Hi,

I am working on a project to optimize the evaporating temperatures, superheat and subcooling on a 2 stage cascade refrigeration cycle using R404a and R744 (CO2). I've setup everything in EES but I have a few concerns regarding the solution I'm getting:

My ltc evap temp is -45C and im testing 10C superheat. When I ran the equations, I got a temperature of 58C at the ltc compressor outlet at almost 4 MPa, which seems a bit high to me, but I really dont know for sure.

I tried to parametrically plot my COP vs. the htc evap temperature, and the COP just keeps going up without bound. I was expecting to see an inverse parabola, but the COP never decreases.

If anyone has any suggestions for me please let me know. Thanks!

David