What is Supercritical?
It's super! It's not critical.
When we look at a phase diagram for a substance,
term "critical point" refers to point at the end of the boundary
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
The SCAMP system
stores air at temperatures that are slightly above those
at which the oxygen in air would liquify at atmospheric pressure.
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
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.
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
of the intricacies introduced by the various substances that make up
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
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
as they exist in the atmosphere. Most other substances (carbon dioxide,
water and organic compounds) are filtered out prior to charging the
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
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
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
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
to store fluids densely are generally incompatible with flexible
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
in the SCAMP.