Calculating Refrigerant charge........

[CamLoch]

When working out the sub-cooled liquid density, do you use the sub-cooled temperature or the saturated pressure liquid density. For example: R134a at 300psig has SCT of 160ºF and liquid density of 61.74 lb./ft^3. However, it is sub-cooled 10ºF. Do I then look up the liquid density at 150º which is 63.81lb./ft^3?

 

[MintJulep]

You'll need both.

Sub-cooling only exists when the system is in operation. Everything is saturated when it's off.

 

[CamLoch]

MintJulep,

To determine the required refrigerant charge for the system I assume a column of liquid in the liquid line. The liquid line would have a fixed volume. To calculate the weight of charge in the liquid line, do I use the sub-cooled temperature density or the saturated temperature density?

 

[imok2]

If you don't have the mfg specs you can use 10-12 degrees of sub cooling sensible heat

 

[CamLoch]

I understand the general requirement for sub-cooling. I guess what I am trying to ask is: Does the density of the refrigerant change as it sub-cooled? Is the density of liquid refrigerant at the saturation point less than the density of the liquid when it is sub-cooled?

 

[imok2]

When liquid refrigerant is sub-cooled before entering the metering device the refrigeration effect per unit mass of capacity is increased because of the decrease in flash gas and a slight increase in density