Limitation of Refigerant

[the-researcher]

Hey Guys,

I am currently in my last year of university and I am researching AC systems and heat recovery. The problem that I am trying to get my head around is the temperature of the refrigerant (ideal) when it leaves the compressor (by the way, I fully understand the refrigeration cycle and what they look like on a T-S and P-H diagram).

Phases:
First: The refrigerant enters the compressor as a saturated vapour (at a low temperature).
Second: The refrigerant is compressed to a super-heated vapour stage before it enters the condenser.

Ok, now say that the ambient air temperature (outside) is 35C, would it mean that the temperature of the super-heated refrigerant can only reach a maximum of 35C or is it possible to reach a higher temperature target?


Thanks in advance

[Brian_UK]

If it only reached a maximum of the ambient air temperature then it would be impossible to condense it by cooling it with the air stream.

[The Viking]

OK,
First, to answer your question, the limiting factor on the discharge temperature is not the refrigerant but rather the oil and the temperature at which the oil will start to break down. Normally the maximum temperature the oil can take is around 115ºC.

Secondly, more research is needed...

The refrigerant entering the compressor must be in a superheated vapour state.

The design of the system is what decide the discharge temperature.
The discharge temperature will always be above the temperature of the media cooling the condenser, therefore the statement in your last paragraph should read: The discharge temperature can never go below ambient temperature.

Happy reading :)

:cool:

[The Viking]

Sorry,
I just saw that you are Australian, as everything is upside down there "down under" you are probably right.

:D

(OK, apologies. I just couldn't help myself)

[the-researcher]

The refrigerant entering the compressor must be in a superheated vapour state.



umm... for an ideal system, shouldn't the refrigerant entering the compressor be in a saturated vapour phase then compressed into a superheated phase?


10735

[Peter_1]

You gave yourself the right answer. Point 2 is higher than ambient.

[the-researcher]

Ahh true, brain is not functioning properly lately. Thanks Peter_1

How does the following answer my initial question anyway, bit confused, can someone clarify?

First, to answer your question, the limiting factor on the discharge temperature is not the refrigerant but rather the oil and the temperature at which the oil will start to break down. Normally the maximum temperature the oil can take is around 115ºC.

[The Viking]

In practice, the compressors are allergic to liquid entering them and to protect them from this happening you will need superheated vapour on their suction side.

And as to the maximum discharge temperature, it doesn't matter what temperature the refrigerant can take, the maximum temperature must be designed to the component of the system that has the lowest maximum temperature and in the case of refrigerant circuits this is generally the oil.

:cool:

[Peter_1]

You can't go as far as you wish at point 2. There are technical limitations for the compressor (both temperature and pressure) and temperature limitations for the oil as The Viking stated. Oil is pumped a little bit mixed together with the refrigerant. If you go too high, oil will carbonize and break down. Do once the exercise with ammonia with a superheat at the inlet of the compressor of 10 K and then with 30K and look once what will happen to the discharge temperature (point 2) So you can heat water higher then condensation temperature but the energy available at a higher temperature is not that high. So post-heating of already heated water at +/- condensation temperature.

[desA]

First: The refrigerant enters the compressor as a saturated vapour (at a low temperature).

It leaves the evaporator with a superheat of between 5 to 7K. Line conditions between evaporator exit & compressor inlet can either raise, or lower the SH value.



Second: The refrigerant is compressed to a super-heated vapour stage before it enters the condenser.

Indeed. The amount of superheat arrived at, will be affected by the efficiency of the compression process. We don't generally see isentropic compressor efficiencies, in practice. So, the compressor basically adds in heat to the vapour stream as a function of both compression & non-ideality.



Ok, now say that the ambient air temperature (outside) is 35C, would it mean that the temperature of the super-heated refrigerant can only reach a maximum of 35C or is it possible to reach a higher temperature target?

Practically, the compressor exit temperature is limited - as mentioned by other esteemed posters, in order to protect the oil from degradation. Many designers would look at a maximum oil temperature of around 110'C - for sustainable system lifespan.

In reality, this limiting compressor exit temperature, will determine the reasonable pressure ratio allowed across the compressor, during operation.