14 replies to this topic

[printz]

hello there!

 

I'm trying to model an actual reciprocating compressor that compresses air at 400 bar with a volumetric flow of 0.45m³/h with 4 interrefrigerated compression stages. 

I have assumed the four compression ratios to be the same (beta_stage=400^1/4) 

problem is I read in this forum the maximum allowable temperature in the whole compression process should not exceed 300F (roughly 150°C) mainly to prevent oil self combustion.

The exiting temperature from my compression stages is always above 250°C (470F). 

the formula I used to calculate the final temperature is

 

 Tout= Tin*((beta_stage)^(epsilon/eta))

 

where epsilon and eta are polytropic exponent and efficiency (0.4(=R'/Cp(T)) and 0.86 taken from an iterative cycle and literature respectively). Is there anything I am missing?

 

here comes the second problem : I assumed the intercooler efficiency to be 0.90% (since the website states that the "outlet temperature is ca. 10-15° above the temperature of the cooling air flow"). I know this intercoolers are made of 7 stainless steel coils cooled by a fan. is this a realistic assumption?

 

many thanks in advance for your time.

ps: I am willing to submit the matlab files of my simulation if it can help 

[Art Montemayor]

Pierpaolo:

 

I don’t know the reason you are “modeling” an air compressor – especially one with what appears to be one with a needle for a piston.  You fail to identify the location of the volumetric flow of 0.45 m³/h, so I have to assume that it is the actual suction to the 1^st stage (as is the custom).  You also fail to state your suction pressure and temperature at the 1^st stage.  I have to assume you are compressing atmospheric air.  If so, then you have an overall compression ratio = (401/1) = 401.  You also fail to identify your pressures as gauge or absolute, so I have to assume you mean gauge – as is the custom in industry.

 

Your difficulty and main problem is in not understanding the thermodynamic concepts and theory of gas compression.  Like the majority of today’s engineering students, you are depending on software like Matlab and simulation programs to do your logic and thinking for you.  This is totally wrong engineering and will never succeed in obtaining the correct resolution to a practical problem.  You must first dominate the theory and concepts of what is occurring in a reciprocating gas compressor.  You must understand and accept the fact that a reciprocating compressor follows an isentropic process closely – and NOT a polytropic process (which is the domain of a CENTRIFUGAL COMPRESSOR).  Please do not take my corrections or even my criticism as a negative response to your request.  On the contrary, I take this time to try to salvage what you have done and steer you in the right engineering direction which is the path that will take you into the engineering profession if that is what you strive for.  Refer to my attached Excel workbook for a sample calculation of what you should be doing with this problem.  Note that I use a logical, sequential algorithm complete with references so that my work can be easily and accurately checked and confirmed as being correct in theory and application.  Neither Matlab or Hysys does this.  They can’t because they are simply very stupid software.  They need accurate and logical data input in order to crunch the correct numbers and spit out the correct calculation.  They can’t think like you are capable of doing with your God-given brain.

 

I don’t understand what you are trying to ask in your second question.  I suspect that you are trying to identify the suction temperatures of the succeeding stages of compression.  If so, then bear in mind that you have to identify what your cooling medium will be in your interstage coolers.  Knowing that, you can then approximate an approach temperature to the cooling medium.  If you have cooling water available at 30 ^oC it is safe to assume you could have 35 ^oC suction gas to the succeeding compression stages (assuming you have well-designed intercoolers).

 

If you are going to do your studies in gas compression and take an interest in doing it correctly and learning a lot at the same time, I highly recommend you obtain a copy of the GPSA Engineering Databook – which you can purchase through the internet.  If you are going to be a professional engineer, you will never regret the money you spent in obtaining this tome.

 

I hope this experience helps you out.

 PierPaolo Air Compressor.xlsx   10.66KB   148 downloads

 

[curious_cat]

 

especially one with what appears to be one with a needle for a piston.  

 

That should be one for the hall of fame! 

 

You made my day. 

[thorium90]

Haha. Evil cat...
Probably a typo on the OP's part. Maybe it was supposed to be 0.45m3/s or something

[printz]

good morning and thanks for the reply. I'm not taking your response badly at all, I'm a student and I need to learn. I hoped you would suggest a book and I'm gonna rent it later on today while I'm waiting for its delivery. 

 

I am indeed an Italian (sorry for the bad english) engineering student, I'm trying to model a compressor for dissertation purpose (I'm trying to model a Compressed Air Energy Storage system of small size, so I will need a model for a compressor and a model for a pneumatic motor too).

 

the compressor I'm trying to model it's a real one: the verticus 5 V15.11-11-5 made by BAUER (specifications on http://www.bauer-kom...uper_silent.php).

the air flow was indeed a typo, it is 0.45m³/min, I'm sorry about that. it is compressing atmospheric air, so all of your assumptions are correct.

 

I have seen the compressor and it is air cooled by a fan blowing air on 7 coils made of stainless steel, I'm attaching a picture where you can see both the coils and the fan. and I am trying to figure out the suction temperatures of the succeeding stages but I was struggling to identify the intercooler efficiency.

 

I have quickly looked at your excel file, where you suggest a 6 stage compressor should be used. does it mean that this specific compressor is destined to fail due to high temp?

 

I'm gonna have a better look at your excel file and the book too and I hope to come back to you with a little more knowledge.

thanks again for your help,

Pierpaolo

 

Attached Files

•  compressor.jpg   65.08KB   6 downloads

[PaoloPemi]

PierPaolo,

have you been in contact with technical office of manufacturer ?

[printz]

I tried but but I didn't get any answer

[PaoloPemi]

if you are working on a real application case (not a virtual project)

yu should contact them to be sure that these packages

are good for your application and know the tested performance.

Edited by PaoloPemi, 02 October 2013 - 06:58 AM.

[curious_cat]

I tried but but I didn't get any answer

 

Not surprising. Sometimes they are overwhelmed with student inquiries. 

[Art Montemayor]

 

Pierpaolo

 

Thank you for the timely response.  Your positive outlook and candid response supports my good opinion of and support for all things Italian.  I am a great fan of the Italian culture, heritage, and the many friends I have.  Do not apologize for your use of the English language; it is quite good and far better than that written by many of my fellow Americans (I’m sad to say).

 

If you can, also try to obtain a copy of:

 

“Compressors Selection and Sizing”

2^nd Edition

By Royce N. Brown

Publisher – Butterworth-Heinemann

Woburn, Massachusetts USA

ISBN 0-88415-164-6

 

The Chapter of interest to you would be “Reciprocating Compressors”.  There you will find a definition of the main parameters and design factors that enter into the design of a reciprocating compressor, such as:

• Cylinder diameter;
• Stroke;
• Rpm;
• Displacement;
• Clearance;
• Volumetric efficiency;
• Single acting vs Double acting cylinders;
• Discharge temperatures;
• Compression Ratios;
• Horsepower;
• Etc.

For a multi-stage compressor compressing a saturated gas (for the worse case condition you have to consider that the atmospheric air you are drawing in will be 100% saturated with water vapor) you will have to take into account that each intercooler and the final aftercooler will have to not only cool the compressed air, but the contained water moisture will also undergo cooling and some condensing.  The heat transfer taking place in each of the intercoolers and the aftercooler will have to take that into consideration and you will also have to ensure that there is adequate phase separation after each intercooler as well as the aftercooler in order to separate out the condensed water moisture.  Additionally, if your cylinders are oil-lubricated (which is the norm for this type of high pressure compressor) then you will also separate out any oil droplets in the separators that follow each of the coolers and you must allow for routine drainage of such liquids.

 

If, by “modeling”, you mean that you are going to simulate the continuous operation of a Bauer multi-stage air compressor then you are going to need all of the design and final dimensions of the Bauer compressor.  Am I correct?  If so, then there is only one way that you can obtain such technical information (besides purchasing a complete compressor package) – and that is by soliciting that information from Bauer Compressors (hopefully the Italian subsidiary company in Nogarazza, outside of Vicenza (refer to my attached Rev1 of the Excel workbook).  As some of our members have inferred, you are probably going to find resistance to obtaining free technical information from Bauer.  Again, go to the attached workbook; I have included all of the technical information that Bauer puts out in their website – and it isn’t very much.  This is not unusual for such a small compressor package where there isn’t much of a profit margin for the manufacturer.  You will note that they make a 5-stage unit that I would select as the most appropriate for what you describe as your process conditions.  You will also note from the workbook that the compressor is V-belt driven.  This means that you can take the V18.1-11-5 model and slow it down from 1,140 rpm to the speed corresponding to the capacity you need by simply changing the size of the sheave(s).  I believe you will find that the 5-stage model will yield a discharge temperature for each stage above the recommended 300 ^oF maximum.  What you can do to alleviate this potential hazard is apply a synthetic oil lubricant that is more tolerant of high temperatures.

 

There is more than one way to skin a cat and I trust Italian ingenuity can be applied by visiting the existing Italian subsidiary of Bauer and frankly explaining how you are a struggling student trying to simulate their compressor and give publicity and recognition to their compressor packages in your dissertation.  Additionally, you can tell them that all work generated by you would be freely shared with them for their use.  But whatever the story, try to win their friendship and cooperation in your project.  Believe it or not, winning over the willingness and cooperation of other individuals is also a large part of engineering – especially when you are a project manager.

 

I hope this helps you out and lets us know if there is anything else we can help with.

 

 PierPaolo Air CompressorRev1.xlsx   461.17KB   76 downloads

[printz]

thank you very much for your help.

this is a lot of information to digest (I thought I was almost through with this work), I will have a lot of stuff to study, I'll come back to you as shortly as I can

 

I guess the only way to know the pressure drops occurring in the coolers the separators and so on is to get them from BAUER isn't it?

I'll keep you posted in the meanwhile!

thanks again

Pierpaolo

[Art Montemayor]

Pierpaolo:

 

Yes.  If you are indeed modeling an existing machine, you have to resort to using the existing data that relates to that specific machine.  Otherwise, you are not modeling; you would be merely theorizing.  And Bauer is the only source for such data.

 

Agreeably, even the Italian Bauer representatives may not have all of the design data.  But their estimates (or best guesses) are better than anyone else's.  You will want to vary such parameters as cylinder clearances, rpm, volumetric efficiency, suction temperatures, final discharge pressure,  suction air humidity, etc. and find out their effects on the machine and its driver.  Therefore, you will need to have some of this data (including such items as intercooler and interstage separators' pressure drops) in order to have a proficient model.

[printz]

good afternoon everybody.

I did some research, and looked over the books you suggested. I'm a little confused though.

 

you said that reciprocating compressors processes should be addressed as isoentropic: does that mean I should use ONLY the isoentropic efficiency to account for friction and losses when computing the work needed?

and I should use the same efficiency to calculate the real outlet temperature, is that correct? or should I use it only once?

T_{out_real}=T_in+T_in*(r^(k-1)/k-1)/n_is

and calculate the work done using this temperature without the efficiency?

W=c_p(T)*(T_{out_real}-T_in)

or use the efficiency again?

W=c_p(T)*(T_{out_real}-T_in)/n_is

or should I use the overall efficiency (which I would determine as isoent_efficiency*mechanical_efficiency*electric_efficiency)for the work calculation?

W=c_p(T)*(T_{out_real}-T_in)/n_overall

 

another question: I'm using a temperature dependent formula to calculate the specific heat for a constant pressure

c_p(Tin)=1057.69-462.92*(T_in/1000)+1182.59*(T_in/1000)^2-853.11*(T_in/1000)^3+198.80*(T_in/1000)^4 

is there anything similar to calculate Z (compressibility factor)?

 

thanks again for the support

Pierpaolo

[Bobby Strain]

You can go to the Ariel website and download their compressor program. It won't calculate anything this small, but it will generate properties and simulate water condensation between stages. It's free, too.

 

Bobby

[printz]

thanks for the reply. unfortunately they do not accept non-corporate-mails to register to their site and one should be a member to download the tool you suggest. plus I need to be able to determine this kind of properties recursively in order to integrate them in my code