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Superheated Steam



To PHYS-L Members,
I am trying again to transmit my article on Superheated Steam. Sometime
ago Kathy Daniel secured help from some of you about steam and
firefighting. I want to ask your help to be sure that the physics is
correct. I am especially interested in knowing if there is any research
that can explain the difference between clockwise rotation of a fog nozzle
versus a counterclockwise rotation. Also a respected fire instructor told
me that steam is not superheated unless it is pressurized. Is this correct?

The Myth of Superheated Steam
by John D. Wiseman, Jr.

Water plays a key role in fires and in firefighting. Water (H20) and
carbon diozide (CO2) are the main products of the combustion process, and
the only products of complete combustion. Since fire temperatures are much
higher than the temperature at which liquid water changes to a gas (steam)
at 212oF (374oC) in a fire water exists largely as steam. Above its
critical temperature of 705oF(374oC) there is no distinction between a gas
and a liquid. Below this critical temperature, water may exist as a liquid
or a gas. Since fires occur at a constant atmospheric pressure, and
confined fires do not significantly increase pressure above atmospheric
pressure, the mixture of liquid and gas is dependent upon temperature (T)
alone.

The concentration of water vapor (gas) is usually measured by vapor
pressure with a greater pressure indicating a greater density of vapor. The
concentration of water vapor increases as T increases since the evaporation
rate increases with T. However, evaporation is a cooling process, so unless
the local temperature is kept constant, the evaporation rate decreases as
it cools.

Likewise, condensation is a warming process. So unless the local T is kept
constant, the condensation rate decreases as T increases. If the vapor
pressure equals the local pressure, then the evaporation rate equals the
condensation rate and there is no net gain of either. This is a state of
equilibrium, and this pressure is said to be the equilibrium vapor pressure.

The phase change from liquid water to steam is a heat absorbing process
(endothermic). This is usually called the latent heat of vaporization, but
the correct name is "enthalpy of vaporization". The enthalpy of
vaporization of water is much greater than that of most other substances.
This is a critical fact for firefighting as well as for fire safety. The
phase change of water at 212oF (100oC) at atmospheric pressure results in
an increase of enthalpy of steam of 971btu/lb (2.5x120>6j/kg). This
endothermic process does NOT increase the T of the steam above 212oF (100oC).

For confined fires, the injecction of a small amount of water into a fire
area produces spectacular results. The T drops rather rapidly (<l min.)
from 1,000oF to 2,000oF to around 300oF. Further at 212oF, one cubic foot
of liquid water expands to 1,700 cubic feet of steam. This near explosion
literally blasts a fire out of existence. Almost immediately as the steam
contacts the cooler surrounding air, the steam condenses producing a white
cloud and a warm moist atmosphere.

There are two tactical requirements for this to work well.
(1) Water must have sufficient velocity near the burning surfaces to
vaporize. In other words, the water must be distrbuted evely throughout the
fire area.
(2) Water must be applied in little drops. This mandates the use of a
standard fog nozzle capable of producing uniform drops of water 0.01 to
0.04 inches ((0.3 to 1.0 mm) in diameter. This increases the surface area
of a given amount of water, compared to solid water, by a factor of
1,400/1. This transfer of heat is proportional to the surface area of the
liquid water.

A fog attack that converts all, or almost all, of the liquid water to
steam is the most effective fire attack for two reasons.
(1) The phase change at 212of (100oC) is well below the ignition T of most
hydrocarbon fuels. Also the rapid drop of T in a fog attack to 300oF is
well below these ignition temperatrues. Therefore a fog attack cools the
fire environment sufficiently to extinguish flaming combustion.
(2)The rapid expansion of steam deprives the fire, momentarily, of the
oxygen needed for combustion. Flaming combustion canot continue if the
oxygen level drops below 15% of air. The rate of heat release in a confined
fire is limited by the amount of oxygen available, so the fog attack
smothers the fire as well as cooling it.

This combination attack constitutes the most effective fire attack available.

The enthalpy of vaporization also has important implications for fire
safety. For this analysis we must consider the mixture of air and water
introduced into a fire by a fog nozzle. There are two numbers needed,
temperature (T) and dew point. Higher temperatures correlate with an
increase in energy. Dew point is the temperature at which water in a
water-air mixture condenses, that is, undergoes a phase chage from vapor to
liquid. If air temperature and the dew point are equal, then this is
familiar to you as 100% relative humidity.

In a fire temperatures range upward from 212oF (100oC). The dew point of
pure steam is 212oF. For an air-water mixture the dew point is still quite
high, ranging from 190oF (50/50 mixture) up to 212oF for pure steam. A
steam-air mixture has a great capacity to heat any surface, or object, that
is cooler than the high dew point of the steamy air. In other words, a
firefighter with a low dew point (<100oF) is at great risk from steam burns
from a high dew point steamy air mixture (>190oF). The condensation of
steam to a liquid at 212oF (or lower T) transfers all the heat to the
cooler object that was used to create the steam by evaporation.

A second safety factor involves the expansion of water to steam by a ratio
of 1,700/1 at 212oF. A firefighter must avoid being engulfed in this blast
of steam. There is a tactic that protects firefighters who are operating a
fog nozzle through an outside window or an inside doorway. The tactic was
created at Iowa State Universsity in the research done there by Keith Royer
and Bill Nelson. They discovered that the best way to use a fog nozzle on a
confined fire is to rotate the nozzle rapidly in a clockwise manner as
viewed from the nozzle position. This clockwise rotation is far superior to
a counterclockwise rotation for the following reasons

(1) Clockwise rotation is safer because it drives smoke, gases, and flames
away from the nozzle. Counterclockwise rotation does just the opposite.
(2)Clockwise rotation produces steam with an active rolling action.
Counterclokwise rotation produces steam with an inactive and lazy action.
(3)A clockwise rotation produces a faster knockdown time.

The scientific reason for these differences is not known.

In this analysis of water behavior and fire behavior, no mention has been
made of superheated steam. It is not necessary to do so since both
evaporation and condensation of water occur at a T of 212oF. It is a fact
that superheated steam is must less dangerous to firefighters than high dew
point steam, or steamy air. "Superheated steam" is a technical term used to
describe a steam-air mixture with a relative high temperature and a
relatively low dew point, compared to a normal steam-air mixture. A good
example is a heating system with a steam boiler. A major difference in this
system is that it operates at a much higher pressure than atmospheric
pressure.

So superheated steam is dry steam, not wet steam. If an object's
temperature (firefighter) is above the dew point, then superheated steam is
no more effective than dry air in heating up the object. So a superheated
mixture with a relatively low dew point is much less dangerous than a high
dew point mixture at the same temperature. It is the transfer of heat by
condensation at the dew point that is the real danger to firefighters.

While "superheated steam" sounds ominous, the actual facts about
superheated steam are much less ominous. There is a real danger to
firefighters, of ourse, but the danger comes from high enthalpy steam,or
high-energy-steam, or high dew point steam.