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Sealed System Basics: Saturation, Subcooling, and Superheat


Son of Samurai

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Sealed system thermodynamics is a deep well, but fortunately, you only need to skim the surface to gain a functional understanding. And as techs, it's this functional understanding that we need to acquire in order to effectively troubleshoot.

To do that, we're going to cover the three central concepts to understanding a sealed system: saturation, subcooling, and superheat. Let's start with the one upon which the other two hinge: saturation.

In order to talk about what saturation is, we first need to have a clear image of a sealed system. Here's a very simple diagram that shows the refrigerant moving through the system and changing state.

Screen Shot 2022-11-30 at 8.44.20 PM.png

Basic things to note here: the compressor and the expansion valve/capillary tube (both serve the same function) divide the sealed system into two different pressure zones: low and high.

Each of those pressure zones has a coil of tubing in it -- the condenser in the high side, and the evaporator in the low side. It's within either of these coils that refrigerant changes state. In the evaporator, the refrigerant changes state from a liquid to a gas (boils), absorbing heat from its surroundings. In the condenser, the refrigerant changes state from a gas to a liquid (condenses), releasing heat into its surroundings.

State changes happen at saturation temperature. In casual language, we would say that saturation is the boiling point. See, saturation is a concept that applies to any kind of matter that exists in liquid and gas forms. For example, the saturation temperature of water at sea level is 212 F. That's when it boils.

Sounds simple enough. But we don't use water in our sealed systems, and not everything boils at 212 F. The saturation temperature depends on both the substance in question and its current pressure.

That bit about pressure is key for refrigeration. The saturation temperature of a substance increases as its pressure increases, and decreases as its pressure decreases. To use water as an example again: if you went to boil water on the top of Mt. Everest, where the air is much thinner and therefore the atmospheric pressure much lower, you would find it boiling at around 154 F. Lower pressure = lower saturation temperature.

To return to refrigerants, here's a chart that shows how the saturation temperature for the refrigerant R134a changes with pressure:

Screen Shot 2022-11-30 at 8.49.51 PM.png

One last important thing about saturation that brings it all together: a substance absorbs heat energy in order to change state, but this does not increase its temperature. This is more intuitive than it sounds if you again think of boiling water. The burner is still on underneath the pot, but the water doesn't get any hotter than 212 F. Instead, all the heat from that burner that enters the water is fueling the phase change from liquid to gas. In other words, it takes energy to fuel the phase change, but that heat isn't felt in temperature. We call this latent heat.

This is the big trick of refrigeration: by lowering a substance's pressure, you can make it boil to absorb heat from one area without getting hotter itself, then move it to another area under higher pressure and let it release that heat, changing state back to a liquid. You can see on the above chart why R134a is a good refrigerant: at low pressures, it will boil at very cold temperatures.

Got all that? Good. Now we can look at a slightly less simplistic diagram of a sealed system that will also introduce us to our next two terms: superheat and subcooling.

Screen Shot 2022-11-30 at 8.47.39 PM.png

Now we get to see not only the state of the refrigerant (liquid or vapor), but we can see where in the system is at saturation vs. where it's superheated and subcooled.

Now that you understand what saturation means, it's relatively easy to explain superheat and subcooling. They just refer to a substance being hotter or colder than the saturation temperature.

Importantly, this does not mean that a superheated substance is necessarily "super hot", or a subcooled one "super cold". For example, if we look at that R134a chart, we see that it boils at around -15 F at 0 psig (atmospheric pressure). This means that R134a at atmospheric pressure that was -5 F would be considered superheated -- not "super hot" at all! Similarly, at that same pressure, R134a at -20 F would be considered subcooled.

As long as you know the pressure of a refrigerant, you know what its saturation temperature is. You can then use that, a temperature measurement, and some basic arithmetic to determine how many degrees of superheat/subcooling the refrigerant has at that point in the system. To use our two examples from the previous paragraph, R134a at 0 psig and -5 F has 10 degrees of superheat, and the same refrigerant at the same pressure but at -20 F has 5 degrees of subcooling.

These concepts of saturation, superheat, and subcooling are the foundation of all sealed system troubleshooting. By applying this knowledge along with clever, non-invasive temperature and amp measurements, you can troubleshoot almost any sealed system problem.

Want to learn how to apply your newfound sealed system knowledge to real-world troubleshooting? Click here to check out the Advanced Refrigerator Repair course over at the Master Samurai Tech Academy.

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  • Team Samurai
Samurai Appliance Repair Man

Posted

Good post, Sam! Just to highlight one important point: the thing that’s so special about saturation is it is the only condition when temperature and pressure move in lockstep with each other. Outside of saturation— superheat or subcooling— pressure and temperature go their separate ways and move independently of each other. 

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On 12/1/2022 at 3:45 PM, Samurai Appliance Repair Man said:

Good post, Sam! Just to highlight one important point: the thing that’s so special about saturation is it is the only condition when temperature and pressure move in lockstep with each other. Outside of saturation— superheat or subcooling— pressure and temperature go their separate ways and move independently of each other. 

I just skimmed this post and will go back and read in depth, but I was just thinking about how you said with refrigerators, you don’t charge based off of sub cool like some ACs, can you tell me why again? It’s been awhile. 

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  • Team Samurai
Samurai Appliance Repair Man

Posted

On 12/6/2022 at 10:16 AM, Tone Blair said:

you don’t charge based off of sub cool like some ACs, can you tell me why again? It’s been awhile. 

Couple reasons.

One is that depending on the size and configuration, subcooling in domestic refrigerator cap tube systems at the condenser will be a couple degrees or non-existent, just saturated liquid. If the system has yoder loops and other heat exchangers, you'll get more subcooling before the cap tube. Even though we don't usually pay attention to subcooling in cap tube systems, it's one of the three thermodynamic states of the refrigerant along with saturation and superheating. So it's part of that context. 

But the biggest reason is that charging cap tube systems using subcooling (apart from the issues mentioned above) is that it's just too inaccurate. Cap tube systems are critically charged to 1/4 of an ounce! So you have to charge by the label weight to get optimal performance. In Part 1 of the Sealed System Diagnostic videos, I show how variations from specified charge weight affects compressor amps and the BTUH (heat) carrying capacity of the system. Spoiler: overcharging is a bigger penalty than undercharging in cap tube systems. So if the label says 44g of R600a, you make your charge as exact to 44g as you can, no more, no less.

You just can't get this kind of charge resolution using subcooling. In AC systems using TXVs, the TXV itself can compensate for variations from rated charge weight. In other words, TXV systems are not nearly as critically charged as cap tube systems. 

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  • Team Samurai
Samurai Appliance Repair Man

Posted

A simpler way to say it: 

As techs, we live and die by specs. I have never seen a domestic refrigerator manufacturer give a spec for subcooling. (AC manufacturers do this but not home refrigerator manufacturers.) However, they do give us specs for the refrigerant charge weight. So we have to work with the specs we're given. 

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7 hours ago, Samurai Appliance Repair Man said:

A simpler way to say it: 

As techs, we live and die by specs. I have never seen a domestic refrigerator manufacturer give a spec for subcooling. (AC manufacturers do this but not home refrigerator manufacturers.) However, they do give us specs for the refrigerant charge weight. So we have to work with the specs we're given. 

It is what it is and that’s all it is. 
amen to that. 
I just felt like it was helpful to know subcooling and superheat, but you’re right, charge it according to spec and done dada. 
 

thanks again! 

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I just want to add my thanks for all the great sealed system theory on the site, and thanks @Samurai Appliance Repair Man for sharing your expertise! Understanding the underlying theory has helped me fine-tune my intuition and develop my Spidey-sense when dealing with sealed system issues.

One example: I was working on a new R600a top-mount Whirlpool Ref. under wty with very weak cooling, Compressor running, very weak frost pattern. I ordered up the Evap. and Compressor.

After purging, blowing everything out, being confident there were no restrictions, and completing the repair, I started recharging the system, think the label charge might have been ~47g R600a. I usually disconnect my high side and tape a thermocouple to the Compressor discharge to watch over the first few minutes. I'm pretty careful about securing my hoses so they don't move and throw off the scale reading, but it's tricky with R600a (such small charge amounts), and we're using very small bottles, like auto parts store refrigerant cans, so it's hard to keep them stable on the scale (unless you buy a fancy scale that has the port to accept the cans directly).

After weighing in what I was pretty confident was the label charge, Compressor discharge temp looked great, but my low side pressure was higher than I expected, something near 15-20 PSI. I checked the Danfoss Ref. Tools slider, saw that 15PSI worked out to a saturation temp of 45*F. Low side pressure might have continued dropping more as the unit pulled down, but that was higher pressure than I'd seen on similar units during pull-down. I bled off some gas until low side pressure was closer to what I've seen in the past, ~5PSI (and dropping, hopefully).

Several months on, unit is running and cooling just fine, checked in with the superintendent several days later and he said it started making ice the next day.

It only occured to me later that this might be one of the Whirlpool units with incorrect charge amount on the label, but I'm pretty sure for whatever reason it was running as an overcharge initially. I'm glad this site helped me develop my skills to be able to recognize that! 

 

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And R290 is just as critical. I just purchased a testo scale that solved my problem of setting a R290 can (like the older liquid propane torches used) that has 14 oz Refrigerant in it. The scale had an option of adding a cut off valve once the programmed weigh in amount is reached. No more getting close on charge. This is the time to be alive and working on these beasts. Now there is no guess work on charging. Plus this tool will work with all refrigerants. Thank you Sam for your help and fine article. 

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