Causes of Negative Subcooling

[HVACR_CA]

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...

[udarrell]

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

[jpsmith1cm]

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.

[frozensolid]

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.

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.

[man from trane]

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.

Yes, I think you're right. I was just trying to think of an extreme example of "negative superheat". Maybe a very dirty condenser or one severely corroded like we get around here would be a better example. Seems like kind of a silly topic anyway...

[beenthere]

Yes, I think you're right. I was just trying to think of an extreme example of "negative superheat". Maybe a very dirty condenser or one severely corroded like we get around here would be a better example. Seems like kind of a silly topic anyway...

A dirty or severely corroded condenser coil won't cause you to have negative SC.

[VTP99]

Sub-cooling no such thing

Sub-cooling no such thing

Sub-cooling no such thing

[tjc76]

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...

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.

[JimmyM]

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...

No sub cooling
Extremely high superheat
No heat being rejected by the condenser
Suction line temp of 57 degrees for a freezer.
= Extremely low charged system

[fotofx]

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...

[ChristopherNJ]

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.

[mcewans]

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.

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.

[kls-ccc]

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*.