We all know how important engine room piece of equipment is the Fresh Water Generator (FWG), as it converse seawater into freshwater by vacuum distillation for the supply of high quality freshwater for domestic and process utilization. About this equipment I have written a previous post which can be found by following this link.
A very important part of this equipment is the brine/air ejector. Ejectors are hydro-pneumatic generators with motor fluid that utilize the transfer of mass and energy between an agent with a high energy potential (pressure) and the working fluid, which has a lower pressure.
The ejectors have a disadvantage in that they produce poor yields, but they have an advantage in that they are highly reliable because there are no moving components in them.
The fundamental idea behind how an ejector accomplishes its work is to transform the potential energy stored in the pressure of high-pressure motive fluid into velocity.
After that, work is done on the suction fluid with high velocity fluid that has been released from a motive nozzle. This work takes place in the diffuser inlet as well as the suction chamber. After then, the leftover energy from the velocity is converted into pressure on the other side of the diffuser. In layman’s words, high-pressure motive fluid is utilized to raise the pressure of a fluid that is currently at a pressure that is significantly lower than the pressure of the motive fluid.
According to the laws of thermodynamics, a high velocity can be attained by adiabatically expanding the motive fluid over the converging/diverging motive nozzle, which lowers the pressure of the motive fluid relative to the pressure of the suction fluid. At the exit of the motor nozzle, the supersonic velocities that are caused by the expansion of the fluid over the nozzle are seen. The converging section of a diffuser reduces velocity as the cross-sectional area is reduced. The diffuser throat is designed to create a normal shock wave. A dramatic increase in pressure occurs as flow across the shock wave goes from supersonic, to sonic at the shock-wave, to subsonic after the shock wave. In a diffuser diverging section, cross-sectional flow area is increased and velocity is further reduced and converted to pressure.
Fluid pressures that are at or above a predetermined minimum have been designed into the system by the maker to ensure that it continues to operate in a steady manner. If the pressure of the supply of motive fluid drops below what was designed for, then a motive nozzle will let through less fluid. In the event that this occurs, the ejector is not supplied with an adequate amount of energy to compress the suction fluid to the discharge pressure that was designed for it.
It is possible for an ejector to function in an unstable manner if it is not provided with an adequate amount of energy to enable compression to its design discharge pressure. Dramatic shifts in the ejector’s operating pressure are one of the telltale signs of unstable ejector functioning.
The Brine Ejector is a device that has been developed specifically to be used with seawater in its operation. It is used for brine elimination from the lower side of the FWG shell and for creating and maintaining the necessary vacuum required for FWG to boil the water by using the jacket water temperature of 70 – 90 ºC and shell temperature of 40-60 ºC.
By forcing sea water to pass through the air/brine ejector and sea water supplied by the ejector pump to be delivered to the ejector, the combined air/brine ejector is able to produce an evaporator chamber vacuum condition. This allows the brine (concentrated seawater) and air to be ejected from the evaporator chamber.
Although ejectors are very reliable, it might happen that sometimes will underperform (poor ejection performance) due different reasons. These reasons mainly are:
- Lower than designed motive fluid (sea water) pressure – obviously in this case there is something wrong with the sea water ejector pump and need to be investigated (fouled filter, worn impeller, damaged mechanical seal etc.)
- Higher than designed motive fluid (sea water) pressure – outboard valve need to be checked, same as outlet piping for any obstacles. If everything is ok the motive fluid nozzle can be replaced with another nozzle designed for a higher pressure.
- Damage of the suction chamber – same need to be opened and investigated for any abnormalities (cracks, corrosion, cavitation etc.). It might be temporary repaired, by using bronze putty or ceramic putty, if spare parts are not available.
- Damage of the nozzle due corrosion or foreign objects – nozzle need to be replaced.
- Damage of the diffuser – same as suction chamber can be temporary repaired, by using bronze putty or ceramic putty, if spare parts are not available.
As part of their maintenance, as ejectors have no moving parts in contact with the process fluids and, as such, they are reliable and require no maintenance.
If you have any questions regarding above, please feel free to use our existing forum Seafarer’s World and will try to answer to all your queries. You can use the feedback button as well!
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Source and Bibliography:
- YouTube video training credit – GEA Group
- Photo credit: Alfa Laval and chiefengineerlog.com