Review

Last newsletter we learned about the fire triangle where we need Heat, Fuel and Oxidizer to create and maintain fire. We discussed where contaminants in the linen (such as spa oils, motor oils or car wash waxy rags) can significantly lower the oxidizing temperature of the linen causing a dryer fire to occur at much lower temperatures. We will discuss the physics of combustion and what we can do to prevent it from happening in our dryers.

What are Oxidation Reactions?

In a general sense, oxidation refers to the combination of a material with atmospheric oxygen, which results in the release of heat. As the temperature increases, so does the rate (or speed) of oxidation. The rate of oxidation is proportional to the surrounding temperature (the higher the temperature, the faster the oxidation rate. For this reason, hours after the last employee has left, heat generation occurs at an accelerated rate and surface temperatures rise more rapidly, which initiates the fire.  

Not all oxidation reactions result in the accumulation of heat. In some cases oxidation which is very slow at the beginning, is dissipated rapidly, and the temperature does not change in a measurable way. A classic example of slow oxidation is the combination of iron with atmospheric oxygen, resulting in the formation of rust. Unlike the oxidation of lint, iron is a good conductor of heat and so the heat is quickly dissipated.

So what is spontaneous combustion?

Spontaneous combustion is the occurrence of fire without application of an apparent heat source. In a hot dryer, oxidation of fabrics occurs more rapidly than at ambient temperatures. As oxidation proceeds, heat accumulates on the garments faster than it can be dissipated. The net accumulation of heat continues until the critical surface temperature is reached. At this point the garments are ignited and burn.

A Closer Look at the Physics of Combustion

Lint has a very fine particle size and this creates a greater surface area. Lint also does not dissipate heat very well and these two factors combine to lower the temperature at which lint can oxidize. For this reason, the small lint particles are more susceptible to spontaneous combustion than the whole garment. Heat can continue to accumulate over a period of hours until the critical surface temperature is reached. A given weight of lint has thousands of times the surface area than a garment of equal weight. Since oxidation takes place at the surface, fine particles also offer more areas at which oxidation can occur simultaneously, which further contributes towards the accumulation of heat. The rate of oxidation depends on the surrounding temperature. As the temperature rises, so does the rate of oxidation. For this reason, instances of spontaneous combustion are greater in summer, and in warm climates.

What can we do to Prevent Spontaneous Combustion?

Make sure that all your dryers have a couple minute cool down at the end of cycle. This will remove the excess heat and energy from the clothes and lower the oxidation rate. Allow enough time for all heat to dissipate before folding or bundling.

• Never allow large loads to remain in dryers after the drying cycle ends.
• Clean the lint screens often.
• Softener sheets can cause a waxy build-up on the lint screen. Make sure that the lint screen is clear of any blockages.
• Do not leave damp items in a warm or hot dryer --- start the drying cycle immediately.

Preventative Maintenance Tips

• Lint Screens - All dryers have a lint screen to capture the lint associated with drying clothing. Lint captured in the lint screen is lint that you can easily dispose of, and doing so will maintain proper airflow (and better efficiency) in the dryer. In coin laundries, some lint screens are designed to be cleaned by the customers, but you should keep a close eye on them to make sure they are being cleaned properly. You also should inspect lint screens or drawers for damage and replace if torn.
• Service Access Plates need to be installed and in place to prevent un-wanted air to enter the dryer.
• Lint Drawer Gaskets need to be in place without damage to prevent un-wanted air to enter the dryer.
• Check Lint Drawer for Lock in place & Operational,
• Dry Wall Screw tears in the top and sides of the dryer cabinet - fix with metal plates to prevent un-wanted air,
• Gas Burner Manifold Burner, (need to be a nice Blue Flame), excess lint will cause the flame to burn yellow.  Remove the burner and clean,
• Check Door Gaskets for proper seal,
• Check Main Exhaust Duct work for clear, un-obstructed airflow,
• Check temperature sensing probe and make sure that it is clean and in the proper position.

The manufacturer designs and builds dryers to specific design specifications. If access panels are removed, gaskets separated from lint compartments or drywall screw tears in the sides and top of the dryer will all allow the infiltration of cold air into the dryer before the temperature sensor. This infiltration of cold air tricks the dryer into thinking that the dryer needs more heat. In certain scenarios dryer cylinder temperature can become 25-50 degrees hotter than normal increasing the potential of a dryer fire. Make sure that your dryers are clean and all in good operating condition. 

additional information: 

 http://www.mcclureindustries.com/Reports/Spontaneous_Ignition.pdf