Hello Mr. Heater, my name is Enthalpy

  • Thu 15 April 2021
  • misc

A month and a half ago, we had a socially distanced going-away party for a colleague, in a pavilion at a park.

Late February in the DC area can be chilly. We had held previous socially lunches at this pavilion and gotten the tsk-tsk from a couple of Arlington County park rangers (yes, such a thing exists) for bringing a fire pit with us. A propane heater is not a fire pit or at least I hoped this logic would work if we met the same folks again so I sourced a three-headed Mr. Heater tank-top heater for the occasion as a warming station.

Mr. Heater was very popular, but if I had it to do again, I'd get a dual head one rather than a triple - the three headed one is a bit unwieldy and they all seem a little fragile. Also notable though is the fact that when set on high it burns propane at a rate that is sufficient to ice up a 20 pound barbeque tank. I've seen this phenomenon before with forges and weed burners - it's generally a sign that you could profit from having a bigger tank so that there's more mass and more surface relative to the draw rate for the propane. But bigger tanks are expensive and a pain in the butt and 20 pound tanks are the coin of the realm.

One of my junior colleagues looked at the layer of frost and asked why that happened. I said "well, it's liquid and phase-changing to gas in the cylinder". Blank look - I guess they don't teach physics in the comp sci program - so out came the smart phones to look up the numbers while I explained that it's called "latent heat of vaporization", how phase change takes a whole lot of energy for most materials, and that this is how refrigerators work.

Apologies for the Freedom Units here, but the heater uses them on the box so that's what we went with for the whole exercise. It was a chilly damp day in the low 40s so we had that propane heater on full power at 45,000 BTU (almost 4x what my barbeque grill, which incidentally doesn't ice up the tank) puts out. Propane is 21600 BTU per pound (assuming a 100% efficient burner which is fine for t-shirt sizing). That means 2.1 pounds per hour, or a 20 pound tank will last a bit less than 10 hours. Propane's latent heat of vaporization is 184 BTU/lb, so 383 BTU/h. Doesn't sound like a lot; a ton of air conditioning is 12,000 BTU, but under the prevailing atmospheric conditions it was enough to drive the surface of the tank not only down below the dew point but below freezing, so we got frost.