Vacuum Technology Know How - Triumf

www.pfeiffer-vacuum.net
The Roots pump advances the water vapor into the condenser, where it condenses. Because
laminar flow prevails there, the vapor flow advances the inert gas being released by the material
to be dried into the condenser.
Were the backing pump to be shut down, the entire condensation process would quickly
come to a stop, as the vapor could only reach the condensation surface area through diffusion.
As the drying process progresses, the volume of vapor declines and less condenses in
the condenser; however the concentration of vapor being extracted by the backing pump will
tend to be larger if the concentration of inert gas decreases. If the vapor pressure in the condenser
declines below the condensation threshold, the condensate will begin to re-evaporate.
This can be prevented if the condensate drains into a condensate storage vessel via a valve
and this valve closes when the vapor pressure declines below the condensation pressure.
In the case of large distillation systems, the pumping speed of the backing pump should be
regulated on the basis of the condensation rate. This can be accomplished, for example, with
the aid of a dosing pump that uniformly discharges the volume of pumped condensate from
the storage vessel. When the concentrate level in the storage vessel declines below a given
level, the backing pump’s inlet valve opens and the inert gas that has collected in the condenser
is pumped down. The condensation rate now increases again, the condensate level
increases and the backing pump’s inlet valve closes again. This arrangement means that the
system pumps only when the condensation rate is too low, and only little condensate is lost.
Summary
When pumping down vapors (drying, distillation), the major pumping effect can be provided
by a condenser. Depending upon pressure and temperature conditions, either one or two
condensers can be employed (Figure 7.3). The condenser between Roots pump and backing
pump is more effective, as the vapor flows into the condenser at a higher temperature and
higher pressure, and a small backing pump evacuates only a portion of the vapor. In distillation,
condensate loss can be minimized by regulating the pumping speed of the backing pump.
7.2.3 Turbopumping stations
7.2.3.1 Evacuating a vessel to 10 - 8 mbar by means of a
turbopumping station
A vessel made of bright stainless steel is to be evacuated to a pressure p b of 10 - 8 mbar in
12 hours. As can be seen from Section 1.3, there are other effects to consider in addition to
the pure pump-down time for air. Both desorption of water vapor and adsorbed gases as well
as outgassing from seals will lengthen the pump-down time. The pump-down times required
to attain the desired pressure of 10 - 8 mbar are comprised of the following:
t 1 = pump-down time of the backing pump to 0.1 mbar
t 2 = pump-down time of the turbopump to 10 - 4 mbar
t 3 = pumping time for desorption of the stainless steel surface
t 4 = pumping time for outgassing the FPM seals
The desired base pressure p b is comprised of the equilibrium pressure caused by gas flowing
into the vessel through leaks and permeation Q l , as well as by gas released from the metal
surface Q desM and the seals Q desK :
Q l Q desM (t 3 ) Q desK (t 4 )
p b = + +
S S S
Page 145
Vacuum
Technology