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Static Fire Stories Articles & Technical Papers Current News

Study on Efficiency of Nanometer Eruptive Generated Aerosol Fireproof and Anti-explosive Agent

(1. Chemistry Laboratory 313, Nanjing University of Science and Technology, 210094)
(2. Jiangsu Police Institute , 210012)

Chao Huang - 1,2

Xujie Yang - 1

Lude Lu - 1

Xin Wang - 1

Abstract: Multi-protection is a principle in security management. When we pay our attention to improve the property of building for fireproof and use more or better fire-fighting appliances, it is also one aspect of it that we use fireproof and anti-explosive agent too. The efficiency of fireproof and anti-explosive agent of nanometer Eruptive Generated Aerosol is analyzed in this paper and it is concluded that this kind of fireproof and anti-explosive agent is better than others. It will fill the gaps in fireproof and anti-explosive agents, and it will play an important role in security management. Keywords: nanometer, aerosol, Eruptive Generated Aerosol, fireproof and anti-explosive agent With the development of science and economy, security has aroused more and more concern of the general public. But fire and explosion hazards occur frequently. As many as 215,863 fire hazards happened in China last year, which caused 2,314 death, 3,752 injured and direct property loss of RMB82.28 million[1]. The great gap between expectation and reality demands further study on fire products. However, the current study mainly focuses on fire-extinguishing products rather than anti-explosive ones. This paper advances the theory of Nanometer Eruptive Generated Aerosols initiatively and analyses their efficiency and functions theoretically. Up to now, such study has not been reported by other people.

1. The significance and functions of fireproof and anti-explosive agent

The policy in firefighting service is "prevention first and combining firefighting with fire prevention". Fire extinguishing is one of the important parts in firefighting, but it is not the principal part. Now much effort has been made to study the extinguishing agents having the qualities of high efficiency and environmental protection. More attention should also be paid to the study and development of fireproof and anti-explosive agents to form multi-protection. It is known that explosion is closely connected with fire. On the one hand, when catching fire, some leaked inflammable materials will explode and produce an explosion temperature as high as 2000?. It is not surprising that such high temperature will cause firehazards. On the other hand, leaked inflammable materials caused by fire hazards can also lead to deflagration. Take the 78 most serious fires listed in the "Fire Statistical Yearbook of China, 1999" for instance. There are 9 cases caused by improper handling of the leaked inflammable materials, which account for 11.5 percent. Being founded, if the leaked inflammable materials had been sprinkled with some fireproof and anti-explosive agents first and then simply handled in time, explosions and fire hazards could have been avoided or reduced. Inflammable materials can be found almost everywhere in such places as factories, warehouses etc. For example, there exist certain kinds of combustible vapor in some places all the time, inflammable liquids in other places and combustible materials or powders in still other places. It is very important to isolate the fire at critical period in some dangerous places, but it is also very important to sprinkle fireproof and anti-explosive agents according to the multi-protection principle. Fireproof and anti-explosive agents play a very important role in avoiding or reducing explosions and fire hazards. Thus they can protect people's life and property from being lost.

2. Main efficiencies that fireproof and anti-explosive agent should have

Fireproof and anti-explosive agent is a substance that can reduce the danger of fire or explosion in the place being protected in certain period of time. This paper discusses the explosion caused by the enclosure to fire of the flammable vapor or the combustible powder combined with air. Such explosion is the same with combustion in principle, being oxidation-reduction reaction and chain reaction mechanics. The difference is that the contact way of oxidant and reducer causes the difference of the reaction rate. Therefore, we can adopt the same method to prevent or reduce the occurrence of fire and explosion. Differing from fire extinguishing, the principle of fireproof and anti-explosive agent has only one method to carry out fireproof and anti-explosion, which is inhibitory method. If the place where the fireproof and anti-explosive agent being used has flammable substances mixed with air, isolation and suffocation method cannot be used. No hazard occurs during the fireproof and anti-explosive actions. The temperature is not high, does not exceed the fire point, and it is not necessary to cool down under the fire point.

The inhibitory agent can prevent the occurrence of fire and explosion. If the outside ignition source decomposes the flammable substance and creates free radical and if the free radical acts with the inhibitory agent and is captured, it will lose the function of transferring the chain reaction. The chain reaction will stop, and burning cannot carry out. Therefore, the effects of preventing the burning and fire can be reached. Such kind of fireproof and anti-explosive agent should have the following main efficiencies:

2.1 Continuousness

Dangers exist in a place sometimes for a short time, such as coincidental leakage of inflammable substances; and sometimes for a long term, such as the existence of suspended materials in warehouses storing inflammable products, coal pits and production workshops of powder flammable materials. After the sprinkling of fireproof and anti-explosive agent, a certain period of effective protection shall be available for the place being protected. The effective protection period does not need be too long for the leakage of flammable matters, as long as it meets with the time demand of treatment on flammable matters. For the place where danger exists for a long time, the effective protection period of the fireproof and anti-explosive agent shall be comparatively long. Therefore, the fireproof and anti-explosive agent should have continuousness.

2.2 Inhibition

The fireproof and anti-explosive agent should be able to catch the transferring agent, the free radical, in the burning chain reaction to inhibit the burning reaction. When the free radical is captured, it can't be used as a transferring agent of the chain reaction and thus no fire or explosion will occur. It is the reason why the fireproof and anti-explosive agent can inhibit fire and explosion.

2.3 Insulating property

The equipment in the place where the fireproof and anti-explosive agent is used for protection need to work normally. Therefore, if the insulation of the agent sprinkled is bad, the equipment cannot work normally.

2.4 Low corrosion

If the fireproof and anti-explosive agent has corrosion, it will reduce the life of the equipment and make the enterprise have loss. Therefore, the fireproof and anti-explosive agent shall be of low corrosion or without corrosion.

2.5 Non-toxicity

The fireproof and anti-explosive agent should be free of toxicity to human beings and animals and have no pollution to the environment. Moreover, the fireproof and anti-explosive agent should have the properties of good flow ability, anti-agglomeration and stability.

3. Analysis on efficiencies of nanometer eruptive generated aerosol fireproof and anti-explosive agent

Nanometer eruptive generated aerosol fireproof and anti-explosive agent is an agent composed of nanometer particles, driven by inert gases, which was sprayed to form aerosol. The analysis on the efficiencies of this aerosol shall start from the functions and positions of eruptive generated aerosol.

3.1 Functions and positions of eruptive generated aerosol

Halon fire-extinguishing agent series will exit from the historical stage of China from 2005 due to its damage to the ozonosphere. In the recent years, the substitutions of Halon series are very popular, in which the eruptive generated aerosol fire-extinguishing agent draws much attention by its excellent functions and low prices. The eruptive generated aerosol fire-extinguishing agent is a solution dissolved from the powder of fire-extinguishing agent, and super-fine powder around 3 millimeters are caused due to the spray dryness, and finally mixed with additives such as dispersant agent, waterproofing agent, anti-static agent and agent that can strengthen fluidity. The powder should be stored in sealed tanks. The sprayed aerosol will submerge the fire and put out the fire, using pressurized gas as driving power. It has overcome the defects that the pyrotechnically generated aerosols fire-extinguishing agent has outward sprayed fire and cause toxic gases. Therefore, it has no bad influence on the environment. It is known to all that for the same type of dry powder fire-extinguishing agent, the fire-extinguishing efficiencies is in the inverse ratio with the size of the particles. The size of particles of the super-slender powder (millimeter level) eruptive generated aerosol fire-extinguishing agent is one of several tens of that of the normal dry powder fire-extinguishing agent, and its fire-extinguishing efficiency is 6 to 10 times of the normal dry powder[2], and 4 to 6 times of the Halon series. The fire-extinguishing efficiency exceeds the pyrotechnically generated aerosols[3].

3.2 Analysis on efficiencies of nanometer eruptive generated aerosol fireproof and anti-explosive agent

The particles of nanometer eruptive generated aerosol are even smaller than the above-mentioned eruptive generated aerosol. Within the size range of the nanometer level, the particles will have some particularity, such as quantum efficiency, small dimensional effect, surface phenomenon and Kubo theory. Besides the functions mentioned in 2.1~2.5, the nanometer eruptive generated aerosol has better functions.

3.2.1 Strong stability of nanometer eruptive generated aerosols

The stability of aerosols is influenced by the diameter of colloid particle. Through experiments, Stokes found that when globular colloid particles fall in still air, their sedimentation velocity, which is called final sedimentation velocity, would become stable soon. Stokes found that this final sedimentation velocity conforms to the following Stokes' Law:


The final sedimentation velocity of nanometer aerosol is 7.3 cm/day[4]. The particle diameter of this aerosol is 100nm. The smaller the particle is, the slower the sedimentation velocity is, and the stronger the stability of the aerosol is and the better the efficiency of fireproof and anti-explosive agents is. The property of nanometer particle is better than micrometer particle.

3.2.2 Strong sorption of nanometer particles

We know, the surface-to-volume ratio of a sphere, The particle sorption relates with the ratio. The larger the ratio is and the stronger sorption the particle is. Since the diameter of nanometer is 1000 times smaller than the micrometer, the sorption is stronger. Sorption can be divided into physical sorption and chemical sorption. The former one is non-optional and the latter one is optional. Analyzing theoretically, it is possible to produce nanometer particles that can absorb free radical so as to gain the ability of partially capture the free radicals.

3.2.3 Strong chemical reaction ability of nanometer particles

When the size of the particle reduces to the nanometer level, one particle might contains more than ten or tens of molecules, so the damaged chain greatly increases and the chemical activity strengthens, which is one of the representations of small size effects. Moreover, for the nanometer particles, the electron levels near the fermi level turn dispersed from quasi-continuous level, and the difference between the levels will enlarge with the decrease of size of particles. A series of abnormal particularities with the macro objects will occur, such as specific heat abnormal, spectral series red shift, and rummy connections between the catalytic activity and numbers of atomicity, which, in a certain extent, showing the quantum effect [5]. It is sure that nanometer particles have comparatively strong chemical reaction activity, so the selection of different materials and crystalline state can reach the purpose of capture the free radicals.

3.2.4 High fire-extinguishing efficiency of nanometer particles

When the particles of dry powder impact with the active transferring agent and absorb its energy and make it inactivation, the effect of fire extinguishing can be reached. The impact is the key step of exertion of functions of dry powder particles. Since the free path of gas molecule is comparatively long, in order to make the comparison easy, suppose the dry powder with different diameters of particles have the same impact frequency of flammable gas molecules, the calculation for the ratio of the fire extinguishing efficiency is as follows:

First, suppose (1) the dry powder particles stay. Since the moving distance of gas molecules in unit time is larger than the movement of the dry powder particles, the influence caused by the movement of dry particles can be omitted. Suppose (2) not consider the chemistry of dry powder particles with the flammable gas molecules. In order to make the analysis easier from the impact aspect and simplify the problem, the chemical action is not considered temporally. Suppose (3) the impact effects of large dry powder particles with the small flammable gas molecules are equivalent.
According to the law of gas molecule movement, the mean free path of gas

molecule is:, the collision frequency of it is:

When the gas molecules collide with powder particles, the probable collision area is:

and the collision frequency is:

Suppose the powder extinguishing efficiency of size r1 is E1=1/a1(m3/g); the powder extinguishing efficiency of size r2 is E2=1/a2(m3/g); is the density of powder particles,

When the collision frequencies of gas particle with particle r1 and particle r2 are the same, from Eq. (2), we can get the Eq.:

Put Eq. (4) into Eq. (3), then or

The recently developed superfine eruptive generated aerosol has an average particle diameter about 3 nanometers, and its fire-extinguishing effect is 4 to 6 times of that of Halon series. From the (5) theoretic calculation, if the eruptive generated aerosol with the average diameter of 100 nanometers is adopted, we know that its fire-extinguishing effect is 30 times than that of the nanometer eruptive generated aerosol.

3.2.5 Efficiency comparison of nanometer eruptive generated aerosol with the pyrotechnically generated aerosol, water mist and CO2 as fire-extinguishing agent when in fireproof and anti-explosion

In the fire-fighting products available, CO2 fire-extinguishing agent and the water mist fire-extinguishing agent can be used as fireproof and anti-explosive agent, however, their effects are not satisfactory. CO2 is not electrically conductive, and has no corrosion. Its anti-explosive mechanism is to lower the content of oxygen. We know that the lowest oxygen content for burning in H2 and C2H2 is 5.9% and 3.7% respectively. When the discharge of CO2 is 3.6 and 5.7 times larger than the space to be filled, the danger of H2 and C2H2 burning or explosion can be released. This is neither economic nor convenient for people to enter the area. The water mist is cheap but of bad insulation, short effective time and the place is restricted. The pyrotechnically generated aerosol will spray fire when use, so explosion of flammable gas might be caused. Therefore, it cannot be used as fireproof and anti-explosive agent. However, the nanometer eruptive generated aerosol does not have the weak points of the fire-extinguishing agent mentioned above. The fireproof and anti-explosive efficiency is excellent.

3.2.6 Removal of eruptive generated aerosol

Even though the nanometer eruptive generated aerosol is not toxic and has no pollution to environment, when people enter such a place, the particles will be absorbed to the lungs, which will do harm to the human bodies. The aerosol can be removed by ventilation or spray of water mist. The influence of aerosol can be also basically removed when the staff wearing wet respirators when entering the places with aerosol.

4. Conclusions

Multi-protections are the basic principles of safety management. However, in the current fireproof and fight fighting work chain, the use of fireproof and anti-explosive agent is in short. As fireproof and anti-explosive agent, nanometer eruptive generated aerosol has incomparable efficiencies than the current products. It can also be used a fire-extinguishing agent, and fire fighting effects are better than that of the super-slender powder eruptive generated aerosol and pyrotechnically generated aerosol, and much better than Halon and normal dry powder fire-extinguishing agent. Economically analyzing, the cost of nanometer eruptive generated aerosol is higher than the super-slender powder eruptive generated aerosol. Used as a fire-extinguishing agent, its quality-price ratio is close to that of the super-slender powder eruptive generated aerosol. Used as a fireproof and anti-explosive agent, its quality-price ratio is much better than that of the super-slender powder eruptive generated aerosol.

 

Reference:
1) Xian xiao. China Fire,2002; (2): 11
2) Xiuzhi shi. Fire Technique and Products Information, 2000; (10): 27-28
3) B.marc ph. Burns, 2001;27(7): 783-784
4) C.N.Davies. Aerosol Science, Chap.12, Academic Press, Inc., New York,1966
5) Kobo R. j. of the Phys. Soc. of Japan, 1962; 17(5): 975~986


 

 

 

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