Checking subcooling and superheat?

[ZeroTolerance]

Ok so ya checking subcooling and super heat is a great method of determining several things BUT how do I know my system needs 10degrees of subcooling, 8 degrees of subcooling, or even 20degrees of subcooling as all systems are different.

My question is how do you know "the sweet spot" (subcooling) for each individual system?

[ZeroTolerance]

Any takers? anyone anyone... Bueller?

[sneep]

Any takers? anyone anyone... Bueller?

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[desA]

My question is how do you know "the sweet spot" (subcooling) for each individual system?

This is a very valid question. The real truth of the matter is - you will never know - unless you designed the system, or have tested it thoroughly.

Each system will balance out differently depending on the components & piping used in the circuit. There will be a few basic rules in regards to superheat & subcooling:

1. SH has a lower limit where control of the TXV becomes difficult;
2. SH has an upper limit where the compressor discharge temp becomes too high;
3. SC has a lower limit of 0K;
4. SC upper limit will depend on the condenser type & liquid line piping;
5. Allowing SC to become too large will push up Tc,sat (high-pressure);
6. SC may rise, or fall over the range of an operating cycle, depending on the condenser & liquid line piping;
7. The liquid sub-cooling at TXV inlet has a lower limit under which it is accepted that too much flash vapour could enter the TXV.

Other posters may want to add to this list & insert the 'norms' of the RHVAC industry. I have particular views on these, based on my own system designs, which may differ to more seasoned veterans of the trade.

[Sridhar1312]

Correct charging a refrigeration system is always achieved by measuring the sub cooling super heat.
However the basis of sub cooling and Superheat for which is designed need to be known.

[desA]

^ The problem is that, very often, I'll bet that the original system designer will not have a real idea of where the sweet-spot is at any particular point in the operating range.

This sweet-spot varies over the heating range of a heat-pump, for instance. A similar effect will be seen for an a/c unit.

I believe that it is a comfortable myth to think that there is a perfect arrangement for each RHVAC system. In my view, there are a number of suitable setup points for each system. Most system maintainers simply do not have the time to find the sweet-spot - they have to rely on rules-of-thumb & then pat themselves on the back when they land inside those. Who knows what happens at other operating points?

[NoNickName]

My rule of thumb is to keep subcooling higher than the temperature difference corresponding to the pressure drop across the liquid line. E.g. measure the pressure at condenser outlet, and convert to corresponding temperature. Do the same at the TXV inlet and convert to corresponding temperature. Subtract the two and find the deltaT corresponding to the pressure drop.
That is your minimum subcooling.

For the superheating, I generally keep it 1.5 times the glide of the refrigerant. If the refrigerant is azeotropic, then my minimum is 3K.