ABSTRACT
Let us consider a tubular heat exchanger, one of the most
simplistic and practical liquid food heating or cooling
devices, operated in continuous and steady-state mode, as
illustrated in Fig. 1.
Two streams of fluids (hot and cold) are separated by
the solid tube wall (non-contact type). One can easily
recognize that the working heat is transferred from the
hotter side through the wall into the colder side (heat
conduction through the solid wall). The solid wall is
usually of stainless steel in the food industry and has a
certain thickness. Before that, however, the working heat is
transferred from the hotter fluid to the solid wall (forced
heat convection because of pumping), and after penetrating
the solid wall, it is transferred to the colder fluid (forced
heat convection as well). The hotter fluid is also likely to
lose part of the heat through the external solid wall of the
annular side into the surrounding spaces as a result of heat
convection because of air movement (natural or free con-
vection). Furthermore, because the outer surface of the
annuli may be hotter than the surrounding surfaces, part
of the heat is transferred to the environment by heat radia-
tion. Should one or two fluids in this system boil or con-
dense on the wall surfaces, the boiling heat transfer or
condensation heat transfer modes must be considered.
