Question about a refrigeration system issue

[dannyzeiss]

Hi Guys,

I'm new to refrigeration, and in fact haven't been trained in repairing refrigerators. However, I am required to work on systems that require a refrigeration system.

For example, I work on instruments in pathology labs, known as "Cryostats". They may be called something else where you are from, but essentially, they are used to precisely slice very thin (4-10 microms) of an autopsy in temperatures of around -20 degrees celcius.

Now, the cryostat that I am working on, has its refrigeration in the back part of the instrument. The current problem with the instrument is that every now and again, there is a clanging sound, kind of like a "jack hammer" sound that occurs, and at first I thought it could be the main contactor that turns on the compressor, however, I don't think it is, because the contacts of the contactor seem fine and are not corroded.

The refrigeration system has two solinoid switches that open and close the gas valves and because the system is atleast 12 years old, I think it could be these that are causing the problem.

However, I really need some advice. Do any of you guys know which of the following would be the most likely cause of this situation:

-Solinoid switch controlling the gas valves
-Main contactor which controls the compressor
-Compressor
-PCB board (called the triac board) controlling the switching of the solinoids and contactor

I can repair/replace the pcb board or the contactor, however, I cannot repair/replace solinoids or compressor, so if I know which are most likely for this "jack hammer" sound, then better prepare for the upcoming repair, rather then groping in the dark.

Let me know if you have any questions, as I would really appreciate any help with this situation.

Best regards,

DannyZeiss

[Brian_UK]

Jack Hammer sounds like the contactor.

You need to check that the coil of the contactor is receiving the correct voltage and that the voltage is steady. Check also that all electrical connectors relating to the coil are 'good'. Loose neutral (0 volt) lines can be a pain in the a****.

It could, of course, be a faulty signal command from controls side. If you can monitor the voltage to the coil then you might get some clues to what is happening. Use an analogue meter if possible unless you have a digital that will show peaks/trends.

Problem is that if it is the contactor then that chattering will also chatter the compressor and could kill it quite quickly.

For the cost of the part, get a new contactor first and start from there.

[dannyzeiss]

Hi Brian,

Thanks for your prompt response. I really appreciate it!.

The Contactor as the cause of the issue does make sense, because of the sound it makes.

I will take your advice and measure the voltage of the contactor and check the connections to make sure nothing is loose as this can be done in 15 minutes. I'm pretty sure the contactor is run on 24 volts. I can't monitor the voltage across the contactor coil as I don't have a monitoring device and the problem occurs to rarely for me to wait for it to happen.

You mentioned another really important point. You said that if it is the contactor, then it could cause the compressor to fail. I have already asked the customer to turn off the cryostat until I replace both the electronics controlling the contactor and the contactor itself. However, let me ask you this: Lets say I do already have a faulty compressor, and I replace the control electronics and contactor. Could a faulty compressor damage the new control electronics or contactor? Or can the damage only be caused from the control electronics/contactor to the compressor and not the other way around?.

By the way, right now, the cryostat is cooling just fine. This chattering sound occurs intermittently, but if the system is cooling, could I rule out a problem with the compressor? I'm assuming that if the compressor is the problem, then it would either making the "jackhammerr" sound all the time, or the system would not cool at all. Is this a correct assumption?

[Brian_UK]

Compressor unlikely to cause damage backwards up the line, it can happen but not often.

If the cooling is OK then we will pray together. :D

[dannyzeiss]

Thanks Brian,

I'm due to work on this when the new parts arrive from Germany in a couple of weeks, so I will post the outcome after my next visit.

Cheers!

[goshen]

Hi
good luck !
but dont forget before replacing the contactor,drop the main switch!!

[Magoo]

get a refrig service tech to look at the system, before you blow yourself up electrically. A dead lab tech is no good to anyone.