Vacuum distillation plant

[jmgurgoiti]

Hi.

I'm looking for information about vacuum distillation plants for ships.

Does anyone know where I can find any technical manuals?
I am interested on controlling the process above all.

Thanks a lot for your attention!!

[ArkSeaJumper]

There is a lot of information out there if you Google 'Fresh water Generator'

Such as:
http://www.separationequipment.com/watergenerators.htm

http://www.wisegeek.com/what-is-a-fresh ... erator.htm

For the most part we use waste heat, and the vacum is generated by edjuctor. There is very little control, mostly, once you have the vacum up (down?) crack on the heat and hope not to lose the vac.
The main control is the salinity level, this level can range depending on use, ie human consumption or steam boiler.

IMHO Alfa Laval make the best and simplest

[conrod]

If you are studying " old school systems " try Butterworths, Marine Auxiliary Machinery, it very clearly describes what we used to just call the vap. But all said and done, its just simple physics, water boils at a lower temperature in a vacuum (or as the book refers - sub-atmospheric pressures).

Reed's General Engineering Knowledge for Marine Engineers also has a small section, with very simple sketches.

From my time deep sea, I recall it being quite the art to get the " vap " flashed up, tweaks here and there to get the balance between heat, vacuum, and salinity............I guess modern systems, although operating on the same priciple, are a lot easier.

[ArkSeaJumper]

Yes I would agree,
The older units were a pain to get going, a little bit of an art form, to balance the heat and vac levels
The newer units are much simpler, and thus require much less fecken around.
They are much smaller for a similer output, and use plate heat exchangers.

[GILLES GAUTHIER]

When i was working on a steam ship, we had vap system, remember doing maths, cost of a glass 250ml of potable water was 5 cent in the 80's
happy hollidays

[The Dieselduck]

Serck Como had some light information on their website at one time, I am not sure if they still do. It was basic, but it was nice to see the diagrams and theory of operation.

They were bought out by Hamworthy, so now that is their website. I am not sure if they still have that info in there, but I could not find it.

The key to proper operation is proper chemical dosing, in my experience. Cannot stress this enough. Check out Ashland.com

[TxMarEng]

Check out Unitest Marine Software site. They used to have a great PC based simulation for the AlfaLaval NIREX unit which is pretty much the same as the ATLAS and SASAKURA distillation units. They now have one for reverse osmosis.

[Pacific Barbarian]

The most crucial part about FWG's is to keep the salinity as low as possible, especially if you use the distillate to feed a boiler

I have witnessed a case where the "engineers" did not do the keep a close eye on the chemical dosing and had the salinometer disabled. The plate type FWG was completely choked up with hard salts in the evaporator (strange they didn't notice the reduction in DW production ) and basically left it like this for about a 7 week round trip.

In the meanwhile the boiler was fed with "DW" in this condition. Salinometer on the hotwell was also not in working condition. Juniors were not testing the boiler water, for run out of testing chemicals.

Well, one thing led to another and had a boiler flame failure in port. Upon inspection of the burner unit in the furnace, when the unit was swung open, out came the flood of water!!!

Boiler had a melt down!

So, lesson to be learnt! Keep salinometers active, regularly descale the evaporator, test that boiler water.

[Big Pete]

The system is pretty much self regulating, if it is properly designed and installed.

The sea water pump cools the condenser and I would expect about 6to 10 C temperature rise across the condenser. Some of this pre-heated water is fed through a flow control valve into the heater of the evaporator shell, the rest goes through the ejector / eductor sucking non condensible gases and steam out of the top of the shell and brine (concentrated sea water) out of the bottom.
If the vacuum is too low there could be air leaking into the vap, or the ejector or the sea waterpump could be worn, or the steam output too great.
If the vacuum is low and the temperature rise in the condenser is higher than the drop in the heater the sea water pump could be worn, or steam output to great.
If the temp drop across the condenser is small and the vacuum low the condenser could be failing to condense the steam, because it is dirty.

Controls:
1)Usually the out put is regulated manually by a bypass valve between the hot water inlet and outlet to the heater, the vap is started with the valve fully opened. It is gradually throttled in to establish the correct temperature differential across the heater usually 6 to 10 C (see maker's manual).
2)The feed water flow control valve has to be overhauled or replaced according to manufacturers instructions if the feed rate appears to be wrong.
3)As other people have said the salinometer is absolutely critical and should be tested for correct operation of alarm and dump valve regularly. Readings should be compared with chemical chloride tests.
4)The only other control is the flow control for the dosing chemical, usually this is a ball in a conical space in a perspex block The viscous drag of the liquid flow past the ball will balance the weight of the ball. When the flow starts the ball will rise until the annular area between the ball and the edge of the cone multiplied by the velocity of flow is equal to the value marked on the scale at that point.

Correct Chemical treatment is very important to prevent scale build up. If you run out of chemical you should open the bypass valve on the heater and reduce the shell temperature, this will reduce ouput but also scale formation.
You will have to chemically clean the heat exchangers afterwards.

WARNING!!!
Last year I joined a ship where the evaporator had not worked for years. When we opened it up we found that all the brass nuts clamping the plates together had rotted out and the plates in both stacks were hanging loose.
At first I assumed that the nuts should't have been made of brass but then I realised that the nuts should be dry all the time and the material should not matter.
I then realised that when the evaporator was shut down, the vacuum had not been released, so it had totally filled the shell with sea water!!

It is very important to shut down in the correct sequence.
1) Stop distilate pump & close valves.
2)Open heater bypass fully and shut inlet & outlet valves,allow shell temperature to fall to sea water temperature.
3)Open vacuum release valve until the evaporator shell is at atmospheric pressure.
4) Stop sea water pump and immediatly close feed water valve then the other sea water valves.
Sometimes the vacuum release valve is included in the safety valve and just requires the operator to rotate a knob on the end of it.

Hope this helps anyone new to "rain makers".

BP