2013



Results 1 to 31 of 31
  1. #1
    Join Date
    May 2009
    Location
    Australia
    Posts
    8
    Rep Power
    0

    Question suction line traps


    Can anyone remind me why have a suction line trap straight out of an evaporator in a room? i.e at the bottom of the riser.



  2. #2
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,340
    Rep Power
    15

    Re: suction line traps

    Traps in refrigerant piping are used for the purpose of providing an area for oil collection. When the oil accumulates in the trap it will eventually create a seal (blocks off the pipe inside diameter). When the pressure behind the seal increases, the liquid oil is pushed up the riser so that the oil can return to the compressor.

    Where the traps are located and their spacing between traps depends on where the equipment is located (above or below the compressor).
    If all else fails, ask for help.


  3. #3
    Join Date
    May 2009
    Location
    Australia
    Posts
    8
    Rep Power
    0

    Re: suction line traps

    Quote Originally Posted by US Iceman View Post
    Traps in refrigerant piping are used for the purpose of providing an area for oil collection. When the oil accumulates in the trap it will eventually create a seal (blocks off the pipe inside diameter). When the pressure behind the seal increases, the liquid oil is pushed up the riser so that the oil can return to the compressor.

    Where the traps are located and their spacing between traps depends on where the equipment is located (above or below the compressor).
    Thanks mate, I knew that much but more to the fact is there any advantage of having the trap at the riser instead of else where in the pipe run.

  4. #4
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,340
    Rep Power
    15

    Re: suction line traps

    The traps are at the riser so that you can move the oil. Otherwise, without the traps you are dependent on the gas velocity to return it in much smaller amounts. It may or may not return in sufficient quantities to maintain the oil level in the compressor.
    If all else fails, ask for help.


  5. #5
    Join Date
    May 2009
    Location
    Australia
    Posts
    8
    Rep Power
    0

    Re: suction line traps

    Thanks for your time mate.

  6. #6
    Join Date
    Oct 2008
    Location
    mallorca, spain,
    Age
    65
    Posts
    768
    Rep Power
    9

    Re: suction line traps

    yes the reason for suction line traps is to assist the oil to return to the compressor if the evap is much lower than the con.d unit the traps are normally 3-4 meters apart, and so the oil slugs up the suction line trap by trap
    Last edited by sedgy; 17-05-2009 at 08:06 PM. Reason: message missed out a line

  7. #7
    Join Date
    Jun 2007
    Location
    Vietnam
    Posts
    67
    Rep Power
    8

    Re: suction line traps

    Dear US Iceman:
    The velocity is important for oil return to compressor, then how can I uncrease the velocity? thanks!

  8. #8
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,340
    Rep Power
    15

    Re: suction line traps

    Quote Originally Posted by Tung View Post
    Dear US Iceman:
    The velocity is important for oil return to compressor, then how can I uncrease the velocity? thanks!
    Make the pipe diameter smaller.
    If all else fails, ask for help.


  9. #9
    Join Date
    Mar 2006
    Location
    Merate (LC) - Italy
    Age
    43
    Posts
    2,554
    Rep Power
    14

    Re: suction line traps

    Or use double riser

  10. #10
    Join Date
    Sep 2003
    Location
    ISRAEL
    Age
    62
    Posts
    3,618
    Rep Power
    22

    Re: suction line traps

    Another useful thing to do is to connect a small diameter pipe [3\8] from the bottom of each oil trap to the top of the next oil trap.

    the pressure in the small diameter pipe is slightly higher and the oil goes up faster.

  11. #11
    Join Date
    Jun 2007
    Location
    Vietnam
    Posts
    67
    Rep Power
    8

    Re: suction line traps

    Dear US Iceman:
    What I mean is if the suction pipe size is fixed, then is there any other method that I can increase the velocity?
    will it work to throttle cooling water to increase the con'd pressure and make the pressure differential between con'd and evap larger?
    I am talking about the chiller with a flooded evap.
    Thanks!

  12. #12
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,340
    Rep Power
    15

    Re: suction line traps

    Quote Originally Posted by Tung View Post
    Dear US Iceman:
    What I mean is if the suction pipe size is fixed, then is there any other method that I can increase the velocity?
    will it work to throttle cooling water to increase the con'd pressure and make the pressure differential between con'd and evap larger?
    I am talking about the chiller with a flooded evap.
    Thanks!
    If you have a flooded chiller that has an accumulator on top of it you do not need traps in the suction line. You need some way to recover the oil directly from the shell of the chiller (assuming the refrigerant is in the shell & on the external surface of the tubes). This is an entirely different issue.
    Last edited by US Iceman; 23-05-2009 at 06:19 PM. Reason: spelling fix
    If all else fails, ask for help.


  13. #13
    John Smith's Avatar
    John Smith Guest

    Re: suction line traps

    Quote Originally Posted by US Iceman View Post
    Traps in refrigerant piping are used for the purpose of providing an area for oil collection. When the oil accumulates in the trap it will eventually create a seal (blocks off the pipe inside diameter). When the pressure behind the seal increases, the liquid oil is pushed up the riser so that the oil can return to the compressor.

    Where the traps are located and their spacing between traps depends on where the equipment is located (above or below the compressor).
    Our new lecturer at bath college taught us that traps in suction lines are only to prevent large slugs returning to the compressor. He said they cannot help oil return and actually reduce oil return and system efficiency. And the trap on a evaporator is only for compressor startup so that oil and liquid dont effect the txv sensor.

  14. #14
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,340
    Rep Power
    15

    Re: suction line traps

    Quote Originally Posted by John Smith View Post
    Our new lecturer at bath college taught us that traps in suction lines are only to prevent large slugs returning to the compressor. He said they cannot help oil return and actually reduce oil return and system efficiency. And the trap on a evaporator is only for compressor startup so that oil and liquid dont effect the txv sensor.
    Sorry to upset the apple cart, but I disagree.

    Think about what a trap is: It is a place to collect liquid (could be refrigerant or oil, but it is intended for oil). In a Freon system the oil is moved by velocity. That is why we go through the trouble of trying to size pipes. We are trying to balance the need for velocity to move the oil versus limiting pressure loss in the piping so that performance or capacity is not affected.

    As the pipe trap accumulates liquid the velocity through the trap increases. If the mass flow decreases due to lower system load, the velocity decreases through the trap and it begins to seal. Once that happens the pressure in the upstream side of the pipe increases. When a sufficient pressure difference is created across the trap, the seal blows out and the oil slug moves downstream.

    The trap is there to allow the oil to collect until the trap seals. All of this is necessary to allow the oil to circulate in the system in a controlled manner so that the oil returns to the compressor.

    You want the trap after a TXV bulb so that the oil does not insulate the bulb and affect its temperature sensing which could affect the TXV (could cause hunting on the valve).
    If all else fails, ask for help.


  15. #15
    John Smith's Avatar
    John Smith Guest

    Re: suction line traps

    Quote Originally Posted by US Iceman View Post
    Sorry to upset the apple cart, but I disagree.
    No it is very interesting. Our new lecturer Marc O'Brien started a lecture telling us that there were still a lot of people who thought that suction traps help return oil. He is a consultant but lectures one day a week for level 3.

    I have typed out the notes from two of his powerpoint slides.

    In the design of high or low pressure vapour lines oil and condensed vapour slugs require management.

    This applies to suction, discharge, hot gas and latent gas lines.

    Generally we want to slope in a downstream direction except in the vicinity of compressors or on the compressor side of surrounding traps where we must always slope away from the compressor for off-cycle protection.

    A suction or discharge riser that fails to properly carry oil will not be improved on by the addition of traps in the rising section, as shown top right. Adding such traps to risers will in fact worsen the oil carrying abilities by adding friction thereby reducing vapour velocities.

    Traps in riser sections only help to limit the size of oil slugs arriving at the compressor during or after the off-cycle.

    Then, seen bottom right, traps are not required if there is an oil separator at the base of the riser. Nor are double risers required.

    If there are problems carrying oil up any riser section, suction or discharge, then apart from the installation of a double riser, the best solution is simply a reducing in the diameter of the riser.

    In the illustration to the right the reducers and elbows are configured so as to minimise the oil quantities draining back upstream from the base bend or draining back down the riser from the top horizontal section.

    Traps along the riser can be added to also provide oil slug size management but their addition will not assist the riser in carrying oil there will be no improvement in oil lift.

    He also says that adding traps to a system that does not return oil does not add a risk of oil slugs hitting the compressor even if the traps are very big.

    I can tell him about this forum if you want.

  16. #16
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,340
    Rep Power
    15

    Re: suction line traps

    Quote Originally Posted by John Smith
    I can tell him about this forum if you want.
    John,

    Don't do anything rash!

    Mr. O'Brien used to be a moderator here and left after some discussion. I'm not going to get into specifics in a public venue about this, and would prefer to let sleeping dogs lie.

    Double risers are only required if the system/compressor has capacity control. If the system load decreases you need to have a method for unloading the compressors (speed or displacement). When this is done the swept volume through the suction line decreases and reduces the piping velocity. That is why you need double risers (suction or discharge) for oil return back to the compressor. Otherwise, the oil will simply lay there until the gas velocity increases to the point where the oil slug moves.

    If you have vapor condensing in a gas pipe you of course need to provide a method for minimizing the liquid slug once it begins to move in the system. This happens a lot in steam system and is called drip draining or something similar to that. This is something different from an oil slug from a P-trap.

    You could say an oil separator at the bottom of the riser would work, if you can return the oil automatically to the compressor. There a lot of ways to manage oil in a refrigeration.

    The best way is to do it such that no gadgets are used or a method that requires human intervention to make it work.
    If all else fails, ask for help.


  17. #17
    Join Date
    Aug 2001
    Location
    UK
    Posts
    1,068
    Rep Power
    14

    Re: suction line traps

    .



    Iceman is right in what he says and there is sufficient information in well respected published books on refrigeration design to support it.
    There's always the need to challenge orthodox thinking on any subject. Some folk say that the Earth is flat, but we know that it is round in reality.

    All oil circulates to a greater or lesser extent through the system. Suction traps are a method of oil return that avoids the expense and considerable flow resistance you get with the other methods, for instance, discharge line oil separators.

    If you have liquid likely to slug back to the compressor you have got a fundamentally serious problem with your evaporation process, either that or you need an accumulator in the event of an automatic defrost cycle. That's a completely different problem and a different solution.

    I'll be quite honest and say that suction traps tend to be an academic subject - taught in colleges because somebody read it in a book and then forgotten about.

    Hands up, how many installations have the height of risers that provide a genuine need for suction traps and then have them installed properly?

    Along with sloping pipe approaches next to the compressor, it's a rule from the text books mostly observed in the breach.

    If I were you I'd agree with your lecturer, the Earth really is flat and then move on.



    .

  18. #18
    John Smith's Avatar
    John Smith Guest

    Re: suction line traps

    Quote Originally Posted by US Iceman View Post
    Double risers are only required if the system/compressor has capacity control. If the system load decreases you need to have a method for unloading the compressors (speed or displacement). When this is done the swept volume through the suction line decreases and reduces the piping velocity. That is why you need double risers (suction or discharge) for oil return back to the compressor. Otherwise, the oil will simply lay there until the gas velocity increases to the point where the oil slug moves.


    If you have vapor condensing in a gas pipe you of course need to provide a method for minimizing the liquid slug once it begins to move in the system. This happens a lot in steam system and is called drip draining or something similar to that. This is something different from an oil slug from a P-trap.


    You could say an oil separator at the bottom of the riser would work, if you can return the oil automatically to the compressor. There a lot of ways to manage oil in a refrigeration.


    The best way is to do it such that no gadgets are used or a method that requires human intervention to make it work.

    Thank you iceman.


    Marc covered double risers in detail in other powerpoint slides. He talks a lot more than he writes about each point so we have a lot of writing to do during lectures. He said you also need to use double risers if you have unloading evaporators even if the compressor does not unload which is rare these days.


    I received an email reply from him today. I asked Marc about the Trane newsletter which is attached.


    Here is Marc O'Briens reply


    Double risers are certainly not a conventional practice no longer needed. They are still as important today as they ever were. But I can imagine an explanation for why the Trane document says so. They say the scrolls they use unload. The scroll compressor then is probably the Copeland digital which disengages the scrolls every 10 seconds for a period inversely proportional to the load proportion. This means the suction riser gas velocities do, for a moment every 10 seconds, rise to design levels.


    Required suction riser velocity can be calculated...
    V = 0.723(gD((Rl/Rv)-1))^(1/2)
    g = gravity
    Rl = density of oil and refrigerant solution
    Rv = density of refrigerant vapour


    The lower the evaporating temperature and the higher the superheat or pipe diameter the higher the velocity required.


    It is a nonsense that a trap atomises a pool of oil better than an elbow or bend does. It is nonsense too anyway that atomising the oil pool even helps. Oil is simply swept up the pipe wall.


    It is certainly a nonsense that during normal operation, unloaded, a slug can form in a trap ready to be suddenly transported on the instance of completing its formation as US Iceman claims.


    In physics the principle by which an oil slug is shot up the riser is called an impulse and an impulse is a near instantaneous change in momentum, which can only happen at a trap on start up or upon a substantial compressor capacity transient. It can also occur with destructive forces on pumped liquid systems after hotgas defrost when the suction stop valve is suddenly opened. You will recall this from the liquid hammer videos given during my lecture on pumped liquid systems.


    Imagine there was no continuous flow of gas behind an impulse driven oil slug. The height it would reach then, in a frictionless system, is simply proportional to the square of its initial velocity. The height reached is a conversion from kinetic energy to potential energy KE=PE so 0.5mv = mgh and in terms of h is 0.5v/g or if we needed to hit 10m the impulse velocity needed would be (2gh)^(1/2) = (2x9.81x10)^0.5 = 14m/s. If we assume the density of the oil is 800kg/m we can estimate that in a pipe trap there might be 18 grams of oil or 0.018kg. Now impulse is the product of force and change in time Fdt. If we assume a time of impulse of 10 milliseconds then the force to accelerate the 18grams of oil instantaneously to 14m/s would be:


    Impulse = m(Vf Vo)/dt = 0.018(14-0)/0.01 = 25N


    The inside area of a outside diameter pipe would be about 10.9mm squared times Pi. Dividing that into 25N gives 66733Pa. That means that an instantaneous or over 10milliseconds at least, pressure of 0.66Bar is required to shoot the slug up 10m. In reality the time over which the force is applied is much longer which compensates for the more gradual, though sometimes not much more gradual, development of pressure differential. Then the gas flow up the pipe continues to also help by the sweeping effect.


    The larger the pipe the greater the impulse force pressure required but we don't see the height between traps being recommended to be less with increasing pipe diameters. Because traps are intended only to limit the size of the slugs potentially arriving at the compressor on start-up.


    So, the numbers above help illustrate the principle of the use of traps only as a means to limit the size of slug arriving at the compressor rather than as a means of assisting oil return during continuous operation.


    You see, if we admit that the size of traps have to be limited to limit the size of slugs arriving at the compressor then we are forced to also admit that oil traps cause oil return, the one is an implied statement of the other. But then their assistance in oil return would only be relevant to systems stopping and starting frequently which is not how we anyway design systems. We instead design systems for as much continuous operation as possible.


    With regard to continuous operation, a trap will not atomise an oil pool better than an elbow will and anyway the oil still has to be swept up the wall atomised or not. And since during normal operation no significant impulse forces occur on account of any rapid or instantaneous pressure transients a trap cannot cause a slug to be transported in any near-whole form, if one could even be formed, we are left then with only the principle of suction line velocities for sweeping oil return. A properly designed system does not allow slugs of oil to collect in any trap other than on the larger trap of a double riser during low loads.



    The maximum pressure that could build up behind a slug is mgh where m is 800kg/m^3 x volume and since a slug would be liquid it would be caused by such a tiny pressure forming to be instantly spread across the pipe wall area otherwise just sit there while the gas bubbles its way through.



    One of my favourite quotes and very relevant here:


    Wittgenstein once asked a friend, "Tell me, why do people always say it was natural for man to assume that the Sun went round the Earth rather than that the Earth was rotating?" His friend replied, "Well, obviously, because it just looks as though the Sun is going round the Earth." To which Wittgenstein responded, "Well, what would it have looked like if it had looked as though the Earth was rotating?"


    You are welcome to copy and paste this to the discussion board.


    I used to police the thinking on that discussion board so it is no longer as sharp as it used to be.

  19. #19
    John Smith's Avatar
    John Smith Guest

    Re: suction line traps

    The attachment did not work. I hope it works this time.
    Attached Files Attached Files

  20. #20
    Join Date
    Aug 2001
    Location
    UK
    Posts
    1,068
    Rep Power
    14

    Re: suction line traps

    .

    You asked for it, Iceman.

    An answer that reads like like 'War and Peace'.
    I should remind you that today, May 25th, is an official Bank Holiday in the UK and the entire country has got bugger-all to do.


    On the subject of deep, deep, philosophy, Monty Python got it about right:

    "And Wittgenstein was a beery swine
    Who was just as schloshed as Schlegel".



    Toodle pip!


    .

  21. #21
    Join Date
    Oct 2003
    Location
    Belgium
    Age
    52
    Posts
    5,496
    Rep Power
    22

    Re: suction line traps

    Quote Originally Posted by John Smith View Post
    Our new lecturer at bath college taught us that traps in suction lines are only to prevent large slugs returning to the compressor. He said they cannot help oil return and actually reduce oil return and system efficiency. And the trap on a evaporator is only for compressor startup so that oil and liquid dont effect the txv sensor.
    If this should be true, then all the refrigeration books from the last 50 years has to be rewritten.
    Why an oil trap should prevent for liquid slugging? It's just the opposite: liquid can accumulate as as soon area is reduced enough, all the liquid is pushed through it at once.
    And the trap on a evaporator is only for compressor startup so that oil and liquid dont effect the txv sensor
    This is not worth arguing on it because both doesn't have a relation with each other.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

  22. #22
    Join Date
    Oct 2003
    Location
    Belgium
    Age
    52
    Posts
    5,496
    Rep Power
    22

    Re: suction line traps

    Quote Originally Posted by John Smith View Post
    <...
    Here is Marc O'Briens reply

    .... They say the scrolls they use unload. The scroll compressor then is probably the Copeland digital which disengages the scrolls every 10 seconds for a period inversely proportional to the load proportion. ....
    The article is from 1998 and the digital scroll wasn't invented those days.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

  23. #23
    Join Date
    Oct 2003
    Location
    Belgium
    Age
    52
    Posts
    5,496
    Rep Power
    22

    Re: suction line traps

    Quote Originally Posted by John Smith View Post

    Required suction riser velocity can be calculated...
    V = 0.723(gD((Rl/Rv)-1))^(1/2)
    g = gravity
    Rl = density of oil and refrigerant solution
    Rv = density of refrigerant vapour


    In physics the principle by which an oil slug is shot up the riser is called an impulse and an impulse is a near instantaneous change in momentum, which can only happen at a trap on start up or upon a substantial compressor capacity transient. It can also occur with destructive forces on pumped liquid systems after hotgas defrost when the suction stop valve is suddenly opened. You will recall this from the liquid hammer videos given during my lecture on pumped liquid systems.

    The height reached is a conversion from kinetic energy to potential energy KE=PE so 0.5mv = mgh and in terms of h is 0.5v/g or if we needed to hit 10m the impulse velocity needed would be (2gh)^(1/2) = (2x9.81x10)^0.5 = 14m/s. If we assume the density of the oil is 800kg/m we can estimate that in a pipe trap there might be 18 grams of oil or 0.018kg. Now impulse is the product of force and change in time Fdt. If we assume a time of impulse of 10 milliseconds then the force to accelerate the 18grams of oil instantaneously to 14m/s would be:


    Impulse = m(Vf Vo)/dt = 0.018(14-0)/0.01 = 25N


    The inside area of a outside diameter pipe would be about 10.9mm squared times Pi. Dividing that into 25N gives 66733Pa. That means that an instantaneous or over 10milliseconds at least, pressure of 0.66Bar is required to shoot the slug up 10m.

    The maximum pressure that could build up behind a slug is mgh where m is 800kg/m^3 x volume and since a slug would be liquid it would be caused by such a tiny pressure forming to be instantly spread across the pipe wall area otherwise just sit there while the gas bubbles its way through.

    Wittgenstein once asked a friend, "Tell me, why do people always say it was natural for man to assume that the Sun went round the Earth rather than that the Earth was rotating?" His friend replied, "Well, obviously, because it just looks as though the Sun is going round the Earth." To which Wittgenstein responded, "Well, what would it have looked like if it had looked as though the Earth was rotating?"

    You are welcome to copy and paste this to the discussion board.

    I used to police the thinking on that discussion board so it is no longer as sharp as it used to be.
    Doubt his students understand anything of these figures. This is not teaching, this trying to overwhelm them, trying to make a good impression. Pathetic, it became even worse after all those days.
    And always ending with a quote found somewhere on internet, ohhh...those good old days.

    It was a little bit to sharp those days.
    Last edited by Peter_1; 25-05-2009 at 07:37 PM.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

  24. #24
    John Smith's Avatar
    John Smith Guest

    Re: suction line traps

    Quote Originally Posted by Peter_1 View Post
    If this should be true, then all the refrigeration books from the last 50 years has to be rewritten.
    Why an oil trap should prevent for liquid slugging? It's just the opposite: liquid can accumulate as as soon area is reduced enough, all the liquid is pushed through it at once.
    And the trap on a evaporator is only for compressor startup so that oil and liquid dont effect the txv sensor
    This is not worth arguing on it because both doesn't have a relation with each other.
    Hi Peter1

    Not liquid slugging. Slugs of oil.

    Marc O'Brien showed us image 19.5 on page 455 in my RJ Dossat. I have the 3rd edition. It shows the txv bulb must be installed 30cm up the vertical riser if a trap cannot be used. If a trap can be used then the bulb can be installed before the trap on a horizontal. The trap collects oil during the off cycle so that it does not interfere with the bulb when the compressor starts.

  25. #25
    John Smith's Avatar
    John Smith Guest

    Re: suction line traps

    Quote Originally Posted by Peter_1 View Post
    Doubt his students understand anything of these figures. This is not teaching, this trying to overwhelm them, trying to make a good impression. Pathetic, it became even worse after all those days.
    And always ending with a quote found somewhere on internet, ohhh...those good old days.

    It was a little bit to sharp those days.
    Thank you Peter1

    Do you think the figures wrong?

    I didn't mean to quote what Marc said about the discussion board not being sharp. I didn't read all I pasted.

    If you think Trane must stop rewriting the books can you explain why they are wrong?

  26. #26
    Join Date
    Oct 2003
    Location
    Belgium
    Age
    52
    Posts
    5,496
    Rep Power
    22

    Re: suction line traps

    Quote Originally Posted by John Smith View Post
    Hi Peter1

    Not liquid slugging. Slugs of oil.
    Quote from your post In the design of high or low pressure vapour lines oil and condensed vapour slugs require management.
    Condensed vapor is liquid for me.
    And an oil bend in the discharge isn't there for oil reasons like he said. (4th line in the same message)
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

  27. #27
    John Smith's Avatar
    John Smith Guest

    Re: suction line traps

    Quote Originally Posted by Peter_1 View Post
    Quote from your post In the design of high or low pressure vapour lines oil and condensed vapour slugs require management.
    Condensed vapor is liquid for me.
    And an oil bend in the discharge isn't there for oil reasons like he said. (4th line in the same message)
    Sorry Peter1. I dont want to scan the page. The traps that prevent liquid slugging are inverted traps. When a evaporator does not pump down. Also before a condenser if the compressor is below, the pipe goes up then back down to the condenser. Most important in discharge lines like multiple compressors. That isnt what Marc is talking about. He is talking about traps in suction risers.

  28. #28
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,340
    Rep Power
    15

    Re: suction line traps

    I used to police the thinking on that discussion board so it is no longer as sharp as it used to be.
    We are not worthy of basking in the light of knowledge.


    I have an issue with instructors who throw a bunch a calculations and fancy stuff at students. The students may not understand what the importance is and how to use it. The second part is this: these are engineering calculations and you need to know what the basis of the assumptions are on whic the calculations are derived. It's like me saying 2+2 = square root of 4 X 2. If you do the math wrong you get a wrong answer and then the customer pays to fix it.

    We started to talk about risers and now we are off on traps, TXV's, oil separators, liquid slugs, etc. That is just too many different subjects to roll into one discussion and have it be meaningful for students (unless the idea is to impress the students).
    Last edited by US Iceman; 26-05-2009 at 12:23 AM. Reason: fix
    If all else fails, ask for help.


  29. #29
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,340
    Rep Power
    15

    Re: suction line traps

    It is certainly a nonsense that during normal operation, unloaded, a slug can form in a trap ready to be suddenly transported on the instance of completing its formation as US Iceman claims.
    This is too easy....

    If the system capacity is reduced, the gas velocity through the evaporator and suction line is decreased. When this occurs the return of oil is minimized as there is insufficient velocity to move the oil. At this time the only thing that can move oil is gravity or miscibility of the oil/gas interface. If the flow is upwards gravity works against you.

    Eventually, if the system capacity stays reduced long enough the oil begins to pool in the trap (if provided). As the oil pool grows the trap is sealed. As soon as the pressure builds up sufficiently to move the oil slug it flows upward. The pressure buildup can occur as a pressure differential consisting of two things: suction pressure drops due to compressor loading up or the evaporator pressure raises just enough to blow the trap out.

    Peter and I are both instructors too and I have a lot of faith in Peters ability and practical experience. We have both worked on equipment and taught others how to and we are still in the same business and still mods here.

    I think its safe to say you have two options: believe everything the guest instructor says or keep an open mind and do some research yourself.
    If all else fails, ask for help.


  30. #30
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,340
    Rep Power
    15

    Re: suction line traps

    Inverted traps do not prevent liquid slugging. Their purpose is to be found on the top of the riser to prevent the oil from draining back down the riser. Follow the oil slug moving upwards. If the oil slug (which is liquid also) flows up the horizontal pipe it is supposed to flow through an inverted trap before the oil reaches the horizontal pipe. This inverted trap keeps the liquid oil from running back down the riser.

    If a sufficient quantity of oil falls back down the riser it is called flooding. In this case the riser diameter is flooded or sealed. The liquid slug falls back down until the gas pressure pushes it back up. This oscillation can generate some funny readings on pressure gauges and how the system reacts to this. This tends to happen in two-phase thermosiphon vent lines if they are not sized properly for annular flow instead of slug flow.

    And that's all I'm going to say on this thread and subject because I am not going to get into a long distance peeing contest with the enlightened one.
    Last edited by US Iceman; 26-05-2009 at 12:16 AM. Reason: spelling
    If all else fails, ask for help.


  31. #31
    Join Date
    Aug 2001
    Location
    UK
    Posts
    1,068
    Rep Power
    14

    Re: suction line traps

    .

    Put simply, the question got sufficient detailed answers - some would argue too much detail - for an obscure detail of pipe design.

    There's enough information for all the posters to make up their own minds.

    I hope that the original poster is not scratching his head and wondering what he started.


    .

Similar Threads

  1. Superheat and Subcooling
    By Chunk in forum Fundamentals
    Replies: 42
    Last Post: 15-01-2011, 02:24 AM
  2. Replies: 5
    Last Post: 14-09-2009, 07:33 AM
  3. Suction line?
    By SkyWalker in forum Fundamentals
    Replies: 6
    Last Post: 18-05-2009, 10:34 PM
  4. suction line velocity
    By Kevin Yeo in forum Supermarket Refrigeration
    Replies: 16
    Last Post: 24-08-2008, 01:26 PM
  5. Benji
    By benji in forum Trouble Shooting
    Replies: 29
    Last Post: 30-01-2008, 02:25 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •