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  1. #1
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    Do you agree with this?



    Do you agree with this?

    Please express your opinion and justify it.

    double riser.JPG
    Regards


    To make progress is never good enough, I want to do better and better and better

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    Re: Do you agree with this?

    Yep!
    It is used when you have capacity control!
    Low capacity, oil and refrigerant goes throughout small pipe. Oil in large trap serve as restriction.
    High capacity, oil is blown from large trap and goes throughout large pipe.

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    Re: Do you agree with this?

    Quote Originally Posted by nike123 View Post
    Yep!
    It is used when you have capacity control!
    Low capacity, oil and refrigerant goes throughout small pipe. Oil in large trap serve as restriction.
    High capacity, oil is blown from large trap and goes throughout large pipe.
    That is at I think...the principle is universal for all systems. The problem is that the trap on the small riser it shouldn't exist...I mean this riser shall not be sealed!!
    To make progress is never good enough, I want to do better and better and better

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    Re: Do you agree with this?

    Yes Sandro, you are right, but if you look on reducing coupling before large trap, you'll see that oil don't flow to the large trap when low capacity (it's wrong reducing coupling). If you install small riser in top of general suction tube then on low capacities oil couldn't lift until capacity not rise. If you use additional small oil trap then oil will lift independently on capacity control.
    In some places will have to think ...

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    Re: Do you agree with this?

    Sorry, i did not paying attention to details.
    First, this arangment is noted as Amonia double riser, for which I am total ignorant.
    Next, from literature i found what you sugesting that there is no traps on small riser:



    Last edited by nike123; 29-03-2011 at 03:13 PM.

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    Re: Do you agree with this?

    Quote Originally Posted by nike123 View Post
    Sorry, i did not paying attention to details.
    First, this arangment is noted as Amonia double riser, for which I am total ignorant.
    Next, from literature i found what you sugesting that there is no traps on small riser:



    That's it, without P trap on the small riser. Even for R717 system wet suction line or dry suction line the principle shall be the same.
    To make progress is never good enough, I want to do better and better and better

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    Re: Do you agree with this?

    Quote Originally Posted by Aik View Post
    If you install small riser in top of general suction tube then on low capacities oil couldn't lift until capacity not rise.
    The oil is already entrained on the vapor-liquid R717. The P trap is only to oblige the fluids go by the small riser, after the P trap create an oil obstruction.
    So when the liquid-vapor R717 up by the small riser will also carryover the oil. If not oil would accumulate further on the horizontal tube below and will promote even more oil entrainment on the R717.
    To make progress is never good enough, I want to do better and better and better

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    Re: Do you agree with this?

    By the way the picture I have sent on the first post is from ASHRAE...
    To make progress is never good enough, I want to do better and better and better

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    Re: Do you agree with this?

    This is ammonia double riser. The purpose of this riser is to prevent(minimize) static pressure not oil return. Not too much oil in ammonia system. This riser can be very useful when load have significant fluctuation. At small load larger pipe will be trapped and small pipe will handle the load with minimum pressure drop.

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    Re: Do you agree with this?

    Straight out of the Carrier manuals for ***** based refrigerants. Never done the double riser for NH3 as I want all the oil in the trap to get flicked back with overfeed rate. Minimal carryover with NH3 unless serious compressor / separator problems.
    Everyone will bag me, for this comment. Cool.

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    Re: Do you agree with this?

    Quote Originally Posted by Segei View Post
    This is ammonia double riser. The purpose of this riser is to prevent(minimize) static pressure not oil return. Not too much oil in ammonia system. This riser can be very useful when load have significant fluctuation. At small load larger pipe will be trapped and small pipe will handle the load with minimum pressure drop.
    You should ensure that oil and liquid R717 get back to the separator, or else you will have bad heat transfer due to lower heat coefficient and as you said static pressure penalization.
    I still maintain my opinion that as the double riser is done the small pipe never will handle the small load. Like it is installed why should the R717 and oil only go through the small pipe rather than also by the larger pipe?
    To make progress is never good enough, I want to do better and better and better

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    Re: Do you agree with this?

    Quote Originally Posted by Magoo View Post
    Never done the double riser for NH3 as I want all the oil in the trap to get flicked back with overfeed rate.
    Sorry, I didn't understand. Please try explain in other way.

    Thanks Magoo
    To make progress is never good enough, I want to do better and better and better

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    Re: Do you agree with this?

    Quote Originally Posted by Segei View Post
    This is ammonia double riser. The purpose of this riser is to prevent(minimize) static pressure not oil return. Not too much oil in ammonia system. This riser can be very useful when load have significant fluctuation. At small load larger pipe will be trapped and small pipe will handle the load with minimum pressure drop.

    Segei is spot on in from my experience.
    Its for liquid overfeed system with low loads like freezer carton tunnel or similar.
    High loads then after load dies off smaller line comes into play.
    In some circumstances if you don't have it big slugs of liquid return all at once due to
    pressure build up (static head).
    Should give better performance at evaporator due to minimizing of pressure drop.

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    Re: Do you agree with this?

    Quote Originally Posted by RANGER1 View Post
    Segei is spot on in from my experience.
    Its for liquid overfeed system with low loads like freezer carton tunnel or similar.
    High loads then after load dies off smaller line comes into play.
    In some circumstances if you don't have it big slugs of liquid return all at once due to
    pressure build up (static head).
    Should give better performance at evaporator due to minimizing of pressure drop.
    Like it is installed how do you guarantee that R717 and oil only go through the small pipe rather than also by the larger pipe?
    To make progress is never good enough, I want to do better and better and better

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    Re: Do you agree with this?

    The way it is drawn: separated liquid refrigerant and any residual oil never gets into the large riser's trap unless you have a real serious overflow, due to the FOT reducer ...So on the restart of something like a spiral or similar process freezer: residual oil from upstream return line will be in the small trap. Assuming for a moment that traps and risers are all in a surrounding at higher temperature than the evaporator temperature: oil has means to return during pull down or otherwise high load because there will be a pressure drop through even the big riser.
    In an overfeed arrangement: if you take the small riser off the top of the main, the upstream pipe will start to act as separator and you will get some unmanageable liquid levels at the pump vessel...Depending on sizes, lengths and similar.

    We don't have any real use for the collection of fittings illustrated...Or put more succinctly: We don't do it this way.

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    Re: Do you agree with this?

    Quote Originally Posted by Sandro Baptista View Post
    Do you agree with this?

    Please express your opinion and justify it.

    double riser.JPG
    Regards
    We have been discuss this mater here:

    http://www.refrigeration-engineer.co...-piping-risers

    maybe is possible to find some answer/s there ...

    Best regards, Josip

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    Re: Do you agree with this?

    Interesting Josip & also oil does not get a mention.

    I think the oil must be mixed with liquid overfeed as ammonia is never really stagnent enough.

    If you have a plant with to much oil in it for whatever reasons it will cause a problem no matter what
    you do.

    The oil selection would also be important as if mineral oil its probably like honey anyway (this
    would be the case in Australia from what I have seen).


    I think an oil drain at lowest point would also be worthwhile.

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    Re: Do you agree with this?

    Quote Originally Posted by sterl View Post
    In an overfeed arrangement: if you take the small riser off the top of the main, the upstream pipe will start to act as separator and you will get some unmanageable liquid levels at the pump vessel...Depending on sizes, lengths and similar.
    l use for the collection of fittings illustrated...Or put more succinctly: We don't do it this way.

    Quote Originally Posted by RANGER1 View Post
    Interesting Josip & also oil does not get a mention.

    I think the oil must be mixed with liquid overfeed as ammonia is never really stagnent enough.

    If you have a plant with to much oil in it for whatever reasons it will cause a problem no matter what
    you do.

    Slope down the main on the direction to liquid separator
    To make progress is never good enough, I want to do better and better and better

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    Re: Do you agree with this?

    I meant the lower main pipe before the risers. The other (upper main, after the risers) also should be sloped as all know.
    To make progress is never good enough, I want to do better and better and better

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    Re: Do you agree with this?

    Purpose of the riser for ***** system is to return oil back to the compressor.
    Purpose of the riser for ammonia overfed system is to minimize static pressure penalties. Usually, this riser installed for production freezer(blast, plate) with significant changes in refrigeration load. Actually, IIAR suggest this riser as well. Small pipe should be sized for 1/3 of the max load and large pipe for 2/3 of max load.
    There are 2 types of flow in the riser. Vapor bubbles flow is low velocity flow. It will give us high static penalties. Liquid froth and mist flow will give us better static penalties, but penalties will increase if velocity become too high.
    At low load small pipe will work with minimal pressure drop(froth and mist) and large pipe will be partly filled with liquid. As soon as load increase, pressure drop in small pipe will increase either and large pipe will pick up part of the load. This is my understanding how it works.

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    Re: Do you agree with this?

    Quote Originally Posted by Segei View Post
    Purpose of the riser for ***** system is to return oil back to the compressor.
    Purpose of the riser for ammonia overfed system is to minimize static pressure penalties. Usually, this riser installed for production freezer(blast, plate) with significant changes in refrigeration load. Actually, IIAR suggest this riser as well. Small pipe should be sized for 1/3 of the max load and large pipe for 2/3 of max load.
    There are 2 types of flow in the riser. Vapor bubbles flow is low velocity flow. It will give us high static penalties. Liquid froth and mist flow will give us better static penalties, but penalties will increase if velocity become too high.
    At low load small pipe will work with minimal pressure drop(froth and mist) and large pipe will be partly filled with liquid. As soon as load increase, pressure drop in small pipe will increase either and large pipe will pick up part of the load. This is my understanding how it works.


    Segei,
    From my understanding small riser is sized on predicted or theoretical lowest load so risers is assured to work under all conditions min & max.
    They are not that common but if put in correctly can keep plant running a lot smoother on low loads.

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    Re: Do you agree with this?

    I think that minimum penalties will be at 1/3 of max load and at max load. At this conditions vapor velocity will be optimum. Between this 2 points, velocities in small riser will be higher than optimum and in large riser velocity will be lower than than optimum. It means that penalties will be higher. However, it will better than have one small or one lager riser.

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    Re: Do you agree with this?

    Barring those who have "smartened up" the liquid feed valves: At less than 30% load on a Recirc Arrangement, The Real Recirc Rate is going to be something like 10 or 12 to 1; presuming the original objective was 3 to 4:1 at design load.

    At low Mass Flux The ratio between whats tolerable as a return main pressure loss and whats required to lift liquid gets smaller as temperatures reduce......That is: if a 12" main is sized for a temperature change of 2-deg at 0, the temperature penalty at Minus 40 is a whole lot larger for the same refrigeration effect.

    At low loads: the smaller pipe takes all the flow, the larger being sealed at the trap. As the load increases, the pressure difference through the small pipe increases and the liquid rises in the larger riser...With a lot of temperature difference available the larger riser will eventually clear; that requires to throw much of its liquid to the higher level.

    If the small pipe is tied to the top of the lower main: vapor will pass over the top of a considerable liquid level to reach the small pipe so liquid will accumulate in the upstream pipe. Once the load increases this accumulation will start travelling again. At a really low percentage of the design load, the smaller pipe will seal at its trap and the upstream pressure will increase until 1) the heat exchanger runs out of temperature difference or 2) the smaller trap gets cleared.

    On the "in between": the bigger pipe successively accumulates; burps clear, flows for some interval depending on the real load, the difference in levels and the available temperature differences; the return main pressure difference almost disappears; the evaporator temperature drops; the "real" load increases; the cooled medium gets cold and the whole cycle starts over again.

    With process freezers: This tends to happen at line breaks that is everybody goes to lunch and the freezer has no "warm" product load. When the line resumes, the product load can also be light until the freezer is back to (say) 30% belt coverage. Then both traps clear as the upstream pressure rises and the vessel at the other end needs to be able to accomodate the liquid...As well as the compressors have to be able to pick up on the higher CFM flow as the vapor from the freezer suddenly "looses" half its evaporating pressure. In worst cases, the compressors will start tracking the liquid accumulation and the changes in return pressure.

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    Re: Do you agree with this?

    I would adjust ratio to 1:1 or 1:1.5 for max load. At 30% load it will be 1:3 or 1:5.
    About the break. It takes time to freeze the product. At least several minutes. This is more than enough time to unload compressor. I saw plant that had 50% of the load is off within 1 second. Properly sized vessel handled this issue.

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    Re: Do you agree with this?

    For me what is logic is a riser as Josip show on his ancient post
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    Re: Do you agree with this?

    Why did you ask this question if you are satisfied with explanation that was given you 5 years ago?

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    Re: Do you agree with this?

    Perhaps he was not aware of Josip explanation until it is linked here in this discussion.
    BTW, why is that bothering you?

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    Re: Do you agree with this?

    Quote Originally Posted by nike123 View Post
    Perhaps he was not aware of Josip explanation until it is linked here in this discussion.
    BTW, why is that bothering you?
    Actually, it is not bother me at all. I gave my explanation how it work. However, I didn't see his.
    BTW, Sandro was aware of Josip explanation. Read ancient post.

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    Re: Do you agree with this?

    I was just curious why you reacted in that manner. I don't need further involvement in that.

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    Re: Do you agree with this?

    Some years ago I have asked someting like that...10/12/2008 (my first post...it was when I came member from the RE) http://www.refrigeration-engineer.co...-piping-risers

    I was the last person to comment that thread and no one give me an answer. Now, after some year I decided to try again to listen your ideas.

    I was reading some posts of that thread (couldn't read all I confess...it was very tedious...) and I got this one amog others. This one gives highlight to my idea...and I share the same opinion as ICEMAN...By the way does some one knows about Iceman? I haven't see him on the RE.
    Re: ammonia suction piping risers

    Originally Posted by Andy
    ...he said that basically if you get the oil moving in the riser, it is drawn up the pipe walls and does not fall out...



    That is true Andy as long as the gas velocity is sufficient to drag the liquid up the riser. This is the annular flow regime for two phase. If the velocity begins to decrease due to a reduction in load, the liquid will begin to shear away from the wall and collapse back down into the riser causing it to flood. This is BAD.

    The liquid flooding the riser causes the evaporator pressure to increase due to static head and you loose temperature control. The boiling in the evaporator can actually stop when the static pressure increases.

    Originally Posted by Andy
    ..., so intermediate traps are a thing of the past. Maybe double risers are also



    Double risers ARE required if the evaporator load changes. And they will certainly do this. At a partial capacity point of operation the volume of vapor being generated in the evaporator can be much less.

    This then decreases the riser superficial velocity. If a single riser is used and it is too big, the riser floods.

    If a single riser is used and it is sized for the part load operation, the pressure drop at full load will be excessive and produce a negative impact on the required suction pressure. The suction pressure will have to be much lower to provide the pressure differential at full load. So, you also loose compressor capacity very quickly.

    The double riser is intended for evaporators where the capacity is variable. At full load both risers are carrying liquid up into the main horizontal pipe.

    At part load the bottom trap in the larger riser (as shown in Josips excellent drawing) will fill with liquid. This is supposed to seal the larger riser. The smaller volume of gas then flows up the smaller riser carrying the liquid with it.

    Each riser of a dual riser piping arrangement must be designed for it's respective gas velocity, which depends on the capacity reduction of the evaporator.


    On interesting point I just thought of. When the liquid fills the bottom trap in the larger riser...


    The trap depth should be deep enough to provide a static pressure (head) greater than the pressure loss through the smaller riser at part load. If not, the liquid can blow out of the trap seal and render the small riser ineffective.


    This is the same principle for water drains on high static pressure air handling units.
    To make progress is never good enough, I want to do better and better and better

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    Re: Do you agree with this?

    I agree with everything except trap depth. I think that trap depth is the hight the ammonia should be lifted. You can't change it. Double riser is self regulating system, because 2 pipes run in parallel. When small pipe handle the load, large pipe is partly flooded. If load increase, pressure drop in small pipe will increase and large pipe will pick up part of the load.

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    Re: Do you agree with this?

    Hi, I have recently read an old yellow paper titled "where did all the liquid go?", written by Arnett Smiley (York worker) in 1979 (A&R Bussiness) (perhaps older people heard about him), and he advised to design properly suction line "to keep the liquid where it belongs". He suggested to not forget the use of suction risers in industrial refrigeration. "The primary reason is to return oil, in DX systems, but it also keeps the liquid refrigerant moving back to the suction acumulator". He also suggested to install P trap before the raise to collect liquid and it will be blown up to the next level.

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