Refrigerants

Background1850’s – 1870’s: ammonia, ammonia/water, CO2

Early 1900’s: SO2, methyl chloride used for domestic refrigerators1930’s: halocarbon refrigerants discovered by Midgley (R-12, R-22, R-114, R-22)

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

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

Ozone Depletion, cont.CFC’s

Most stable – remain in atmosphere for many years, allowing them to diffuse to high altitudesCFC’s break down, and Cl combines with and consumes some ozone

HCFC’sHydrogenatedNot as stable – most of it breaks down before reaching high altitudesLess damaging to ozone

Ozone Depletion, cont.

HFC’sContains no ClCauses no depletion of ozone

Montreal Protocol (1987)

Called for curtailment of production of CFC’sFollow-up conferences (London & Copenhagen)

Complete cessation of CFC productionEventual discontinuance of HCFC production

Global WarmingShort 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.)

TEWITotal Equivalent Warming ImpactConsider 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.

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.

Numerical Designation of Refrigerants

1st digit on right is number of F atoms in compound2nd digit from right is number of H atoms + 1 in compound3rd 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 seriesInorganics – 700 series

Selection CriteriaPhase-out due to ozone depletionGlobal warming (TEWI)EfficiencySafetyContainment/Vessel construction reliabilitySizeAvailability/PriceFuture ConversionSaturation Pressures and TemperaturesMaterial CompatibilityLow Freezing Temperature

Saturation Temperatures and Pressures

Operating pressureLow enough to use pipe & vessels of standard wall thicknessesBelow atmospheric pressure undesirable because air may leak in

Should have 5-10 degree temp difference between refrigerant and medium.

SafetyAmerican Conference of Governmental Industrial Hygienists – Threshold Limit Values

1st column for a 40-hr work week2nd column for short-term exposure

Refrigerant Blends

Azeotropes – the blend acts as a single, different refrigerantZeotropes – 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, CO2, propane, and water are being used more.Our legal system makes flammable refrigerants questionable in the US.