108 Chapter 4
cold medium removes heat from a dairy fl uid.
This cool medium may be incoming cold raw
milk (as is the case of the regeneration section
of a pasteurizer) or chilled water. Chilled
water is produced by contacting water with a
refrigerant (commonly ammonia in the
United States). The apparatus in which
heating or cooling takes place is generically
called a heat exchanger.
Calculating the heat transfer area required
for a particular operation is a complex process
involving product fl ow rate, physical proper-
ties of the fl uid being heated and the heating
medium, the temperature program necessary
for the operation, allowed pressure drops,
design of the heat exchanger, sanitary require-
ments, and necessary operational time. The
product fl ow rate depends on the operating
capacity of the dairy factory.
Density and specifi c heat and viscosity are
important parameters that defi ne the physical
properties of the fl uids. The temperature
program depends on the legal requirements
and temperature differentials between the
medium being heated and the heating medium.
Temperature changes (often referred as Δ t)
depend upon the inlet temperatures of the
medium being heated and the heating medium.
The design of the heat exchanger includes
the fl ow of the fl uid being heated in relation
to the fl ow of the heating medium. Such fl ows
can be countercurrent (Figure 4.6 ) or concur-
rent (Figure 4.7 ), meaning the fl uid being
heated fl ows against the fl ow of the heating
medium or in the same direction as the heat-
ing medium, respectively. The design also
includes the physical nature of the heating
apparatus, including plate heat exchangers or
tubular heat exchangers, and in some cases
scraped surface heat exchangers.
The ability to effectively clean and sani-
tize food contact surfaces is vital in the food
industry and therefore the design of a heat
exchanger must take this into consideration.
The necessary operational time is the length
of time the equipment can be operated
without cleaning, and it depends on a numbertransferring cream and cultured products.
Eccentric screw, piston, and diaphragm
pumps are also positive displacement pumps
that are used for specialized purposes in dairy
plants.
Heat Transfer Operations
Heating and cooling are two common opera-
tions in any dairy plant. Collectively these
operations involve the transfer of heat from
one medium to another. Transfer of heat can
be routinely achieved through indirect contact
of a hot medium against a cool medium. In
the case of heating dairy fl uids, the hot
medium is hot water. Boilers produce steam
that is directly injected into the water and the
result is hot water. In the case of cooling, a
Figure 4.5. Lobe rotor principle. Reproduced with
permission from Tetra Pak.