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the-researcher
10-05-2014, 07:14 AM
Hey Guys,

I'm having a hard time finding out the repercussions of have a refrigerant (e.g. R410a) being in the supercritical stage, for a standard AC system. At the moment, I only know that it is hard to define whether the refrigerant is a vapour or liquid form when it is supercritical. Apart from this what are some other problems?

Thanks.

Rob White
10-05-2014, 12:40 PM
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Supercritical? Do you mean when the refrigerant is above the critical point?

We call that transcritical.

From what I know 410a does not react well to being transcritical (supercritical).

Some natural refrigerants like C02 can run at transcritical temps/pressures but
man made HFC's burn out or thermally decompose if they are exposed to the
critical temp/pressure for prolonged periods of time and if you run HFC's at transcritical
pressures/temps it will almost certainly "burn out".

With transcritical pressures you don't have a change of state like you do in subcrital
systems, the change in state only happens with a reduction of the pressure.
So that means you don't have condensers, you have gas coolers and then a
pressure reducing valve to create the phase change from vapour to liquid.

Regards

Rob

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the-researcher
11-05-2014, 01:56 AM
.

Supercritical? Do you mean when the refrigerant is above the critical point?

We call that transcritical.

From what I know 410a does not react well to being transcritical (supercritical).

Some natural refrigerants like C02 can run at transcritical temps/pressures but
man made HFC's burn out or thermally decompose if they are exposed to the
critical temp/pressure for prolonged periods of time and if you run HFC's at transcritical
pressures/temps it will almost certainly "burn out".

With transcritical pressures you don't have a change of state like you do in subcrital
systems, the change in state only happens with a reduction of the pressure.
So that means you don't have condensers, you have gas coolers and then a
pressure reducing valve to create the phase change from vapour to liquid.

Regards

Rob

.

Hi Rob,

Thanks for the reply, yes I meant when the refrigerant was about it's critical point.

What do you mean about the refrigerant thermally decomposing or 'Burn Out', can you go into a bit more detail?

Thanks.

Rob White
11-05-2014, 12:05 PM
Hi Rob,

Thanks for the reply, yes I meant when the refrigerant was about it's critical point.

What do you mean about the refrigerant thermally decomposing or 'Burn Out', can you go into a bit more detail?

Thanks.

If it runs too hot for too long it breaks down thermally,
just the same as if you were to burn the refrigerant with
a flame.

Have you ever brazed a pipe with some refrigerant inside?
the results can be quite extreme, the refrigerant burns
and the burnt gasses react differently (they are also toxic
and generally nasty).

But in a sealed system those "burnt out" gasses can't go anywhere
so you have a refrigerant that no longer works the same as the original
refrigerant that was put in there in the first place.

HFC refrigerants do not react too well to being run too close to the
critical temperature and if they are ran too close to it they break down
or "burn out"

Regards

Rob

.

the-researcher
11-05-2014, 12:12 PM
Thanks for all the help Rob, that makes it very clear.

the-researcher
13-05-2014, 12:25 PM
Hi Rob,

Just one last question on refrigerants being in transcritical stage. You stated that HFC are prone to being "burn out", how about natural refrigerants such as R290 going close to or beyond it's critical temperature and pressure, do they also "Burn Out"?

Thanks.

Rob White
13-05-2014, 02:59 PM
Hi Rob,

Just one last question on refrigerants being in transcritical stage. You stated that HFC are prone to being "burn out", how about natural refrigerants such as R290 going close to or beyond it's critical temperature and pressure, do they also "Burn Out"?

Thanks.

Natural refrigerants are less problematic and C02 is perfect for it.

Consider using C02?

Regards

rob

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