the pressure and temperature and in some cases the flow rate of fluids such as polymers are controlled by reducing the cross - sectional area of the conduit carrying the fluid to provide a flow channel communicating with the inner surface of the conduit whose cross - sectional area is less than that of the conduit . a valve housing is provided containing a chamber which communicates with the flow channel . the pressure and temperature and in some cases the flow rate of the fluid is controlled by adjusting the position of a valve member slideably disposed in the chamber . as the valve member slides within the chamber , it enlarges or reduces the cross - sectional area of the flow channel to change the flow resistance of the channel , thereby changing the pressure and temperature upstream of the channel and in some cases the flow rate through the channel . in applications where the material at the upstream end of the channel is wet and at a substantially greater pressure than the downstream end , it is desirable to also provide an elongated chamber whose inlet port communicates with the downstream end of the channel and which contains means for advancing and compacting material emanating from the downstream end of the channel . in cases where fragmentation of the material is produced by a rapid , and sometimes explosive , vaporization of the moisture associated therewith upon exposure of the material to the low pressure at the downstream end of the channel , the chamber contains the explosion , collects the resulting fragmented materials and compacts the fragments into a form more convenient to handle .