Wizard_of_Frobozz
Well-Known Member
You could calculate the number of BTUs needed, I think--In my system, with a fairly rigorous boil, the effluent was about 140 degrees. I was boiling off 1 gallon per hour, and the water spray is about 9 gph. The water starts out about 50 degrees or so. It takes a BTU to raise a pound of water 1 degree fahrenheit (about--that's at 39 degrees IIRC, but close enough for purposes here). Ten gallons is 84.3 pounds. Nine gallons is 75.06 pounds.
This is where it's early, I haven't had enough coffee, and I'm sure there's an engineer out there who can more accurately figure this out, but I'm thinking that if 75.06 pounds of water is raised in temperature from 50 to 140, that would be a capture of (75.06 pounds of water x 90 degrees of temp change) of 6755 BTUs of energy.
Engineers, is that at least in the ballpark? Or has the lack of coffee sapped my brainpower?
Your math is not the whole story. You are correctly calculating the heat required to raise the water spray from 50F to 140F BUT you are also adding the condensed steam to that water stream, so it doesn't cover the entire system. You have to add the 1 gal x 8.3 lbs/gal of condensed steam to the mix, the latent heat removed by condensing, and the sensible heat removed to bring it down from 212F to 140F.
You really don't need to do all that however. It's pretty easy to calculate how much heat you need to remove in order to condense the steam:
1 lb of 212F steam = 970 btu's (heat of vaporization of water)
1 gal of water = 8.3 lbs (yes, this changes depending on temperature, but it's close enough...)
If you boil off one gallon during the boil, you need 8.3 x 970 = 8,051 btu's.
Ideally, the steam slayer effluent only needs to be below boiling temperature - any extra means you are flowing more water than you need to. Practically, you probably want it a good amount cooler to make it safer to handle/dispose of. I've seen PVC drain lines melt because 200F+ condensate was put down it for a long time.
I also think the traditional 1 gal/hour boil off rate is way off. Most people that are using the steam slayer are finding the boil rates are much lower, which makes sense, because you'd never be able to condense 8.3 lbs of steam in an hour and get the effluent at 140F with only 6755 btu's.