May 21, 2017

[2.70] Seek and Geek #13: Peristaltic pump

A peristaltic pump is used for supplying precise fluid volumes and flow rates without exposing/contaminating the fluid with pump components. They're commonly used for pumping sterile fluids in hospital settings and for pumping abrasives or aggressive solvents in industrial machines.
Peristaltic pumps!
from reddit.com/r/mechanical_gifs
Fluid flow is driven by rollers periodically compress a tube, which does two things. Increased pressure pushes the fluid forwards, and tube compression prevents backflow. As the roller rotates and releases pressure on the tube section, vacuum pressure draws more fluid into the tube.

Slow-motion gif showing the mechanism
from wikipedia
Because all fluid is contained within the tube, this pump doesn't have to worry about finding chemically-compatible o-rings or seals. The only component of concern is the tube itself.

Tubes in peristaltic pumps need to be elastomeric to sustain millions of compression cycles, which rules out common polymer choices like PTFE or PVDF. The most common material choices for peristaltic tubes include natural and synthetic rubbers (good fatigue resistance and chemical compatibility) and silicone for water-based fluids.

The ideal peristaltic pump, one that minimizes tube compression-fatigue and pulsations, has infinite diameter pump-head and rollers (so impossible). Pumps that approach this ideal often use designs like asymmetric heads and offset rollers to increase effective diameters.

Wikipedia's example of an asymmetric peristaltic pump
from wikipedia

The maximum compression force a pump can apply on the tube is determined by the minimum gap between the roller and the housing wall. There's an inverse relationship between pumping force and tube life, bounded by minimum-acceptable-tubing-life (maximum squeezing) and force required to prevent backflow (minimum squeezing). This squeeze force is called 'occlusion', and is usually 20 to 40% of the tube wall thickness:

$ F = \frac{2t - g}{t} \cdot 100% $
F = occlusion, t = wall thickness, g = minimum gap size

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