I'm just curious but does anybody know for sure if they pressurize a sub just to keep stress on the hull down a bit? Or is the amount they can pressurize it negligible compared to the outside pressure that they do not even bother?
Question about pressure, submarines and deco.
- Thread starter Buccaneer
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Don Burke
Contributor
The pressure inside a submarine does not vary beyond a couple of inches of mercury from 1 ATM.partridge:
Pressurizing the inside would put the crew into saturation pretty quickly.
Imagine trying to recover from a fire with days of deco obligation to get to the surface. Not pretty.
Don Burke
ETCS(SS) ret
Never even thought of the sefety escape issue. thanks
Don Burke
Contributor
Escaping is not the issue. Getting rid of the smoke and replacing it with clean air is the issue.partridge:
cool_hardware52
Contributor
This is a little off topic here, but it was referrenced early in the thread. The reason Airliners cabin altitude is ~8000 and not closer to Sea Level is the cost of maintaining the pressure, not the effects on hull life. You can bet they'd run it at 12k if they could do without too many complaints. The differential pressure between 40k and 8 k is not a lot less than between 40k and sea level.
An airliner is not just a simple closed tube full of people. The air is circulated and filtered and conditioned (cooled / heated) and most importantly a certain % is replaced with fresh outside air. All airplanes also leak, door seals, wiring penetrations, controls etc.
The outside air at say 40,000 ft is at a lower pressure than the 8000 ft cabin. To achieve a cabin pressure of 8000 this fresh outside air has to be compressed. On modern jets this done by bleeding air from the compressor section of the engines, i.e. the first set of fans, before the fuel is introduced. This compressor bleed air reduces the fuel efficieny and costs the airlines more money.
Ever had stale air in an airliner? This is because of reduced fresh air.
For general aviation (little planes) the compressed air is furnished by turbo on the engines, I've never seen a "P" model general aviation airplane that was not turbo'd.
The non turbo'd gen aviation guys are sucking ABO at altitude
Just more trivia for those interested.
Regards,
An airliner is not just a simple closed tube full of people. The air is circulated and filtered and conditioned (cooled / heated) and most importantly a certain % is replaced with fresh outside air. All airplanes also leak, door seals, wiring penetrations, controls etc.
The outside air at say 40,000 ft is at a lower pressure than the 8000 ft cabin. To achieve a cabin pressure of 8000 this fresh outside air has to be compressed. On modern jets this done by bleeding air from the compressor section of the engines, i.e. the first set of fans, before the fuel is introduced. This compressor bleed air reduces the fuel efficieny and costs the airlines more money.
Ever had stale air in an airliner? This is because of reduced fresh air.
For general aviation (little planes) the compressed air is furnished by turbo on the engines, I've never seen a "P" model general aviation airplane that was not turbo'd.
The non turbo'd gen aviation guys are sucking ABO at altitude
Just more trivia for those interested.
Regards,
ChrisA
Contributor
Buccaneer:
You are close, but not quite right.
The air in a 3000PSI tank remains at 3000PSI not matter where you put the tank, in the bottom of the ocean at 600ft or in outter space.
Thinks about it.... It the valve is closed what is going to change? Same air in the tank,the tank is the same size (OK assume the temperature remains constant.)
Back to the submarine: It's just a big SCUB tank filled to 1 bar or 14 PSI. As long as you don't open a valve it will always have 1 bar in it
As for the air being compressed and more dese at depth, yes it is but you don't breath the air in the tank directly. You breath from that airspce inside the reg just in front of the moulthpiece. THAT air is kept at the same pressure as the "outside" and does change when "outside" changes.
ChrisA
Contributor
There is another reason. The airliner is typicaly rated at about 6PSI.
So if you tried to maintain 1ATM inside you would either
1) Have to fly relativly low alitudes or,
2) Burst the presure shell
They could build airplanes to hold 1atm at cruising altitude but then the structure would weigh more and the weight would cut into the allowed payload. So yes it boils down to cost.
So if you tried to maintain 1ATM inside you would either
1) Have to fly relativly low alitudes or,
2) Burst the presure shell
They could build airplanes to hold 1atm at cruising altitude but then the structure would weigh more and the weight would cut into the allowed payload. So yes it boils down to cost.
cool_hardware52:This is a little off topic here, but it was referrenced early in the thread. The reason Airliners cabin altitude is ~8000 and not closer to Sea Level is the cost of maintaining the pressure, not the effects on hull life. You can bet they'd run it at 12k if they could do without too many complaints. The differential pressure between 40k and 8 k is not a lot less than between 40k and sea level.
An airliner is not just a simple closed tube full of people. The air is circulated and filtered and conditioned (cooled / heated) and most importantly a certain % is replaced with fresh outside air. All airplanes also leak, door seals, wiring penetrations, controls etc.
The outside air at say 40,000 ft is at a lower pressure than the 8000 ft cabin. To achieve a cabin pressure of 8000 this fresh outside air has to be compressed. On modern jets this done by bleeding air from the compressor section of the engines, i.e. the first set of fans, before the fuel is introduced. This compressor bleed air reduces the fuel efficieny and costs the airlines more money.
Ever had stale air in an airliner? This is because of reduced fresh air.
For general aviation (little planes) the compressed air is furnished by turbo on the engines, I've never seen a "P" model general aviation airplane that was not turbo'd.
The non turbo'd gen aviation guys are sucking ABO at altitude
Just more trivia for those interested.
Regards,
Darn it! Now look what you've done!
Here's what's really going on. The mouse dies of oxytox. If you opened the valve too quickly before that happened he'd be bent too. Submarines maintain close to 14.7 psi, but certain evolutions can affect this such as running the air compressors, the diesel generator, screwing up the ventilation line up, flooding etc. The air pressure inside the pressure vessle of a sub has absolutely nothing to do with its bouyancy. Bouyancy is controlled by pumping water into trim tanks. An emergency blow, which displaces water from the ballast tanks with 4500 psi air is only in emergencies, and will make the sub float, not become neutral.
Here's what's really going on. The mouse dies of oxytox. If you opened the valve too quickly before that happened he'd be bent too. Submarines maintain close to 14.7 psi, but certain evolutions can affect this such as running the air compressors, the diesel generator, screwing up the ventilation line up, flooding etc. The air pressure inside the pressure vessle of a sub has absolutely nothing to do with its bouyancy. Bouyancy is controlled by pumping water into trim tanks. An emergency blow, which displaces water from the ballast tanks with 4500 psi air is only in emergencies, and will make the sub float, not become neutral.
Buccaneer:
Forget for a few moments about breathing. Supose you take a full cylinder, say 200 ATA. It doesn't have a breathing regulator, only a pressure gauge attached to it. At Surface the SPG shows 200 ATa. Take this cylinder to 20, 100 or probably 1000 meters- SPG will still show 200 ATA (assuming temperature doesn't change). Inside a submarine it is the same, but instead of 200ATA it keeps 1ATA, so submariners don't need decompression stops when they want to get out of the sub. If the sub was compressed in order to keep water out (such as in the tunnels), then they would need deco stop too before getting out.
When you are breathing from the cylinder, you consume the air/gas contained, and as a consequence, the pressure inside the cylinder drops a little with every breath. Since you breath at ambient pressure (thanks to the regulator 2nd stage, which is cool since your lungs will be in ambient pressure and thus do not collapse), it makes a difference if you are at surface or submerged, say at 30 meters. Because if you fill your lungs (let's assume they are 5 liters) at surface, or fill them at 30meters with ambient pressure gas (4ATA), then at 30meters you need four times more molecules of gas with every breath. Therefore, you end up consuming more gas at depth.
Hope ot helps clarify some things.
Great discussion! This is what this board is best for! Hopefully even the most novice layperson can understand this topic now. Buccaneer, please do go back and read the 1st 3 chapters of the divers encyclopedia. These are very important concepts that you want to pass on to new divers/students correctly. One caveat, the regulator doesn't change the pressure per se (such as a computer controlled pressure valve), but allows the ambient water pressure to act on the compressed air leaving the tank (not directly but through a piston or diaphragm) thereby allowing you to breath air at the ambient pressure of your particular depth. The 2nd stage is just a demand delivery device that allows you to receive the ambient air from the 1st stage. Y'all can just slap me for nitpicking.
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