11.   It has the disadvantage that it can be used only in closed systems, not in flows where fluid is continuously entering and leaving.

12.   And, they say that even mills with supposedly closed systems often discharge pollutants into the surrounding watershed.

13.   Because the firebox is a closed system, most of the energy released by burning the gas remains in the room.

14.   Because Kesterson is a fairly closed system and no longer receives drain water from San Joaquin Valley farms, it has no obvious contaminant sources.

15.   Besides, what could be worse than the cynicism bred by a closed system which essentially gags hundreds of thousands of voters?

16.   A closed heating system is not really closed, and picks up odor from the basement.

17.   A closed system would be self-sufficient and would not need to be resupplied with additional air or oxygen.

18.   A plethora of technologies enables open networks, like the Internet, to supplant closed systems, like cable outlets and TV shows.

19.   A tray of ice cubes is something of a closed system.

20.   But the firewalls around Web sites that allow for interaction by users, like downloading files, are more vulnerable than those protecting closed systems.