Layer of particles, which remains during or after filtration on the filter medium. In case of crossflow filtration particles cannot hindered completely from sticking on the surface of the membrane and thus forming a thin layer of particles. Scraper discharge of filter cakes from peeler centrifuges or pan filters need a remaining heel to protect the filter cloth from becoming destroyed. The heel becomes blocked by small particles after some time and than must be removed or regenerated.
Abbreviation for „High Efficiency Particulate Air Filter“. Special air filter to remove particles down to 0.3 μm and separation efficiency of 99.9 %. HEPA-Filters are made of cellulose, glas or synthetic fiber layers. They are used e.g. for clean room technology.
High Gradient Magnetic Separation (HGMS)
Kind of deep bed filter with a very open and thus permeable structure or ferromagnetic fibers. If this structure is exposed to a magnetic field the streamlines of the magnetic field are strongly deformed and high magnetic forces are affecting magnetizable particles in a slurry, which is led through the filter. The magnetizable particles are separated in the filter and must be back-washed periodically after turning-off the magnetic field.
One speaks of hindered sedimentation if the particles in a fluid, i.e. a liquid, are not able to settle entirely on their own, but being influenced by each other. Resulting effects start already at a few volume percent solids content in a suspension.
Hollow Fiber Module
Special arrangement of membrane in modules for crossflow filtration. The membrane consists of hollow fibers with a diameter of less than 1mm, arranged as a bundle in a pipe. These hollow fibers are flown through by the liquid to be concentrated, leaving the module as a concentrate. The permeate flows radially through the membrane walls of the hollow fibers, and is collected inside the enclosing pipe. Hollow fiber modules are utilized for the microfiltration, but particularly for ultrafiltration, because they clog easily.
Through and through the same likeness, without distinct places (isotropic).
Hot Filter Press
Combined mechanical-thermal demoisturing process for industrial steam pressure filtration. It is a special type of membrane filter press, equipped with alternating heating and membrane filter plates. The heating plates produce a filtrate steam cushion. This steam then displaces mechanically the main portion of the liquid in the filter cake. Following this the cake can be more efficiently thermally contact dried. A similar, however, not as powerful but frequently offered combination of filter presses and thermal demoisturing is realized when after a conventional demoisturing step a small amount of heat is transfered and the press chamber is evacuated. In this application the drying process takes several hours compared to one hour in the previously mentioned equipment.
The characteristic mean pore diameter dh of a bulk, that would apply if one permeated a bundle of cylindrical capillaries instead of the investigated solid system at hand. Analogous to fluid mechanics the following can be defined for a bulk:
d32 = Sauter diameter, Sv = specific surface, ε = porosity, φ = form factor. For sand applies for example: φ ≅ 1.4, ε ≅ 0.4 ⇒ dh ≅ 0.33d32
Cylindric-conical, non-moving separation device, into which a suspension is pumped under pressure tangentially into the cylindrical part. The liquid develops a potential vortex flow, wherein the particles in the flow are exposed to a centrifugal force. Particles up to a certain cut size are spun out to the wall and leave the hydrocyclone at the lower, conical end through the apex nozzle. The major liquid volume flow including the fine particles is extracted upwardly through a central vortex finder. These machines are especially suited for the classifying of suspensions. They can be operated with pressures of up to 4bar and cut sizes of from 5 – 500 μm. Hydrocyclones are employed for enrichment, thickening, de-gritting, classifying, and de-sludging. Due to their simple design they can be manufactured readily in extremely different materials of construction as required by the process.
Hydrogen molecules for example, due to their extremely strong polar covalent bond, are definite permanent dipoles that strongly attracting each other. These dipole forces are considerably larger than the Van-der-Waals forces, because their charge distribution is contrary to induced dipoles permanently asymmetrical. Frequently such dipole forces can evolve when a hydrogen atom is bonded with a strongly electron-attracting (i.e. electron negative) atom like F, O, or N. The subsequently positively polarized hydrogen atom acts because of its extremely small size especially strong attracting upon another, negatively polarized atom. The resulting bond is called a hydrogen bridge.
(fr. Gk: hydor = water, lyein = dissolve). Hydrolysis is the splitting of a chemical bond by the addition of water.
(fr. Gk: hydor = water, philos = friend). Water-attracting wetting behavior of water against a solid, where the wetting angle has to be δ < 90°. An example for a hydrophilic system is water and glass.
(fr. Gk: hydor = water, phobos = fear). Water repelling-wetting behavior of water against a solid, where the contact angle has δ more or equal 90°. An example for a hydrophobic system is water and Teflon.
Hydrostatic Cake Formation
Phenomena appearing especially at disc filters with large disc diameters. Due to the depth of immersion of the disc into the suspension a hydrostatic pressure is created, which in combination with the atmospheric pressure on the filter cell builds up a pressure difference, that starts a cake formation on the filter medium before the actual vacuum build-up in the cell begins. As the hydrostatic cake formation increases with progressing depth, the commonly occurring problem of uneven cake formation on the filter cells also increases.
(i.e. water attracting). Hygroscopic substances (e.g. sulphuric acid, calcium chloride) attract humidity out of the surrounding air.
Class of continuously working overpressure cake filters, developed in Karlsruhe, Germany. Hyperbaric filters are characterized by a drum filter or disc filter, which is mounted mostly complete with its drive in a man-sized pressure vessel. There is a manhole for maintenance and service. The discharge of the filter cake from the pressure chamber is accomplished by a chamber sluice as for example double-flap sluice gates. Hyperbaric filters are preferably employed in the mineral, ore, and coal processing industries, as well as in the food and chemical sector. They operate with pressure differences of typically up to 6 bars and can hold up to 168 m2 filter area in a single pressure vessel. Hyperbaric filters are built by few manufacturers, among others by the BOKELA company with their BoHiBar Filter technology.