Design temperatures in a cascade NH3/CO2 system

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

This sounds very much like an assignment ;)

for an application where you need -40 on the low side, a simple ammonia system would have done, running on -0,3 bar on the suction, and 7-10bar on the discharge (18-28 C), I wouldn't have complicated it unless I needed to get the temperature down to -50/60C ;)

Other than that, best of luck on your assignment :)












(oh and pay attention in class ;))

Hi Tycho and thanks for the reply!

I'm trying to design a state-of-the-art system and thought that a cascade CO2/NH3 would do the job in the temperature interval. It is a quite large project but the focus have not been on the system components but rather the system's interaction with the surrounding energy system, looking at energy prices and thermal storage. However I would like to make an fairly good model of the compressors to be able to decide which loads to run then on. I forgot to mention that there are refrigeration loads on the high temperature side as well and one of the reasons for CO2 is that it is used for short term low temperature thermal storage in a receiver.

How did you come up with the pressures and temperatures above?