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Primary / Secondary Basics

Hi all ,

I have a question regarding the attached sketch. Since the loops are                  " hydraulically disconnected " won't the secondary heat exchanger loop flow like a Ferris Wheel regardless of which pipe is entering first ? Even it is reversed from the flow in the primary pipe it should flow backward in the main. I have all of the books but when talking with others on this issue they act as though the fluid needs to " slinky " in the first pipe and if it can't do that than it must be wrong. I say that either way the only thing that matters for their purpose is to cool the fluid between the branches and either way the result is the same , if not better the way that we have it. Thanks for replies.  
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Comments

  • ZmanZman Posts: 2,194Member ✭✭✭
    I'll take the picture on the right.

    I'm not sure what you mean by "slinky" and "ferris wheel".

    You have it piped backwards.

    Carl
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  • ChrisChris Posts: 2,869Member
    edited August 2013
    What Goes Into A Tee

    Must leave a tee. Yes you will have a mix water temp in pic one..



    Say the Main loop is flowing 8gpm of 180 degree water. The secondary loop is taking 4gpm of X...For design purposes we designed for a 20 degree delta-t on the secondary for a 180 supply and a return of 160 degree water.



    (8*180) + (4*160) = 12X in the primary loop

    1140 + 640/12 = 148 Degree Water blending in the primary loop before the secondary could takes its needed under these design conditions.



    So you wouldn't have 160 degree water returning in the secondary because you can only make 148 degree water to supply the secondary. In other words, you'd never get the supply water temp you needed through the heat exchanger. You'd get the flow you wanted but that's about it.



    Don't confuse flow with btu/hr because your just moving it. What the HX can emmitt/transfer is based on the water temp/flow that btu/hr is flowing in.
    Post edited by Chris on
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
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  • hvacfreak2hvacfreak2 Posts: 5Member
    edited August 2013
    Common Piping

    If the tees were 20 ft apart it would be easier for me to see the discharge of the secondary loop being swept away into the building. These tees are 8 inches apart ( 6 inch steel pipe ). Since we can't have less in the common piping than we do on each side of it I say that the common pipe has to be used by the secondary loop during it's operation regardless of flow in the primary. I'm thinking that there would be a " mixed temperature " entering the secondary loop either way. The application is cooling , but no need to flip the math. But if we started with 8 , lost 4 , go back to 8 gpm I say that can't happen. That 4 is still there and in use by the secondary is my thinking. I'm sure I'm wrong on this Chris.



    The welders piped correctly to the piping drawing and did not see the controls diagram that the engineer is hanging on to ( go figure ). But thanks for the help Zman.
    Post edited by hvacfreak2 on
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  • ChrisChris Posts: 2,869Member
    edited August 2013
    In Diagram 1

    You have 8 leaving the primary circ. You pick up 4 at the secondary return so you have 12 flowing in between the tees, 4 leaves to the secondary supply tee leaving 8 flowing back down the primary return.



    In diagram 2. You have 8 leaving the primary circ. 4 Enters the secondary supply so you have 4 left flowing in between your tees. 4 comes back at the secondary return and 8 flows back down the primary return.



    Always remember. What enters a tee must leave a tee.



    Either way you have the same 4gpm flow into the secondary. The question again is what is that water temp? You are move 4gpm of x piped either way. Diagram 1 is going to get you in trouble from a water temp standpoint not from a flow standpoint.
    Post edited by Chris on
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
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  • hvacfreak2hvacfreak2 Posts: 5Member
    Well

    I'll be on floor 2 with the 2 inch fire hose if anyone needs me.
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  • ChrisChris Posts: 2,869Member
    In Diagram 2

    Your mixed water temp is going to happen where the secondary return enters the primary return. In diagram 1 your mixed temp is going to happen after the secondary return which is going to drop the temp entering into the primary supply before the secondary supply.. Not a good thing..Your secondary is always going to consistently see cooler water. Might not be a bad thing for radiant cooling.. All depends I guess. What is the design water temp flowing out the primary circulator? What was the water temp design flowing to the secondary? GPM design for both would also be help. It's just simple math.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
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  • hvacfreak2hvacfreak2 Posts: 5Member
    It's a repipe

    And I am almost certain that I will see a lower temperature entering the secondary versus leaving the pump ( mixed temperature ) after we correct the piping to the controls diagram. But I have no numbers to support my suspicion so that means drain , cut , weld.

    Just so you know the main is 10 inch and the secondary is 6 inch weld pipe. The purpose is free cooling of the main chilled water loop using condenser water as the primary cooling medium. It is not going to work regardless of how it is piped so I'll be on the second floor dumping the 10 inch main down so that we can open the system .
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  • ChrisChris Posts: 2,869Member
    edited August 2013
    Based On Diagram 2

    You won't see a mix in the secondary. The mix will happen where the primary return and secondary return intersect.



    See if I can make this easy for you to see



    10gpm 100 degree water leaves Pri Circ Pump

    5 gpm of 100 degree water enters the secondary

    5gpm of 100 degree water flows down the primary between the two tees

    5gpm of 80 degree waters leave the secondary return

    10gpm of X is now in the primary return



    (5 x 100) + (5 x 80) = 10X

    500 + 400 = 900/10 = 90 Degree Water is in the primary return at the point both the primary return and secondary return intersect.



    Do you know the water temp and flow rate leaving the pri circ?

    Do you know the flow rate entering the secondary?

    Do you know the design delta-t in the secondary?



    Answer these and I'll give you the temp the math is here for you if you want to do it yourself.
    Post edited by Chris on
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
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