A B727 CREW EXPERIENCED PRESSURIZATION PROBS DURING CLB OUT OF CHICAGO; RETURNED TO DEP ARPT.

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IT WAS A HOT DAY. ACFT HAD R PACK DEFERRED AND OUR FLT PLAN WAS FILED FOR A 1-PACK OP. THE REMAINING PACK HAD RECENTLY BEEN WRITTEN UP AND SIGNED OFF; SO WE BRIEFED IT AND KEPT A CLOSE WATCH ON IT. AN AIR CONDITIONING CART WAS HOOKED UP TO THE ACFT WHILE AT THE GATE AND CABIN TEMPS WERE COMFORTABLE. AFTER ENG START; THE CABIN WAS RPTED AS EXTREMELY WARM. HOWEVER; WE HAD BRIEFED THE FLT ATTENDANTS OF THE 1-PACK OP. DURING TAXI OUT; THE SUPPLY DUCT INDICATED WARMER THAN NORMAL. CREW AGREED IT SHOULD COOL DOWN ONCE AIRBORNE. AFTER DEP; IT WAS IMMEDIATELY NOTICED THE CABIN WAS FAILING TO PRESSURIZE AND THE OUTFLOW VALVE WAS CLOSED. WE CONTINUED TO CLB THE ACFT AS CREW MONITORED CABIN ALT AND TRIED TROUBLESHOOTING. WE HAD SELECTED FULL COOL WITH THE CABIN TEMP SELECTOR. IT WAS NOTED THAT THE SUPPLY DUCTS WERE VERY WARM; WELL ABOVE NORMAL FOR THE COOL SETTING SELECTED. BTWN THE CABIN TEMP CONTINUING TO GET WARMER; NOT BEING ABLE TO OBTAIN COOL DUCT TEMPS AND UNABLE TO PRESSURIZE; I ELECTED TO RETURN TO OUR DEP ARPT. WE NOTIFIED ATC; OPS AND FLT ATTENDANTS; WHO SAID IT WAS EXTREMELY WARM IN THE CABIN. WE RECEIVED CLRNC AND VECTORS BACK. AS WE DSNDED; THE FO THOUGHT HE HAD SMELLED SOMETHING FAINT WHICH FURTHER CONFIRMED OUR DECISION OF RETURNING TO OUR DEP ARPT. WE CONTINUED INBOUND AND WHEN WE WERE ON FINAL; THE FLT ATTENDANT IN THE REAR SAID SHE SMELLED A FAINT ODOR ALSO. WE LANDED UNEVENTFULLY AND DEPLANED THE PAX. WE LATER FOUND OUT FROM MAINT THAT THEY HAD TO REPLACE THE L AIR CYCLE MACHINE.