We"ve every heard about glide, but what is it really, and also how does it influence our system?
Glide, or temperature glide, is the difference between the bubble point and the dew point of the zeotropic refrigerant mixture.
You are watching: Define temperature glide as it pertains to a refrigerant blend
Well, that wasn"t really helpful, to be it? All we did to be introduce brand-new terms without defining them, i m sorry further puzzled the issue.
So, let"s begin with “zeotrope” or “zeotropic mixture.” A zeotropic mixture is a chemistry mixture that never has actually the same vapor phase and liquid phase composition at the vapor-liquid equilibrium state. Quiet unhelpful? I thought so, too, for this reason let"s look in ~ what it method to us quite than what the publications say.
A zeotrope is a refrigerant mixture or blend the boils across a selection of temperatures at any type of given pressure. So, uneven water that boils at a consistent temperature of 212°F at atmospheric pressure, a zeotropic mixture will boil across a selection of temperatures at the same single pressure. Making use of R407a together an example, in ~ atmospheric pressure, the fluid would begin to cook at -49°F and will proceed to boil till the critical droplet boils far at -37.5°F. I recognize that it’s type of weird come think that the procedure of boiling choose that, yet that"s what is happening with a zeotrope. Boiling takes location over a selection of temperatures.
That temperature selection is referred to as the glide.
Now the we"ve acquired a basic concept that we deserve to work from, we deserve to start to understand glide and ultimately obtain to how it affect a refrigeration system. Let"s start with balloon point. Because we should have actually a solid expertise of claims of matter and also the change between liquid and also vapor, let"s i think we have R407a refrigerant in a 100% liquid state in ~ 140 PSIG. If we start at 66°F, we"ll be simply slightly subcooled, which is a perfect starting point because that this example. If we start to add heat and also raise the temperature of our refrigerant when holding our pressure constant, a single bubble will show up in the refrigerant as it starts to boil. That allude is called the balloon point. For our purposes, we can specify the bubble allude of a zeotropic refrigerant blend as the suggest at i m sorry the very first bubble appears.
Am ns still making sense? ns hope so.
Continuing through our R407a in ~ 140 PSIG example, we"re walk to continue to add heat to the refrigerant v the same constant pressure. The refrigerant continues to boil, but as the mixture the refrigerant changes, the boiling suggest changes, progressively rising as the fluid boils away. Eventually, we will have included enough heat to with a point where one critical droplet exists; that suggest is dubbed the dew point. As we did with bubble point, let"s state one operating meaning for balloon point. The dew allude is the point at which the last droplet that the liquid evaporates. Because that our example, the temperature is 75.5°F or really near that. Due to the fact that it"s boiling end a selection of temperatures, the is additionally true that the refrigerant condenses end the same selection of temperatures together we remove warmth from it. That will take place in turning back of the procedure I just described.
What go this typical for the company guy?
Obviously, these various values impact our superheat and subcooling readings. Because the dew suggest is the suggest where the critical droplet that the fluid boils off, we require to understand that value to measure and also calculate superheat. Similarly, through the balloon point, we require that to calculate subcooling. These are the values discovered on the PT charts and also programmed right into your digital manifold gauges.
In refrigeration work, evaporator coil temperature can be supplied for several things. Most commonly, us will use it to manage fixture temperature and to terminate defrost. It supplied to be simple to know what our evaporator temperature is. We looked at the gauge and transferred that number come a PT chart. We deserve to no much longer look in ~ our evaporator pressure and also know what our corresponding evaporator temperature is in rather the same way.
Let"s look at numbers. Suppose the manufacturer says that you require an 18°F coil temperature. V R22, you merely look at her trusty PT chart, discover 40.9#, and work native there. Simple enough, right?
Now, let"s look in ~ the very same coil v R407a. We have two clues that are 18°F. The dew point (40#) and also the bubble allude (52.5#), therefore which one do we choose?
The correct answer winds up being neither one. In between manufacturer"s recommendations and also field experience, I"ve uncovered it best to usage something closer to the average of dew and also bubble allude to discover the actual, practical temperature of the evaporator.
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52.5+40 = 92.5. 92.5/2=46.25
Looking in ~ a PT chart, this shows us 13°bubble allude and simply over a 23° dew point. If you look, 18° will land right about in the middle. This isn"t constantly a perfect setting, yet it"s as an excellent a place to start as you deserve to find. Collection the manage valve there and fine-tune that as needed to get the performance the you need. If we should use a push reading to terminate defrost, us will should reference bubble allude because it is the cooler of the two temperatures and will for sure a complete defrost. If we offered dew point, the inlet that the evaporator would be several degrees colder than the outlet, and also frost may still be very present.