Ferrofluidic is the
brand name
A brand is a name, term, design, symbol or any other feature that distinguishes one seller's good or service from those of other sellers. Brands are used in business, marketing, and advertising for recognition and, importantly, to create an ...
of a staged
magnetic liquid rotary sealing mechanism made by the Ferrotec Corporation and was first patented by the predecessor company's co-founder R.E.Rosensweig (USP 3,620,584). The seals are used in rotating equipment to enable rotary motion while maintaining a
hermetic seal
A hermetic seal is any type of sealing that makes a given object airtight (preventing the passage of air, oxygen, or other gases). The term originally applied to airtight glass containers, but as technology advanced it applied to a larger categor ...
by means of a physical barrier in the form of a
ferrofluid
Ferrofluid is a liquid that is attracted to the poles of a magnet. It is a colloidal liquid made of nanoscale ferromagnetic or ferrimagnetic particles suspended in a carrier fluid (usually an organic solvent or water). Each magnetic particle ...
. The
ferrofluid
Ferrofluid is a liquid that is attracted to the poles of a magnet. It is a colloidal liquid made of nanoscale ferromagnetic or ferrimagnetic particles suspended in a carrier fluid (usually an organic solvent or water). Each magnetic particle ...
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 pressure
Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases ...
, 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
In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...
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
Ultra-high vacuum (UHV) is the vacuum regime characterised by pressures lower than about . UHV conditions are created by pumping the gas out of a UHV chamber. At these low pressures the mean free path of a gas molecule is greater than approximately ...
(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. This is due to the premature failure of the ferrofluid seal when it seals a liquid. However, a lot of research is conducted in order to solve this problem.
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.
References
External links
Video demonstration of a magnetic liquid rotary seal
Seals (mechanical)
Magnetic devices