John Anton

20-04-2016, 02:24 PM

Hello guys!

I'm an undergraduate student and this is my first post in the forum, please bare with me.

I'm trying to achieve a simple design of a cascade refrigeration system. The required fluid temperature on the low temperature side is -40 C and on the high temperature side the condenser temperature, on the shell side, varies between -20 C and +25 C with an yearly average of +8 C. I'm considering using a CO2/NH3 cascade system and are know looking at design temperatures for the CO2/NH3 heat exchanger and landed on -7 C / -12 C. Is this resonable temperatures?

Second question, how do one design temperature and pressure after the compressor? In the case above with CO2 on low temp side I have a suction temperature of TL1 = -40 C. The goal condenser temperature TL2_goal = -7 C, corresponding to a condensation pressure of 28.83 bar. To calculate the temperature after the compressor I used TL2/TL1 = (p2/p1)^((k-1)/k) resulting in a T2 of 20 C. And to calculate the work done: W = cp (TL2 - TL1). Assuming isentropic conditions is this the correct procedure? If I instead do the calculations in reverse order, assuming T2L=T2L_goal, I would have a p2 not corresponding to the condensation temperature sought. Any advice is appreciated!

\John A

I'm an undergraduate student and this is my first post in the forum, please bare with me.

I'm trying to achieve a simple design of a cascade refrigeration system. The required fluid temperature on the low temperature side is -40 C and on the high temperature side the condenser temperature, on the shell side, varies between -20 C and +25 C with an yearly average of +8 C. I'm considering using a CO2/NH3 cascade system and are know looking at design temperatures for the CO2/NH3 heat exchanger and landed on -7 C / -12 C. Is this resonable temperatures?

Second question, how do one design temperature and pressure after the compressor? In the case above with CO2 on low temp side I have a suction temperature of TL1 = -40 C. The goal condenser temperature TL2_goal = -7 C, corresponding to a condensation pressure of 28.83 bar. To calculate the temperature after the compressor I used TL2/TL1 = (p2/p1)^((k-1)/k) resulting in a T2 of 20 C. And to calculate the work done: W = cp (TL2 - TL1). Assuming isentropic conditions is this the correct procedure? If I instead do the calculations in reverse order, assuming T2L=T2L_goal, I would have a p2 not corresponding to the condensation temperature sought. Any advice is appreciated!

\John A