A. R. KING and I. S. WALKER
ALLAN R. KING is a senior research officer and IAN S. WALKER a research officer in the Division of Physical Chemistry, Commonwealth Scientific and Industrial Research Organization (OSIRO), Melbourne, Australia.
Using the infrared reflecting properties of aluminum
THE CONTROL and suppression of wildfires in forest, scrub, and grassland areas requires the use of men who work at the fire edge under trying and sometimes dangerous conditions. Apart from the strain imposed by hard work, the energy of these men is further taxed by emotional strain and the heat of the fire. Although some part of these effects can be offset by insuring an adequate amount of water, salt, food and rest for the fire fighter (King, 1962), the heat stress and emotional factors, particularly in potentially dangerous situations, present problems which have had to be accepted as an unfortunate but necessary condition of the fire fighter's job. There is a considerable loss of efficiency in fire fighting operations, as a result of the additional exhaustion due to the external heat stress and also from the justifiable reluctance of the fire fighter to fight the fire in locations where his life might be in danger if the fire developed extreme violence. An attempt has therefore been made to find methods of increasing the protection and safety of the fire fighter while working at the fire front.
It has been shown (OSIRO, 1962) that the principal means of heat transfer from the fire to its cooler surroundings (i.e., unburnt fuel, fire fighters and their vehicles) is by radiation from hot carbon particles in the flames. More than 90 percent of this radiation lies in the infrared region of the spectrum; the remainder dies in the visible region and can be seen as the typical red to yellow luminosity of the flame mantle. The relative unimportance of convection of hot air from the fire in the heat transfer process has been illustrated by the fact that the air temperature near violent flame fronts is usually never more than 30°F (16.7°C) above the ambient air temperature well removed from the fire (CSIRO, op. cit.).
Although air at 200°F (93°C) can be breathed and borne by the body for half an hour before physical distress becomes apparent (Buettner, 1950), it will be seen from Figure 1 that the radiation intensity close to what would be regarded as relatively mild flame fronts is high enough to cause unbearable pain to the exposed portions of a fire fighter's skin in a matter of 20 seconds or less. Under the same conditions, the clothing worn by the fire fighter also absorbs a large proportion of the radiation and, in spite of the fact that it initially provides a shield against direct transmission of radiation to the skin, at least 50 percent of the heat that it absorbs is eventually transferred to the body by re-radiation, convection and conduction.
The effect of external heat stress on the fire fighter will be considered under the two headings of efficiency and survival.
Efficiency
Even under vigorous working conditions in hot weather the body's cooling system can just dissipate the heat produced in the body by physical exertion and absorption of heat from the sun and the warm air. The additional heat absorbed as a result of radiation from fire overtaxes the cooling system to such an extent that the successive symptoms of heat exhaustion become increasingly apparent within a comparatively short time. The initial effect on the fire fighter is an increasing drop in working efficiency; the eventual effect is, in some cases, actual collapse.
Further, there are often occasions when the fire fighter would like to work close to the fire front at a distance of 2 to 4 feet (60 to 120 centimeters) for a short time, such as 2 minutes, when the effect of the radiation is to cause unbearable pain within a few seconds to the unprotected portions of the skin - that is, the hands holding equipment and the face, which cannot be completely protected by the upturned collar and the pulled down hat or helmet. These are situations which can mean the difference between holding and losing a fireline.
Survival
On many occasions when a fire fighter is trapped by a fire he may be exposed to radiation intensities sufficient to cause excruciating pain within 2 seconds and even though he might be where the fire can never actually reach him - for instance, on a roadway in a pine plantation - he dies. The strain on the body's cooling system is sufficient to cause heart failure, even though the period of intense radiation may only last for 1 or 2 minutes. Alternatively, the unbearable pain may cause panic and the victim starts to run (further taxing his cooling system) and he possibly attempts to run through the deep flame fronts that are usually associated with these really dangerous situations.
If the effects of radiation on the fire fighter are to be minimized, it is obvious that some method must be found to see that he does not absorb the radiated heat. Insulating materials - for example, the fire fighter's clothing - are only successful until they themselves become hot, since they rely for their effectiveness on their low thermal conductivity. If possible, it is far better, therefore, to reject the incident radiation by reflection.
A number of highly polished metals have the ability to reflect infrared radiation. Of all these metals aluminum suggested itself as likely to be the most useful, since, apart from its cheapness, it is readily available as thin sheet, foil or powder - the powder being in the form of thin flakes which lie flat when rubbed or brushed on to a surface if dispersed in a suitable medium. In these various forms aluminum is capable of reflecting between 50 and 99 percent of the infrared radiation incident upon it. These properties have been used for the protection of the fire fighter and his vehicle in the various ways indicated below.
Under working conditions
A skin cream has been developed for the protection of the hands and wrists and, if desired, of the face and neck. This cream consists of a commercial barrier cream (essentially lanoline emulsified in water) broken down with water until it contains 82 percent of water. Into this binding agent aluminum flake (200 to 300 Tyler mesh) is stirred until the cream formed contains 27 percent flake (2 parts of the flake to 1 part of barrier cream solid).
Some commercially available "aluminum pastes" composed of flake, lubricant, and a petroleum solvent have fairly good properties when used as cream, the main objections to them being that they can be extremely irritating if allowed to touch the eye and that they can also lead to dermatitis. It is possible that these pastes could be altered slightly to give a product that is superior in all respects to the formulation mentioned.
The advantage of such a cream is illustrated in Figure 1, on which is plotted the time which can elapse before severe pain is felt versus the level of radiation intensity experienced when the exposed skin is bare or covered with creams which reflect either 50, 65, 75, or 80 percent of the radiation. In addition, typical intensities of radiation that could be experienced in various working positions are marked on the graph. For example, whereas a fire fighter could endure working for only 9 seconds with his hands 4 feet (120 centimeters) away from flames of an equal height he could remain in this position for a minute if they were coated with a cream which reflected 65 percent of the radiation. Such a cream, in addition to its use in protecting against pain, will also lessen the heat load on the body and thus the heat fatigue.
The cream should be rubbed onto dry skin with the hands to form a film that is dry and lustrous and which completely hides the color of the skin.
There is no convenient method of measuring accurately the reflectivity of films on the skin, but another method (King, 1961) has been used satisfactorily. According to this, the receiving element of a special radiometer is coated with the cream and the transmission plus absorption are measured when exposed to a known intensity of radiation. Then:
Reflectivity (%) = 100 - transmission (%) + absorption %.
On using this method the formulated cream reflected approximately 65 percent of the radiation from a heat source maintained at approximately 1,200°F (650°C).
Field tests have supported the claims made for the cream though initially fire fighters were reluctant to use it on the face because of its silver color. However, as the face can be protected by inclining the head, especially if a hat is worn, a great advantage may be gained by applying the cream only to the hands.
Under survival conditions
A device described as a "survival tent" was developed to protect the fire fighter if he should become trapped by fire. The tent is constructed from a special laminated fabric consisting of 0.002-inch (0.05-millimeter) aluminum foil and 0.002-inch (0.05-millimeter) glass cloth bonded by an adhesive which liberates nontoxic vapors when heated. This fabric is sufficiently stiff for the tent to be self-supporting.
The tent has an inverted-vee shape with a base of 8 x 4 feet (240 x 120 centimeters), the height to the ridge being 2 feet (60 centimeters) (Figures 2 and 3). The ends are virtually closed, although small gaps are purposely left to act as sight holes. The occupant lies on tapes which run across the width of the tent, and extensions of the tent ends are available to be folded under and anchored by the weight of the occupant. This anchoring is very necessary, since drafts generated by the fire may lift the tent away from the fire fighter and expose him to intense radiation.
The following facts were taken into consideration when designing the tent:
1. Though the fire fighter may be insulated from a large proportion of the heat by the shielding effect of the aluminum tent, evaporation of sweat from the body should not be impeded. Confinement of the air in the tent and restriction of its movement should therefore be avoided.2. The air near the ground is usually fresher and cooler than that higher up and the temperature of the soil is not appreciably increased by incident radiation. (The authors know of no evidence to support allegations that the air near a fire becomes oxygen deficient to the extent that life cannot be supported or that fire fighters are exposed to poisoning by carbon monoxide generated by the fire).
3. Under most circumstances in Australia a fire fighter trapped in a dangerous situation would have at roast 3 minutes before the conditions became unbearable. During this time he would be able to find or prepare a clear space at least 12 feet (3.6 meters) across devoid of fuel. Such a fuel-free area could be a road, track or camp site, or a space cleared by raking or burning.
4. To be practical, the survival device should be light in weight, not bulky, able to stand limited rough treatment, easily erected, and cheap.
Tents of the type outlined above have been success fully tested in Australia using human occupants and under conditions where survival would normally be impossible (CSIRO, op. cit.). While the tent greatly lessens the risk of death, it is not anticipated that it will necessarily be adequate at all times. One further factor that emerged from the field testing of the tent is the necessity for the occupant to wear a simple mask, such as that suggested by King, 1962, to diminish discomfort occasioned by smoke.
FIGURE 2. - The survival tent.
Motor vehicles engaged in bushfire fighting should be protected against radiant heat, since at times they need to be used close to intense flames, and in an emergency they may be used as refuges for fire fighters. Three major problems arise from radiation.
1. The tires may decompose somewhat, causing rapid loss of strength with the likelihood of deflation; occasionally tires may actually ignite.2. The body panels of the vehicle may be heated to a temperature at which paint finishes decompose and give off unpleasant or toxic vapors both inside and outside the vehicle; occasionally, the physical deformation of the metal work will cause doors and windows to jam.
3. The motor may overheat, resulting in loss of power.
Although the second and in part the third problem could be largely overcome by using aluminum in the bodywork construction of fire fighting vehicles, a more practicable suggestion is to paint them, including the side walls of their tires, with aluminum paint. The reflectivity of suitable paints was tested in the same way as that of the aluminum cream mentioned above.
The protection likely to be afforded by aluminum paint to the vehicles' tires was tested by subjecting tire sections, untreated and painted with two suitable paints to two different intensities of radiation (Table l). After 40 minutes of exposure the untreated tire showed much greater deterioration (Figure 4). This lengthy exposure would only be experienced in unusual situations, for example, during mopping-up procedures when vehicles may be parked adjacent to burning heaps of heavy fuel.
In many cases it is quite possible that the inner tube would fail some time before the tire became seriously decomposed, since the tube is more sensitive to heat, which is conducted to it not only through the tire wall but also by the hot wheel rim. Also over a period of time the lives of both tires and tubes are likely to be shortened because of frequent exposure to even comparatively low radiation intensities.
FIGURE 4. - Effect of irradiation on tire sections untreated and protected with aluminum paint.
TABLE 1. - Heat irradiation tests on 6-inch (15-centimeter) sections of 2.7 x 19-inch (5.3 x 49-centimeter) tires
|
Radiation intensity and tire treatment |
Time until |
||
|
smoking |
cracking |
glowing |
|
|
Minutes |
|||
|
0.35 cal./cm²/sec.-1 |
|||
|
Untreated |
1/3 |
3 |
3 ½ |
|
Paint A 5 |
1 ¾ |
5-6 |
8 ¼ |
|
Paint B 6 |
1 ¾ |
8-9 |
11 ½ |
|
0.17 cal./cm²/sec.-2 |
|||
|
Untreated ³ |
¾-1 |
5 ½ |
4 10 ½; 15 ¾ |
|
Paint A 5 |
4 ½ |
21 |
4 40 |
|
Paint B 6 |
3 ¼ |
18 ¼ |
4 40 |
1 Radiant intensity equivalent to the average usually encountered 1.6 meters (6 feet) above ground at 1.5 meters (5 feet) from 3- to 6-meter (10- to 20-foot) flames.
² Radiant intensity equivalent to the average usually encountered at ground level 1.5 meters (5 feet) from 3- to 6-meter (10- to 20-foot) flames.
³ These are the results of two tests; the time to glowing is governed In part by the specific way In which the surface cracks.
4 Pilot ignition Immediate for the untreated tire and at 40 minutes took 4 and 6 seconds respectively for paints A and B. Pilot ignition is that brought about by direct contact of the inflammable material with a separate flame, as opposed to spontaneous ignition, which takes place when flaming is initiated by the interaction of the hot vapors from the decomposing tire with the oxygen in the air.
5 Average heat reflection, 66 percent.
6 Average heat reflection, 60 per gent.
In an emergency, when the vehicle becomes a refuge, the outer surface of the windows could well be rendered reflecting by paint applied either by brush or, less desirably, by spray (paint contained in a pressure pack). A more appropriate method would perhaps be to cover windows with pieces of thin - 0.005- to 0.020-inch (12- to 50 millimeter) - aluminum foil which could be precut to size. In most emergency situations there would be sufficient time for the use of such protective devices on the windows. Two objections have been raised to painting vehicles with aluminum paint. These are that the vehicle becomes less easily seen in dense smoke and that the glare from the bonnet and body tends to dazzle the vehicle driver and other persons in the vicinity. However, actual experience with one vehicle of the Mobile Laboratory, CSIRO Bushfire Section, has shown that these problems are not serious.
BUETTNER, E. J. 1950. Amer. Med. Ass., 144: 732.
COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH 1962 ORGANIZATION (CSIRO). 1962. Preliminary report on a survival tent for rural firefighters, Division of Physical Chemistry, Melbourne, Australia.
KING, A.R. 1961. Brit. J. Appl. Phys., 12: 663.
KING, A. R. 1962. The efficiency of rural firefighters. Common wealth Scientific and Industrial Research Organisation Chemical Research Laboratories, Technical Paper No. 4.
