AN MD80 CREW; AFTER DEPARTING EWR; EXPERIENCED A LOSS OF CABIN PRESSURE; SPAWNING AN EMER DSCNT TO 10000 FT AND A RETURN TO EWR.

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FO HAD JUST LEVELED OFF AT FL240 WHEN THE FLT ATTENDANTS CALLED US AGAIN FOR THE THIRD TIME SINCE 10000 FT SAYING THEY WERE STILL FREEZING. I HAD ALREADY MADE 3 ADJUSTMENTS IN AUTO MODE TO WARM UP THE CABIN. I MADE THE NEXT ADJUSTMENT TO THE R PACK IN MANUAL MODE SO I COULD CTL AND SEE IF WARMER AIR WAS BEING DELIVERED TO THE CABIN SINCE IT WAS NOT HAPPENING IN AUTO MODE. WITH A SMALL INCREASE IN HEAT TO THE R PACK; I GOT A R PACK TRIP AND FLOW LIGHT 'ON.' THE L PACK WAS STILL IN AUTO; BUT COULDN'T MAINTAIN CABIN PRESSURE. THE CABIN STARTED TO RAISE SLOWLY. I HAD THE FO STOP CLB; WHICH HE DID THROUGH ATC AT FL270. WITH THE CABIN STILL CLBING; I ASKED ATC FOR A CLRNC TO 10000 FT BECAUSE OF PRESSURIZATION PROB. ATC GAVE ME A CLRNC DOWN TO FL180; BUT SAID IT WOULD BE A FEW MORE MINS BEFORE HE COULD GIVE ME LOWER. I DIDN'T WANT TO LEVEL OFF AT FL180 TO SEE IF I DID GET LOWER BEFORE I GOT TO FL180; SO IN THE INTEREST OF SAFETY UNDER THE CIRCUMSTANCES; I DECLARED AN EMER AND ASKED FOR AN EMER DSCNT DOWN TO 10000 FT; WHICH HE GRANTED. WE REACHED 10000 FT BEFORE THE CABIN OXYGEN MASK DROPPED. I INFORMED THE FLT ATTENDANTS WE HAD PRESSURIZATION PROB AND WE WERE DSNDING DOWN TO A LOWER ALT. ASKED ATC FOR A CLRNC BACK TO EWR AS A PRECAUTIONARY MEASURE AND A SAFE COURSE OF ACTION TO TAKE. WE DID NOT HAVE ANYONE HURT OR INJURED; LNDG IN EWR WAS NORMAL. ON THE RETURN TO EWR; I CALLED OPS AND MAINT ABOUT OUR SIT AS WELL; AND THE FLT ATTENDANTS AND PAX WERE INFORMED.