Gas Injection System Design |
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This page explains
some of the issues associated with the design and construction of the
high pressure gas injection system used in my magnetoplasmadynamic
thruster design. Since this design does involve gas flow issues, it is
suggested that you also read up on compressible
flow first if you're not already
familiar with it.
The first
design
parameter that needs to be decided is the desired flow rate of the
system. In my case, the flow rate of the injection system was the
limiting factor in other areas, so flow rates as high as possible
within budget constraints were sought. This also suggests a high
delivery
pressure, so this portion of the design is limited only by the
components and materials that can reasonably be obtained that can
withstand these pressures.
The design started with the choice to make the working gas Argon. Among
its various properties that made it attractive as a propellant, the
final decision came down to availability and cost. Argon is used in
welding applications such as the electric arc welding of aluminum and
when mixed with carbon dioxide for electric arc welding of steel, so it
was readily available at a reasonable price at welding supply shops in
compressed gas cylinders. Cylinder pressures for Argon are typically
around 2,000-3,000 PSI, which requires a pressure regulator in order to
supply it at lower pressures. I found a good Harris 3500-125 high
flow Argon regulator on eBay for approximately $40 (what a steal!) The
maximum delivery pressure on this unit is about 125 PSIG (i.e. PSI as
read from a gauge. This reading is the pressure above ambient, so a
reading of 0 PSIG means at ambient pressure. Ambient pressure is about
14 PSI, which could be expressed as 14 PSIA, or 14 PSI absolute.) Let's
take a quick look at the flow data from the Harris datasheet for this
regulator. [1]
Fig. 1: Argon
Regulator
Flow Data
This
datasheet assumes that the tank can deliver at least 2000 PSIG to the
regulator. If we read across from 125 PSIG delivery pressure, we find
that this regulator can deliver Argon at a rate of about 1000 SCFH
(standard cubic feet per hour. A standard cubic foot is the volume of
the gas at standard temperature and pressure. Standard temperature and
pressure are 20 degrees Celsius and 1 atmosphere, respectively.) This
is equal to a little more than 0.25 standard cubic feet per second, and
isn't very much. We can get around the regulator's limited flow rate by
using a surge tank, which is simply a large pressure vessel connected
downstream of the regulator. This works by allowing the regulator to
(relatively) slowly fill the surge tank until it reaches a high
pressure, which will support much higher flow rates for the brief
period of time that it is needed. The folks at Aimtek in
Auburn,
Massachusetts were kind enough to sell me a used E size oxygen tank
which worked very nicely as a surge tank.
Fig. 2: Argon
Cylinder, Regulator, and
Surge Tank Assembly
The
next major component selection was the gas solenoid that would control
the gas flow from the surge tank and regulator. In light of the 125 PSI
maximum working pressure from the regulator, this was relatively easy.
Not surprisingly, I found a something on eBay - an AAA SS40 gas
solenoid
with a 160 PSI maximum working pressure for about $50. The solenoid is
shown below connected to the surge tank.
Fig. 3: Gas
Solenoid
Connected to Surge Tank
For
the purposes of analyzing the gas injection system, we can consider the
solenoid as emptying directly into ambient air pressure since the gas
solenoid presents a critical orifice to the gas flow. This greatly
simplifies the job of calculating the gas flow rate, since this data is
directly available from the SS40's datasheet. [2]
Fig. 3: SS40 Gas
Solenoid
Datasheet Excerpt
Using
this data, I calculated the gas flow rate from the solenoid to be
approximately 327 SCFM or 5.45 SCFS. This is much better than the 0.25
SCFS obtained directly from the gas regulator
Fig. 4: Complete
Assembly
Warning - Suffocation Hazard |
Working
with compressed inert gases such as Argon is generally safe, however,
leaks or accidental discharges can quickly displace oxygen leading to
death by suffocation. Always work in a well ventilated area, check for
and fix leaks, and handle gas cylinders with care. Dropping a gas
cylinder can shear off the valve, releasing large amounts of gas and
propelling the cylinder to dangerous velocities.
I hope to raise the
working pressure of this system significantly, and thereby increase the
gas flow rate. The largest obstacle at this point is finding a gas
solenoid that can work at considerably higher pressures, with large
diameter connections, for a reasonable price. Unfortunately, these sort
of things don't show up on eBay very often. I'll also need to find a
high pressure inert gas regulator, which is a significantly easier
though much more expensive task. The hosing will also need to be
replaced, but this is something that I can deal with when I get there.
References
[1]
Tech Data, Regulator Technical Data 3500, Harris Calorific, Inc.,
Mason Oh.
[2] Valve Catalog #28, Complete Air Valve Catalog, AAA Products
International, Dallas Texas.
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visitors since September 2007
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