Telemark TVP-2000, Руководство по эксплуатации, установке и обслуживанию

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TVP Manual (full) March 2007 CE rev 03.doc
4.60 The design and placement of cryo-coils
Caution
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, BOC Edwards
and other general vacuum equipment suppliers.
It is important to appreciate that a TVP 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 TVP 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 TVPs 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.
Maximum recommended coil surfaces and theoretical pumping speed for water vapour at –
105
o
c coil average temperature
Unit
Suggested cryo-
surface Theoretical H
2
O pumping
speed
Sustainable H
2
O pumping
speed (30% deration)
TVP2000 1.6 M
2
238,400 l/S 166,880 l/S
TVP3500 2.4 M
2
357,600 l/S 250,320 l/S
The value for the sustainable pumping speed represents a 30% reduction to account for
factors such as proximity of system components reducing conductance and the accumulation
of ice reducing heat transfer. The de-ration is subjective and unique to each installation and
process.
Since the pumping action is a function of temperature the cryo-surface must be designed to
minimise heat gains from sources of external heat. In a perfect system all of the heat load
placed on the TVP would come from the condensation of water. Internal heat from the mass
of the coil also must be removed before the system is capable of pumping therefore a design
which uses a thermally conductive material of low mass should be selected.
The construction ideal cryogenic surface
Appropriate surface area for system
Copper or Aluminium construction
Thin wall
Large surface area volume ratio
Internal volume of not less than 0.3 Litre
A pressure drop between the inlet and the outlet of less than 0.3 bar or equivalent to 20M
(TVP2000) or 40M (TVP3500) of 12.5mm i.d smooth tubing.
Capable of handling a pressure of 17 bar (250 psi) over temperature range +150
o
C to
–160
o
C.
Simple to clean – coating processes often create large amounts of powdery deposits, which
will reduce the efficiency of the TVP.
The most common form of cryo-surface and the simplest to work is 16mm 5/8” refrigeration
grade copper tubing the only significant limitation is that as a consequence of a very thick wall
the thermal mass is usually high slowing the rate of cooling significantly.