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air purging

edow Member Posts: 17
why is air purging of a hydronic system is important?

because the pumps are not strong enough to pump the air?  i know you cannot have air in the system


  • hot_rod
    hot_rod Member Posts: 20,762

    air can cause the circulators to cavitate. Cavitation comes from the root word cavity. A cavity being a hole. A hole in water would be an air bubble. If you have large air bubbles the pump may fail to circulate the fluid.

    Small air or entrain air, that milky colored fluid, will also cause numerous problems from noise, to poor heat transfer, corrosion, etc. Bubbles or even a layer of air in the boiler HX can cause poor heat transfer possibly over heating. The same for air in the heat emitters being radiant tube, radiators, air coils, etc.

    It really pays to have a good micro bubble air elimination device in any system. They do a much better job than ramp or scoop type air purgers, especially on systems with glycol. Glycol is a tough fluid to remove that entrained air from.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • icesailor
    icesailor Member Posts: 7,265
    Air Purging/Pump Cavitation:

    Water pumps pump water. Air pumps, pump air. A table fan is an air pump. Water pumps are water compressors. Air pumps compress air. Cavitation or turbulence are found in both. Water pumps can't compress water. Just move it along.

    In the beginning, if there is no air in a system, the pump (circulator) will move the water. If there is air in the system, the "normal" head circulator will not have the ability to overcome the resistance of the air trapped in the system. When you purge through the return, you are making the system think it has more flow pressure  and volume flow than would be found if it was the circulator pump alone.

    Domestic or any single stage water pumps won't pump much over 50# or a 115' head. 007 pumps won't come close to that. But the higher the resistance, the higher the pressure (head) m must be and can create all kinds of problems. Cavitation being the worst. Take your average 3-speed pump. Turn it on "low" and listen. Quiet? Turn it to "medium" and listen. Quiet? Turn it to "High" Can you hear it? It's probably cavitation you are hearing. It's the pump, trying to make head pressure that causes the cavitation. The fluid always want to stay at the same pressure. If you pressurize part of it, an equal part will have its pressure lowered. If the lower pressure goes below the boiling point, you get cavitation. Or turbulence. The pump caused the cavitation and air bubbles are the result. Sparco or Micro-Bubble removers are just a tool to try to repair a problem that didn't exist when systems were designed for easier flow.

    The circulator pump won't pump air but it may "make" air through cavitation. Not the other way around.

    Submarines under water have serious prop cavitation. It makes noise. Sonar listens for prop noise. Cavitation. Going deeper, increases the pressure on the prop blades and will stop it. So will slowing the prop down. Consider the amount of power and thrust the sub prop must generate to make a 400'+ multi-ton vessel, under the water, go over 30+ Knots. Next time you see a photograph of a Nuclear Sub, leaving a sub base from the air, notice the long wake in the background. Notice the white area in the middle of the wake track. That's from prop cavitation. It's making bubbles. If a submarine made those same bubbles when it was at 150 feet, to find it, all you would need to find it would be to look for the bubble trail. Lower depth, higher pressure, less or no cavitation.

    I've seen a lot of over-pumped systems that cavitated because the piping was poorly designed and needed high head pumps to overcome the resistance.

    Except on start up, I personally don't even see the need for an air scoop on a properly designed and installed heating system. All the float vents leak so get screwed down, and the systems work fine. If I see air, it isn't because there were no open float vents. I find that PRV's that don't open soon enough (if at all) with system drop in pressures are a greater cause of air in systems than any other cause. Or, open float vents on air handlers in attics where the system pressure isn't high enough to keep the fluid up. So air gets sucked into the system.

    Cavitation, boiling water is a bigger cause.

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