Tag: blend

I am consistently surprised by how much false information still circulates out in the field and one of the ones I hear often is the idea that you cannot or should not “top-off” or recharge R410a systems on top of an existing charge of R410a when the system is low.

So to be clear before we move on, it is 100% OK to add to an R410a charge without fear of any significant fractionation. If you doubt me, you can read THIS from Dupont/Chemours.

R410a is a near-azeotropic blend of 50% R32 and 50% R125. This means that while it has a tiny amount of temperature glide you can still work with it like a zero glide (azeotropic) refrigerant for all practical purposes.

The fear that some have is that if the refrigerant leaks out in vapor phase, one refrigerant will leak at a higher rate than another which could change the blend as it leaks.

While this can (and does) occur with high glide refrigerants, it has been proven that this is most likely to occur in very slow leaks during long periods of storage when the refrigerant is not moving. An example would be a high glide blend in a tank with a slow leak at the valve on top. This is the worst case scenario and an example of where fractionation can be a real issue.

In a running system or a system that runs most of the time, it is unlikely that fractionation would pose an issue because the movement of the refrigerant in the circuit mixes the refrigerant and prevents one part from leaking significantly faster than another. This study by Purdue covers this as it relates to flammability risks.

The practice of charging blends in liquid phase still makes good sense because fractionation, to the extent it occurs is still most likely to pose an issue in a static vessel like a tank and charging in the liquid state is just cheap insurance against fractionation.

But once again… It does no harm to top off an R410a system with R410a. This is NOT to say I’m advocating recharging systems without finding and repairing leaks where possible, just that fractionation isn’t a reason not to do so.

— Bryan

We’ve been pretty spoiled in residential and light commercial in the USA because we haven’t needed to deal with glide much. R22 has no glide and R410a is a near-azeotropic blend which means it has almost no glide.

The days of being able to ignore glide are coming to an end.

Carrier has announced their replacement for R-410a will be R-454b which they will call “Puron Advanced” which still has very little glide (only 0.2°F), but many of the other options (like R-407c shown above) have a rather severe glide.

Glide comes down to the fact that some blended refrigerants boil and condense over a range of temperatures rather than at a single pressure/temperature point.

The point at which it is fully liquid before subcooling (or the point of the very first bubble in the liquid) we call bubble point and we use the bubble point to calculate subcooling.

The point when the mixture becomes fully vapor before superheating (or the first drop of liquid dew in a vapor) we call the dew point and we use it for calculating superheat.

Zeotropic blends (blends with glide) have several impacts on the system, but the one we notice most is in the evaporator. When blend with glide enters the evaporator coil, it will start by boiling at a lower temperature, and as it moves through the coil, the refrigerant temperature will increase until it hits the dew point before it starts to superheat. This means that neither the dew or the bubble temperature is REALLY the evaporator temperature, the true effective evaporator temperature is somewhere in the middle, we call this the mid-point.

Because some of the refrigerant flashes off right at the start of the evaporator the effective midpoint isn’t really the middle between the dew and bubble, it tilts more towards the dew and Emerson recommends a more accurate estimate would account for that “inlet quality.” So merely multiply bubble by 0.40, dew by
0.60 and add the two together to get a more accurate evaporator midpoint.

But let’s say you connect to a system that is off or connect gauges to a tank and want to know for sure that that refrigerant you think is in the tank or system is what you think it is?

Do you use bubble, dew or mid-point for static pressure?

The answer is you use bubble. Now I’ve not had anyone fully explain why to me but it stands to reason in my head that in the static state the majority of the refrigerant mass in the system (or tank) is in the liquid state and since it is neither in the process of boiling or condensing then it would be at the bubble point. That’s probably a very unscientific way of thinking about it, but it’s what I’ve got for now.

— Bryan

P.S. – Totally unrelated but my friend Andy Holt is putting on a Soft Skills training “camp out” seminar in Orlando starting on 4/1/19, and I will be stopping by to do some technical training as well. Follow THIS LINK to learn more.

 

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There are all sorts of complicated refrigerant acronyms.. HFC, HCFC, CFC and well as the mythical Zeotropic, Azeotropic and near Azeotropic… Let’s simplify

CFC = Refrigerant that is really bad for the Ozone. They are almost all gone. R-12 and R-11 are examples

HCFC – Refrigerants that are bad for the Ozone but not as bad as CFCs. Most common is R22.

HFC – Refrigerants that aren’t bad for the Ozone but they do add to global warming through the greenhouse effect. Most common is R410a.

When it comes to the whole zeotropic, azeotropic thing the main thing you need to know is that older refrigerants were often just one type of molecule. That meant that they condensed and evaporated consistently and it didn’t matter if you added them to the system via vapor or liquid. These simple refrigerants were known as PURE refrigerants.

Today we mostly work with HFC and HCFC blends. These blends can be Azeotropic, which means they blend together and act as one refrigerant or Zeotropic which means they have “glide” resulting in different boiling and condensing temperatures of the refrigerants mixed in. Rubber meets the road in a refrigerant with high glide when you need a separate “condensing” and “boiling” termperatures on the PT chart. R407C is an example of a high glide zeotropic refrigerant where R410a has nearly 0 glide. While R410a is TECHNICALLY zeotropic is is so close to being Azeotropic that the industry coined the phrase near-azeotropic.

In all blends, you must charge the refrigerant as a liquid to prevent the refrigerants from separating in the vapor state. As always, when charging liquid in the suction line, add it slowly and carefully, allowing all the liquid to boil off before entering the compressor to prevent flooding/slugging.

— Bryan

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