I theorize that once the water molecules are "attached" to the silica gel, the gel provides a "heat sink" that prevents the water molecules from warming up enough to boil away.

As far as buying a compressor to run a pneumatic vacuum pump, I personally feel that this is really serious false economy. You can make a really decent vac pump for $15 out of an old refrigerator compressor. You can buy a small Robinaire used for $100. Either of these will do a MUCH better job than a venturi pump.

My $0.02,