rrat- the reason for my pointing out the succinct answer at #8 was that it was almost instantly obvious that the OP had very little understanding of aircraft pressurisation - for everyone to launch off into complicated explanations of systems is all very interesting, but wasted on the OP. To me the question 'how can an aeroplane pressurise with the packs off' is a pretty broad hint.
This is a recurring event here - we recently had a question from someone who all could see plainly had not much idea of the effect of a moving air mass on an aircraft in flight but was presented instantly with a trigonometric solution to the query about the effect of wind on a curved flight path, when all that was needed was 'do you actually understand drift'?
bio and fred were plainly 'not understanding' hence my early replies to probe the level of knowledge.