Ferrofluidic seal
Encyclopedia
Ferrofluidic is the brand name of a magnetic liquid rotary sealing mechanism made by the Ferrotec Corporation. Magnetic liquid seals are used in rotating equipment to enable rotary motion while maintaining a hermetic seal
by means of a physical barrier in the form of a ferrofluid
. The ferrofluid
is suspended in place by use of a permanent magnet.
, oil-based liquid held magnetically between the rotor and stator. Therefore the operating life and equipment maintenance cycles are generally very long, and the drag
torque
very low. The magnet material is permanently charged and requires no electrical power or other re-energizing or maintenance. Ferrofluid-sealed feedthroughs reach performance levels that other technologies can't achieve, by optimizing features such as ferrofluid viscosity and magnetic strength, magnet and steel materials, bearing arrangements, and water cooling for applications with extremely high speeds or temperatures. Ferrofluid-sealed feedthroughs routinely operate in environments including ultra-high vacuum (below 10−8 mbar), temperatures over 1,000 °C, tens of thousands of RPM, and multiple-atmosphere pressures.
Magnetic liquid seals are engineered for a wide range of applications and exposure but are generally limited to sealing gases and vapors, not direct pressurized liquid. Each particular combination of construction materials and design features has practical limits with respect to temperature, differential pressure, speed, applied loads and operating environment; however, these limits can generally be overcome by carefully selecting or designing the device for the application. Necessary features may include multiple ferrofluid stages, water cooling, customized materials including metals, permanent magnets and ferrofluid, and exotic bearings. Ferrofluid-based seals have extremely low leak rates - almost immeasurable with laboratory equipment - however they cannot reach the levels of welded connections or other all-metal, static (non-rotating) seals.
Hermetic seal
A hermetic seal is the quality of being airtight. In common usage, the term often implies being impervious to air or gas. When used technically, it is stated in conjunction with a specific test method and conditions of usage.-Etymology :...
by means of a physical barrier in the form of a ferrofluid
Ferrofluid
A ferrofluid is a liquid which becomes strongly magnetized in the presence of a magnetic field.Ferrofluids are colloidal liquids made of nanoscale ferromagnetic, or ferrimagnetic, particles suspended in a carrier fluid . Each tiny particle is thoroughly coated with a surfactant to inhibit clumping...
. The ferrofluid
Ferrofluid
A ferrofluid is a liquid which becomes strongly magnetized in the presence of a magnetic field.Ferrofluids are colloidal liquids made of nanoscale ferromagnetic, or ferrimagnetic, particles suspended in a carrier fluid . Each tiny particle is thoroughly coated with a surfactant to inhibit clumping...
is suspended in place by use of a permanent magnet.
Benefits and limitations
Magnetic liquid rotary seals operate with no maintenance and extremely low leakage in a very wide range of applications. Ferrofluid-based seals used in industrial and scientific applications are most often packaged in mechanical seal assemblies called rotary feedthroughs, which also contain a central shaft, ball bearings and an outer housing. The ball bearings provide two important functions: maintaining the shaft's centering within the seal gap, and supporting external loads. The bearings are the only mechanical wear-items, as the dynamic seal is actually a series of rings made of ultra-low vapor pressureVapor pressure
Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...
, oil-based liquid held magnetically between the rotor and stator. Therefore the operating life and equipment maintenance cycles are generally very long, and the drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
very low. The magnet material is permanently charged and requires no electrical power or other re-energizing or maintenance. Ferrofluid-sealed feedthroughs reach performance levels that other technologies can't achieve, by optimizing features such as ferrofluid viscosity and magnetic strength, magnet and steel materials, bearing arrangements, and water cooling for applications with extremely high speeds or temperatures. Ferrofluid-sealed feedthroughs routinely operate in environments including ultra-high vacuum (below 10−8 mbar), temperatures over 1,000 °C, tens of thousands of RPM, and multiple-atmosphere pressures.
Magnetic liquid seals are engineered for a wide range of applications and exposure but are generally limited to sealing gases and vapors, not direct pressurized liquid. Each particular combination of construction materials and design features has practical limits with respect to temperature, differential pressure, speed, applied loads and operating environment; however, these limits can generally be overcome by carefully selecting or designing the device for the application. Necessary features may include multiple ferrofluid stages, water cooling, customized materials including metals, permanent magnets and ferrofluid, and exotic bearings. Ferrofluid-based seals have extremely low leak rates - almost immeasurable with laboratory equipment - however they cannot reach the levels of welded connections or other all-metal, static (non-rotating) seals.
External links
- This video demonstration of a magnetic liquid rotary seal can help explain how the seal functions.
- Rotary Seal
- Rotary Feedthrough