Modern Engineering Thermodynamics

568 CHAPTER 14: Vapor and Gas Refrigeration Cycles
WHAT ARE “SOFT” DRINKS AND WHY ARE THEY
SERVED COLD?
The soda fountain is a unique 1920s American invention at which ice cream and carbonated nonalcoholic
(i.e., “soft”) drinks were sold. The soft drinks were usually cooled to increase the solubility of the
CO 2 used in their carbonation (Figure 14.26). This enhanced their flavor and created a unique
tingling feeling in the mouth and throat.
In 1767, Joseph Priestley invented carbonated water, a key component of soft drinks. He added carbon
dioxide to water by suspending a container of water above a beer vat at a local brewery.
In 1881, the first cola-flavored beverage was introduced. In 1885, Charles Aderton invented “Dr Pepper”;
in 1886, Dr. John S. Pemberton invented “Coca-Cola”; and in 1898, “Pepsi-Cola” was invented by
Caleb Bradham. Until 1905, Coca-Cola contained extracts of cocaine (from coca leaves) and
caffeine (from the kola nut) and was marketed as a medicine that could cure various ailments.
FIGURE 14.26
A soft drink bottle.
14.12 PART II. GAS REFRIGERATION CYCLES
Reversed gas power cycles have the same potential for producing cooling that we see with reversed vapor cycles.
But not all gas power cycles have a reversed cycle refrigeration analog. For example, we do not yet know how
to reverse internal combustion gas power cycles, because to do so would require the development of rapid
endothermic (heat-absorbing) chemical reactions similar to the exothermic combustion reactions used in the
power cycles. However, all external combustion gas power cycles have effective reversed cycle refrigeration
technologies.
The rapid development of vapor and gas power cycles forever changed the world. They improved productivity
and efficiency in agriculture, transportation, textiles, manufacturing, and in many other areas that affect the way
we live. The social impact of the reversed power cycles, limited primarily to food preservation and environmental
control, was less influential. Though we have known for a long time that many gas power cycles could be
reversed to produce cooling, their development into viable technologies had less social impact and consequently
grew less rapidly. Part II of this chapter focuses on the technology of gas refrigeration cycles plus a few miscellaneous
refrigeration technologies that were not derived from reversed power cycles.
14.13 AIR STANDARD GAS REFRIGERATION CYCLES
The working fluid in gas refrigeration cycles is less complex than in gas power cycles. For example, refrigeration
cycles do not involve internal combustion processes that change the working fluid during the cycle, but they
can be either open or closed loop cycles. Therefore, there is much less need for a working fluid simplifying
model like the air standard cycle (ASC). Nonetheless, we use its simplifying characteristics in analyzing gas
refrigeration cycles in which air is the working fluid. In particular, all gas refrigeration cycles are assumed to be
closed loop cycles when an ASC analysis is used. In addition, the following assumptions apply to gas refrigeration
ASC analysis:
1. The working fluid is a fixed mass of air that obeys the ideal gas equation of state.
2. All inlet or exhaust processes in open loop systems are replaced by heat transfer processes to or from the
environment.
3. All processes within the cycle are reversible.
4. The air has constant specific heats. 5
ASC refrigeration analysis yields reasonably accurate results for most cycles using air as the working fluid. One
notable exception is in the area of throttling or Joule-Thomson cooling, in which the amount of cooling
depends exclusively on real gas behavior. This is illustrated later in this chapter.
5 Since the temperature variations within a gas refrigeration cycle are not nearly as large as those within a gas power cycle that contains
a combustion process, there is no practical need to distinguish between a hot refrigeration ASC, in which temperature-dependent
specific heats are used, and a cold refrigeration ASC, in which constant specific heats are used.