Regional Jet 200 ER/LR (CRJ200)
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical
Air Traffic Control Issued New Clearance; Flight Crew Took Evasive Action; General Declared Emergency
Aircraft
The aircraft had two MEL's: the autopilot system and the Right Pack.  During departure and climb-out; the single pack bleeds open procedure was accomplished per the MEL. Shortly after accomplishing the procedure we had the Left Pack HI PRESS Caution message appear.  From what I can remember I had done the procedure just as it was written. When this occurred I quickly double checked my work. From what I could tell the 10th Stage Bleeds were properly configured per the MEL. Without doing anything to the switch positions the Left Pack HI PRESS Caution message was cleared. The captain was flying at this time with the Auto Pilot off.  A few minutes later we reached the cruising altitude of FL250. Shortly after reaching cruise I noticed the Cabin Alt. indication climbing approximately 700ft/min. The cabin altitude was around 6;700 -7;000 feet. I quickly looked up and saw that the Left Pack was off even though the switch position indicated that it was on. I immediately brought this to the attention of the captain. Within about 40 seconds or less the Cabin Alt. was approaching 8;000 feet. I immediately reached for the QRH. With only 2;000 feet left before we had to deploy the crew oxygen masks and the rate of climb being a steady 700ft/min I knew we didn't have much time. I suggested to the captain we start an immediate descent. He agreed and we began a descent after receiving a clearance from center. At this point the cabin altitude reached 10;000 feet. We then deployed the crew oxygen masks and declared an emergency. I performed the EMERGENCY procedure for CABIN ALT (Warning Message) or Emergency Descent Procedure. During this start of this procedure we were already around 17;000 feet and descending very quickly. I believe I noticed the rate to be around 4;500-5;000 ft/min in the descent. I was only able to accomplish the boxed items because I needed to communicate with the flight attendant and ATC. We reached 10;000 feet and leveled off. We removed the oxygen masks and the CABIN ALT WARNING message cleared. I then referred to and accomplished the CABIN ALT; Abnormal Procedure.  Furthermore; I accomplished the Unpressurized Flight Procedure (PACKS off). I believe the Cabin Altitude began to stabilize around 9;000 feet. At this time we were about 15 minutes from the airport. No further emergency procedures were accomplished. We briefed the Flight Attendant multiple times and the Captain also briefed the passengers.The root cause was due to an inoperative PACK system. This emergency was due to mechanical issues and therefore was unavoidable.
CRJ-200 First Officer reported loss of cabin pressure after they were dispatched with one pack inoperative; and the remaining pack failed at FL250.