AN MD80 FLT CREW RPTED THAT THEY EXPERIENCED A PRESSURIZATION FAILURE DURING A FLT FROM DFW TO SAT; WHICH CAUSED THEM TO DECLARE AN EMER WITH ZHU AFTER WHICH THEY MADE A NORMAL LNDG AT SAT.

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WHILE AT CRUISE AT FL350; THE #4 FLT ATTENDANT CALLED THE COCKPIT AND ASKED IF WE COULD WARM UP THE BACK A BIT. THE FO MADE A SMALL ADJUSTMENT TO THE R TEMP CTL; THEN CHKED THE CABIN AND SUPPLY TEMPS. THE CABIN TEMP WAS NORMAL; BUT THE SUPPLY TEMP WAS PEPPED AT 150 DEGS. THE FO THEN ADJUSTED THE CONTROLLER TO A COOLER SETTING; WATCHING THE VALVE MOVE TO A COOLER POS. THE SUPPLY TEMP REMAINED PEGGED AT 150 DEGS. THEN THE FO NOTICED BOTH FLOW GAUGES SHOWED ZERO FLOW AND THE CABIN ALT WAS RISING THROUGH APPROX 8500 FT. WE IMMEDIATELY REQUESTED A DSCNT FROM CTR. CTR REPLIED WITH 'STAND BY.' AT THIS TIME WE DECLARED AN EMER AND INFORMED CTR WE WERE DSNDING BECAUSE OF LOSS OF PRESSURIZATION. CABIN ALT AT THIS TIME WAS APPROX 9500 FT WHEN WE INITIATED THE EMER DSCNT PROCS. CABIN ALT CONTINUED TO CLB DURING OUR DSCNT UNTIL WE REACHED EQUILIBRIUM AT APPROX 15000 FT CABIN ALT; THEN THE CABIN DSNDED WITH THE AIRPLANE. THE CABIN OXYGEN MASKS DID DEPLOY AT APPROX 14000 FT CABIN ALT. WE CONTINUED DSCNT TO AN UNEVENTFUL LNDG AT SAN ANTONIO; TAXIED TO A GATE; WHERE ALL PAX DEPLANED UNDER THEIR OWN PWR AND APPEARED CALM AND UNHARMED.