Enthalpy calculations with R134a refrigerant 2

[WelshEngineer87]

Hi all,

I'm sure there is a really easy answer to this question but can't figure it out at the moment. I'm trying to workout what is the enthalpy condition in the suction line of an AC compressor to calculate isentropic efficiency.

The condition i have is 2.5bar suction pressure at a temperature of 6degC, based on the pH diagram this would relate to an enthalpy of around 400-410Kj/kg, however in the superheated tables for R134a at the same pressure and temperature the value is more like 250-260kJ/kg.

I have no idea why it's different on the chart than it is in the table. Any suggestions?

Regards

 

[WelshEngineer87]

Here is the condition on the pH chart.

 

[Snickster]

The chart I have shows that at 2.5 bar the saturation gas temperature is -5 C with an enthalpy of 395.34 kJ/kg. So if extend it out at constant pressure to 6 C and add the increase in sensible heat then you would get close to in the PH diagram you have attached.

Note that to find enthalpy of your state point I believe you need to travel straight down and not down following a constant temperature line of 6 C. To do this would give you the state at 0.5 bar at 6 C not 2.5 bar at 6 C.

 

[WelshEngineer87]

@Snickster,
Ok thanks for the information, that was my thought on the ph chart which matches with the compressor data i have.

I still don't understand why in the superheated tables at the same pressure and temperature the enthalpy is more like 250-260kJ/kg.

 

[Snickster]

I have found two tables that agree with the PH chart you have and other table that does not. Don't know why there is a difference. Attached is one that agrees with the chart and can be found here:

https://www.freon.com/en/-/media/files/freon/freon-134a-si-thermodynamic-properties.pdf