[knowledge] Fire Rescue: Does the fire service wear wet or dry?

As many people are now arguing about the issue, if the firefighters' equipment is wet, will it be better than the dry heat resistance? This issue has not only been discussed at home for a long time, and even there are quite a lot of experimental studies abroad, because Firefighters often wet their clothes in various situations on the fire, such as shooting water, sweating, and so on. In order to understand the mysteries, the author discovered two similar articles in the United States and translated and collated them for your reference.

In the issue of moisture for firefighters, some firefighters will also display information related to moisture and heat resistance in the instructions. Some are correct, while others are misleading. This is because the moisture is actually resistant to moisture. There is no certain answer to the influence of heat. Sometimes when the humidity reaches a certain degree of saturation, it will increase the heat-resistance effect of the fire protection clothing; sometimes the humidity will cause danger. Perhaps the only conclusion that can be written is that "dry firefighters can expect firefighters whose heat resistance is relatively high." The same characteristics can be applied to gloves, fire shoes and hoods.

Head cover experiment

The article on this page mentions that there was a large unit in the United States a few years ago, and the players inside were just beginning to familiarize themselves with the use of the headgear. The method used was to wet the headgear. After this practice spread in other teams, it was unanimously considered that the mode of use was unsafe. After that, most firefighters chose not to wear headgear, instead of choosing not to wear wet headgear. In order to verify which method is correct, they have convened experts and scholars to conduct experiments and classified the headgear as “dry”, “wet” and “saturated wet”, aiming at the flow of heat from low radiation to extreme flashes. The heat flow began as a heat-resistant experiment.

In this experiment, an interesting finding was that the dry headgear was better than the wet headgear in the case of the lowest thermal radiation, but the saturated wet headgear was also better than the wet headgear; the moderate heat radiation Under the condition of fire flow, the three kinds of protection effects of dryness, moisture and saturated humidity are equivalent; in the case of extreme flash heat flow, the saturated wet headgear "slightly wins" the damp headgear, and the dry headgear performs the worst. On the whole, as the heat radiation increases, the three kinds of heat resistance are also lower. Finally, the test results of the human body model show that under the high radiant heat test that occurred during the flash burn, the face, neck, and ears that were not protected by the headgear were burned.

The above-mentioned experimental results, the most definite conclusion is that without wearing a headgear must be worse than wearing a headgear, and then there is a different degree of moisture, for the protection of the heat resistance of the equipment will have different performance.

Changeable Fire Test Results

In general, the composition of the fire clothing is composed of three layers of outer layer, impervious (or fully waterproof) layer and heat resistant layer. A study within 30 years has pointed out that the heat resistance of fire protection clothing will change depending on whether the outer layer, the inner layer or the inner and outer layers are all wet. This experiment also mentions that when the inner layer is wet but the outer layer is dry, the fastest heat conduction warning can be given before the firefighters are burned; and the inner material of the completely waterproof material is more water-retentive than the impervious water. Material has more warnings.

In recent years, there have been more studies to verify the thermal energy storage and conduction of clothes under relatively long periods of low radiative heat exposure. In these experiments, it was found that when the water-blocking layer is a completely waterproof material, it will result in the worst heat-resistance result under a certain humidity.

The reader can quickly discover that there are some contradictions in the above conclusions. In fact, these experiments can't say any specific conclusions. The point here is to tell the reader to write wet conditions from a set of two conditions. The effect of gas on firefighters is actually quite difficult. Why is it so complicated? You have to understand how water plays its characteristics and influence in firefighters.

Where does the water come from?

There are many opportunities for fire fighters to wet themselves. For example, when shooting fire at a fire spot, some of the water mist will be scattered back and sprinkled on the firefighters; some water droplets attached to the ceiling will be dripped and attached to the nozzle; even When firefighters feel that the radiant heat is overheating, sprinkle water on themselves. The outer layer of the new fire protection jacket will be waterproofed, but this kind of waterproof treatment will be quickly washed out.

When water is attached to firefighters, the equipment will start to become bulky and the anti-skid devices of the gloves will be passivated. Then water will transfer the water through the outer layer to the fibers on the surface of the anti-dip water layer. At this time, the surface will absorb some water. Then, the water will be blocked by the anti-dip water layer, unless it continues through the collar, cuffs, clothing and other openings to the inner layer.

However, when firefighters come in contact with wet areas, they will have a new pipe into the inner layer of water. For example, in terms of knees, even if the knees are treated with waterproof when walking in a kneeling or crawling position, the water will follow the knee to the bottom of the bottom of the trousers when it comes into contact with moisture, and the fibers at the bottom of the trousers will be Water is sucked into the inner layer; when the fire shoe is stepped into the water, even if the sole is waterproof, water will lead the water through the seams of the shoes and the trousers to the inner layer of the fire protection pants.

Finally, we must mention the firefighters must have sweat in any case. Sweat is constantly generated. Even if the reader reads in a chair now, the sweat is not seen because the air between the clothes and the skin is dry compared to the clothes, so the sweat evaporates; When the humidity between the clothes and the skin reaches saturation, evaporation will decrease and sweat droplets will form. In the same concept, when the firefighters wear a fire-fighting jacket, the inner heat-proof layer is in contact with the skin. At this time, the sweat of the fire-fighter will face two conditions. If the waterproof layer is a material that is relatively easy to absorb water, the fire-fighters The sweat will be absorbed by the heat-resistant layer, and then diffused in the heat-dissipating layer; if the material is less absorbent, sweat will flow down the shoe into the shoe according to the gravity characteristics, which is why some firefighters will complain. Feeling of sweat flowed into their shoes.

The mediating role of water

Fire protection clothing, as mentioned above, can easily get wet under various conditions. However, once the fire protection jacket is in a wet state, it will affect its heat resistance performance. Why? Because the water in the fire-fighting coat became an important medium. When water becomes a medium, there are two very noteworthy characteristics. The first is the high heat capacity of water. Because of the high specific heat of water and the latent heat of vaporization, water can store more heat energy. Therefore, wet clothing can store much more heat than dry clothes.

One noteworthy feature is the conductivity of water. Compared to air, water is more conductive than air. One of the reasons why the heat-resistant layer can resist heat is that there is a gap between the fibers of the heat-resistant layer, and the air between the gaps makes it difficult for heat to conduct into the skin layer, and when the water fills the gap, it will make it resistant. The thermal layer forms a new thermal conduction path.

The high heat-absorbing capacity of water and the high heat-conducting properties are very important concepts. When wearing a wet fire suit in the fire, water becomes an extra heat-absorbing material. However, when water continuously absorbs heat but does not disappear through other means, the heat may reach the temperature of the burn. The water attached to the surface of clothing is the closest to the heat source, so the temperature rises very fast. At the same time, these waters will have two paths. One is to evaporate from the surface, and the other is to penetrate through the path that can penetrate the inner layer. The layer moves, and it is possible to come into contact with the skin and cause injury.

Of course, water is by no means the only factor that affects the protectiveness of fire protection clothing, but the variability of water added to fire protection clothing cannot be ignored. How firefighters can effectively manage the moisture of fire protection clothing is a matter that cannot be ignored.

Practical applications

From the above explanation, we can clearly understand that water adhered to fire protection clothing may cause fire protection clothing to have different changes in heat resistance.

When water inside a fire suit can continue to absorb heat and heat is continuously removed in various ways, then water may become an addition protection for fire protection clothing; in contrast, when the high heat absorption capacity of water cannot be effective When removed, then its high thermal conductivity will quickly transfer heat to the body's skin, increasing the likelihood of burns.

Of course, the aforementioned experiment also mentioned that when water stays in a fire suit, it is also because of its highly conductive nature that when a high temperature exists in the scene, the firefighter feels that the fire suit with a hot dry time will be faster.

At the beginning of the article, it was mentioned that dry fire protection clothing with a relatively warm resistance to heat may be expected because wet fire protection clothing will vary depending on the degree of moisture, the heat radiation of the fire field, and the material of the clothes. There are many changes that may make the security of the fire field unpredictable. In practice, there are also actual cases because of accidental burns and injuries caused by the wetness of fire suits.

About the author: Chief of Disaster Rescue Section of Yunlin County, Taiwan.