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Refrigeration Turn-Down Capacity


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#1 CHEMENGR2811

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Posted 26 October 2018 - 05:30 AM

Dear sirs,

 

We had a 38 TR -40°C refrigeration screw compressor. Refrigerant is Ammonia. We would like to operate the same compressor at -10°C instead of -40°C. Now my doubts are

 

1) Is the compressor capacity is 38 TR or not after operating the compressor at -10°C  ?

2) How to find out the Motor shaft load of changing the temperature from -40°C to -10°C?

 

Plz guide me for the above doubts.

 



#2 shan

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Posted 26 October 2018 - 08:34 AM

I guess TR = Ton Refrigeration (1TR = 12,000 Btu/Hr).

 

1. If you achieve -10 C by reducing the compressor discharge pressure, you will not have 38 TR chilling capacity.

2. You may check the difference of compressor motor power consumptions to figure out the shaft load change between - 40 C refrigerant temperature and -10 C refrigerant .   



#3 Art Montemayor

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Posted 26 October 2018 - 01:56 PM

CHEMENGR2811

 

These questions are, in my opinion, student type of questions and are not a matter of “guiding” you through the answers.  If you are a practicing engineer (I am assuming you are, since you are posting in this Industrial Professional Forum) and not a student, then I believe the best way to assist you in these questions is to advise you to inform yourself in the basics of ammonia mechanical refrigeration which can be found in many text books.  Additionally, you should have a complete and firm understanding of the basic Thermodynamic theory that serves as the basis for the ammonia refrigeration cycle.

 

Please refer to the attached workbook and advise me if you understand what I’ve presented.  I have developed and given tutorials in the past on the subject and I can offer you this piece of information to allow you to learn and understand the following:

  • The “TR” of a refrigeration system is the “Tons of refrigeration” applied by the designed system.  It represents the amount of heat removed from the refrigerated product in the related ammonia evaporator.  In the USA, it is equal - as Shan states - to 12,000 Btu/hr.  It is defined as the rate of heat transfer that results in the melting of 1 short ton (2,000 lb) of pure ice at 32 °F in 24 hours.
  • Unfortunately, the remainder of Shan’s comments is erroneous.  You cannot reduce the compressor’s refrigerating capacity - as he infers - by reducing the compressor’s discharge pressure.  The discharge pressure has nothing to do with the refrigerating capacity obtained at the ammonia evaporator.  As clearly seen in any Thermodynamic explanation of the mechanical refrigeration process, the compressor discharge pressure is fixed by the condensing temperature of the refrigerant.  Given a fixed condensing temperature, the ammonia compressor’s discharge pressure will be the same at any downstream evaporator refrigeration loads - regardless of what they may be.

Briefly, I address what I believe are your basic questions:

  1. You can reduce the amount of refrigeration obtained by your ammonia system.  To do this, your system must be capable of maintaining and controlling a higher evaporator pressure (approx. 2.9 barA) instead of the present one (approx. 0.715 barA).  Refer to the attached workbook and see the saturated pressures corresponding to your mentioned saturated temperatures.  We don’t know how you presently control your evaporator pressure at 0.715 barA, so we can’t tell if you can adjust the controls so that you get the desired 2.9 barA.
  2. To identify the required new motor HP you calculate the required electric motor load for the lower refrigeration load by doing the appropriate calculations.  Again refer to the workbook to get a hint of how this is done.  You will be doing less refrigeration work with the evaporator operating at a lower refrigeration load, so your present motor should work OK.

However, before you go about and start changing the load on your ammonia system I highly recommend you contact the manufacturer and designer of your system and request their advice and consent that you can lower the refrigeration load with the equipment and instrumentation that you presently have.  Basically, you must control the evaporator at a higher saturated pressure.  Since I don’t have the detailed P&ID nor do I know who designed, fabricated, and sold you the equipment, I can’t be specific on what you can and can’t do.  First, get the approval of the designer/fabricator of the equipment after you have thoroughly studied and understood exactly how your refrigeration system works and how to control it.

 

Attached File  Ammonia Mechanical Refrigeration.xlsx   553.42KB   19 downloads



#4 CHEMENGR2811

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Posted 29 October 2018 - 12:54 AM

Sir,

Thanks for comments by Art Montemayor sir & Shan sir,

 

Im a Practising Process Engineer in a Pharma company ( Chemical Engr) . First time im studying the different type of problems in Refrigeration department. That's why posted  a question to get the suggestion from our Chemical Engg forum.

Thanks

CHEMENGR2811






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