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  1. #1
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    Variable Primary Flow

    I have a bit of a problem job. I have three chillers in parallel and each has a dedicated chill water pump on a drive and a chill water isolation valve on the cooler inlet. If one machine is running with its pump @ 100% I get roughly a 2-3 pound drop through the barrel of that chiller(The design delta is 8.6'). On a call for the second chiller the primary pump for that chiller starts and the isolation valve opens. Here is the problem, with both pumps running at 60hz and two isolation valves open I get much closer to design flow @ around 1-2 pounds differential through the two evap barrels and I can't get the DP switch to reliably read at pressures that low. What I want to do is find another way to prove flow through the coolers of these machines.

    I'm sure many of you have seen this problem before, what solutions did you come up with?

    Thanks

  2. #2
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    Put paddle switches on there i always think they are better than DP`s.

  3. #3
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    McDonnell and Miller FS4-3 paddle flow switch for low pressure drop situations.
    "Wheel" mechanics work on "Wheel" chillers

  4. #4
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    Check out the "IFM efector" flow sensor. Same as what York is using on their chillers. Easy to retrofit....1/2" threadolet insted of 1-1/4" and no paddles to break off.

  5. #5
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    Use a thermal Dispersion Flow Switch. I'm interested about the less than 1/2 flow, how old is the system and has it been like that from the start. One pump running can't do the job, and chiller can't give proper tonage. Is there a check valve leaking by?

  6. #6
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    Quote Originally Posted by chillerfreak View Post
    Check out the "IFM efector" flow sensor. Same as what York is using on their chillers. Easy to retrofit....1/2" threadolet insted of 1-1/4" and no paddles to break off.
    Carrier also uses the IFM thermal dispersion switch and it only needs a 1/4", which like you say no moving parts to break and no condensate issues to rust out paddle later on.

  7. #7
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    I guess I don't understand the system, aren't you concerned the gpm's are so far away from design? Is this a variable speed chiller?

  8. #8
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    variable primary flow is what is replacing constant primary/variable secondary systems. cost savings....one less pump and less piping as well as energy savings on reduced speed primary pump. If I remember correctly most chillers can operate down to roughly 30% of design water flow....also 1/2 differential through the bundle is not 1/2 flow as this is a square root function PD2= PD1* (gpm2/gpm1) squared.

    The problem I have seen with these systems is that the controls geeks set the controls to where chilled water pump speed changes to quickly...then the chiller controls go nuts.....then the valves at the coils go nuts ....and so on and so on.....
    Last edited by chillerfreak; 04-05-2008 at 12:49 AM.

  9. #9
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    [QUOTE=chillerfreak;1819968]variable primary flow is what is replacing constant primary/variable secondary systems. cost savings....one less pump and less piping as well as energy savings on reduced speed primary pump. If I remember correctly most chillers can operate down to roughly 30% of design water flow....also 1/2 differential through the bundle is not 1/2 flow as this is a square root function PD2= PD1* (gpm2/gpm1) squared.

    The problem I have seen with these systems is that the controls geeks set the controls to where chilled water pump speed changes to quickly...then the chiller controls go nuts.....then the valves at the coils go nuts ....and so on and so on.....[/QUOTE

    That makes sense, We had a dozen or more buildings changed to constant primary, variable secondary back in the 90's to save energy without too many problems, but we did have that problem with the chiller not being given enough time to unload even with the decoupling loop. I can see where a suddenly reduced flow through the chiller would be even worse. I still don't understand why Mallron never acheives design flow, is that normal?

  10. #10
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    Thread Starter
    Thanks for the replies guys. I'll research the IFM switch.

    ZZZRSC/need more time- I actually have a bit more than design flow. In my original post I stated that design is 8.6'. If you convert that to psi your delta P is actually less than a pound. The problem is that DP switches do not read realiably when pressure is that low.

  11. #11
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    Quote Originally Posted by mallron View Post
    Thanks for the replies guys. I'll research the IFM switch.

    ZZZRSC/need more time- I actually have a bit more than design flow. In my original post I stated that design is 8.6'. If you convert that to psi your delta P is actually less than a pound. The problem is that DP switches do not read realiably when pressure is that low.
    Mallron 8.6 feet= 3.7 psig

    1 psig = 2.31 ft water column....not trying to be a no it all.....I just can't understand why Engineers can't use PSIG instead of feet???? my feet are what are tired at the end of the day!

    I think you will be happy with the IFM....Good Luck!

  12. #12
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    Thread Starter
    Is this the switch you guys are talkin' about? If so, then I guess I'll need the model with AC relay output to prove flow to the machine.


    Flow sensors with rugged stainless steel housing, easy setup and bar graph display - Series SI5


    DC switching output

    AC relay output

    Flow transmitter with easy setup and bar graph display - Series SI


    DC analog output

  13. #13
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    Thread Starter
    I guess I was taught/remember the conversion incorrectly. I look it up. Thanks.

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