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Is there a significant pressure drop/loss here?

  1. Jun 28, 2011 #1
    I've got natural gas flowing to a chamber with a regulator set at 9 inWC. The gas then goes through a mass flow meter with a 1/4" diameter hole, which causes a pressure loss of about 3 inWC (to verify this, I have another pressure gauge after the flow meter which accurately reads 6 inWC... After that gauge, it goes through a gas tube with quick-connects directly into a water heater. The tube is hefty, some kind of semi-rigid metal (not sure of the kind, can look it up) and the tube has a diameter of 3/4", and is about 8 feet long with two bends (90 degrees, but not sharp).

    My question is - would this pipe cause significant pressure loss? The water heater needs at least 4.5 inWC to operate. I wouldn't imaging this tube casing a 1.5 inWC loss, but I could be wrong. Any insight? Thanks.
  2. jcsd
  3. Jun 28, 2011 #2
    This is easy to compute.

    What is your flow rate? What is the wall thickness of the 3/4" tube?
  4. Jun 28, 2011 #3
    well, I think therein lies (one of) the problem(s)... the flow meter is giving us numbers, but we're not sure if they're correct. so we really don't know what the flow rate is.
  5. Jun 28, 2011 #4


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    My calculation for flow through an orifice gives about 2 SCFM. Problem is your measurements aren't very accurate so that's just a ball park guess. But it's probably accurate enough to answer your question about the pressure drop through the 8 foot of tube. If it's tube and not convoluted metal hose, then the pressure drop shouldn't be more than about 1/4 inWC. But if it's convoluted metal hose, it could be much higher and pressure could drop below 4.5 inWC. Convoluted hose has an equivalent length that's a function of the ID, number of convolutes, depth of convolutes, number of convolutes per inch, etc... but in general for hose that small, pressure drop is about 10 times higher than for tube of the same diameter (very roughly).
  6. Jun 29, 2011 #5
    Ah, I see... the hose is, in fact, convoluted... Next time I am over in the lab, I will check to see the brand/type so I can give a more accurate description. It's nominal size is 3/4"... I'm not sure what the actual ID/OD measurements are, but the OD definitely looks < 1".

    Perhaps it's worth it to put a pressure gauge at the end of the hose, even though that might be a pain in the butt, calculations with a convoluted hose seem like they have too many variables that could make your answers inaccurate.
  7. Jul 3, 2011 #6
    I will preface my response by saying that I am not an ME or a physicist. I am a Master Plumbing, Heating, and Air Conditioning contractor. My son visits this forum frequently and we were discussing this topic.

    I do not understand your need to know the velocity of gas flow to the water heater. Way beyond my education. All I can comment on is the actual application.

    There is no appreciable pressure drop between the gas valve and the orfice. If you are interested academically, then it is possible to install a gauge in the orfice into the orfice tapping. However, from a purely combustion standpoint, the pressure drop is not important.

    The supply regulator at the supply point is adjustable, but only as far as the reliability of the measuring equipment that the installer has available. The most reliable that I am aware of is in the +/-.03 " w.c.

    Is this water heater a boiler or a commercial water heater application?
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