I am exploring various concepts to convert the typical main deck into a "Class C" compartment carrying cargo. Its for a new combi configuration and its my understanding that
during normal operation, air enters into the main deck cargo compartment and exits via the air grilles near the floor. During main deck fire fighting mode, the fresh air inflow has to be shutoff so that halon would not get diluted.

Following are some queries:

1. Should the air grilles in the main cargo compartment be shutoff as well to prevent halon from escaping into the lower lobes?

2. What is the most effective way of preventing halon from escaping out of the main cargo compartment - is there a simple way to shutoff the air grilles?

3. Can the air grilles be re-located to the ceiling since halon is heavier than air so they shouldn't escape readily?

If anyone out there has past experience or proven concepts to share, it'll be most appreciated.

I am not a Subject Matter Expert in this particular area, but I think I can provide an answer to ONE of your comments that will be of value to you.

You stated in item 3:

3. Can the air grilles be re-located to the ceiling since halon is heavier than air so they shouldn't escape readily?

I would say that is very unlikely to certify.  One of the primary purposes of those air grilles is to facilitate rapid airflow from the upper deck to the lower deck or vice versa in the case of an explosive decompression.  This prevents the build-up of destructive forces on the floor structure that could otherwise cause catastrophic failure of the airframe.

I suspect that you need to provide airflow into both compartments, so that either can be shut-off in case of fire.

the ECS (Air Cycle Machines) typically delivers air into the cabin through PSUs (Passenger Service Units), in the overhead bins.  thuse you have a distributed airflow into the cabin.  you might be able to group some PSU inlet plumbing together to give you a single point to valve (i guess you'll need only one close-off valve, since you'll only ever want to shut-down the cargo compartment ... the cabin will fight fires with the flight attendants<gate valves>).  but i think you'll need to provide an outflow for the cabin (assuming the original outflow valve is in the cargo compartment) since you'll need to isolate teh cargo compartment.  this implies that the cargo compartment should be reasonably isolated from the cabin ... i don't think it needs to be air-tight, but i think you need to demonstrate (by test) that your system delivers a volume of Halon and sustains a sufficient concentration for a sufficient time.

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