BOC Edwards 18B4B, Instruction Manual, Page: 8 / 62

Issue A Feb 04
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18B4B Vapour Booster Pumps
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The ejector nozzle (8) is housed in the ‘T’ piece (38),
which is bolted to the side cone (14) into which the
ejector discharges. The side cone ends in a vertical
backing condenser (17).
The pump fluid is heated and vaporised by electric
heaters (27). There are three heaters in the pump,
rated the same for all voltage variants. The boiler
(33) has a dipstick/fluid filler connection (21) and a
fluid drain connection (29).
The electrical supply cables from the heaters are
connected into a terminal box (31) on the base of
the pump, for ease of connection to your electrical
supply.
The pump boiler assembly (33) has a cleaning port
cover (10). You can remove this cover and use the
cleaning port to remove any residue build up from
the inside of the boiler. You should regularly clean
the boiler, to maintain the pump performance at its
optimum level. A radiation shield (22) underneath
the heaters reflects heat from the underside of the
heaters (27) to the boiler, and so increases the
efficiency of the pump.
The pump is cooled by water, which flows through
copper cooling-coils (13, 19, 41) wound around the
top cone, the side cone and the backing condenser.
The cooling couplings (36, 40) connect these coils
together, to form a single cooling circuit around the
pump.
The pump can be supplied with one or both of the
following optional items:
• A fluid low level switch (12): see Section 1.6.
• A Pt100 temperature probe (30): see
Section 1.7.
1.4 Principle of operation
Refer to Figure 1. Pump fluid is heated in the boiler
(33) to produce a vapour, which passes up through
the interior of the diffusion pumping stages (42, 6, 5)
and emerges from the gaps in the jet caps (43, 44,
47) as high velocity vapour streams. The vapour
streams condense on the cooled walls of the top
cone (45) and the side (ejector) cone (11), then
drain into the boiler (33) for recirculation.
A portion of system gas which arrives at the pump
inlet (1), is trapped in the vapour stream from the
first stage jet cap (47) The gases are compressed and
transferred to the next stage. The gases then pass
through the ejector nozzle (8) and into the backing
condenser (17) where they are removed (through
the outlet, 16) by your backing pump.
The cooled surface of the guard ring (2) above the
top jet cap (47) traps and condenses pump fluid
vapour and minimises any backstreaming of pump
fluid vapour into your vacuum system.
1.5 Over-temperature protection
1.5.1 Cooling-fail thermal snap-switches
Refer to Figure 1. The pump has two cooling-fail
thermal snap-switches (7, 9) on the cooling-coils.
These snap-switches protect the pump against
damage due to failure of the cooling-water supply.
Both snap-switches have normally closed contacts
that will open if the temperature of the cooling coils
increases above 46
o
C. The snap-switches will reset
automatically when the pump cools down.
You must configure the snap-switches to switch off
the electrical supply to the pump heaters when the
snap-switch contacts open. You must also interlock
the snap-switches so that the pump cannot
automatically restart after the snap-switch contacts
have opened and the pump has then cooled again,
until the problem has been rectified.