Air Jet Ejectors are based on the ejector-venturi principal and operate by passing motive air or gas through an expanding nozzle. The nozzle provides controlled expansion of the motive gas to convert pressure in to velocity which creates a vacuum with in the body chamber to draw in and entrain gases or vapours. The motive gas and suction gas are then completely mixed and then passed through the diffuser or tail, where the gases velocity is converted in to sufficient pressure to meet the predetermined discharge pressure.
Vacuum Ejectors are used in a variety of applications in the process, food, steel and petrochemical industries. Typical duties involve filtration, distillation, absorption, mixing, vacuum packaging, freeze drying, dehysrating and degassing. Ejectors will handle both condensible and none condensible gas loads as well as small amounts of solids or liquids, however accidental entrainment of liquids can cause a momentary interruption in vacuum but this will not cause damage to the ejector.
More recently Air Ejectors have been used for leak testing of under sea pipework, where high levels of vacuum are applied and held on several hundred kilometers of pipe for several days. For these applications several Air Ejectors (either single stage or two stage systems) are used in parallel in reduce the overall duty cycle time.
Primary advantages over other vacuum pumps can be seen below:
Air Jet Ejectors range from Single to Two Stage units. Using additional stages is possible, however the Ejectors rapidly become larger in size with each additional stage. This is purely due to being unable to use interstage condensors to remove a proportion of the gases from each stage, hence each subsequent Ejector must handle the entire discharge load of the preceeding stage.
Single stage Vacuum Ejectors generally cover vacuum ranges from 30-40mm HgA up to atmospheric pressure. To maximise performance eight different designs are available with each ejector being optimised to operate in a specific vacuum range. This allows the motive air consumption to be kept at a minimum for the selected ejector, and also ensures that operation will be stable. All single stage ejectors are designed to discharge either at or slightly above atmospheric pressure. Sizes range from 1 Inch to 6 Inch, however large size are available if required. Standard materials of construction are carbon steel or stainless steel, both of which are fitted with a stainless steel nozzle.
Staging of Ejectors is required for more economical operation when the required absolute vacuum level is reduced. Two stage Vacuum Ejectors generally cover vacuum ranges between 3-6mm HgA to 130mm HgA, however depending up on actual operating conditions a Single Stage may be more economical if at the upper limit of the operational envelope
In operation a two stage system consist of a primary High Vacuum (HV) Ejector and a secondary Low Vacuum (LV) Ejector. Initially the LV ejector is operated to pull vacuum down from the starting pressure to an intermediate pressure. Once this pressure is reached the HV ejector is then operated in conjunction with the LV ejector to finally pull vacuum to the required pressure.