Charging formula

[piashaw]

Hi Can somebody please remind me what the formula for charging a system on R22 using superheat is?

When Indoor DB and WB, Outdoor DB and suction line pressure are known.

Thanks

[lana]

Hi Can somebody please remind me what the formula for charging a system on R22 using superheat is?

When Indoor DB and WB, Outdoor DB and suction line pressure are known.

Thanks

Hi and welcome to the RE forums.

Would you please clarify what you mean by your question?
Do you mean the refrigerant charging amount?
Cheers

[piashaw]

Hi

Thanks for the welcome.

Yes it is for the refrigerant charging amount.

I want to use it as an accurate way to test refrigerant levels in split type AC units as well as for refilling when the data plate has been made illegible. I'm in Greece and outside temps in one day can vary from 35C to 18C. DB vs WB is also enormously variable here as well, as I'm close to the sea and changes in wind direction can have a rapid effect on the readings.

Thanks

[Tesla]

Hi Piashaw
Its pretty simple. Take suction pressure at compressor then allow for suction line pressure drop of say 1 to 2 psi. So if you got 60 psi at compressor your suction pressure at evap would be about 61 psi. Look at a refrigerant pressure/temperature chart and you take the equivalent temp which in this case is 1.5deg c. Then take a temp of evap outlet. The difference between these two temps is the superheat, and should be 4 - 8 k for a recip. This is the old way and not very accurate. I use a superheat calculation tool from yellow jacket ritchie which measures the refrigerant temp directly via schrouder fitting. Don't forget to monitor head pressure and motor current whilst charging. And I think the best time to check superheat is toward the end of the cycle.
Good luck

[coladin]

hi all

i'm a newbie, I come form vietnam, my company produce air-conditioner, so that I hope I can learn moreknowledge form RE forum for my job.

coladin

[lana]

Hi piashaw,

There is no formula for charging.

The charging procedure is relatively simple if you know what you are doing. If there is a receiver and TEV in the system then this is much easier. If there is no receiver and there is cap tube (like splits) then charging becomes very critical.

You have to observe the followings:

1- Suction pressure (LP)
2- Find the corresponding temperature for LP (te)
3- Discharge pressure (HP)
4- Find the corresponding temperature for HP (tc)
5- Measure evaporator outlet gas temperature (t1)
6- Measure condenser outlet liquid temperature (t2)
7- Find the superheat (SH) = t1-te
8- Find the subcooling (SC) = tc - t2
9- Make sure that all condenser fans are operating.
10- Measure air inlet temperature into the condenser (ta)
11- Add 10°C to this air temperature (ta+10) = tcc
12 - This new temperature (tcc) must be near your tc.

Now all of the followings must comply :

1- SH around 5-8°K.
2-SC around 4-7°K.
3- te around 2-5°C.
4- tc around tcc.

Simple guide : if SC is low then the refrigerant charge is low. If SC is high then it is overcharged.

You see this procedure is not a formula and you have to observe many things together. If you have enough experience and practice then this is not so difficult as it seems.

Cheers

[sapper]

hi piashaw ...understand the problems you are having as i am working in cyprus and therefore have the same climate variables. if you follow what lana is telling you then you really can t go wrong. also worth checking how the frost behaves on suction line.... nothing more than an indication you understand...... when frost melts off out here then your close.......... just saves you taking loads of readings

[collo]

Gday Piashaw,
Collo from Australia. Re your Super heat problem. If you are working on Wall splits you would have realised by now that generally there is only a schrader valve on the suction line at the condensing unit. Occasionaly there is a valve on the liquid side as well but the pressure will be nearly identical to the suction side as both capiliaries are in the condensing unit, before the service valve/s. As a rule on these little systems Up to 10Kw capacity the charge is critical, and I mean critical. I would follow Tesla's advice as there usually isnt enough service points or access to the ends of the coils to follow Lana's advice. Myself I convert the pressure from the low side guage to temperature as read on the gauge, and add or subtract from the suction line temperature to give SH. This works,just make sure that you have run the system for at least 20m mins to give time for the system to settle in before adding any refrigerant.
that my bit to add to the confusion.
Regards
Collo