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

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

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

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).

More about Radon in boil-off

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

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 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:

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.

3) if the new dewar isn't pressurized, open the pressure builder for 10-20 minutes to get the pressure >50psi.

4) turn down the primary dewar's regulator to get <12psi at the tee.

5) turn up the new (backup) dewar to 12psi.

6) turn the primary dewar back up to 15psi

7) verify that the flow rate is ~8scfh (16scfh for Syracuse)

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

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 auto-

dialer when both dewars have emptied and lost pressure.

The Auto-Voice Dialer

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

If the system gets down to 6 psi and the auto-

dialer 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.

Possible Improvements

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