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Refrigerants

Refrigerants. Background. 1850’s – 1870’s: ammonia, ammonia/water, CO 2 Early 1900’s: SO 2 , methyl chloride used for domestic refrigerators 1930’s: halocarbon refrigerants discovered by Midgley (R-12, R-22, R-114, R-22)

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Refrigerants

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  1. Refrigerants

  2. Background • 1850’s – 1870’s: ammonia, ammonia/water, CO2 • Early 1900’s: SO2, methyl chloride used for domestic refrigerators • 1930’s: halocarbon refrigerants discovered by Midgley (R-12, R-22, R-114, R-22) • Halocarbon advantages – stable compounds, favorable thermodynamic properties, safer than existing refrigerants • Ammonia still used for large low-T industrial plants

  3. Ozone Depletion • Molina & Rowland (1974) hypothesized that Cl in CFC’s contributed to depletion of ozone (O3) in upper atmosphere. • CFC’s, HCFC’s, HFC’s

  4. Ozone Depletion, cont. • CFC’s • Most stable – remain in atmosphere for many years, allowing them to diffuse to high altitudes • CFC’s break down, and Cl combines with and consumes some ozone • HCFC’s • Hydrogenated • Not as stable – most of it breaks down before reaching high altitudes • Less damaging to ozone

  5. Ozone Depletion, cont. • HFC’s • Contains no Cl • Causes no depletion of ozone

  6. Montreal Protocol (1987) • Called for curtailment of production of CFC’s • Follow-up conferences (London & Copenhagen) • Complete cessation of CFC production • Eventual discontinuance of HCFC production

  7. Global Warming • Short wavelength radiation from sun passes easily through atmosphere. • Earth emits long wavelength radiation. • Greenhouse gases block transmission of long wavelength radiation, causing the earth to retain more heat. • Greenhouse gases are removed from the atmosphere by natural processes at varying rates. • NOTE THAT GLOBAL WARMING AND OZONE DEPLETION ARE DIFFERENT PROBLEMS WITH DIFFERENT CAUSES. (A lot of people mess up on this on exams.)

  8. TEWI • Total Equivalent Warming Impact • Consider direct contributions to global warming (refrigerant emissions) and indirect (CO2 emitted due to electrical energy usage) • Indirect contributions are up to 98% of the total contribution. • System efficiency is important! • After redesign, most HCFC and HFC systems now have lower a TEWI than CFC systems.

  9. Kyoto Protocol (1998) • Requires reduction in six greenhouse gas emissions to a level seven percent below what existed in 1990. • Signed by US in 1998 but not ratified by Senate.

  10. Numerical Designation of Refrigerants • 1st digit on right is number of F atoms in compound • 2nd digit from right is number of H atoms + 1 in compound • 3rd digit from right is number of C atoms –1 in compound. If zero, this digit is omitted. • 4th digit from right is number of unsaturated C-C bound in compound. If zero, it’s omitted. • Azeotropes – 500 series • Inorganics – 700 series

  11. Selection Criteria • Phase-out due to ozone depletion • Global warming (TEWI) • Efficiency • Safety • Containment/Vessel construction reliability • Size • Availability/Price • Future Conversion • Saturation Pressures and Temperatures • Material Compatibility • Low Freezing Temperature

  12. Saturation Temperatures and Pressures • Operating pressure • Low enough to use pipe & vessels of standard wall thicknesses • Below atmospheric pressure undesirable because air may leak in • Should have 5-10 degree temp difference between refrigerant and medium.

  13. Safety • American Conference of Governmental Industrial Hygienists – Threshold Limit Values • 1st column for a 40-hr work week • 2nd column for short-term exposure

  14. Refrigerant Blends • Azeotropes – the blend acts as a single, different refrigerant • Zeotropes – the constituents remain at least partially separate

  15. Current/Future Refrigerants • R-134a has emerged as the primary substitution for many CFC’s. • HCFC-22 and –123 are viable alternatives for now but will eventually be phased out. • In Europe, natural refrigerants such as ammonia, CO2, propane, and water are being used more. • Flammable refrigerants are questionable.

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