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  1. #1
    Join Date
    May 2019
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    Differential pressure in Variable primary CHW system

    Hello all, and thank you ahead for any responses. I recently started a new position operating a building BMS system (Ahu's, 3 water cooled chillers, cooling towers, High pressure boilers, HX's). I'm a little confused on the topic of system Differential pressure in my CHW system. It's a variable primary system.
    From my understanding, as CHW valves close DP rises. As they open, DP will drop. CHW pumps will vary their speed accordingly.
    -Will my condenser pumps also vary their speed based on system DP?

    -What factors will determine what my system DP set point should be?

    -I also have a DP reading at my evaporator on all three chillers. Is this DP telling me the water flow through my evaporator ?

    -What factors will determine what my evaporator DP should be?

  2. #2
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    Aug 2006
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    Mixing oil and fire with a big spoon.
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    1. Your condenser pumps typically will not change speed. Yours could but you would be one of the only ones that would have variable condenser water flow.

    2. Your chilled water differential pressure stepping should be fixed and is determined by the design of the system so it was supposed to be set by the design engineer.

    3. The differential pressure across each chiller will tell you what your flow rate is, however, you will need many more details to make that determination. The main goal of the differential pressure across each chiller is to know when you are near the minimum flow rate for that barrell. When you are near the minimum, you should be opening up a bypass valve. This will help maintain the minimum flow rate through the chilled water barrel.

    4. The manufacturer of the chiller should be able to tell you what the minimum flow rate is along with what that differential pressure would be at that minimum flow rate.
    "Right" is not the same as "Wise".

    Don't step on my favorite part of the Constitution just to point out your favorite part.

  3. #3
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    Jul 2018
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    SYS-APM001-EN Chiller System Design and Control, Tranes Applications Engineering Manual - is a good reading, particularly from page 55 - Variable-Primary-Flow Systems.

  4. #4
    Join Date
    Feb 2016
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    Louisburg Kansas
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    Sounds like a well engineered system. Get a copy of the balance report. It should answer all your questions concerning dp.
    No man can be both ignorant and free.
    Thomas Jefferson

  5. #5
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    May 2019
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    Thread Starter
    Thank you all for your replies and for that explanation. I will look at that manual and at the balance report also. Thanks again.

  6. #6
    Join Date
    Mar 2005
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    Dover, Pa.
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    1. Dp setting are obtained by the designer and more importantly by the start up and water balancing.
    2. Dp set point will be determined by opening all chw valves while still being able to have enough pump to provide gpm to all coils.
    3. Dp sensor will be placed 2/3rd down stream in the loop, or somewhere as specified by the designer.
    4. Cooling towers can have constant flow to the tower while also utilizing a 3 way valve to bypass the tower when water temp drops in the loop.

    If you can provide more information, I’m sure more answers will follow.
    Good mechanics are getting harder to find.....have you encouraged or discouraged a young mechanic today?

  7. #7
    Join Date
    Mar 2005
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    excellent concise answers , real pros
    Keep it simple to keep it cool!

  8. #8
    Join Date
    Nov 2006
    Location
    connecticut
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    If you have a variable flow configuration chiller plant the bypass is control by the chiller flow not the DP . It's the most complex system to understand & control this type of chiller plant. How I know is by learning & reading because where I were work chillers program was completed wrong.We had the bypass looking at the DP over ride to 75% and the chiller pump was over ride to a % on the BAS system we had some many meeting after this new building was open nobody could understand after 3 years I took over the plant and it was very hard to do any PM set up like this in till we started to reading and get all the proper people involve to change the program.

  9. #9
    Join Date
    Feb 2016
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    Louisburg Kansas
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    Did the balance report answer your questions? If not let us know and we will give you more info.
    No man can be both ignorant and free.
    Thomas Jefferson

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