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  1. #1
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    Low Delta T Syndrome Question

    Does low delta t syndrome reduce the usable capacity of individual compressors within a given circuit? Ie. Will it rob the lead compressor of some of its useful capacity causing the controller to bring up the second compressor when the lead is only loaded to, say, 70% of its capacity?

  2. #2
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    What in the world is low delta T syndrome?

  3. #3
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    Quote Originally Posted by klove View Post
    What in the world is low delta T syndrome?
    My understanding is that this occurs when a large amount of chilled water bypasses the AHU coil via the mixing valve resulting in a lower RWT and (if the LWT is at a fixed setpoint) a operating temperature differential across the chiller. (Maybe there's a more common name for this phenomena...)

    As I understand it, low delta-T syndrome creates a situation where the chiller cannot be loaded to 100%. This affects the entire chiller, but my question was attempting to discover if it also affected each compressor on a chiller.

  4. #4
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    Quote Originally Posted by klove View Post
    What in the world is low delta T syndrome?
    Uhm yeah, low delta where?

    Significantly vauge questions can draw significantly complex wrong answers.

  5. #5
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    Quote Originally Posted by fbcit View Post
    My understanding is that this occurs when a large amount of chilled water bypasses the AHU coil via the mixing valve resulting in a lower RWT and (if the LWT is at a fixed setpoint) a operating temperature differential across the chiller. (Maybe there's a more common name for this phenomena...)

    As I understand it, low delta-T syndrome creates a situation where the chiller cannot be loaded to 100%. This affects the entire chiller, but my question was attempting to discover if it also affected each compressor on a chiller.
    I believe someone has tried to make things look like rocket science to you in order to show how smart they are (you may read as much sarcasm into that statement as you like). There's no such thing as "Low delta T syndrome".

    The scenario you describe is simply a system not operating at full load, thus the chiller doesn't need to operate at full load, so it doesn't. There's no "syndrome" to it. The controller sees what the system needs based on it's error from setpoint for the fluid temp and reacts accordingly to match the capacity of the chiller with the needs of the system. There's all kinds of "what if"'s that you could get into based on single compressor, multiple compressor, PID control, proportional control, leaving or return temp control, variable fluid flow, and any of 1000 other wrinkles you might want to throw into the scenario. You can also get into compressors operating at higher or lower capacities based on differing compression ratios even though they're rated at XXX.X tons. Lots of things come into play with the actual capacity being used to match the load, and it can get somewhat complicated depending on how deep you want to go, but generally, there's no need to go there.

  6. #6
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    'Low Delta T Syndrome' is a real issue, however, it is something that is very rarely due to an engineering design flaw.

    'Low Delta T' is when the difference between the Supply Water Temperature and the Return Water Temperature is less than design. This happens all of the time during a part load condition.

    the 'Syndrome' part is when the coils do not supply the cold (or hot) air to meet design. this can be due to dirty coils (inside the coil piping or in the air side or both), improper piping design, poor control valve selection/operation, etc.

    this causes the return water to not absorb as much heat as normal. since the coils are not satisfying the load, the BAS increases the water flow rate in the secondary side of the system. greater flow rates in the secondary system causes more water flow in the decoupling piping. since the chiller has a low entering chilled water temperature, it maintains its setpoint easily. the problem is that we are now mixing lots of return water and a little chilled water and the water temperature going out to the secondary is warmer. so the BAS then starts another chiller even though the first chiller is not trying hard.

    this is usually a maintenance issue. clean the coils, replace control valves that don't work properly, verify that the air flow is correct, etc.
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  7. #7
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    i gave this issue some thought last night and read a few articles...
    low delta t syndrom is associated with primary/ secondary (decoupled) piping systems...
    this problem can be escallated if the plant is not staging the chillers correctly...
    there can be times when the secondary side of the system is requireing more flow than the primary...
    when this happens the excess flow is deverted back thru the bypass mixing with the chilled water supply providing warmer water than desired to the coils...

    there are several fixes for this...
    install a check valve in the bypass line to eliminate the secondary return water from mixing with the primary supply...
    or have your chiller(s) setpoint control to the secondary supply temperature (which can be problematic)...
    the idea is to keep the excess flow on the primary side of the system so only true chilled water supply temperature is sent to the air handlers...
    this is usually not an issue with comfort cooling applications but sometimes in process applications it means running more chillers than whats required for tonnage but nessiary for keeping excess primary flow...

  8. #8
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    I've ran into the problem but never heard that term. Good explaination guys.

    I was thinking "low delta T syndrome" was something I could get from drinking the water here around TMI!

  9. #9
    sounds like u have low flow due to pour loading i have a few sites like that one of them is a nursing home so the people like it warmer but it drives the chiller nut it starts and stops about 20 times a day we had to set a 15 degree diff in the software of the chiller

  10. #10
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    Look on page 80 - 84, this explains it pretty good!
    Attached Files Attached Files

  11. #11
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    hello chad thanks for posting the mcquay document... good perspective...
    in reading near the end a very important point is mentioned that engineers, technicians and facilities operation personnal often overlook when setting up chiller plant's staging and operations... running the chillers in the "max cap range" can help remidy low delta t syndrome if you end up over pumping the primary to solve this issue (this can be a good thing)... if we all spent more time with this concept we could change the way buisness is done with commercial air conditioning and dramatically reduce the amount of energy this country consumes with a few setpoint/ logic changes... staging the chiller plant and running optimization logic with condenser flow and tower temperature control in specific ways can help improve the preformance of the system and save a ton of energy!... installing a variable speed chiller is one way to increase the systems effieciency but condenser relief per the attached is by far the best best way to get the most performance out of the existing plant for minimal investment... the savings are huge and usually a good utility will incintivize implementation... there are platforms out there that charge big bucks for this but its really quite simple and does not need to be so complex if you understand were a chiller can and cannot operate... be careful with this as many senarios will end up overdriving the tower which then waists energy... we should always evaluate a new or existing system using this type of approach and thinking... being that our company primarily installs oil free variable speed equipment, we are not bound by lift related problems like oil return... but we always set up the system the maintain motor cooling requirments for hermetic type machines... 5 to 10 years from now i feel that any system we walk up to should/ will have 10 - 16 F water temp delta's regardless of machine capacity with supply reset... if thats not enough to make a technician pucker on a troublshooting call, i dont know what is... i was at a national chiller function a few years back and ended up having dinner with this gent... turns out he was the key note speaker for the next days agenda and i was blow away with his presentation and concept...

    Operating Chillers in the "Max Cap" Range
    enjoy...!

    Chiller Rob
    Attached Files Attached Files

  12. #12
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    Aug 2009
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    Well, one compressor and another cracked compressor head later, I've gotten to catch up on all of the great feedback in this thread.

    Thanks to all who posted documentation (which I've taken time to read through). This system has one 200 ton chiller with only about 35 tons of load at peak on it, and I was wondering if this might result in a variation of this phenomena when staging compressors. However, it appears that that is really an academic exercise at best with little real-time effect on the system. The greater issue is simply the constant under loading of the chiller.

  13. #13
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    Apr 2010
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    Florida
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    One 200 ton chiller with 35 tons at peak!! I am going to guess that this is an air cooled chiller as well. Chiller loops, piping, require that the volume of the system be enough to accomodate a 2-minute loop. If you have an air cooled system, and low peak loads with a large chiller, then you migt want to check the volume of the piping system to ensure that the compressors are not cycling excessively. If it is less than 2-minutes you will need to add a tank to increase the volume and therefore increase the run time of the compressor(s). The tank should equal the amount necessary to bring the entire system to a 2-minute loop.

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