Understanding RefrigerantBlend Performance

1 UnderstandingefrigerationrSince the phaseout ofCFCs more than 10 yearsago, refrigerant blendshave become common-place in the rmarket for both retrofitand new installations. Equipment thattraditionally used R-12 or R-502 isnow running on one of approximately13 commercially available you consider the pendingphasout of R-22, another three orfour blends get thrown into the addition, contractors and ser vicetechnicians must know the pitfalls ofefrigerant blends. Fortunately, wehave lear ned much about blend per-for mance during the last 15 refrigerant blend has its ownunique properties that are somewhatdifferent from the original productthey are intended to replace.

2 Byunderstanding how blends differfrom single-component refrigerants,contractors and technicians can bet-ter identify or avoid blend -relatedproblems when installing or servic-ing and temperature glidewill affect system operation, controlsettings and service/troubleshootingpractices. Different blends will showdifferent amounts of fractionation ortemperature-glide effects, though theimpact on a system will be similar forall are blends?Blends are made up of two or moresingle-component refrigerants. EachJanuar y 2006 ~RSES Jour nal29trsingle-component refrigerant has itsown pressure-temperature relationshipand unique physical properties, such asdensity, heat of vaporization and heat-transfer coef ficient.

3 To match the prop-erties of a single refrigerant with ablend, the individual components mustbe mixed in the right are a special case inwhich the refrigerants combine in aunique way . There is some attraction/interaction between the differ ent typesof molecules, which causes uniquepr oper ties within the blend . Since azeo -opes, such as R-500, R-502, R-503,R-507 and R-508B, do not allow com -ponents to separate, they will not beKnowing how and whyblends differ from single-component refrigerantscan help you betteridentify or avoid problemswhen installing or servicingequipment. Part one of athree-part series coversfractionationBy Jim Lavelleincluded in this have a pressure-tempera -ture r elationship that is a natural com -bination of the components proper-ties.

4 There is no interaction betweenRefrigerant BlendPerformancePA R T O N EPressureFigure 1:New variable different types of molecules. Thepressure for the blend falls between thepressures of its components. But as wewill see later, the vapor compositionwill become a problem. If we mix a blend of refrigerant Aand refrigerant B, we usually talkabout the higher-pressure componentfirst, in this case A. In general, if agreater amount of A is mixed with B,then the blend will have a pressurecloser to A. If a greater amount of B isin the mix, then the blend will have apressure closer to B. If you mix equalamounts, the blend will fall in betweenthe pressures of A and blend compositions havebeen adjusted so the resulting blendproperties fall exactly where the man-ufacturers intended.

5 The problem,however, is that not all of the proper-ties can match the original refrigerantunder all conditions. Composition is a concernOnce a blend is mixed at a given com-position, the pressure-temperaturerelationships follow the same generalrules as for pure components. Forexample, the pressure goes up whenthe temperature goes three blends containing dif-ferent amounts of refrigerants A andB, the pressure curve is similarlyshaped (see Figure 1). But the result-ing pressure will be higher for theblend that contains more of the A(higher pressure) component. Refrigerant blends that are intended tomatch some other product (R-12, forexample) will rarely match the pres-sure at all points in the desired temper-ature range.

6 What is more common isthe blend will match in one region andthe pressures will differ blend with composition 1matches the pure refrigerant at coldevaporator temperatures, but thepressures run higher at condenserconditions. The blend with composi-tion 2 matches closer to room tem-perature and might show the samepressure in a cylinder being stored,for example. The operation pressuresat evaporator and condenser temper-atures, however, will be somewhatdifferent. Finally, the blend at composition3 will generate the same pressures athot condenser conditions, but theevaporator must run at lower pres-sures to get the same will see later that the choice ofwhere the blend matches the pres-sure relationship can solve (or cause)

7 Certain retrofit-related graph also illustrates that if ablend loses some of the higher-pres-sure component, the remainingblend will have to achieve a loweroperating pressure in order toachieve the same this around, a system oper-ating at the same pressure actuallywill boil refrigerant at a higher blends fractionateFigure 2 illustrates two basic behav-iors of refrigerant molecules that willhelp explain why fractionation pure refrigerant, A or B, exerts pres-sure on the cylinder (or a system)because the molecules are in motion. At higher temperatures the mole-cules move faster, which increasespressure. At lower temperaturesthere is less movement and lowerpressure.

8 Different refrigerants havedifferent energies at the same tem-perature, and, therefore, generatehigher or lower pressures at thesame of refrigerant are con-stantly moving from liquid to vaporand vapor to liquid at the surface ofthe liquid. Vapor and liquid at equi-librium transfer the same number ofmolecules back and forth. Boiling liquid transfers more fromliquid to vapor and condensing refrig-erants transfer more from vapor toliquid. Different refrigerants transferback and forth to the vapor at differ-ent rates and ultimately have differentnumbers of molecules in the vapor atthe same refrigerants A and B aremixed together and they don t form anazeotrope, the individual refrigerantmolecules behave as if the other type isnot there.

9 The refrigerant A moleculesbounce harder than the refrigerant Bmolecules, contributing more pressureto the blend . The composition can beadjusted so that the combined pressurefrom the two types of moleculesmatches the desired importantly, as the tworefrigerants move in the cylinder,the A s transfer back and forth tovapor faster than the B s. This meansthere will be a higher concentrationof A s building up in the vapor com-pared to the B s. When liquid andvapor are together at equilibrium, itis always the vapor that goes to thewrong blends fractionate Looking at the containers in Figure3, you can see that when vapor isremoved from a cylinder or systemcontaining a zeotropic blend , twothings will happen: 30 RSES Journal~ January 2006 Figure 2:Behavior of individual refrigerant moleculesIn zeotropic mixtures,the refrigerant A moleculesmove independently from therefrigerant B of refrigerant A is higher(more movement)Combined pressure: refrigerant A more activethan refrigerant BPressure of refrigerant B is lower(less movement)+1.

10 The vapor being removed is atthe wrong composition. The vaporwill have more of the higher pres-sure/higher capacity refrigerant com-ponent compared to the liquid The liquid that is left behindboils more of the higher-pressurecomponent out of the liquid toreplace the vapor. Eventually, the liq-uid composition changes becausemore of the A component leaves thecontainer compared to the bulk liq-uid is the change incomposition of a blend because one(or more) of the components is lostor removed faster than the other(s).A larger difference between the pres-sures of the starting componentswill cause a greater difference in the vapor composition compared toliquid.