Steam & Air Vacuum Ejectors
Vacuum Ejectors are based on the ejector-venturi principal and operate by passing motive steam 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 stream 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.
Primary advantages over other vacuum pumps can be seen below:
- No Moving Parts - Ejectors are exceedingly simple and reliable. There are no moving parts to wear or break in a basic ejector.
- Low Cost - Units are small in relation to the work they do and cost is correspondingly low.
- Versatile - Various piping arrangements permit adapting to environmental conditions.
- Self Priming - Ejectors are self-priming. They operate equally well in continuous or intermittent service.
- Easy to Install - Relatively light in weight, ejectors are easy to install, and require no foundations. Even multi stage units are readily adaptable to existing conditions.
- Corrosion and Erosion Resistant - Because they can be made of practically any workable material, or coated with corrosion-resistant materials, ejectors can be made highly resistant erosion and corrosion.
- High Vacuum Performance - Ejectors can handle air or other gases at suction pressures as low as 3 microns HgA.
Ejectors range from Single upto Six Stage units, and can be either Condensing or Non-Condensing types. The number of Ejector stages required are usually determined by the economy of the ejectors and the level of vacuum required. The operating range for each stage of Vacuum Ejector using motive steam can be seen below, also for reference 1 BarA = 760 mm HgA.
1st Stage : 810mm HgA - 30mm HgA
2nd Stage : 130mm HgA - 3 mm HgA
3rd Stage : 25mm HgA - 0.8mm HgA
4th Stage : 4mm HgA - 75 microns HgA
5th Stage : 0.4mm HgA - 10 microns HgA
6th Stage : 0.1mm HgA - 3 microns HgA
Air driven units tend to be limited to Single or Two stage units and are usually of the Non-Condensing types. Air Ejector are less efficient than steam driven units but provide a viable alternative when steam unavailable or where large quantities of unused compressed air.
Single Stage Ejectors
Single stage Vacuum Ejectors generally cover vacuum ranges from 30mm 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 gas 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.
Two Stage Ejectors
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 3mm 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, or a Three Stage Ejector System if conditions are at the lower end.
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.
Two stage systems can also be either Condensing or Non-condensing types depending upon the motive gas used. Condensers can be used as pre-condensers, inter-condensers, and after-condensers, all of which help to reduce the gas load being passed on to the next ejector stage. This helps to reduce motive consumption and also allows smaller ejectors to be used with in the system. Depending up on the application Non-condensing systems can also be used, however this can be less efficient than Condensing Types as each ejector must entrain the full gas load from the previous stage. This can lead to ejectors becoming large and also increases motive consumption. Non-condensing types are usually used where it is not feasible to install condensers, or where service is intermittent, making operating costs a secondary consideration.
Three Stage Ejectors
Three stage Vacuum Ejectors generally cover vacuum ranges between 0.8mm HgA to 25mm HgA, however depending up on actual operating conditions a Two Stage Ejector system may be more economical if at the upper limit of the operational envelope, or a Four Stage Ejector system if conditions are at the lower end.
In operation a Three Stage system consist of a primary Booster, a secondary High Vacuum (HV) Ejector, and a tertiary Low Vacuum (LV) Ejector. As per the Two Stage System, 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 pull vacuum to the lower intermediate pressure. Finally the Booster is operated (in conjunction with the HV & LV Ejectors) to pull vacuum to the required pressure.
Three stage systems use steam as the motive gas and are also usually of the Condensing type. Again as per the Two Stage system, condensers can be used as pre-condensers, inter-condensers, and after-condensers in order to reduce the gas load being passed on to the next ejector stage. Depending up on the application Non-condensing systems can also be used however this is less efficient than Condensing Types as each ejector must entrain the full gas load from the previous stage.
Four, Five & Six Stage Ejectors
These systems are similar to Three Stage Systems, however they include additional boosters which are equipped with Steam Jackets to prevent ice forming with in the ejectors. These systems are usually of the Condensing type to increase efficiency and reduce motive steam consumption.