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Influence Of Natural Ventilation Of Structures

In general, the accidental release of hydrogen in confined environments will be affected by ventilation streams coming from other rooms or from the atmosphere. As a general safety rule: “Any structure containing hydrogen system components shall be adequately ventilated when hydrogen is in the system” (NASA:NSS:1740:16:1997). The amount of ventilation required will vary in each case depending on the total supply of hydrogen, the rate of generation, and the venting arrangement from the process or hydrogen system. The goal of any hydrogen ventilation system is to keep the concentration below the lower flammability limit (4% at normal conditions).

Ventilation systems use vents, ducts, heat exchangers, fans and other components. They are based on the principles of natural or forced convection as described above in this chapter. Under most common conditions, hydrogen has a density lighter density than air and tends to rise upwards when in contact with air. On the other hand, the air temperature affects the ventilation behaviour: warmer air is less dense than cooler air, and therefore warm air tends to push upwards when in contact with cooler air.

The use of a fan, as a forced ventilation system, has to supply approximately 25 times the volume of hydrogen to maintain a safe concentration of hydrogen. The reliability of this system depends on the eventual event of a mechanical or electrical failure.

In order to avoid these reliability problems, passive systems are usually established on hydrogen applications in enclosures. A typical configuration is based on using high and low vents on walls. Under most common conditions, hydrogen has a lighter density than air and tends to rise upward when in contact with air. Moreover, warmer air is less dense than cooler air, and so warm air tends to push upwards when in contact with cooler air. In general, a ventilation system is driven more by air temperature differences than by hydrogen concentration, and can be affected by the difference of temperature between the enclosure and the external environment.

On a cool day, when the inside temperature is hotter than outside, the lighter warm air mixed with the lighter hydrogen in the enclosure will rise together out the high vent, drawing fresh cool air in through the low vent. Both the temperature of the warm air and the presence of hydrogen will drive the ventilation rate. Under these conditions, the hydrogen amount in the enclosure will decrease. On a warm day, the direction of air flowing through the vents can reverse. When this happens, warm air trying to enter the top vent pushes back the hydrogen trying to rise out the same vent, causing the hydrogen to stagnate and build up inside the enclosure. On this condition the hydrogen is trapped within the enclosure and the molecular diffusion is the only mechanism to mix the hydrogen. Therefore, the explosive conditions could be not avoided.

Other systems use tubes as a small chimney that catches the hydrogen at some elevation. The use of these tubes in combination with low or high vents, being set at the same height on the wall, prevents some of the unwanted thermal convection described above. However, the combination of these systems has to be studied in detail, considering the effect of the external conditions in order to avoid failures in the ventilation behaviour under certain circumstances.


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Page last modified on December 04, 2008, at 06:29 PM