8 replies to this topic

[Rudra]

Hello everyone,

During one of my projects, i came across a condenser design operating at low temperature of the range of -50 C.

I am not familiar with the refrigeration loop. Please help me in understanding it.

What i understand is this:

1. Refrigerant is evaporated at low pressure thereby condensing the process vapor. The pressure of evaporation is decided by the process requirements

2. The evaporated refrigerant is compressed, condensed at higher pressures. How this pressure is decided. I suppose the compressor outlet pressure is decided such that the vapor can be condensed using available cooling media cooling water, chilled water or brine.

3. The condensed liquid is then sent to expansion valve where pressure is reduced and hence the temperature is also reduced to the evaporation temperature. The liquid is recirculated back in the evaporator.

Is my above understanding is right? Presently i am working on a system that involves ethylene as refrigerant. I tried to do simulation as described above. Attached here is a sketch showing my simulation parameters. Please guide me.

Regards,

Attached Files

•  querry_about_refrigeration_loop.xls   895.5KB   320 downloads

[Art Montemayor]

Rudra:

A quality, well written and articulate query deserves a detailed response. The quality of the response should be proportional to the quality of the query.

Please refer to the attached Rev1 of your excellent submitted illustration of your query and comments. I hope I have detailed out all – or most – of your concerns and questions.

I would not attempt a simulation of such a cycle unless I thoroughly understood and dominated the concept and principles behind the process. The un-witting use of simulation to substitute for human logic and ingenuity is a grave and silly mistake being undertaken everyday by more and more engineering students and young graduates. This is an unfortunate and often serious flaw in their engineering preparation and learning. Process simulation is a tool – not an answer - to an engineering problem. A simulator is nothing more than a fancy hand calculator that is incapable of thinking or using the necessary logic and expert ingenuity that only the human brain possesses.

Note that I have arrived at the same answer of 66% ethylene vapor formed by isenthalpic expansion through an expansion valve. The calculation is found in the ethylene thermodynamic properties tables that I have included in the workbook revision for your use and learning.

I hope that I have succeeded in explaining the refrigeration basic principles (although, not all of them) and that I have addressed all of your concerns. In the future, please NEVER use a pressure value without labeling it accordingly as a gauge or an absolute one. I have assumed you meant to use barA (absolute pressure) and not gauge. This is very important when dealing with thermodynamic databases. As you can see from the NIST database print out tables, it makes a big difference when trying to interpret the properties of the refrigerant – especially in the evaporator.
 Refrigeration_Loop_Query_Rev1.xls   992KB   325 downloads

[Rudra]

Dear Art,

First of all, i thank you very much for sparing time and for giving such a nice and elaborated response for my query. It gave me good understanding of a refrigeration loop.

I agree that the concept itself was misunderstood by me.

Regarding the pressure units, i fully agree that at this point of time in my career i should not do such a silly mistake of pressure units. I will keep this in mind from next time.

Thank you very much

[Rudra]

Dear Art,

I have followed your guidelines and done some calculations for my case.

I am working on a feasibility study where the process column conenser temperature is -51C.

I have done calculations for ethylene refrigerant and are attached in the spreadsheet.

The power requirement and utility consumption which i am getting is too high and with these numbers, the project becomes non feasible.

Please take a look and let me know if i am wrong somewhere. If the calculations are right then the operating cost i am getting is 40 Rs./TR.

I have not done calculations for a refrigeraation loop before and hence a bit confused with the numbers i am getting.

Please guide me

Attached Files

•  querry_about_refrigeration_loop_Rev_2.xls   895.5KB   135 downloads

[Art Montemayor]

Rudra:

I have looked at your Rev2 workbook and I don’t find any calculations that you may have done. All I’ve found is a result of what appears to be a computer simulation run – which are not calculations done by you.

If you have done any calculations, please show them or submit them if you want me to review them and guide you. No one can guide you when all you submit is the results, but not the calculations – or even the algorithm employed to obtain the stated results.

I have done some calculations – which I include in the attached Rev3 workbook – and they are self-explanatory since I employ Excel and show all my references and assumptions. These are the quality of calculations that can be reviewed and analyzed as to correctness and accuracy.

Your confusion may be due to the fact that you are obtaining results from some calculation done by a simulation program and which are not explained nor do they reflect the logic and reasoning behind the algorithms used to obtain the answer.

Note that I did not calculate the compressor power requirement nor the Condenser duty. I simply do not have any basic data regarding the compressor used. I don’t know if it is multi-staged, centrifugal, or reciprocating. Therefore, I can’t calculate the discharge temperature. If I can’t calculate the discharge temperature, I can’t calculate the total condenser duty. I hope you are becoming aware of how critical it is to always furnish COMPLETE basic data when submitting your engineering work and calculations for review or checking.

My preliminary calculations reveal that your results are probably correct with respect to the refrigeration duty on the Evaporator. Note that my results are done based on a different Evaporator pressure because that is the thermo data that I generated for what you originally gave us. If you use the conditions you now state, you will probably find that my method is as accurate as your results given.

Without ALL the basic data and your calculations, there is not much more I can contribute to your application.
 Refrigeration_Loop_Query_Rev3.zip   275.31KB   166 downloads

[Jiten_process]

Rudra,

i have done once refrigeration process calculation for one of my projects where we have to supply chilling plant approx 50 TR. And i did not use any simulator, just as art said manual excel calculation.

Find calculation sheet attached, In first sheet ('DATA') of the workbook you have to provide all the basic refrigerant data needed for refrigeration calculation and u can see the final result in summary sheet, you can refer the steps and assumption of calculation in calculation sheet. This sheet is for ammonia refrigerant however if you provide all the relevant data of other refrigerant it will work for it too. Use this sheet to cross check your result with simulation output. I have used this sheet for estimation purpose.

see whether it appears useful to you or not.

good luck...

Attached Files

•  refrigeration_calculation.xls   50.5KB   182 downloads

[Art Montemayor]

Jitan:

I have not had too much free time to thoroughly review and check out all of your calculations on the ammonia cycle you submitted. However, I have some comments which I have attached and also have added a NIST database worksheet on Ammonia that is far more detailed than the one you supplied.

Basically your refrigeration effect calculation is essentially correct. However, you fail to give a PFD and Mollier diagram to define your cycle and therefore, your calculations are very difficult to follow. Besides, you have made some errors in your calculations which I have noted. Additionally, some of the write-up is confusing. Do you mean to say "Refrigerating (or Refrigeration) Effect"? If so, then your calculation for the same is wrong.

Your calculations and the method employed are basically very sound, except that there may be some wrong understanding of the refrigeration cycle. The workbook needs further work and improvement.
 Refrigeration_Calculation.xls   156.5KB   163 downloads

[Rudra]

I used the calculation sheet to compare my simulation results. They are almost similar except one thing.

THe compressor super heat is assumed as 5 C which may be correct for ammonia. In my case degree of superheat at compressor outlet is very high.

But except this, compressor power and condensing load is matching.
Thanks for the spreadsheet

[Jiten_process]

Dear art,

thanks a lot for deep review. Actually this sheet was not meant to be used as a workbook for generalize case. I knew the refrigeration cycle so i did not incorporate it. Ya, but there are mistakes in writing as u pointed out, however the values are correct coz when i cross checked compressor power, compressor outlet temp, compressor suction flowrate and condenser design data with vendor's offers it was exactly matching so then i didnt look further to correct my mistakes and prepare esthetically good spreadsheet. Also this sheet used assuming no economizer. If economizer introduced further rigorous calculation required to be performed.

I have given asnwers of your comments in attached sheet. And now i will try to incorporate more detail and correct information in it. One thing also i wanted to incorporate is COP(coefficient of performance) which is very much important as far as refrigeration cycle is concern.

Rudra,
I think you have misinterpreted suction super heating temp. Actually 5°C was not compressor superheat. Superheated compressor outlet temp. needs to be found out which i did in my calculation sheet assuming adiabatic compression.

5°C is a compressor suction superheat on above evaporating temp. Refrigeration cycle was designed for 5°C( or less) super heating of refrigerant vapor to ensure no liquid particle in to the compressor. But this is not the rule i suppose.

Hope it might helped u....
Good luck...

Attached Files

•  Refrigeration_Calculation_REV_1.xls   157KB   231 downloads