Section I
Table of Contents
Section III 
Section I
Table of Contents
Section III
Ozone is a gas, slightly bluish in color, with a pungent odor. It consists of three atoms of oxygen in each molecule; the oxygen we breathe contains two atoms in each molecule. Chemically, oxygen is O2, and ozone is O3. The "ozone layer" consists of ozone in the stratosphere, high above the Earth at an altitude of between 7 and 28 miles. It is formed by ultraviolet light (UV) from the sun acting on oxygen molecules. The ozone layer absorbs and scatters ultraviolet light from the sun, preventing harmful amounts of ultraviolet light from reaching the Earth. For this reason, it is often referred to as the ozone or protective shield.
Ozone is also found at times in the lower atmosphere where we breathe it. Here it is caused by ultraviolet radiation from the sun acting on smog and air pollutants on hot summer days. This situation should not be confused with the protective ozone layer in the stratosphere. Ozone at ground level is a harmful pollutant; in the stratosphere it is a protective shield.
In June 1974, Professor Sherwood Rowland and Dr. Mario Molina of the Department of Chemistry at the University of California at Irvine, first proposed the theory that certain chlorine-containing compounds could pose a threat to the stratospheric ozone layer above the Earth. The Rowland-Molina theory states that CFCs would ultimately cause damage to the stratospheric ozone layer, which protects the Earth from harmful levels of ultraviolet radiation from the sun. What follows is a summary of the current theory held by the EPA.
Refrigerants that contain chlorine but not hydrogen are so stable that they do not break down in the lower atmosphere even one hundred years or more after being released. These chemicals gradually float up to the stratosphere, where the chlorine or bromine react with ozone, causing it to change back to oxygen.
The "Ozone Hole" is a thinning in the ozone layer over Antarctica, which occurs during the Antarctic spring season (autumn in the Northern Hemisphere). It occurs over the Antarctic continent due to the unique climate in that part of the world. Powerful winds encircle Antarctica during its winter, isolating the continent from warmer winds that would otherwise migrate from lower latitudes on the Earth's surface. The continent is in darkness during the winter. These two effects combine to produce the coldest temperatures on Earth; colder than the Arctic. The stratosphere is normally too dry to form clouds, except at the bitterly cold temperatures reached during the Antarctic winter. At these frigid temperatures, clouds of ice and nitric acid, called polar stratospheric clouds (PSCs), form in the stratosphere over the continent of Antarctica. Chemical reactions take place on the surfaces of these clouds, converting chlorine and bromine from forms that do not react with ozone to other, less stable forms that readily break up in the presence of sunlight and go on to destroy ozone. Both cold temperatures and sunlight are critical to the ozone depletion process. So it is in the spring, when the sun again rises and when the PSCs are still present, that the Antarctic ozone hole is found. As the sun warms the region in spring, the clouds dissipate.
This area is being carefully monitored for the degree to which the ozone thins out because it has been found to lead to ozone depletion in other parts of the world as well. Significantly reduced ozone levels were detected in 1985, and high chlorine levels were found in 1986. Since that time, aircraft and ground-based instruments have indicated that the ozone depletion problem may be more serious than initially thought.
When ozone depletion occurs, more UV radiation penetrates to the Earth's surface. Moreover, because of the long atmospheric lifetimes of CFCs, it will take many decades for the ozone layer to return to past concentrations. As stated earlier, bromine-containing compounds, which are contained in typical Halon fire extinguishers, react the same way as chloride atoms in destroying the ozone. In the years since the theory was first proposed, substantial scientific research has supported the general concern that increased concentration in the stratosphere of chlorine and bromine pose substantial risks of ozone depletion resulting in harm to human health and the environment.
The CFC refrigerants and the halons have been assigned a factor that represents their relative ability to destroy stratospheric ozone. Called the Ozone Depletion Factor, or Ozone Depletion Potential (ODP), this scale is based on CFC-11 having been assigned a factor of 1. CFC-12 has an ODP of 1.0, HCFC-22 has an ODP of 0.05, and HFC-134a has an ODP of 0.0.
Shielding the Earth from much of the damaging part of the sun's radiation, the ozone layer is a critical resource safeguarding life on this planet. Should the ozone layer be depleted, more of the sun's damaging rays would penetrate to the Earth's surface. Some scientists have claimed that each 1% depletion of ozone increases exposure to damaging ultraviolet radiation by 1.5-2%. EPA's assessment of the risks from ozone depletion focus on the following areas:
Unlike many other environmental issues, stratospheric ozone protection is a global problem. CFCs and halons are used in many nations, and because of their long atmospheric lifetimes, they become widely dispersed over time. As a result, the release of these chemicals in one country could adversely affect the stratosphere above other countries and therefore the health and welfare of their citizens. Many developed and some developing countries produce CFCs and halons. Most consume the chemicals in a variety of different products. The United States, for example, consumed 29% of the world's CFCs. Other developing nations were significant users. To protect the ozone layer from the damage that may be caused by CFCs and halons, an international solution was critical.
Hydrofluorocarbon (HFC) refrigerants such as HFC-134a, contain no chlorine and therefore have a zero ozone-depletion potential (ODP), however they can still contribute to the global warming problem caused by green house gas emissions. The potential of these refrigerants to contribute to global warming is expressed in a relative scale, known as the Global Warming Potential or GWP. Europe is currently taking steps to meet the Kyoto Protocol, which calls for quantitative reduction of greenhouse gases (including HFCs) for the period 2008-2012. Therefore Europe will be restricting the use of any refrigerant with a Global Warming Potential (GWP) in excess of 150. This means that even new residential air conditioners using HFC-410A and MVAC systems using HFC-134a will be banned. As a point of reference, HFC-134a has a GWP of 1725, HCFC-22 has a GWP of 1500, and HFC-410A has a GWP of 1890.
To address the global warming issues, researchers are looking at several potential refrigerant changes. These refrigerants would not be drop in replacements for R-12 or R-134a but instead would require completely new system designs. Some of the refrigerants that are being considered because of their low GWP (and zero ODP) are propane (R-290), n-butane (R-600), isobutane (R-600a), propane/butane blends, R-152a (difluoroethane)hydrofluoro-olefin (HFO)-1234yf, and carbon dioxide (R-744). HFO-1234yf has a GWP of 4, Propane has a GWP of 11, carbon dioxide has a GWP of 1, and R-152a has a GWP of 140. Note that HFO-1234yf, R-290, R-152a, R-600, and R-600a are all flammable refrigerants. Automobile manufacturers are currently considering HFO-1234yf, R-152a and carbon dioxide. Propane and butane is used in Europe for air conditioning applications and is more closely related to R-22's saturation pressure temperature curve. The pressure temperature saturation curves for theses refrigerants is presented in Figure 1. Table 2 contains the pressure temperature behavior of R-152 and Table 3 contains the pressure temperature behavior for carbon dioxide. The carbon dioxide data stops at 85°F, because 87°F is the critical point temperature of carbon dioxide, also note the much higher pressures of the carbon dioxide at the same temperature.
Figure 1. Saturation Temperature Pressure Relationship for
Some Possible Refrigerants
| Table 2. Saturation Pressure Temperature Table for HFC-152a (1,1-difluoroethane) |
||||
|---|---|---|---|---|
| Temperature (°F) |
Pressure (psig) |
Temperature (°F) |
Pressure (psig) |
|
| 20 | 15 | 80 | 76 | |
| 25 | 19 | 85 | 84 | |
| 30 | 22 | 90 | 92 | |
| 35 | 26 | 95 | 100 | |
| 40 | 30 | 100 | 110 | |
| 45 | 35 | 105 | 119 | |
| 50 | 39 | 110 | 129 | |
| 55 | 44 | 115 | 140 | |
| 60 | 50 | 120 | 151 | |
| 65 | 56 | 125 | 163 | |
| 70 | 62 | 130 | 175 | |
| 75 | 69 | |||
| Table 3. Saturation
Pressure Temperature Table for Carbon Dioxide (R-744) |
|
|---|---|
| Temperature (°F) |
Pressure (psig) |
| 20 | 407 |
| 25 | 441 |
| 30 | 476 |
| 35 | 513 |
| 40 | 553 |
| 45 | 594 |
| 50 | 638 |
| 55 | 684 |
| 60 | 733 |
| 65 | 784 |
| 70 | 838 |
| 75 | 895 |
| 80 | 955 |
| 85 | 1018 |
| 87.8 Critical Point Temp | |
The following recent publications more fully discuss possible new refrigerants for air conditioning and refrigeration systems:
R-152a is 1,1-Difluoroethane, also called simply difluoroethane. R-152a is a chemical compound composed of carbon, hydrogen, and fluorine. Its molecular formula is C2H4F2 and it is a flammable liquid and gas under pressure. Thermal decomposition yield toxic products which can be corrosive in the presence of moisture. There are no known toxicological effects and no known ecological damage caused by R-152a. At standard temperature and pressure, it is a colorless gas. It is classified as a halogenated aliphatic. In use as a refrigerant, it has a low global warming potential, and has recently been approved for use in automobile applications as an alternative to R-134a. It is also commonly found in electronic cleaning products, and many consumer aerosol products that must meet stringent volatile organic compound (VOC) requirements. New HFC-152a MVAC systems are acceptable under the condition that MVAC systems must be designed to avoid concentrations in the passenger cabin that are above 3.7% for more than 15 seconds. Recovery machines specifically designed for this refrigerant will need to be developed, SAE is currently developing standard J2851 relating to performance standards for recovery machines for R-152a.
Carbon dioxide (R-744) is also a possible refrigerant, however the much higher pressures would preclude the use of flexible hoses, and the refrigerant would be operating above the critical point at the heat rejection temperatures. This translates into a much lower efficiency cycle. It is extremely unlikely that a shaft-driven traditional MVAC system would be developed if CO2 were utilized as a refrigerant. It appears that R-152a is much more likely. However, if a CO2 MVAC system were developed, the CO2 system would be acceptable to the EPA under the condition that MVAC system was designed to avoid concentrations in the passenger cabin that are above 3% for more than 15 minutes.- Ozone in the stratosphere above the Earth consists of molecules containing 3 oxygen atoms (O3).
- It is the chlorine and bromine in refrigerants that cause stratospheric ozone depletion.
- CFCs are chemically very stable, and they do not dissolve or break-down in water (so they are not removed by rain). Because of this chemical stability, they are able to reach the stratosphere.
- CFCs have the highest ozone depletion potential (ODP) and are the most harmful to stratospheric ozone.
- The ozone layer protects the Earth from ultraviolet radiation from the sun. Skin cancer, increased cataracts, and damage to crops are just some of the results of damage to the Earth's ozone layer.
- Capturing and ultimately eliminating the use of chlorofluorocarbons is being done in the United States to stop damage to the stratospheric ozone layer.
- When addressing consumer complaints regarding additional service expense due to recovery efforts, the technician needs to explain to the customer that recovery is necessary to protect human health and the environment.
- After reaching the required recovery vacuum on an system, you should turn off the recovery device, (isolate the system), and wait for a few minutes to see if the system pressure rises, indicating that there is either refrigerant in liquid form, refrigerant trapped in the oil, or a leak in the system.
- Non-condensables in a refrigeration system result in a higher discharge pressure.
Section I
Table of Contents
Section III