FLC OF ACR MLG ACFT LOST CABIN PRESSURIZATION DURING CLBOUT AND RETURNED TO LAND.

Narrative

FLT TOOK OFF FROM IND BOUND FOR MCI AND ALL SYS WERE NORMAL. AT APPROX FL190; CABIN PRESSURE ALT BEGAN CLBING RAPIDLY. FO ATTEMPTED TO CTL CABIN ALT; WHILE I SIMULTANEOUSLY ASKED CTR FOR LEVEL OFF AT FL200. WHEN IT BECAME APPARENT THAT WE WERE LOSING PRESSURIZATION; I IMMEDIATELY ASKED FOR AND RECEIVED A DSCNT CLRNC TO 14000 FT. WE MADE A REASONABLY AGRESSIVE DSCNT TO THAT ALT. WHILE DSNDING TO 14000 FT; CABIN ALT EXCEEDED 14000 FT; SO ALL THE PAX OXYGEN MASKS DEPLOYED. SINCE WE WERE AT A 'BREATHABLE ALT;' I INSTRUCTED THE PAX NOT TO DON THE OXYGEN MASKS. ALL APPLICABLE CHKLISTS WERE RUN AND THE DECISION WAS MADE TO RETURN TO IND IN AN UNPRESSURIZED CONDITION. AT THAT POINT; I REQUESTED 10000 FT FROM CTR AND RECEIVED CLRNC TO DSND. SINCE THERE WAS NO APPARENT STRUCTURAL PROBS AND CTR WAS ACCOMMODATING ALL OF OUR REQUESTS; I DID NOT DECLARE AN EMER. RETURN TO IND AND SUBSEQUENT LNDG WERE BOTH NORMAL. WE RETURNED TO GATE AND 'WRITE UP' THE AIRPLANE. WHEN WE FINALLY ARRIVED IN IND; MECHS SHOWED US THAT WE HAD 'BLOWN' A PNEUMATIC/AIR CONDITIONING DUCT BEHIND THE REAR PRESSURE BULKHEAD.