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  1. #1
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    Need help with tower bypass valve piping

    So I looked at a tower today for a customer, although that wasn't what I was there for. Carrier 30HXC.

    He said over the winter a tower bypass 3-way valve was installed, and this is it's first season in service. He said in March the chiller would go off on high head pressure multiple times per day. It's not doing that anymore because it's hot enough outside now that the bypass stays open. But now he has issues with it getting air locked every day and he has to go to it every morning to cycle the pump a few times to get the air out. I manually started actuating the valve and sure enough the pump started gurgling.

    The chiller and tower are right next to each other on the roof. The piping isn't run so that it would create any air traps, it is lower than the tower supply the whole short run it has. The tower is plenty full of water, not getting low and sucking air through the strainer.

    So I started searching on-line because I didn't think the bypass valve was installed correctly. I found quite a few posts on these forums talking about it. One said if it were piping as mixing valve rather than a diverting valve it tends to build up air, which wasn't making much sense to me. But the way they have this one installed, as the valve starts to bypass, it is bypassing the chiller instead of the tower. So it is reducing the flow through the chiller while maintaining full flow through the tower.

    I thought it was supposed to be the other way around. So I started looking for diagrams on-line. Everything I found said to always use a diverting valve, not a mixing valve, and always maintain full flow through chiller. Which made sense to me. So I ended up finding the 30HXC install manual, and sure enough they show the piping the exact way it is installed now: with a mixing valve and bypassing the condenser. So I wanted to post here and see what you guys though. I am including a picture of the actual manual diagram, and one modified for the way I thought it was supposed to be.

    Please tell me if I am completely wrong on this or what I am missing here. I didn't have time to look into the programming while I was there but maybe it is just programmed wrong.

    EDIT: I just added another picture, it is from a Daikin manual (two diagrams), with the piping the way I saw it everywhere else on the internet, and it mentions in the text above (not in the picture) about needing a check valve to prevent air from getting into piping an pump. In those it's either a diverting or a mixing but in both cases it maintains full flow through condenser.
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  2. #2
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    I agree on the the "way it should be" diagram.
    I have a system with 2 York's piped in the other way (condenser bypass). Yeah, have issues there. That building does have free cooling, but if memory serves me right, there is already a 3 way valve for that off the main, not at each chiller.
    That may work in your application, but I would think that a manual stop needs to be there so it maintains a minimum flow through the condenser.
    Looking at it more, it's acting kind of like a head pressure regulating valve... but keeps full flow to save the pump. I would imagine the modulating signal would keep a minimum condenser water outlet temp.
    -----Stop, step back, relax and have another go at it.-----

  3. #3
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    Reducing the water flow going through the cooling tower will increase the tower fill fouling also the tower water in the sump will be stagnant lacking proper treatment. The air in the pump could be the result of water temperature and pressure changes with air being absorbed then liberated.
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  4. #4
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    Quote Originally Posted by valdelocc View Post
    Reducing the water flow going through the cooling tower will increase the tower fill fouling also the tower water in the sump will be stagnant lacking proper treatment. The air in the pump could be the result of water temperature and pressure changes with air being absorbed then liberated.
    Reducing condenser flow isn't good either! Well, as long as it's not for an extended period of time. Flow should only be reduced until the loop is up to temp, then wide open (design flow). I would imagine it should be the same for tower bypass.
    -----Stop, step back, relax and have another go at it.-----

  5. #5
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    Quote Originally Posted by BennyD View Post
    Reducing condenser flow isn't good either! Well, as long as it's not for an extended period of time. Flow should only be reduced until the loop is up to temp, then wide open (design flow). I would imagine it should be the same for tower bypass.
    Around here I see way more reduced/modulating condenser water flow versus tower bypass, why do you think its not a good thing?
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  6. #6
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    Quote Originally Posted by valdelocc View Post
    Around here I see way more reduced/modulating condenser water flow versus tower bypass, why do you think its not a good thing?
    Isn't there more condenser tube fouling with low flow?
    -----Stop, step back, relax and have another go at it.-----

  7. #7
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    Quote Originally Posted by BennyD View Post
    Isn't there more condenser tube fouling with low flow?
    Thats a valid point although I have not notice much differences in the other hand a low flow tower would make stalactites quick. are you in NYC?
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  8. #8
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    Other side of the state, Buffalo. Still battle hard water.
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  9. #9
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    Does the pipe between the tower sump and the condenser rise above the tower sump at any point?

  10. #10
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    Quote Originally Posted by WAYNE3298 View Post
    Does the pipe between the tower sump and the condenser rise above the tower sump at any point?
    I would need to double check that, but I believe not. The suction side of the pump comes right off the tower. It comes out of the tower, down to the ground, through the wall, directly into the pump. The discharge of the pump goes up about four feet, over and back down into the condenser, and also into the bottom of the bypass valve (as the piping looks in the carrier chiller diagram). The problem they reported was noise coming from air stuck in the pump, and they would have to cycle the pump a few times to get the air out. But if the pump comes directly off the tower sump, and is lower than the sump, where does the air come from? Is there any other way air could get in there other than the sump getting low on water and sucking air through the sump? I didn't think about what would happen if the pump shut off if it's interlocked with the chiller controls. He said it happens every morning so I wonder if when it gets cooler at night the chiller satisfies and shuts off and turns the pump off. But even still, the pump and all suction piping is lower than the sump.

    If there isn't a check valve in the line that goes back to the top of the tower, that would actually drain back down to the level of the sump if the pump shut off wouldn't it? The line coming out of the condenser to the mixing valve is higher than the sump on the other side of the wall. That still doesn't explain why the pump would get air in it being at the very bottom of everything.

  11. #11
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    I take it that there probably is not a VFD on the condenser pump? I'm wondering if there is a good negative pressure on the inlet to the pump on startup. That would cause air to escape from the water, kinda like what valdelocc was getting at. Not sure if it would be enough to cause the pump to cavitate to that extent. Once it establishes flow, it goes away.

    Maybe try starting the pump with the discharge valve 3/4 closed and slowly open once it starts.

    Does the pump continually sound like that if there was no interference or does it go away over time? How is the strainer in the tower sump?
    -----Stop, step back, relax and have another go at it.-----

  12. #12
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    Thinking about it more...
    When the pump starts, is the new valve in the bypass position? There may be less head pressure since the water is not flowing through the chiller. This might be the change that your seeing. The pump has less head, causing an even lower pressure on the inlet.

    Another thing to try is to have the valve in the position to have full flow through the chiller BEFORE the pump starts.
    -----Stop, step back, relax and have another go at it.-----

  13. #13
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    Quote Originally Posted by BennyD View Post
    I take it that there probably is not a VFD on the condenser pump? I'm wondering if there is a good negative pressure on the inlet to the pump on startup. That would cause air to escape from the water, kinda like what valdelocc was getting at. Not sure if it would be enough to cause the pump to cavitate to that extent. Once it establishes flow, it goes away.

    Maybe try starting the pump with the discharge valve 3/4 closed and slowly open once it starts.

    Does the pump continually sound like that if there was no interference or does it go away over time? How is the strainer in the tower sump?
    Quote Originally Posted by BennyD View Post
    Thinking about it more...
    When the pump starts, is the new valve in the bypass position? There may be less head pressure since the water is not flowing through the chiller. This might be the change that your seeing. The pump has less head, causing an even lower pressure on the inlet.

    Another thing to try is to have the valve in the position to have full flow through the chiller BEFORE the pump starts.
    If I ever go back for that I will check that. No VFD. I just looked at it briefly while I was there for something else and upon looking at how the bypass valve was installed I assumed it was wrong but apparently it can be done that way. I still think it might be programmed wrong seeing as it was going off on high head several times a day in March (when it would have actually needed to bypass) and was probably cutting the flow back too far through the chiller. Definitely need to go back and do some tests starting and stopping the pump like you described. I did look in the tower and the strainer looked clear, no debris.

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