Refrigerant quantity vs efficiency etc?

ChrisJ

I have a few questions that I can't quite wrap my head around.

There's been a constant trend to try and minimize the amount of refrigerant in systems.

My understanding is there's several reasons for it, but the main one is apparently for efficiency.

How and why does a large refrigerant charge effect system efficiency? I'm not asking how an undercharged or overcharged system behaves, but why a system designed to use a smaller charge operates more efficiently than one designed around a larger charge? Assuming you've got minimal SH and reasonable SC why does the quantity matter in regards to efficiency?

Also,

Why do some refrigerants require less of a charge I.E. R134A vs R12 in a retrofitted system, or R152A in a R134A system. Why does the system suddenly need less liquid in areas? Does the liquid weigh less, so it's technically the same volume but less weight? Or is there something else going on?

The next question is probably going to be above my head but I'm going to try…..

Why does it seem like some refrigerants, like 410A that run at a much higher pressure actually work more efficiently than others like R22? Or, is that simply because higher pressure means more compressed and the actual pressure difference between high and low is similar? I've never actually checked that……

mattmia2

is the 410a vs 22 efficiency the refrigerant or the system design, was higher efficiency required around the time r22 was phased out? in 2001 when i bought my split system if i recall there was not much in the way of efficiency requirements, you could buy systems with different efficiencies but i don't think it was mandated yet.

you can find the densities of different refrigerants on wikipedia.

the evaporator has to remain full of liquid but the amount of liquid and gas in the condenser could vary with different refrigerants.

if there is less refrigerant you have to pump less stuff but i don't know if that is the efficiency reason. maybe a higher ratio of tube surface to tube volume means the compressor has to move less stuff but that is pure speculation.

EdTheHeaterMan

A smaller refrigerant charge by itself does not magically improve efficiency. What improves efficiency is the system architecture that allows the machine to operate well with a smaller charge: smaller internal volumes, shorter lines, better coil circuitry, microchannel or optimized tube coils, lower hold-up volume, tighter control, and less “extra” refrigerant sitting in places where it is not doing useful heat transfer. A better question might be: Why can a system designed around a smaller charge often be more efficient than one designed around a larger charge?

EdTheHeaterMan

@ChrisJ asked "Why do some refrigerants require less of a charge I.E. R134A vs R12 in a retrofitted system, or R152A in a R134A system. Why does the system suddenly need less liquid in areas? Does the liquid weigh less, so it's technically the same volume but less weight? Or is there something else going on?"

Yes the refrigerant is less dense, for example: In many R-12 automotive retrofits, the recommended R-134a charge was often roughly 80–90% of the original R-12 charge by weight.

A major reason is that R-134a liquid is less dense than R-12 liquid at typical A/C conditions. So if the condenser, liquid line, and evaporator contain roughly the same liquid volume, the R-134a charge will weigh less.

R-152a is even more dramatic. It has a much lower molecular weight and lower liquid density than R-134a. R-134a has a molar mass of about 102 g/mol, while R-152a is about 66 g/mol. CoolProp’s R-134a data page lists R-134a’s molar mass as 0.102032 kg/mol.

That does not mean you use exactly 66/102 of the charge, because charge is not based only on molecular weight. But it helps explain why the required weight can drop a lot.

With R-152a, you may need enough refrigerant volume to wet the evaporator, maintain a liquid seal to the metering device, and establish proper subcooling, but that same volume weighs much less.

But that is not the only factor…. there is much more going on, however the difference in density is a big reason.

ChrisJ

https://forum.heatinghelp.com/discussion/comment/1899499#Comment_1899499

Actually,

I guess I'm wrong because the biggest way you improve efficiency appears to be larger condensers and evaporators. A larger condenser probably doesn't hold much more, but the evaporator must.

mattmia2

The condenser i have is the most efficient system Lennox had in 2001 without going to 2 stage and it is the size of a small porch. All the parts are scaled up so it doesn't look big but it is huge for a 2.5 ton condenser.

ChrisJ

https://forum.heatinghelp.com/discussion/comment/1899509#Comment_1899509

What's its EER / SEER?

EBEBRATT-Ed

I don't claim to be smart enough to comment but I will anyhow.

One reason for limiting the amount of charge in a system (in the old days at least) was to minimize the amount of liquid refrigerant in the system. They talked about this in many of the older books including Copelands

By limiting the charge it reduced or prevented liquid getting back to the compressor, slugging and liquid migration are reduced preventing compressor damage.

I too wonder about the pressures required with different refrigerants.

Some like R-12 run at lower pressures than like 410A whose suction pressure (for AC systems) is probably higher than R-12s discharge pressures.

Common sense you would think R-12 with lower pressures would require lower HP compressors than a 410A unit but

That just shows I don't know what I am talking about.

Line set sizes between refrigerants doesn't seem to change all that much in spite of the higher pressures.

I always refer to my old Copeland books for stuff like this. The density of the refrigerant as @EdTheHeaterMan mention is part of the discussion and section 1 & 2 in this book go into detail

AE101_R2.pdf

GGross

I think the larger units we see are mostly due to less than perfect engineering. we had older 18 SEER units that felt like they were twice the size of the current 20 SEER2 units. (not really but they were bigger overall) at our most recent roundtable with a manufacturer they even discussed this directly, that manufacturing the newer A2L units had been progressing and we would see smaller units shipping out at some point. You also have to remember that if you are comparing ratings from today with older units the numbers are not exact, if you see a 13 on todays equipment that means SEER2 which is different from SEER..

back to the original point, somewhat offtopic but related… Personally I think the most important aspect of lower refrigerant charges in the current age has to do with the rating of the refrigerant. You can build an R290 mono bloc A2WHP with such a tiny amount in it that there is virtually no risk of fire if it leaks. IMO systems like this where you have self contained miniscule refrigerant charges located outside and only hot/chilled water flowing through the house will gain traction over time, or maybe just wishful thinking…

Here in the US using an A2L if you can keep total system charge below i think 4lbs the RDS that everyone is freaking out about is no longer required.