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A review of alternate / D f -

refrigerants and near-azeotropic refrigerant mixtures

Refrigerant blends

by John A. Tomczyk

ecent environmental scares and legislation have accelerated research to find environmentally safe refrigerants R Research focuses on blends as "drop-in" or near drop-in replacements

-for CFC-12, CFC-502, HCFC-22, and other refrigerants. Major chemical companies mathematically model near-azeotropic refriger-

ant mixtures (NARMs). Researchers use the computer models to tailor refrigerant-blend characteristics to give maximum system efficiency and performance.

Vapor pressures can be adjusted by varying the percentage of each constitu- ent in the blend. In fact, many refrigerant manufacturers will use the same blend, but vary the percentage of its constituents to target different evaporating temperature applications. Percentage composition changes also can lower compression ratios and discharge temperatures.

Refrigerant blends can be HCFC based, HFC based, or a combination of both. The HCFC-based blends are only interim replacements because of their chlorine content. Blends with a major percentage of HCFCs have lower ozone depletion and global warmingpotentials than most CFC refrigerants that they arereplacing.TheHFC-based blendswill belong-term replacements for certain CFCs and HCFCs, until researchers can find pure compounds to replace them. There are major advantages in most refrigerant blends on the market today:

Blends have comparable capacity and efficiency when measured against CFCs and HCFCs.

.Blends can use oils that are already on the market (certain synthetic alkylbenzenes and esters). Blends are compatible with most materials of construction in today's

MIXTURES AND GLIDES Refrigerant blends are not new to

hvacr . Two popular azeotropic binary blends - CFC-502 and CFC-500 - have been in use for a long time. These binary blends consist of azeotropic mixtures of HCFC-22KFC- 1 15 and CFC- 12/HFC- 152a respectively.

Azeotropic mixtures are mixtures of two or more liquids, which, when mixed in precise proportions, behave like one compound when phase changing from liquid tovapor and fromvapor to liquid. The resultant boiling point of the azeotropic mixture will be either above or below the boiling points of the indi- vidual liquid components.

There will still be only one boiling point or one condensing point for each given pressure. The molecules all vapor- ize at the same rat: and the liquid and vapor have the same composition.

However, many of the replacement blends in research, and some that are on themarket today, arenot pureazeotropic mixtures. Many are near-azeotropic ter- nary and binary mixtures or blends.

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Pressure/Enthalpy Diagram Near-Azeotropic Blend

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ENTHALPY

Pressure/Enthalpy Diagram Azeotropic Blend(CFC-502)

These blends have a temperature glide when they boil (vaporize) and condense. Temperature glide is when the blend, at a given pressure, has a range of temperatures as it evaporates and condenses. Temperature glide oc- curs because the phase change takes place at different pressure/temperature relationships for each component in the blend. So, as the refrigerant phase changes along the length of the heat exchanger, there will be a range of boiling or condensing points for each pressure (Figure 3).

An examination of Figure 3 shows that the liquid phase and vapor phase of this blend have two distinct tempera- tures for one given pressure. In fact, at 100 psia (6.89 bar), the liquid tempera- ture is 67.79"F (19.88"C), and the va- por temperature is 7508°F (23.93"C).

Temperature glide may range from 2 to 12 degrees Fahrenheit depending on specific blends and system conditions. On the other hand, a pure compound like CFC-12, boils at a constant tem- perature for each pressure (Figure 4) . True azeotropic blends like CFC-502 and CFC-500, also boil at a constant temperature for each given pressure.

A look at a near-azeotropic blend's pressure/enthalpy diagram also shows temperature glide wi t h sloped is0 t herms under the saturation dome (Figure 1). Pure azeotropic blends, like CFC-502, will exhibit level isotherms under their saturation domes (Figure 2).

Temperature glide is design depen- dent and may not seriously affect system performance. Often the designer will have to take a conservative approach and make sure condenser exit tempera- tures are a bit lower (more subcooling), and evaporator exit temperatures are slightly higher (more superheat) tocom- pensate for temperature glide. Most of the blendsexhibit low temperatureglides and system performance will not be effected. By all means, evaluate system design conditions when retrofitting with a near-azeotropic blend.

A somewhat similar, but not exact conditionexists whena purecompound

or true azeotropic blend experiences pressure drop through the length of a heat exchanger.

BLEND FRACTIONATION Another phenomenon of near-

azeotropic refrigerant blends is fraction- ation. Fractionation is a change in com- position of a blend by preferential evapo- ration of themorevolatile components, or condensationof theless volatilecom- ponents. Fractionation occurs when one or more refrigerants of the same blend may leak at a fasterrate than otherrefrig- erants in the same blend. This different leakage rate is caused from the different partial pressures of each constituent in the near-azeotropic mixture.

Fractionation was initially thought of as a serviceability barrier because the original refrigerant composition of the blend's constituents may change over time from leaks and recharges. Depend- ing on the blends'make-up, fractionation may also segregate the blend to a flam- mablemixtureif one or two constituents in theblendareflammable.Whenleaked, refrigerant blend fractionation may also result in faster capacitylosses than single component pure compounds like CFC- 12,502, or HCFC-22.

However, research proved that most blends were near-azeotropic enough for fractionation to be managed (Figure5). Figure5 shows a pure compound (CFC- 12) and a near-azeotropic HCFC-based refrigerant blend consisting of HCFC-

34%). This HCFC-based blend is cur- rently on the market and is an interim replacement for CFC-12 in medium- and high-temperature applications.

The graph shows pressure changes resulting from five vapor leaks from a confined container of CFC-12 (straight line), and the refrigerant blend (jagged lines). Each leakwas a 50% weight loss followed by a recharge of the original refrigerant or blend to their initial weights.

Notice that CFC-12 did not lose any vapor pressi.ire when leaked and re- charged. This is because i t is a pure

22/HFC-152dHCFC-124 (53%/13%/

compound wit ha constant boilingpoint for any certain pressure. The blend, however, did lose some vapor pressure after each leak sequence. This is an example of blend fractionation.

However, once recharged with the original blend to its initial weight, the vapor pressure did recover somewhat, but not quite to its original vapor pres- sure. After five consecutive leaks of 50% of its weight with recharging to its initial weight, the blend lost less than 10% of its original vapor pressure. This is considered satisfactory and will only slightly impact performance.

One has to consider the severity of this leakhecharge test when trying to put a system in perspective. To keep fractionation at aminimum, a refrigera- tion system incorporating a near- azeotropic blend should be charged with liquid refrigerant whenever possible.

Leaks of thismagnitude should never occur because an environmentally con- scious service technician should leak- check hvacr systems before adding any refrigerant. It may soon be illegal to top- off leaky systems with refrigerant with- out first repairing the leak. The costs of refrigerants, servicecallbacks, and com- pany reputation will also deter techni- cians from intentionally topping-off leaky systems.

SERVICE DILEMMAS The service and retrofit markets also

face the refrigerant and oil dilemma. Retrofitting an existing CFC-l2/min- era1 oil system to a HFC-l34a/polyol ester system will require removing from 95% to 99% of the mineral oil. This could require at least three oil flushes with polyol ester oil, and is a labor intensive retrofit.

Joint efforts between compressor manufacturers and oil companies have produced written retrofit procedures.

The automotive industry has used polyalkylene glycol (PAG) as the first- generation lubricant with HFC-134a. PAGs are extremely hygroscopic and need a controlled handling environment. Itisexpected that theautomotiveindus-

try may soon move to a polyol ester lubricant for use with HFC-134a be- cause esters are less hygroscopic and will tolerate more chlorine contami- nants for aftermarket retrofits and ser- vicing. Also, polyol ester lubricantswill not form peroxides and acids from inci- dental exposures to air.

HFC-134A'S ROLE Blends are strong candidates to be

bothinterimand long-termreplacemcnts for HCFC-22, CFC-502, CFC-12, and many other CFC and HCFC refrigerants in use today. Many refrigerant blends are experiencing accelerated research or are currently on the market.

HFC-134a containsnochlorineinits molecule and thus has a zero ozone- depletion potential. Its global warming potential is 0.27.

Polarity differences between com- monly used organic mineral oils and HFC refrigerants make HFC-134a in- soluble, and thus incompatible, with these oils. HFC-134a systems must em- ploy synthetic polyol ester (POE) or polyalkylene glycol (PAG) lubricants.

As a pure compound, HFC-134a is the alternate refrigerant of choice to replace CFC- 12 in many medium- and high-temperature stationary applica- tions and in automotive air conditioning.

HFC-134a suffers capacity losses when used as a low-temperature refrig- erant. HFC-134a can also replace CFC- 12 and CFCdOO in centrifugal chillers. In many chiller applications, efficien- ciesareimproved, but notwithout some reduction in capacity.

Perhaps more importantly, HFC- 134a is also a constituent in refrigerant blends or mixtures of refrigerants.

One promising mixture to replace HCFC-22 is a binary blend of HFC-32 and HFC-134a. Another is a mixture of HFC-152a/HFC-32. The major draw- back in these binary blends is that R-32 and R-152a are flammable. A third major candidate, a near-azeotropic blend of HFC-32/HFC- 125/HFC-l34a, will replace R-22 in air conditioning and heat pump applications.

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The other possible HCFC-22 replace- ments to be tested, with constituent percentages in parentheses, are:

Propane, R-290, a pure

Ammonia, R-7 17, nitrogen

.'R-32/125 (60/40), an azeotropic

R-32/134a (30/70) R-134a (100) R-32/125/134a (10/70/20) R-32/125/134a (30/10/60)

hydrocarbon

and hydrogen

mixture

R-32/125/134a/290 (20/55/20/5) R-32/227~ (35/65) R-32/134a (25/75).

All refrigerants and blends listed above to replace HCFC-22 are void of chlorine, thus have a zero ozonedeple- tion potential. Propane (HC-290) is not a strong stand-alone candidate because of its extreme flammability.

CFC-502 REPLACEMENTS CFC-502 has aninterim HCFC-based

temary blend replacement consisting of HCFC-22A-IFC- 125/HC-290. Propane, being a pure hydrocarbon, can assist in oil solubility and simultaneously con- tribute to the refrigeration effect. The propane is in such small proportions that the blend is not flaqmable even when fractionation occurs.

This blend will provide a 90% im- provement over CFC-502 in ozone deple- tion and global warming potentials and is presently on the market.

An HFC-based temary blend con-

sisting of R-l25/143a/134a (44/52/ 4), will be the long-term replacement for CFC-502 until a pure compound can be found. Expanded field-testingof this blend will continue through 1993.

Thus, the transition away from CFC- 502 may consist of a two-step proce- dure: the first step is use of the HCFC- based blend and the second use of the permanent HFC-based blend.

Other chlorine-free CFC-502 long- term replacement candidates are; R- 125/143a (45/55), an azeotrope; R- 125/143a/134a (44/52/4); R-32/125/ 143a (10/45/45);andHFC-125 (100).

Because HFC-125 has such a low critical temperature, questions arise about it being a strong stand-alone re- placement candidate for CFC-502. Ap- propriate high-side system coolingmus t be incorporated if using HFC- 125 as a stand-alone replacement to stay below the refrigerant's critical temperature.

CFC-12 REPLACEMENTS As noted above, CFC-12 has a long-

term high- and medium-temperature replacement, HFC-134a. Near-azeo- tropic interim replacement refrigerant blends consisting of HCFC-ZZ/HFC- 152a/HCFC-l24withvaryingpercent- ages for different temperature applica- tions are also replacing CFC-12. These consist of constituent percentages of (53/ 13/34) for commercial refrigera- tion, (33/15/52) for automotive air conditioning,and (614 1/28) fortrans- portation refrigeration applications.

CONCLUSIONS Widespread research on refrigerant

blends and pure compounds continues as you read this article. Most emphasis is placed on fmdmg long-term HFC- based replacement blends andpurecom- pounds for CFG502, CFC-12, and

HFC-134awillbethelongtermCFC- 12 replacement in most high- and me- dium-temperature refrigeration and air conditioningapplications. However, be- cause of polarity differences between some HFC-based refrigerants and com- monly used organic mineral oils, newer synthetic poly01 ester and polyalkylene glycol lubricants have to be incorpo- rated with most HFC-based refrigerants to meet solubility requirements.

Refrigerant manufacturers may vary some or all constituents in the blend to diversify their temperature applications or lower compression ratios and dis- charge temperatures for system effi- ciencies. Other properties can also be altered by changing the composition percentages. Blend temperature glide and fractionation are two phenomena which occur and have to be managed when incorporating near-azeotropic re- frigerant blends inrefrigerationsystems.

Chemical companies, government agencies, engineers, designers, contrac- tors, educators, and serviceman are but a few who are facing the refrigerant and oil dilemma that is impacting the hvacr

HCFC-22.

and other industries today. IB

John A. Tomnyk is an associate profes- sor in the Energy Systems Technology Program, Femes State University, Big Rapids, MI. Research on alternative re- ftigerants and oils reported in this article is the result of a cooperative effort be- tween Ferris State University, Du Pont Chemical's Fluorochemicals division, and Castrol Oil Co. A faculty research grant from Ferris State University to the author of this article made most of the research possible. Special thanks to Mr. C. Curtis Lawson of Du Pont Chemicals Fluorochemicals Laboratory.