Aerospace Technology...
for Air, Land and Sea
A Supercritical Air Mobility Pack

"Supercritical" Explained
Why is it super? It certainly isn't critical!

About Niwot Technologies

Niwot Technologies, LLC.
P.O. Box 526
Niwot, CO 80544

Tel: (303)678-7135
Fax: (303)581-1046

Contact: Harold L. Gier



What is Supercritical?

It's super! It's not critical.

When we look at a phase diagram for a substance, the term "critical point" refers to point at the end of the boundary between gas and liquid phases. "Supercritical" refers to the characteristics of a fluid that exists at a temperatures and pressures above and beyond this point, i.e., where the distinction between liquid and gas phases no longer exists.

In the Niwot Technologies SCAMP, supercritical air behaves very much like compressed air - if you release the pressure, it expands into ordinary, but rather cold, air. The supercritical air is stored at very low temperatures. This allows a lot of air to be stored in a small tank, without resorting to the high pressures associated with compressed air. In other words, we store more air, but at a lower pressure.

The SCAMP system stores air at temperatures that are slightly above those at which the oxygen in air would liquify at atmospheric pressure. Because the air is never liquified, some of the problems associated with using liquid air systems are avoided.

The SCAMP system does not require sophisticated systems, elaborate controls, or complicated mechanisms in order to supply breathing air. Air flows out of the air tank under its own pressure, through a first stage pressure regulator, through two heat exchangers that have no moving parts, and through a second stage regulator that is identical to those used in conventional compressed air systems.

The air is stored at a low temperatures, and the outflow of very cold air travels through heat exchangers to provide cooling. This cooling capability is used to reduce heat stress, improving and extending physical and mental performance.

Phases of Air

Phase Diagram Sketch

A phase diagram shows the boundaries between the phases a substance may assume, typically as a function of temperature and pressure.

The above sketch represents a phase diagram of a typical substance. Because air is a mixture, a single phase diagram does not illustrate all of the intricacies introduced by the various substances that make up air in the atmosphere. Nevertheless, the illustration is useful.

Ordinary air is a mixture of nitrogen as N2, oxygen as O2, water (H2O), carbon dioxide (CO2), argon, and a spectrum of other substances. Some of these (water in particular) can interfere with breathing systems, and are typically removed in the preparation of air for compressed storage. Others are more of a problem when the air is to be chilled to very low temperatures, such as with cryogenic systems and the SCAMP. The two substances that form breathing air that is suitable for most purposes are oxygen and nitrogen. The SCAMP is designed to operate using a mix of oxygen and nitrogen in roughly the same proportions as they exist in the atmosphere. Most other substances (carbon dioxide, water and organic compounds) are filtered out prior to charging the SCAMP.

Oxygen and nitrogen do not share the same characteristics under all conditions. At very low temperatures and pressures, nitrogen can be a gas while oxygen is a liquid. The "triple point", where a substance can simultaneously exist as a gas, a liquid, and a vapor, occurs at a different temperature and pressure for oxygen than nitrogen. Therefore, the triple point shown in the above sketch is fictitous - there is no distinct triple point for air.

In other conditions, a mixture of oxygen and nitrogen does behave uniformly. The SCAMP takes advantage of this by operating in a region where the mixture exists as a single-phase, either as a supercritical fluid, or as a gas at lower pressures. Certain undesirable properties of the air mixture are avoided by avoiding the liquid phase in which they present themselves.

A Single-Phase Air Supply

When air is stored as a cryogenic liquid, a portion of air exists in the vapor phase. When the liquid is drawn down, the vapor phase fluid expands to fill the available space, resulting in a tank that is filled partly with liquid and partly with vapor. The liquid has a much higher density than the vapor, which can cause several problems.

Under gravity, the more dense liquid will settle in the bottom of the storage vessel, with the less dense vapor on top. If the system is designed to use vapor, rather than liquid, this works well unless the system gets flipped upside-down. Upside-down happens. Spraying liquid nitrogen into a face mask is not tolerable.

Also, because nitrogen boils easier than oxygen (i.e., has a lower boiling point), the nitrogen tends to vaporize more readily than the oxygen, so the initial outflow tends to have a higher concentration of nitrogen and corresponding lower concentration of oxygen. As more vapor is drawn out of the tank, the nitrogen becomes depleted, and the vapor then becomes oxygen rich - not exactly an ideal situation in a fire.

The SCAMP solves both of these problems by operating in a range of temperatures and pressures where air never exists as a liquid. In the supercritical and vapor phases, the oxygen and nitrogen in air remain well mixed, and the density at the top of the tank is virtually the same as that in the bottom of the tank. At any given point in time, the air in the SCAMP remains in a single phase.

Single-Phase in Orbit

Gravity is not useful in maany spacecraft applications. If a fluid were to be stored as a liquid, removing it from a tank would be challenging. Pouring the liquid out, or draining out of the bottom of the tank would not work without gravity. If the tank were pressurized, liquid and vapor would sputter out in an uncontrollable mix - or the liquid would simply stick to the sides of the tank. Although the phases could be separated by a bladder, the extremely low temperatures that are necessary in order to store fluids densely are generally incompatible with flexible materials. Reaching into the tank and scraping the globs of fluid out is not reasonable, nor is storing all fluids in squeeze tubes.

Spacecraft applications commonly use supercritical-phase fluids to avoid the issues associated with liquids. Niwot Technologies applies experience gained from the Apollo and Space Shuttle programs that utilized supercritical fluids in the SCAMP.

Copyright 2006 Niwot Technologies, LLC. All rights reserved.