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Thread: gas pressure

  1. #1
    can anyone tell em why lp requires a higher pressure than natural gas when the heating value of lp is so much higher?

  2. #2
    Join Date
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    welcome to h-talk,mbjeepn

    I look at it this way:
    Higher heat value, so less fuel needed per hour, so smaller orifice.Smaller orifice so more pressure needed to deliver fuel to burner.

    also it is a liquid fuel and therefore it just is....

  3. #3
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    LP does not REQUIRE a higher pressure,its pressure is relative to temperature like refrigerant.

  4. #4
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    Originally posted by wallynut
    LP does not REQUIRE a higher pressure,its pressure is relative to temperature like refrigerant.
    Wally, I think Mustbjeepn wanted to know "why is the regulated manifold pressure of an LP heating appliance more than 3x of that for natural gas?"

    I don't think pressure/temperature is really an issue, once it's regulated.

    I suspect the delivery pressure to the burner needs to be higher in order to increase the velocity as it flows through the orifice & into the burner.

    Higher velocity would help entrain enough primary air into the venturi to support complete combustion for the btu-rich LP.

    Jacob, LP is only liquid while it is still in the tank, isn't it? It's drawn out as a vapor.

    Anybody, please feel free to correct anything I might say.

  5. #5
    Originally posted by wallynut
    LP does not REQUIRE a higher pressure,its pressure is relative to temperature like refrigerant.
    What he's getting at is the manifold pressure is higher than it is for Natural gas.

    Therefore, it is a HIGH pressure compared to it's alternative fuel.

  6. #6
    Originally posted by bwal2
    Jacob, LP is only liquid while it is still in the tank, isn't it? It's drawn out as a vapor.
    Yes, LP is drawn from the tank as a vapor. There is, liquid LP burners on some different applications. But, this is not the normal residential furnace we would be talking about!

    The pressure in the tank is a high pressure (the tank should have a high pressure regulator), then you'll have a low pressure regulator at the residence (this being a two-stage system).


    [Edited by jultzya on 03-28-2005 at 01:22 PM]

  7. #7
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    read the installation guides, not all lp furnaces require a higher pressure.
    i wanted to put a picture here

  8. #8
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    tlcartman is correct check the manufacturers conversion instructions. If you look for gas valve conversion on new Bryant package units you won't find one just orfices. The burner pressure is 3-1/2 inches wc.
    Aircraft Mechanical Accessories Technician. The Air Force changed the job title to Air Craft Environmental Systems Technician. But I've decided I'll always be a Mech Acc.

  9. #9
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    LP gas is heavier than natural, therfore, natural takes less pressure to deliver the necessary volume than does LP

  10. #10
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    In a colder climate a higher pressure for LP could prevent the vapor from liquifing.Propane's boiling point is aprox. -44 below zero whereas natural gas is something like -250,so there is no chance nat. gas would liquify.Also propane is heavier so maybe more gas velocity is needed to draft naturally in the older models.Anyone have any other ideas?

  11. #11
    Originally posted by majormickey
    In a colder climate a higher pressure for LP could prevent the vapor from liquifing.Propane's boiling point is aprox.
    Would you care to expand on the above statement?

  12. #12
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    gas properties

    Due to the denser fuel, it takes more pressure to squirt that LP through the lines than NG. The LP molecule also has more friction loss(static pressure) against the walls of the tubing.

    Since a propane molecule has 10 bonds compared to the 4 of a methane molecule, propane on a volume basis has ~2.5 times more energy released than NG and therefore requires ~2.5 x more air. You must match the stated BTU input firing rate. This is why the orifice is derated so much smaller for LP while the air shutter (primary aeration) is usually wide open or wider than NG. As previously noted, the heavier LP molecules do entrain more air than NG. However, on some burners the port loading changes so some units require a different burner for LP.

    However, comparing specific gravities, LP, at 1.50-1.56 is about 2.5 x heavier than NG at 0.6-0.64. FYI, we usually refer to this number as sp. gr. when technically this is the vapor density. The actual sp. gr. refers to the liquid state. If you ever see a shipping manifest from an LP bulk supplier, they usually use the vapor density at 60F.

    In an LP tank, the fuel is liquid up to a max. of 80% by volume and at low pressure. The space above this "lake level" is the high pressure vapor. This pressure fluctuates throughout the day from maybe 60-80 psi on a cold morning to well over 100 psi when warmed by the sun. Even if you could find a way to overfill a tank and have liquid phase fuel flow into the first stage regulator, it would freeze up the regulator as it vaporized as the pressure dropped from ~100 psi to a couple of psi or at the second stage where it was knocked further down to 11-14 wci. It is hard but not impossible to flow liquid phase propane. We used to do it at the Fire Academy during live burn drills. Believe me, when that fuel expands 270 times, you'll learn real damn fast to respect it.

    HTH

  13. #13

    i agree

    I agree with alomost everyone. The most logicall explanation I could find is that the gas pressure is higher to draw in more primary air. Natural gas only requires 10 cuft of air for complete combustion and propane requires 23.5. It has to get this extra air somewhere so i am assuming by increasing the gas pressure it does speed up the velocity and pulling more air in. The post about the higher specific gravity might be correct also

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