A DC9 IN CRUISE AT FL290 DECLARED AN EMER AND DIVERTED DUE TO LOSS OF CABIN PRESSURIZATION. CAUSED BY 2 FAILED PNEUMATIC DUCTS IN THE AIR CONDITIONING PACKS.

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WE NOTICED A PRESSURIZATION PROB INBOUND DURING DSCNT THROUGH FL220 TO 16000 FT. CABIN SUDDENLY CLBED FROM 6500 FT TO 9500 FT. THE OUTFLOW VALVE WAS CLOSED AND L AND R SUPPLY 25 PSI. AROUND 12000 FT; SYS NORMALIZED. I WROTE IT UP AND MAINT CTLR MEL'ED AUTOPRESSURIZATION. SYS OPERATED NORMALLY ON OUTBOUND LEG TO FL330. 50 MINS INTO OUR FLT; WE NOTICED CABIN ALT OF 8000 FT; BUT CLBING 200 FPM (ICE OFF). OUTFLOW CLOSED PACK OUTPUT 25 PSI. WE DID NOT LOCATE A COM PROC FOR THIS. ATC CLRED US FROM FL330 TO FL290. THE CABIN CONTINUED TO CLB UNLESS THE L AIR CONDITIONING SYS WAS ALMOST FULL HOT. PAX AND FLT ATTENDANT COMPLAINED SO WE LOWERED TEMP AND CABIN CLBED. ATC WOULD NOT GIVE US LOWER THAN FL240. AS WE RAISED TEMP ON CABIN; WE LOST L PACK AND LOST CABIN. R PACK WAS NOT DOING ANYTHING. WE PERFORMED EMER PROCS; BUT DID NOT REGAIN CABIN. WE DECLARED EMER AND DSNDED TO 10000 FT. OXYGEN MASKS IN CABIN DEPLOYED. WITHOUT PRESSURIZATION; ICE PROTECTION; AND TEMP CTL; WE DIVERTED. WE CHANGED ACFT AND CONTINUED. MAINT DISCOVERED THE ACFT HAD A BLOWN DUCT (2 LOCATIONS). I FEEL THE DUCT BLEW DUE TO A FROZEN WATER SEPARATOR BAG. THUS; WE OPERATED ON 1 PACK WHICH ALSO HAD A FROZEN WATER BAG. THAT IS WHY USING HIGH TEMP (BYPASSES BAG) HELD CABIN. I FEEL THE BAGS WERE PROBABLY DIRTY; WHICH CONTRIBUTED TO FREEZING AND PLUGGING. WX WAS 79 DEGS F AND HIGH HUMIDITY. HAD I KNOWN THE EXACT NATURE OF THIS PROB IN MSY; I WOULD NOT HAVE ACCEPTED THE ACFT.