Water in an enclosed container will exert a pressure equivalent to the waters vapor pressure. The space above the liquid level is completely filled with water vapor. When a vacuum is pulled on the chamber, vapor is removed from the chamber. The vapor space now needs to be replenished with vapor. The water  forms vapor bubbles that replenishes the vapor space.


Boiling liquids need nucleation points in order to begin vapor formation. Vapor therefore begins forming on parts within the chamber. The best nucleation points are in crevices and tight areas where a high solid surface to liquid volume can be found. The interior of small lumen tubes is an ideal location for vapor bubble formation. The vapor bubbles begin to form throughout the lumens and liquid is forced from the interior creating fluid motion within the tube. Stopping vacuum reverses the process and fresh fluid enters the tube. This process is cycled until the tube is cleaned.