temp / pressure relationship

[wilks]

hi guys ,

i was working with one of my colleagues the other day when we had to recharge a display fridge with r134a , he charges SYSTEM by relating to the temp pessure relationship , he adds the gas until he gets correct temp on gauges ? say

r134a low side temp is at -5 oC (nowing that the refridgerant will take out 10oC leaving us with a dislay fridge at + 5 oC) so he charges untill he gets the correct temp which then translates to 1.5 bar = -5 oC is this correct ?

rnt we reading the suction pressure that depending on pipe runs etc , couldnt they add up to higher superheat so therefore giving higher pressure readings???

bit confussed !

and trust me it doesnt take much !!

[lana]

Hi there,

The method you described is correct BUT :

This is right if every component in the system is properly sized and there is no fault in the system.

I explain it to you by an example :

If there is a problem with the filter, say it is blocked, then suction pressure goes down. Now, if you only rely on the LP and charge the system, you will end up by overcharging it.

Charging a system with correct amount is a relatively simple procedure, if you know what you are doing.
These parameters must be checked simultaneously :
LP, HP, ambient temp., super heat, sub cooling.
If all these parameters are within the correct range then the charge it OK.

Cheers

[taz24]

hi guys ,
r134a low side temp is at -5 oC (nowing that the refridgerant will take out 10oC leaving us with a dislay fridge at + 5 oC) so he charges untill he gets the correct temp which then translates to 1.5 bar = -5 oC is this correct ?
bit confussed !

and trust me it doesnt take much !!

Critical charge systems will work like this.
as Lana states there are other factors but it does work.
Weather conditions can influence the outcome of the charge. If you charge to the correct level during the cold winter months It could be under charged in the warm summer months ( or the odd day if in the UK)

cheers taz.

[wilks]

Critical charge systems will work like this.
as Lana states there are other factors but it does work.
Weather conditions can influence the outcome of the charge. If you charge to the correct level during the cold winter months It could be under charged in the warm summer months ( or the odd day if in the UK)

cheers taz.

thanks guys . what would be the best way to check for blockages , would putting OFN in the high side only and checking guages to see if we have any pressure on the low side ?

[BigJon3475]

thanks guys . what would be the best way to check for blockages , would putting OFN in the high side only and checking guages to see if we have any pressure on the low side ?

Superheat and subcooling

[wilks]

so ,

what temp / pressure im getting at guages on low side is telling me what temp refridgerant is evap at ?

thanks guys , still learning done basics with my company brewery but now there trying to get us techs to do more , we have other refrigeration techs if we need back up , but to be honest dont think they have the knowledge you guys have ,

[BigJon3475]

It's telling you the evaporation temperature. So if you were to take the temp. of the line where your gauge is hooked up to the system and you have R-22 in the system at 68.5 psi or 4.72 bar. The the temp correlation for that is 40ºF or 4.444ºC. You then know if you record the temp/ of the line set and you get 4.44ºC you have NO superheat. You would have liquid at the place your gauge is attached. If you get a temp. of 7.22ºC then you have 2.78ºC worth of superheat. Read over Lana's post carefully until you understand it completely. This is the most important part to understand to understand the ref. cycle. I'm in the US so if my ºC is off sorry I don't normally it (disclaimer )

[wilks]

It's telling you the evaporation temperature. So if you were to take the temp. of the line where your gauge is hooked up to the system and you have R-22 in the system at 68.5 psi or 4.72 bar. The the temp correlation for that is 40ºF or 4.444ºC. You then know if you record the temp/ of the line set and you get 4.44ºC you have NO superheat. You would have liquid at the place your gauge is attached. If you get a temp. of 7.22ºC then you have 2.78ºC worth of superheat. Read over Lana's post carefully until you understand it completely. This is the most important part to understand to understand the ref. cycle. I'm in the US so if my ºC is off sorry I don't normally it (disclaimer )

thanks jon ,