appliance  10  as shown in  FIG. 3  is provided by way of example only, and other locations may be used as well. 
     With continued reference to  FIG. 3 , appliance  10  may include temperature sensors and relative humidity sensors that provide temperature (e.g., dry bulb temperature) and humidity measurements to controller  56  from certain locations in the air flow along path  58  during a drying cycle. More particularly, appliance  10  includes a temperature sensor  104  and a relative humidity sensor  105  placed at the outlet  54  of drum  26  (having compartment  25  for receipt of a load of articles for drying) in order to measure the temperature and relative humidity of the air exiting drum  26 . Such air is received from compartment  25  and may be MLA or moisture laden air, particularly in the earlier time period of a drying cycle of wet laundry articles. In this embodiment, in terms of the air flow along path  58 , temperature sensor  104  and relative humidity sensor  105  are downstream of drum  26  and upstream of evaporator  82 . Based on their location relative to drum  26  and the direction of air flow, temperature sensor  104  and relative humidity sensor  105  may also be referred to herein as the drum outlet air temperature sensor  104  and drum outlet air relative humidity sensor  105 . 
     Appliance  10  also includes a temperature sensor  106  and a relative humidity sensor  107  placed upstream of the drum  26  and at the outlet  87  of condenser  86  to measure the temperature and relative humidity of the after treatment by condenser  86  and before entering drum  26 . Such air is supplied to compartment  25  and may be DA or relatively dry air from which water vapor has been removed as previously described. Based on their location relative to drum  26  and the direction of air flow, temperature sensor  106  and relative humidity sensor  107  may also be referred to herein as the condenser air outlet temperature sensor  106  and the condenser air outlet relative humidity sensor  107 . 
     As an alternative, or in addition thereto, appliance  10  may include another placement of a temperature sensor and/or relative humidity sensor for measurements of air that is suppled to compartment  25 —placement that is downstream of condenser  86  and located just before entering drum  26 . As shown in  FIG. 3 , appliance  10  may include a temperature sensor  108  and relative humidity sensor  109  placed at the inlet  52  of drum  26 . Based on their location relative to drum  26  and the direction of air flow, temperature sensor  108  and relative humidity sensor  109  may also be referred to herein as the drum inlet air temperature sensor  108  and drum inlet air relative humidity sensor  109 . 
     Other locations for both temperature sensors  104 ,  106 ,  108  and relative humidity sensors  105 ,  107 , and  109  may also be used provided that such allows for measurement of the temperature and relative humidity of air supplied to, and air received from, compartment  25  of drum  26 . 
     Appliance  10  may also include means for determining the average moisture extraction rate (MER) from a load of laundry articles place in the compartment  25  of drum  26  during a drying cycle. The average moisture extraction rate or MER will be understood as the average rate of removal of moisture from articles in drum  26  by the air circulated therethrough during a drying cycle. For example, appliance  10  may include a load sensor  110  on drum  26 . Load sensor can measure the weight w of laundry articles place in drum  26  at certain times t over the course of a drying cycle and provide this information to controller  56 . As moisture is removed from the laundry articles during the drying cycle, the weight of laundry articles in drum  26  will decrease. This is illustrated in  FIG. 4 , which depicts the weight w of a laundry load in drum  26  over time t during a drying cycle for appliance  10 . Controller  56  can calculate an average MER by dividing the change in weight of the laundry articles by the elapsed time during which such weight changed occurred, as represented by Equation 1: 
     Eq. 1—average MER 
       average MER=( w   2   −w   1 )/( t   2   −t   1 ) 
     The average MER may, for example, be expressed as pounds of water per minute, kilograms per second, or other mass per time units that may be used as well. Notably, as shown in  FIG. 4 , the average MER (e.g., the slope of curve  103 w) becomes relatively constant once steady state conditions are reached. 
     Alternatively, for determining an average MER, in another exemplary aspect appliance  10  may use a flow meter  112  that measures the volumetric flow of condensate from drain line  92  and provides the same to controller  56 . In still another exemplary aspect, condensate from evaporator  82  may be collected in a reservoir  116  and a pressure sensor or float  114  would measure the amount of condensate collected over a given time interval or determine when a predetermined amount of condensate has been collected in reservoir  116  and provide such information to controller  56 . Using the teachings disclosed herein, one of skill in the art will understand that other techniques may also be used to determine the average MER. 
     As previously mentioned, filters  46  and/or  110  can accumulate lint and eventually create an undesirable pressure drop during operation of appliance  10 .  FIG. 4  illustrates a plot during a drying cycle of the volumetric air flow rate in cubic feet per minute (CFM) through air flow path  58 . Plot  800  represents a lint filter that was about 25 percent blocked whereas plot  802  represents a lint filter that was about 75 percent blocked (the percentages were determined relative to the desired unblocked airflow, which was considered to be 100 percent). The volumetric air flow rate is higher for the less clogged filter represented by plot  800 , and the volumetric air flow rate decreases over the time t of the drying cycle operation as lint accumulates. 
     For a laundry dryer such as appliance  10  having a heat pump system  80  for providing heat to the drying air, the temperature of compressor  84  (as measured e.g., by temperature sensor  91  will increase undesirably as the airflow is reduced by one or more clogged lint filters such as filter  46  and/or  110 . Once the temperature of the compressor exceeds a certain predetermined threshold, certain actions will be taken in response by appliance  10 . The responses, referred to herein as “compressor temperature response” (CTR), can include one or more of a variety of actions depending upon the design of appliance  10 . As used herein, compressor temperature response or CTR means the action appliance  10  undertakes in an effort to lower the temperature of compressor  84 . The temperature of a compressor may be measured at the compressor&#39;s shell, which is simply a temperature measurement at an exterior wall or shell of the compressor. 
     For example, if compressor  84  is a variable-speed type, then the compressor temperature response or CTR may include appliance  10  (e.g., controller  56 ) operating compressor  84  at a lower speed to maintain the superheated state and the inlet air temperature to drum  26 . The lower compressor speed means the mass flow rate of refrigerant (as measured by e.g., meter  83 ) through refrigerant circuit  90  will decrease.  FIG. 5  provides a plot of refrigerant mass flow rate over time as measured e.g., by refrigerant mass flow rate sensor  83 . Plot  900  represents appliance  10  having 25 percent filter blockage whereas plot  902  is for appliance  10  having a  75  percent filter blockage (the percentage being determined with respect to no lint present (0 percent) and total blockage (100 percent). As shown, the mass flow rate of refrigerant in either case first reaches a steady state condition during a drying cycle. Thereafter, the mass flow rate of refrigerant is significantly less for plot  902  where the lint filter(s) of appliance  10  is significantly more clogged. In this example, the difference in plots  900  and  902  is particularly evident around the 45 to 50 minute mark one the mass flow rate has stabilized. 
     If compressor  84  is a single-speed type, then compressor temperature response or CTR may include controller  56  activating an auxiliary condenser (connected in parallel or series with condenser  86 ) in order to maintain the superheated state and the inlet air temperature to drum  26 . Alternatively, or in addition thereto, the compressor temperature response or CTR may include controller  56  activating a cooling fan  89  for compressor  84  in order to maintain the superheated state and the inlet air temperature to drum  26 .  FIG. 6  provides plots of shell temperature of compressor  84  over time as measured by e.g., temperature sensor  91 . Plot  1000  is for appliance  10  having 25 percent filter blockage whereas plot  1002  is for appliance  10  having a 75 percent filter blockage (the percentage being determined with respect to no lint present (0 percent) and total blockage (100 percent). As shown, the compressor shell temperature fluctuates as e.g., cooling fan  89  is cycled on and off and/or as auxiliary condenser is employed in an effort to regulate the shell temperature of compressor  84 . In addition, the frequency of the temperature regulation increases during a given drying cycle as lint accumulates in the filters(s), and the frequency of the temperature regulation is higher for the filter that is more clogged (plot  1002 ). As shown, the frequency of the temperature regulation by cooling fan  89  is about 50 percent higher when the filter(s) are 75 percent blocked as opposed to 25 percent blocked. An appliance  10  with a variable speed compressor  84  may also be equipped to utilize an auxiliary condenser or cooling fan as an alternative, or in addition to, compressor speed control for purposes of compressor temperature response or CTR. 
     In one exemplary aspect, the present invention utilizes the compressor temperature response or CTR to determine the condition of the one or more lint filters in air flow path  58 . Based on the frequency of the compressor temperature regulation responses or CTRs, one or more actions may be undertaken by controller  56 . Referring to  FIG. 7 , an exemplary method of  200  operating appliance  10  will now be described. Using the teachings disclosed herein, one of ordinary skill in the art will understand that other methods within the scope of the invention and claims that follow may be applied as well. 
     After start  202  of a drying cycle for appliance  10 , a determination is made in step  204  at a time after start-up of the drying cycle as to whether steady state conditions in appliance  10  have been reached. For this exemplary embodiment of the invention, determining steady state conditions may be important so that changes in the compressor temperature and/or mass flow rate of attributed to the accumulation of lint or clogging of the filter instead of being affected by transient changes that occur before appliance  10  reaches steady state. A variety of different techniques may be used to determine whether appliance  10  has reached steady state conditions. 
     For example, during a drying cycle of appliance  10  with a laundry load present in compartment  25  of drum  26 ,  FIG. 8  depicts the temperature measurements  108   T  from drum inlet air temperature sensor  108  and temperature measurements  104 T from drum outlet air temperature sensor  104 . For the same drying cycle as  FIG. 8 ,  FIG. 9  depicts the relative humidity (RH) measurements  109   RH  from drum inlet air relative humidity sensor  109  and relative humidity (RH) measurements  105   RH  from drum outlet air relative humidity sensor  105 . As shown in  FIG. 8 , temperature measurements  108 T from drum inlet air temperature sensor  108  changed rapidly during the first approximately 20 minutes of the drying cycle. The relative humidity measurements  109   RH  from drum inlet air relative humidity sensor  109  also changed rapidly during the first approximately 10 minutes of the drying cycle. 
     One or both of the measurements depicted in  FIGS. 8 and 9  may be used by appliance  10 , and specifically controller  56 , to determine when steady conditions have been reached. For example, controller  56  may be configured to simply delay a predetermined period of time t initial  after appliance  10  has been operating before proceeding to use compressor temperature response or CTR to determine the condition of the one or more lint filters in air flow path  58 . In one embodiment, t initial  might be preset as 20 minutes, after which in step  202  the controller  56  proceeds under the assumption of steady-state conditions. Other time periods for t initial  may be used as well. 
     In another embodiment, controller  56  would determine whether the rate of change (ROC) of the temperature measurements  108   T , relative humidity measurements  109   RH , or both, has fallen below certain predetermined threshold values before proceeding to use compressor temperature response or CTR to determine the condition of the one or more lint filters in air flow path  58 . As used herein, rate of change or ROC means the change in a measured value of a certain interval of time. For example, as indicative of a steady state condition being reached, controller  56  might monitor the temperature, relative humidity, or both, of air supplied to compartment or drum  26  to determine when the rate of change has reached or dropped below a predetermined threshold value, ROC THR . In one embodiment, controller  56  may monitor temperature measurements  108   T  and determine that a steady condition in drum  26  has not been reached until the rate of change (ROC) for temperature measurements  108   T  is less than an ROC THR-T  of 5 degrees per minute, less than 3 degrees per minute, or less than 1 degree per minute. Other values for ROC THR  may be used as well. 
     In still another example, controller  56  may monitor relative humidity measurements  109 RH and determine that a steady condition in drum  26  has not been reached until the rate of change (ROC) for relative humidity measurements  109   RH  is less than an ROC THR-RH  of 10 percent per minute, less than 5 percent per minute, or less than 1 percent per minute. Other values for ROC THR  may be used as well. In still another embodiment, controller  56  might monitor both temperature and relative humidity measurements until the ROC for both the temperature measurements  108 T and relative humidity measurements  109 RH are each below certain predetermined threshold values, ROC THR . By way of further example, controller  56  might also use measurements from sensors  106  and  107  in addition to, or instead of, measurements from sensors  108  and  109 . 
     In still another example, controller  56  might also use measurements from refrigerant mass flow rate sensor  83  to determine that steady state conditions have been reached before proceeding to use compressor temperature response or CTR to determine the condition of the one or more lint filters in air flow path  58 . Accordingly, controller  56  might monitor the rate of change (ROC) for mass flow rate measurements and determine that steady conditions have not been reached until the ROC for the mass flow rate measurements is less than an ROC THR-MF  of 10 percent per minute, less than 5 percent per minute, or less than 1 percent per minute. Other values for ROC THR-MF  may be used as well. 
     At about the same time or shortly after steady state conditions are determined, in step  206  controller  56  ascertains whether the frequency of the CTRs (CTR F ) is at, or exceeds, a predetermined frequency threshold (CTR F-THR ) as an indicator of the conditions of lint filters  46  and/or  110 . Returning to  FIG. 6 , due to the shell temperature of compressor  84  repeatedly exceeding a certain temperature, appliance  10  (e.g., controller  56 ) has repeatedly initiated certain compressor temperature response or CTRs—such as activating cooling fan  89 . For plot  1002 , where the lint filter(s) are 75 percent blocked, the frequency of CTRs has increased about 1.5 times over the 25 percent blocked condition depicted in plot  1000 . 
     For example, CTR F-THR  might be a fixed value such as CTR F-THR ≥5 CTRs per hour or CTR F-THR ≥10 CTRs per hour. Other values may be used as well. Alternatively, controller  56  might determine CTR F-THR  has been reached based on a predetermined amount of increase in the frequency of CTRs after reaching steady state conditions. For example, controller  56  might determine CTR F-THR  has been reached if the frequency of CTRs increases by more than 50 percent. One of skill in the art will understand, using the teachings disclosed herein, that other methods for determining CTR F-THR  may be used. 
     In the case of a variable speed compressor, if appliance  10  (e.g., controller  56 ) ascertains that the frequency of CTRs or CTR F-THR  has met or exceeds a certain predetermined frequency threshold (CTR F-THR ), then in step  208  appliance  10  proceeds to detect if the refrigerant mass flow rate (RMF), as may be measured by mass flow rate sensor  83 , is at or below a certain predetermined threshold value (RMF THR ). If so, then appliance  10  can undertake one or more corrective actions. 
     In step  210 , for example, as a corrective action appliance  10  can provide a notification to the user that one or more filters need to be cleaned. In another exemplary embodiment, where compressor  84  is not a variable speed compressor, then appliance  10  may skip step  208  and proceed directly to undertaking one or more corrective actions such as providing a notification to the user as in step  210 . Such alerts or notifications may be a visual and/or audible signal at the end of the previous drying cycle, could be provided when the user is about to initiate another drying cycle, or a combination thereof. 
     As will be understood by one of skill in the art using the teaching disclosed herein, other corrective actions may be utilized as alternatives, or in addition, to providing a notification (e.g., visual and/or audible) to the user. For example, if the frequency of CTRs or CTR F-THR  has met or exceeds a certain predetermined frequency threshold (CTR F-THR ) and/or the refrigerant mass flow rate (RMF) is at or below a certain predetermined threshold value (RMF THR ), then appliance  10  may stop the current drying cycle of appliance  10 . If the frequency of CTRs or CTR F-THR  has met or exceeds a certain predetermined frequency threshold (CTR F-THR ) and/or the refrigerant mass flow rate (RMF) is at or below a certain predetermined threshold value (RMF THR ), then appliance  10  may provide a notification to the user regarding a lint filter to indicate e.g., that such filter should be cleaned and/or stop operation of appliance  10 . If the frequency of CTRs or CTR F-THR  has met or exceeds a certain predetermined frequency threshold (CTR F-THR ) and/or the refrigerant mass flow rate (RMF) is at or below a certain predetermined threshold value (RMF THR ), then appliance  10  may undertake an automated cleaning cycle of one or more lint filters. 
     Still other exemplary methods of operating appliance  10  may be employed with the present invention as will be understood using the teaching disclosed herein. For example, appliance  10  may be equipped with an oversized lint filter  46  that does need to be cleaned with every drying cycle. Instead, appliance  10  and particularly controller  56  may be configured to estimate when lint filter cleaning will be needed depending on variables such as load types and sizes. The degradation of the filter  46  can be correlated to the degree of filter loading for various load types and sizes, which can be determined based on user selection and load size determination as previously described. When a remaining interval for cleaning of lint filter  46  is less than a certain threshold value, appliance  10  can alert the user that one or more lint filters or e.g., lint filter  46  should be cleaned. The process  200  described in  FIG. 7  may be provided as a back-up to such estimations. 
     In still another example, CTR F-THR  and/or RMF THR  may include a first set of predetermined values based on a lint filter that is only at e.g., 75 percent clogged so that the corrective action of appliance  10  includes providing the user with a notification that the lint filter should be cleaned while continuing to allow operation of appliance  10 . A second set of values for CTR F-THR  and/or RMF THR  (indicative of e.g., 90 percent clogging of the lint filter) might be used to initiate a subsequent corrective action where a drying cycle of appliance  10  is terminated until the lint filter can be cleaned. Thus, if the first set of values for CTR F-THR  and/or RMF THR  are reached, then controller  56  can provide an alert or notification to the user that the lint filter should be cleaned before starting another drying cycle, but controller  56  would only prevent another drying cycle if the second set of values for CTR F-THR  and/or RMF THR  has been reached. 
     Accordingly, the process set forth in  FIG. 7  is exemplary and representative only. Using the description provided herein, one of ordinary skill in the art will understand that other steps may also be used within the scope of the invention and claims that follow. The order of certain steps may be changed and operations described or claimed as a single step herein may actually be executed in multiple steps or operations. The invention includes an appliance having one or more controllers, microprocessors and/or other elements configured to operate a drying appliance as previously described. Also, while exemplary aspects of the invention have been described using English units (e.g., in the equations above), such is by way of example only and one of skill in the art will understand that e.g., the International System of Units (SI) may be used as well. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.