superheat and subcooling

[Airconking]

when taking the above readings is there general temps to achive for both s.h and s.c?
i.e s.h 7K s.c 5K???
ive read loads of posts on here and asked several other engineers and had several different answers.
i understand the reasons why we need both s.h and s.c and know how to take both the readings but just want a rough temp to work to for both.
also glide on r407c 7K???

cheers.

[Magoo]

There is no rule of thumb, or average. As every system is totally different. Whether aircon or commercial or industrial refrigeration applications. If you understand the principals and relationships and methods of checking you are fine. Treat each system as an individual application.

[Goober]

Well said Magoo, and I might add, that superheat and subcooling is as much about black magic as they are based on engineering fact. In my humble opinion, or if you are of the texting culture IMHO

[The Viking]

IMHO, there are a few basic principles within the refrigeration circuit...

- All expansion devices needs a solid liquid supply to perform to their design capacities.
- No compressor benefits from pumping liquid.
To me these are 2 of the non negotiable principles.

There is no magic involved in this, black or otherwise, it is pure engineering facts...


Now,
In order to ensure all the refrigerant reaching the expansion device is in an liquid state it needs to be cooled below it's dew point, just to make sure there isn't too much of a carry over of non condensed refrigerant (gas). This additional cooling is what we refer to as sub cooling and is is there for a reason.

Similarly, in order for us to be sure there is no liquid refrigerant getting sucked in to the workings of the compressor the refrigerant needs to be heated to above the point where all the refrigerant enter it's vapour state. Let us call this super heat shall we...


No liquid to the expansion device = not enough refrigerant gets through = no performance.
Liquid enters the compressor = extremely high wear and tear = high costs
(OK, in most cases it would read,
Liquid enters the compressor = compressor failure = replacement compressor)


So back to the original question, what should the sub cooling and super heat be?

Back in the good old days, before accountants ruled the world and when the controls were electromechanical 99% of all normal refrigeration circuits were designed with 6-9K sub cooling and 4-6K super heat.
This would allow for the imbalances of the mechanical expansion valve, and other parts, working and responding to varied running conditions.

Nowadays we got electronic controls / regulators and more forgiving compressors, this combined with the ongoing penny pinching results in less margins being needed and therefore also built in to the design.

A modern EEV feeding a Scroll compressor might operate on as little as 1K superheat, this will allow for a smaller evaporator being needed which in turn would save money and quite possible award additional green credentials.
The EEV operating on so tight tolerances also reduces the variations of the refrigerant flow, therefore the sub cooling can also be reduced which enables further cost savings as the condenser also can be reduced...

.

[Airconking]

Thanks for the replies.

[Magoo]

Hi Airconking.
if you use the forum site search at top right of screan, there is a truck load of info available.

Hi Viking. I liked your comment about accountants, Oh so true.