Shutdown AG4108 SNF 2008-07-30 22:42:57: plate overtemp...

shott at snf.stanford.edu shott at snf.stanford.edu
Sat Aug 2 16:54:24 PDT 2008


This is a "provisional" clear of the shutdown condition.
I did find that the chiller had tripped due to evaporation in the reservior and it has been refilled and turned back on.
Because we had some replacment temperature click switches from AllWin,, I also did a comparison of their trip point relative to that of the installed trip switches thinking that they might be tripping too early.  As described below, that is not the case so we need to look more carefully at the coolant flow in the 7 parallel cooling lines to see if one or more are starved ...
Preliminary investigation of this problem:
We got some replacement overtemp snap switches from AllWin.  I mounted some of them on an aluminum block with silicone heat sink compound and then also placed one of the in-use ones (the most readily accessible top plate unit) on the same block.  I then heated the aluminum block on the digital hot plate in litho in 5 deg C increments.  All three switches tripped at an indicated hot plate temperature of 75 degrees C within 1 degree C.  I was surprised how close they were.  The replacement ones seem to indicate a trip point of 150F which is about 65C.
In any event, I think that our premature over temp is NOT a temperature switch problem and is likely indicative of a real cooling problem.
As soon as we can, I would like to place flow meters temporarily in each of the 7 cooling legs that run in parallel to see how the 2-2.5 gal/min cooling flow is divided amongst the 7 paths.
My guess is that it is not terrribly even.
John




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