There has been a lot of publicity, and rightly so, about fires in domestic tumble dryers caused by the ignition of lint on the heating elements. This article deals with an entirely separate cause of fires involving tumble dryers and these fires are becoming more common.
Before discussing domestic tumble dryers, commercial launderers have known for a long time that some fabrics that have been heavily soiled with vegetable oils, for example cloths used in commercial kitchens, or towels from spas might catch fire after being removed from hot dryers. These fires start because oils, still present after the washing process, undergo a self-heating process at the elevated temperatures of the dryer.
Self-heating leading to “spontaneous combustion” is a well known cause of fires: the hazard posed by cloths impregnated by linseed oil being one of the best documented examples. A self-heating reaction involving drying oils, such as linseed oil spread on cloths, can start from room temperature, with the heat of that reaction leading to an increase in the temperature of the cloths. As the rate of a chemical reaction roughly doubles for every 10°C increase in temperature, the increase of the temperature of the cloths leads to the reaction progressing faster. That, in turn, produces more heat, with the reaction then going still faster and producing still more heat. Under certain circumstances this process can continue until the heating leads to the spontaneous ignition of the cloths.
With the more common kitchen oils, the rate of the reaction at room temperature is normally not significant, with any heat produced being dissipated to the surroundings. Indeed, normal quantities of soiled washing do not self-heat whilst they await laundering. As such, the propensity for self-heating of kitchen oils is different from the group of materials referred to as “drying oils”.
During tumble drying the ambient temperature is typically increased, at times, to the temperatures of commercial laundries. However, spontaneous ignition happens only rarely because most modern tumble dryers have a cool-down period at the end of the drying cycle. Indeed one set of Operating Instructions notes "in many programmes, the heating phase is followed by a cooling down phase to ensure that the items are not too hot to handle when you remove them (this also avoids the danger of the laundry self-igniting). The programme is not finished until the cooling down phase is complete. Ensure that you always wait until the end of the programme before removing the laundry." However, the cooling down phase will not be completed if, in the mean time, the appliance is switched off or the door opened. There may also be an "anti-crease phase" that can last for an hour or so, when the tumbling of the laundered items should prevent any localised build-up of heat within the load but that can also be stopped in the same ways.
In a 2006 paper entitled "Spontaneous Combustion Tendency of Household Chemicals and Clothes Dryers – Part 2", investigators from Exponent Failure Analysis Associates investigated spontaneous combustion as a cause of fires in domestic tumble dryers. They reported that spontaneous ignition was noted only in the tests where the "thermal safeguards" on the appliance had been by-passed in order to obtain a higher initial drum temperature. Additionally, the dryer was stopped in each of the tests "prior to the cool down portion of its cycle.”
There are a number of possible reasons why these fires are becoming more common. A number of commercial kitchens now choose to use their own appliances, typically domestic models, to launder items such as kitchen cloths, with tumble dryers themselves becoming more prevalent in society. On a related point we have seen self-heating of cloths at a house used by an aromatherapist, who worked from home. Also more economic, lower temperature washing cycles and more ecological detergents may be impacting on the amount of residual oils left on the cloths. It may even be that some safety concerns lead to tumble dryers being turned off when unattended instead of being allowed to complete their cycle.
The accompanying video shows items being removed from a commercial dryer and left bundled together. There is no evidence of any combustion event, such as a smouldering fire, to attribute the cause to smoking materials. Instead the fire starts seemingly “spontaneously” some 4½ hours after the items were removed from the dryer.
Bob Goudsmit, Basingstoke Office
 Delmar Morrison, Yee San Su and Mark J. Fecke, "Spontaneous Combustion Tendency of Household Chemicals and Clothes Dryers, Part 2" APPLIANCE magazine (July 2006).