surface finish requirements for
wetted parts of mixer, requirement for
heating or cooling the mixer contents,
operation of mixer under vacuum and
pressure conditions, preferred sealing
materials and sealing technology for
mixer, equipment maintenance related
requirements and so on should be clearly
specified.
(f ) If the end user has a previous
experience of a similar or identical
process, it is imperative that the relevant
information is provided, along with the
areas that may require improvement.
The end user needs to ensure that the
equipment manufacturer has understood
his requirements and is equipped with the
technology and resources to deliver the
desired performance.
4) W HAT ARE SOME PITFALLS YOU SEE
END - USERS COMMONLY ENCOUNTER
IN FLUID MIXING APPLICATIONS ? H OW
CAN END - USERS BEST AVOID AND /
OR RESPOND TO SUCH APPLICATION
PITFALLS ?
The awareness of the end-users about
42
PECM Issue 29
the different types of agitator designs
available, and their applications need
to be increased. Moreover, a better
understanding is required on the
concepts of flow and shear, produced by
the impellers. More often than not, end
users prefer to duplicate their operational
systems, even for new requirements
which may be different from the existing.
The effects of improper selection are
realized much later, during actual
operations.
I would like to bring to you attention a
very recent example, where in the works
manager from a chemical company called
me and asked me to quote for a “power
saver” agitator for a 12,000 litres vessel.
When, I enquired about his process, I was
told not to worry about the technical
aspects, since they felt that the “power
saver” impeller was their best option.
They termed the “hydrofoil impeller” as
the “power saver” impeller, and chose it
because someone told them that they
would reduce on power consumption
during agitation. Very few users know
that hydrofoil impellers are available in
different designs, solidity ratios, each
of which is best suited for a particular
kind of application. Users therefore need
to consider selection of the agitator
types, keeping in mind their process
requirements, properties of process
materials and other operating conditions.
It is recommended that the selection of
agitators be carried out in consultation of
the agitator specialist.
Besides selection of impellers, scale-up
of agitation equipment offers several
challenges in fluid mixing. While scaling
up, design factors such as the physical
configuration of mixing vessel (off-bottom
distance, spacing between impellers and
the ratio of impeller diameter to tank
diameter), impeller diameter, speed,
power consumption, and so on need to
be considered.
Application pitfalls can be avoided
by ensuring that the equipment
manufacturer knows everything about
the process, including what could
go wrong in case there is a deviation
in the process conditions. In case
of new processes, applications, it is
recommended that laboratory, pilot scale
trials be carried out to establish process
objectives and equipment design and
performance parameters.