SPI Supplies
16
SPI-DRY Critical Point Dryer
4.4
Physical Description
The SPI Supplies SPI-DRY Critical Point Dryers are comprised of cylindrical, water heated/cooled
pressure chambers, horizontally mounted, with attached gauges and valves.
Three manually operated valves, positioned around the chamber, are provided to control the flow of
inlet and exhaust gases, as well as the venting of the system. The pressure system incorporates a
bursting disc, rated at 1850 psi, located in the gas line between the chamber and the pressure gauge,
see figure 7.1. The disc is designed to rupture in the unlikely event of an excessive build up of pressure
in the chamber. Should this happen, all gas in the chamber is safely vented away.
All chambers and seals are pressure tested by an independent authority before shipment to the
customer. Each chamber bears the serial number of that test.
Temperature and pressure gauges allow for constant monitoring of chamber conditions.
The complete assemble consists of the following:
Pressure vessel assembly with integral water jacket, endplates (2), window, specimen loading
door, and water connectors (2)
Control valves (3)
Thermometer
Pressure Gauge
Safety Valve
Baseplate
Sample holder assembly
High pressure transfer hose
Spare parts kit
4.4.1
Pressure Vessel
The pressure vessel is machined from a solid brass bar to form a cylindrical chamber with a wall
thickness of approximately 1” (25mm). The vessel wall acts as a water jacket, with a series of narrow
bores drilled lengthwise that allow water passage for heating and cooling. The ends of the water jacket
are sealed with two annular end plates and gaskets. Two water connectors are screwed into the vessel
wall (one at each end) to allow for connection of the water supply.
Both ends of the vessel are internally threaded. The specimen loading door screws into one end, with
the viewing window located at the other end, held in place with a retaining ring. The window consists
of a 1” (25mm) piece of toughened glass, covered by a ½” (12.7mm) plastic shield, which acts as a
safety guard in the unlikely event of the glass cracking.
4.4.2
Control Valves
There are three high pressure valves (right-angle type) fitted to the pressure vessel
using ¼” BSP
threaded unions. The valves seal by contact between a ground steel cone and a brass knife edge.
NOTE: Do not overtighten these valves, as doing so will impair the efficiency of the metal to
metal contact.
The
INLET
and
VENT
valves are screwed directly into the top of the pressure vessel, and seal against
O-rings. The O-rings are located on their seats by small stainless steel inserts. When the valves are
initially installed, they are adjusted so that they face the desired direction once they have been screwed
into the vessel. Locking nuts ensure they do not rotate further.
The
DRAIN
valve located at the bottom of the vessel is installed in a slightly different manner. A ¼” BSP
plug is sealed into the female part of the valve body with an adhesive, and this plug butts against an O-
ring seal. This method allows the valve to be conveniently mounted in the horizontal postion.
4.4.3
System Gauges
The thermometer is the bi-metallic type, and measures the temperature of the pressure vessel brass
wall. It is attached to the vessel by means of a push-fit plug inserted in the vessel wall. The gauge fits
