Figure 69  General Gas 12 – Fumigation Vented Purge

At the completion of the fumigation exposure period, the system is switched to vented purge mode to safely remove residual phosphine gas from the grain and bin. The recirculation return valve is closed and the fresh air inlet valves are opened. The fan draws in fresh outside air and pushes it upward through the grain mass via the Supply Header. Fumigant-laden air exits the bin through the open roof vents and is safely discharged to atmosphere via the tall exhaust stack.The system operates in closed-loop recirculation mode. 


For pressure loss calculations the gas mixture behaves essentially as air. The purge stack and roof vents are open to atmosphere, providing a low-resistance discharge path. 

The headspace at the top of the bin is modelled with a fixed gauge pressure of 0.3 kPa (300 Pa) during purge simulation. This small positive pressure represents the static pressure developed by the fan that is transmitted upward through the grain column, providing the necessary driving force for flow to travel down the return riser and then up the tall stack to atmosphere.


A valve on pipe 14 is used to adjust system resistance so the fan continues to operate close to its Best Efficiency Point (BEP) during purging. 

The fitting named ‘DP Set’ with a K factor of 10 remains in the model to represent this balancing valve.