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  1. #1
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    Market Rack Drop-Legs.

    Havent had much to do with them but are being retrofitted to existing Markets.

    What are they for ?
    What do they do ?

    Any reliable information source to obtain some information ?

    TIA.
    The primary function of the design engineer is to make things difficult for the fabricator and impossible for the serviceman.

  2. #2
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    Quote Originally Posted by fridg View Post
    Havent had much to do with them but are being retrofitted to existing Markets.

    What are they for ?
    What do they do ?

    Any reliable information source to obtain some information ?

    TIA.
    In market refrigeration, the condenser dropleg is a term that refers to the liquid drain from an air-cooled or evap-condenser. A single circuit condenser, usually the smaller ones under 10 tons, will have a dropleg sized one pipe size larger than the main liquid line to allow for bi-directional flow of the condensed liquid from the condenser and the vapor from the receiver.

    Larger condensers, typical of market parallel rack systems, will have two circuits which must be piped together. To offset any pressure drop differences in these two circuits, a liquid dropleg of sufficient depth is needed so the pressure drops with be equalized by the weight of the height difference of the liquid in the droplegs. This depth is usually 6 feet as a minimum recommendation.

    Name:  split-condenser-circuit-piping.png
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    Note that on system with split condenser control for cold weather operation, the droplegs are fitted with two liquid solenoid valves and two check valves. The active solenoid valve is normally closed and the other is normally open and is there simply to help ensure an equal pressure loss in the two droplegs.

    Some additional diagrams of droplegs are shown here, starting on page 2-5:

    https://icemeister.net/backroom/wp-c...anual-2013.pdf

    Another method of split condenser pressure equalization is to utilize what I call a seal pot receiver. It's a small horizontal receiver tank about 5-6 dropleg pipe diameters by 2-3 feet long with two top inlets and one top outlet with a dip tube. These are handy when the condenser is located below the receiver.

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  4. #3
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    Quote Originally Posted by icemeister View Post
    In market refrigeration, the condenser dropleg is a term that refers to the liquid drain from an air-cooled or evap-condenser. A single circuit condenser, usually the smaller ones under 10 tons, will have a dropleg sized one pipe size larger than the main liquid line to allow for bi-directional flow of the condensed liquid from the condenser and the vapor from the receiver.

    Larger condensers, typical of market parallel rack systems, will have two circuits which must be piped together. To offset any pressure drop differences in these two circuits, a liquid dropleg of sufficient depth is needed so the pressure drops with be equalized by the weight of the height difference of the liquid in the droplegs. This depth is usually 6 feet as a minimum recommendation.

    Name:  split-condenser-circuit-piping.png
Views: 128
Size:  59.8 KB

    Note that on system with split condenser control for cold weather operation, the droplegs are fitted with two liquid solenoid valves and two check valves. The active solenoid valve is normally closed and the other is normally open and is there simply to help ensure an equal pressure loss in the two droplegs.

    Some additional diagrams of droplegs are shown here, starting on page 2-5:

    https://icemeister.net/backroom/wp-c...anual-2013.pdf

    Another method of split condenser pressure equalization is to utilize what I call a seal pot receiver. It's a small horizontal receiver tank about 5-6 dropleg pipe diameters by 2-3 feet long with two top inlets and one top outlet with a dip tube. These are handy when the condenser is located below the receiver.
    Condensers have already been Split for Heat Reclaim. An Outfit has been back to install Drop Legs to some older Plants.

    From what I have been reading if Drop Leg Setpoint is 30C & Refrigerant Temperature is at or below this it will ByPass the Reciever & send the Liquid straight to the Coils.

    Am I correct in thinking this ?
    The primary function of the design engineer is to make things difficult for the fabricator and impossible for the serviceman.

  5. #4
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    Over the years there have been a number of different methods to control the receiver bypass for ambient subcooling, but with a basic liquid return line (AKA dropleg) temperature control I have found the most common setting to be around 24C (75F). You need to sense this temperature upstream of the condenser flooding valve if used.

    Ideally, if you have more than 5-6K of subcooling you want to bypass the receiver, but unless you have a control that can calculate your subcooling then you have to make do with the fixed temperature method of control.

  6. #5
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    As liquid refrigerant expands through the TEV port, some percentage of the liquid will flash into a vapor. This flashing is the vehicle that allows the remaining liquid to drop to the saturation temperature at the new lower pressure.

    The remaining percentage of liquid after the flashing occurs is the usable portion of the mass flow that allows for heat transfer in the evaporator.

    As the difference between the liquid temperature and evaporator SST is reduced, the the percentage of liquid flashing is reduced.

    As the percentage of flashing is reduced, the usable percentage of mass flow entering the evaporator increases.

    This is the benefit of subcooling....it allows the expansion process to become more efficient, and results in a reduced overall mass flow to accomplish the load requirement.

    Now, if you can provide subcooled liquid without any added load to the system (ambient subcooling)....voila! You have a true gain in system efficiency.

    In the receiver you have liquid and vapor present, which means it will be at a saturated condition. When subcooled liquid refrigerant enters the receiver, an equilibrium condition (saturated condition) will have to be reached, and the benefit of the subcooling will be lost. So the receiver has to be bypassed to keep the liquid refrigerant in a subcooled condition.

  7. #6
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    Thanks for the explanation guys ,,, makes much more sense now.
    The primary function of the design engineer is to make things difficult for the fabricator and impossible for the serviceman.

  8. #7
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    Quote Originally Posted by bunny View Post
    ...In the receiver you have liquid and vapor present, which means it will be at a saturated condition. When subcooled liquid refrigerant enters the receiver, an equilibrium condition (saturated condition) will have to be reached, and the benefit of the subcooling will be lost. So the receiver has to be bypassed to keep the liquid refrigerant in a subcooled condition.
    There are many here who disagree with you about the loss of subcooling in a receiver, me included. But I'm not up to re-opening that topic. We have a couple of past threads here which went on and on for dozens of pages and I can't bear to go through all that again.

    Anyway, the surge receiver concept is based on the premise that subcooling is lost, so I think for the sake of this thread we should just keep it that way. If anyone wants to challenge what bunny said here, then please start a new thread.

  9. #8
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    Good point Ice...I remember at least one of those very lengthy threads.

    I will say in theory, yes...you’ll lose the subcooling when it enters the two state receiver.

    In a real and dynamic system, yes, it’s a little different. But to prevent another lengthy discussion, perhaps we can agree that you’ll get more benefit from the ambient subcooling by bypassing the receiver, rather than allowing it to enter the receiver. How much more.....depends on a myriad of variables.

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