Steam loco with on board desalination?

[Talltim]

Was just thinking today about locos working in areas with little water and googling the African locos with huge condensing tenders when I started wondering if there had ever been any experiments with building a desalination plant into a steam loco? Obviously it would need a fair amount of extra kit, but one of the main issue with distillation desalination is the power consumption boiling the water, but in a steam loco you are doing this anyway.

[jjb1970]

A lot of modern desalination plants use reverse osmosis which is far more efficient and avoids the whole apparatus of boiling and condensing, usually requiring pressure vessels to boil the water under vacuum to reduce the temperature needed. In much of Africa the main problem seems to be availability of water full stop, you could put a desalination unit on a steam locomotive but if you used a traditional shell and tube evaporator it'd be very bulky and you also need a source of coolant for heat rejection when you condense back to water. It'd be very complex but I'm sure it could be done if you wanted to. I think a much better way to achieve the same end (i.e. reduce watering requirements and avoid water wastage) would be to install a steam condenser and operate a closed steam/feed loop. The condenser would also give you a big boost in efficiency and potentially either a more compact or more powerful engine too.

[EddieB]

I’d say that the main reasons were that there was very limited available technology during the steam era, that there was limited demand (and funding) in Africa and, as mentioned above, there wasn’t an over abundance of saline water where fresh water supplies were limited.

 

The only African use of condensing steam was in South Africa, for the Karoo main-line, where rainfall is low and seasonal and there is little surface water. Extensive use is made of borehole water in that region, which is usually slightly brackish. By the time the single class 20 was fitted with condensing apparatus and custom tender in 1948, Henschel had already produced successful condensing locomotives for Argentina and Germany. Trials of the modified locomotive were successful, leading to the development of the famous class 25 condensers. Elsewhere in South Africa, the provision of supplementary water tanks seems to have obviated the need to modify existing locomotives.

[Edwin_m]

As far as I can see a conventional steam locomotive would theoretically work if supplied with salty water, as the steam that comes off will not contain salt.  However the salt will build up in the boiler and elsewhere, no doubt causing no end of problems. 

[Talltim]

Sounds like no-one ever tried, which I find quite surprising considering all the weird and wonderful experiments there have been over the years.

The situation I was thinking such a concept might be useful was on islands with low rainfall where you are never far from the sea, but fresh water is a precious comodity.

Interestingly, although I may be googling wrong, every history of desalination starts with ancient times and says that commercial (distilation) desalination was quite common by the late C19, yet the only pictures are of modern plants.

[jjb1970]

As far as I can see a conventional steam locomotive would theoretically work if supplied with salty water, as the steam that comes off will not contain salt.  However the salt will build up in the boiler and elsewhere, no doubt causing no end of problems. 

 

Running a boiler on salt water will lead to boiler failure extremely quickly, the salt deposition on tubes destroys heat transfer, you get corrosion, it promotes water carryover and other undesirable things. It isn't just an operational issue either as the failure modes can be catastrophic, even in extremis it is not advisable.

[asmay2002]

If you can it makes far more sense to desalinate the water before you put it in the locomotive.