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issue: December 2009 APPLIANCE Magazine

APPLIANCE Engineer - Testing & Certification
Fire Containment and Clothes Dryers


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Trey Morrison, managing engineer, Thermal Sciences Practice, Exponent Inc.

 A revised fire safety standard for clothes dryers creates several design challenges for appliance engineers.

Figure 1. Example test configuration with appliance draped in cheesecloth.

Recently, Underwriters Laboratories Inc. (UL) issued an amended version of the safety standard for listing of electric clothes dryers—UL 2158 Electric Clothes Dryers.1 This new version of the standard includes aggressive fire tests that all clothes dryers will have to successfully pass by March 20, 2013 to receive the UL listing. This article discusses one interpretation of the rationale behind the new tests, the challenges that these tests pose to the safe design of clothes dryers, and suggested approaches to meeting the design challenges.

Description of UL 2158 Fire Containment Tests

This section does not attempt to completely describe the criteria for the UL 2158 fire containment tests, but it does intend to provide an overview of the test conditions as an aid to discussion. The new fire containment provisions are described in detail in clauses 19.6, “Load fire containment,” and 19.7, “Base fire containment,” of the standard. Each of these two clauses calls for a static test with the dryer energized but not tumbling, and a test with the dryer energized, heating, and tumbling. Therefore, the test regimen requires a total of four separate appliances since each test may cause sufficient damage to prevent reuse of each appliance as indicated below.

1. Static Test – Load Fire

2. Dynamic Test – Load Fire

3. Static Test – Base Fire

4. Dynamic Test – Base Fire

The titles for each test clause indicate where the test fire will be ignited: either on the test load in the drum or in the mechanical compartment in the base of the cabinet below the drum (i.e., tumbler). Once the test fire is ignited, the test continues until failure or to a maximum of 7 hours (without failure). A small negative draft through the appliance is maintained through all tests using a fan blowing air at 3.3 L per second (L/s). This is used to simulate natural ventilation in the residential installation. The failure criteria for the fire tests are common to similar types of appliance fire standards in which the appliance is draped/enclosed with a single layer of cheesecloth, which, if ignited, indicates failure of the test. Cheesecloth is a good indicator for flame escape from the appliance because it is readily ignited by flame contact. An example of the test configuration is depicted in Figure 1.

 

Tests for both clauses incorporate a standard towel load with a dry weight of 0.016 kg per liter (kg/L) of drum volume. The towel specifications are described in the standard, and they may be commonly referred to as “UL towels” or “UL ballast.” In the load fire tests, the towels in the drum are directly ignited through the use of a hand-held propane torch. After ignition, the drum door is closed and the test is commenced. In the case of the dynamic (tumbling) test, the dryer is started after the load is ignited. It is fairly typical for the test load to have a total towel mass between 4.5 and 6.8 kg. When loosely tumbled, this load will produce flames that project from the opened door and can extend up the front face of the appliance. However, once the door is closed, the flames can be observed to die down almost immediately in many cases. In the enclosed drum, the burning rate of the towels is controlled by ventilation of fresh air into the drum, which may be due to the appliance fan, the test fan, or natural ventilation. Thus, with the door closed and ventilation limited, the towel load can burn in a smoldering or intermittently flaming fashion with a much lower heat release rate. In some instances, it has been observed that the tumbling action of the towel load in the dynamic load may be sufficient to extinguish the towel load. This is perhaps one supporting reason for conducting the test in a static mode as well.

The two tests where the fire is ignited in the base require an additional fuel source to the towel load. Commonly, the mechanical compartment in the base of an appliance may contain plastic air ducting and a plastic fan housing. These fuels may burn given the right conditions, but the likelihood of ignition of these components is increased through the placement of cheesecloth in the base of the appliance. The cheesecloth can be considered as a rough analog to excessive lint accumulation in the base of the appliance. The base fire clause requires a layer of cheesecloth eight sheets thick to be draped over the horizontal surfaces, components, and wiring located within 15 cm of the base of the appliance. The cheesecloth must be continuous across these surfaces, yet it cannot impede or interfere with mechanical functions or components’ ventilation. The cheesecloth is directly ignited in these tests, and the fire is allowed to spread within the mechanical compartment.

 

Possible Fire Containment Test Failure Modes

There are many possible failure modes for an appliance in these fire tests. Some possibilities include the following:

  • Flame escape through gaps in cabinet panels.
  • Flame escape through door due to hinge-mount or door-latch failure.
  • Flame escape through seals or gaps due to intermittent flaming (e.g., puffing).
  • Flame penetration through polymeric user interface panels due to melting.
  • Dripping of molten flaming plastic through the cabinet base.
  • Sparks and molten metal globules created by fire attack on energized conductors.

There may be other possible failure modes not listed above, but this list could be considered a set of likely test failures during the appliance design process.

Relation to Other Clauses of UL 2158

The objective of the standard is to set a minimum threshold for safe performance of the appliance. The standard addresses details of the construction, electrical safety, thermal safety, and human safety in various clauses.

In general, the appliance cabinets are designed with ventilation openings and are constructed with steel walls—both factors assist in containing an internal fire. Other specialized openings, such as those for ventilation and the drum door itself, may provide means for fire escape during the containment tests. UL2158 dictates criteria for several aspects of appliance design that may constrain the problem of fire containment. Among the most obvious test clauses are the following:

  • 20.6 Entrapment
  • 22.5 Bottom openings
  • 22.9 Ventilation openings
  • 32 Polymeric materials

Entrapment

One of three alternative design approaches is allowed; the intent of each is to mitigate the likelihood of child entrapment and suffocation within the appliance drum. The first approach is to limit the door opening force to 67 N (15 lb force), thus simulating a minimum threshold for pushing the door open from inside the drum. The second approach suggests that natural ventilation for the drum be investigated, presumably to provide for a minimum natural ventilation rate to support respiration. The final alternative requires a two-step approach, where the dryer door can be fastened shut only through simultaneously holding the door closed and activation of a secondary locking control away from the door.

Manufacturers that choose the first or second approach to meeting the entrapment clause may encounter distinct challenges. In the case of low opening force, the latch system may fail due to either direct fire attack or the effects of intermittent flaming or puffing of the burning materials inside the dryer cabinet. If the door opens during the test, it will lead to test failure. In the other case, where a minimum natural ventilation rate is designed, this design will also feed air to the fire, causing enhanced burning. The enhanced burning rate may lead to compromising other components within the appliance to allow flames to escape and fail the test. Given the three alternatives, the sequential door locking mechanism may prove to be the most robust method to meet the entrapment clause without defeating the fire containment clauses.

Bottom Openings

The standard dictates that the bottom (base pan) of the dryer cabinet shall be constructed in a manner “…to reduce the likelihood of molten metal, burning insulation, or the like…” from falling through the bottom of the mechanical compartment in the base of the dryer. Openings in the base pan may be the result of machine tooling during fabrication or may serve as points for securing internal components in the mechanical compartment.

Under the previous versions of the standard, the most likely materials to pass through such openings would be failed electrical components or materials ignited by such failures. With the new fire tests, the likelihood of burning materials passing through existing cabinet openings is enhanced because a greater quantity of burning materials is present. The cheesecloth and thermoplastic components in the mechanical area in the base of the dryer are new fuels that were not considered under the prior version of the standard. Thus, openings in the bottom of the appliance that may not have been a concern under the prior version of the standard may now directly cause failure of the test due to allowing the passage of molten plastic, fire-induced arcing of wiring, or passage of other burning materials. One potential design method to mitigate the effects of bottom openings would be to remove or shield those openings.

Ventilation Openings

Ventilation openings are openings on the sides or top of the appliance other than those considered “bottom openings” that may allow passage of burning materials. The standard requires that these openings have barriers or louvers that limit the size of the openings to mitigate the likelihood of burning materials from passing through. With the fire containment tests, these louvered openings have the potential to be exposed directly to the flames from materials burning inside the appliance. If the ventilation openings allow flames to pass outside the appliance cabinet, it will lead to failure of the test. This may lead to relocation or redesign of ventilation openings such that they remain low enough in the appliance cabinet that they will only experience the inflow of cool air from the surroundings and not outflow of hot gases or flames during a test. The optimum location for such openings may be specific to both the geometry of the given appliance and the type of fire test being conducted.

Polymeric Materials

The standard requires polymeric materials to meet one of three different UL 94 classifications—HB, 5VA, or 5VB—depending upon their use in the appliance. Current enclosing parts such as electronic component housings must meet the higher fire resistance of 5VA or 5VB, but other components such as the drum baffles, air ducting, lint screen housing, fan housing, and fan must only meet the HB classification.

The HB designation refers to the horizontal burning rate test from UL 94 in which a horizontal specimen will burn at a specified minimum rate. These types of plastics may be readily ignited by the burning cheesecloth in the base fire tests or the burning towel load in the drum fire tests. These plastics are typically thermoplastics that melt and flow in a fire environment and may allow burning drips to escape from the cabinet through either small holes in the base pan or gaps in the cabinet walls. Also, the molten plastic may flow to other areas inside the dryer cabinet where openings or susceptible components have not been removed or protected. Some appliance styles incorporate thermoplastic leveler legs that could melt away and create a hole for passage of flaming materials. An individual manufacturer may have to modify the type, quantity, and location of plastic materials inside the appliance to pass the fire containment tests.

Potential Design Strategies

Each manufacturer will likely undertake a different design process to either modify their existing appliance configurations or to develop new configurations that will pass the fire containment tests. Based upon the review and discussion above and good engineering practices, there are many different approaches that can be taken to designing a suitable appliance. Overall, the objectives of such a design approach should be some combination of hardening the appliance against internal fire attack.

Examples of possible alternatives that may individually or in combination achieve the fire containment objective for a given appliance are listed below:

  • Eliminate HB-rated thermoplastic components.
  • Increase the fire resistance of plastic components.
  • Improve seals and eliminate potential openings in the cabinet and base.
  • Improve the fire resistance of door hinge/latch assemblies or door locking.
  • Install rigid non-combustible shielding to block openings or remove openings.
  • Limit fire-induced natural ventilation.
  • Install active internal fire suppression systems (e.g., sprinklers).

This list is not exhaustive but suggests several general design alternatives. An individual manufacturer must undertake the analysis and testing to decide which, if any, alternatives are suitable and acceptable for their products. As an example, consider an individual appliance that contains several internal components such as the air duct and fan housing that are currently manufactured from HB-rated polypropylene. The manufacturer may find that those parts melt and form a flaming pool of plastic inside the appliance that sometimes flows out of the dryer cabinet in the fire tests.

In looking at the list of alternatives above, one may be able to rank them as potential solutions based upon manufacturability and economics. For example, replacement of the part in-kind with a V0-rated plastic may not be reasonable due to the manufacturing requirements for the part. A more-successful approach may be to install barriers in the cabinet base that prevent molten plastic from flowing out of the appliance. Another option may be to redesign the component so that it can be manufactured from a different material. Successful design for the new fire containment tests will require some measure of iterative fire testing coupled with fire dynamics analysis to identify appropriate design features for each appliance.

Conclusion

In summary, the UL 2158 Electric Clothes Dryers standard has recently been revised to include clauses requiring fire containment tests on listed clothes dryers. The fire containment requirements may reduce the ultimate number or severity of residential fires related to clothes dryers. The new test requirements will take effect in 2013, but the design challenges that the new testing imposes may take significant investment of time and resources to overcome.

References

1. UL 2158 Electric Clothes Dryers. Northbrook, Illinois: Underwriters Laboratories Inc., March 20, 2009.

Trey Morrison is a managing engineer in the Thermal Sciences Practice at Exponent Inc. He consults on, investigates, and performs research in the areas of chemical process safety and appliance fire safety. He is currently a member of the UL Standards Technical Panel for UL2158 and has participated in many related clothes dryer fire safety initiatives. If you would like to respond to Morrison’s article, please post your comments on the APPLIANCE Talk blog: ApplianceMagazine.com/blog

 

 

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