The Nitrogen Purge System
By Ryan Badman
Purpose: To keep the hardware
inside the cleanroom in a pure
nitrogen environment that
protects the hardware from the
normal radon levels in air.
Theory
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Highly pure inert gases such as argon or nitrogen can be used
to purge an environment of radon (called nitrogen stripping
when N2 is used). Synthetic air can also be ordered, but this is
a much more expensive method. The nitrogen flows from the
cabinet to a charcoal bed that filters out the radon.
Berkeley System Schematic
The Syracuse System
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For the most part, our system at Syracuse emulates the
Berkeley system. However, we use only one cabinet, instead of
four, and the cabinet is 16 ft^3 in volume. Our flow rate
(16scfh)is twice as high as Berkeley’s (8scfh).
(scfh=standard cubic feet per hour)
According to Berkeley, one exchange rate per hour is needed
because radon gas can penetrate hermetically sealed purge
cabinets
Calculations for flow rate
Cabinet Volume= 4’ x 4’ x 1’ = 16’
Liquid N2 capacity = 230 L = 60.4 gallons
1 gallon = 93 cubic feet of N2 gas boiloff
N2 boiloff flow rate= 16 cubic feet per hour to make one
exchange occur per hour
Total cubic feet of gas from a 240 L dewar = 60.4 * 93 =
5614
1 dewar lasts 5614 / 16 = 351 hours = 15 days
Liquid Nitrogen Dewars
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The system uses two 230L ultra
high purity liquid nitrogen dewars.
Berkeley recommends using
dewars that are able to have a
pressure of 50-100psi at the top
of the dewar. Berkeley used 235
psi dewars.
Boiloff gas is used instead of
cylinders of gas because cylinders
get contaminated easily at low
pressures.
Questions that need to be answered
* How much radon is in nitrogen boiloff?
(No definite amount at this point, but the two next
slides gives some information).
 * How does it depend on how full the dewar is?
(This would depend on the radon concentration in
the air outside the dewar and how much air is
leaking into the dewar as boil-off gas flows out).
 * What happens if both dewars run dry ?
(This is answered in the section about the back-up
system).
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More about Radon in boil-off
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The boiling point of radon is -61 degrees
Celsius and the boiling point of nitrogen is -195
degrees Celsius. Radon boils at a higher
temperature and is heavier than nitrogen.
Theoretically then, in a tank of liquid nitrogen,
most of the radon would remain in liquid form
and evaporate much slower than the liquid
nitrogen, making the boil-off gas sufficiently
pure.
Pressure Regulator
Pressure Regulator
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One is at 15 psi for the primary dewar supplying gas. The
second is set at 12 psi. The two dewars are connected by a tee
after the pressure regulators. Gas only flows out of the second
dewar when the first dewar drops below 15 psi.
Setup Procedure
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According to Berkeley:
“Check the dewars daily to see if the primary has run out. If so, increase the backup dewar
pressure regulator to attain 8scfh flow rate (16scfh for Syracuse) and order a replacement dewar
for the primary or refill it.
To replace or refill an empty dewar:
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1) Turn the regulator all the way down (counterclockwise) on the dewar you are about to replace
(empty) and close that dewar's gas valve.
2) Disconnect the regulator from that (empty) dewar and connect it to the new (full) dewar and
open the new dewar's gas valve.
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3) if the new dewar isn't pressurized, open the pressure builder for 10-20 minutes to get the
pressure >50psi.
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4) turn down the primary dewar's regulator to get <12psi at the tee.
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5) turn up the new (backup) dewar to 12psi.
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6) turn the primary dewar back up to 15psi
7) verify that the flow rate is ~8scfh (16scfh for Syracuse)
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The dewar safety valves will release at 235 psi. Avoid letting the pressure get near this, or safety
valve could be releasing slowly (hissing constantly) and top of dewar will ice up and dewar will
only last a few days. “
The Pressure Switch
The Pressure Switch
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Repeatability: ±1 psi or ±5% of actuation point, whichever is
greater
Pressure Sensing Element: Buna-N diaphragm-sealed piston
Pressure Connection: Brass NPT male
Case Material: Brass
Electrical Switch: SPDT, 10 amps at 125/250 VAC
Temp. Ranges: Ambient: -40° to +250° F; Process: -40° to +250°
F
Set pressure dial to the pressure you want the auto-dialer to notify
you at. which is 6 psi in our case. The switch only sets off the autodialer when both dewars have emptied and lost pressure.
The Auto-Voice Dialer
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http://www.globalw.com/products/
ad200.html This is the link from
where it was purchased.
It takes one alarm input and will
dial up to four phone numbers. For
an alarm it sends a prerecorded
voice message.
The auto-voice dialer is activated by
an electrical pulse from the
pressure switch
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If the system gets down to 6 psi and the autodialer calls, then you have less than an hour to
change in a new dewar to avoid contamination!
The Adjustable Flow Meter
The Adjustable Flow Meter
This device controls the flow rate from the
dewar. The meter has a range of 2 to 20 scfh.
 We set it to 16 scfh.
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Possible Improvements
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To further lower radon in a nitrogen purge
system, a cooled charcoal radon trap is often
added. It has not been decided whether such a
trap will be added to the Syracuse system.
LINKS
http://cdms.berkeley.edu/cdms_restricted/UC
B_labs/facilities/cleanroom/facilities/N2_purg
e_system/N2_purge_system.html#5._Required
_Parts_:
 Pocar Thesis from
http://cdms.syr.edu/internal/radon_mitigation.
html
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