Instrument Details

1. Make & Model: Linde Cryogenics USA, Model 1410
2. Installed on: May 2009
3. Purchased under the Scheme: ISPC, thrust area

Technical Specifications

1. 14 litres per hour in pure gas mode
2. 30-40 litres per hour with liquid nitrogen pre-cooling.

Working Principle

J-T principle: A compressed gas when allowed to expand get cooled. If the same cycle is repeated, then the gas can be cooled down below its liquefaction temperature. In this plant, the helium gas is compressed to high pressure of ~230 psi and allowed to expand in the cold box. With proper heat exchangers and valves, the helium gas finally cools below 4.2 K and gets liquefied. This liquid is collected in a separate container (helium dewar) for storage and transfer.

• Produces liquid helium from helium gas. Liquid helium can be used for cooling down to 2 K.

J-T principle: A compressed gas when allowed to expand get cooled. If the same cycle is repeated, then the gas can be cooled down below its liquefaction temperature. In this plant, the helium gas is compressed to high pressure of ~230 psi and allowed to expand in the cold box. With proper heat exchangers and valves, the helium gas finally cools below 4.2 K and gets liquefied. This liquid is collected in a separate container (helium dewar) for storage and transfer.

FREQUENTLY ASKED QUESTIONS ABOUT LIQUID HELIUM PLANT:

Q: What do I need to do to get Helium from the facility?

A: You need a clean and leak proof helium recovery system or a thoroughly checked return gas line from your laboratory to the helium plant.

Please note that we cannot give Helium to laboratories without a proper recovery system and flowmeter tested by us.

Q: Why can’t you just give me some Helium in a dewar?

A: The cost of liquid Helium is largely (~80%) the cost of the pure gas. In fact if you buy liquid Helium from industrial suppliers you will find that the cost of 1 liter of liquid is largely the cost of the equivalent amount of pure gas. It is 800-1000 Rs/liter at current rates. Thus without recovery, the cost of 100 liters of Helium would be close to 80,000 Rs. Since we recover ~95% of the gas and re-liquefy the recovered gas, the cost comes down enormously – because the same volume of gas gets recirculated many times.

The cost of liquefaction itself is a lot less than the cost of the gas.

Please treat Helium from the facility like a library book. If users do not return books (Helium loss) or return them in badly damaged state (contaminate the gas) then the library will not be able to sustain itself!

Q: How much gas is really needed to produce one liter of liquid?

A: 1 liter of liquid Helium weighs 125 gms (0.125 kg) = 31 moles = 31 x 22.4 = 694.4 liter at NTP (approx). More accurately it is 750 liters or 0.75 cu mt.

Q: How do you calculate the losses and quantify this?

A: We take the amount of liquid given out during the whole year. Then take how much fresh (pure) gas we had to put in to keep the circulation going. This fraction should be as low as possible.

For example In the period OCT 2017 – SEPT 2018 we disbursed 14,463 liters of liquid to ten different systems. The total “loss” was 732 liters. So the lost amount was 5.06% of the amount given to users.

The actual production by the liquefier is about 20% more since a certain amount goes into precooling the dewars during every transfer into smaller dewars.

Q: If I bring some Helium gas in a cylinder, can you liquefy it for me?

A: This is not as simple as it sounds. A small amount of gas cannot be liquefied in isolation. The gas that you have brought in general will require to be mixed with our pool of gas. This means that unless we are 100% sure about the purity of the gas that you have brought we cannot allow this. There is a danger of impure gas contaminating our stock. So in general, this will have too many potential problems that can cause difficulties with regular operation.

So this will work only for very large volumes of gas which have to be carried in very heavy cylinders, under pressure. In fact 100 litres of liquid can be carried in a container whose tare weight may not exceed 100 kgs. If you want to carry the same amount in gaseous form, you would require at least 10 standard cylinders (gas stored at 150 atmosphere pressure), with each cylinder weighing more than 50 kgs! So bringing “some gas” in cylinders is not at all an easy proposition!

Q: What do you mean by a “good and efficient” recovery system?

A: A good and efficient recovery system must be “Helium leak-tight” to a leak rate of 1e4 mbar-lt/sec. Remember that Helium atom is the smallest “freely floating” atom with a size of 30 picometer (approx.). Hydrogen exists mostly as H2 molecule which is larger in FAQ : Liquid Helium Plant : November 2018 P a g e | 3 size than this. Helium gas can diffuse through the pores of many materials even when other gases cannot, because the Helium molecules are smaller in size. So some seals which may be leak tight for Nitrogen are not necessarily “leak-tight” for Helium.

To know whether a recovery line is good for Helium or not – the line has to be pressurized to approximately 5 atmospheres. Then all joints, seals and valves are tested by a “Helium leak-detector” in its “Sniffer mode”.

Overall the recovery (over a month) should be more than 95% of the Helium given to your laboratory.

The impurity in the returning gas needs to be kept to 1-2%. The efficiency of the liquefier degrades drastically as the impurity increases.

Q: What kind of material should I use (and not use) in building a recovery line?

A: Ordinary rubber types (Neoprene, Nitrile etc) are porous to Helium. If you use gaskets made out of these to seal any joint or valve, Helium will leak through. You should use gaskets, O-rings etc made of VITON or BUNA-N. For all joints you should use a KF type coupling and either Swagelok (or equivalent) and Globe valves.

The brazed or welded joints must be Helium leak-tested and should be done by people qualified to do this.

Q: Why are you so paranoid about impurities?

A: Impurity in a Helium line (pump oil, Air etc) degrade the performance of the liquefier drastically. While the liquefier will ultimately filter these out, in doing so it will collect some of the impurities in its circulation circuit. The cleaning process of these requires almost 7 days.

So essentially any one user introducing excessive impurities in a Helium recovery network is like one computer user introducing a virus in a network. Everyone will get badly affected and we do not want this to happen!

Q: Why do you insist that the user must return the dewar with some liquid Helium in it?

A: The liquid Helium containers must be kept (as far as practicable) cold. “Cold” means that at least a small amount of liquid must be there in it. There are two problems that arise if this practice is not followed.

First, when liquid is being transferred to the dewar (typically 60 or 100 lts capacity), the amount required to cooldown the dewar itself would be very large if the dewar is warm. For example, our experience suggests that if a 100 lt capacity Helium dewar is “warm” then 40-50 lts of liquid is necessary to precool the inner walls of the dewar before liquid Helium can “collect” in it. Thus to fill a 100 lt dewar, nearly 150 lts would have to be used. This loss reduces a lot if a small amount (5-10 lts) is already there at the bottom of the dewar. Thus the utilisation of the liquid is more efficient.

Second, the vacuum between the inner walls (at 4Kelvin) and the outer wall (at room temperature) is extremely important for providing thermal isolation. Repeated warming of the inner wall degrades this vacuum and the dewar would then require a vacuum jacket evacuation with a turbo molecular pump for 2-3 days. This is a long process which needs to be done by trained people. It will also reduce the performance (holdtime, boil off rate) of the dewar, if this has to be done repeatedly.