gas pressure

[mustbjeepn]

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

[jacob perkins]

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....

[wallynut]

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

[Wild Leg]

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.

[Jultzya]

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.

[Jultzya]

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]

[tlcartman]

read the installation guides, not all lp furnaces require a higher pressure.

[MechAcc]

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.

[bornriding]

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

[majormickey]

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?

[Jultzya]

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?

[hearthman]

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

[mustbjeepn]

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

[Wild Leg]

nevermind...

[karsthuntr]

Originally posted by hearthman
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

I was working at an airplane fire training office while they were doing a live burn. 175gpm (that's minute) of liquid propane. I got a good movie and some pics during the burn and while the fire truck was putting it out, in less than a minute.




The mat under the flames is 100' x 80'.

[majormickey]

jultzya; By running higher pressures from tank to second stage regulators the flow of propane will not be effected by cold temps. We use vaporizors that draw from the liquid level of the tank and convert into vapor via a burner inside the vaporizor unit. I thought the same could apply between the second stage and appliance by using 11" WC versus the output of a natuaral gas regulator which is 7" WC. Could be out to lunch on this one but I thought the higher second stage pressure could be an advantage to battle exteme temps.

hearthman; One application where we use liquid propane is on the family farm for the grain dryers. The liquid is heated up within the burner through coils before ignition.

[Jultzya]

MM, I have never seen any home setup that was pulling liquid from the tank. I do know about the liquid setups for the grain driers.

The main reason for a 2-stage setup for the home, is to be able to supply a larger amount of fuel through a smaller line. Otherwise, you would end up with a hefty line from the tank all the way to the home.

[hearthman]

Requires 2 stage regulation on residential applications.

Maj. Mick, if the LP is in liquid phase as it enters the burner, what's to allow it to vaporize and expand the 270x before ignition? Also, what safeguards are there should this vaporization & expansion are delayed? That could look impressive!

[majormickey]

sorry hearth, the liquid is vaporized a split second before ignition, my mistake.

jultzya; the liquid goes to vaporizor then through a first stage reg., then to the second stage, then to furnace. Most installations it's not neccasary only when trying to demand a large volume. It is used more in the northern part of our province. Temps here can dip to -50 celsius or more. Pretaining to the original thread I thought a higher second stage pressure could help maintain the flow of vapor in those cold temps.