About gas blend properties (how can high pressure components condense?)

[DaBit]

I have another question which is probably basic stuff for many of you: What mechanism causes the effect that high pressure components in a refrigerant blend can condense at lower pressures?

Example: take a commercial hydrocarbon blend such as CARE 50 from Calorgas. This is a blend of R290 (propane) and R170 (ethane), meant to be used as a drop-in for R22 and R407C, and featuring pressures similar to R22.

This specific refrigerant blend contains a component (ethane) that has a very high vapour pressure at common R22 condensing temperatures, and it's low critical temperature of 32C (90F) even seems to prevent it from condensing at condensing temps > 32C / 90F.

But somehow the blend is able to condense, otherwise the high side pressures would go through the roof due to ethane accumulating in the condenser.

I wonder what causes this. I cannot explain it as a chemical bond between the two components; as far as I know there exists none. I am thinking in the direction of partial pressures of the molar fractions, but it doesn't explain this either.

I also wonder why it works for some mixtures, but apparently not for others. If I mix helium and butane, I probably won't see this effect. Why?

If this is decribed in the ASHRAE Fundamentals manual, an elaborate answer is not needed since I ordered my copy a few weeks ago. Still hasn't arrived, though.

[RogGoetsch]

Deep study. Term is azeotropic behavior (R-502 is an azeotrope, the new blends are only near-azeotropic, hence the "glide".)

Azeotropic properties of substances are listed in CRC Handbook of Chemistry & Physics.

But why or how it happens?? You are the quickest study around, DaBit. When you find out how it works, please explain it to me!

Rog

[DaBit]

Originally posted by RogGoetsch
Azeotropic properties of substances are listed in CRC Handbook of Chemistry & Physics.

Well, at least THAT book on my list has arrived yet. Just got mail form Amazon that the ASHRAE manual is delayed. Again.

I will look into the CRC Handbook and see whether or not it comes up with anything useful.

But why or how it happens?? You are the quickest study around, DaBit. When you find out how it works, please explain it to me!

Oh, if I only had access to the information I needed.... It's the bad thing from not being on university anymore; no access to all those paper jewels.

C'mon Prof, where are you?

[Prof Sporlan]

LOL!!! The Prof may be adept at using the thermodynamic and thermophysical properties of refrigerants. But as they relate to zeotropic and azeotropic blends, he is no better than that early ape man trying to understand the strange black monolith before him...

[DaBit]

Originally posted by RogGoetsch
Azeotropic properties of substances are listed in CRC Handbook of Chemistry & Physics.

I checked, but I haven't found anything useful. There are indeed tables with azeotropic properties of various substances, but I haven't been able to distillate any rule from the raw data.

Originally posted by Prof Sporlan
But as they relate to zeotropic and azeotropic blends, he is no better than that early ape man trying to understand the strange black monolith before him...

LOL! One fool can ask more than the wisest can answer

But still I find it hard to believe that this is unexplored territory. Many of you have added substance X to refrigerant Y to improve a property (propane/pentane added to R503/R13/R23 to faciliate oil return being an example). If I did such a thing, I would want to know how the physical and thermodynamic properties change.

Originally posted by Marc O'Brien
Gosh, perhaps the higher pressure component gets dissolved?

I suspect this is what's happening.

Does the higher pressure component actually condense? Or does it drift around with the condensed components as a dissolved component?

If it did not condense, the enthalpy of evaporation (and thus condensation) of a mixture would be quite low compared to the separate components. Which is, as far as I know, not the case.

When you calculate compression power, superheated discharge temperatures etc. you use the mean of the molor proportion values of each.

Does other properties such as vapour pressure, heat capacity, PT relationship, etc. also follow this rule?

[saratoga]

From what i remember:

An azeotropic mixture is just something in which one fluid dissolves another. Intermolecular forces keep the two fluids from seperateing easily (there is a favorable and nontrivial enthapy of solution). Thus the PV and entrophy curves of the new mixture are not the same as either individual gas and the mixture cannot easily seperate due to the energy gained from mixing.

[Dan]

Makes me think about how we weren't supposed to be all that concerned about bubbles in the sightglass with terniary blends. Also hearkens me back to shared observations of bubbles in supposedly 10 deg subcooled azeotropes or even "pure" refrigerants. Does this even furthermore tie in with CO2 transcritical refrigeration cycles?

Mixtures are not the same as solutions. Is this similar to zeotropes and azeoptropes.. but we never paid attention to the relative changing of state? I know there are some fine thinkers pondering Dabits simple question. I am suggesting we find the early question.
Perhaps one step before favorable nontrivialities?

[Prof Sporlan]

The Prof can imagine getting a phone call from a service mechanic in the not too distant future with the following service problem...

"Hey, I've got this self contained trans-critical CO2 beer cooler that I've just installed, and it ain't keeping the beer cold. I'm not sure if the TEV is feeding quite right, and I see all these bubbles in the sight glass. I can't measure and pressures since I don't have gauges that can handle the CO2 pressures, but I charged the unit per the manufacturer's instructions. What do you think is wrong?"

With any luck, the Prof will be retired by then, and enjoying cold ones from his still operating CFC beer cooler...

[chemi-cool]

hello Prof,

I was told once that new blends like: R-134a and R-404a should be charged in the liquid phase and from that reason in automotive AC, it is not advised to add gas but to chage it.

could you or any one else explain?

chemi

[DaBit]

R134a is a pure substance, so with R134a it would not matter much if you charge either liquid or gas.

With the 4-series of refrigerants (R404a, R407C, R410A, etc), this is different. If you charge vapour, the highest pressure components boil off first, and the lowest pressure components stay behind. So the mixture you get in the refrigeration system has changed from what's in the bottle.

If you have a leak in the system, the same thing happens. The high boiling point components leak the most, leaving the low boiling point components behind. This lowers the head pressure and evaporator pressure, and it lowers the refrigerants volumetric efficiency. If you top up with fresh gas, the imbalance stays.

[chemi-cool]

thanks DaBit.

got it.

chemi

[Prof Sporlan]

Vapor charging a non-azeotropic blend will cause the refrigerant to fractionate, i.e., the more volatile components of the blend tend to boil off first, leaving the less volatile components in the refrigerant cylinder.

If you happen to be charging the system with all of the refrigerant in the cylinder, then vapor charging is ok since all of the components will make their way into the system.

BTW, R-134a (tetrafluoroethane) is a single constituent refrigerant like R-22, and there is no problems vapor charging with it. The small cap 'a' indicates it is an isomer (there is more than one way to attach four fluorine atoms on ethane). R-404A is a refrigerant blend of R-125, R-143a, and R-134a. The large cap 'A' indicates it is a refrigerant blend. Other formulations of R-125, R-143a, and R-134a recognized by ASHRAE would be given subsequent letter designations, i.e., 'B', 'C', 'D'. At this time, ASHRAE recognizes only one formulation of R-125, R-143a, and R-134a.

[chemi-cool]

thanks Prof for for a few educating moments.

everything will be taken in future charging considerations.

chemi

[frank]

I'm confused Prof - did you mean a non-zeotropic blend?

[Prof Sporlan]

Perhaps a better term for "non-azeotropic" blend is a "zeotropic" blend, which simply means the refrigerant blend isn't azeotropic, i.e., it doesn't act like a single constituent refrigerant and thus can fractionate.

[RogGoetsch]

I recommend the following experiment for those still concerned about azeotropes:

Probably the oldest commonly used azeotrope in human history is derived when distilling ethanol from a solution of ethanol and water. An azeotrope of 95% ethanol and 5% water has a lower boiling point than either, hence is the most concentrated form of ethanol attainable (without an extraction involving benzene, I think it is).

Acquire a quantity of said azeotrope. Injest small amounts, perhaps blended with any other aqueous solution preferred. Continue injesting until either: 1) a deep understanding of the problem is acquired or 2) a deep indifference to the problem is acquired.

Report on the results of your experiments so the data can be verified and independently confirmed by similarly scientifically-minded individuals.

Rog

[shogun7]

Azeotropes are refrigerant blends which evaporate and condense exactly like a pure component at some temperature and pressure. They may not behave in this fashion at all temperatures and pressures but they will be close.
Example: R-502

Zeotropes are refrigerant blends which show some amount of temperature glide when evaporating or condensing. In other words they may not always have a precise Temperature/Pressure relationship like a pure refrigerant always has. Some may act like Azeotropes and glide may not be noticeable. Zeotropes with glide greater than 3 ºF will have one end of the evaporator warmer than the other which may affect system performance. If retrofitting to a Zeotrope it may be necessary to change a pressure activated operating control to a temperature activated operating control to avoid temperature swings in the refrigerated space. Other controls may have to be adjusted as well.
Example: R-401A (MP39)

[Prof Sporlan]

Probably the oldest commonly used azeotrope in human history is derived when distilling ethanol from a solution of ethanol and water.

Commercially available brand for purposes of experimentation: Everclear