Cooling spray has a bad temper and may be flammable and explosive!

Even though the beginning of autumn has passed, the heat of summer is still here. Although the temperature is high, you can't give up your work; air conditioners are great, but you can't take them with you everywhere. Alas, you can't help yourself in the world, it would be great if there was a cooling tool that could always be with you! What's more, the "cooling spray" that is popular online recently is just like this god-like existence? Just spray it anytime, anywhere to cool down instantly, and the price is not expensive, it is simply a must-have in summer!

Wait, do you really know this so-called "cooling spray"? Don't rush to place an order, let's take a closer look at its background first!

The firefighters have tried, so don't try it...

01

The surface area of ​​water atomization increases exponentially

Amplify the cooling effect of evaporative heat absorption

Using spray cooling is a very common large-area cooling method, which can achieve good cooling effects in a short time and can be used for both indoor and outdoor cooling.

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In fact, its working principle is easy to understand: Have you ever had this experience: when you go to the hospital to draw blood or get an injection, you will feel very cool when the nurse applies disinfectant alcohol on your skin. This is a typical phenomenon of evaporation and heat absorption. The same is true for spray cooling, which makes us feel cool. Water passes through the atomizing nozzle under high pressure to form fine granular droplets. The droplets are in full contact with the air, and the purpose of cooling is achieved by using the principle of evaporation and heat absorption.

But since it is also evaporation, why is it that putting a basin of water at home does not have such a good cooling effect? ​​This is because the surface area of ​​the droplets is large, evaporation is fast, and heat absorption is also fast, so the cooling effect is better. Take 1 gram of water as an example. The volume of water in the liquid state is 1 cubic centimeter, and its total surface area is 6 square centimeters (cube model); if all of it is converted into 100-micron-sized droplets, the total surface area becomes 600 square centimeters, which is 100 times that of liquid water, equivalent to a 99-fold increase in the contact area, and the smaller the droplets, the larger the total surface area. Therefore, the sharply increased surface area makes the evaporation heat absorption process extremely fast. Under normal circumstances, the heat taken away by the evaporation of each gram of water can cool down each cubic meter of air by about 1.5-2 degrees. However, the effect of spray cooling is greatly affected by the humidity of the air. The higher the relative humidity, the more difficult it is for the droplets to evaporate, and the worse the cooling effect.

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The main ingredient of the popular cooling spray on the Internet is water, in addition to a small amount of ethanol (alcohol), menthol and other organic substances. Since organic substances such as ethanol have a low boiling point and absorb heat faster than water, the cooling effect is more obvious. There are also some special cooling sprays that claim to be able to instantly cool down by dozens of degrees. The main ingredients of this type of spray are high-pressure liquefied gas such as propane (boiling point -42.1°C) and butane (boiling point -0.5°C). Compared with water, ethanol, etc., they have a lower boiling point and can instantly vaporize and absorb a large amount of heat, allowing the surface of an object to cool down rapidly, and the local temperature can even drop below zero.

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02

Flammable, frostbite, slow evaporation...

There are many safety hazards of cooling spray

Although cooling spray does have a real cooling ability and can bring a sense of comfort on a hot summer day, there are pros and cons, and we cannot ignore the hidden safety hazards. For example: According to the introduction of the spray composition, no matter what kind of cooling spray it is, it contains a part of flammable gas (ethanol, menthol, or propane, butane, etc.), so you should be extra careful when using it and must stay away from fire; the organic substances in the spray are not only flammable, but may also cause skin allergies in some sensitive people; sprays containing ultra-low boiling points such as propane and butane can reduce the local temperature to below zero when used, and direct spraying may cause frostbite on the skin; in addition, some organic substances may still remain after using the spray for a period of time... Based on these issues, it is recommended that you pay attention to the following points when using cooling spray:

1. When using the cooling spray for 30 minutes, open flames are prohibited in the use area;

2. It is forbidden to store in high temperature environment above 50℃ to prevent overheating and explosion;

3. Do not spray directly towards the flame;

4. Do not spray directly on human skin to prevent frostbite;

5. It is strictly forbidden to use in confined spaces to prevent respiratory problems caused by inhalation of excessive volatile organic compounds.

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03

The cooling effect of the cooling spray does not last

If you are allergic to alcohol, you might try a small fan

Since cooling spray has so many safety risks, can it still be used? The answer is yes. However, considering the composition of the spray and the precautions for use, it is recommended to buy sprays with low alcohol content. For high-pressure liquefied gas (propane, butane) sprays, personal use is not recommended due to the high risk.

However, we should also maintain an objective judgment on the cooling effect of cooling spray. For example, some product advertisements even claim that their cooling spray can "keep cooling for several hours." Through the previous analysis of the principles and ingredients of cooling spray, it is not difficult to find that this statement is too exaggerated. Whether it is alcohol or other organic matter, the boiling point is relatively low, and it will generally evaporate completely within 30 minutes to 1 hour after spraying. After these substances evaporate, they no longer have a cooling effect, and the temperature will gradually rise.

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In fact, most cooling products on the market absorb heat through evaporation. Menthol and alcohol in cooling sprays are also used in other products, such as floral water, cooling oil and wet wipes, so these products can help us cool down and avoid the heat. If you are allergic to alcohol or menthol, you might as well prepare a small portable fan to carry with you and blow it anytime and anywhere on your way to and from get off work, which is also quite comfortable.

Author: Yuan Zhiqin, Associate Professor, Beijing University of Chemical Technology

Review | Xiong Bin Associate Professor, College of Chemistry and Chemical Engineering, Hunan University

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