Factors defining spontaneous heating and ignition of Hay

The ways in which heat balance, water balance and aeration interact in the self‐healing of hay beyond 70°c were examined in the laboratory. A temperature‐controlled environment is essential for the temperature of small hay samples to exceed 70°c; the rate at which airflows through the apparatus must be slower than that required for self‐heating below 70°c, in order to restrict loss of latent heat caused by increased water vapour transfer; otherwise, air at the same relative humidity as the hay must be used. Some water is essential for hay to self‐heat beyond 70°c, but self‐ignition occurs only when most of the water has been ha at or near 100°c. Theoretical considerations show that the latent heat transfer exceeds the thermal conductivity of hay above 20°c, increases by one order of magnitude by 65°c, two by 95°c and infinitely by 100°c.