Lower pressure peristaltic pumps typically have dry casings and use rollers along with non-reinforced, extruded tubing. This class of pump is sometimes called a "tube pump" or "tubing pump". These pumps employ rollers to squeeze the tube. Except for the 360 degree eccentric pump design as described below, these pumps have a minimum of 2 rollers 180 degrees apart, and may have as many as 8, or even 12 rollers. Increasing the number of rollers increase the frequency of the pumped fluid at the outlet, thereby decreasing the amplitude of pulsing. The downside to increasing number of rollers it that it proportionately increases number of squeezes, or occlusions, on the tubing for a given cumulative flow through that tube, thereby reducing the tubing life.
There are two kinds of roller design in peristaltic pumps:
1. Fixed occlusion - the rollers have a fixed locus as it turns, keeping the occlusion constant as it squeezes the tube.Chemical pumps This is a simple, yet effective design. The only downside to this design is that the occlusion as a percent on the tube varies with the variation of the tube wall thickness. Typically the wall thickness of the extruded tubes vary enough that the % occlusion can vary with the wall thickness (see above). Therefore, a section of tube with greater wall thickness, but within the accepted tolerance, will have higher percent occlusion,Self-priming pumps which increases the wear on the tubing, thereby decreasing the tube life.Tube wall thickness tolerances today are generally kept tight enough that this issue is not of much practical concern. For those mechanically inclined, this may be the constant strain operation.
2. Spring-loaded rollers - As the name indicates, the rollers are mounted on a spring. This design is a bit more elaborate than the fixed occlusion, but helps overcome the variations in the tube wall thickness over a broader range. Irrespective of the variations, the roller imparts the same amount of stress on the tubing that is proportional to the spring constant, making this a constant stress operation. The spring is selected to overcome not only the hoop strength of the tubing, but also the pressure of the pumped fluid.
The operating pressure of these pumps is determined by the tubing, and the motor's ability to overcome the hoop strength of the tubing and the pressure.

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2011-11-15

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