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) Halocarbon advantages – stable compounds, favorable thermodynamic properties, safer than existing refrigerants Ammonia still used for large low-T industrial plants

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

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

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

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

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.)

TEWI Total Equivalent Warming Impact Consider direct contributions to global warming (refrigerant emissions) and indirect (CO 2 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.

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

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

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

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.

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

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

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, CO 2, propane, and water are being used more. Our legal system makes flammable refrigerants questionable in the US.