Pressure switches piston type

A piston compressor (in case of a Stirling type PTR) or a combination of a compressor and a set of switching valves (GM type PTR) are used to create the pressure oscillations. The regenerator of a PTR stores the heat of the gas during a half-cycle and, therefore, must have a high heat capacity, compared to the heat capacity of the gas. 

Dual-head reciprocating pumps offer lower solvent pulsation at the expense of mechanical complexity. Here two pistons fill and pump 180° out of phase and in theory provide pulseless flow. Dual-head pumps are more expensive and have either two check valves (series heads) or four check valves (parallel heads). Both designs generally provide for some type of pressure or flow feedback control to further compensate for minor flow variations during switching from one head to the next. A detailed discussion of the mechanics of these and other pumping systems is provided elsewhere (13). 

The control panel in a swivel-type version is usually placed next to the piston extmder and displays the essential parameters such as piston speed, radial pressure, vacuum, position of the piston, etc., moreover it contains the control elements (for example at the operating panel) for altering the piston speed, on-off, emergency off-switch, etc. 

There is a variety of valves available for PGC. These are classified according to the mechanism by which the ports are switched and include rotary, piston, slider, and diaphragm valves. The different types of valve all have strengths and weaknesses in terms of operating lifetime, ease of maintenance, and the ability to handle various types of sample reliably. Valveless techniques, based on pressure balancing, are also seeing some application in PGC for column switching. 

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