123
New Brunswick Scientific
User’s Guide
20.2
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/CelliGen 115 fermentor is supplied with two properly sized Rushton
Impellers. Placement of the impellers should be as indicated in
Figure 3
.
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.
For example, the standard impeller used on the 10-liter BioFlo/CelliGen 115 has a
3.24-inch
(
±
0.015) diameter. If three impellers are to be used, 3.06” diameter impellers are
required. This size impeller is normally used in a 7.5L BioFlo/CelliGen 115 vessel.
These impellers should be placed such that the bottom impeller is placed one impeller
diameter from the bottom of the vessel. The second impeller should be placed one
impeller diameter above the bottom impeller, and the third impeller should be placed
one impeller diameter above the second.
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
