June 15, 2008

Submarine Has to Solve Three Basic Problems

By Marshall Brain

Submarines have been a little mysterious since they were invented more than two centuries ago. That's because most submarines have been owned by the military. But recently, subs have started to get friendlier. There is a growing market for personal subs, and many coastal resort areas now have tourist subs that let visitors see underwater reefs and wrecks in air-conditioned comfort. Let's take a look at how these different kinds of subs work.

Any sub, from smallest to largest, has to solve three basic problems. First, it has to be able to move through the water. Second, it has to be able to sink down below the surface of the water and then rise back again. And third, it has to provide an environment that keeps the humans on board the sub alive.

Moving through the water is probably the simplest part, because most submarines use a propeller just like a boat would. The interesting difference between a sub and a boat is the lack of air. A boat's motor, being above the water, can use air freely in a gasoline or diesel engine. Because a sub is underwater, it loses access to free air. Subs have two ways to solve this problem.

The more common approach uses a large bank of batteries and an electric motor to spin the propeller. That is all that's needed for a tourist sub. When the sub returns to dock, it can plug in to recharge the batteries. But a military sub might need to stay out for a month or more. Therefore, military subs that use the battery approach have a diesel engine and a generator. The sub surfaces, runs the diesel engine long enough so that the generator can recharge the batteries, and then can dive again.

The most advanced military subs are gigantic -- so big that they can house a small nuclear power plant on board. Because a nuclear reactor needs no oxygen to generate electricity, these subs can stay underwater for months at a time. The only real limitation is the need to resupply food for the crew members. A nuclear sub can run for years before it needs to refuel.

Getting the sub to sink and rise is straightforward. A sub has tanks known as ballast tanks that allow the sub to float when they are full of air. By letting the air out of the ballast tanks and filling them with water, the sub gains enough weight to sink. When the sub wants to surface, high pressure air can force out the water in the tanks, allowing the sub to rise again.

Keeping the air inside the sub safe for humans to breath is the third challenge, and might be the most complex. The air we breathe on Earth is made up of three important gases: nitrogen (about 80 percent), oxygen (about 20 percent) and carbon dioxide (0.04 percent or so). Plus there is almost always some amount of water vapor in the air.

When we breathe in air, our bodies consume its oxygen and convert it to carbon dioxide. Exhaled air contains about 5 percent carbon dioxide. Our bodies do not do anything with nitrogen. We breathe it in and out without changing it.

A submarine is a sealed container that contains people and a limited supply of air. There are three things that must happen in order to keep air in a submarine breathable. First, the oxygen has to be replenished as it is consumed. If the percentage of oxygen in the air falls too low, a person suffocates. Next, carbon dioxide must be removed from the air. If the concentration of carbon dioxide rises, it quickly becomes a toxin that will kill everyone on board. Then the moisture that we exhale in our breath must be removed, or the humidity will get so high that water starts condensing on the walls.

Fresh oxygen can come either from pressurized oxygen tanks, an oxygen generator (which can form oxygen from the electrolysis of water) or some sort of "oxygen canister" or "oxygen candle" that releases oxygen through a chemical reaction. Oxygen is either released continuously by a computerized system, or it is released in batches through the day.

Carbon dioxide can be removed from the air chemically using soda lime (sodium hydroxide and calcium hydroxide). The carbon dioxide gets trapped in the soda lime by a chemical reaction.

The moisture gets removed by a dehumidifier or chemically.

By combining ballast tanks, a motor and propeller and an air- handling system in a water-tight tube, you have a submarine.

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