Conditions in which an air should be preferred over steam as motive fluid for ejector
1. Low capital cost required for the relevant infrastructure:
In plants where a steam supply is absent, installation of a steam ejector will require an erection of steam supply apparatus.
This steam supply will include a boiler apparatus, condensers and a water treatment plant which will raise the total capital cost. This infrastructure will also create a large space requirement.
Whereas an air supply will only require an air compressor. Air compressors are generally available in almost all industrial plants.
Other than a compressed air supply Air ejectors just need the inlet and outlet lines, no extra equipment such as cooling water connections, condensers are required. All these factor help to keep the overall capital cost low.
2. Low cost in case of Corrosive media due to different materials of construction:
In case of steam as the steam expands in the nozzle a part of it condenses due to release of energy. The liquid phase mixes with the process media inside the ejector and the wet process media is then discharged from the ejector.
If the process media is corrosive, the wetting leads to formation of acid inside the ejector and some of this acid is also discharged from the ejector. The acid being in liquid phase has a severe corrosive tendency compared to it being in gas phase.
In case of air ejectors the chances of formation of acid are eliminated, hence the corrosive activity of the process inside an air ejector is considerably less than in a steam ejector.
Hence the material of construction in case of steam ejector has to have a higher corrosion resistance than an air ejector.
The higher corrosion resistance material increases the capital costs for Steam Ejector and can make an Air ejector a more cost effective proposal.
3. Where the process media can be let off to the atmosphere:
Air in an ejector can be discharged to the atmosphere directly as it isn’t contaminated in an ejector.
If the process media is also air or any other medium that can be discharged directly into the atmosphere, an air ejector can turn out to be a better solution as a steam ejector in any case will require an after-condenser to separate the steam from process which will further add to the costs.