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Ventilation system design of hydrogen-related EMC darkroom

Release time:

2023-04-24 15:18

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Hydrogen energy is recognized as a clean energy, which has many advantages over other energy sources, such as: first, the energy density is high, and the calorific value of hydrogen is about three times that of oil; second, there is zero pollution, and the product is only water, which can decompose hydrogen again to achieve a virtuous circle; third, it is rich in reserves, which can be developed in seawater, and the reserves of seawater resources on earth are inexhaustible.

A hydrogen energy vehicle is a vehicle that uses hydrogen to provide energy. Its operating principle is mainly to convert hydrogen into energy through a fuel cell, and finally the battery gives the vehicle power. Hydrogen is stored in the hydrogen tank in the form of high pressure, which is compressed by the hydrogen compressor to the high pressure state required by the engine and enters the fuel cell stack. The fuel cell stack has the ability to dissociate hydrogen ions. The hydrogen ions enter around a carbon rod containing air, and an original charge generates a further current. Furthermore, the generated current is further converted into power. According to the generated current, the running speed of the car can be controlled, and the running power of the car will be further converted into the car's stroke and running noise will also change accordingly. Finally, when the car stops using, the hydrogen in the hydrogen tank will decompose through the fuel cell stack to produce oxygen, which is released in the exhaust pipe. Not only is there no pollution, but the water will turn the original oxygen into water. Under the condition of temperature, it will be converted into steam, that is, water vapor. It can be seen that hydrogen energy cars have no pollution at all during the fuel combustion process of the engine in the car room, it is a clean and pollution-free mode of transportation.

Compared with other energy supply vehicles, hydrogen vehicles have some advantages that other functional methods are difficult to match, such as low pollution, high efficiency, low cost, etc., but there are also such as large initial investment, high risk of use, and charging time. Disadvantages such as long and small hydrogen capacity.

Much of the public's concern about the safety of hydrogen, and even a general fear, stems from a famous air crash. On May 6, 1937, the Hindenburg Zeppelin, which was sailing from Frankfurt, Germany to the United States across the Atlantic, had an accident when it was about to land at a naval air station in New Jersey. It crashed in just over 30 seconds after the fire. The accident caused 35 crew members on the airship and 1 person on the ground, a total of 36 people died. Since commercial travel across the Atlantic at that time mainly depended on cruise ships, each cross-flight of Zeppelin always attracted a large number of journalists waiting to report at the landing site. When the air crash occurred, a total of four media carried more than 20 movie cameras and cameras arranged in all directions, waiting to take the landing images, and there were broadcasts broadcasting the grand occasion of the airship landing to audiences on both sides of the Atlantic. The sudden disaster was broadcast to a wide audience on both sides of the Atlantic, and the subsequent exposure of the accident images brought strong sensory and psychological stimulation to the public in the era when there was no television. Since Zeppelin provides buoyancy through hydrogen, its huge airbag is full of hydrogen. The huge hydrogen flame that appeared above the airship after the airship caught fire makes the disaster concrete and deeply buried in the hearts of the audience. From then on, the concept that hydrogen will bring huge disasters has become a public subconscious, which continues to this day and cannot be changed.

Although the image of the danger of hydrogen did not become public awareness until after the Hindenburg airship crash in 1937. The history of human understanding and utilization of hydrogen can be traced back to several centuries ago. As early as 1671 British chemist Robert. Boyle produced hydrogen in chemical experiments, the first time hydrogen was discovered and collected by humans. 1766 British scientist Henry. Cavendish identified hydrogen as an element. In 1783, the French chemist Lavoisier named the gas, which was previously called "combustible gas", Hydrogen, a combination of the Greek "composition of water. The Chinese word "hydrogen" takes the meaning of hydrogen "light" characteristics. In the same year, French scientist Jacques Charles invented the hydrogen balloon. In 1852 the French engineer Henri Giffard built the first airship. In 1898, British scientist James Dewer successfully liquefied hydrogen, and the following year produced solid hydrogen.

In 1900, the Zeppelin was born. After continuous improvement, the Zeppelin was put into commercial passenger operation. In total, more than 35000 passengers took the Zeppelin from 1910 to 1914. After World War I, the operation of the Zeppelin was stopped for a time due to the defeat of Germany, but the commercial operation of the Zeppelin was revived in the 1920 s and reached its peak through the transatlantic route in the 1930 s until the Hindenburg air crash. Although airships later replaced hydrogen with non-combustible helium to save the public's safety confidence crisis, the entire airship transportation industry was quickly replaced by emerging civil aircraft after World War II.

In China, the United States and Europe, pure electric vehicles are the mainstream development direction. Japan has put the treasure of the development of new energy vehicles on hydrogen fuel vehicles, with the highest enthusiasm for development, accounting for more than 90% of the world's hydrogen vehicle patents.

As we all know, hydrogen is the first element in the periodic table and the lightest element. The molar mass of a single hydrogen atom is 1.008g/mol, and the hydrogen molecule is composed of two hydrogen atoms with a molar mass of 2.016g/mol, so hydrogen is very light, only 1/14 the mass of air, and is also the lightest gas in the world. Hydrogen rises and diffuses at a very fast rate of about 20 m/s.

For the "test site requirements for fuel cell products-hydrogen leakage prevention" has been elaborated in the special article, this article is no longer wordy, the link is as follows:

　　http://www.emc-mall.com/news/151.html

It is precisely because the extremely fast rising speed of hydrogen can reach 20 m/s, just like the speed of a class 8 typhoon, the overall ventilation of the ordinary darkroom is not less than 5 times/hour, and the wind speed is only 2-5 m/s. Such ventilation design obviously cannot catch up with the rising speed of hydrogen itself, so hydrogen will rise to the top of the leakage point very fast. If the emission of hydrogen is not considered in the early stage, then within a short period of time, hydrogen will form a relatively strong accumulation at the top of the dark room.

Hydrogen is a combustible gas, and its upper flammable concentration limit is 75%, and its lower flammable concentration limit is 4% (volume ratio to air). The minimum ignition temperature is 573.6°C. The minimum ignition energy is very low, and it is one of the typical gases of the combustible gas group IIC. The combustion of pure hydrogen emits ultraviolet light, which is difficult to see with the naked eye in natural light. When the concentration reaches the flammable range and does not get a quick emission, the risk naturally comes.

The following are two comparative photos of simulated hydrogen (specific gravity lower than air gas) emission for regular darkrooms. The first is the aggregation in darkrooms with reflective cover plates, and the second is the situation in darkrooms without reflective cover plates:

Picture 1 Gathering of a darkroom with a reflective cover

Picture 2 Gathering in a darkroom without a reflective cover

It can be seen that the contrast difference between the two photos is very large. In picture 1, the aggregation is very serious, while picture 2 is much better.

The main reason for this is that under normal circumstances, the air flow rate in the dark room is extremely low compared with the hydrogen flow rate, which has no fundamental impact on the rise of hydrogen, and hydrogen cannot be driven to the ventilation waveguide window according to the preset to discharge the dark room. Even if strong exhaust and strong exhaust facilities are installed, the rise speed of hydrogen 20 m/s cannot be reached.

The fundamental reason why the gathering situation in the dark room without reflector is much better than that in the dark room with reflector is that the air in the gap between the wave-absorbing materials can flow horizontally, and the air inlet effect of the wave-guiding window under the ventilation effect of the wave-guiding window will form a thrust in the horizontal direction to introduce hydrogen into the ventilation wave-guiding window, thus reducing the gathering concentration of hydrogen, due to the height problem of the wave absorbing material, the main thrust formed by the exhaust wave guide window is in the vertical direction. In the horizontal direction, due to the existence of the wave absorbing material and the white cover plate, the flow rate is extremely low, and an effective horizontal thrust cannot be formed, resulting in the failure to discharge hydrogen in time, which is easy to cause the concentration to reach the ignition point range, and the risk is extremely huge.

In a regular darkroom equipped with ferrite, there will be relatively small mm-level gaps in the middle of ferrite modules. Strictly speaking, these gaps need to be glued and leveled to meet the requirements of the GB4962-2008 "Safety Technical Regulations for Hydrogen Use". The internal plane of the building top should be leveled to prevent hydrogen from gathering at the top. For darkrooms without ferrite, it is also possible to consider filling and plastering the gaps in the assembly of steel structures to ensure the flatness of the top plane.

The adoption of such a flat top plane design is based on the reason of driving hydrogen in the horizontal direction. Fundamentally speaking, it is a remedial measure rather than a fundamental solution. For the extremely fast rising speed of hydrogen, the best way is to design direct exhaust in the area directly above the hydrogen leakage point and in the area properly expanded, which is a more reasonable exhaust design, because of the extremely fast rising characteristic of hydrogen, the rising process is close to vertical rising without blocking, but it will only form aggregation and diffusion under blocking. Even if strong exhaust and strong intake measures are added, the rising channel of hydrogen will not be fundamentally trapped, because the air flow in the dark room is relatively unable to be completely controlled, on the contrary, the strong horizontal airflow is easy to disturb the rising channel and accumulation area of hydrogen, forming more risk points.

The regular solid wave absorbing material cannot design more emission wave guide windows in the area above the hydrogen leak point, and the lack of large-area wave absorbing material has defects both in appearance and performance.

Frankonia hollow absorbing material in this case can play its own characteristics, absorbing material can be designed as a honeycomb, forming a direct exhaust channel, the physical map is as follows:

For the area behind the wave absorbing material, a flat ventilation module can be installed to ensure that the internal plane of the building roof should be flat to prevent hydrogen from gathering at the top as required in the GB4962-2008 Safety Technical Regulations for Hydrogen Use. The physical diagram is as follows:

This design can ensure that most of the hydrogen directly enters the exhaust waveguide window when the hydrogen rises at a very fast speed, ensuring that the hydrogen concentration is not more than 4%, and ensuring absolute safety.

The hydrogen test results of Toyota's fuel cell vehicles also showed that although the flames leaking hydrogen were violent and frightening, the flames were all above the vehicles and the damage to the vehicles themselves was not significant. However, fuel leakage and combustion of fuel vehicles will often completely damage the vehicle, because the fuel will adhere to the vehicle body material, the flame will spread, and the materials encountered will be completely burned. Therefore, for wave absorbing materials, it is also very necessary to adopt a completely non-burning A2 level. Typical A2 wave absorbing materials only have high-temperature carbonization without any open flame during the combustion test with a 1000-degree flame gun.

The combustion results of a typical B2 grade absorbing material with a 1000 degree flame gun are as follows:

Therefore, reasonable ventilation design, coupled with the non-combustible characteristics of wave-absorbing materials, can ensure the safety of hydrogen-related laboratories.

Of course, due to the flammability of hydrogen (the minimum ignition energy MIE of hydrogen is 19uJ) and the very wide explosive concentration range, hydrogen is still a very dangerous gas. And hydrogen has a strong diffusion, easy to leak, easy to gather in a confined space to form an explosive environment, once the ignition source will form a strong explosion. Safety with hydrogen must not be taken lightly.

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