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  1. #1
    Join Date
    Jul 2010
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    Denver, CO
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    433

    Reverse Return, Unbalanced Flow

    I recently (2 months ago, its been busy) started up a pair of Carrier 30MPW Chillers, 45 tons a piece.
    I piped these chillers in a reverse return configuration on both the condenser and evaporator. The chilled water side is primary secondary with a hydrolic separator. When I first started these chillers the number 1 chiller was alarming low flow on the evap side. I had to pinch down the outlet valve (approximately 20%) to make the flow switch close and allow the chiller to run. I had a Carrier rep onsite for the whole start up, though he did not understand reverse return. I believe that the flow switch is not calibrated correctly and needs to be replaced but he does not agree. Reverse return = equal flow through all equipment. The other chiller runs just fine and has adequate flow. The pump is sized correctly, and probably about 25-35 gpm oversized. I have yet to have a flow balance (don't kick me on that). I am just curious what y'all think about this situation.

  2. #2
    Join Date
    May 2009
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    South of Heaven
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    Until you measure the pressure differential through your machine you have no idea where you are at regarding flow.

  3. #3
    Join Date
    Jun 2013
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    NORTHERN
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    T45 - lotta cents saved with that and sense...

    we do rev ret all the time in GT ground loops to expensive double supply header to single return (all rev ret) in custom "old style" boiler -copper-linie distribution, to cast iron bbd when so...

    but if much over 4 ft per second fluid velocity, fluid inertial forces by pass "seemed logical" layout
    of T-off's,

    since T45 is clear:
    YMK (YouMayKnow)
    it SHOULD do exactly what it is doing, b/c all is in place for what is happening to should be occuring
    Quote Originally Posted by tech45 View Post
    Until you measure the pressure differential through your machine you have no idea where you are at regarding flow.
    ... however, much work still needs to be done.
    CLOSED LOOP newer ratings are listed, but in numerical EER's Closed- is posted below OPEN LOOP EER's:

    http://www.energystar.gov/productfin...r=0&lastpage=1

  4. #4
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    Jun 2013
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    Quote Originally Posted by GT1980 View Post
    T... all is in place for what is happening to should be occuring
    can you then measure the PD across the pump, too for total GPM per non-defective OEM fluid ratings being presumed (if you can presume such)?

    and then
    water btuh= 500 x Td x GPM
    solve
    btuh/500/Tdiff= GPM
    ie glycol may be a constant of 485 or what it is; and knowing the net btuh would be another NEAR guess to get perhaps within 3% -5% of reality based on OEM listed absorption-per-situation
    without
    any flow gauge nor pdrop known- all things not being OEM/installer-defective: HX paths, etc.
    ... however, much work still needs to be done.
    CLOSED LOOP newer ratings are listed, but in numerical EER's Closed- is posted below OPEN LOOP EER's:

    http://www.energystar.gov/productfin...r=0&lastpage=1

  5. #5
    Join Date
    Dec 2008
    Location
    Dixiana, AL
    Posts
    2,609
    Reverse return will make the system "inherently self-balancing" per the engineering guides, but that doesn't mean it totally balances itself to the correct flow rates, just that it gives each piece of equipment the same amount of piping. As you can surmise, this only goes just so far to doing your balancing job for you.

    As 45 said, you have to measure the flow thru the machines individually and set them correctly or you're just guessing.

    My question to you: You said that you had to "pinch down on the outlet valve to make the flow switch close and allow the chiller to run". Does anything seem strange about that to you?

  6. #6
    Join Date
    Jul 2013
    Location
    Illinois
    Posts
    33
    Must Amit, rev return is a new term to me, just when you think you know what you are doing someone proves you wrong...thus my user name... Not to disrupt the thread and help this person out, but if anyone can send me anything to educate me, I may wake up a little smarter than I went to bed.

  7. #7
    Join Date
    Jan 2011
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    1,040
    Quote Originally Posted by humbled daily View Post
    Must Amit, rev return is a new term to me, just when you think you know what you are doing someone proves you wrong...thus my user name... Not to disrupt the thread and help this person out, but if anyone can send me anything to educate me, I may wake up a little smarter than I went to bed.
    I don't know if I have any literature per se, but York uses what they call series counterflow to get large delta Ts in some of their larger equipment. I started a plant that had four 2500 T. nominal chillers piped in pairs in a series counterflow arrangement. The entering condenser water goes through the condenser of chiller B (2 pass) and then goes into the condenser of chiller A (also 2 pass) and returns to the tower. The chilled liquid is entering chiller A (2 pass) and upon exiting then enters chiller B. The theory is the colder condenser water is getting the colder chilled water and vice versa. In the case I'm citing the chillers had different gear sets so that they had the lift to do the job. The customer wanted to chill approximately 6400 GPM from 57F to 40F. The only way to do this economically was to run this series/counterflow arrangement. The City Center in Las Vegas employs the same strategy.

  8. #8
    Join Date
    Jul 2010
    Location
    Denver, CO
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    Quote Originally Posted by GT1980 View Post
    can you then measure the PD across the pump, too for total GPM per non-defective OEM fluid ratings being presumed (if you can presume such)?

    and then
    water btuh= 500 x Td x GPM
    solve
    btuh/500/Tdiff= GPM
    ie glycol may be a constant of 485 or what it is; and knowing the net btuh would be another NEAR guess to get perhaps within 3% -5% of reality based on OEM listed absorption-per-situation
    without
    any flow gauge nor pdrop known- all things not being OEM/installer-defective: HX paths, etc.
    I did use this calculation during start-up and found I had about 15-20 gpm more through chiller 1 with the valve pinched down. I was unable to get a delta without the valve pinched since the chiller wouldn't run, so I will have to do it by p drop. Like I said I calculated more flow, though as you said its a 3%-5% error factor. At 115 gpm 5% would only be 6 gpm which tells me I still have more flow through this chiller.

    Quote Originally Posted by klove View Post
    My question to you: You said that you had to "pinch down on the outlet valve to make the flow switch close and allow the chiller to run". Does anything seem strange about that to you?
    Yes, it does seem very strange as I should have equal flow or pretty close to equal flow. Pinching down the valve, from my knowledge, is actually reducing GPM but increasing velocity. That seems counter intuitive to me, but it worked for the time being till I can figure out exactly what is going on. Is this what you were thinking or am I missing something?

    Quote Originally Posted by humbled daily View Post
    Must Amit, rev return is a new term to me, just when you think you know what you are doing someone proves you wrong...thus my user name... Not to disrupt the thread and help this person out, but if anyone can send me anything to educate me, I may wake up a little smarter than I went to bed.
    Reverse return is essentially the first one in is the last one out. I typically use it on a primary/secondary system but you can use it in all applications, down to hot water base board. In my applications I have 1 chiller pump on my primary chilled water side and system pump on my secondary side. Instead of balancing valves my inlet water goes to chiller 1 first and chiller 2 second, my outlet water comes from chiller 2 first and chiller 1 second. See the diagram I attached.
    Attached Files Attached Files

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