That's right.
He measured wrong, it's negative superheat....
Either way it still would not be negative sub-cooling.
A Diamond is just a piece of coal, that made good under pressure!
That's right.
He measured wrong, it's negative superheat....
I'm still stuck on the guy saying it's normal for a compressor to pump liquid in some applications.
You'll never convince a compressor manufacturer of that. Liquid refrigerant + hot oil = cold oil. That's not a good thing. Better put an accumulator in front of the compressor if your superheat is less than 5 Deg. That's what minisplit manufacturers do.
If such a thing existed wouldn't they make instruments to read it ?
It exists, and it doesn't. It's not called "negative superheat". If you are above saturation temp, it's called superheated. Below saturation temp is subcooled. At saturation is called vapor pressure equalibrium. It's just a matter of using the correct terms.
If your condensing temp is 100 degrees and your liquid line temp is 105 degrees that is "superheat", not "negative subcooling". The condenser is not operating at correct conditions due to corroded piping, low airflow, dirt, fan turning backwards, etc. It could also be undercharged, but probably not with the readings I gave unless you're working on small refrigeration equipment.
Hot gas enters the condenser at very high temps, over 200 degrees in some cases. Of course it can make it through to the outlet and still be superheated. Just disconnect the fan for a few seconds and see what happens. Actually, no- don't. But believe me, subcooling disappears fast with no airflow.
For more context:
I'm a student in a refrigeration/ac/water technology program. Last week, we were working on a unit for a walk-in freezer (uses 402a). The pressure at the discharge line was 205 psig, which corresponds to approx 85 deg F, if I'm not mistaken. Then we took our temp at the liq line just before the metering device (txv maybe?): we read 86 deg F, or -1 deg subcooling. I noticed the condenser fan was rejecting ambient-ish temp air, which I thought was odd.
Curiously, our evaporator pressure was 16 psig (-23 deg F), which seems very low, and our suction line temp was 57 deg. So that put our s/h at 57 - (-23), or 80 deg. I just shook my head at that point...
Liquid lines running over extremely hot tar-roofs will superheat, & superheated vapor can even replace liquid going to the metering device.
There are extreme heat-source situations where liquid lines should be insulated from heat sources. - Darrell
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udarrell
Get that post count up and get a pro membership. It's free.
In the meantime, put a sightglass in the liquid line and install a few more pressure taps. It would be nice to measure the liquid pressure where you are taking the temp for best accuracy.
I have to disagree with that. If you are dealing with a properly charged system lets say its 12 degrees subcooled. When you disconnect the fan subcooling will increase very rapidly. The sudden increase in pressure would increase the condensing press/temp right away but it would take time for the liquid to catch up so to speak.
This would result in a net increase in sub-cooling.
If all that liquid flashed to gas instantly and superheated I would expect catastrophic condenser failure from the pressure rise.
Maybe I am not just thinking right so please correct me if I am wrong.
A Diamond is just a piece of coal, that made good under pressure!
Sub-cooling no such thing
Sub-cooling no such thing
Sub-cooling no such thing
With a saturation temperature of 85 and a line temperature of 86, it is too close to call. Using an analog gauge it is very difficult to say it is actually 205psi and not 207 psi, and a digital thermometer has a range of accuracy, usually plus or minus 2 to 3 degrees. I would recheck the readings. In refrigeration we have two types of vapor- saturated and superheated. We have two types of liquid- saturated and subcooled. Neither superheat or subcooling readings can truly be negative. Sounds like you have flashgas present at the txv with that 80 degree superheat. But it is definetely not negatively subcooled.
Had a 20 year old lennox that I hooked my digital gauges up to with temp sensors. Without getting into all the specifics the guages were showing 14 degrees superheat and -58 degrees subcooling.... Totally corroded out condenser...
I had unit yesterday, that with my didgicool, measured -78 SC and 20* sh. High discharge pressure and high suction, going off on high head safety and compressor overheating. So I figured the condenser wasn't rejecting heat, although the fan was working. Cleaned condenser coil bc it looked pretty restricted, but shortly thereafter compressor seized up. 8year old rtu that had no PM.
Saw this yesterday. Replaced a condenser motor that was not turning at all. Compressor was running and sending "hot gas" from the discharge line back to evaporator coil. The blower motor was "condensing" the hot gas to a liquid and "flooding" the compressor with low superheat. I did not check it with temperature probe, but when I purged my gauges, liquid refrigerant came out of the suction line.
Stuart
Lack of airflow destroys compressors.
Correct me if I am wrong but when I think about negitive SC or SH I think of it being reversed of normal readings. In otherwords if you have a air cooled condenser operating in 95* amdient and have R-22 at 225 head with saturation temp of 110* and a 115* LL temp I would call that 5* negitive SC as it went reverse or negitive of the saturation temp. Or say you have a R-22 system with a 70# suction and 35* line temp. In either case there was actually no SC or SH as the temp increase so there was no subcooling in the 1st example and the temp decreased in the second example so there was no superheat.
I have also heard negitive subcooling used in a situation where the LL temp went below condensing media temp like 95* ambient with a line temp of 90*.
The compressor creates heat. It does not absorb heat! Not unless the entering gas temperature from the suction line is hotter than the compressor.
The refrigerant gas returning to the compressor either absorbs the heat produced by the compressor, or it will be to super-heated to absorb that heat, thus not helping cool the compressor. In short, the refrigerant removes the heat created in the compressor, (ie: Heat of compression), or the compressor will most likely overheat. (Assuming it does not have some other external means of cooling the compressor).