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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?
Does it give the same changes in Enthalpy?
Maybe different people wrote those parts of coolpack?
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
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.
Same topic discussed in earlier thread here: https://www.refrigeration-engineer.c...ative-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