R-421A Dew Point

[Rob White]

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A useful chart :-)

When I said agresive it was because I struggled to explain
What I meant. By agresive I meant large. We always teach
for the temperature difference across the evaporator to be
about 10K (when air is used as the eat exchange medium).

So when I said agresive I meant a large temperature difference
and that would then ensure the 125 is vaporised before the 134.

If there is a chemical bond would that reduce the glide?

Regards

Rob

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[Saturatedpsi]

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If there is a chemical bond would that reduce the glide?

Regards

Rob

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I don't have a clue... ..but azeotropic blends experience some sort of bonding and don't have the glide.

I think I asked the wrong question. I'm quite sure my area of confusion is in understanding just what refrigerant blend 'bubble and dew' points are, and how they are detemined.

Dew point is stated as "vapor with a dew drop" and bubble point "liquid with a bubble". The "dew drop" is a drop of what? And the bubble point "bubble" is a bubble of what? Is the answer to both questions, some mixture of both 125 and 134A?

I believe the answer to the question is "yes", but don't understand why. The explanation for zeotropes say the components act independent of each other. And if that's the case, why doesn't all the 125 vaporize before the 134A, leaving only 134A liquid?

And if the 'drop' and 'bubble' are some ratio of both refrigerants, it would seem some attractive 'force' is bonding them together since they have very different boiling points at a given pressure...

I saw some mathematics for calculating dew point (I think) which are atrocious...partial pressures, mole fractions, Raoult's Law, funky looking equations...jeeeze.

Guess I'm just trying to find someone who can put it in practical terms and mechanic's language, all the way from the bubble point state, beginning at the evaporator inlet, to dew point phase...without any mention of Raoult's Law.

Now, if you can manage that...

I appreciate your continued patience with my ignorance...

[Saturatedpsi]

Dew point is stated as "vapor with a dew drop" and bubble point "liquid with a bubble". The "dew drop" is a drop of what? And the bubble point "bubble" is a bubble of what? Is the answer to both questions, some mixture of both 125 and 134A?

Well, as the saying goes, had it been a snake, I'd have been bitten... You can't loose sight of the fundamentals.

Many of the blend discussions include this image for a contained mixture at static/equilibrium conditions. (You may have to click on it for a larger view)

RefBlend.jpg

There's always some back-and-forth phase change taking place at the surface of the liquid...molecules of both A and B components move from the liquid phase to the vapor phase and back to the liquid phase. Applying the same logic to an evaporator, regardless the change in ratio and temperature, a "drop" of liquid will contain both components. As to "why?"...just because.

The "dew drop" of 421A is some ratio of 125 and 134A...the "bubble" at bubble point is some ratio of 125 and 134A.

Agreed?