Student Project (Cascade Refrigeration System)

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

I'm not sure what exactly you input in the calculation, but I'm pretty confident that 58°C at compressor discharge is quite normal, provided the condensing temperature is subcritical (let's say around 50K discharge superheat)

ok, thats good to know, i wasn't sure.

But now, my problem is that when I plot the COP vs. the cascade temp for a specific set of design conditions, the COP just increases almost linearly without a limit. Any ideas?

That's also normal. As the evap temperature increases, the COP increases as much, and the reason is the higher mass flow at suction together with a better volumetric efficiency. Please note that the evap temperature in a real case scenario is bound to the physical constraint of the testing setup

hey buddy may i have ur analysis on EES..??? if u dont have any prob plz send it to me....i thnk i m working on somewhat similar project so i need ur help

is there any way to plot property curve (say T-s diagram) for two different fluids on same graph, i.e. overlaying one's property plot over another in 'EES'?
if anyone knows plz reply.
actually i need combined T-s diagram for ammonia and ethane on one plot.

Coolpack can assist. It's a free download based on the EES platform.

Hey guys,

I am also working on a project based on CO2 cascade refrigeration systems......if anybody can send me the code pertaining to the optimisation of cascade system in terms of intermediate temp.(IT) and overlap temp(DT) I would be highly thankful.

I am also working on a project based on CO2 cascade refrigeration systems......if anybody can send me the code pertaining to the optimisation of cascade system in terms of temp(DT) .