Working out charge amount R134A

[icesav]

Hi Guys,

Here is a question that has probably been put forward a number of times before.

I work with small split systems mainly Cornelius units i.e Maxi 110. 210, 310 300, Classic 1000, Polarflow rage etc etc.

Now and then we do get some fridge decks that come with no charge amount so we pretty much have to guess the charge. I will clamp on and reclaim any refrigerant in the system and weigh it to try and get some indication but some systems come almost empty because of leakage etc.

So here is an example of what I do.

1. Run the unit to see if the comp runs and see if the condensor gets hot, high side and check for signs of sweat on the evap coil.

2. Locate charge line and clamp on piercing pliars and reclaim the refrigerant, check my gauges to see what pressure is in the system if any and note the refrigerant weight.

3. Fill the system with OFN @ 350 psi to leak test useing soapy water, locate and repair any leaks on the system and replace filter drier.

4. Vac the system out to approx 5.5 -2 and turn off vac pump and ensure it holds negative pressure to check again for any signs of leakage.

5. Because I wont know the charge amount I set me digi scales to 0 and begin to charge the system 30g at a time and then run the comp until the return on the comp begins to sweat.

Dont many like this with no problems however I cant help but feel I should be doing it the "proper way".

I have only done ACRIB and Industrial Brazing, and BRA stainless steel brazing so I am still learning alot about refrigeration.

Can anyone advise me ?

As a note all the work carried out is always factory based and always on second hand units which are sent to us for repair.

Cheers Guys

[jeds1972]

what is the suction pressure when you are finish with the charge???

You have to rely on the suction pressure and evaporation temperature to charge properlly this domestic fridges.

[Peter_1]

Very important, capillary or TEV and if TEV, with or without receiver?
Cornelius units doesn't ell me anything.
What amount of gas ware we talking about?
If capillary, you're very close to the right handlings

[icesav]

99% of the Units I work on are capillary and 9 times out of 10 the suction pressure is normally about 1 to 1.5 Bar! While Running.

Hope this help guys.

Evap temperature I couldnt say Im sure there is a accurate way to measure ? Thermomotor with sensative sensor ??

Cheers guys

[nike123]

4. Vac the system out to approx 5.5 -2 and turn off vac pump and ensure it holds negative pressure to check again for any signs of leakage.

What these numbers mean?

[keepmecool]

Evaporator temperature can be measured with a thermocouple placed inside the unit (during the production) and linked to a meter, or alternatively you can measure the temperature in the freezer compartment using a thermocouple at the back wall touching the liner where the evaporator coil is and then you should account for a few degrees loss to get a rough idea of the evap. temperature inside the pipes. For example, if the inside temp on the back of the liner is -20 degrees centigrade, then it is likely about -25 degrees C in the pipe.
Michael

[Deniver45]

I would think a 8 to 10 superheat at a 70F degree ambient would be an adequate charge in a 134a system!

[icesav]

Thanks for the posts guys, really good info. Ok let me put it like this, I give you a fridge system with no charge left in it, how do you work out the correct amount of refrigerant to put in it?

[nike123]

Note how high and low pressure changes during pull down when correctly charged

[Peter_1]

Which book is this figure coming out Nike?

[Peter_1]

I not fully agree with the figure, if you look to the undercharge situation, then LP must go lower.
A good diagram should be with temperatures

[nike123]

Which book is this figure coming out Nike?

Modern refrigeration and airconditioning

[desA]

Thanks for the posts guys, really good info. Ok let me put it like this, I give you a fridge system with no charge left in it, how do you work out the correct amount of refrigerant to put in it?

As a starting point, the evaporator can only be full of liquid refrigerant, up to its exit port. This would be the static, or filling critical charge.

So, under that condition, knowing refrigerant density & evap volume, you would have a starting mass to work with. The rest would come from test & observing SH & SC values in the system. If the suction line begins to ice up under operation, then you will need to back off charge a tad. If SC is almost 0K, you may want to consider charge addition.

In the end, though, there may very well be manufacturer charge information which can be useful - based on their internal experimental results. If you compile enough of this information, you will quickly have enough information to estimate a reasonable starting point. The problem really lies in the fact that at different operating temperatures, the required charge may vary slightly.

[icesav]

Great discussion guys, thank you for the input.

Ok new one for you but il start a new thread, thank you.