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Making Compressed Gas Cylinders
Name: Michele V.
Status: other
Age: 30s
Location: N/A
Country: N/A
Date: 10/15/2004
Question:
How do they get the helium into the cylinders?
Replies:
Helium gas is a minor constituent of natural gas that is obtained from gas
wells. The helium is cryogenically separated from the natural gas. It is
then pushed into the cylinders with powerful pumps.
Bob Erck
One story is to find, collect, and purify the helium.
But that is not what you asked about. so I will assume that is done.
The helium gas is waiting in a large tank at some not-very-high pressure
such as 1 atmosphere, (15 pounds/square-inch absolute, "psia").
Fairly ordinary piston-pumps are used to compress it
and gradually fill the bottle to about 2500 psia, or 150-200 atmospheres.
The pump's shape is optimized for such high pressures:
thick metal walls and narrow pistons are used.
If the piston is the size of a pencil, only 1/4 inch wide,
then the area of its end face is only 1/20 of a square inch,
and only 125 pounds of force are needed to push it forwards.
A medium-large 1000-Watt motor can do more than 10,000 strokes in an hour,
roughly the number of small strokes needed to fill a whole tank.
People in scuba-shops do this all the time, with air cylinders for divers.
They find that the compressed air and its bottle gets hot as the gas is
compressed,
and the heat must be carried away so the job can be finished at room
temperature.
The tank is sometimes sitting in a large barrel of water while it is filled.
In the gas industry I am not sure what they use, maybe water spray,
maybe fans and automatic valves doing delayed topping off of batches of
bottles.
Maybe they just leave the fill valve open a long time, until the tank
cools down.
In practice I think that just as soon as they get any helium
they pressurize it in stacks of rather large bottles,
say a pyramid of 6 bottles, each 15 feet long and 1 foot wide. Maybe
you've seen one somewhere.
Then each consumer bottle can be filled by merely connecting a thick metal
hose
and opening a valve and filling to 2500psia.
Then it's hot, so they close the valve and put the bottle aside while it
cools down.
As it cools down, the pressure of the gas trapped inside the bottle decreases,
and later they top off each of the bottles again.
It is usually shipped around in rail-cars and tanker-trucks.
I am not sure what pressure those carry.
But the tanks behind the truck are quite narrow
compared to tanks for gasoline or chemicals, so I think it is fairly high
pressure.
There are a bunch of ways to run that business.
Jim Swenson
Helium is compressed into the cylinders using a motor driven piston. Actual
compressors are fairly complicated but they all operate on that basic
principle.
Vince Calder
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Update: June 2012
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