B737-800
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical
Flight Crew Returned To Departure Airport; General Declared Emergency; General Maintenance Action
Aircraft
Departed with one A/C pack inoperative; after a few minutes at cruise the 'Pressurization Flow' light came on and the cabin was showing a slow (300 FPM) climb. After QRH procedures complied with still unable to stop the cabin climb. With outflow valve closed and manually locked; airfoil switches off; and pneumatic crossfeeds closed the cabin continued to climb.  We were at FL240 and descended to FL180 before the cabin climb rate slowed down. I knew we cannot make it to [destination] at FL180 so we elected to return to [departure airport]. We declared an emergency for the overweight landing.
B737-800 Captain reported pressurization problems after departure with one pack deferred. Flight returned to departure airport.