Information regarding spontaneous combustion in laundries:
Each year, hundreds of fires occur in laundries, some of which were found to have started in dryers, under counters or in laundry carts. These fires often occur during the night, hours after the last employee has left the premises. They can occur anywhere from coin laundries to tunnel finishers. This article looks at the chemistry and physics of spontaneous combustion and how to prevent fires in laundries.
HOW FLAMMABLE IS FLAMMABLE
Materials which can catch fire and burn are classified as either flammable or combustible, depending on the temperature at which they ignite. When dealing with liquids, this temperature is referred to as the flash point. The lower the flash point, the higher is the degree of flammability. For example, liquids which ignite at temperatures below 38 oC, are considered to be flammable whereas those which catch fire and burn at temperatures above 38oC are combustible. For solids, such as cotton towels, rags or lint, the ignition point is referred to as the critical surface temperature.
Cotton, which is combustible, starts to decompose when the surface temperature reaches approximately 95°C and in the process, generates its own heat. This decaying process is accelerated when the fabric is tightly folded and placed in a confined area. Because it is hot, moist, and has no possible way to dissipate its heat, the oxidation continues to build upon itself, until the garment reaches the critical surface temperature and bursts into flames.
THE FIRE TRIANGLE
Generally, there are three requirements necessary to support combustion as shown in the diagram:
- Oxidizing Agent
- Heat Source
In a dryer, the fuel can be garments, or more likely lint, which is easier to ignite. The oxidizing agent is oxygen, which is present in air, so where does the heating source come from, hours after the dryer is turned off? To better understand the process we have understand what is meant by oxidation.
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 of oxidation, since the speed of oxidation is proportional to the surrounding temperature. 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 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
The finer the particle size, the slower can accumulated heat be dissipated from it. For this reason, lint is more susceptible to spontaneous combustion than the whole garment from which it was derived. 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 surfaces exposed to air, 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 final 10 minute cool down cycle with dampers set to deliver air at ambient temperature. 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 screen after every load.
- Softener sheets can cause a waxy build-up on the lint screen. Wash the lint screen in warm soapy water and dry it completely before replacing it in the dryer.
- Do not leave damp items in a warm or hot dryer --- start the drying cycle immediately and keep the exhaust opening clutter free.