EjectorsEjector is a type of pump that uses the Venturi effect of a converging-diverging nozzle to convert the pressure energy of a motive fluid to velocity energy which creates a low pressure zone that draws in and entrains a suction fluid.
- 1. The motive steam with high pressure and low velocity is supplied in the steam chest which exits the steam nozzle at design suction pressure and supersonic velocity. (Where velocity of the steam up to Mach number 3.0 or 4.0 is achieved by Converging & diverging nozzle).
- 2. The nozzle throat diameter is designed to control the amount of steam to pass through the nozzle at a given pressure and temperature.
- 3. And then motive stream is injected into a suction chamber. The suction chamber has the lowest static pressure (approximately equivalent to the suction pressure Ps).
- 4. As the combine flow of suction fluid and the motive fluid (steam) move through the converging section of the diffuser its pressure increases with decrease in its velocity.
- 5. At some point in the throat of the diffuser, their combined flow attains the exact speed of the sound. A stationary, sonic-speed shock wave is formed there which gives a sharp rise in absolute pressure. The shock wave in the diffuser throat changes the velocity from supersonic to sub-sonic.
- 6. The velocity of the mixture is sub-sonic in the diverging section of the diffuser. The increasing cross sectional area increases the pressure but further decreases the velocity.
- 7. The net result of these energy transformations is an increase of the absolute pressure of the mixture on discharge to several times the pressure at which it entered the ejector inlet.
Vaccum ejector or simply ejector
A vacuum ejector, or simply ejector is a type of vacuum pump, which produces vacuum by means of the Venturi effect.In an ejector, a working fluid (liquid or gaseous) flows through a jet nozzle into a tube that first narrows and then expands in cross-sectional area. The fluid leaving the jet is flowing at a high velocity which due to Bernoulli's principle results in it having low pressure, thus generating a vacuum. The outer tube then narrows into a mixing section where the high velocity working fluid mixes with the fluid that is drawn in by the vacuum, imparting enough velocity for it to be ejected, the tube then typically expands in order to decrease the velocity of the ejected stream, allowing the pressure to smoothly increase to the external pressure.The strength of the vacuum produced depends on the velocity and shape of the fluid jet and the shape of the constriction and mixing sections, but if a liquid is used as the working fluid the strength of the vacuum produced is limited by the vapor pressure of the liquid (for water, 3.2 kPa or 0.46 psi or 32 mbar at 25 °C or 77 °F). If a gas is used, however, this restriction does not exist.If not considering the source of the working fluid, vacuum ejectors can be significantly more compact than a self-powered vacuum pump of the same capacity.
Types of ejectors
Liquid jet ejectors
In a liquid Jet Ejector, the motive fluid is a non-compressible liquid (generally water) with no heat energy hence with a single stage centrifugal pump we can not increase its pressure much.
As a result motive fluid velocity at the nozzle tip is very less and it affects its non-condensable load handling capacity drastically.
Steam Jet Ejectors
In a steam jet Ejector, the motive fluid is a jet of high pressure and low velocity compressible steam which exists from the nozzle at the designed suction pressure and super sonic velocity thus entraining the vapour in to the suction chamber.
Due to the super sonic velocity of the steam at the tip of the nozzle, its non-condensable load handling capacity is much higher than liquid jet ejectors.
Combination of Both
In this type of ejector system, both Liquid jet and Steam jet Ejectors are employed simultaneously to produce low absolute pressure.
The suction of Liquid Jet Ejector is connected to the discharge of last stage of the steam jet ejector.
On basis of number of nozzles
Single Nozzle Ejector
- Single Nozzle ejectors can be used for either critical or non-critical flow, but usually preferred for one set of design conditions only.
- Its compact size is the main advantage of the single nozzle ejector.
- It is generally used where very large compression ratios are required.
Multi-Nozzle Ejectors
- The uniqueness of Shail Vac’s multiple nozzle ejector is in its design and performance.
- A multiple nozzle ejector usually has one nozzle on centre while remaining of the nozzles equally spaced peripherally around it.
- In most of the cases, multi nozzle ejectors designed for the same conditions usually reduce steam consumption by 10% to 15% as compared to Single Nozzle Ejector.
- The advantage of multiple nozzle ejectors is its much higher efficiency compared to single nozzle ejectors.
- It is also considerably shorter in length than an equally rated single nozzle ejector.
