Telemark Cryogenics Microprocessor Based Cryochillers
Revision 1.0.7 October 2018
6
DESIGN OF CRYO-COILS
The design and placement of a cryo-coil is a specialised task, which must be
carried out by a skilled vacuum practitioner. Free technical help is available
directly from Telemark. Telemark also manufactures cryo-coils to order to
suit your installation.
It is good practice for cryo-coils with attached feed-through to be
installed from inside the chamber so that there are no connections
within your vacuum system, verify that moving parts within your
vacuum chamber do not interfere with the cryo-coil. Telemark
supplies a range of high quality vacuum feed-throughs, which
compliment its range of refrigeration lines
The cryo-coil should be more than 1 diameter away from the
chamber wall or any other object including other parts of the cryo-
coil. It is bad practice to mount the cryo-coil directly onto a metal
plate. The ideal cryo-coil should have a large surface area but also
be of low mass. The cryo-coil should be capable of accepting a
working pressure of 275 psig. If chamber radiant heating is being
used aluminium foil placed between the cryo-coil and the heat
source is a very effective heat shield, however it may reduce vapor
trapping speed. Radiated heat from any source in a vacuum
chamber will have a detrimental effect on the cryochiller.
The design and placement of cryo-coils assumes a basic understanding of the
principles of vacuum pumping, many good texts on the subject are available
from Leybold™, Edwards™ and other general vacuum equipment suppliers.
It is important to appreciate that a unit will only pump water efficiently in
conditions of molecular flow i.e. In vacuums of less than 1 x 10
-3
mbar. Where
the pressure exceeds this value before the system is switched to cool there is a
risk of over loading and saturating the cryo-surface, which may lead to poor
process quality and slower pump downs. The temperature at which the unit
may be placed in cool is known as the crossover pressure and is analogous to
the same criteria, which is applied to other forms of secondary pump.
A units performance is conductance limited i.e. Its maximum pumping speed
is directly related to the exposed surface, because of the high efficiency of the
unit it is generally possible to use larger cryo-surfaces than traditional systems
of equivalent power consumption.
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