Aaron:
You fail to submit a Process Flow Diagram (PFD) of the mechanical refrigeration cycle that you are working with. This is what you should do first in order to fully explain your queries. Kabtik shows you a very simple block diagram, but what you need to supply is a PFD with the flow conditions at each location within the cycle. A drawing is far better than your written explanation. For example, we don’t know if you are using an economizer or not.
What I understand is that you are to determine the Propane refrigerant flow rate within the cycle (you don’t say where in the cycle). To calculate this, you need to know the evaporator pressure or temperature on the refrigerant side as well as the duty the evaporator handles. Lacking the duty, you have to calculate it using the process fluid’s (ethane) flow rate and the difference in temperature it undergoes. What doesn’t make for good sense is that you claim you can’t calculate an “accurate” answer to the ethane flow rate. There can be only ONE answer when done properly. How it that you know your answer is wrong? Where did you get the “right” answer? We can’t comment on your “answer” if you don’t submit your detailed calculations (in an organized and orderly electronic spreadsheet) we can’t check them or comment on your inability to get the “right” answer.
I have developed detailed spreadsheets calculating various refrigeration cycles – including many propane examples – in the past and have uploaded them here in our Forums on many threads. To obtain this information all you have to do is use our SEARCH engine, study the threads and download all the workbooks. One such example is found at:
http://www.cheresour... +refrigeration