Peristaltic Pumps: How Do They Work?

Peristaltic pumps are a type of positive displacement pump that use the progressive compression and relaxation of a flexible tube to move fluid along. These pumps mirror the biological process called peristalsis. This is where the rhythmic contraction of muscles is used to transport fluid through the body, such as in the digestive tract. In the laboratory, peristaltic pumps have become critical instruments for precise fluid handling, particularly in areas where sterility and high levels of chemical compatibility are required. The reason for this is that the fluid being transported is isolated completely from the pumping mechanism.

How Do Peristaltic Pumps Work?

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The basic working principle behind a peristaltic pump is the progressive deformation of a flexible tube and the sweeping of fluid via this motion. Typically, a system will use a flexible tube mounted within a circular casing, this tube is then compressed by a roller, typically a bearing attached to a rotating arm connected to a shaft. Each time the bearing performs a full rotation it will displace a fixed volume of solution.

As the length of tube that is in swept by the roller must always be less than the circumference of the tubing, there is always a point within each rotation of the arm where pumping is not occurring. This results in a pulsed flow. However, this pulsed flow can be reduced by increasing the number of rollers causing compression.

Peristaltic Pump Designs

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There are two main designs that are used within peristaltic pumps: fixed or variable occlusion. Occlusion, in relation to peristaltic pumps, refers to the compression mechanism for the tubing. A fixed occlusion system uses a fixed roller where the distance and compression force is maintained and cannot be modified. While for a variable occlusion pump allows for the adjustment of compression force, this is typically done through the use of a compression spring where the compression of this spring can be adjusted.

Another important design consideration for the system is the number of rollers, one of the main drawbacks of peristaltic pumps is that the flow is pulsed; to reduce pulses in flow the number of rollers can be increased. It should also use an even number of rollers in a symmetrical design to ensure that a fixed number of rollers are sweeping at a given time.

Choice of Tubing Materials

Both the performance and maintenance frequency of a peristaltic pump is heavily dependent upon the choice of tubing material. The process of peristalsis results in the repeated compression and expansion of the tubing. The stress strain relationship during compression follows Kelvin-Voigt model and can be represented simply by the equation:

Where the stress (σ) is dependent on the product of elastic modulus (E) and strain (ε) and the product of viscosity (η) and the rate of change in the strain. Here the amount of stress on the tubing is dependent on the properties of the tubing itself but also the rate of compression and relaxation of the tubing.

High speeds, more rollers, and a higher elastic modulus contribute to higher levels of stress within the tubing material and faster degradation. However, higher elastic modulus also improves the recovery time of the tubing after deformation.

How to Determine Flow Rate in a Peristaltic Pump

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For any application of peristaltic pumps understanding how to determine the flow rate of the pump and accurately control it is critical. The flow of the pump is governed by the compression of the pipe by the roller and the total distance it has swept. If we look at a single roller to begin with the total volume displaced by the roller is given by the equation.

Where the d is the internal diameter of the tube and L is the length of tubing swept by the roller. The flow rate is then controlled by varying the frequency of rotation of the roller via a motor, therefore the flow rate (Q) for a given number of rollers becomes.

Where ω is the rotational frequency and n is the number of rollers used. Although the relationship that governs the flow rate is relatively simple theoretically there are several factors that need to be accounted for to properly calibrate and determine the real dispense rate of a system.

Flow Rate Corrections in Peristaltic Pumps

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The main correction factor that results in variation between the theoretical and measured dispense rates for a peristaltic pump is called the slip factor. This is a correction factor that is based upon the properties of the tubing material, and occurs due to an incomplete recovery of the tubing from a compressed state and any back flow present in the system. This value is typically around 0.85 to 0.95 and can result in a 5% to 15% drop-in flow rate for a peristaltic pump.

In addition to the slip factor, peristaltic pumps also must correct for temperature variations between when the system is calibrated and when the system is in operation. As the temperature varies the internal diameter of the tubing will change resulting in a variation in the total volume swept each time a roller passes across the tube. In addition to the internal diameter changes the wall thickness will vary and the elastic modulus of the tubing will vary, this has additional impacts on the slip factor of the tubing as it impacts the recovery of the tubing to compression.

Uses of Peristaltic Pumps

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Peristaltic pumps see significant use in areas where sterility and low levels of contamination are critical. Due to the design of the pump the separation of the pumping mechanism from the solution itself ensures that no contaminants can enter the system during operation. Areas such as cell culture systems and analytical chemistry see widespread use of peristaltic pumps. In addition, peristaltic pumps are preferred over other sealed displacement pumps such as syringe pumps when it comes to equipment that requires the delivery of large volumes over time as syringe pumps are typically limited by the volume of the syringe itself.

Why Use Peristaltic Pumps

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Peristaltic pumps provide a simple yet elegant solution to precision fluid handling for both scientific and industrial applications. The design and operating principle of the pumps provide unique and unmatched advantages in both the deliver of sterile, low contaminant, and sensitive materials. By understanding the underlying physics and engineering principles it is possible to select the best pump for your application and also see how variations in environmental conditions and material selection can impact the performance of your peristaltic pump.

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