Cryogenic: How to repair the compressors of JT type dilution frigdes

Here we write how to repair the compressor (KNF N145ANE) when the diaphragm is broken in the operation of the dilution refrigerator.

The priorities in the repair are to prevent rapid vaporization of the mixture, which damages the refrigerator and to minimize the loss of 3He.

0. Prepare a spare diaphragm and a tool for the repair shown in the right figure when you operate the refrigerator.

Speedy repair is important to minimize the loss of 3He. Prepare tools in advance.

1. Check what happens.

If the diaphragm is broken, the pressure at the storage and the injection will rise because air is mixed into the 3, 4He line and the LN2 trap is blocked. The pressure in front of the compressor will be 1 atm.

To confirm the break of the diaphragm, decrease the pressure in front of the compressor by opening the storage valve. If the diaphragm is really broken, the pressure will soon return to 1 atm when we close the storage valve again.

2. Decide where to store the mixture during the repair.

If the mixer temperature is kept low, store the mixture in the refrigerator during the repair. Close valves in front of the gas handling system.

If the mixer temperature is rising, start recovering the mixture into the storage during the repair. Temporarily close the valves in front of the gas handling system and change the 3, 4He line to connect directly to the storage at the back of the circulation pump. Then open the valves in front of the handling system and start recovering.

3. Remove the upper cover of the compressor.

Remove the pipes and the screws indicated in the right figure. Remove the upper cover.

4. Remove the broken diaphragm.

Remove the screw which fixes the diaphragm by using the tool.

The diaphragm will be broken as the right figure.

5. Set the new diaphragm.

The smooth side is the upper side.

6. Set the upper cover of the compressor.

This cover works as a valve. Clean the cover with ethanol before setting.

7. Change the 3, 4 He line as the way it was.

8. Recover the mixture into the storage.

But if the mixture temperature is still kept low, there is no need to recover the all mixture at this stage. Recovering later will make it easy to purify the mixture.

9. Purify the mixture stored in the storage.

Recover the LN2 trap by warming with hot water and vacuuming the trap.

Set the LN2 trap again and circulate the mixture only in the gas handling system. Because the air is mixed, the trap may be blocked. In that case, recover the trap again.

Repeat purifying and recovering the gas into the storage. If purifying is not enough, the pressure of the storage decreases by the repetition. Repeat the steps until the pressure becomes stable.

10. Recover all of the mixture into the storage and finish the operation of the refrigerator.

The pressure of the storage will be smaller than the value before the operation. This corresponds to the loss of the mixture.

If the decrease is large, we need to add the gas. The loss will be mainly 3He.

11. Warm up the refrigerator.

The 3, 4He line in the refrigerator will be blocked by air. Warm up the refrigerator and vacuum the line.

To prevent that the pressure of the line becomes too high, vacuum the line immediately after pulling up the refrigerator with a dry pump.

12. Operate the refrigerator again and check the work.

If the refrigerator is not damaged and the loss of the mixture is small, the lowest temperature is not changed from the value before the trouble.

If the refrigerator does not work, we need to repair the refrigerator or adjust the amount of the mixture.

In troubles of the dilution refrigerator, ask experts for help.

Although troubles of the refrigerator are bummers, don't give up your experiments!