Flammability Limit Dependence on Temperature

In general, the flammability range increases with temperature.5 The following empirically derived equations are available for vapors  

Zabetakis, S. Lambiris, and G. S. Scott, Flame Temperatures of Limit Mixtures, in Seventh Symposium on Combustion (London Butterworths, 1959), p. 484. 

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Flammability Limit Dependence on Temperature In general, as the temperature increases, the flammability range widens, i.e., the LFL decreases and the UFL increases. Zabetakis et al. (Zabetakis, Lambiris, and Scott, Flame Temperatures of Limit Mixtures, 7th Symposium on Combustion, Butterworths, London, 1959) derived the following empirical equations, which are approximate for many hydrocarbons ... 

The flammability limits depend on the temperature and pressure. At elevated temperatures and pressures, a given mixture may be flammable, even if this is not the case at ambient conditions. It is a key objective of the design of the inlet zone of a CPO reactor to prevent ignition and thermal combustion. 

Flammability limits are not absolute, but are dependant on temperature, pressure, and other variables. Care must be exercised in using flammability limit data when conditions are different from ambient. For example, in reactors and thermal oxidizers. 

Flammability limits of vapor Dependence on temperature T, °C Dependence on pressure... 

The autoignition temperature is the minimum temperature required for self-sustained combustion in the absence of an external ignition source. The value depends on specified test conditions. Tht flammable (explosive) limits specify the range of concentration of the vapor in air (in percent by volume) for which a flame can propagate. Below the lower flammable limit, the gas mixture is too lean to burn above the flammable limit, the mixture is too rich. Additional compounds can be found in National Fire Protection Association, National Fire Protection Handbook, 14th ed., 1991. 

An alternate method for flash point prediction is the method of Gmehling and Rasmussen and depends on the lower flammabihty limit (discussed later). Vapor pressure as a function of temperature is also required. The method is generally not as accurate as the preceding method as flammability limit errors are propagated. The authors have also extended the method to defined mixtures of organics. 

The flash point of an oil is the temperature at which it gives off, under specified conditions, sufficient vapor to form a flammable mixture with air. This is very different from the temperature of spontaneous combustion. The test is an empirical one and the result depends upon the instm-ment used and the prescribed conditions. For example, the flash point may be closed or open , depending on whether the test apparatus has a lid or not. As far as lubricating oils are concerned, the test is of limited significance, although it can be indicative of contamination (for example, the dilution of crankcase oil by fuel). 

An increase in temperature tends to widen the flammable range, reducing the LFL. For example, the LFL for methane in air is commonly quoted as 5%. As the temperature of methane increases to autoignition temperature, the LFL falls to around 3%. Stronger ignition sources can ignite leaner mixtures. Flammability limits also depend on the type of atmosphere. Flammability limits are much wider in oxygen, chlorine, and other oxidizers than in air (NFPA, 1997). 

Flammability limits. The range of flammable vapor-air or gas-air mixtures between the upper and lower flammable limits. Flammability limits are usually expressed in volume percent. Flammability limits are affected by pressure, temperature, direction of flame propagation, oxygen content, type of inerts, and other factors. The precise values depend on the test method. 

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