Steam Jet Ejectors
Steam jet ejectors are a simple alternative to mechanical pumps for vacuum - raising applications. The steam jet ejector utilizes high pressure steam to compress low pressure vapours or gases.
The major advantage of ejectors over vacuum pumps is the fact that they have no moving parts and are thus require little maintenance. They also require very little space and are easily and cheaply installed. However an ejector has to be designed for specific conditions of pressure both for the motive fluid and the vacuum conditions, and its efficiency falls off rapidly outside these conditions, see disadvantages below.
High pressure motive steam at pressure P0a expands through a nozzle and exits at low pressure and at high velocity. The low pressure induces a flow of load vapours at the suction pressure (P0b) into the ejector. The two streams ie the low pressure - high velocity steam from the nozzle and the entrained load vapours mix as they converge into the throat of the ejector. Upon leaving the throat, the gases slow down and regain pressure to a pressure P03.
The load gases have been compressed from their original pressure P0b to a new pressure, P03. We define the compression ratio for the ejector as P03/P0b. The suction pressure to motive pressure ration is P0b/P0a
If Wa kg/hr motive steam are required to entrain and compress Wb kg/hr of load vapours, we define the entrainment ratio for the ejector as Wb/Wa.
Using the ratios defined above and the graph below a preliminary estimate of the ejector specification can be done. Please request a detailed specification and quotation
In the case of a batch vaccum pan the pressure conditions change widely over the pan boiling cycle. So an ejector should not be specified for batch pan vacuum systems. In a sugar factory, ejectors are better suited to evaporator and continuous pan vacuum raising duties. But is must be borne in mind that the vacuum conditions in contious pans and evaporators change slowly as the heatings surfaces become fouled over time. And if the ejector is sized for clean conditions it may not work so well just before the vessel is due for cleaning.
If leaks develop in the vacuum vessels and the volume to be removed increases substantially the performance of the ejector will probably decrease to the point where an early stop to find and repair the leak is essential. A liquid ring vacuum pump
Steam jet ejectors are noisy, noise originated both from the body of the ejector and from the jet of vapours leaving the discharge nozzle. Silencing by encasing the ejector body in acoustic insulation and by means of a silencer on the discharge is essential.
Steam jet ejectors are not energy efficient, Liquid ring pumps are more energy efficient than ejectors by a factor of about three under the conditions used in pans and evaporators.