Powder Layer Tests

Air Over Layer Test

The powder layer test follows a procedure described by John Abbot in his book Preventing Fires and Explosions in Dryers. It simulates the conditions in dryers, such as cross flow, tray and band dryers, in which hot air circulates above a layer of material and also simulates the condition of deposits on the internal surfaces of all types of dryers. A thin layer of material (usually 15mm) is placed in a small test tray layer and heated by passing heated air around it at a fixed velocity. Thermocouples are used to monitor the temperature of the air and within the sample layer. The test may be carried out isothermally (fixed temperature) or ramped (increasing temperature):

Screening test (ramped

temperature)

An initial screening procedure, in which the temperature of the air is increased at a rate of 0.5 o C per minute from room temperature up to 400 o C. This will give an initial estimate of the onset of exothermal activity.

Isothermal test (fixed

temperature)

The more usual test where a sample is subjected to heating at a fixed temperature for a prolonged duration (between 8 and 24 hours). The test is repeated successively at different temperatures until the critical ignition temperature is identified with the required accuracy (2-10 o C) A safety margin of 20 o C is usually recommended to minimise the risk of decomposition, but should not be used as the sole basis of safety.