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.
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?
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.
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