Modern Engineering Thermodynamics

552 CHAPTER 14: Vapor and Gas Refrigeration Cycles
EXAMPLE 14.5
As a technical expert in a multibillion-dollar lawsuit, you are asked to determine the refrigerant numbers for the following
refrigerants by the prosecuting attorney:
a. Chloroform, CHCl 3 .
b. Chlorotetrafluoroethane, CHClFCF 3 .
c. Octafluoropropane, CF 3 CF 2 CF 3 .
Solution
Being totally unimpressed by the prosecuting attorney’s aggressive questioning, you calmly reply as follows:
a. “Chloroform contains one carbon atom (a = 1), one hydrogen atom (b = 1), and three chlorine atoms (c = 3), and no
fluorine atoms (d = 0).” Making a quick calculation in your head using Eq. (14.8), you arrive at R-(a − 1)(b +1)d = R-(1 − 1)
(1+1)0= R-020 = R-20 (dropping the leading 0). Then you reply, “So, the refrigerant number for chloroform is R-20.”
b. “Chlorotetrafluoroethane, on the other hand, contains two carbon atoms (a = 2), one hydrogen atom (b = 1), one
chlorine atom (c = 1), and four fluorine atoms (d = 4).” Using Eq. (14.8) again you find R-(2 − 1)(1 + 1)4 = R-124,
and you reply, “So, its refrigerant number is: R-124.”
c. “Now octafluoropropane is a very interesting compound in that it contains three carbon atoms (a = 3), no hydrogen or
chlorine (b = c = 0), and eight fluorine atoms (d = 8).” (Thinking, again using Eq. (14.8), R-(3 − 1)(0 + 1)8 = R-218.)
“Consequently its refrigerant number is: R-218.”
Exercises
13. Suppose the prosecuting attorney in Example 14.5 asks you for the refrigerant number of carbontetrachloride CCl 4 . What
would you say then? Answer: Your response: “The refrigerant number is: R-14.”
14. “Aha!” the prosecutor in Example 14.5 exclaims, “You seem pretty confident of yourself, don’t you? Well, then, can you
tell me what substance has refrigerant number 720?” (Recall that 700 series refrigerants are inorganic compounds and
the last two digits of the R number correspond to the molecular mass of the compound.) Answer: Your response: “The
compound is neon.”
15. The prosecuting attorney in Example 14.5 vociferates, “You don’t say, then give me the chemical formula and refrigerant
number for trifluoromethane!” Answer: Your response: “CHF 3 which is R-23.”
14.8 CFCs AND THE OZONE LAYER
Ozone (O 3 ) in the upper atmosphere absorbs ultraviolet radiation from the sun and prevents much of it from
reaching the surface of the Earth. Exposure to ultraviolet radiation is a known source of skin cancer and other
biological problems.
All chlorofluorocarbons (CFCs) are combinations of chlorine, fluorine, and carbon atoms. After 1950, the use of
chlorofluorocarbons dominated the domestic and automotive refrigeration and air conditioning markets. In the
1950s and 1960s, inexpensive chlorofluorocarbons found use as a propellant in aerosol spray cans for paint,
deodorant, hair products, and so forth.
In 1974, Professor Sherry Rowland at the University of California—Irvine and her postdoctorate student Mario
Moline postulated that chlorofluorocarbons are so chemically stable that they can exist in the atmosphere for hundreds
of years, eventually diffusing into the Earth’s stratosphere, where ultraviolet radiation decomposes them to
release chlorine atoms. The chlorine atoms then catalyze the conversion of ozone into oxygen as follows:
O 3 + Cl ! O 2 + ClO
ClO + O ! O 2 + Cl
with the chlorine atom being regenerated. The overall reaction is then
O + O 3 ! 2O 2
The CFC production in 1974 was 1 million pounds per year, and the shear volume of CFCs released through
spray cans and leaking refrigeration systems could possibly destroy the ozone layer faster than it is created by
ultraviolet radiation acting on oxygen molecules. Rowland’s hypothesis alluded to a massive global problem,
and it had profound impact on CFC use.
But, what will replace the banned CFCs? It was not too difficult to find safe propellants (such as CO 2 ) for use in
aerosol cans, but finding suitable replacements for refrigerants such as R-11 (used in large building air conditioning
systems) and R-12 (used in domestic refrigerators and air conditioners and automotive air conditioners)
was much less obvious.