Calculating Blower CFM

[twofer]

As I mentioned in the other thread I've ordered an Inferno without the Inferno equipment (just want the air tight shell). :)

I've been working on a few equations to calculate the necessary cubic feet per minute so I can spec the correct blower. Hopefully you guys can look over my math below and give some feedback.

Pounds of wood per hour = Gallons of water evaporated per hour * BTU per gallon for evaporation * (1.0 / overall evaporator efficiency) / BTU per pound of wood

CFM = Pounds of wood per hour * Cubic feet air necessary for combustion / 60 minutes per hour

Using those equations and the following constants I'll show my math for my 2.5x10 evaporator. All of these constants are


Gallons of water evaporated per hour = 75
BTU per gallon for evaporation = 8800
BTU per pound of wood 20% MC = 7700
Cubic feet air necessary for combustion = 70
Evaporator efficiency.


Pounds of wood per hour = 75 * 8800 * (1.0 / .5) / 7700
Pounds of wood per hour = 171

CFM = 171 * 70 / 60
CFM = 199.5

One of the many things that I'm not sure about with this math is at what pressure should the blower be capable of the CFM. I'm also not totally sure about the constants (especially evaporator efficiency) so hopefully someone can correct me if I'm wrong or reaffirm my research.

This just might be the most boring post of 2010. :)

[maple flats]

Too much calculation. I just hooked up a high pressure blower of unknown CFM and it is likely 75-100 yrs old so specs are not available BUT it worked great on my 3x8 with air over and under the fire.
If you want a chart for the over/under high pressure air I will look up a link to get you the info and will post it in this reply when I find it. Try this http://www.uvm.edu/~pmrc/Combustion.pdf

[twofer]

Too much calculation.

...

http://www.uvm.edu/~pmrc/Combustion.pdf

Yeah.......but I'm a freak when it comes to efficiency. In my home I have a gasification boiler with 500 gallons of thermal storage. Monitoring and logging of the temperatures throughout the system is performed by a microcontroller sytem that I built. It allows me to fine tune the loading of the thermal storage, as well as, graph the overall system performance over the long term. This has helped me fine tune the primary air and secondary air settings to further increase efficiency. When I get the evaporator up and running I plan on implementing the same type system so I can measure efficiency and further fine tune it. Yep, I'm a freak. :)

I've been working off that document from the Proctor Research Center but it is light on technical details because it is only "guidelines".

[maple flats]

Due to varying types of wood being burnt the burn would not be a constant and while boiling it would be difficult to weigh each charge. Being an efficiency "freak" as you say it might not fit here. You have too many things to watch to control the fuel input by pounds. First of all, it will not need exactly the same amount each time.
The first thing however is to get the highest efficiency you will need a high pressure blower and I doubt Bill Mason is going HP. I would tell him to go HP or not include the blower and that you are going to be getting a HP blower with an over the fire manifold and valve regulation to regulate under and over distribution. I found that I almost stop over air to light and little under air. As the fire gets going I open the under about every minute until it is at half valve open (both of my valves are 3" ball valves with a 4" PVC coming from the blower outside and splitting just before the valves). I then open the top about 25% (about 3-4 minutes after lighting and as the bottom is opened to 50%) in a minute or 2 I go to 60% open on top and close the bottom to about 30%. From there it is left the same until shut down at the end of the boil. After last fueling at the time when I would have added more wood, I slightly close the top (to about 30%) and open the bottom to about 50% to burn the last bit of fuel faster. All percentages are approximate and are % open not % closed. My burn times went from an arm load every 5 minutes (3x8 evap) to a slightly larger load every 15. My boil rate increased 10% and gal of syrup per full cord increases 30-35%.
As for the guideline from Proctor, I think you will need to develope your own level of "tech specs" if you feel them necessary. I just look at the bigger picture, more syrup per cord, faster boil and less wood to cut for next season on a projected super season preparedness. In fact I now have enough wood cut, split and stacked for 2 average seasons with my vacuum projections for increased sap included.
Good luck.

[twofer]

With those equations I just wanted something that could get me or anyone else in the ballpark. IMHO I would rather have a blower that is sized just slightly larger than I need rather than slap a blower on there that I have choke way down all the while burning more electricity.

I'd disagree with this being a bad fit for being an efficiency freak. Just because it's a game of inches and the fuel source is variable doesn't mean you can't get an idea of how well the evaporator is running. This is where the temperature logging/graphing comes in. If you graph the system performance over the long term and pair it with some light loading notes on wood and operation you can begin to see if the changes you have made are making a difference.