NEW BRUNSWICK SCIENTIFIC BioFlo 415, Manual To Operations

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New Brunswick Scientific User’s Guide
22.2 Some Factors that Affect OTR And Horsepower
Many factors influence OTR, not the least of which are type, size and placement of
impellers in the reactor. (Factors which effect OTR are vessel dimensions, impeller
diameter, type of impeller, i.e. turbine, marine, pitched blade, etc.). New Brunswick
Scientific selects and recommends the placement of impellers in the vessel to attain a
minimum of 350 mM O
2
/L/hr of OTR.
The BioFlo 415 fermentor is supplied with two or three (19L) properly sized Rushton
Impellers. Placement of the impellers should be as indicated in
Figure 2
.
In some processes, users may wish to use a third impeller. Should this be the case,
however, a smaller impeller diameter is required, since the systems are specifically
designed such that the vessel diameter, motor, impellers, to produce a specific OTR.
When any of the factors is changed, other features may also change.
To determine the horsepower utilized by a given number of impellers, the following
formula can be used. The impeller diameter varies to the 5
th
power with respect to
horsepower. A very slight change in the diameter of an impeller can make a great deal
of difference in the HP required to drive that impeller.
The approximate horsepower utilized to drive a given set of impellers is determined as
follows:
HP = D
5
x RPM
3
x (4.5 x 10
-13
) x I
Where:
HP = Horsepower
D = Impeller diameter in inches
RPM = Agitator speed in RPM
4.5 x 10
-13
= Constant (factor based on unaerated water at 20
°
C
with a six-bladed Rushton impeller)
I = factor based on the number of impellers used in the
vessel:
•
Use 1 for one impeller
•
Use 1.8 for two impellers
•
Use 2.4 for three impellers
#
NOTE:
The HP requirements are substantially affected by aeration. An airflow
rate of one vessel volume per minute (VVM) may produce as much as 40%
reduction in the horsepower used. It is required that some air/gas flow be
utilized when running at speeds above 750 RPM. The relationship in the
reduction of horsepower when gas is added into the system is not linear.
A small amount of air can produce a 20% reduction in horsepower.