A320 cargo smoke and ventilation doubt

[capt.anjum.chhabra]

Hi i want to clarify something about Airbus320 cargo smoke and ventilation...I am new to the aircraft... if anyone could help...

Air from the cabin goes via the inlet isolation valve to the aft cargo compartment, driven by an extraction fan...
Now lets say that there is smoke in the cargo compartment.. checklist says Aft isolation pb off..and then agent discharge...and land ASAP

ventilation chapter says air from cabin comes to cargo compartment with the help of extraction fan and then thrown outside...so that menas the same air is not circulated back....
Now putting isolation PB to off closes all the inlet and outlet valves and stops extraction fan..thereby isolating the cargo compartment...now if we close the valves the air is trapped and with the help of extinguisher it can be extinguished..
Lets say we do not close the isolation valve so in that case also smoke filled air cannot come in the cabin..and if extraction fan runs that smoke air will be thrown out..
Now are we closing the valves to deprive them of oxygen so that it dosent ignite more...or some other concept...

THanks if someone coulp put some light to it..
also in B747 SAA flight 295..investigation revealed that crew followed the checklist and there in that combi aircraft the cargo compartment air filled in the cabin also...somehting of that sort...i was watching the mayday episode...and somehow was confused with our aircraft...

Regards

[Dream Land]

(e) Class D. Cargo and baggage compartments are classified in the "D" category if they are so designed and constructed that a fire occurring therein will be completely confined without endangering the safety of the airplane or the occupants. Each Class D compartment must comply with the following:

(1) It must have a means to exclude hazardous quantities of smoke, flames, or noxious gases from entering any compartment occupied by the crew or passengers.

(2) Ventilation and drafts must be controlled within each compartment so that any fire likely to occur in the compartment will not progress beyond safe limits.

(3) It must be completely lined with fire-resistant material.

(4) Consideration must be given to the effect of heat within the compartment on adjacent critical parts of the airplane.

[TO MEMO]

No other concept there. Actually it`s a simple concept. Closing the valves you cut off the airflow and isolate the cargo compartment. After deploying the fire agent the agent/air mixture will keep constant and that will ensure that the fire will not ignite again. But as always there are some small leeks and the thruth is that the agent quantity will gradually decrease and also lose eficiency. That`s why you have a second bottle on ETOPS equipped airplanes with a meetering device that will keep on adding agent for the next 240 minutes (depends on the kind of bottle, ie time of ETOPS aproval) once activated, wich is usually 1 hr after the first bottle discharge.

hope that helps