Page 74 - Plant design and economics for chemical engineers

56 PLANT DESIGN AND ECONOMICS FOR CHEMICAL ENGINEERS

The flammability limits of mixtures can be estimated from the data for
individual fuels by using le Chatelier’s principle
c (y,,‘LFL,) = 1.0

where yi is the mole fraction of each component of the fuel in the air and the
LFL, is the corresponding LFL value for each component. A similar relation-
ship can be used to estimate the UFL for a gas mixture. If the concentration of
a mixture of fuel gases is known, the LFL for the mixture can be approximated
from

(LFL),,, = lOO/c (p,/LFL,) (2)
where pi is the percentage of fuel in the original mixture, free from air and
inert gases. The two preceding relationships provide reasonably good LFL and
UFL values for mixtures of hydrocarbon gases and mixtures of hydrogen, carbon
monoxide, and methane. The relationships provide poorer results for other gas
mixtures.
If the concentration of fuel is within the flammability limits and the
temperature of the mixture is high enough, the mixture will ignite. The temper-
ature at which ignition will occur without the presence of a spark or flame is
designated as the autoignition temperature (AIT). If the temperature is less than
the AIT, a minimum amount of energy (as low as a few millijoules for
hydrocarbons) is required for ignition of flammable mixtures.
When the fuel is a gas, the concentration required for flammability is
reached by allowing more fuel to mix with a given quantity of air. However, if
the fuel is a liquid, it must first be vaporized before it will burn. When the vapor
concentration reaches the LFL, the vapor will ignite if an ignition source is
present. The liquid temperature at which the concentration of the fuel in the air
becomes large enough to ignite is labeled the flash-point. The latter is a
measure of the ease of ignition of a liquid fuel.
Prevention of fires is best accomplished by keeping all flammable materials
under close control. In most industrial operations, once the confined materials
are released, it becomes very difficult to keep air from mixing with the
materials to form a flammable mixture. It is then essential to eliminate as many
ignition sources as possible. In fact, a’ number of codes, like the National
Electrical Code promulgated by the National Fire Protection Association
(NFPA), specify in NFPA Standard 70(1) the elimination of all ignition sources
or the use of protective devices to prevent potential ignition in areas where
flammable mixtures are apt to occur. However, damage from the release may
make this difficult. Thus, most designers of fire-protection systems assume that
ignition generally will occur when a flammable material is released.
The heat-transfer rate in a fire depends on two mechanisms: convection
and radiation. Calculation of the heat-transfer rate must be made by consider-
ing each of the mechanisms separately and then combining the result. If the fire 1