Page 216 - Fluid mechanics, heat transfer, and mass transfer

MIXING
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& Another option is use of rectangular tanks since the materials, seal requires external flushing and clean-
corners act as baffles. ing with product compatible fluid.
& The most common impellers for slurries are marine & Side-entry seals are also immersed in the liquid

propellers, four-bladed/45 pitched turbines, and contents of the agitator and require similar
hydrofoils of proprietary designs. Two or more im- treatment.
pellers may be required, either on separate shafts or . What are the different types of seals used for mixers?
on the same shaft, to provide suspension of solids, as & Compression Packing or Stuffing Box: Consists of
well as mixing or blending.
a series of rings of packing material arranged
. What are the ways to resuspend solids after a power between the rotating shaft and the stationary tube
failure? attached to the mounting flange. The number of rings
& Agitators are often unable to resuspend the solids used determines the pressure rating, which is limited
once they have settled. to 6 bar gauge.
& Most pumps are designed for light slurry loads and & This type of seal can be sterilized with steam injec-
cannot handle the settled solids, usually due to high tion and can be lubricated or operated dry.
viscosities. & Single Mechanical Seal: Consists of two sealing
& Sparging with a liquid is the best solution for faces, one rotating with the shaft and the other
agitation. Liquid jets can recover the solids sus- stationary. These seals give lower rates of leakage,
pension, provided the solids are not extremely are more expensive, and have longer life. Should not
cohesive. be used on sensitive or pathogenic cultures or toxic
& Nozzles should have the capability of either backflow contents of the vessel.
or solids-free liquid injection after power failures. & Single Fully Split Mechanical Seal: Similar to single
& Positive displacement pumps can handle very high mechanical seal but with the addition of a longitu-
solids concentrations (maybe as high as 95%), but dinal split line for ease of maintenance. Disadvan-
add considerable costs. tage is lower pressure rating with higher leakage
rates.
& Dual Pressurized (Double) Mechanical Seal: A bar-
rier liquid is introduced into the sealing chamber for
6.2.8 Mixer Seals
lubrication, which isolates process fluid and vapors
. What are the functions of agitator seals?
from the environment.
& Containment of vessel contents to save valuable
& Gas-Lubricated Seal: A thin gas film separates the
materials, from safety considerations involved with
sealing faces. Designed to run dry without face
toxic and flammable materials.
contact and involve no wear. Dry N 2 is used as the
& Avoidance of contamination and corrosion problems.
separating gas.
& Static and dynamic sealing to maintainvesselpressure.
& Combination of Contact Dry Running and Gas-
. What are the entry aspects that influence seal selection? Lubricated Seals: Can reduce problems of N 2 accu-
& Entry of the shaft of an agitator can be top entry (most mulation in unvented closed building spaces.
common), side entry, or bottom entry. & Modular Cartridge Seals: Preassembled self-con-
& In top entry, seal is not wetted by the liquid contents tained seals.
and agitator is in contact with only vapor. & Magnetic Drives: High levels of containment and
& Bottom-entry seal is immersed in the liquid contents. isolation. High initial and long-term maintenance
This may provide seal lubrication. For abrasive costs.