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issue: June 2004 APPLIANCE Magazine

2004 IATC Best Paper Winner
Detection of Abnormal Operating Conditions in Electric Clothes Dryers


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by Randy Butturini, PE, electrical engineer, Arthur Lee, electrical engineer, U.S. Consumer Product Safety Commission, Bethesda, MD, U.S.

Advances in sensor technology have created opportunities to address many hazards that products can present to consumers, and this paper reports the results of experiments performed on an electric clothes dryer. The test clothes dryer was instrumented with a number of sensors and operated under normal and abnormal operation conditions. The recorded sensor data were analyzed to determine if undesirable conditions that could lead to impending hazardous conditions could be detected before developing into a consumer hazard.

Background

Electric and gas-fueled clothes dryers continue to be associated with a large number of fire incidents in the U.S. In 1998, an estimated 15,600 clothes dryer fire incidents occurred, resulting in 20 deaths, 370 injuries, and U.S. $75.4 million in property damage [1].

With approximately 80 million clothes dryers in use in the U.S. [2] and an average product life of about 13 years, the potential for future fire incidents exists. The operation of an electric clothes dryer involves unattended operation, high temperatures, high voltages and currents, and a potential fuel source (i.e., the materials being dried). Since these conditions are intrinsic to clothes dryer operation, any attempts to reduce the consumer risk associated with this product must take these factors into account.

IATC Winning Paper

The following is an edited version of a paper delivered at the 55th Annual International Appliance Technical Conference (IATC), held March 29-31, 2004 in Lexington, KY, U.S.

The paper’s authors were awarded the Dana Chase, Sr. Memorial Award for the best paper presented at the conference.

 

Prior studies by the U.S. Consumer Product Safety Commission (CPSC) staff [3] and The National Fire Prevention Association (NFPA)[4] have documented the scenarios surrounding reported clothes dryer fires. The reports showed that elevated temperatures could be present in a clothes dryer with no warnings directed at the consumer.

CPSC researched how new and emerging sensor technologies could be used to reduce hazards associated with consumer products. The study identified a variety of sensor types with the potential to prevent, correct, or alert the consumer of an incipient hazard condition [5].

A research project was undertaken to examine how sensor technology could be applied to a clothes dryer to reduce the risk of fire. This project had three primary objectives to demonstrate the following: the application of new and emerging sensor technologies can be a means of detecting abnormal operating conditions that might lead to a risk of fire; the use of multiple sensor inputs of different types can be used to distinguish specific abnormal operating conditions; and condition-based monitoring can be used to assess clothes dryer operation outside of normal ranges.

Methodology


Figure 1: Normal Operation Temperature Profile. CLICK to see the full-size image.

For this project, an electric clothes dryer was instrumented with a variety of sensors. The dryer was operated under normal and abnormal conditions, during which time the sensor outputs were recorded. Sensor data were examined to establish the normal operating characteristics of the clothes dryer, and to determine if an abnormal operating scenario or a condition that could lead to an abnormal operating scenario could be discriminated prior to its creating a potential consumer hazard.

Clothes Dryer

The clothes dryer used in these experiments is a typical retail model. The appliance consists of a rotating tumbler, a removable lint screen, an electric heating element, and a blower that creates negative pressure in the tumbler as it exhausts the heated and moistened air out the exhaust ducting. The model clothes dryer used has a lint screen accessible from the top and located in the rear of the appliance, not on the dryer door.

The clothes dryer is equipped with two auto-reset temperature-limiting controls. The first device is the control thermostat, positioned just after the blower exhaust. During normal operation, this thermostat cuts electric power to the heating element once the exhaust airflow temperature reaches a preset level. When the airflow has cooled, the thermostat resets and re-energizes the heating element. The second device is a high-limit thermostat, positioned at the heating element air intake. A temperature increase at the high-limit thermostat for any reason (such as a reduced airflow) will activate the thermostat and de-energize the element. This thermostat also re-energizes the heating element after cooling down.

Sensors

The following nine sensing technologies were integrated into the clothes dryer :

Temperature Sensor—Temperatures were monitored by sensors positioned to detect the following: heating element intake air, tumbler intake air, control thermostat (blower exhaust air), tumbler surface, exhaust duct air, control panel interior air, clothes dryer, interior air outside the tumbler, and ambient room air.

Infrared Sensor (IR)—An IR sensor was installed in the stationary rear wall of the tumbler to monitor the temperature of the load being dried.

Humidity Sensor—The relative humidity of the exhaust air, the clothes dryer interior (outside the tumbler), and the room ambient air were monitored.

Rotation—Tumbler rotation was monitored with a Hall Effect sensor. Thirty-two magnets were attached to the tumbler rim. As the magnets passed by the sensor, it would emit voltage pulses. The rotation rate of the tumbler was calculated from the number of pulses observed during a fixed time interval.

Pressure Sensor—The air pressure drop across the lint screen was measured with a solid-state, differential pressure sensor capable of detecting very small pressure changes.

Vibration Sensor—The vibration of the clothes dryer’s motor was recorded with two single-axis accelerometers mounted perpendicularly on the motor mount. The accelerometer outputs were sampled 128 times a sec. A Fast-Fourier Transform (FFT) was performed on the data to generate a power spectrum with 0.125-Hz resolution and a 64-Hz bandwidth.

Current Sensor—The electric currents drawn by the motor and the heating element were monitored separately with toroidal current sensors.

Gas Sensor—Exhaust air concentrations of various gases were measured with three sensors. Carbon monoxide (CO) and carbon dioxide (CO2) were sensed separately. Volatile organic compounds (VOC) were sensed with a broad-spectrum sensor located in the dryer exhaust. This device responds to many organic complexes, alcohols, and chlorinated compounds.

Airflow Sensor—Airflow in the exhaust was measured using a hotwire anemometer. Data collection was performed with personal computers connected to multi-input data acquisition modules. Spreadsheet software was used to perform the calculations (averages, slopes, durations, etc.) and to generate graphs of the recorded information. Graphical icons represent sensors that are linked to storage files and displays. The dials, gauges, and charts displayed data in real-time as the load was dried.

Testing Plan


Figure 2: Pressure Change During a Drying Cycle. CLICK to see the full-size image.

A standard test load of eight all-cotton bath towels was chosen for the testing. The load was washed in a household clothes washer before drying in the instrumented clothes dryer. The load was dried for 60 min, the time necessary to fully dry the towels.

Both “normal” and “abnormal” tests were conducted. Normal testing was defined as ordinary operation without any modifications to the clothes dryer. Abnormal testing involved modifying the clothes dryer or the load to simulate conditions associated with lack of maintenance, misuse, improper installation, component failure, or end-of-life circumstances.

The following nine types of abnormal operating conditions were defined for this testing:

Overfilled Tumbler—A wet load of 15 or 20 towels was placed in the tumbler, and the clothes dryer was operated for 60 min.

Modified Electric Heating Element—A partial short-circuit was created in the heating element to increase power dissipation by 60 percent.

Blocked Lint Screen—Lint screens with 25 percent, 50 percent, 75 percent, or 100 percent of the screen area covered were installed in the clothes dryer.

Blocked Exhaust Duct—An orifice was installed in the exhaust ducting, and restrictions of 25 percent, 50 percent, 75 percent, and 100 percent of the cross-section of the duct were created.

Air-Leak Gaps—Air-leak gaps of 1/4 to 1/2 in were created in the airflow path of the clothes dryer. The gaps were created between the dryer exhaust and the exhaust ducting at the blower intake housing, at the ducting behind the tumbler and before the lint screen, and at the gasket between the tumbler and the stationary dryer wall.

Combustible Vapors—Samples of volatile, flammable chemicals were added to wet or dry towel loads, which were then dried.

Smoldering Combustion—Sample materials (cotton rope) with non-flaming combustion were tumbled separately in the clothes dryer or added to a dry towel load.

Flaming Combustion—Samples of flaming cotton towel were tumbled separately in the clothes dryer.

Spontaneous Combustion—Soybean or linseed oil was added to a towel load, which was then dried and left in the tumbler after stopping the dryer before its cooldown cycle.

For the combustion tests, the door of the clothes dryer was replaced with a plastic shield.

Sensor Responses

During the testing, all the sensor inputs were recorded. The data were examined to determine signal responses during normal testing and their deviations from the normal responses during the abnormal tests. Not every sensor’s response changed from its nominal value when an abnormal test was executed. Only those sensors with the most significant normal to abnormal variations were considered.

Normal Operation


Figure 3: Tumbler Intake Air Temperatures. CLICK to see the full-size image.

Nominal sensor responses to clothes dryer operation were established to fully characterize the appliance’s operation and to provide a measure against which abnormal test sensor outputs could be compared.

Temperature

The cycling of the control thermostat modulates air temperatures in the tumbler. As a consequence, many other measured temperatures track the air temperature around the control thermostat. During normal operation, the air temperature at the control thermostat rose to about 80°C. Then, the thermostat activated and de-energized the heating element. Once the air temperature cooled to about 55°C, the control thermostat reset, which energized the heating element and started another heating cycle. Another air temperature of interest is at the tumbler intake. The air temperature in this area rose to about 175°C during normal operation [see Figure 1].

Airflow

Airflow through the exhaust duct was characterized by a constant flow, unaffected by the accumulation of small amounts of lint in the lint screen. The average exhaust airflow was about 1,337 ft per min (fpm). This flow remained substantially constant throughout the drying cycle and was not affected by the wetness of the towel load.

Pressure Drop Across the Lint Screen

When the clothes dryer was started, there was an immediate increase in the pressure drop sensed across the lint screen. The pressure change across the lint screen increased by about 0.025 in of water (6.2 Pa). As the load was dried, the pressure typically increased by another 0.043 in of water (10.6 Pa). The pressure change was not linear. Rather, the pressure difference increased very slowly for about the first 30 min, then increased more rapidly for the last half of the drying cycle. The turbulence of the air in this region of the clothes dryer is evident in the signal variability [see Figure 2].

Exhaust Gases

Ambient atmospheric levels of CO2, CO, and VOC were observed during normal drying. For CO and VOC, regular towel loads with or without detergent during the washing generated predictably low signals. For CO2, ambient background levels were measured. These values varied from around 330 to 800 ppm.

Detection of Abnormal Operation


Figure 4: Pressure Sensor Response to Abnormal Test Conditions. CLICK to see the full-size image.

All of the abnormal operating conditions were detected by the sensor instrumentation as a variance (either increase or decrease) from previously established nominal values, conditional on the test being executed and the sensor being examined. Depending on the amount of “abnormality” in the testing parameters, the sensor response magnitudes ranged from small to large.

Tumbler Intake Temperature

The air temperature at the intake to the tumbler was a sensitive indicator of abnormal operation. This sensor responded strongly to air leaks, exhaust-ducting blockages, and blocked lint-screen conditions. The rapid temperature changes during the abnormal test are caused by the cycling of the high-limit thermostat. The tumbler intake air temperature can indicate abnormal operation in two ways. One method is to detect air temperatures much higher than the normal values. The other method is to monitor how frequently the high-limit thermostat cycles. Rapid cycling of the high-limit thermostat is indicative of high air temperatures in the area of the tumbler intake [see Figure 3].

Exhaust Airflow

A magnitude change in the exhaust airflow was shown to indicate some types of abnormal operation. An increase in the exhaust airflow was a general indication of an air leak inside the clothes dryer or a missing lint screen. The magnitude of the change from the nominal flow of 1,337 fpm was between +8 percent and +12 percent for the conditions tested.

Exhaust ducting blockages decreased the airflow to 1,120 fpm (-16 percent) at a 50-percent blockage and to 625 fpm (-54 percent) at a 75-percent blockage. Overfilled tumbler conditions did not affect the measured airflow in this clothes dryer model. Air entered the tumbler, traveled along the rear wall, and exited to the blower without mixing with the towel load. Clothes dryer designs with lint screens located in the front might show reduced airflow under overfilled tumbler conditions.

Pressure-Drop Across the Lint Screen

The differential pressure across the lint screen was an indicator of some abnormal testing conditions. Depending on the test executed, either the static pressure (the pressure change from off to on) or the dynamic pressure (the pressure change from the start to the end of the test) varied from the nominal case. The large static pressure increase associated with a partially blocked lint screen is readily apparent. The smaller magnitude dynamic pressure change seen with the missing lint screen is more subtle [see Figure 4].

Exhaust Gases

Sensing CO was shown to be a more sensitive indicator of combustion occurring inside the clothes dryer than detecting either CO2 or VOC. The relatively variable background levels of CO2 sometimes made the detection of combustion products difficult. In the case of CO, however, the combination of low background levels and large magnitude changes during an abnormal test rendered detection unambiguous [see Figure 4].

The IR sensor plus the CO sensor can be used to distinguish flaming combustion from smoldering combustion. If a high signal is read from the IR sensor and the CO sensor reports elevated levels, then flaming combustion might be occurring in the tumbler. A low signal from the IR sensor combined with detection of high levels of CO is more indicative of smoldering combustion [see Figure 5].

Sensor Fusion


Figure 5: Carbon Monoxide Detection of Smoldering Combustion. CLICK to see the full-size image.

Sensor fusion is the combination of signals from different types of sensors to determine an operating state. For some test conditions, combinations of multiple different sensor readings are needed to more precisely identify a particular abnormal operating condition. If only a general indication of a normal/abnormal operating condition is desired, the number of sensors required for sensor fusion purposes can be minimized.

There are various possible sensor combinations. Table 1 lists some of the strongest-responding sensors for the selected test condition. Logical inferences can be made between a combination of sensor responses and the functional condition of the clothes dryer. Algorithms using the logical AND, OR, and NOT operators may be combined with sensor readings (e.g., nominal, higher than nominal, lower than nominal) in a truth table format to identify particular clothes dryer operating states. This lends flexibility to product designers in their implementations of sensor fusion to distinguish abnormal operation. In some cases, the absence of a sensor output’s change from the nominal value coupled with another sensor’s changed output identifies a particular abnormal operating condition.

Condition-Based Monitoring


Figure 6: Condition-Based Monitoring of Increasing Exhaust Duct Blockage. CLICK to see the full-size image.

The “normal” operating values of some parameters in a clothes dryer vary from unit to unit. Aspects such as component tolerances, installation factors, and ambient environmental conditions combine to establish particular normal operating characteristics for each clothes dryer. Perceiving a change from normal operation requires establishing unit-specific normal values for comparison purposes.

Condition-based monitoring is one technique that may be used to define normal operation for an individual appliance and detect operational changes from normality. Long-term monitoring of clothes dryer performance is useful to identify slowly changing conditions that take an extended period to become evident. Comparing current performance to previous operation allows changes to be detected before clothes dryer operation affects consumer safety.

As an example, the exhaust airflow in a clothes dryer can be considered for condition-based monitoring. Variations in the exhaust ductwork and the site altitude preclude the use of one normal airflow value for all clothes dryers. After dryer installation, an initial airflow value can be established in one of many ways. A standard type of load can be dried to set up the initial value. Alternatively, the clothes dryer can automatically average airflow values from the first “N” loads dried to define the starting value. Other methods are available.

Subsequent clothes dryer use generates airflow values that are compared to the initial measurement. Slow, long-term changes in the airflow are easier to detect when the monitoring system retains previous readings for comparison.

In the case of exhaust airflow, detection of a decreasing airflow can be indicative of an increasing exhaust duct blockage. Conversely, a general increase in the exhaust airflow over time may point to an air leak inside the clothes dryer. In either instance, condition-based monitoring methods can be used to detect ever more abnormal operation for the monitored appliance before conditions develop that might lead to a safety hazard. Depending on the variables considered for condition-based monitoring, the comparison value can be periodically updated to reflect ordinary changes during the appliance life or preserved as was first calculated.

Data from successively blocked exhaust ducts (blue line) were fitted with a trend line (black line) [see Figure 6]. At a fixed offset from the installed clothes dryer’s nominal airflow, a shaded detection zone is established in which the monitoring system detects the reduced flow and alerts the consumer, begins to monitor other sensors to preclude a hazardous condition from developing, or takes some other action. Once the lower operational limit of airflow is reached (red line) for this clothes dryer, the condition-based monitor acts to prevent further development of an incipient hazardous condition.

Conclusions


Table 1. CLICK to see the full-size image.

An electric clothes dryer was instrumented with a variety of sensors and operated under normal and abnormal conditions. Analysis of the sensor outputs shows that abnormal operation can be distinguished from normal operation. Multiple sensor outputs can be logically combined to uniquely identify particular abnormal circumstances. The use of condition-based monitoring procedures may be used to identify clothes dryer operation outside of previously established normal ranges.

The use of sensors and data analysis techniques can potentially be used to identify conditions that may lead to consumer hazards before the hazard is manifested. This holds the promise of reducing the risk of fire or other hazards in future products.

References

1. Mah, J., Table 1, “Estimated Residential Structure Fires Selected Equipment 1998,” 1998 Residential Fire Loss Estimates, Directorate for Epidemiology, U.S. Consumer Product Safety Commission, 2001.

2. Appliance.com, 2002.

3. Kadambi, S., Final Report on Electric And Gas Clothes Dryers, Directorate for Engineering Sciences, U.S. Consumer Product Safety Commission, 2000.

4. National Fire Protection Association, The U.S. Home Product Report (Appliances & Equipment Involved in Fires), January 2002.

5. Butturini, Randy, Sensor Technologies to Reduce Consumer Product Hazards, Proceedings of the 54th International Appliance Technical Conference, West Lafayette, IN, U.S., 2003.

 
About the Authors
Randy Butturini is a project manager and electrical engineer in the Directorate of Engineering Sciences at the U.S. Consumer Product Safety Commission. He has B.S. and MSEE degrees from the University of Tennessee and the Rochester Institute of Technology, respectively, and holds several patents. Prior to joining the commission, he worked at Eastman Kodak Company, Cornell University, and FileTek, Inc. Arthur Lee is a project manager and electrical engineer for the U.S. Consumer Product Safety Commission. He has B.S. degrees in electrical engineering and mathematics from the University of Maryland. Prior to joining the commission, he worked for the U.S. Department of Navy.

 

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