temperatures when air lock doors are opened and when heated trays are present. The air locks are provided with a nitrogen purge gas supply. Exhaust gas from the air locks flows to the MPT BOX unit. The main chamber is heated by two 300-kW water-cooled induction coils wrapped on the outside of the main chamber wall. Each coil heats one of the two zones in the main chamber. Gases generated by the heating of the projectiles or waste in the main chamber are swept from the chamber using low-pressure, superheated 1000 °F–1200°F steam generated by the electrically heated MPT steam superheater. The steam and gases flow out of the main chamber through a header in the top of the chamber to the BOX unit along with purge gases from the inlet and outlet air locks. Ducting for all gases flowing to the BOX is insulated and trace heated to prevent a condensation of tars. In the BOX, the combustible gases generated in the MPT are oxidized and then flow through a cyclone before being discharged to the downstream equipment of the OTM. Additional MPT system details are provided in the following discussion of MPT system operation.
All agent-contaminated solid materials are fed to the MPTs in trays specifically designed to support the rapid and uniform heating of the materials. Because of the varying off-gas generation rates, the feeding of secondary waste, EBH residue, and projectiles into the MPT are controlled by software that, at any one time, allows the MPT main chamber to process only the same type of material in Zones 1 and 2. A zone is defined as a position within the main chamber where a tray is heated for a set period of time. Interlocks are provided to prevent the mixing of feed materials in the main chamber.
Before a tray is transferred into the MPT inlet air lock, the vent valve from the inlet air lock is closed, the air lock nitrogen purge valve remains open, and the main chamber in-gate remains closed. The MPT main chamber pressure is also lowered before the main chamber inlet gate, or in-gate, is opened. These actions minimize the intrusion of main chamber gases into the air lock. Gas flow generally is from the inlet air lock to the main chamber.
The inlet air lock in-gate is then opened and a tray is transferred into the inlet air lock; then the inlet air lock in-gate is closed. The air lock is continuously purged with nitrogen to reduce its atmosphere to below 3 percent oxygen. The estimated time to purge the inlet air lock is approximately 20 minutes. Because some air in-leakage from door seals is expected, the nitrogen purge is continuous. The inlet air lock is maintained at a negative atmospheric pressure of −0.25 in. water column