Idle Time

The unproductive time of a discontinuous separation apparatus. This includes e.g. times for the filling with suspension, for the solids discharge and for the preparation of the apparatus for a new filling. The total batch time then results out of the sum of idle time and separation time


refers to a bulk’s behavior of not getting compressed by pressure. Incompressibility is exhibited especially with coarser particles in the region over 100 μm. Very small particles or flocks, however, tend to form bulks with a distinct compressibility. Incompressible bulks cannot be demoistured by press filters, and the entrained liquid has to be removed by overcoming the capillary pressure by means of gas or centrifugal pressure.

Inert Gas

is a gas concerning the solid-liquid separation that does not react with the mixture to be separated. Generally, this is a nitrogen or helium atmosphere, which requires an enclosing of the separation machine. An inert gas atmosphere is especially necessary if oxidation processes ranging up to explosions by atmospheric oxygen in the air are to be avoided.

Inlet Cone

Form of feed distributor, frequently used in filtering centrifuges, for the even charging of the suspension into the centrifuge drum. If the inlet cone rotates, then it serves simultaneously for the pre-acceleration of the suspension to the rotational drum speed.

Inner Liquid

is the liquid bound in fine hair cracks or inside isolated voids of particles in a bulk. The volume share of this intra-particle liquid is extremely high in biological cells, which for the largest part contain water isolated from the outside by the cell membrane. The inner liquid is not accessible for mechanical demoisturing without particle destruction.


is the qualitatively unimpaired condition of a filter element, ensuring a safe functioning of the element in critical filtration processes, such as are demanded in the pharmaceutical industry.

Interfacial Tension

Reversible, isothermal work that is necessary at constant temperature and mole number to enlarge an interfacial boundary surface A by the amount dA. As the molecular attraction forces at the boundary of two immiscible substances (at least one liquid) do not compensate, but form instead a resulting force pointing to the inside of the homogeneous phase, work is necessary to transport additional molecules into the interface. Surface-active substances, also called tensides, reduce the interfacial tension. It is also decreased by a temperature increase. The interfacial tension is measured as force per unit length and is quoted in N/m or mN/m, resp. Water at 20 °C possesses an interfacial tension of 72 mN/m; it decreases with a temperature increase. Often the commonly valid term of interfacial tension is synonymously used with the expression „surface tension“. However, the latter applies according to its strict definition only to the surface of a substance against a vacuum. In view of measuring accuracy these agree mostly with surfaces against their own respective steam or a gas.

Interior Drum Filter

Drum filter with the filter area located on the interior. The suspension is fed internally and retained in the drum with a flange ring. Such filters are suited for rapidly settling feed suspensions. Nowadays, they are rarely employed in Europe.

Intermediate Suspensions

Suspension concentration region within the range of hindered sedimentation where instabilities of the settling process can appear in the form of channeling.

Interstitial Liquid

remains due to attracting capillary forces (capillary pressure) at the contact points of particles after a mechanical demoisturing process. Depending on the geometric conditions and the wettability (wetting) of the solids, the fraction of interstitial liquid held in the bulk can reach 8÷20 % of the saturation.

Interstitial Liquid Demoisturing

While it is not possible with a gas differential pressure field to demoisturing the interstitial liquid held at the contact points of particles in the bulk due to pressure compensation around the contact points, a certain part of the liquid still can be removed by inertia force demoisturing in centrifuges operating at extremely high C-values. The demoisturing of interstitial liquid is characterized as the so-called 4th region of the Bond-curve.

Inverting Filter Centrifuge

A discontinuously working filter centrifuge, with a drum insert where the filter cloth is attached to at one end like a cuff. After the cake demoisturing process, this drum is hydraulically pushed out of the drum in axial direction. This turns the filter cloth inside out and the filter cake that had formed on its inside, is now on the outside and can be cast off the filter cloth. An advantage here is the complete removal of the filter cake without a remaining cake layer, as required with peeler centrifuges. Also, it cleans the cloth extremely well. As the sieve basket rotates in an enclosed housing, the centrifugation can also be superimposed by a pressure filtration or steam pressure filtration by pressurizing the drum’s internal space. This internal space is sealed against the housing by the front plate of the drum insert. The main field of application of this relatively complicated machine is in fine chemicals and pharmaceuticals.

Isoelectrical Point

In colloidal ampholytes the mobility caused by electrophoresis drops to zero and the zeta potential disappears. The ability of suspended particles for agglomeration (agglomerate) reaches a maximum.

Isokinetic Sampling

refers to taking a fluid sample with minimal interference by the sampling device on the flow. It has to be designed in such a manner that the flow velocity at its intake corresponds exactly to the one of the fluid surrounding the device.


A bulk material is defined as isotropic if the center of gravity of each particle has the same probability to be located at any random location in the bulk at any random point in time. An isotropic bulk material is entirely evenly mixed and does not display any predominant direction.

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