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  1. #1
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    capacity problem



    Hi guys,

    I am presently working on a 52 tonnes freezer -4dF, holding charge. I was not responsible of the capacity, it has been given by the customer. But I check and it maybe a bit short, because my load calcuation are of 55 tonnes, but it is not a major difference.

    There is the result received from the field:
    R404a
    room temp 1.2dF
    succion main header 18.2psig
    compressor succion = 15 psig
    superheat main header 10dF
    succion pressure at evap 21psig
    outside temperature= 46.5dF
    Liquide temp after drier=86.3dF
    Liquide temp in evap= 76dF
    In temp condenser= 122dF
    out temp condenser= 87.6dF
    Discharge pressure= 208psig

    The evap are supposed to give me a 7.5dF, at 8 dF superheat.
    Parrallele of 4 open type compresser. and 4 evap.with 2 circuit each, and about 140 ft of line. the line is calculated at one psig drop per 100ft.
    I have a flocon valve for the hotgas defrost back to the succion, which desigh give me a 3 psig pressure drop. My compresseur are about 40ft lower than the evap..

    I have a accumulator for each compressor, and a complet oil control system.

    The major problem are my compressor are not sollicitated, when the 4 evap are feeding, one is often stop are run unloaded. The compressor were design to give 52 tonnes at 6psig pressure drop.

    I am not able to take the pressure at the inlet of evap, but I wonder if I could have flash gas because I only have 4dF of subcooling after the drier. The liquid line is cooled down in the freezer but I have a rise of 34 ft on the liquid line.


    Kathleen, P.eng
    www.refri-ozone.com

  2. #2
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    Hi, Kathleen
    I have read your post a number of times, it's stsrting to make sense, but what is the flowcon function, a crankcase pressure regulator.
    As for the flash gas if it was the problem the superheat at evap outlet would be higher, as the gas would take up space in the liq line that liquid should take, reducing the feed to the expansion valves and increasing the superheat. Is the superheat the same at each evap outlet, measuring the common header superheat will not tell you if one evap is not performing.
    Also you say the compressors have been sized at 6psig suction line losses, but you have not said what the pressure is at the compressor inlets, maybe the pressure drop in the suction line (inc flowcon) is so great that you are stepping the compressors of too soon. You can either reduce the suction pd or lower the copmpressor set point and make then run to acheive greater duty.
    I take it you are trying to acheive a room temp of-4df not 1.2df.
    I would investigate the pd through the flow con it may be under sized.
    Hope this helps. Regards. Andy.

  3. #3
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    Andy,

    Flocon valve is a parker succion dual pressure regulator valve, that regulate my defrost pressure cycle. I already verify with the manufacture and the valve is supposed to be well sized. They are supposed to give 3psig pd for a 12 tonnes capacity. One per evap.

    I should have my superheat at the evap tomorrow morning, because my mechanics are back to the project.

    My succion compressor is 15psig, I will have then check the pressure at inlet of each compressor. Total presure drop =6psig


    Kathleen
    Kathleen, P.eng
    www.refri-ozone.com

  4. #4
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    There is the result received from the field:
    R404a
    room temp 1.2dF
    succion main header 18.2psig
    compressor succion = 15 psig

    ---------------------------------

    So there is some pressure drop through the accumulator? Nothing bad here. The compressors are doing -20 Deg F duty. The main header is indicating -18 deg F suction temperature. Even at a 10 deg F TD at the evaporators, you should be able to achieve a -8 deg F temperature in the room.

    -----------------------------------
    superheat main header 10dF
    succion pressure at evap 21psig

    ------------------------------------

    I think perhaps we have a problem with these measurements, although they look sensible.

    A suction pressure at evap of 21 psig indicates -14 deg F coil evaporating temperature. A superheat of 10 deg F at the main header indicates
    -8 deg F. A warm-up of 6 deg F.

    I think you have some coils starving and at least one coil overfeeding. Your superheat should be higher at the main header. Check each coil's suction line temperature. I think you will find the gremlin.

  5. #5
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    Hi, yep I agree with Dan, some of the coils are starving and at least one is overfeeding.
    Just a thought, what way are the coils placed in the room. Also seeing it is a freezer are the coils all ice and frost free. Again is the plant sized for any product load and is the product at storage conditions before entering the room.
    I personally would check and note the air on/off each evap along with the saturation at each cooler outlet and the superheat. At the same time the coils can be checked for ice and put through their respective defrosts, with termination press/temp noted.
    More info please, sounds like the evaps are not doing the duty.
    Regards. Andy

  6. #6
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    I am starting to think my expansion valve are causing me problem. Because I ask to the mechanics to give me information of the valve and they are OSE12 , therefore 12 tonnes and the coil as a capacity of 12 tonnes total, 6 tonnes per circuit.

    They might cause me hunting problem.
    Kathleen, P.eng
    www.refri-ozone.com

  7. #7
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    OSE-12 at -20degF has NOT a capacity of 12 tons. according to the Sporlan "201", the table information @ -20degF capacity is 8.46 tons R-404A.

    I assume that each evap or circuit that is served by each TEV is 6 tons, if I understand you correctly.

    If you are going to employ the Sporlan product, that would be the selection. the next "lower" capacity Sporlan valve would be an OSE-9 which is rated @ 5.31 tons @ -20degF (not a good selection).

    I would feel more comfortable with an Alco TCL-7R. But, I'm sure that the Professor would indicate that the OSE-12 is appropriate, and if I was involved in your situation, I would continue looking for the cause.

    I have not yet noticed (in any post) that you went back and checked the superheat (suction line temp) at the outlet of each evaporator circuit. Have you? You must do this.

    Last edited by herefishy; 20-11-2002 at 04:07 PM.

  8. #8
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    Evaporator Outlet Superheat

    Hi, Kathleen
    I would agree with Herefishy superheat measurement at the evap outlets is essential. These would need to be taken after initial pulldown when the plant is running at steady state, this will show straight away if you are hunting.
    Regards. Andy

  9. #9
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    I t seems likely that one TXV is overfeeding, robbing the other TXV's of refrigerant. Superheat readings at coil outlets are essential here.

  10. #10
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    I would feel more comfortable with an Alco TCL-7R.
    Alco ??

    But, I'm sure that the Professor would indicate that the OSE-12 is appropriate, and if I was involved in your situation, I would continue looking for the cause.
    Absolutely. The Prof would want to confirm that a 'Z' or 'ZP' thermostatic charge is being used here. An incorrect thermostatic charge can also lead to problems....
    Prof Sporlan

  11. #11
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    Alco ??


    Dear Prof, I was by no means indicating a preference for a manufacturer. Furthermore, the OSE-12 absolutely is the proper component for Kathleen's application.

    If I were to spec the application from scratch, I might be attracted to the 7.31 ton rating of the ......uh,....... "other thing". That's all.

    Last edited by herefishy; 22-11-2002 at 08:23 PM.

  12. #12
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    Dear Prof, I was by no means indicating a preference for a manufacturer.
    Alco and Sporlan go back a long ways. In fact, Sporlan's founders, Harry Lange and Hermann Spoehrer, both worked for Alco in the early 1930s, and were minority stockholders. They formed Spoehrer-Lange Co. (later to become Sporlan) in 1934.

    Below, a photo of the Alco clan in 1930 with Harry Lange and Hermann Spoehrer.

    Prof Sporlan

  13. #13
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    Hello long time for a reply

    Sorry guys but end of years is probably to all of you a busy time.

    I also had problem to find a good equipment to do monitoring of my superheat that stand -4dF in a room without sending false readings due to batteries.

    I do not know how accurate but the reading we received was of 4dF of superheat at a room near 0dF, but without a monitoring device. Therefore I can not be sure I do not have large difference went the TXV operate.

    I had installed last week a service valve at the inlet of each evaporator to be able to take my subcooling at the entering of the evaporator.

    My expansion valve operate at 64% of its capacity by using selection program.

    Each evaporator are dual circuit, and the superheat are taken in the main line of each evaporator, I wonder if each circuit operate well.

    Because my concerned is why my compressor are able to maintain the pressure at 17psig, 21 psig at the coil, when each coil seems to feed. I made some test and when I stop the last compressor my pressure increase due to the demand, at 20psig, 25psig at the coil with four evaporators feeding.


    Merry Christmas to all of you

    Kathleen
    Kathleen, P.eng
    www.refri-ozone.com

  14. #14
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    Originally posted by Kathleen
    Each evaporator are dual circuit, and the superheat are taken in the main line of each evaporator, I wonder if each circuit operate well.


    .................I give up.......................

  15. #15
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    Merry christmas Herefishy

    I feel like doing so also, but that what make us love refrigeration.


    Take care
    Kathleen, P.eng
    www.refri-ozone.com

  16. #16
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    Hi, Kathleen
    nice to here from you. If your superheat reading at evaporator outlet is 4 Deg F then the valves will be hunting, this to me is two small a setting, I would go for 6 to 8Deg F. The valve selection seems rather too large also, with the valve satisfying the load at only 64%, again this is only my opinion but I would select the valve for around 80-90% open at design conditions, this is more than able to satisfy the pulldown load.
    I would take the following course of action.
    [list=1][*]Close down the expansion valves to obtain a supheat setting of around 8 Deg F[*]If this doesnot work I would check the phail placement with the valve Manufacturer[*]If the phail is correctly placed and the valve still hunts re-select the valves at 90% open[/list=1]

    Try this to see if it helps, if not post again and we will get their eventually.
    Regards. Andy.

  17. #17
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    Also you need to measure the superheat at the outlet of each circuit [B]Not the common suction as it is impossible to determine which valve/ coil is malfunctioning.
    Regards. Andy

  18. #18
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    Originally posted by Andy
    [B]Also you need to measure the superheat at the outlet of each circuit Not the common suction as it is impossible to determine which valve/ coil is malfunctioning.
    Regards. Andy
    Hi Andy,

    We will try to take the superheat, but their is no space in this 2 circuits coils. I dislike to circuit coil. The distance between each circuit and the main head of the coil is mininal.

    Happy new years
    Best regards

    Kathleen
    Kathleen, P.eng
    www.refri-ozone.com

  19. #19
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    Hi, Kathleen
    Happy New Year yourself
    If the design of the coil is such that you are having problems measuring the superheat, how is the expansion controlling the superheat of each individual coil, or is this the problem in that the phails of the expansion valves are attached on the main suction not the circuit they are controlling. Or alternativly the phails are in close proximity to each other and to the mass of the suction header and by this reading a false superheat signal.

    Regards and good luck Andy

  20. #20
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    Hi, Kathleen
    checked out your pressure drop in the suction line and sized the suction line using DirCal.
    As follows.
    Evaporation -37 Deg F
    Suction line 8"n.b.
    Velocity at 100% in suction line 21.4ft/s
    Velocity at 100% in flowcon 34.8ft/s
    Design velocity 32.8ft/s
    Them flowcon's is a lot of the pressure drop lossing you 6 deg F, the same as 140 ft of suction line.
    Although you have designed the compressors for the 6psig drop, I still think that is excessive, the company I work for designs on 2-3 psig pressure drop. We would have fitted a motorized valve to control the evap pressures during defrost, or a hot gas powered valve.
    I would get rid of those nasty valves and fit something with less pressure drop, it's going to increase your COP substantially.
    I can see a cure for you problems close at hand.
    Please feel free to post again, and we will help you out.
    Also what part of Canada are you in, it's a very big country and quite stunning.
    Regards. Andy

  21. #21
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    Hello Andy

    thanks for your reply, I am part of the french part of Canada, quebec province. It is a very nice country but quite cold, probably such as Ireland.

    I know my flocon is probably not the best choice because of the pressure drop. But I should get back my capacity at the compressor in relation to the pressure drop which I do not.

    But since your last reply I ask many question to the mechanic, to realize that it was us that did the joint connection on the both circuit of the coil. It was wrongly done. He did not separate each circuit with a p-trap and he put the expansion bulb and external at the outlet of each circuit. Therefore the upper circuit give wrong signal to the expansion valve.

    I wonder If I could change only the buld of the both external and install them at the main suction before the p-trap, and use a common external would do so. Or I should do the piping once and for all.


    Best reagards

    Kathleen
    Kathleen, P.eng
    www.refri-ozone.com

  22. #22
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    Hi, Kathleen
    Ireland is cold and damp, but generally the temperature even in mid winter would not drop below freezing for more than a few days at a time. -14 deg C is the lowest temperature I have ever experienced in Ireland.
    I was in Ontario about three years ago, I plan to go back in a two years time, my wife has friends in Montreal which we will vist then.

    Back to refrigeration, I am not clear from your post what way your evaps are piped, but I would pipe each coil to a u trap then usually rise to above the main suction header and drop down into the main suction in a inverted u configuration. That is what is called a single riser, a double riser is necessary where the rise exceeds 3m or the velocity calculated in the riser is insufcient to carry oil up into the header.
    If you need details of a double riser please post again.
    Generally this consists of two risers, one sized at 25% duty and one at 75%. The smaller riser is taken off the top on the horizontal outlet suction from the evaporator, then piped into the top of the mains suction in a inverted u.
    If the rise is greater still than say 6m traps are required in both pipes at 3m intervals.
    As for the external equalizer connection they must be placed after the bulb of the appropiate expansion valve.
    I would try the Sporlan site for information about the above if in doubt.
    Hope this helps. Regards. Andy

  23. #23
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    Boy!!!! I sure am glad we got to the bottom of this.

    I was entirely convinced that the superheat of each circuit was in need of evaluation. When Kathleen explained that the readings of each circuit could not be properly checked due to piping configuration, that's when I just had to throw in the towel, because the scenario was getting impossible!!

    Hurray for Kathleen!!!!

    (She) must have really grilled the engineers to get the truth out of 'em !!! I couldn't imagine the (psychological) atmoshpere in my office if such an occurence took place in my shop!

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