© Edwards Limited 2007. All rights reserved.
Page 19
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Installation
B722-01-880 Issue K
3.3.5
Backing connection
Use suitable vacuum tubing and connectors to connect the NW flange of the backing-port to your backing-pump. If
necessary, use flexible pipe or bellows to reduce the transmission of vibration from the backing-pump to the EXT
pump.
We recommend that you use an Edwards two-stage backing-pump. The backing-pump can also be controlled by the
EXC Controller. The minimum size of the backing-pump required is given in
. You may have to use a larger
backing-pump if you run the pump at a high inlet pressure or high throughput or if you purge the pump with more
than 25 sccm (0.42 mbar l s
-1
, 42 Pa l s
-1
) of purge gas.
Do not use the EXT pump with a backing pressure below 1 x 10
-4
mbar (1 x 10
-2
Pa). Lower backing pressures will
increase the evaporation rate of the lubricating oil and so will reduce the life of the bearings.
3.4
Vent-valve connection and control
When you design your system and when you install a vent-valve, take note of the information in
and in
. You can vent the EXT pump and your vacuum system by any of the following methods:
z
Use the manual vent-valve supplied.
z
Use a TAV5 solenoid vent-valve accessory (see
) in place of the manual vent-valve, together with
a vent-restrictor, if necessary.
z
Use a TAV5 vent-valve connected to a convenient flange on your vacuum system, with a vent-restrictor, if
necessary.
z
Use an alternative valve connected to your vacuum system, together with a vent-restrictor, if necessary.
If you use the manual vent-valve, you must open the vent-valve only after the EXT pump speed has fallen to 50% of
full rotational speed.
If you use the TAV5 vent-valve, you can only vent the EXT pump when it is at full speed if the vacuum system has a
volume of 5 litres or more. If the volume of your vacuum system is less than 5 litres, you can incorporate a suitable
restrictor (see
) and vent the pump when it is at full speed. Alternatively, if you do not fit a restrictor, you
can use the EXC Controller to control the vent-valve and configure the EXC Controller to open the vent-valve after
the EXT pump speed has fallen to 50% of full rotational speed: do not select the ‘Vent On Stop’ option (refer to the
EXC Controller instruction manual for more information).
If you use another vent-valve, you must ensure that you have a suitable vent-restrictor fitted to suit your vacuum
system to limit the rate of pressure rise: refer to
. If you do not have a suitable vent-restrictor fitted, you
must open the vent-valve only after the speed of the EXT pump has fallen to 50% of full rotational speed.
If you connect the vent-valve to your vacuum system, select a point upstream of the EXT pump to prevent
backstreaming of oil from the backing pump. Do not connect the vent-valve to the backing pipeline. Connect the
inlet of the vent-valve to the vent gas supply (refer to
for the vent gas specification).
Note:
If you use a vent-restrictor, you may find that the time required to vent your system is unacceptably long.
You may be able to reduce the vent time if you use an unrestricted vent port without a vent-restrictor and
wait until the pump speed has fallen to 50% of full rotational speed before you vent the pump.
Table 9 - Vent-valve orifice diameter (with atmospheric pressure at the inlet of the vent-valve)
Vacuum system volume (l)
Orifice diameter (mm)
< 20
< 10
< 5
< 2.5
< 1.25
≤
1.0
≤
0.7
≤
0.5
≤
0.35
≤
0.25
Summary of Contents for EXT250 Series
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