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thebrockenshire
08-04-2017, 10:49 AM
hey guys

If you heat a gas above its critical temp the refrigerant will not condense no matter what the pressure is, but if you reduced the temperature back down below critical temp, will it the be able to change state again?

why i ask is that back at TAFE me teacher told me the gas would be "burnt" and result in the gas never being able to change state again and would need to be replaced.
i work with another technician who disagrees. what are your thoughts?

joe-ice
08-04-2017, 11:56 AM
I dont think thats necessarily true about going over the critical temperature , but i can tell you that if 404 is run at high pressure and temps (blocked condenser) for long period of time it loses its efficiency and ability to evaporate at low temperatures, this is always accompanied by a burnt smell from the gas in the system.

cduque
10-04-2017, 12:40 PM
Hi,
The critical point is above, both a certain temperature and pressure. At low pressure when you cool the gas obviously it will condensate again.
Above critical point you need to reduce pressure in order to be able again to condensate and evaporate the fluid.
If you take a look at the PH diagram (Mollier chart) this relations become more obvious.
If you heat it too much maybe you can transform the substance and if it degradates maybe you can not achieve again the original state, that I dont know.
Regards,
CDuque14737

Rob White
10-04-2017, 01:14 PM
.

Hello

I think the issue is with man made refrigerants (synthased) HFC based refrigerants.

Natural refrigerants like R744 (C02) tend not to burn out.

Rob

.

mikeref
11-04-2017, 08:59 AM
Critical temperature is where conventional refrigerants degrade.

One instance was an R134a vehicle air-conditioner that had a bypassed HP/LP and a seriously inefficient condenser. ( Blocked with grass seed and Insects.) Cleared that drama but the A/C wouldn't perform.

Some of the refrigerant condensed. Liquid pressure and temperature remained high.:confused:
Pulled the R134a, replaced the drier, and vacuum. Fresh charge of refrigerant. Customer is happy with the outcome.
Compressor oil contamination? I Have No Idea.

mikeref
11-04-2017, 09:17 AM
Hi,
The critical point is above, both a certain temperature and pressure. At low pressure when you cool the gas obviously it will condensate again.
Above critical point you need to reduce pressure in order to be able again to condensate and evaporate the fluid.
If you take a look at the PH diagram (Mollier chart) this relations become more obvious.
If you heat it too much maybe you can transform the substance and if it degradates maybe you can not achieve again the original state, that I dont know.
Regards,
CDuque14737

You are on the right Path though Charts can't predict molecular transformation.

cduque
11-04-2017, 02:19 PM
There in no critical temperature but a critical point and it is established both in temperature and pressure.
So it is not possible to heat the gas over the critical point if the pressure is not high enough.
Also there is not any connection between one fluid "degradation" or "burn out" point and it's critical point.
Several fluids, for instance R744 (CO2), doesn't burn or degradate much over it's critical point.

thebrockenshire
13-04-2017, 09:32 AM
cool guys, thanks for the input.

the example with R744 is perfect. i have been referencing "critical point" with "refrigerant degrading", which has been incorrect. although with R404A these two points seem to be close.

it would be very advantageous to have a chart with some temperatures at which some of these refrigerants begin to de-grade

cheers everyone

chemi-cool
13-04-2017, 02:25 PM
It happens a lot with 404 for unknown reason to me.

I have came across quite a few systems that had a full charge and all seems to be ok but no cooling.
replacing the charge made it work fine again, so to my humble opinion, your teacher is right.

hyperion
13-04-2017, 04:26 PM
Many years ago an experienced engineer referred to the refrigerant becoming 'tired' and would therefore require replacing. Whether this was convenient bulls*** for the customer or not, I am not sure. But maybe there was a minor element of truth in what he was saying.

thebrockenshire
14-04-2017, 11:32 AM
Many years ago an experienced engineer referred to the refrigerant becoming 'tired' and would therefore require replacing. Whether this was convenient bulls*** for the customer or not, I am not sure. But maybe there was a minor element of truth in what he was saying.

Refrigerant being "tired", I call BS on that one.

it seems to always occur due to high head issues, the gas always stinks too.

HVACRsaurus
14-04-2017, 12:17 PM
HmmKay,

So - I'm thinking you are dealing with Two different matters (critical point and then "decomposition")

Let's talk about critical point - let's talk about something called R134a, some people may have heard of it. Critical point is 101 C. Let me ask you a question "in your educated opinion, could we heat refrigerant r134a to 102 Celciuls, then cool it down & continue to use it per usual"

Well yes, I think so. I could put tap water in a pressure cooker, heat that to 102 Celsius then dunk a tea bag. Just real hot tea - wait 'till it cools down same same.

Now, second matter - decomposition. Let's remember that refrigerants are made from elements from the periodic table. Now let's imagine what might happen should All the tetras and all the flouros and all the ethanes were "separate" into their respective elements.... Would that make for some "tired" frigo. Pretty sure if my body decided to separate into its periodic elements I'd be a bit tired..

Now could refrigerant actually separate?

This a a question that many like to sweep under the carpet..

I'm sure you can make your own judgement.... (hint, think about what might happen during winding burnout or extended operation at elevated temperatures)

Ps, I know what happens to a piece of wood when it operates at elevated temperatures (it cannot retain its original form for very long, before turning into smoke...)

Grizzly
14-04-2017, 12:40 PM
I like your thinking HVACR.
If refrigerant doesn't split! Then how come some have to be charged in their Liquid state only.
In fairness to all concerned!
If as we know some refrigerants will split into their constituent gases under certain conditions. Then surely they
could leech out of a system at variable rates.
Resulting in a refrigerant which is no longer the same as original!

Either way anything used at the extremes of their operating range, will suffer.
A very interesting post all the same.
No-one has yet mentioned that the refrigerants at the elevated temps, will likely contain oil in them as well.
Unless it's a Turbocore.
Grizzly

Grizzly
14-04-2017, 12:47 PM
Refrigerant being "tired", I call BS on that one.

it seems to always occur due to high head issues, the gas always stinks too.
5 posts and you are already accusing other valued posters of "Bull ****".
A bit harsh for one so new to the forum don't you think?
whether you call something Tired or degraded or split or stinky!
It's only a term to describe something not correct.
So why the accusation of BS?
Grizzly

RANGER1
14-04-2017, 10:32 PM
Some information on the subject


http://fire.nist.gov/bfrlpubs/build92/PDF/b92004.pdf

cadwaladr
15-04-2017, 04:33 AM
Yes I agree although I have health issues etc,resorting to words that are best kept to oneself as grizzly says is out of order! I visit many social media sites but bad language is a no no,it does not respect intelligence and if you have to resort to it then think before sharing/ commenting but hey it's only my opinion!

thebrockenshire
15-04-2017, 06:59 AM
5 posts and you are already accusing other valued posters of "Bull ****".
A bit harsh for one so new to the forum don't you think?
whether you call something Tired or degraded or split or stinky!
It's only a term to describe something not correct.
So why the accusation of BS?
Grizzly

My apologises, my comment being a bit heavy handed.
my interpretation being the refrigerant becoming "tired" due to general use. which I cannot relate to.

hyperion
16-04-2017, 03:58 PM
Ranger1,
Interesting article.