What is steam jet vacuum system and how does it operate?

Answer 1

a steam jet vacuum system works according using a venturi following the principle described in the Bernoulli fluid flow equation
(see wikipedia article, link below)

A venturi can be use to pull a vacuum by forcing a fluid through a venturi (a tapering restriction that then tapers back out to a larger diameter). The small diameter area of the venturi is at a lower pressure than the inlet and outlet, and can be designed to be a very low pressure

http://en.wikipedia.org/wiki/Bernoulli's_principle

steam vacuum jets use steam as the motive fluid and are often use in industrial applications to provide vacuum

Answer 2

The high velocity of a steam jet through a venturi creates low pressure and entrains gas at the choke in the venturi.