SPI Supplies
21
SPI-DRY Critical Point Dryer
Ensure that all connections are properly made.
Close all valves on the pressure vessel (do not over-tighten, as this will damage the seals).
Turn on the cold water supply to cool the pressure vessel.
Open the CO
2
cylinder control valve.
Open the
INLET
valve on the pressure vessel. Listen for any sign of leaks. If a leak is detected, the
associated connection should be re-tightened.
With the
INLET
valve open, and the
VENT
and
DRAIN
valves closed, the pressure vessel will begin to
fill with liquid CO
2
. The rate and extent of the filling can be increased by slightly opening the
VENT
valve
on top of the unit. This allows any air trapped in the vessel to be flushed out.
At this point it is helpful to experiment with the manipulation of the various control valves. It should be
noted that if the
INLET
and
DRAIN
valves are both opened, a constant level of liquid CO
2
can be
maintained, as seen through the viewing window. This process is referred to as
FLUSHING
. Sometimes,
it may be necessary to slightly open the
VENT
valve as well, if the liquid level begins falling.
Once comfortable with flushing and filling the chamber, adjust the liquid level so the chamber is half full,
and then close all of the valves.
Turn off the cold water, and turn on the hot water (35C-40C) to slowly raise the temperature of the
chamber. There may be some minor turbulence in the chamber. If there is excessive turbulence, this
indicates that the chamber is heating too rapidly, and the water temperature should be lowered.
While the chamber is warming, observe the liquid level through the window, while also monitoring the
TEMPERATURE
and
PRESSURE
gauges. As the temperature of the chamber nears 30C, the pressure
should rise to approximately 1100 psi. Once at this point, the surface of the liquid will start to dissolve.
Once the pressure passes 1200 psi, the meniscus of the liquid will have disappeared completely,
indicating that the CO
2
has passed through its critical point. There are actually three indications that
critical point has been passed:
The visible effect of the meniscus disappearing
The
PRESSURE
gauge reading above 1200 psi
The
TEMPERATURE
gauge reading above 32C (although this is the least reliable sign, due to
the possibility of thermal lag)
NOTE: The pressure reading will unreliable if the liquid level is above the inlet to the PRESSURE
gauge during the heating process.
With the chamber above the critical point temperature (above 32 C), experiment with releasing the
pressure by use of the
VENT
valve. If the pressure is released too quickly, re-condensation of the gas
will occur by adiabatic cooling. The
VENT
valve should be adjusted to give a vent time of 4-5 minutes.
After a critical point run, cool the chamber to 20 C before attempting another run.
Repeat the above procedure several times, until comfortable with the workings of the critical point dryer.
WARNING
The CPDA is a pressure vessel and operates at pressures in the range of 800 -
2000 psi (54 - 136 bar) which is potentially dangerous if handled carelessly.
WARNING
HAZARD TO HEALTH!
Risk of asphyxiation in poorly ventilated rooms.
As carbon dioxide is heavier than air, the concentration of exhaust gas will be
greater at ground level and the oxygen concentration correspondingly reduced.
CAUTION
Before attempting to re-tighten a transfer pipe union, close the cylinder control valve
and release any gas pressure in the pipe by opening the CPDA inlet and drain valves.
