1. Field of the Invention
This invention relates to a method and apparatus for circulating a fluid within a closed system.
2. Description of the Related Art
It is often desirable to circulate fluids within a closed system to prepare fluid mixtures, to equilibrate fluid phases, or for other purposes. For example, in gas-solid adsorption equilibrium testing, a gas mixture containing a chemical vapor and an inert gas is circulated through an adsorber packed with adsorbent particles. Magnetic piston pumps have traditionally been used in such applications, since such pumps require no physical connection between the piston and the driver driving the piston.
Such conventional magnetic piston pumps generally include a ferromagnetic piston disposed within a cylinder defined by a pump body. The pump further includes a driver including a magnet that is disposed external to the pump body. As the driver is moved in a first direction with respect to the pump body, the magnetic attraction between the driver and the piston urges the piston to move within the cylinder commensurate with the driver. In this way, fluid is drawn into the cylinder. Once the cylinder has been filled, the direction of the driver is reversed. The magnetic attraction between the driver and the piston urges the piston to move within the cylinder commensurate with the driver. The fluid, previously drawn into the cylinder, flows through the piston, through a series of check valves, and out of the pump.
Conventional magnetic piston pumps are generally capable of producing a maximum flow rate of about 100 cm3 per minute, which is insufficient for many applications. Their low flow rate capability is due in large part to limitations of the magnetic attraction between the driver and the piston and to their ability to pump fluid only during one stroke of the pump cycle. Further, check valves that are integral with the pump often wear quickly and typically require the pump to be disassembled for repairs to be made. The piston of such a pump also often wears quickly, since it typically slides directly against an interior wall of the cylinder in which it operates. This wear often produces minute particles of the piston and/or cylinder wall, which may contaminate the fluid flowing through the pump.
The present invention is directed to overcoming, or at least reducing, the effects of one or more of the problems set forth above.