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Introduction
B344-21-880 Issue N
1.3 Construction
The Diffstak is a three-stage fully fractionating diffusion pump with an ejector-type third-stage. The pump-body is
water-cooled and made of stainless steel with a mild-steel nickel-plated base.
The inlet-flange has an integral roughing pipeline into which a combined backing/ roughing-valve can be connected
in the Diffstak 63/150, 100/300 and 160/700 pumps. The Diffstak 250/2000 is designed to use separate roughing and
backing pipeline-valves. You may fit a vacuum gauge head to the coupling provided on the roughing pipeline. Housed
within the inlet-flange is the low-impedance high-vacuum isolation butterfly-valve. Sectional views of the Diffstak
pumps are shown in Figures 1 to 4.
Located below the valve is a baffle which consists of a water-cooled flanged cylinder with a detachable cover. The
baffle also retains the jet assembly by spring pressure (except in the 250/2000 pump in which the interior is retained
by a tie-rod).
The interior jet system consists of a first, second and third-stage jet assembly with fractionating tubes. Above the
jet system is a detachable first-stage jet-cap. The jet system is clamped together by a central tie-rod.
The boiler is located in the base of the pump. The pump fluid is heated and vaporised by plate-type electric heater(s)
(one or two, this depends on the size of the Diffstak). The boiler has a fluid filler connection and a drain-plug. The
electrical supply cables from the heater are brought out to a terminal-box at the bottom of the pump for ease of
connection to your electrical supply.
1.4 Principle of operation
Pump fluid is heated in the boiler to produce a vapour which passes up through the interior of the jet assembly and
emerges from the jets as high-velocity vapour streams. The vapour streams condense on the cooled pump-body wall
and drain into the boiler at the bottom of the pump for recirculation.
A portion of system gas which arrives at the Diffstak pump-inlet is trapped in the vapour stream from the first-stage
jet. The gases are compressed and transferred to the next stage. This process is repeated through the pump
jet-stages until the gases are removed by the backing pump through the cooled backing-condenser.
The guard-ring above the top-jet is designed to minimise any back-streaming of pump fluid vapour. The guard-ring
traps the fluid vapour and the vapour condenses on the cooled surface of the guard-ring.
The fractionating feature of the Diffstak design provides a means of purging the fluid charge of undesirable light
fractions and foreign matter and the process helps significantly towards the attainment of a low ultimate vacuum.
The ejector jet also purges the fluid of any contaminants as it returns to the boiler and at the same time ensures a
high critical backing pressure even when you use low vapour pressure fluids.
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