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Blends: Bof fractionation inside ful to review a few …

14 RSES Journal~ February 2006 Because temperature glideresults from the effectsof fractionation insidethe evaporator, it is use-ful to review a few keypoints from last month sarticle on fractionation . When two (or more) refriger-ants are mixed and they don t forman azeotrope, the vapor and liquidcompositions tend to differ. Thisoccurs because the higher-pressurerefrigerant tends to jump into thevapor faster than the other(s) andtake up more space. When individual componentshave extremely different pressures,the vapor composition will be muchdifferent than the liquid composi-tion. When the pressures are similarthe vapor composition will be closeto that of the liquid. When vapor is taken awayfrom the liquid, more liquid will boilto replace the vapor and more of thehigh-pressure component boils outof the liquid. This causes the liquidcomposition to change, becomingmore concentrated in the lower boil-ing component(s).

February 2006 ~ RSES Journal 17 forming a few drops and later becom-ing all liquid, the condensing temper-ature will drop by an amount similar to the glide seen in …

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Transcription of Blends: Bof fractionation inside ful to review a few …

1 14 RSES Journal~ February 2006 Because temperature glideresults from the effectsof fractionation insidethe evaporator, it is use-ful to review a few keypoints from last month sarticle on fractionation . When two (or more) refriger-ants are mixed and they don t forman azeotrope, the vapor and liquidcompositions tend to differ. Thisoccurs because the higher-pressurerefrigerant tends to jump into thevapor faster than the other(s) andtake up more space. When individual componentshave extremely different pressures,the vapor composition will be muchdifferent than the liquid composi-tion. When the pressures are similarthe vapor composition will be closeto that of the liquid. When vapor is taken awayfrom the liquid, more liquid will boilto replace the vapor and more of thehigh-pressure component boils outof the liquid. This causes the liquidcomposition to change, becomingmore concentrated in the lower boil-ing component(s).

2 Changing the liquid composi-tion causes the boiling point temper -ature to andtemperature glideThe key point with fractionation isthat the vapor above a large pool ofliquid turns into the wrong composi-tion. When considering temperatureglide in the evaporator, however, thereis no longer a large pool of , there will be a smallamount of liquid that we will follow asit travels down the length of the evap-orator coil and eventually becomesvapor. Figure 1 shows a blend of 50percent refrigerant A and 50 percentrefrigerant B, with A being the higher-pressure refrigerant. The refrigerant flows into theevaporator as liquid then it leaves asvapor. It is the local fractionationeffect on each portion of the refriger-ant that causes the shift in liquidcomposition along the way, whichthen causes a rise in the boiling tem-perature. In the example given inGlideRefrigerantBlends: TemperatureThe Impact ofThe second report in a three-part series onrefrigerant blends addresses how temperatureglide occurs in the evaporator and its effecton system operation and controlsBY JIM LAVELLEE qual vapor and liquidLiquid = 37/63*Vapor = 65/35*Temperature = 5 F Vapor and lastfew drops of liquidLiquid = 25/75*Vapor = 50/50*Temperature = 10 F Figure 1 Part 2 Liquid with afew bubblesLiquid = 50/50*Vapor = 80/20*Temperature = 0 FThe diagram shows the relationship between blend fractionation andtemperature glide through an evaporator coil.

3 *The number left ofthe slash represents the percentage of refrigerant A and the numberright of the slash represents the percentage of refrigerant ~ RSES Journal 15 Figure 1 the blend begins boiling at0 F and ends boiling at 10 F. Thatmeans there is a 10 F temperatureglide across the the beginning of the coil, theblend is mostly liquid with a fewbubbles in it. The liquid is composedof half refrigerant A and half refrig-erant B (the correct composition)and the vapor in each bubble con-sists of 80 percent refrigerant A and20 percent refrigerant this example, the 50/50 liquidis boiling at 0 F. As the blend movesdown the coil, more of the refriger-ant A molecules will shift to thevapor and the liquid compositionwill begin to change (see sidebarbelow for an analogy). Somewhere in the middle of thecoil, the blend will become part liq-uid and part vapor. Both of thesephases are at the wrong compositionbecause of the uneven shift of refrig-erant A molecules to the vapor.

4 Theliquid composition has changed to37 percent refrigerant A and 63 per-cent refrigerant B. The correspon-ding boiling point has gone up toabout 5 F. At the end of the coil, most of therefrigerant is now vapor at the cor-rect composition of half of each ofthe two refrigerants. The last fewIf traffic is stopped, then allowed tomove suddenly, the cars will takeoff quickly and the trucks will takesome time to rumble up to acts quite similarly. This is like the molecules in refrig-erant A jumping quickly into the bub-bles, while the molecules in refriger-Effects of temperature glide Thermostat placement in air stream. Ice machine: ice formation and harvest control setting. Figure 2 Frost formationColder regionAverage temperatureWarmer regionSuperheat settingdrops of liquid are now at 25 percentrefrigerant A and 75 percent refriger-ant B, and boiling at 10 on system operationFigure 2 shows the effects of temper -ature glide on system the previous example, theaverage evaporator temperature willbe 5 F.

5 The first part of the coil iscolder and the last part of the coil iswarmer. But if you blow a fan over theentire coil, the air coming off theother side will look like it went overant B stay in the liquid. The averagespeed is low, just as the boiling pointis lower at this you stand about one mile downthe road, you will see a bunch of carsgo by with a truck or two, then youwill see a bunch of trucks with a caror two. This is like the liquid andvapor being at different composi-tions. The average speed is higher, asthe boiling point is getting higher. Go 10 miles down the road and thetrucks have caught up to the , the refrigerant A and B mole-cules come together as vapor. The speedis higher still, like the boiling can behave like stopped traffic a 5 F coil. There are several systemoperation issues that will be differentwith blends than they would be withsingle refrigerants: Frost operat-ing below 32 F probably will showfrost formation, and the colder a sys-tem runs the more likely there willbe a defrost timer and some sort ofdefrost mechanism (electric coil,hot-gas bypass).

6 Systems with a sin-gle refrigerant, like R-12 or R-22, willshow an even frost pattern over theentire surface of the coil. High-glide blends, however, willof those formation and hot or coldspots will need to be addressed out-side of the refrigeration loop byadjusting the defrost timer and prod-uct placement, for example. Changingthe thermostat setting to avoid frostor a cold spot would make the boxrun too machine retrofit study A retrofit study was performed on aManitowoc 200 ice machine usingthe popular R-12 retrofit blends R-401A, R-409A and R-414B. Original baseline R-12 operation: Suction pressure:dropped tojust below 15 psig before harvest. Suction temperature: around10 F across entire coil before harvest. Cycle time: about 20 minutesper block. Ice weight:entire block weighedabout 30 ounces after retrofit, each blend performedsimilarly: Suction pressure:dropped tojust below 15 psig before harvest. Suction temperature:R-401 Aaveraged 9 F.

7 R-409A and R-414 Baveraged F. Cycle time:about 20 minutes(after adjustment of thickness sensor). Ice weight:about 30 ouncesafter key difference from R-12 tothe blends was in the profile of icethickness from the top of the blockto the bottom. Figure 3 shows a sideview of the ice plate showing howthe thickness of the ice grew awayfrom the coil. For R-12, with a constant temper -ature profile, the ice sheet grewsteadily and uniformly until third row from the top showedthe most weight of ice compared tothe other rows. This also happenedto be where the thickness sensor waslocated. With each of the high-glide blendsthe general result was the same:there were dimples in the first fewrows of ice, and the majority of theweight of ice was in the lower fiverows. The first cycle after retrofittingtook over 30 minutes to harvestbecause the thickness sensor was notshow thicker frost formation towardthe valve, where the actual boilingtemperatures are lower than at theend of the coil.

8 After a retrofit to ahigh-glide blend , it is possible thatthe first part of the coil could beblocked by frost before the originaltimer setting calls for defrost. Youmay need to adjust the defrost timerto avoid this condition. Temperature controls or is possible that thermostatbulbs or case temperature indicatorscould be placed close enough to thecoil to be affected by temperatureglide. If the bulb is located nearer thecolder part of the coil, then the sys-tem may shut off early (or show acolder temperature). If the bulb iscloser to a warmer part of the coil,then the system may run longer (orshow a warmer temperature). In general, temperature sensorsshould be located far enough fromthe coil that they will read the bulkair temperature in the case or box,not the air temperature comingdirectly off a part of the coil. Thermostatic expansion valve(TXV) sensor bulbs. These are locat-ed on the suction tubing after theoutlet of the evaporator.

9 The springFigure 3 Side view of an ice machine grid/ice profile near harvest R-12 andhigh-glide on a TXV is adjusted to makesure that superheated vapor is mov-ing up the suction line, not liquid. Because the boiling temperatureof the refrigerant gets warmertoward the end of the evaporator, andthere is still liquid present, you needto check the superheat setting of theTXV and possibly adjust it higherafter a retrofit to a high-glide blend . Iwill address superheat adjustment inpart three of this series, in the Marchissue. Ice icemachines have an evaporator coilrunning vertically behind a cube-making plate. A single refrigerantwill produce a constant temperatureacross the entire face of the plate,whereas a high-glide blend will becolder at the bottom than at the 3 summarizes a retrofit studyfor high-glide blends used in an R-12ice machine. Generally speaking, a high-tem-perature glide does not necessarilyaffect a system s ability to removeheat from the air or from a likely, the glide will affect theresponse of various controls on thesystem and retrofitting with high-glide blends will require adjustment16 RSES Journal~ February 2006R-12 High-glide blendsThicknesssensorThicknesssensor noadjustmentNeeds to bemoved inR-401A: 13 FR-409A: 15 FR-414B: 15 FR-401A: 9 FR-409A: FR-414B: FR-401A: 5 FR-409A: 2 FR-414B: 2 FSuction pressure = 15 psigIce weight = 30ouncesCycle time = 20 minutes10 F10 FRefrigerant flowWater flow10 FFebruary2006 ~ RSES Journal 17forming a few drops and later becom-ing all liquid, the condensing temper -ature will drop by an amount similarto the glide seen in the evaporator.

10 Air-cooled condensers will operatesimilarly to how they did with a singlerefrigerant. Water-cooled condensersmay gain or lose efficiency dependingon which way the water and refriger-ant flow relative to each other, butgenerally they will behave as they didwith the original fractionation as refriger-ant moves through the heat exchang-er will create temperature glide. Theglide may cause the system to behavedifferently than it did with a changes, however, usually willinvolve adjustment of a control. Affect-ed controls typically include the defrosttimer, thermostat bulbs, ice machinethickness sensors, and superheat andpressure control settings, which I willaddress in part three in next month sissue of RSES Journal. Part three also will cover howtemperature glide affects the infor-mation given on a pressure-tempera-ture (PT) chart. Pressure gauges andPT charts are used to check for cor-rect or abnormal system operation,set superheat and subcool tempera-tures and set pressure blends require specialattention for these Lavelle is technical sales managerat National Refrigerants the ice after 20 minutes.


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