Modern Engineering Thermodynamics

14.16 Miscellaneous Refrigeration Technologies 577
14.16.2 Refrigerating Mixtures
Any endothermic (heat-absorbing) chemical reaction can be used to produce refrigeration. The simplest refrigerating
reactions occur when salts with endothermic heats of solution are dissolved in water. The resulting saltwater
(brine) mixtures can become cold enough to freeze ice on the outside of the container and are known as refrigerating
mixtures. Sodium nitrate (NaNO 3 ), potassium nitrate (KNO 3 ), ammonium nitrate (NH 4 NO 3 ), and calcium
chloride hexahydrate 8 (CaCl 2 · 6H 2 O) were all used as refrigerating mixtures in ancient times. Refrigerating
mixtures were used in ancient times to produce artificial ice to cool drinks. More recently, it was discovered that
most of these salts also reduce the freezing point of a saltwater mixture, so if one of these salts is added to ice or
snow at 0°C (32°F), the salt melts some of the ice or snow, producing a brine solution. Chemical equilibrium
requires that thermal energy be added to the endothermic process of dissolving the salt, plus additional thermal
energymustbefoundtomelttheiceusedtoproducethebrine. If the mixture container is insulated, then the
required thermal energy must come from the mixture itself. This results in lowering the mixture temperature to
the freezing point corresponding to the brine concentration. For example, a saturated solution of common
hydrated calcium chloride (CaCl 2·6 H 2 O) can produce an ice-brine mixture temperature of −55°C (−67°F) in an
insulatedcontainer.Thissaltisalsousedtomelticefrom sidewalks and roads during winter, so long as the ice
temperature is above −55°C (−67°F).
14.16.3 Surface Evaporation
The ancient Egyptians knew that, when water seeped through porous pottery and evaporated from its surface, it
would cool the contents. Evaporative cooling is the method warm-blooded animals use to control their body
temperature. The evaporation of perspiration from the surface of our skin is one of the main cooling mechanisms
of humans. Dogs and cats pant, causing evaporation directly from the surface of their lungs.
HOW DO YOU MAKE ICE CREAM?
Have you ever tried to refreeze melted ice cream? Refreezing
changes the texture of the ice cream from smooth and creamy
to coarse and unpleasant. This is because refrigerator freezers
freeze foods slowly, producing large ice crystals that result in
the coarse texture in refrozen ice cream (it also destroys the
cellular structure of meats frozen this way).
Cooled or semi-frozen foods containing milk or cream have
been known since ancient times. Initially reserved for the
wealthy, who could afford such extravagance, they finally
became universally available when a Spanish doctor, Blasius
Villafranca, discovered that cream could be frozen in a container
surrounded by a mixture of saltpeter (potassium
nitrate, KNO 3 ) and snow in 1550. Nancy Johnson, a farmer’s
wife, developed an ingenious home ice cream freezer using
salt and ice in 1846, and in 1851 Jacob Fussel established
the first ice cream company in Seven Valleys, Pennsylvania.
By the late 1800s, it was commonly known that mixing table
salt (NaCl) or hydrated calcium chloride (CaCl 2 · 6H 2 0)
with solid ice or snow in an insulated container produced
subzero temperatures cold enough to make good quality
homemade ice cream by quick freezing (Figure 14.34).
FIGURE 14.34
A 19th century home ice cream maker.
Around 1890, a small ice cream vendor decided to stimulate weekend sales by adding chocolate or fruit syrup to ice
cream sold on Sunday. At first this was known as Sunday ice cream, but to avoid religious conflict, the name and spelling
were subsequently changed to ice cream sundae. The first ice cream on a stick was produced commercially in 1904.
8 Pure anhydrous calcium chloride (CaCl 2 ) has an exothermic heat of solution due to the strong exothermic hydration process that
occurs during solution. However, common solid calcium chloride salt is already hydrated as CaCl 2 · 6H 2 O, and has an endothermic
heat of solution, since the exothermic hydration process is not required.