Steam Jet Ejector
An ejector has two inlets. One to admit steam, and the other to admit the gas/vapor mixture to be evacuated. Motive steam, at high pressure and low velocity, enters the inlet 1 and exits the steam nozzle at design suction pressure and supersonic velocity, entraining the vapor to be evacuated into the suction chamber through inlet 2. The nozzle throat diameter controls the amount of steam to pass through the nozzle at a given pressure and temperature.
The entrained gas/vapor flow and the motive fluid (steam) flow mix while they move through the converging section of the diffuser, increasing pressure and reducing velocity. The velocity of this mixture is supersonic and the decreasing cross sectional area creates an overall increase in pressure and a decrease in velocity. The motive fluid slows down and the inlet gas stream picks up speed and, at some point in the throat of the diffuser, their combined flow reaches the exact speed of sound. A stationary, sonic-speed shock wave forms there and produces a sharp rise in absolute pressure. The shock wave in the diffuser throat changes the velocity from supersonic to sub-sonic.
Then, in the diverging section of the diffuser, the velocity of the mixture is sub-sonic and the increasing cross sectional area increases the pressure but further decreases the velocity. 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. The choice of the ejector system will primarily depend on the final vacuum to be achieved. The presence and, indeed, the amount of condensables in the gas stream will influence the choice of system, as will the operating costs and capital available.
Water Jet Ejector
Pressurized motive fluids other than steam, such as ethylene glycol, alkaline solutions or water, follow the same operating principles as steam ejectors. The system consist of closed circuit recirculation system for driving fluid. The vacuum produced is lower in this case (50 mm Hg). It can be used as acidic gas scrubber.
These are available in single & double stage and are oil sealed, rotary vane type positive displacement pumps where, not only complete assembly is immersed in oil but shaft seal is also designed in such a way that it totally eliminates the possibility of suction of air through it. These are designed to achieve maximum vacuum in a very short time and rise in temperature of the unit also remains under check, which in turn reduces the wear and tear of its components and makes it suitable for noiseless and trouble free performance for years and thus helps in reduction of repair bills.Compressor
These pumps are very compact and are so simple in design that all minor repairs, in case of need, can be carried out easily without removing the main housing from its base plate and without disturbing the alignment. In industrial models, or in vacuum pumps of higher capacities which are operated continuously for very long spells of time the oil gets heated either on account of friction or on account of heavy intake of hot mixture of gas and vapours, the performance of the pump drops. In order to keep it cool, a jacket is constructed around three sides of the pump housing, in which water is made to circulate for keeping the temperature of vacuum pump under control and retaining the performance at its best. In this model an inbuilt device, known as Gas Ballast is provided, which effectively checks to a large extent condensation of the water vapours which otherwise would not only had contaminated the oil, but had also diminished the performance of the pump.
These are normal compressors. The vacuum line is connected to the inlet of compressor (instead of air filter) and discharge is left to atmosphere. The vacuum produced is lower in this case (10 mm Hg). It can be used where the air contains water vapor (which will dilute oil in case of vacuum pump). It is inexpensive as compared to vacuum pump, for operation.