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I was intrigued knowing from this SE-Aviation post that modern, aircraft Emergency Evacuation-Slide inflation-cylinders are often filled with a CO2+N2 mixture.

See here:

Which gas is used to fill the evacuation slides of modern aircraft?

Intuitively the reason behind the CO2 addition instead of using simply pure N2 is not very obvious to me. Digging deeper into the technology, Wikipedia reveals that "The CO2 is used to slow down the rate at which the valve expends the gases."

Any idea about the science behind this? Why would CO2 slow down the inflation rate? And why do we want to slow the inflation in the first place? Fabric damage?

Also, what is the CO2 percent in the mix, anyone know?

curious_cat
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2 Answers2

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Let me first give some background information on the art of inflating an evacuation slide. There are two ways to fill them:

  1. Expand gas from a high-pressure bottle: This will cool the gas as it expands, so ice may form at the aspirator outlets where sucked-in air and the expanding gas mix, blocking the flow of outside air into the slide. Also, storing compressed gas needs bulky and heavy steel vessels.
  2. Pyrotechnic gas generators which use an exothermic reaction to create a large amount of hot gas. They are more compact and lighter than a pressure vessel for the same amount of gas produced, but their hot gas could scorch the slide material and will contract when it cools down after the filling process, so a slide filled by a pyrotechnic gas generator will sag after a while.

Because of the disadvantages of both pure stored gas and pure pyrotechnic inflation devices, current state of the art emergency evacuation slide systems use a hybrid inflator, which comprises a stored compressed gas together with a pyrotechnic gas generator. The pyrotechnic gas generator augments the stored compressed gas by providing additional gas as well as heat to counteract the effects of the expansion-induced cooling of the compressed gas as it expands out of the pressure vessel.

Operation of a hybrid inflator

The typical gas generator uses solid ammonium nitrate which is electrically ignited and produces nitrous oxide (N$_2$O) and water (H$_2$O) when heated. Nitrous oxide will exothermically decompose into nitrogen and oxygen when heated above 650°C or 1200°F, so even more energy and gas is produced. Note that the decomposition of two moles of nitrous oxide will produce two moles of nitrogen (N$_2$) plus one mole of oxygen (O$_2$), thus adding 50% more gas volume. This hot mixture of nitrogen, oxygen and steam is then piped into the pressure vessel and mixes with the gas flowing out from it, thereby heating the emitted gas.

The pressure vessel contains gasses like nitrogen or carbon dioxide. Other candidates are argon or helium, but their higher price has held them back. To increase the yield from the pressurized gas, the trick with the nitrous oxide can be repeated here: By storing nitrous oxide in the pressure vessel, again 50% more gas volume can be produced from what is stored, while the exothermic decomposition produces additional heat that offsets, to some extent, the heating of the stored gas.

Nitrous oxide is also attractive because it offers the best manufacturability, storage and handling of the possible decomposable gas source materials. Although classified as an oxidizer, in practice it is generally nontoxic and non-corrosive and is relatively inert up to temperatures of about 200°C. Also, nitrous oxide, as compared to gases such as oxygen, nitrogen and argon, liquefies relatively easily at ambient temperatures - helpful when the gas in the pressure vessel is stored in liquefied form to reduce its bulk.

Heated to its decomposition temperature, pure nitrous oxide will undergo a rapid disassociation chain reaction unless a coolant gas is added to absorb some of the heat generated by the exothermic reaction. Therefore, the mixture in the pressure vessel contains 50% or more of carbon dioxide which has a latent heat of vaporization of approximately 66 Calories per gram. As a three-atomic gas it is better suited for this task than two-atomic alternatives. Thus, it does not serve as a diluent (as aeroalias wrongly states), it absorbs heat and stabilizes the disassociation process.

Why a mixture of nitrogen and carbon dioxide

Now we can answer your question: Nitrogen is contributed both by the pyrotechnic gas generator and by the compressed gas and is produced from the dissociation of nitrous oxide (N$_2$O). Carbon dioxide is added to dampen the exothermic dissociation and absorb some of the heat produced. Without it, the gas generator would experience a runaway chain reaction! Its concentration in the compressed gas is between 50% and 75%, so the concentration in the gas generator mix is around 35% to 50%.

The high speed flow of gas from the pressure vessel is now piped through an aspirator, which sucks in ambient air by using the Venturi effect, and from there into the slide. This contributes up to three quarters of the total gas in the slide, so most of the nitrogen in the slide is from ambient air. In the slide, the concentration of carbon dioxide is diluted to around 10% to 15%.

Aspirator

Aspirator (from BF Goodrich's US Patent 4368009). High-speed gas from the gas generator (red arrows) enters from the angled pipe on the left and pulls ambient air (blue arrows) with it when shooting from the nozzles on the right. Contrary to aeroalia's claims, an aspirator is not just another name for a gas generator, it is an entirely different thing altogether.

Peter Kämpf
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    Excellent answer, thanks! So much complexity hides in the most unexpected places. :) – curious_cat Nov 22 '15 at 17:07
  • As an aside, why not use the typical automotive airbag inflation mix? I recall that's a mixture of sodium azide (NaN3), KNO3, and SiO2? – curious_cat Nov 22 '15 at 17:16
  • So prior to activation what is the physical state of the CO2 in the system? Liquid? Soild? – curious_cat Nov 22 '15 at 18:45
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    @curious_cat sodium azide decomposes explosively. That's perfect for an airbag which inflates then immediately deflates all in under a second, but not what you want for an escape slide that inflates over the course of several seconds and stays inflated for many minutes. Also, sodium azide in the quantities required for an escape slide would be downright dangerous. – Level River St Nov 22 '15 at 19:19
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    @curious_cat: Normally gaseous, but the latest innovation is to use partly liquified storage to make the bottle smaller. No solid storage, because ammonium nitrate does not give off carbon dioxide and is the only solid source of gas in this process. Regarding complexity: The first evacuation slides more than 50 years ago simply used compressed air, until people found that storing oxygen at high pressure might not be such a brilliant idea. – Peter Kämpf Nov 22 '15 at 20:31
  • @PeterKämpf Storing air at high pressure might not be brilliant but Nitrogen seems safe enough. Only downside is the weight of a pressure vessel I suppose. – curious_cat Nov 23 '15 at 04:51
  • @PeterKämpf Just to clarify my understanding: If the CO2 stored is normally gaseous then how does the latent heat of vaporization that you've mentioned in your answer matter? CO2 would moderate the reactive exotherm via the latent heat mode only if it were solid / liquid pre-inflation, correct? – curious_cat Nov 23 '15 at 04:53
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    @curious_cat You're correct about the gaseous nitrogen. Storing it as a compressed gas is safe, but needs a heavy pressure bottle. Now to nitrous oxide: If heated above 650°C it will exothermically split into nitrogen and oxygen, and it is this extra heat which needs to be kept in check by the carbon dioxide. To get away with smaller bottles, the most recent systems store both gasses in liquid state and pressurize the vessel with argon or nitrogen. – Peter Kämpf Nov 23 '15 at 09:08
  • @PeterKämpf My point was the using CO2 to moderate the exotherm only makes sense if the CO2 is stored as liquid or solid under pressure. Otherwise the heat moderation benefit offered by CO2 as gas is identical to that offered by gaseous N2. Do we agree on this? i.e. Since you mentioned the "Latent Heat" of CO2 that comes into play ONLY if CO2 is stored as a Liq. Right? – curious_cat Nov 23 '15 at 09:28
  • @PeterKämpf They should have tried pressurized Hydrogen. It heats when it expands through a nozzle. No ice crystals problem. Just kidding of course, but it'd sure solve the ice crystallization problems in the aspirator! – curious_cat Nov 23 '15 at 09:34
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    @curious_cat ... and the slide would float better when used as a raft. But as a gas it should also cool when expanded - it is a two-atomic gas and behaves like all other gasses. Note the difference in the specific heats between CO$_2$ and N$_2$ on the linked page, though. They do have their own properties, but I still struggle myself to fully comprehend the reason for selecting CO$_2$ over N$_2$ as a heat sink. – Peter Kämpf Nov 23 '15 at 10:47
  • @PeterKämpf About H2 also cooling when expanding, I think you are wrong. Not all gases cool when expanding (I hope I'm not being condescending by writing stuff you already know! ). Most gases do cool when expanding but this is only because they are below their Joule Thomson inversion temperature. Very few gases have their Joule Thomson inversion Temperature below Room Temperature. Hydrogen is one of those. So unless, you were expanding hydrogen through a nozzle at cryogenic temperatures in my opinion it would heat up when expanding. I could be wrong. Would love to hear your opinion. – curious_cat Nov 23 '15 at 11:30
  • @PeterKämpf Of course, I'm implicitly assuming that the nozzle in a evacuation gas pressure vessel leads to a Joule Thomson expansion i.e. an isenthalpic expansion. If it were an isentropic expansion then you would be correct: even Hydrogen would cool. But I think it is a good assumption to think of our example as isenthalpic and not isentropic. Do you agree? – curious_cat Nov 23 '15 at 11:33
  • @curious_cat: Uhh - isenthalpic sounds like no change in the kinetic energy, which means the pressure is converted into frictional heat. That leaves too little energy for the aspirator to do its job, and now you need to have three to four times as much gas to fill the slide. Using hydrogen for this will need a laaaarge bottle ... – Peter Kämpf Nov 23 '15 at 13:35
  • @PeterKämpf Indeed. I think you are right. I was mentally modelling the aspirator as a throttling valve. In hindsight it seems more like a nozzle / steam ejector. So, yes, even hydrogen would cool in an isentropic expansion. :) – curious_cat Nov 23 '15 at 14:28
  • @PeterKämpf I was reading the link you sent about specific heats. It shows CO2 as 0.84 versus N2 as 1.04. In that case, wouldn't N2 be a better heat sink than CO2? What gives? – curious_cat Nov 24 '15 at 07:17
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    @curious_cat: I'm still struggling to understand it myself. Maybe the reason is that carbon dioxide cools less in an adiabatic expansion than do two-atomic gasses. – Peter Kämpf Nov 26 '15 at 02:24
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    @PeterKämpf One thought: We are looking at per kg specific heats. We should be comparing per volume specific heats which are essentiall comparing per mole specific heats. If you do that CO2 has a higher specific heat than N2. So adding CO2 helps more than N2. Does that make sense? – curious_cat Nov 26 '15 at 15:25
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    @curious_cat: Yes, it does make sense if volume is restricted and the increased mass per heat capacity can be accepted. Since the slide canister on the inside of the door should indeed be as small as possible, your reasoning is sound. – Peter Kämpf Nov 28 '15 at 21:04
  • Why not use gas stored in a heated high-pressure bottle, to counteract adiabatic cooling? – Vikki May 25 '18 at 00:08
  • @Sean: Do you want to wait until the bottle is heated (while flames might consume the interior of the cabin), or do you want to keep the bottle permanently heated (and make it a fire hazard)? No, I'm afraid that heating the gas bottle first is not a good way to counteract adiabatic cooling - unless you use a gas generator as current slides do already and mix hot gas to what is in the bottle already. – Peter Kämpf May 25 '18 at 07:01
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The $\text{CO}_{2}$ is used as a diluent.

  • The pressure vessels (for inflating the slides) usually have nitrous oxide (or similar) compounds, that undergo exothermal thermal decomposition during inflation. For example, $\text{NO}$ reaches temperatures of $\sim 650\,^{\circ}\text{C}$ during thermal decomposition. As $\text{CO}_{2}$ absorbs heat during its expansion, this prevents any damage to the fabric.

  • For the same reason, the presence of $\text{CO}_{2}$ controls the reaction rate of the decomposition of the nitrous oxide as it is usually impossible to prevent decomposition from generating unpredictable amounts of gas.

The $\text{CO}_{2}$ forms around 50% to 75% of the mixture.

The amount of gas required for inflation of the slides in the aircraft is quite high. If all these are carried in pressure vessels, either of the following has to be done

  • increase the pressure (already $\sim200$ bar) of the gas in the vessel (its usually in liquid form), which increases the pressure vessel thickness and consequently the weight, or

  • increase the number or size of pressure vessels, which again increases the weight.

Also, a gas generator (usually pyrotechnic) can be used, which generates gas by exothermic reaction. This too, has its disadvantages:

  • The gas generated is at high temperature (via thermionic reaction), resulting in damage to the slide fabric.

  • Also, as the gas cools, the evacuation slide may sag (as gas contracts). This is not good as the slides are expected to act as life rafts.

The size of the gas bottle can also be reduced by using an aspirator, which draws in ambient air to mix in a ratio of about two-thirds air to one-third gas from the bottle.

To overcome these, the aircraft slide inflation system uses is hybrid system that uses two techniques:

  • Instead of carrying the gas itself, carry a compound capable of undergoing thermal decomposition such that the number moles of gas produced by the inflator is greater than the number of moles of gas stored. Nitrous oxide is used because of manufacturability, storage and handling concerns.

    Two moles of Nitrous Oxide $\text{N}_{2}\text{O}$, when undergoing thermal decomposition, forms two moles of diatomic nitrogen ($\text{N}_{2}$) and one mole of diatomic oxygen ($\text{O}_{2}$)

$2\text{N}_{2}\text{O} \ = \ 2\text{N}_{2} \ + \ \text{O}_{2}$

Because the universal gas constant is the same for all gases, the three moles of nitrogen and oxygen produced by the decomposition of two moles of nitrous oxide occupy 50% more volume than the two moles of nitrous oxide would have occupied at the same temperature and pressure. Thus, the size of the container is reduced by a third (compared to situation where we use undecomposed gas).

  • A pyrotechnic gas generator (or an aspirator) that augments the stored compressed gas by providing additional gas.

For more details, see US patent application US 6877698 B2, Aircraft evacuation slide inflation system using a stored liquified gas capable of thermal decomposition

In case the amount of gas required is low (for example, in some helicopters), only the pressure bottles are carried.

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aeroalias
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    Interesting. So, this mixture you describe seems totally different from the one described on Wikipedia as well as the other SE-Aviation Thread? That one had N2 in it & no NO. – curious_cat Nov 22 '15 at 06:05
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    As an aside, why the NO & exothermic decomposition? Why won't a simple compressed gas do? e.g. compressed N2 – curious_cat Nov 22 '15 at 06:06
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    @curious_cat I've edited the answer as I thought its better to answer them there. Have a look. – aeroalias Nov 22 '15 at 08:36
  • Wait what? I thought that 2NO2 = N2 + 2O2? – Zizouz212 Nov 22 '15 at 16:04
  • @Zizouz212 I made a typo. Nitrous oxide is N2O, not NO2. Corrected it. Thanks for catching. – aeroalias Nov 22 '15 at 16:11
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    Ah, okay. I began to think that my chemistry course is a lie xD – Zizouz212 Nov 22 '15 at 16:12
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    No, it is not used to dilute anything. Please try to read and understand the patents before you cite them. Your answer is full of factual mistakes. – Peter Kämpf Nov 22 '15 at 21:27
  • @curious_cat: Expanding a compressed gas will drastically cool it. You'd get frostbite - not good either. A careful mix like this will balance the heating from decomposition with the cooling from expansion. – MSalters Nov 23 '15 at 02:09
  • @MSalters Does anyone care about frostbite? Who's touching the valve? The one reason I'd care about is if I was aspirating external air then I'd worry about moisture freezing up & choking the ports. – curious_cat Nov 23 '15 at 04:49
  • @curious_cat: The gas itself is that cold too. And yes, you are in fact pulling in moist external air as well. (See Peter kampf's answer). Ice would clog up the aspirator. But I now realize that there's an even bigger risk. When you cool down the inside of the slide to -40 or so, the material becomes brittle. – MSalters Nov 23 '15 at 13:09