SPI Supplies

SPI Critical Point Drying Apparatus

Detailed Description of the Apparatus and Service Information: Installation and trial run
Choice of drying liquid

At one time there were two drying liquids in general use and they were liquified carbon dioxide and liquified Regrigerant 13. But because of the more environmentally acceptable carbon dioxide, there has been a shift away from the fluorocarbon based systems to the liquid carbon dioxide approach. The characteristics of the two choices are: 

Carbon dioxide critical point  31.5 °C      1100 psi
Refrigerant 13 (CClF3) -"-     28.8 °C      560 psi

The critical pressure of Refrigerant 13 is lower than that of carbon dioxide but this gives no particular advantage. The usual deciding factor is the price; Refrigerant 13 is when it is available is about 20 times more expensive than carbon dioxide. And then coupled with the fact that there is great environmental concern about the use of these materials when alternatives are indeed available, this is why liquid carbon dioxide has become the most widely used drying liquid.

The transfer pipe provided with the apparatus is suitable for use with a carbon dioxide cylinder of the siphon type. If a standard cylinder is used, it should be inverted to obtain liquid.

The liquid carbon dioxide should be essentially water-free. It is important to specify dry carbon dioxide when ordering from your vendor.

For the rest of our discussion, we will assume that carbon dioxide is indeed the drying liquid.

Important note:Flexible High Pressure Coupling:

Cylinder connection threads tend to vary from country to country. If the user finds it necessary to fabricate his own transfer pipe, he should take advice from a local supplier of high pressure fittings. The thread on the apparatus is 1/4" NPT (National Pipe Thread) of the U.S.

The water supply

It is convenient to attach a water mixer at the hot and cold water supplies in the laboratory. The apparatus requires both heating and cooling water during the cycle: cooling to allow easy filling of the liquid carbon dioxide ; heating to take the carbon dioxide above its critical point. Good control of the water temperature is of great assistance to the operator.

Installation

  1. Choose a site for the apparatus which is conveniently near the hot and cold water supplies and the vertical standing carbon dioxide cylinder. The cylinder (5 feet high) will need changing regularly (every 20-30 runs), so leave some access to the cylinder. If mixer taps are not already fitted to the hot/cold water supply, it is useful to fit a 'Y piece.
  2. Connect the flexible transfer pipe to the inlet valve on the apparatus. A fiber washer is provided to ensure a good seal. The wrench (spanner) here is 7/16" BSF. Tighten to about 20 ft. lb. You want it to be "snug" but at the same time, don't over do it.
  3. Connect the other end of the flexible transfer pipe to the cylinder. No gasket is required. Do not rotate the pipe while tightening the connection: A ¼" BSF wrench is used to grip the pipe end while tightening the large nut (11/16" BSF).
  4. These two unions can be tested by closing the inlet valve on the apparatus and then opening the tap on the cylinder. Any leaks will be audible (or visible if severe). Leaks in this area are cured by further tightening of the joints.
  5. Connect the mixer tap or 'Y piece to the water inlet with ¼" bore neoprene or polythene tubing. Connect the water outlet to a convenient drain.
Trial run
  1. Close all three valves on the pressure vessel. Do not overtighten as this will damage the seals.
  2. Open the tap on the liquid carbon dioxide cylinder.
  3. Open the inlet valve on the pressure vessel. Listen for slight leaks. If any seals have loosened in transit, they should be tightened. (Note: Before disturbing seals first close the cylinder and vent all carbon dioxide from transfer pipe and vessel through drain valve with inlet valve open).
  4. With the Inlet open and the other two valves closed the pressure vessel will fill slowly. The rate of filling can be increased by opening the vent valve in the top of the apparatus. This removes the trapped volume of air.

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Thursday October 16, 2008
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