11.   The purpose of a sealed length of pipe is to hold trapped air on the inside.

12.   The system has to be bled to allow the trapped air bubbles to escape.

13.   The trapped air will be forced out through the air vent, the small metal cylinder, on the side of each radiator by the incoming steam.

14.   The trapped air is really the insulator, because it blocks heat losses that normally occur from convection.

15.   When this happens the trapped air will remain in the radiator and keep the hot steam out.

16.   When plastic butter is creamed or beaten, it incorporates air, and the trapped air cells expand and produce volume during baking giving cake its texture.