Get an understanding of the main factors to consider when specifying multichannel peristaltic pumping systems for use in industry and research.
Introduction Fluid Dynamics
Pump Heads Occlusion
Motor Torque Drive Selection
It is sometimes important in research laboratories and production plants to move several fluid channels simultaneously. This can save considerable time and resources, while greatly improving process efficiency.
Controlling multiple channels together can provide a number of benefits. Varying the speed of the motor will at once vary the flow in all channels. The use of one motor can significantly reduce the cost per channel. It can also reduce the total energy requirements, the number of moving parts, and the size of the pumping system. The combined result will be an economical fluids handling system to acquire, operate, and maintain. Multichannel pumping is generally carried out using peristaltic pumps. These are available with a wide range of interchangeable multichannel pump heads, drives, and tubing. Special pump designs can deliver over 40 channels simultaneously.
Multichannel peristaltic pumps are used successfully in many industries including printing, pharmaceutical, chemical processing, agriculture, and water treatment. Applications, besides general fluid transfer duties, include metering and dispensing. (Fig. 1).
The most important advantage of the peristaltic pump lies with the use of tubing as the pump chamber. The fluid being pumped remains inside the tubing at all times, so it cannot escape and cause contamination. And it will not become contaminated by the pumping operation, assuming of course that the tubing material has been properly specified. The pump provides gentle fluid transfer; and it is capable of handling a wide variety of fluids including gases, viscous liquids, and mixed phase fluids such as gas/liquid and solid/liquid combinations.
Pump selection criteria include: fluid flow rates, fluid characteristics, ease of tubing changes, flexibility in pump design (for example, stackable versus cartridge heads), accuracy between channels, torque requirements, and drive control mechanism. Let us look at these selection criteria in more detail.
The flow rate and fluid dynamics through a peristalitic pump are affected by tubing size and tolerance, and by the design of the pump manifold, tube bed, and roller. As a result, each pump head transfers fluid slightly differently.
Tubing size is directly proportional to the flow rate in all pump heads. Larger tubing sizes and rotor diameters have a greater "pillow" volume (the fluid space in the tubing between adjacent rollers in the pump head). This volume determines the flow per revolution of a given pump head.
For high flow accuracy, it is important to use high tolerance tubing and calibrate the pumps. "High tolerance" means that the internal diameter and wall thickness of the tubing have been kept within narrow tolerance limits during manufacture. This type of tubing reduces the variation in flow rate beween channels. Calibration of the pump, by adjustment of occlusion, will reduce the variation even further.
All tubing materials have a break-in period, during which their shape and flow rate will change. The length of this time, and amount of change in the tube, varies greatly depending upon tubing size and material. For high accuracy, operate a pump fitted with new pump tubes for several minutes before calibrating the flow through individual channels.
- Manifolds It is possible to use the flow channels of multichannel pumps independently, or to manifold them together with Y-connectors. When a manifold is constructed to provide a common inlet for several tubes, it reduces the number of plumbing lines to a reservoir. When channels on the discharge side are combined, it increases the flow rate. With a special offset pump head, a combination of channels can reduce flow pulsation (Fig. 2). Remember to take care and use larger tubes when manifolding on either suction or discharge side, so that you minimize pump cavitation.
- Bed Designs
There are a variety of tubing bed designs that affect the flow dynamics of a pump. Two types are most common, arcuate and tangential. Arcuate tube beds pivot with a hinge on one side and the adjustment mechanism for occlusion on the other. The tubing bed swings in an "arc" radius. This design delivers more flow with greater pulsation in one direction, while providing less flow with reduced pulsation in the other direction.
Tangential designs support the occlusion bed equally on each end. The point of occlusion is at a right angle to the tubing. Flow rates and pump performance are identical in both directions.
Pumps designed with planetary driven rollers offer the best flow accuracy. In this design of pump, a central gear meshes with individual gears at the end of the rollers. The rollers not only rotate, but literally roll across the tubing. This limits the distortion of the tubing, which is normally caused by dragging a roller over it.
- Type of Fluids
Fluid consideratons are the same for single and multichannel pump applications. Water-like fluids move easily through many different sizes of tubing. Viscous fluids need larger tubes and slower pump operation speeds.
Two types of pump head adapt easily to multichannel applications: the stackable and cartridge. Both cartridge and stackable designs provide for many tubing channels within a relatively small space.
Frequent tubing changes greatly affect pump set-up and maintenance times. Some stackable pump head styles require the pump head to be dismantled in order to replace the tubing. Newer stackable head styles (such as the Masterflex Easy-Load pumps) and most cartridge pump heads are easier to set up and service (Fig. 3). Operators can remove and replace the tubing without dismantling the pump.
- Stackable Type
Stackable pump heads can be added or removed as needed for different applications. Stackable pump heads are therefore popular for applications that require flexibility to frequently reconfigure the pump design. Stackable heads are preferred for larger tubing sizes, and when the application involves high suction lift and/or discharge pressure. They require special multichannel mounting hardware for stacking, but provide better control of occlusion and tubing retention.
Individual pump heads can mount with others of the same type or with pump heads of vastly different types and styles. This is a valuable benefit for proportional flow applications: you can mix and match tubing sizes or pump head styles to get the proportional flow rates required. Proportional flow rates of over 100:1 are possible (Fig. 4).
| Figure 4 |
Stackable pump heads incorporate an offset roller design. These staggered rollers limit the torque loading on the drive circuit when several pump heads are mounted together. Staggered roller systems may deliver differing volumes if used for only short dispensing times (depending upon the final position of the rollers). This limitation disappears when the pumps are running continuously.
- Cartridge Type
Cartridge pump head designs accept a predetermined maximum number of channels: Any number of these channels can be used, up to the capability of the head and the drive. Cartridge pump heads are available separately, or integral to the design of the pump.
Cartridge pumps have long rollers that provide synchronous fluid delivery between the cartridges. These pumps are recommended for low volume fluid transfer applications. Many designs incorporate the motor and pump head in one package to further reduce size.
Fixed occlusion pump heads are most popular for applications where channel-to-channel accuracy is not critical. These pumps are the easiest to use. You simply load the tubing into the pump head, then begin pumping.
The flow rate in different tubes of a multichannel pump can be matched by adjusting the occlusion. Pump manufacturers provide a variety of mechanical levers, screws, and knobs to adjust the level of occlusion. The procedure is as follows. After loading the tubing, start the pump. Increase the occlusion on the tubing until there is flow, then adjust a few degrees more. After priming all tubes, adjust the flow rate of individual tubes by further increasing or decreasing the level of occlusion. Increasing the occlusion will reduce the pillow size and the flow rate in the tube. Flow adjustments of up to 5% are possible with this method.
Spring-loaded tubing beds and a calibrated torque wrench offer a method of consistent occlusion adjustment. This system will apply the same degree of occlusion to each channel regardless of small variations in wall thickness. Installing high tolerance flow-rated tubing in this type of pump allows delivery of fluids with a high degree of flow accuracy.
The flow rate and the type of tubing material required are the major considerations when calculating motor torque requirements. High flow rates require larger tubing sizes. Larger tubes have a larger tubing-to-roller interface and more material to compress. Harder durometer tubing materials require more energy to squeeze the tubing. The combination of larger sizes and harder durometers greatly increases the torque requirments of the pump head. Softer durometer tubing materials like silicone, with a durometer less than 55 shore A, need comparatively small motors, and provide substantial savings in terms of pump purchase price and operating costs.
Rotor design, tube bed design, suction lift, and discharge pressure also affect torque loads. The torque load for the total system is a multiple of the requirements for each tubing channel (Fig. 5).
Select a drive with sufficient torque and speed to meet the flow and pump head requirements. Fixed-flow drives are fine for basic transfer applications (Fig. 6). They are compact, with low energy requirements. Different gear ratios provide a range of torque outputs.
Variable-flow drives adjust the flow rate simultaneously for all the channels. The secondary channels flow proportionately to the first channel.
Digital drives provide the best pump control, with built-in tachometer feedback. Flow rate display is possible, although this will be for only one channel unless a number of flowmeters have been built into the system.
Fixed occlusion pumps are popular for their consistent tube loading and pumping capability. Used with precision extruded tubing, these pumps can deliver fluids with an accuracy of 3 to 5% between channels. This degree of performance is adequate for heat exchangers, printing presses, and pigment additions.
Cartridge pump heads are popular for accurate sampling, dispensing, and metering of fluids (Fig. 7). The synchronous rollers provide coordinated fluid delivery between each channel in the pump head. With adjustable occlusion, these pumps can deliver flow accuracies of 1% between channels. Analytical techniques in research, production, and environmental monitoring require this high level of performance.
The peristaltic pump provides a special combination of fluid handling characteristics and design flexibility for multichannel applications. A variety of interchangeable tubing sizes, tubing materials, pump heads, and drives optimize the pump at an economical price. Multichannel pumps are being adapted to many applications with tremendous savings in time, resources, and process.