# propane conundrum

Here’s a sizing chart;

Ok, they don't show 100# tank, but extrapolating from 200# looks like it might be a stretch for cooktop that might be pulling 50Kbtu/hr or so with all burners in use.

100lb tanks have a really small surface area. The tank is tall and thin so there isn’t much surface to evaporate from. A fatter tank will be able to evaporate more.

100lb tanks have a really small surface area. The tank is tall and thin so there isn’t much surface to evaporate from. A fatter tank will be able to evaporate more.
I'm not sure; a fatter (more resembling a sphere) tank will have less surface area relative to its volume.
The same volume in a cylinder has more surface area.

It’s not the sides that matter. It’s the circle where the liquid propane can evaporate. The top of the liquid. A wider tank has a circle with a larger area. I’m probably not saying this right.

It’s not the sides that matter. It’s the circle where the liquid propane can evaporate. The top of the liquid. A wider tank has a circle with a larger area. I’m probably not saying this right.
No, it's the heat input that creates evaporation. The boiling does not happen at the surface of the liquid, it happens at the interface between liquid and metal tanks - that's where the heat comes in.
Like water boiling in a kettle or pot: the bubbles get formed at the bottom where the heat input is.

The surface area of the tank determines the heat input from the ambient environment. Bubbles will form at that interface (walls and bottom), and will rise to whatever small "top of the liquid surface" there is.

Same as having a beer bottle on a stove; the boiling will happen regardless of how narrow the neck is where the (small) surface of the liquid is.

It's based on the surface area of the propane tank exposed to the outdoor temperature that allows the liquid propane to absorb heat from the surroundings.

A couple square inches of surface area is all that is needed if the temperature is sufficient to boil the propane.

Keep in mind that chart is for steady state operation. A propane tank that is only used occasionally, say for half an hour a day to run a couple stove burners, can supply the required gas solely by its own internally stored heat.

We use 500k btu propane torches on 20lb bottles all the time, the torch works great for the first 5 or 10 minutes until the entire bottle cools off, then the rate of gas boil off is proportional to the amount of heat the bottle can absorb from its surroundings.

No, it's the heat input that creates evaporation. The boiling does not happen at the surface of the liquid, it happens at the interface between liquid and metal tanks - that's where the heat comes in.
Like water boiling in a kettle or pot: the bubbles get formed at the bottom where the heat input is.

The surface area of the tank determines the heat input from the ambient environment. Bubbles will form at that interface (walls and bottom), and will rise to whatever small "top of the liquid surface" there is.

Same as having a beer bottle on a stove; the boiling will happen regardless of how narrow the neck is where the (small) surface of the liquid is.

Interesting thoughts about phase change. I hadn't thought too hard about it and believed all the folks talking about "wetted" surface area. As though somehow the interface area between liquid and gas governed the rate of boil.

So really, a spherical tank would be the least ideal. You actually want an inefficient shape like tall and skinny.

We use 500k btu propane torches on 20lb bottles all the time, the torch works great for the first 5 or 10 minutes until the entire bottle cools off, then the rate of gas boil off is proportional to the amount of heat the bottle can absorb from its surroundings.

I'm sure you noticed that you could turn that torch onto the propane tank to warm things up!

I'm sure you noticed that you could turn that torch onto the propane tank to warm things up!

I plead the 5th...

Interesting thoughts about phase change. I hadn't thought too hard about it and believed all the folks talking about "wetted" surface area. As though somehow the interface area between liquid and gas governed the rate of boil.
Wetted surface area to me means the area of the metal that is wetted by the liquid.
So really, a spherical tank would be the least ideal. You actually want an inefficient shape like tall and skinny.
Only if you need a high BTU gas flow for the volume of tank that you have

Wetted surface area to me means the area of the metal that is wetted by the liquid.

Only if you need a high BTU gas flow for the volume of tank that you have
Correct, only the surface area that contacts liquid inside transmits heat to the liquid. That’s why you can see the condensation line or frost area on the outside of a cylinder under load where the liquid is in contact with pressure vessel.

Horizontal tanks have more surface area across the bottom touching liquid as it gets low, which raises the pressure like a large pan with a little water across the bottom of a large surface area. The tank can still supply a 100,000 BTU furnace at 1 or 2 % where an upright cylinder may need 5 or 10% to maintain pressure at or below freezing temperatures. At -45*f propane ceases to boil, pressure drops, and you only have liquid in the tank with no vapor pressure, just like water below its boiling point in a bucket.

Corn dryers with very high BTU requirements use a supply tank heater to induce heat into the fuel to vaporize it. Literally heating the tank with a burner to boil the liquid faster than heat from the atmosphere can.

Liquid in the tank or cylinder cannot ignite since it is 100% fuel with no oxygen. A tank rupture that expels expanding ignited liquid into the atmosphere is called a bleve. An expanding fireball.

It is easy to experience pressure drop when changing valves and appurtenances on cylinders and ASME tanks. You open the outlet to expel vapor as fast as possible. (In a remote location) A few minutes roaring out, the wetted surface area frosts up. Tank wall is so cold the heat from atmosphere no longer boils liquid inside. Pressure drops and vapor slows. You remove the valve (above liquid level) and replace it in the tank or cylinder. Tank warms up coming back up to pressure. With smaller cylinders you can pour the liquid propane out and instantly freeze things, such as weeds or leaves, and shatter them with a gloved hand. It is clear liquid that bubbles and dances on the ground like pouring water on a hot stove boiling rapidly.

Fun stuff us propane guys do.

stoveliker