Vacuum Systems > STEAM JET EJECTORS
 

PRINCIPLES OF OPERATION
Steam jet vacuum ejectors are compressors that utilize high pressure steam as the motivating fluid to compress gas to atmospheric pressure from a sub-atmospheric pressure (vacuum) at the inlet of the system. Since the compression of a single ejector is limited, jet ejectors are commonly staged by connecting the ejectors in series to produce a lower pressure (higher vacuum) at the inlet of the system. The stage discharging to atmospheric pressure is referred to as the ‘Z’ Stage. The stage connected to the ‘Z’ Stage is referred to as the ‘Y’ Stage, the stage connected to the ‘Y” Stage is referred to as the ‘X’ Stage, and so on for W, V, and U Stages. Multistage systems often incorporate condensers between the ‘X’ and ‘Y’ Stages, the ‘Y’ and ‘Z’ Stages, and after the ‘Z’ Stage to lower motive steam requirements for the system and to reduce emissions to the atmosphere. Type 733, Type 741, and Type 756 Ejectors are the names for the ‘U’, ‘V’, ‘W’, or ‘X’ Stages, the ‘Y’ Stage, and the ‘Z’ Stage respectively. Type 733, Type 741, and Type 756 Ejectors are also referred to as boosters, high vacuum ejectors, and low vacuum ejectors respectively.

The high pressure motive steam expands through a specially designed nozzle that converts the pressure to a high velocity jet of steam at the exit of the nozzle. The high velocity steam entrains and mixes with the suction gas. The combined flow passes through the venturi tail where the gas is compressed to an intermediate pressure between the suction and motive pressures.

A single stage ejector is capable of providing a suction pressure of 50 Torr when discharging to atmospheric pressure. However, at a pressure of approximately 100 Torr or lower, and where cooling water is available, a multistage ejector system with a condenser between the stages is often used to lower overall steam usage. At pressures below 50 Torr condensers are almost always used between the ejector stages to lower motive steam consumption.

Barometric installation is required for gravity draining of the condensate from inter-stage condensers. Based on water drainage, barometric height is 34 feet (10.36m). However, a minimum of 38 feet (11.58m) is recommended to account for barometric pressure fluctuations, tail pipe pressure losses, and vena-contracta losses at the drain connection of the condenser. If barometric installation is not possible, specially designed low level operating systems are available.

APPLICATION
Steam jet vacuum ejectors provide a low cost solution for most applications requiring a vacuum. Applications for which vacuum ejectors can be used to provide the vacuum inside the metal degassing, freeze drying and vacuum drying operations, vacuum distillation, desalination, refrigeration, crystallization, and removal of air from turbine condensers.

Since steam jet ejectors have no moving parts, they may provide a reliable alternative to maintenance intensive mechanical pump systems where downtime could be costly. Steam jet ejectors can also be constructed of exotic metals.

CONSTRUCTION
The standard construction materials are steel and stainless steel. The ejectors can also be machined or fabricated in a variety of corrosion resistant materials such as Titanium, Hastelloy, Brass, Inconel, Teflon, Graphite, and FRP.

Suction and discharge connections are normally flanged. Motive steam inlet connections are threaded for small ejectors and flanges for larger fabricated ejectors.

In technical collaboration with Severn Trent Services