Air chamber--drop elbow vs drop tee?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by Neptune, Nov 3, 2012.

  1. Neptune

    Neptune New Member

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    Oct 31, 2012
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    Illinois
    Why does it seem customary to install an air chamber as a water hammer arrestor using a brass drop elbow with a tee fitting beneath it to branch off to a separate arm, like a saquaro cactus? Wouldn't it be easier just to use a brass drop tee and extend the piping up to form the air chamber? When the stub out the wall is just a copper pipe, it seems common to do this with just a sweat fitted tee.

    I'm aware of the debate as to whether air chambers are useless, anyhow, but I intend to install one for the feed to a new toilet, just in case. I want to make sure I'm not overlooking something on how this should be done.
     
    Last edited: Nov 3, 2012
  2. LLigetfa

    LLigetfa DIYer, not in the trades

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    Are you talking about a low-tech hammer stop that is just open-ended? Those will waterlog over time as the air gets absorbed. They should be mounted on the top of a Tee so as to collect any air that may be in the water stream.

    The better hammer stops have a piston with a pre-charged, closed air reservoir that are not prone to waterlogging so the air supply need not be replenished by accumulating air. They can and should be installed as you say, with an elbow so as to not accumulate air, thereby keeping the O-ring seal on the piston lubricated.
     
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  4. hj

    hj Moderator & Master Plumber Staff Member

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    Three things;
    1. You already know that air chambers are only effect for a short time, until the water absorbs the air.
    2. It is almost impossible to recharge an air chamber, regardless of what anyone tell you about turning off the water and draining the pipes.
    3. A toilet is the one fixture that seldom, if ever, needs an air chamber.
     
  5. LLigetfa

    LLigetfa DIYer, not in the trades

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    1. Assumes low tech, not pre-charged piston type.
    2. May depend on how and where they are installed.
    3. Very true.
     
  6. Neptune

    Neptune New Member

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    Thanks for your observations.

    I agree that an arrestor of any sort is probably not necessary, but I happen to have on hand a drop ear tee but not a drop ear elbow. So, I'm thinking--why not just use that and add another 14" of pipe with a cap, just to cover old school practice?

    This, then, led me to wonder why you see the "cactus arm" configuration so often, which takes extra fittings and joints, and not nearly as elegant as using a tee.

    Maybe the answer has nothing to do with flow characteristics or the like, but simply that plumbers typically have in their trucks a supply of drop elbows but not drop tees!
     
  7. Terry

    Terry Administrator Staff Member

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    It's been a long, long time since I've installed an "air chamber"

    In the plumbing code; if there are installed, there must be a way of draining them down.
    What is approved is hammer arrestors. In a tight (closed) system, those would be installed on quick closing valves.
    Ice Maker
    Dish Washer
    Clothes Washer

    All of these have solenoids.

    And yes, a hammer arrestor can fail too.
     
  8. hj

    hj Moderator & Master Plumber Staff Member

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    quote; 1. Assumes low tech, not pre-charged piston type.
    2. May depend on how and where they are installed

    1. Mechanical styles are "shock absorbers" NOT air chambers
    2. A filled air chamber is like a straw with your finger over the end of it. The liquid will NEVER drain out until air can get into it, and there is NEVER enough "pressure" to force air in, AND even if there were few systems are installed so that ALL the water can be drained.

    As far as the location, good hydraulics specifies that the inlet to the shock absorber be in line with the direction of water flow.
     
  9. Holeshot

    Holeshot New Member

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    Air chambers can be drained. Air chambers should not be compared to a drinking straw. Shock absorbers have o-rings that will eventually fail and could toxic like other old o-rings that melts on our fingers turning it black. But some are saying air chambers can't be recharged?

    Yes, if the lines teeing off to the air chambers are full of water, the air chambers will never drain. If the lines are empty of water and air pressure is open to the atmosphere, it will drain. Law of physics tells us it must drain.

    Using a drinking straw, the first thing to ask is what keeps the water from dripping down? The partial vacuum on the upper part of the tube can't be the full answer, because it doesn't explain why little droplets don't simply detach from the bottom side and fall without changing the size of the vacuum. The key physical ingredient of the explanation must be surface tension, the extra free energy that's needed per area of water-air interface. Formation of a droplet increases the surface area, so unless the gravitational force is strong enough, it will take extra energy for a droplet to form, and the water will stay stuck in the tube.

    How does that limit the tube radius, R? I'll do a crude calculation, not keeping track of small numerical factors, i.e. using some dimensional analysis. The extra energy required to form a drop goes as its area times the surface tension s, very roughly sR2 for the biggest drop that would form from a tube of radius R. The energy lost by lowering the water in that drop by a distance comparable to its size is roughly ρgR4, where ρ is the mass density and g is the gravitational acceleration. So for the bottom surface of the water not to shed drops we need roughly sR2>ρgR4 or R2< s/ρg.

    The surface tension of water near room temperature is about s= 70 ergs/cm2, ρ=1 gm/cm3, g=980 cm/s2, so we end up with roughly R2< 0.07 cm2. That would correspond to a tube with diameter of about 5mm. Given that I haven't tried to solve the problem exactly, that answer could easily be off by a factor of 3. Is it anywhere near the measured cutoff between tubes that hold the water and those that don't?

    Mike W.

    p.s. A first test showed that ~3mm diameter gave stability and ~ 10 mm diameter was unstable, so this calculation seems to be pretty close.
     
  10. kreemoweet

    kreemoweet In the Trades

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    Seattle. WA
    Air chambers are great. Simple, indestructible, and effective. As opposed to made-in-China mechanical devices
    that WILL fail, perhaps already failed outta the box, and most of the time installed in a non-accessible and
    non-repairable manner.

    I'm surprised that any plumber would think there will be a drinking-straw kind of effect with normal sized
    plumbing pipes. Simply not gonna happen.

    I've not been able to find any actual data at all on how quickly an air chamber will "waterlog". The common experience
    of many sez that it can be many years until that happens. I suppose it depends on the normal air content of the house
    water supply. In my experience, it is frequently saturated with air, and hot water will cloud up with air bubbles when
    drawn from a tap. Needless to say, air-saturated water is not going to absorb any air out of the air chambers.
     
  11. asktom

    asktom Member

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    Victor, MT
    Listen to hj.
     
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