>Attila:

We have had turbomolecular pumps in our lab for 14 years and have had
little or no problems with them if they are water cooled. We originally
took delievery of a Balzers THP 170 in 1983 on our MAT 251, but immediately
took off the air cooling fan and installed a water chiller (NESLAB) with a
water circulation interlock.  This interlock will not let the turbo run
unless cool water is circulating, there is a flow and temperature switch.
This old style 170 pump, with regular 6 month wick oil changes ran
effortlessly for 13 years and is still in good shape.  We replaced this
pump last year with a 260 turbo-drag pump so that we could eliminate the
oil diffusion pump behind the turbo for running H/D.  This new pump has
reduced the H3 factor and backgrounds tremendously on the 251.

When we took delivery of the Kiel carbonate device 3 years ago, the Balzers
065 turbo was air cooled and failed in the first 3 months.  It was replaced
under warranty and we immediately hooked it up to the water chiller.  The
replacement has run flawlessly for 2 3/4 years.  We find that the pump down
time is greatly reduced if you have a large backing pump (>200 l/m)

On the other hand we have had many problems with oil diff pumps, primarily
they contaminate everything when the cooling circulation fails.  This has
only happened to us once, but once is enough!  When turbos fail due to
bearing failure or controller failure they spin down and vent or go to
forevac pressure.  If you have a edwards shutoff valve the system will just
vent without contamination.  Diff pumps also don't work very well if the
oil gets oxidized when exposed to a leak or repeated vents to atmosphere.

You can avoid many problems by water cooling your turbos.

Cheers,

Bill



-------------------------------------
William J. Showers
Dept of Marine Earth & Atm. Sciences,
North Carolina State Univ.

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