Causes of Negative Subcooling

[Savant]

A compressor is a pump for reducing volume and increasing the pressure of gases in order to condense the gases and according to gas laws the caused an increase in temperature not heat content. While some heat is added due to the electro-mechanical processes in the compressor it is NOT the compressors job to reject heat or even absorb it for that matter. The condenser rejects heat. If someone is seeing negative subcool what is that really ............ superheat. Since we are doing all we can to reject the heat one of the premises is wrong, so what should we do now................ a contradiction. Go back and check one of your premesis, one is wrong. Check the accuracy of your gauges and your temperature measurements. If that is not the problem then you likely have either the wrong refrigerant or non-condensables in the system

[VTP99]

A compressor is a pump for reducing volume and increasing the pressure of gases in order to condense the gases and according to gas laws the caused an increase in temperature not heat content. While some heat is added due to the electro-mechanical processes in the compressor it is NOT the compressors job to reject heat or even absorb it for that matter. The condenser rejects heat. If someone is seeing negative subcool what is that really ............ superheat. Since we are doing all we can to reject the heat one of the premises is wrong, so what should we do now................ a contradiction. Go back and check one of your premesis, one is wrong. Check the accuracy of your gauges and your temperature measurements. If that is not the problem then you likely have either the wrong refrigerant or non-condensables in the system

Well said please post more often.

[hvacrmedic]

First of all, is it possible to have negative subcooling (ie liquid line temp is greater than temp of condensation)? Second, what could cause this to happen?

For a pure refrigerant that means that either all, or at least the majority of the refrigerant, is still in the form of superheated vapor as it leaves the condenser. This can be caused by a low charge, a dirty condenser coil, condenser air recirculation, excessively high ambient, etc. Measurement error is also a possibility.

Savant, not likely noncondensables, since noncondensables artificially elevate subcooling readings.

[scooters45]

Technically a compressor does reject heat.
Follow me and I'll try to make sense.
The motor windings are cooled by the returning suction vapor. Since the windings are hotter than the suction vapor the compressor rejects heat to vapor. Because heat allways moves from a warmer to a less warm (colder) object.

Of course this dosen't matter to us for field work only when they design a compressor.

There is also the latient heat of compression that is added to the compressed super heated gas during compression. Then the gas is condensed in the condenser and subcooled.
Thats why the coils on air cooled equipment allways seem to loose there fins in the first 3 or 4 rows from the bottom of the coil because thats where must of the work (of chaing state ) is taking place.

I have seen higher subcooling at the AHU then at the condenser but only several times. They where all in resi systems that either the liquid line was to small or the line set was run thru and attic or both. It was only a couple of degress but was there.

JMHO

Scooter

[frozensolid]

First of all, is it possible to have negative subcooling (ie liquid line temp is greater than temp of condensation)? Second, what could cause this to happen?

I was going to saying nothing but I guess a torch could make it happen. Negative sub-cooling does not exist.

Discharge gas could be superheated But I don't think it could get all the way through the condenser coil and remain that way.

[davo]

There could be several reasons this could happen. He never said what type of system he was working on.

What if ther were a head master stuck open, bypassing the condensing coil with 100% discharge gas?

In most likelyhood, his guages/thermanometer are out of calibration or his temp probe is on the discharge line.

[mike dixon]

I figgered it out. hes working on a heat pump, he forgot.

[frozensolid]

Either way it still would not be negative sub-cooling.

[jpsmith1cm]

That's right.

He measured wrong, it's negative superheat....

[man from trane]

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.

[jpsmith1cm]

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.

...and he designs this stuff.

[VTP99]

If such a thing existed wouldn't they make instruments to read it ?

[man from trane]

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.