The conditions after the dryer stops are also important. I have had cases where the towels were left in the dryer after it stopped. I have had other cases where the towels were removed, folded, and stored in the trunk of a car parked under the hot sun in the summer. The phenomenon of self-heating requires a porous pile to be present. The theory of self-heating to thermal runaway assumes an non-insulated porous pile in the open. If you provide any insulation to the exterior of the pile, this will reduce the heat losses at the pile and increase the propensity for self-heating and reduce the time to thermal runaway. This insulating effect can be important and can occur as a result of other items in the dryer on top of the oil impregnated towels or the dryer enclosure itself. Compression of the pile also can increase the insulating effect and enhance the propensity to self-heat.

Experimental results of oil impregnated towels can be used to determine if thermal runaway can occur with some conservative assumptions. If it can occur in an open air environment, then it will occur in the dryer.

In some limited cases, the time for thermal runaway to occur can be estimated. The problem is that the conditions need to be close to the critical temperature for the specific pile size. I doubt you have those conditions here. The dryer scenario may produce conditions well above the critical temperature. So the dryer scenario may not be fully responsible for producing thermal runaway since it can occur without the dryer (only takes longer), but may significantly shorten the time for thermal runaway to occur.

Batt201 Wrote:
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> I am currently working a case where spontaneous
> ignition is being considered as a potential
> ignition source. Information I am trying to find
> involves testing of the required time needed for a
> load of towels, contaminated with self-heating
> oils to reach a temperature for ignition to occur
> once the external heat source is taken away. I
> have found a great deal of information detailing
> requirements for ignition to occur, however, none
> of these tests describe the time component. Does
> anyone have any information on time requirements
> for spontaneous ignition involving self heating
> oils?
>
> Thank you for any help.

Douglas J. Carpenter, MScFPE, CFEI, PE, FSFPE
Vice President & Principal Engineer
Combustion Science & Engineering, Inc.
8940 Old Annapolis Road, Suite L
Columbia, MD 21045
(410) 884-3266
(410) 884-3267 (fax)
www.csefire.com