Effect of excessive Sub Cooling

Hi all,

During system integration of R290 Split AC System, I observed sub cooling above 15 Deg C
Can anyone explain what should be ideal sub cooling and what are the harms in excess sub cooling ??

From my observations a clear sight glass on a standard refrigeration system starts with subcooling at 2 or 3K.
More subcooling the better because you get a lower proportion of flash gas in the expansion valve. Well, it's something like that anyhow but it's been a while since I've looked at the books! On a pressure enthalpy it's the region on the left where you go down to evaporating pressure, you start evaporating with a higher % of liquid.
In practice if subcooling is over 8K generally that's an indication of overcharging as refrigerant is backing up in the condensor. Also lots of subcooling can (I think!) cause head pressure to drop then that can cause too low a pressure difference across the expansion valve leading to poor cooling.

Now, how that relates to a split AC unit is a bit harder to get ones head around because there are a few variables, well compressor speed is the only real difference as I'd imagine you have an inverter system. They come precharged so quite possibly you could have an overcharged system like how I described earlier.
Hope that makes sense, most of the info came from Garys TECH method so I can't take all the credit. :-)

Cheers,
Andy.

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The way I look at it is any sub-cooling is good.

If you want to evaporate at a given temp then the liquid has to be cooled to that temperature before
it can start to evaporate and remove heat from the product.

If you are evaporating at -5 DegC and your liquid is at +15 DegsC, then the first part of the evaporation
process requires the refrigerant to evaporate (flash off) just to cool itself down. Only when the liquid is at
the evaporating temperature will it start to remove heat from the product.

I try not to give specific answers to open questions because there is never one stock answer that answers all
scenarios, sort of how long is a piece of string question.

Sub-cooling is designed for specific applications and for the most part sub-cooling is free cooling so rarely will
it be lower than the ambient air temperature. The more sub-cooling and the lower it is in relation to the ambient temp generally means the system is performing at optimum efficiency.

Rob

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By the book, subcooling is ideally 2-4K, 34degrees C condensing temp and 30-32 degrees C liquid line temp in the UK design ambient of 19C. How often we see design conditions is highly speculative and high and low ambient temperatures can drastically change system performance in my experience.

Do some units have pre-expansion from condensing units?

Hi

The chances are your actually reading the flash/evaporating gas temperature (as the small pipe normally on the outdoor unit will be the outlet of the EEV) the only way you will be able to get a sub cooling reading will be at the bottom of the outdoor heat exchanger (condenser) or before it enters the EEV, not after it as that will be classed as part of the evaporator.

CB