Effect of temperature on flammability

Figure 4.15 Effect of temperature on flammability for a given pressure (taken from Zabetakis [5])...

Effect of temperature on flammable limits. The higher the temperature at the moment of ignition, the more easily the combustion reaction will propagate. Therefore, the reference temperature (initial temperature) of the flammable mixture must be stated when flammable limits are quoted. There are not a lot of data for flammable limits under different conditions of initial temperature. The behavior of a particular mixture under different conditions of initial temperature usually must be determined by tests. 

Fig. 2 Effect of temperature on flammability limits of a combustible vapor in air at constant initial pressure. (From Ref.. ) (View this art in color at www.dekker.com.)...

Fig. 4.9 Effect of temperature on flammability limits of hydrogen in air (pressure 100 kPa).

The atmosphere in a plant is normally air, but if the oxygen content of the atmosphere is reduced below 21% the flammability limits narrow progressively. Below about 8% oxygen the mixture will not bum whatever the concentration of vapour or gas in the atmosphere (unless the material decomposes to release its own oxygen). The minimum oxygen concentration to ensure nonflammable conditions depends on the temperature of the mixture and this, together with the effect of temperature on flammability limits, is illustrated in Figure 7.2. 

Figure 7-45. Effect of temperature on limits of flammability of a combustible vapor in air at constant initiai pressure. By permission, U.S. Bureau of Mines, Bulletin 627 [43].

Figure 7-48 [43] shows the effects of temperature on limits of flammability at a constant initial pressure. As temperature is increased, the low er limit decreases and the upper limit increases. This is extremely important in... 

It is a non-flammable colourless gas which is twice as dense as air, and slightly soluble in water, forming sulphurous acid. It is readily liquefied as a gas under its own vapour pressure of about 35psig (2.4 bar) at 21°C. Figure 8.5 depicts the effect of temperature on vapour pressure Table 8.12 lists the physical properties. Cylinders tend to be protected against over-pressurization by metal plugs melting at about 85°C. 

Liquid carbon dioxide produces a colourless, dense, non-flammable vapour with a slightly pungent odour and characteristic acid taste . Physical properties are given in Table 8.5 (see also page 277). Figure 8.1 demonstrates the effect of temperature on vapour pressure. 

Figure 5. Effect of temperature on the flammability of ethylene-acrylic rubber samples.

In view of the accelerating effect of temperature on chemical reactions, it is reasonable to expect that limits of flammability should be broadened if the temperature is increased. This trend is confirmed experimentally. The increase is slight and it appears to give a linear variation for hydrocarbons. 

The effect of temperature on the upper limit of flammability may also be obtained from the modified Burgess-Wheeler law if we assume the heat release at the upper limit is equal to that at the lower limit. In this case, we have ... 

The effect of temperature on the lower limit of flammability of ethylene in air may be obtained from Equation (4.9). This yields... 

W. R. Rolingson, John MacPherson, P. D. Montgomery, and B. L. Williams, Effect of Temperature on the Upper Flammable Limit of Methane, Ammonia, and Air Mixtures, J. Chem. Eng. Data, 5 349-351 (1960). 

Pressure apparently has an effect similar to that of temperature on flammability, with the effect being more marked on the upper limit than on the lower. Considerably less work has been carried out in studying the effect of pressure on the limits, with some inconsistencies apparent in the data being reported. Consequently, simple models have not been developed to enable the effect of pressure to be predicted with sufficient accuracy. 

This approach has not been tested for any dusts that bum heterogeneously such as some metal dusts (A-6-1.2). Also, this large an effect of temperature has not been found by some other workers, it has yet to be confirmed that the effect is not due in part to loss of moisture (6-1.6) or evolution of flammable gas (6-1.3), both of which can have large effects on MIE. 

In coal pulverizers it is desirable to supply heated air to vaporize the water in the coal. However, the temperature of this heated air should not be so high that ignition could occur. Craig (1G) in studying this problem discusses the effect of coal concentration of flame velocity. Maximum flame velocity occurs at an air-fuel ratio of about 5 to 1. Godbert and Greenwald (3G) report the effect of fineness on the flammability of coal dusts. 

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