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Author Topic: Why do diesel submarines surface when they need to recharge the batteries?  (Read 4450 times)

Tafara Bare

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Tafara Bare  asked the Naked Scientists:
   
Hi Chris,
 
I always listen to you on Redi Direko's show, but never able to get my question through. Just wanna ask, why do diesel submarines surface when they need to recharge the batteries, why not recharge when submerged?
 
Thanks,
Tafara

What do you think?
« Last Edit: 01/10/2010 17:30:02 by _system »

Geezer

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Diesel engines need a lot of air to allow the combustion of the diesel fuel, and the combustion produces a lot of exhaust gas.

A submarine could not carry enough air to run the diesel engines for very long, and it could not easily expel the exhaust deep under water, so it needs access to the atmosphere for an air supply, and somewhere to send the exhaust.

I think some submarines don't actually have to surface. They have a snorkel device that lets air in and exhaust out for the diesels without having to surface the sub.

LeeE

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The largest marine diesel engines have a capacity of nearly 25,500 litres and operate at around 100 rpm, so in just one minute it would get through nearly 2,550,000 litres of air (the volume of fuel used in comparison to the air needed is tiny, otherwise fuel tanks would have to be impossibly large).

I couldn't find a figure for the capacity of typical submarine diesel engines but I would guess that they range up to 1/10th or more of the capacity of the largest engines (the sort of submarines we're talking about here will be military vessels, so will require high performance).  Being smaller engines though, submarine diesel engines will run faster, probably at several hundred rpm, so we might see something like 300 rpm x 2000 litres capacity = 600,000 litres of air per minute.

Like I say, I couldn't find any specific figures but I've also read accounts of submarine operation with a snorkel in WWII and it seems that snorkels could only be used with the submarine fully submerged if the waves are relatively small: the snorkel incorporated a non-return valve that prevented water from being drawn in if a wave passed over the snorkel head but this meant that the engine needed a 'relief' valve to temporarily allow it to draw air from the internal cabin supply when the non-return valve closed and this often resulted in the crew getting punctured eardrums due to the sudden drop in pressure.

imatfaal

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I think the submarine engines were another order of magnitude smaller - about 100-200 litres - for the U-class and V-Class British Navy Submarines.  But they ran a little faster, about 600 rpm so your estimation is valid.
History of the Engine Maker and More history

On your comparison with massive commercial motor diesel engines - I know of ships with engines about half this size (I have never understood why Maersk specced such huge engines for  "relatively" small ships ie the E-series container ships) which would burn in the region of 75-150,000 litres of fuel per day.

peppercorn

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Just out of interest how much free oxygen is there in seawater?

imatfaal

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About 0.2 to 0.4 moles per cubic metre read off from map half way down this page.  Very rough calculations to follow
VLCC engine takes in 904 m3 of air per minute. 
thats 190m3 of oxygen
it uses about one third of that
thats 63m3 used oxygen
equiv at RTP (and I realise its colder than RTP...)
2625 moles of oxygen
you would thus need to completely strip the oxygen from 6.5-13,000 cubic metres of water per minute.

I think you would leave some very irate, and very soon to be dead fish in your wake

peppercorn

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I think you would leave some very irate, and very soon to be dead fish in your wake

:o ...  :D  Er, Okay - Whoops!

I was thinking more of hydrogen fuel cells, but still!

yor_on

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The
Just out of interest how much free oxygen is there in seawater?

This one is rather long, but as it's a subject I'm interested in too?
Read it at your peril :)

There are areas that have no free oxygen at all, and they are growing as as we use 'nutrients' as fertilizers that follows the groundwater and streams into the oceans. "The largest dead zone covered 70,000 square kilometres (27,000 mi˛). A 2008 study counted 405 dead zones worldwide." From Dead zones. Add to that the accumulation of man-made CO2 in the oceans introducing acidity and further heating of the oceans, freeing even more oxygen (and C02) from the surface, and also reducing its capacity to take up atmospheric CO2. The greatest uptakes of CO2 and oxygen comes from the cold water in the arctic and antarctic and those uptakes have been reduced significantly according to measurements made by several researchers.. Ocean acidification.

Then there is one point more. I don't know how the increased accumulation will change the chemistry of the oceans, that is how much we can expect it to accumulate, it also depends on if the salinity and temperature and that changes with the underwater circulation and streams, as/if the warming disrupts them, and change their revolutions around the globe.

For example, we have streams transporting oxygen-full cold water from both the arctic and the antarctic to our warmer waters, they also bring with them krill etc which is the basis for the marine food chain. Would they to change you will find a accelerating scenario with fish death and more oxygen free areas, as a guess. Also it will heat the oceans even further and just as a hot soda-can almost will explode if left to long in the sun so the ocean then will free both CO2 and oxygen creating an even more unhealthy environment for marine life.

Then we come to the oxygen in the atmosphere. It seems to be falling according to Professor Ralph Keeling from Scripps Institute..

The Keeling curve from climatecentral.

What control the distribution of carbon in the oceans are called the 'solubility pump' and the 'biological pump'. Cold polar waters contains double the amount of CO2, as compared to the warmer waters (tropical) and their streams, like our gulf stream, are the mechanisms for distributing it into the cold denser depths of the oceans, that then can hold greater amounts for a very long time, assuming that those depths don't get warmed up too that is. The 'biological pump' is made out of plants (phytoplankton) that needs the CO2 for convertion. They mostly grow close to the surface as they also need the sunlight. They are the main food for zooplankton that in its turn becomes the food basis for krill, and so are the start of the marine food circle.

There are some that expect CO2 to help our flora/fauna to grow, but that is a very delicate balance. Take a look here if you're interested about the newest finds. Another thing about the acidification of our oceans is that it seems to kill of the reproductivity of several species of marine life, according to Jonathan Havenhand at the University of Gothenburg in Sweden.  "Experiments show that the struggle by copepods, snails, sea urchins and brittlestars to balance the changing pH inside their bodies impairs their ability to reproduce and grow. Many species are unlikely to genetically adapt to ocean acidification, because the change is occurring too quickly." So it's not only coral reefs 'withering away/dissolving' here.

Heh, too long by far huh :)
==

« Last Edit: 06/10/2010 17:14:22 by yor_on »

bobwilliam

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I think there are involved in the rate of computational fluid dynamics, and therefore it depends on the shape of the submarine. As the submarine's ballast water and air blowing out of the rising increase in theory should be able to point to the same speed, a bubble will rise from the depths. But even so, there are different interest rates, because the foam is compressed at a deeper depth, it rises slowly compared to less bubble closer to the surface gets.

 

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