A B737-200 IN CRUISE AT FL310 DECLARED AN EMER AND DIVERTED DUE TO THE LOSS OF CABIN PRESSURE CAUSED BY A PARTIALLY OPEN RADIO RACK COOLING OUTFLOW VALVE.

Narrative

ENRTE FROM ATL TO PHX THE CREW DISCOVERED THAT THE CABIN WAS CLBING AT APPROX 100 FPM AND THE CABIN WAS JUST BARELY OVER 9000 FT. IT SHOULD HAVE BEEN AT 4100 FT. I CALLED FOR THE LOSS OF PRESSURIZATION CHKLIST. ALL ITEMS WERE COMPLETED BUT CABIN WAS STILL CLBING AT 100 FPM. THE INTERMITTENT PRESSURIZATION HORN WENT OFF AND I INITIATED AN EMER DSCNT TO 10000 FT AND RECEIVED VECTORS FROM ATC FOR DIRECT DFW AS THIS IS A LARGE ACR HUB AND A GOOD PLACE FOR REPAIRS AND PAX HANDLING. WE LANDED DFW AT XA00 AND ENDED UP CANCELING THE FLT. THE PAX WERE PUT ON ANOTHER CARRIER AND OUR ACFT TAKEN TO THE HANGAR FOR REPAIRS. WE DID DECLARE AN EMER TO DFW ATC. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED WHEN LOOKING AT THE PRESSURIZATION PANEL IT WAS NOTED THE CABIN HAD CLBED TO 9000 FT AND SLOWLY INCREASED 100 FPM. THE RPTR SAID AN EMER DSCNT WAS MADE AND ADVISED ATC OF NEED TO DIVERT TO DFW. THE RPTR STATED THE CAUSE OF THE PRESSURE PROB WAS THE RADIO RACK COOLING OUTFLOW VALVE WHICH IS NORMALLY CLOSED TO ALLOW WARM AIR HEATED FROM THE ELECTRONIC EQUIP TO HEAT THE FORWARD CARGO COMPARTMENT. THE RPTR SAID THIS VALVE WAS CRACKED OPEN ENOUGH TO ALLOW THE CABIN TO CLB 100 FPM.