3 replies to this topic

[Amr Mohamed]

Hello everyone,

I will go through explaining my point directly...

 

I'm working on a project now for conditioning of C3/C4 Feed to remove sulfur mercaptan (MEROX Unit). We get an inquiry from our client that Feed coming temp. may be reaching 115 F which is unacceptable for Merox unit specs. (Inlet Temp. is 95-105 F or there will be additional heat duties requirements) so we are going to design/size single stage propane refrigeration cycle to cool feed (C3 or C4) from 115 to 95 F.

 

Actually its the first time for me to work on refrigeration cycle and herein my procedure/trials/Way of thinking for designing refrigeration cycle (Followed by my Questions):

 

Objective/Constrains/Basis of cycle: Cooling Feed (C3 case and C4 case) from 115 F to 95 F

                                                    Propane will be used (97 mol%) as Refrigerant in single stage refrigeration cycle.

                                                    Design reboiler duty required will be 21.8 MMBtu/h (Based on C3 Feed case)

                                                    Screw Oil Flooded Compressor will be used.

Procedure:

1. Assume 20 F temp. approach in Reboiler so C3 refrigerant upstream of reboiler will be at 75 F.

2. Dry Air bulb Temp. is 94 F, assuming 15 F temp. approach so Condenser outlet conditions will be sat. Liquid @ 109 F.

3. As its not pure C3 so evaporation isn't occur on constant temp. so instead of defining evaporator outlet conditions as sat. Vap @ 75 F, I specified outlet conditions as Sat, vapor and specify pressure drop through kettle shell side as 0.1 psi.

 

For my understanding degrees of freedom for the loop is now zero and can be solved ! Attached my hand calc. using P-H diagram for C3 (Results will be slightly changed when using actual composition for refrigerant) to get C3 mass rate, Compressor power, Air cooler Fan power and %sat. Liquid if Reboiler upstream.

 

4. Finally, start adding pressure drops for Air cooler (5-10 psi), piping bet. compressor and Air cooler (5 psi) and piping bet. Reboiler and Compressor (5 psi) and run simulation.

 

Questions based on above case (constrains/basis):

1. Procedure above for solving problem is right or needs to be modified?

2. I read something about adding a dummy exchanger - it's not real, in warm climates, system will gain heat from environment before it gets back to compressor. Amount of heat will depend on piping length, insulation type/extent, ambient conditions/Refrigerant temp. ?? Did I need to count for that ? or it will be done in detailed design or Vendor will take care or that ? On contrast, temp. will be decreased due pressure drop through piping bet. Evaporator and Compressor So to be conservative assume delta P with constant temp. (temp. reduction through piping will be balanced with heat absorbed from outside environment) ?

3. When using Oil flooded Screw compressor; I don't know if I'm true or not; Circulated Lube oil will absorb amount of heat so discharge compressor temp. will be decreased which will reduce power needed for Air cooler fans ?!! If that's true, how can I count for that ?

4. Oil Flooded Screw compressors; Is there any kind of equation to calculate its efficiency or just assume 60% will be fine?

5. I don't have such experience about which is more better (on cost and delivery time basis) to ask vendor supplying whole package or order each equipment separately? and if separately who will guarantee cycle performance, Compressor vendor?

6. This cycle will be operated based on Feed coming temp. if its below 95 F so it won't work, So typically how long compressor will need to startup?

7. I know York and Salof as a vendors, Are there other famous vendor names?

 

General Questions (In other words by removing constrains and thinking backward about basis itself !!)

1. Did I need to consider using Butane as refrigerant and see its effect? (as its Mwt is higher than C3 which might reduce compressor power)

or C4 cycles isn't commonly used?

2. In above case, compression ratio is less than 2 so single stage is sounding reasonable.. Just for my knowledge what if I want to design 2 stage refrigeration? (may be you can consider same case with using C3 at -40 F) How to define mid point compression? What I will need to add on procedure to solve such problem?

3. I heard something about cycle optimization, what exactly I will need to do in order to optimize cycle? (Can you recommend some useful articles talking about optimization)?

 

Thanks a lot and Sorry for this long mail

Your quick response is highly appreciated !

 Hand Calculations_Single Stage C3 Refrigeration.pdf   222.09KB   139 downloads

 

[Bobby Strain]

You really should have vendors supply the whole system, including the chiller and refrigerant condenser. So give them a duty spec along with design ambient conditions. You might want to look at a swamp cooler, too.

 

Bobby

[Art Montemayor]

Amr:

 

You are making your assignment overly complicated and you are getting all mixed up with a simple refrigeration cycle.  You do not have to write a book to explain this simple engineering application.

 

Simply resort to generating good, clear, professional-grade  sketches of your proposed system.  This is part of engineering because it is the accepted and simplest method to communicate with other engineers (and suppliers as well).  Your T-S chart and calculations are not professional at this date in life.   That's what we did 50 years ago.  Today, you have a spreadsheet workbook to do all your calculations, estimates, filing, sketches, and graphs.   You have to use Excel - just like the attached workbook I did for another student recently.  I am supplying you with this head start (when in reality it should have been your obligation and job to do it) to stop you from writing a book and saving a lot of useless time writing posts on this forum.  Use this workbook to label your streams just as I have started them for you.  Then study the calculation algorithm and the actual calculations and done for another propane refrigeration project.

 

I don't have the slightest idea of what you call a reboiler.  Look at the refrigeration cycle on the workbook.  There are 4 major components for any mechanical refrigeration cycle:

1. compressor;
2. condenser;
3. expansion valve; and
4. evaporator.

There are no reboilers.  Once you arrange this workbook to suit your application, return to this thread and upload your version as Rev1 of the workbook.  We will then at that time address all your questions and concerns and mark up your workbook with revisions in order to keep track of the work as it progresses.  That is the engineering way.  I refuse to work in any other, disorganized manner where we have to manually check all calculations.  Organize your work correctly and professionally.

 

Additionally, you never use 95% propane as a refrigerant.  You have to use R-290.  If you don't know the difference, look it up in Goggle or Bing and find out all about Propane refrigerant.

 

I will await your reply and return with the work you have done.

 Propane Single Stage Refrigeration.xlsx   875.81KB   187 downloads

[shan]

Where is the plant located?  Based on my knowledge, cooling water is able to lower process stream temperature to 95F in most parts of the world.