Superheat

[crocens]

Hi there!

As usualy i have an unusual question ;)

We all know (or should to know) how to messure superheat.

I was woundering why does it must be at least 4-5K ? (for EEV)

As i know untill whole liquid does not evaporate there will be no superheat becouse all energy will be used in evaporation proces.

So it should be obvius that even 1K of superheat should ensure that there is no liquid in line. But we all know there is if superheat is to low. Where does this liquid come from, how it is possible that liquid and superheated vapor, both at the same pressure in the same pipe exists?

Have a nice day, Marcin

[The Viking]

Speed and carryover...

True, at 0.1K superheat the liquid will have changed state in to vapour but the problem is the speed the gas is travelling at, it will carry droplets of liquid way pass this point.

Imagine that you carry a loose bag of marbles on the roof of your van going 110km/h. Now you slam on the breaks and the van stops after 50m, the bag will probably be found at around 60m but there will be marbles that would have traveled way over 100m due to the momentum they got...
Now replace the motorway with a pipe and the marbles with droplets of liquid. Got the idea?

:cool:

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[Sandro Baptista]

Hi there!

As usualy i have an unusual question ;)

We all know (or should to know) how to messure superheat.

I was woundering why does it must be at least 4-5K ? (for EEV)

As i know untill whole liquid does not evaporate there will be no superheat becouse all energy will be used in evaporation proces.

So it should be obvius that even 1K of superheat should ensure that there is no liquid in line. But we all know there is if superheat is to low. Where does this liquid come from, how it is possible that liquid and superheated vapor, both at the same pressure in the same pipe exists?

Have a nice day, Marcin

If you want guarantee that with 1K there wouldn't have droplets you would have a long suction pipe after the pipe to "give time" that droplets will evaporated (it is necessary guarantee time and sufficient energy to ensure that all refrigerant is in vapor state).

[crocens]

Thamk you for your replays.
It is quite clear for me, i just thought that evaporation process is rather rapid. But ok, that answers satysfy me, thank you :)

[frank]

It's given that 3K superheat is sufficient to ensure that liquid does not reach the compressor inlet and this is called 'Useful Superheat' and can be included into acceptable energy calculations.

Where a system has 6K superheat, the first 3K is 'Useful' and the next 3K is 'Unuseful' as it just adds to the work the compressor has to do.

[Rob White]

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Superheat can be as low as 1K and as explained above there are
reasons why 1K superheat is not desirable.

One big reason though, not mentioned yet is TEV design. Traditional
expansion valves had a built in safety factor, to prevent flooding back.

Manufacturers traditionally set valves to about 6 to 8K superheat and
that was purely for safety reasons. The last thing a valve manufacturer
wanted was a claim for damaged compressors due to liquid carry over.

With the use of EEV's the need for a wide superheat is not as important,
but if an EEV manufacturer got a reputation for damaging compressors
they would soon lose money, so as a safety feature they set superheat
wider than necessary purely for safety reasons.

Regards

Rob

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[Rob White]

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Another point to consider also is "glide".

Some refrigerants have a wide glide and the setting up of
superheat is very important.

R407c for example has about 6K glide. Now if you set superheat
to 4 or 5K and if you measure the superheat from the bubble
point as apposed to the dew point, you have the real risk of carrying
saturated liquid over into your suction pipe.

Regards

Rob

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[crocens]

Hey, thanks.

I know how does TEV and EEV works, are build and what is a minimum superhaet for them.

I just wanted to know how can both superheated vapour and liquid exists in one pipe, i was always sure that liquid should quite fast vaporize. This proces should be fast becouse of turbulent flow in pipe.

[Rob White]

Hey, thanks.

I know how does TEV and EEV works, are build and what is a minimum superhaet for them.

I just wanted to know how can both superheated vapour and liquid exists in one pipe, i was always sure that liquid should quite fast vaporize. This proces should be fast becouse of turbulent flow in pipe.


Subcooled liquid exists inside the receiver underneath superheated vapour.

There are lots of anomalies inside a refrigeration circuit and most would
take a professor of thermodynamics to explain them :eek:

Regards

Rob

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