Enthalpy of air from air tables differ from enthalpy from psychrometric chart

[800Logs]

Hi,
I have uploaded two images. One is a table of Low Pressure Air Properties. The other is a typical psychrometric chart.

If you look up the enthalpy of air at 60F (520 R) in the low pressure air table, you get an enthalpy of 127.27 btu/lbm.

If you look up the enthalpy of air with zero humidity (dry air) in the psychrometric chart at 60F, you get an enthalpy of roughly 13 btu/lbm.

I know I am making some basic mistake here. Can anyone spot why these values do not match or what mistake I am making?

Thanks

 

[EnergyProfessional]

enthalpy (and entropy) have arbitrary zero points. Different systems (or tables or charts) can have that "zero" point at different conditions.
your table may have zero at actual zero °Rankin or Kelvin. the HVAC chart may have it at 0°F or so.

 

[800Logs]

Thank you, that answer sounds correct to me and also possibly explains why on some tables the enthalpy is shown as a negative number once you get below a certain temperature.

On the tables I looked up the following
520 R (60F) ==> h =124.37 btu/lbm
545 R (85F) ==> h approx 130 btu/lbm

delta h (low pressure air tables) approx 5.73 btu/lbm

On the phychrometric chart
60 F ==> h approx 14.5
85F ==> h approx 20.5

delta h (psychrometric chart) approx 6 btu/lbm

The two enthalpies are likely the same and have a different zero point. Thank you very much, that really helped me out.

 

[EnergyProfessional]

this thermodynamics book by Klein and Nellis really explains it all well