Under Comparison on Cycle, Compare with offers only the saved cycles it can draw honestly on the same P–h diagram. The line under it, starting “Not offered:”, counts what it left out and why.
Why Kelvin refuses
Enthalpy is measured from each refrigerant's own datum, so an absolute enthalpy means something only against its own refrigerant. Put two refrigerants on one enthalpy axis and the distance between their traces is an artefact of two arbitrary zeros. A difference in enthalpy compares; an enthalpy across two refrigerants doesn't.
The same holds for one refrigerant on two property models. A revision saved on the earlier property model measures enthalpy from a different datum than the tables Kelvin solves on now.
The reasons it gives
- “a different refrigerant”, or on a cascade “a different high-stage refrigerant”.
- “the earlier property model (re-save it to compare)”.
- “not a cascade” or “a cascade”: a cascade is compared only with another cascade, both circuits drawn.
- “no saved cycle results”.
What to do
- 1
For a project saved on the earlier property model, open it on Cycle and save it again, and it can be compared. An earlier revision in a design's version history keeps the model it was saved on.
- 2
To weigh two refrigerants against each other, read each design's own results, such as its COP and compressor power, rather than laying their cycles on one diagram.
The Reports screen applies the same rule when a cycle report carries a comparison, and says what it left out.