Compressed air car

[Silas Sparkhammer]

Originally Posted by Il-Mari

Do they?

Do compressed air tanks explode? They most certainly can; it happened here in California, in the recent fires. My sister's pump shed burned, and the water tank -- water with a bubble of compressed air -- let go with a great bang. We found fragments dozens of yards away, and found splinters deeply embedded in nearby trees.

However, it also occurs to me that a compressed air tank could probably be manufactured with a specific "blowout valve," an intentionally-place weak seam, so that when the structure starts to fail, this part will direct most of the blast (presumably downward, toward the street, rather than upward, toward the passengers.)

Silas

Originally Posted by Silas Sparkhammer

Do compressed air tanks explode? They most certainly can; it happened here in California, in the recent fires. My sister's pump shed burned, and the water tank -- water with a bubble of compressed air -- let go with a great bang. We found fragments dozens of yards away, and found splinters deeply embedded in nearby trees.

Is a water tank equivalent to a compressed air tank? And why was it the compressed air that caused the explosion? Sealed containers full of water that is brought to a boil is a great way to get good rupture of some type.

[BoKu]

It may be a UL that poking a hole in a SCUBA tank, or breaking the neck off of a high-pressure tank will cause it to explode. However, it's very much within the realm of possibility that a compressed air (or other gas) tank can rupture with enough energy to propel shards with lethal force. Gasses can store a fair amount of energy when they're compressed.

Here's a good start: Rust compromises an old air compressor. Imagine standing near that.

Another: Car Wash Supervisor Died When an Air Tank Exploded in a Car Wash Equipment Room

Another: Maintenance Worker Struck in Head by Exploding Pressurized Tank

Another: Three hurt after air tank explodes at fair

Another: Third person dies in Mojave Airport explosion, names released

Bob "Builds his own O2 systems" K.

[Dropbear]

Originally Posted by Silas Sparkhammer

However, it also occurs to me that a compressed air tank could probably be manufactured with a specific "blowout valve," an intentionally-place weak seam, so that when the structure starts to fail, this part will direct most of the blast (presumably downward, toward the street, rather than upward, toward the passengers.)

Silas

Given that compressed air cars would necessarily be made of quite light materials this could have unintended consequences.

"Police today recovered the body of a man killed when his compressed air car's fuel tanks ruptured following an accident. The recovery was made more difficult by the car's location on the roof of a nearby apartment block."

Dropbear

[jimmy101_again]

Another problem with any type of compressed gas storage system is the large amount of energy that is lost as heat. CO2, Air, nitrogen etc. all heat up a lot when compressed. The heat is transfered to the tank then lost to the environment. That's basically 100% wasted energy. As you draw compressed gas from the tank the remaining gases (and perhaps liquid in the case of CO2) cool off. You can recover some of the energy from the environment if you take the compressed gas from the storage tank very slowly, slow enough that heat can flow into the tank from the environment.

Hence, CO2 powered paintball markers ice up (ice forms on the outside of the gun) if fired to fast. And, the liquid CO2 in the bottle cools so much that the pressure in the tank drops below a useable level.

In an engine, there may be strategies to minimize the energy lost as heat during fueling and to maximize the rate of heat absorption during operation but heat loss will still represent a pretty large loss of efficiency.

[Silas Sparkhammer]

Originally Posted by jimmy101_again

Another problem with any type of compressed gas storage system is the large amount of energy that is lost as heat. CO2, Air, nitrogen etc. all heat up a lot when compressed. The heat is transfered to the tank then lost to the environment. That's basically 100% wasted energy. . . .

Could this loss be reduced by centralized compressing? Instead of small compressors aboard cars, big compressors at service stations, or even bigger compressors serving entire cities? The heat could be put back to use (?) at those locations (co-generation plants, perhaps?)

Don't modern air-conditioning plants, for large buildings, recover some energy that way, whereas, for instance, the AC in a car or small apartment only produces waste heat?

Or am I barking up the wrong tree?

Silas (or merely barking?)

[BoKu]

Originally Posted by Silas Sparkhammer

...Don't modern air-conditioning plants, for large buildings, recover some energy that way, whereas, for instance, the AC in a car or small apartment only produces waste heat?...

Air conditioning is just another way of moving heat around, and compressing and releasing gasses is how you usually do it. So the heating that occurs when you compress the refrigerant gas is not a waste byproduct, it is central to the process.

Basically, you compress a gas so that it gets hotter than the ambient air. Then you run the compressed gas through a radiator called a 'condenser' and transfer the heat to the ambient air, cooling the gas to near-ambient temperature while making the heat somebody else's problem. Then you vent the compressed gas to a lower pressure inside another radiator (called an 'evaporator'). Releasing the pressure reduces the gas's temperature so that it is cooler than ambient, and it cools the evaporator. Then you use a fan to push some air past the coils of the cold evaporator and the air comes out cold.

That's the way AC works in your car, and that's the way it works in just about every modern installation except maybe the old ammonia icehouse systems (I don't know how they work). The effeciency of the system scales pretty well. The major issues with car AC systems is that cars generally aren't insulated very effectively, and they tend to absorb a lot of heat from the engine that has to be gotten rid of as well.

Thanks, Bob "BoKu" K.

[ASL]

Originally Posted by Dropbear

Given that compressed air cars would necessarily be made of quite light materials this could have unintended consequences.

"Police today recovered the body of a man killed when his compressed air car's fuel tanks ruptured following an accident. The recovery was made more difficult by the car's location on the roof of a nearby apartment block."

I don't see that as particularly likely, although I suppose to be sure you'd have to do some calculations based on the max energy released in an explosion taking say a second. (But I'm not een going to try because I don't think it's worth it. You'd need to make assumptions about the mass of the car, the length of time the explosion takes, the amount of compressed air stored in the car, and the location of the air tank.) But in any event it seems like it would be less likely to send a car sky high than an explosion of gasoline which has a much higher power density and although I realize that the idea of cars exploding at the slightest provocation is untrue, surely some must still expxlode every now and then due to, say, a fire? Even if it did release just enough energy to do it, it's still more likely that it would just flip the car over or onto it's side unless the tank is placed at the center of mass and all the air that werw are counting on being released is directed straight downard rather than outward.

[jimmy101_again]

Originally Posted by Silas Sparkhammer

Could this loss be reduced by centralized compressing? Instead of small compressors aboard cars, big compressors at service stations, or even bigger compressors serving entire cities? The heat could be put back to use (?) at those locations (co-generation plants, perhaps?)

Partially. You are right that a well designed compressor station may well recover some of the heat generated by the compression. Like many things, you can increase the efficiency of a process if you spend enough money.

Even a small home compressor system could be built to recover some of the waste heat but it'll cost more to build. A home system could pump the excess heat into the ground then use a grond source heat pump to extract the energy back out for heating in the winter time. Or the excess heat could be pumped into the hot water heater.

[DemonWolf]

Originally Posted by Il-Mari

Do they?

Can you provide some references for this, since when the mythbusters tested shooting a SCUBA tank whily trying to cause an explosion like in Jaws, all they got was it rocketing around for a while (which is still dangerous, but not an explosion). The author of the book, Peter Benchley, also thought that the tank exploding was too unbelievable, but Spielberg insisted on that being the method by which the shark was to be dispatched.

The same thing happened when they showed that a valve getting knocked off a compressed air tank can make the tank itself go through a brick wall.

- Il-Mari

I think "explode" is too dramatic of a word. "Burst" is probably better. Like a soda bottle unter too much pressure, it will burst at the wakest point and the force will be expelled. That is why there is a safety valve.

[Il-Mari]

Originally Posted by DemonWolf

I think "explode" is too dramatic of a word. "Burst" is probably better. Like a soda bottle unter too much pressure, it will burst at the wakest point and the force will be expelled. That is why there is a safety valve.

Yeah that's what I was thinking too - and like I said, it would certainly still be very dangerous even without shrapnel, since, as the Mythbusters showed, a compressed air tank can easily go through a brick wall and cause a decent dent in the reinforced concrete wall behind it.

- Il-Mari

[Alchemy]

Originally Posted by Silas Sparkhammer

However, it also occurs to me that a compressed air tank could probably be manufactured with a specific "blowout valve," an intentionally-place weak seam, so that when the structure starts to fail, this part will direct most of the blast (presumably downward, toward the street, rather than upward, toward the passengers.)

High-pressure gas cylinders are cold-worked from a single piece of steel (plus a reinforcing steel collar) so there is no seam at all. The valve stem has a pressure relief valve. Low-pressure containers, like propane tanks and some air compressor setups, are usually two or more welded pieces.

Originally Posted by Silas Sparkhammer

Could this loss be reduced by centralized compressing? Instead of small compressors aboard cars, big compressors at service stations, or even bigger compressors serving entire cities? The heat could be put back to use (?) at those locations (co-generation plants, perhaps?)

Any large chemical process plant will utilize heat integration in its design. A gas compression plant would not be very profitable unless it found some use for the heat produced during compression cycles. Using boiler feed water as a coolant is one way to go about it.

[Troberg]

Can you provide some references for this, since when the mythbusters tested shooting a SCUBA tank whily trying to cause an explosion like in Jaws, all they got was it rocketing around for a while (which is still dangerous, but not an explosion).

I wasn't talking about a neat puncture, I was talking about a real rupture, such as if the tank is squished in an accident and cracks all the way. Remember, the tank has to be large to take enough air for even a short trip, so it will be a very large part of the car, and thus likely to get damaged in any accident.

Even if the tank itself does not explode, all that air released at once will make every object nearby into deadly shrapnel.

Is a water tank equivalent to a compressed air tank? And why was it the compressed air that caused the explosion? Sealed containers full of water that is brought to a boil is a great way to get good rupture of some type.

No, liquids don't compress, which is why hydroforming is a perfectly safe method of making complicated parts. As it will not compress, no energy will be stored.

http://en.wikipedia.org/wiki/Hydroforming

Idea is coming around again. This article explains a little more about the science behind it. Not saying I understand it - just that it surprised me to see that this had been going around since 2000 and this article is in the last couple of years: http://www.themotorreport.com.au/573...r-blow-me-down