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  1. #1
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    Jul 2015
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    Variable Primary Chiller Piping

    Me again!

    We have a 110 ton McQuay air-cooled chiller on campus serving a classroom/office building that trips on low flow or low evap temp fairly often. I've learned quite a bit from you guys about hydronics piping since I started working here, but this on is a little different than most of our other systems. Seems to happen at fairly random times and I've only observed it last fall and now in the last couple weeks. From viewing the trend data in the BAS, it appears to usually not make it through an entire day.

    I'm currently waiting on O-rings to pull the strainers on the pump diffusers (as I'm sure it has never been done before), so I thought I'd come on here to try and understand the piping of a system like this better.

    Little history:

    5 years ago the building had a major overhaul where most everything in the CHWS was replaced except for the piping (including the chiller and pumps). Sounds like the chiller has been an issue pretty much since then. A co-worker mentioned something about the chiller control board being changed out and a strainer being cleaned multiple times. Have no records to look at from this time period, so pretty much starting from scratch here.

    In looking at the system, the piping seems to be different than what I'm finding on piping examples from various sources.

    Here is what we have:
    Name:  Piping-asis.jpg
Views: 3587
Size:  59.1 KB


    I honestly can't remember right now which side of the chiller the light green strainer is on. I'm not even sure right now what kind of strainer it is exactly. I think this is the one that my co-worker was talking about being cleaned out because the insulation has obviously been redone since it was installed.

    The B&G SRS has a compression tank fitted with an airtrol fitting attached to the air removal port. Instructions for the SRS show it as needing to be installed just before the pumps, ours has the chiller between it and the pumps. Is this set-up OK or should it be changed?

    The pumps currently pump "away" from the chiller. Every piping diagram I can find on variable primary systems with a bypass show the pumps pumping "into" the chiller. Which way is best (does it make a difference)? In my head, the by-pass would work better if the system was pumping "into" it instead of "pulling" it through the chiller and this could possibly be part of the flow issue. Or am I over-thinking it like usual?



    From what I've read, it seems like the system should flow this way and the SRS should be moved:
    Name:  Piping-should.jpg
Views: 1182
Size:  58.3 KB






    Here is the SoO for the system.
    Name:  SoO.jpg
Views: 952
Size:  92.3 KB


    Chiller minimum flow is 170 GPM

    It is a McQuay AGZ125CHSNN-ER10

    S/N: STNU100300221

    Sticker on unit says it has Benshaw controls. Our BAS is Siemens Apogee and it communicates with the chiller VIA BACNET.


    Being primarily a classroom building, there can be a very low heat load in this building at times during the day. A couple times that I noted a few times in the alarm history was around 1130am and 230pm. Might coincide with classes letting out and the building more or less being empty, but I haven't looked that far into it yet.


    Am I right to suspect the piping to be an issue or do you think the strainers and low heat load are the most likely culprit? If it's mostly due to the low heat load, any suggestions on a work-around? The only reheat in the building is VAV's supplied from heat recovery our data center Lieberts.

  2. #2
    Join Date
    Sep 2002
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    Is this a DX chiller?

    I dislike varying fluid flow through a chiller as it makes it hard to achieve stabilized operation at anything other than full flow rate.

    Did the previous system have a primary and a secondary loop? With independent pumps?

    I like substantial buffer tanks but even an H connection would be better than nothing.

    PHM
    ------




    Quote Originally Posted by DDCSD View Post
    Me again!

    We have a 110 ton McQuay air-cooled chiller on campus serving a classroom/office building that trips on low flow or low evap temp fairly often. I've learned quite a bit from you guys about hydronics piping since I started working here, but this on is a little different than most of our other systems. Seems to happen at fairly random times and I've only observed it last fall and now in the last couple weeks. From viewing the trend data in the BAS, it appears to usually not make it through an entire day.

    I'm currently waiting on O-rings to pull the strainers on the pump diffusers (as I'm sure it has never been done before), so I thought I'd come on here to try and understand the piping of a system like this better.

    Little history:

    5 years ago the building had a major overhaul where most everything in the CHWS was replaced except for the piping (including the chiller and pumps). Sounds like the chiller has been an issue pretty much since then. A co-worker mentioned something about the chiller control board being changed out and a strainer being cleaned multiple times. Have no records to look at from this time period, so pretty much starting from scratch here.

    In looking at the system, the piping seems to be different than what I'm finding on piping examples from various sources.

    Here is what we have:
    Name:  Piping-asis.jpg
Views: 3587
Size:  59.1 KB


    I honestly can't remember right now which side of the chiller the light green strainer is on. I'm not even sure right now what kind of strainer it is exactly. I think this is the one that my co-worker was talking about being cleaned out because the insulation has obviously been redone since it was installed.

    The B&G SRS has a compression tank fitted with an airtrol fitting attached to the air removal port. Instructions for the SRS show it as needing to be installed just before the pumps, ours has the chiller between it and the pumps. Is this set-up OK or should it be changed?

    The pumps currently pump "away" from the chiller. Every piping diagram I can find on variable primary systems with a bypass show the pumps pumping "into" the chiller. Which way is best (does it make a difference)? In my head, the by-pass would work better if the system was pumping "into" it instead of "pulling" it through the chiller and this could possibly be part of the flow issue. Or am I over-thinking it like usual?



    From what I've read, it seems like the system should flow this way and the SRS should be moved:
    Name:  Piping-should.jpg
Views: 1182
Size:  58.3 KB






    Here is the SoO for the system.
    Name:  SoO.jpg
Views: 952
Size:  92.3 KB


    Chiller minimum flow is 170 GPM

    It is a McQuay AGZ125CHSNN-ER10

    S/N: STNU100300221

    Sticker on unit says it has Benshaw controls. Our BAS is Siemens Apogee and it communicates with the chiller VIA BACNET.


    Being primarily a classroom building, there can be a very low heat load in this building at times during the day. A couple times that I noted a few times in the alarm history was around 1130am and 230pm. Might coincide with classes letting out and the building more or less being empty, but I haven't looked that far into it yet.


    Am I right to suspect the piping to be an issue or do you think the strainers and low heat load are the most likely culprit? If it's mostly due to the low heat load, any suggestions on a work-around? The only reheat in the building is VAV's supplied from heat recovery our data center Lieberts.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  3. Likes WAYNE3298 liked this post.
  4. #3
    Join Date
    Dec 2014
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    New Mexico
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    Make sure the flow meter is reading the correct flow, may be out of calibration.
    Would like to see the minimum flow valve farther way from the chiller , so when it opens it gets to mix with more of the return. This will help keep cold water from hitting the chiller. Check to see if bypass valve is the right size for the 175 gpm . If it is the same size as main lines when it opens it can dump off to much cold water at one time. Hard to control the amount.
    Have no less then three gals. of water per ton, may need a tank right before the chiller.


    Sent from my iPhone using Tapatalk

  5. #4
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    Feb 2016
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    Louisburg Kansas
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    I wouldn't have piped the system as shown but the existing piping config should work. I can't tell how the min flow through the chiller is controlled but for best results it should be controlled by chiller differential pressure.

  6. #5
    Join Date
    Jul 2015
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    South Dakota
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    Thread Starter
    Quote Originally Posted by Poodle Head Mikey View Post
    Is this a DX chiller?

    I dislike varying fluid flow through a chiller as it makes it hard to achieve stabilized operation at anything other than full flow rate.

    Did the previous system have a primary and a secondary loop? With independent pumps?

    I like substantial buffer tanks but even an H connection would be better than nothing.

    PHM
    ------
    I want to make sure we're on the same page with the term DX chiller. Do you mean a chiller with a remote evaporator in the building, or an air cooled chiller? I'm still not great with some of the terms!

    The current chiller has the chilled loop piped outside to the unit that had an on-board evaporator. The previous chiller had the refrigerant lines piped into the building where the evaporator was located. It is air-cooled.

    Looking at the demo page in the prints, the previous system was primary/secondary with two chilled water system pump (duplex) and a separate evaporator loop.

    The current system is set-up so that the bypass is supposed to maintain ~175 through the chiller by modulating that bypass valve.

    Sent from my MotoG3 using Tapatalk

  7. #6
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    Jul 2015
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    South Dakota
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    Thread Starter
    Quote Originally Posted by Chev5372 View Post
    Make sure the flow meter is reading the correct flow, may be out of calibration.
    Would like to see the minimum flow valve farther way from the chiller , so when it opens it gets to mix with more of the return. This will help keep cold water from hitting the chiller. Check to see if bypass valve is the right size for the 175 gpm . If it is the same size as main lines when it opens it can dump off to much cold water at one time. Hard to control the amount.
    Have no less then three gals. of water per ton, may need a tank right before the chiller.


    Sent from my iPhone using Tapatalk
    Main chilled lines are 5" and the bypass is 2 1/2".

    By tank, do you mean one in addition to the air compression tank?

    I'll get the flow verified. Thanks!

    Sent from my MotoG3 using Tapatalk

  8. #7
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    Thread Starter
    Quote Originally Posted by WAYNE3298 View Post
    I wouldn't have piped the system as shown but the existing piping config should work. I can't tell how the min flow through the chiller is controlled but for best results it should be controlled by chiller differential pressure.
    It is controlled by an electromagnetic flow meter. Trying to figure out where the probes are located now.

    Sent from my MotoG3 using Tapatalk

  9. #8
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    Thread Starter
    Some more information. The mystery strainer is located on the line going into the chiller. It is not shown in the blueprints.

    The SRS that I thought was in the wrong spot is shown in the prints as being the only thing between the chiller and the pumps on the intake side of the pumps.

    Bypass is shown as being right after the pumps and into the main return line that leads straight out to the chiller.

    Compression tank has 144 gallon volume and does not have a bladder.

    Sent from my MotoG3 using Tapatalk

  10. #9
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    Jul 2015
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    Thread Starter
    Heres a quick shot of the prints.

    Hope it came out clear. The chiller system pumps are the ones above my finger. The other pumps are for the Liebert heat recovery reheat loop.

    Sent from my MotoG3 using Tapatalk

  11. #10
    Join Date
    Jan 2008
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    Phoenix Arizona
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    The bypass and flow meter need to be setup and calibrated to ensure minimum flow...What type of compressor is on this chiller? Sounds like a screw, but I don't know from the model# alone...
    "If history repeats itself I am so getting a dinosaur"

  12. #11
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    Aug 2009
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    Prattville, Alabama
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    Quote Originally Posted by DDCSD View Post

    The current system is set-up so that the bypass is supposed to maintain ~175 through the chiller by modulating that bypass valve.

    Sent from my MotoG3 using Tapatalk
    Probably the VFD-driven pump and the bypass are used together to maintain a minimum of 175 GPM thru the chiller.

    There is likely a maximum rate of change of chilled water flow that the chiller can handle. Ensure this rate of change is not exceeded. In other words, with a significant drop in load, the chiller must be allowed to keep pace - or given time to unload and match the new load. If not, the evaporator becomes too cold, until the chiller is able to "catch up". This can result in a low evaporator temp.

    Or, it is possible that this is all due to a low flow condition at the minimum programmed flows. In which case you would be on the right track with cleaning the strainer.

    Could also be borderline conditions at the minimum flows, and cannot tolerate an otherwise acceptable strainer condition.

    Hopefully others here, with experience on this model chiller, can help with rate of change question. Could be in a manual, or a service bulletin.

  13. #12
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    Mar 2015
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    Medford, N.Y.
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    Hi DDCSD. In that fancy BAS/Chiller thing is the water temp entering the chiller recorded or displayed? Is the BypassValve an open/closed or modulating? KindaSorta full flow is 330gpm and lo flow is 170gpm so thats close to a 50/50 operation.Would the recording equipment show when each circ was running?

  14. #13
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    Dec 2014
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    The tank is called a buffer tank. It has to be a certified pressure tank. The water flows in one side and out the other. It is there to add volume to the loop.
    A lot of the top used to slow down the rate of temp drop so the chiller can back normally.


    Sent from my iPhone using Tapatalk

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