Example 3- Figure 3A

A battery enclosure using low vents and a riser tube is shown in  Figure 3A & 3B. This type of ventilation system makes an attempt to  use the properties of natural convection as described in Example 2.  Placement of the inlet and outlet vent tubes at the same height (low on  the wall) prevents some of the unwanted thermal convection described in  Example 2 which causes too much air flow and excessive temperature  extremes inside the box.

Since Hydrogen tends to rise inside the box, this system works  against the natural forces at work in the battery box. A high vent is  really needed to effectively remove hydrogen from the top of the box by  natural convection.

Figure 3A- Hydrogen Ventilation Using Low Vents and Riser Tube, Cold Day Convection Trap.

Figure 3A shows that on a cool day, the lighter warm air and lighter  Hydrogen will rise together to the top of the box and be trapped there  creating a very dangerous explosion hazard. Natural diffusion of the  hydrogen will cause it to mix with the air throughout the box. Neither  the warm air nor the light-density hydrogen gas will sink down the riser  tube by natural convection. 

Example 3-Figure 3B

Hydrogen Ventilation Using Low Vents and Riser Tube, Warm Day Hydrogen Trap.

Figure 3B shows that on a warm day, when the inside temperature is  cooler than outside, air flow through the vents may occur, driven  entirely by the temperature difference between the inside and outside  air. The Hydrogen will tend to rise and be trapped in the top of the  box. Natural diffusion will spread the hydrogen throughout the box and  the airflow on a warm day may dilute it, but not with any reliability or  efficiency. Under the right conditions, this system can cause explosive  levels to be reached inside the box.