Student Project (Cascade Refrigeration System)

[gth724a]

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

[NoNickName]

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)

[gth724a]

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?

[NoNickName]

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

[tctarunchauhan]

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

[rockon605]

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.

[desA]

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

[pal.of.pinaki]

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.

[jonbonraki]

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) .