Processing Principles of Dairy Ingredients 107They are useful for higher viscosity fl uids; at
lower viscosities they may exhibit some slip
as the pressure increases. The net result is a
reduction in volumetric fl ow on each stroke.
Throttling by fl ow control valves at the dis-
charge end of the pump should be avoided,
and these pumps must be fi tted with a pres-
sure relief valve. Flow control in positive
displacement pumps is achieved by control-
ling the speed of the motor or adjusting the
volume of reciprocating pumps. Positive dis-
placement pumps must be placed as close to
the feed tank as possible and the dimensions
of the pipes should be large relative to those
of centrifugal pumps. If pipe diameters are
too small, the pressure drop may be high
enough to cause cavitation in the pump.
Positive lobe pumps generally have two
rotors, with three lobes on each rotor (Figure
4.5 ). A vacuum is created when the lobes
move, causing the process fl uid to fi ll the
cavities of the lobes. The process fl uid is then
moved along the outer walls of the pump
toward the discharge end. The rotors are
driven independently by reducing gear
motors. The lobes do not touch each other
or the walls of the pump casing. These
pumps are used when the viscosity of the
process fl uid exceeds 300 cP, such as whencircular motion to the liquid. The liquid exits
the pump at a higher pressure than that at the
inlet. Centrifugal pumps are useful for trans-
ferring liquids that are not very viscous.
These pumps are widely used in most appli-
cations in a dairy factory due to their lower
costs (compared with positive displacement
pumps). Centrifugal pumps are not suitable
for high viscosity liquids or those products
that require care in handling, for example,
fl uids in which structures should not be dis-
turbed or ingredients whose identity is criti-
cal to product appeal. Flow control is
achieved by three different means. Throttling
is expensive but offers the greatest fl exibility.
Changing the impeller diameter that is the
most economical but the least fl exible.
Installing an electronic speed controller is
both economical and provides fl exibility.
Positive Displacement Pumps
Positive displacement pumps work on the
principle of positive displacement in which
each rotation or reciprocating movement
results in a fi nite amount of fl uid is being
pumped, regardless of the manometric head.
The main types of positive displacement
pumps are rotary and reciprocating pumps.
Figure 4.4. Centrifugal pump showing (1) delivery line, (2) shaft seal, (3) suction line, (4) impeller, (5) pump
casing, (6) back plate, (7) motor shaft, (8) motor, (9) stainless steel shroud and sound insulation. Reproduced
with permission from Tetra Pak.
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