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  1. #14
    Join Date
    Mar 2003
    Posts
    7,314
    times used to be, primary flow was greater than or equal to secondary. these days, it really depends on system design, delta t etc. engineers, and regular joe's like me play with delta t, pipe sizing to do different things. this is even more true on boiler systems running multiple temps on secondary loops. you need to look at the requirements of your primary equipment(chiller), and that of the secondary loop(s).

  2. #15
    Join Date
    Dec 2003
    Posts
    2,266
    If the secondary flow is greater than the primary flow the return from the loads will mix with the supply from the chillers. This will raise the temperature of the supply water to the loads. This doesnt seem like a good thing.

    I always thought the primary should be more than the secondary. Some engineers like a check valve in the decoupler that will not allow the secondary flow to be greater than the primary.

  3. #16
    Join Date
    Mar 2010
    Location
    Morgan Hill Ca.
    Posts
    1,171
    Quote Originally Posted by bigtime View Post
    If the secondary flow is greater than the primary flow the return from the loads will mix with the supply from the chillers. This will raise the temperature of the supply water to the loads. This doesnt seem like a good thing.

    I always thought the primary should be more than the secondary. Some engineers like a check valve in the decoupler that will not allow the secondary flow to be greater than the primary.
    While in a common sense world I would agree with you, however, with some of today's systems and controls schemes things have really been set outside the box.

    The one I did recently had two chillers, building divided into two separate loops and all tied together using four pumps. Basically with three way valves at the control level, I can literally run both building sections on a mild day with less than half of one chiller.

    The return water temperature sensor controlling the chillers is literally in the decoupling run. This creates a blended return temp of both buildings and it's reset by building total load (using a return water sensor for each building section). The energy used for cooling has dropped in half and the second chiller seldom runs for more than a few hours on the warmest of days.

    The system used to be one chiller, one building section with a single pump with full flow going to each chiller/building section.... Ran that way for 12 years. I replaced one of the 40 ton air cooled chillers and talked them into piping it this way and never looked back.

    Don't get me wrong, I see your point, but it is not that "cookie cutter" anymore.

    I could tell you about the 20 ton FHP system we did, but you wouldn't believe that it actually works and I am too tired to try and explain it right now....
    If a day goes by and you have learned nothing, I hope you got a lot of sleep.

  4. #17
    Join Date
    Dec 2003
    Posts
    2,266
    Sounds like you definately improved things. I would enjoy seeing a schematic of the system.

    I'm just have a hard time seeing how it can be a good thing to have return water from the ahu's mixing with supply from the chillers, and then going back to the ahu's. Oh well, Im not a cw piping expert by any means, just trying to think through it.

  5. #18
    Join Date
    Jun 2010
    Location
    Miami
    Posts
    257
    Bigtime, just all depends on conditions (temps/humdity/etc). You could get away with stuff in some parts of the country that would never fly in others.

    These days I wouldn't even design with a primary and secondary loop if I had a remotely constant load.

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