Volumetric clearance, starting

This means that a substantial volume of gas remains trapped between the cylinder head and the piston, before the travel of the piston is reversed. This volume of gas, called the starting volumetric clearance,... 

The proper way to reduce the volumetric efficiency is to increase the starting volumetric clearance. This is done with an adjustable unloading pocket, as shown in Fig. 29.4. This device, also called the head-end unloader, works by increasing the starting volumetric clearance. This is known as the volume of gas trapped between the cylinder head and the piston, when the piston position is at bottom dead center, as explained in Fig. 29.2. 

Starting volumetric clearance The space between the piston and cylinder header at the end of the stroke in a reciprocating compressor. 

Figure 6.22 depicts schematically the flow configuration. Two identical rolls of radii R rotate in opposite directions with frequency of rotation N. The minimum gap between the rolls is 2H0. We assume that the polymer is uniformly distributed laterally over the roll width W. At a certain axial (upstream) location x = X2 (X2 < 0), the rolls come into contact with the polymeric melt, and start biting onto it. At a certain axial (downstream) location x A), the polymeric melt detaches itself from one of the rolls. Pressure, which is assumed to be atmospheric at X2, rises with x and reaches a maximum upstream of the minimum gap location (recall the foregoing discussion on the pressure profile between non-parallel plates), then drops back to atmospheric pressure at X. The pressure thus generated between the rolls creates significant separating forces on the rolls. The location of points A i and X2 depends on roll radius, gap clearance, and the total volume of polymer on the rolls in roll mills or the volumetric flow rate in calenders. 

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