Tag: glide

 

We’ve all heard about glide, but what is it really and how does it affect our system?

Glide, or temperature glide, is the difference between the bubble point and the dew point of the zeotropic refrigerant mixture.

Well that wasn’t very helpful, was it? All we did was introduce new terms without defining them and further confused the issue.

So, let’s start with zeotrope or zeotropic mixture. A zeotropic mixture is a chemical mixture that never has the same vapor phase and liquid phase composition at the vapor-liquid equilibrium state. Still unhelpful? I thought so, too, so let’s look at what it means to us rather than what the books say.

A zeotrope, is a refrigerant mixture or blend that boils across a range of temperatures at any given pressure. So, unlike water that boils at a constant temperature of 212°F at atmospheric pressure, a zeotropic mixture will boil between across a range of temperatures at that same single pressure. Using r407a as an example, at atmospheric pressure, the liquid would begin to boil at -49°F and will continue to boil until the last droplet boils away at -37.5°F. I know that it’s kind of weird to think of the process of boiling like that, but that’s what is happening with a zeotrope. Boiling takes place over a range of temperatures.
That temperature range is called the glide.

 

Now that we’ve got a basic concept that we can work from, we can start to understand glide and ultimately get to how it affects a refrigeration system. Let’s start with bubble point. Since we should have a solid understanding of states of matter and the transition between liquid and vapor, let’s assume we have r407a refrigerant in a 100% liquid state at 140 psig. If we start at 66°F, we’ll be just slightly subcooled which is a perfect starting point for this example.. If we start to add heat and raise the temperature of our refrigerant while holding our pressure constant, a single bubble will appear in the refrigerant as it begins to boil. That point is called the bubble point. For our purposes, we can define the bubble point of a zeotropic refrigerant blend is the point at which the first bubble appears.
Still making sense? I hope so.

 

Continuing with our r407a at 140 psig example, we’re going to continue to add heat to the refrigerant with the same constant pressure. The refrigerant continues to boil, but as the mixture of refrigerant changes, the boiling point changes, slowly rising as the liquid boils away. Eventually, we will have added enough heat to reach a point where one last droplet exists, that point is called the dew point. Like we did with bubble point, let’s state an operating definition for bubble point. The dew point is the point at which the last droplet of liquid evaporates. For our example, that temperature is 75.5°F or very near that. Since it’s boiling over a range of temperatures, it is also true that the refrigerant condenses over the same range of temperatures as we remove heat from it. That will happen in reverse of the process I just described.

 

What does this mean for the service guy?

 

Obviously, these different values affect our superheat and subcooling readings. Since the dew point is the point where the last droplet of liquid boils off, we need to know that value to measure and calculate superheat. Similarly, with the bubble point, we need that to calculate subcooling. These are the values found on the PT charts and that are programmed into your digital manifold gauges.

In refrigeration work, evaporator coil temperature can be used for a number of things. Most commonly, we will use it to control fixture temperature and to terminate defrost. It used to be simple to know what our evaporator temperature is. We looked at the gauge and transferred that number to a PT chart. We can no longer look at our evaporator pressure and know what our corresponding evaporator temperature is quite the same way.

 

Let’s look at numbers… say the manufacturer says that you need an 18°F coil temperature. With R22, you simply look at your trusty PT chart, find 40.9# and work from there. Easy enough, right?
Now, let’s look at the same coil with r407a. We have 2 points that are 18°F. The dew point (40#) and the bubble point (52.5#), so which one do we choose?
The correct answer winds up being neither one. Between manufacturer’s recommendations and field experience, I’ve found it best to use something closer to the average of dew and bubble point to find the actual, functional temperature of the evaporator.

 

52.5+40 = 92.5. 92.5/2=46.25

 

Looking at a PT chart, this shows us 13°bubble point and just over a 23° dew point. If you look, 18° will land right about in the middle. This isn’t always a perfect setting, but it’s as good a place to start as you can find. Set the control valve there and fine-tune it as needed to get the performance that you need. If we need to use a pressure reading to terminate defrost, we will need to reference bubble point because it is the colder of the two temperatures and will ensure a complete defrost. If we used dew point, the inlet of the evaporator would be several degrees colder than the outlet and frost may still very present.

–Jeremy Smith

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.

 


Over the years I have heard technicians say that refrigerant can wear out or “lose it’s blend” by sitting in a tank.

This does not happen… at least not like that

What can and does happen is called “Fractionation”. Refrigerant blends that are composed of a mix of refrigerants with different vapor and liquid PT characteristics known as Non-azeotropic, Zeotropic or in some cases near-azeotropic. All fancy words to mean that these refrigerant blends must be added or removed completely or in the liquid state to prevent more / less of one refrigerants in the mix to be added or removed than the other.  If the refrigerant is allowed to fractionate and some of it is added in the vapor only state both the refrigerant left in the tank, and the refrigerant added to the system will no longer have the designed properties of the listed refrigerant.

If one of the refrigerants in the blend leaks out faster, what you have left isn’t the same refrigerant

While all blends should all be charged in the liquid state, some refrigerants are more likely to be impacted by fractionation than others.

For example, R-410a  (50% R-32 & 50% R-125) has very little “glide” between liquid and vapor and so while it is a blend, it is less likely to fractionate severely when charged in the vapor phase (which you still shouldn’t do).

A refrigerant like R-407c ( a mixture of R32/125/134a) will fractionate much more easily resulting in far greater pressure/temperature swings and poor performance when it occurs.
Fractionation will often happen for three reasons

  1. A technician charged the system in vapor phase (tank upright) instead of in liquid phase (upside down)
  2. The tank had a small leak while stored upright
  3. The system has a significant leak.

The particular case of fractionation being caused by a system leak depends on many factors including what part of the system the leak occurs, the physical location of the leak and how much refrigerant leaked out. There was a study done at Purdue that shows that fractionation after leakage can be a factor in high glide systems like R407c.

The ramifications of this depend on the specific situation, but in some cases, the only viable option will be to completely recover and recharge with a virgin charge. This is not because refrigerant has “lost its mix” from sitting, but rather because some of the”mix” has left the tank or system at a different rate, leaving an improper mix behind.

— Bryan

 

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