What we don’t know yet
If we wish to prevent a specific peak from disappearing
into column B once it has passed through column A, we
need a separation between the two columns and also an
outlet for the cut. The solution is the live T-piece.
We can imagine two combined back T-pieces with a
coupling capillary in their common center wall.
Straight: new pressure conditions
The coupling capillary in the live T-piece has a higher
flow resistance than a section of column of the same
length. Our pressure response therefore has a kink.
That doesn’t worry us because solenoid valve MV
Cut
is
closed. The result is initially the same as for straight
operating mode on the previous page.
And now the cut
We open the solenoid valve MV
Cut
. The result is shown
on the left. The components are flushed into open air as
soon as they leave column A.
What we don’t like
If we open the solenoid valve MV
Cut
, the pressure there
drops to atmospheric. That is not what we want. We
therefore fit a resistance, e.g. a needle valve, down-
stream of the solenoid valve.
The pressure on the solenoid valve then only drops a
little when we open it. But the cut functions neverthe-
less: because the pressure to the left of the coupling
capillary is lower than on the right, all gas from column
A must pass to the cut outlet!
Resulting design of switching
This switching is known as Deans switching.
P
A
P
M
MV
CUT
Column A Column B
Live T-piece
P
P
A
M
Atm.
MV
CUT
Column A
Column B
P
P
A
M
Atm.
MV
CUT
Column A
Column B
P
P
A
M
Atm.
MV
CUT
Column A
Column B
Four pages ... - and you know how live switching functions!
live.chp, Issue August 17, 1999
6 - 5
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