Coolpack error

[Peter_1]

Hello colleagues,

Draw a cycle in Coolpack
R134a, TC 40°C; 5K SH and 5 K SC, TE= -5°C.
In Cycle analysis, enthalpy at end of evaporator = 257 kJ/kg
Same cycle drawn in Refrigeration Utilities of Coolpack and enthalpy is there at the same point around 400 kJ/kg (zoomed in)
Where's this difference coming from?

[seanf]

Does it give the same changes in Enthalpy?
Maybe different people wrote those parts of coolpack?

[MrFreez717]

Agree with seanf, check to see if the enthalpy CHANGES are the same. The "zero point" for enthaply seems to be an arbitrary decision, so the values at a given point may differ, but the change between two states should yield the same result. I'm old so use tables mostly, but my favorite defines enthalpy at 0 for -40F (or -40C) saturated liquid.

MrFreeze

[Peter_1]

Hello, change in enthalpy is indeed the same for both codes but anyhow, a given refrigerant has at a certain point in a log ph always a fix enthalpy which should be always the same.

[MrFreez717]

Same topic discussed in earlier thread here: https://www.refrigeration-engineer.com/forums/showthread.php?11427-negative-enthalpy

As stated, -40C is "typical" place to assign 0 enthalpy for saturated liquid, but you could use a different temperature for this, as some sources do. Since the change in enthalpy is the point of interest, it does not matter where you assign a zero value - the difference between state points is what defines the work done.

In my most used source, all refrigerants (ammonia, CO2, R507, etc) have 0 enthalpy for -40F saturated liquid. This is not a crazy coincidence, but rather a conscious choice to use a common reference point. Obviously all these different substances to not have the exact same internal energy as each other.

MrFreez717