ACR MLG LOST CABIN AIRFLOW AT FL370. HAD TO DSND TO 11000 FT AND DIVERT TO AN ALTERNATE.

Narrative

DURING CRUISE FLT AT FL370 I NOTICED A NO MODE ANNUNCIATION. I LOOKED UP AT THE OVERHEAD PANEL AND OBSERVED THE AMBER NO FLOW LIGHT ILLUMINATED AND THE CABIN RATE OF CLB INDICATED THE CABIN WAS IN A CLB WITH A CABIN ALT OF APPROX 9500 FT. WE IMMEDIATELY SWITCHED FROM PRIMARY TO STANDBY ON THE PRESSURE CTLR. THE CABIN MOMENTARILY STOPPED CLBING THEN RESUMED. WE THEN SWITCHED TO THE MANUAL CTLR BUT COULD NOT STOP THE CABIN FROM CLBING. BY NOW THE CABIN WAS ABOVE 10000 FT AND THE CABIN ALT LIGHT WAS ILLUMINATED. WE DONNED OXYGEN MASKS. THE CAPT TOOK CTL OF THE ACFT WHILE I COORDINATED AN IMMEDIATE DSCNT WITH ATC (ZDV) AND BEGAN RUNNING THE EMER CHKLISTS. DENVER IMMEDIATELY CLRED US TO 11000 FT. AFTER WE WERE LEVEL AT 11000 FT AND ALL CHKLISTS WERE COMPLETE I RESUMED FLYING THE ACFT WHILE THE CAPT MADE A PA ANNOUNCEMENT TO TELL THE PAX WHAT HAD HAPPENED. WE WERE VECTORED TO A VISUAL APCH TO 17R AT DENVER AND LANDED WITHOUT FURTHER INCIDENT. WE HAD FLOWN THIS ACFT FROM MSP TO LAS AND WERE ENRTE BACK TO MSP WHEN THIS INCIDENT OCCURRED. ALL NIGHT LONG WE HAD BEEN HAVING PROBLEMS WITH THE AIR CONDITIONING SYS. EVEN IN FULL HOT THE PAX AND CABIN CREW COMPLAINED THAT IT WAS TOO COLD. THE CABIN DUCT TEMP WAS READING 150 DEGS (TOP OF THE GAUGE). I THINK THAT WHAT HAPPENED WAS THAT THE SUPPLY DUCT TEMP ROSE ABOVE 190 DEGS AND WE EXPERIENCED A SHUTDOWN OF THE AIR CONDITIONING PACKS. SINCE THE THERMAL SWITCH SHUTS DOWN THE PACKS AT 190 DEGS DUCT TEMP I THINK IT WOULD BE HELPFUL IF THE DUCT TEMP GAUGE READ UP TO 190 DEGS RATHER THAN JUST 150 DEGS.