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> 09-19-05: Science in action!
post Sep 20 2005, 09:16 PM
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Science In Action!

How fresh water is produced on the USCGC Healy by desalination
Fresh (Potable) water system on the USCGC Healy for” Dummies”

142 people have to drink water, cook and shower. Where is the water coming from on the USCGC Healy?

We are sailing the Arctic Ocean which has a high salinity. Ocean water is not drinkable. So, where is the fresh water coming from for the 142 people aboard during our two month cruise? With this question in mind I explored the engineering department to find a knowledgeable person. The Hot and Cold Potable (fresh) Water System (PW) belongs to the auxiliary division of the ship. The auxiliary division of the ship is the responsibility of MKCS Joseph Bisson. He gave me a tour through the ship.

The PW system provide for the treatment and storage of water for drinking and cooking and other domestic uses. Mazeing through areas of the ship with lots of pipes, pumps, gauges, heat and noise guided by MKCS Bisson I got an idea how fresh water is produced. Here my easy version of a complex system.

Distilling Plant
For long cruises like ours with 142 people onboard it is necessary to produce our own fresh water continuously out of the oceans saltwater.

In order to do so the ship has two distilling plants that produce fresh water. By vaporizing and condensing water the salt is left behind and distilled, fresh, clean water is produced.

Fresh water is need for:
• Drinking, cooking, sanitation
• Engine cooling system
• Cycloconverter de- ionized water system and other auxiliary equipment

How does distillation work on the ship?

1. Seawater intake
The ship has 2 seawater feed pumps (big!). They provide the sea water for conversion to fresh water.

IPB Image

2. Transport
The sea water flows to various locations (lots of pipes) and is preheated with steam before it enters the first flash chamber (steel box under vacuum) of the distilling plant.

IPB Image

3. Vaporization of seawater in flash chambers
The flash chambers are the heart of the distilling plant. The cambers are under vacuum (low air pressure) to enable the heated seawater to vaporize at a lower temperature than under normal atmospheric air pressure. (Physics one to one- The vaporization point of a liquid depends on air pressure, cook things longer on a high mountain because the boiling temperature of water is below 212 F!!) At a temperature of 168F (kills most bacteria) the seawater is vaporized.

IPB Image

4. Brine is removed
The vapor flows though a mesh separator where the droplets of salt are removed. The left over that does not condensate is called brine (Supersaturated salty liquid) which is pumped overboard.

5. Distilled Water Circuit
The vapor produced in the flash chambers is cooled down. The resulting condensate or distillate is collected, tested for salinity and treated with bromine (prevent bacteria growth) and pumped to the potable (fresh) water tanks.

6. Storage
There are two 15460- gallon fresh (potable) water tanks located on the 3rd deck. They can be filled by either of the ship’s distilling plants or from shore supplied water.

Science talk

Liquid to Gas Vaporization (Process needs heat)

Gas to liquid Condensation (Process gives off heat)

In the process of distillation the salt of the ocean water is left behind (brine) and fresh water is produced!
Special thanks to MKCS Joseph Bisson for taking the time showing and explaining the ship’s auxiliary system to me.

Best regards and study you science,
U. Kaden
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