Air Ejector Theory
It works on the principle of convergent /divergent nozzle as it provides the venturi effect at the point of diffusion as the tube gets narrows at the throat the velocity of the fluid increases and because of the venturi affect it pressure decreases, vacuum will occur in the diffuser throat where the suction line will be provided.
An air ejector which uses the high pressure motive fluid such as air or steam to flow through the convergent nozzle the function of the convergent nozzle is to convert the pressure energy of the motive fluid into the velocity energy.
As in convergent nozzle the following effect takes place,
P1-pressure of the fluid entering the nozzle.
V1- velocity of the fluid entering the nozzle.
P2- pressure of the fluid leaving the nozzle.
V2- velocity of the fluid leaving the nozzle.
By Bernoulli’s theorem:
P1 × V1 = P2 × V2.
As the pressure energy before entering the convergent nozzle is greater and the velocity is less for the fluid. At the point of discharge the pressure energy is converted into the velocity so the velocity will be greater and the pressure will be less during the discharge.
Divergent nozzle the opposite effect takes place velocity energy is convert into pressure energy, at the point of diffusion there is a low pressure or vacuum is created which is used to suck the other fluid for the motion.
In the fig: C- Convergent Nozzle.
D- Divergent Nozzle.
V- venturi Point or Diffuser Point.
Appliction of Air Ejectors:
1.Steam Jet Air Ejector:
It is one of the types of air ejector which is used in the steam like near the condenser to remove the non condensable gases and some vapour entering into main condenser by an air ejector and it is cooled by the main condensate and released in the ejector condenser.
A steam is used as the motive fluid to withdraw air and dissolved gases from the condenser by the ejector action. In each stage of the steam jet ejector, high pressure steam is expanded in a convergent /divergent nozzle. The steam leaves the nozzle at a very high velocity in the order of 1220 m/s and a proportion of the kinetic energy in the steam jet transferred by interchange of momentum to the body of air which entrained and passes along with the operating steam through a diffuser in which the kinetic energy of combined steam is re-converted to pressure energy.
The maximum pressure ratio that can be obtained with a single stage is roughly 5:1 and consequently it is necessary to use two or even three stages in series to establish a vacuum in the order of 724mm of Hg with reasonable steam consumption.
There are a variety of ejectors designed in service which work on the same principle. Older unit have heavy cast steel which serves as a vapour condenser and also contains diffusers. These are arranged vertically the steam entering at the top. More recent design has the diffuser arrangement externally and vapor condenser shell is some what lighter in construction.
Horizontal singe element two stage air ejectors this unit comprises a stack u-tubes contained in a fabricated mild steel condenser shell on which is mounted a single element two stage air ejector.
The condensate from the main or auxiliary condenser is used as the cooling medium. High velocity operating steam emerging from the 1 st stage ejector nozzle entrains the non-condensable and vapour from the main condenser and the mix discharge into the inter condenser.
Most of the steam and vapour is condensed when it comes into contact with the cool surface of the tubes, falls to the bottom of the shell and drains to the main or auxiliary condenser.
The remaining air and water vapour are drawn into the second stage ejector and discharged to the steam drain tank and non-condensable gases are at last discharged to the atmosphere through vacuum retaining valve.
2.Fresh Water Generator:
The next main application of the air ejector in marine field is in fresh water generator as it is used to remove the air and non condensable in the evaporator chamber so as to maintain the vacuum inside the chamber. Thus the efficiency of the generation increases at low temperature of the sea water.
3. Self Priming of Centifual Pumps:
It is also employed in priming of the centrifugal pumps by the air ejector, which removes the air inside the casing of the pump by the suction effect created by the air ejector thus by flooding casing with the liquid so that it helps in stating of the pump.