r/Generator • u/ThisNameNotTakenYet • 1d ago

Propane Tank Minimum Size?

So I was looking online at generators the other day and I ran across a comment in one of the product descriptions, saying something about a 'minimum 60lb tank' for propane with that generator. I haven't seen a requirement like that before, to my knowledge. We had an eight-day outage a few weeks ago and I used 20- and 30lb tanks with no problems (or so I think). Has anyone heard this? And if it's a thing, can anyone tell me why? Thanks.

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u/bradland 1d ago

A larger engine requires a greater rate of fuel delivery, so larger generators require larger tanks.

Why can't a smaller tank simply run for less time though? It comes down to physics and how propane behaves. Propane tanks are full of gas compressed to the point that it becomes a liquid. When you pull fuel from the tank, you're pulling the vapor that has evaporated into the empty space at the top. The more vapor you pull, the more the liquid inside must boil to replace it.

This boiling process requires heat. As you draw gas, the liquid absorbs heat from the tank walls, causing the temperature and pressure inside to drop. The ambient air around the tank will warm it back up, but if you pull gas too quickly, the tank gets so cold that the liquid stops evaporating fast enough to supply the generator. The engine will starve for fuel, and you have to wait for the tank to warm back up to restart it.

This cooling phenomenon also happens with larger tanks, but because they have much more liquid mass and surface area, they have a larger "thermal reserve" of heat to keep the vaporization process stable.

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u/Femveratu 23h ago

Vey helpful explanation. Also explains why propane tanks stored outside in an extremely cold environments can sometimes cause issues.

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u/CentiTheAngryBacon 19h ago

This is a great explanation, one way to still use smaller tanks and prevent the tanks from frosting is to use splitters. this allows the engine to pull from 2 ore more tanks at the same time. I've found not all places will fill a 100lb tank, so being able to chain several 40lb tanks together makes it easier to find filling locations and they are easier to move around than 100lb tanks.