Capsule pipeline systems pick-and-mix different technology, depending on what the system is designed to do. There is no single method or approach, although some techniques are common to many systems.
Pneumatic Capsule Pipelines (PCP)
Simple PCPs follow conventional fluid mechanics principles. Air is blown down and / or extracted from the pipeline, propelling the capsule along the pipe. Both ends of the pipeline are sealed during transport, allowing the air behind the capsule to be above atmospheric pressure, and / or air in front of the capsule to be below atmospheric pressure. Simple PCPs involve a limited number of capsules in the system at any one time (normally just one). This reflects the inefficiency of creating sufficient pressures to propel multiple capsules, and the difficulty in retrieving one capsule from the end of the pipeline while a second capsule was being propelled.
Diagram showing Carsten's theory as applied to a simple capsule pipeline system (Source: Based on Carstens).
Modern PCP large diameter systems utilise through flow booster pumps, also known as jet pump injectors. These create the pressure differentials required to propel multiple capsules through a pipeline, while allowing both terminals at atmospheric pressure. This is done by placing a booster pump midway along the pipeline, and designing it in such a way that capsules can pass through the pump. [1] gives a technical explanation of theory associated with this. On long lengths of pipeline multiple booster pumps can be placed at regular intervals to allow many capsules to be propelled simultaneous without significant inefficiencies in the system. This type of PCP is still restricted in the number of capsules which can be conveyed at any one time, unless booster pumps are placed at very regular intervals. Uneven flow (irregular capsule movements) or very high speeds tend to require the system to be designed in a way which makes inefficient use of energy. The best technical/mathematical summary of research into large diameter PCPs is [2].
Small diameter capsules rely on a felt or plastic ring at each end of the capsule to provide both a seal between the capsule and the pipe wall, and to reduce friction of the capsule against the pipe wall. In larger diameter systems, or where heavier loads are conveyed alternative means of reducing this friction are required. Early PCP systems tended to mount capsules on railway tracks within the pipeline. Later PCP systems used clusters of wheels mounted at each end of the capsule. The wheels face onto the pipeline wall, and are arranged at even intervals around the circumference of the capsule. The capsule normally pivots between its two clusters of wheels, allowing the wheel clusters to move freely in the pipe without up-turning the contents of the capsule. Alternative means of reducing friction have been proposed (for example by using opposing magnets on the capsule and pipeline to suspend the capsule in mid-air within the pipeline [3]), but none have been implemented.
Hydraulic Capsule Pipelines (HCP)
In HCP, capsules are conveyed in a flow of water along a pipe. At low water speed the capsule slides along the floor of the pipe, however once the speed of flow is sufficiently high lift is generated (similar to that in an aircraft) and the capsule becomes waterborne. Once this is achieved transport of the capsule only required 10-30% more energy than would be required to move the water alone. HCP has spawned the concept of Coal Log Pipelines (CLP). These utilise the same principles as HCP except the capsule is made up of a 'log' of the cargo itself, with no enclosing membrane as such. Coal is crushed and compacted into a capsule shape. The capsule is then feed into a pipeline containing a flow of water. On arrival at its destination the coal log is crushed. The coal is then 'de-watered' by a mix of sedimentation and flocculation. CLP is still under development, although the first commercial installation is likely in the next few years. The most recent technical/mathematical summary of research into HCP is probably [4].
Electro-Magnetic/Linear Induction
The use of linear induction and / or linear synchronous propulsion has been proposed as an alternative means of moving capsules within an air filled pipeline [5]. An electromagnetic thrust would be induced in each capsule as it passed over magnetic induction coils set in the pipeline. In the case of linear induction propulsion these coils would be spaced at intervals within the pipeline. Linear synchronous propulsion would involve a continuous line of coils. As capsules pass through the pipeline at the same speed as one another, a constant volume of air remains between capsules. The proposed system would have a greater capacity than booster pump based PCP, and seems better able to attain high speeds (up to around 60 mph) and deal with uneven flow efficiently.
Notes
- Carstens, M., (1970), 'Analysis of a Low-speed Capsule-Transport Pipeline', Hydrotransport 1: Warwick, BHRA.
- Round, G. F., Marcu, M. I., (1987), 'Pneumocapsule Pipelines: Potential for North America', Journal of Pipelines, 6, 221-238, also presented to the Fifth International Symposium on Freight Pipelines, Philadelphia, Oct 13-16 1985.
- Knoll, E. G., Knolle (sic) Super Pipe, US Patent No. 4,024,947.
- Capsule Pipeline Research Center, http://www.missouri.edu/~cprc/, [cached text].
- Vandersteel, W., US Patent No. 4,458,602. A US Patent is also held by the University of Missouri relating to PCP systems based on linear induction motors.