help with Dan's Formula

jpf321

I'm looking at the formula on page 158 of TLAOSH .. can someone tell me where he gets the numbers in Step 2  ... 19.27 and 966 .. 966 sounds like latent BTUs of steam .. 19.27 escapes me .. thanks.

jpf321

looked everywhere...

i looked everywhere for an explanation of those numbers .. i couldn't figure it out .. i did find this very useful site: http://www.comfort-calc.net/Steam_Formulas.html .. but no mention of 19.27 or 966

lutorm

Probably heat capacity

Somewhere you need to know how much heat it takes to heat iron from 55 F to 219F. That must be that number, though with the English units I'm not sure:



Specific heat capacity for iron is 450J/kg*K, that would be 0.107btu/(lb*F), multiplied by 219-55=164F, you get 17.5 btu/lb. Not exactly right, but who knows what roundings went into the 19.27...

jpf321

sounds like a possibilitiy

does anyone else want to chime in on the origin of 19.27? thx

superMARKet

Carbon steel, not iron?

You’re right on the money with 966. That represents the latent heat of vaporization (condensation) of water (steam) in Btu/lb. (or something real close, depending on how precise you want to get).



As for 19.27 Btu/lb., lutorm is on the right track, but I think you’re really working with steel here, regardless of what it’s called. If you do that, you can get real close to 19.27 Btu/lb., at least.



The specific heat of carbon steel is 490 J/kg·°C. That’s 0.117 Btu/lb.·°F. Therefore, it takes 19.19 Btu/lb. to raise the temperature by 164°F (∆T = 219°F - 55°F).



(Or, to get 19.27 Btu/lb.-°F more exactly, you’d use 492 J/kg·°C.)



Is that close enough for you?

jpf321

thanks

i appreciate the comments .. i now understand better the concept.