Evaporator Efficiency

[Helicopter Seeds]

Such a simple title of a new thread, but it can mean so many things. One of my fascinations as an engineer doing this hobby is that I contemplate many types of efficiencies, calculate little, and then hack at attempts to experiment anyway. I see many questions and comments on boil rate, and the answers are always.. it depends. As people respond to boil rate by talking about fuel size and rate, airflow, and the time cost and effort to improve is not always captured. This dawned on me the other night when I decided to chill out and boil slower, rather than keep splitting and feeding at 9 PM. I also saw my friends setup, which I call brute force boiling over a large open fire with six or seven large pots on a massive grate. Time efficient, yes, heat efficient? not so much - but with abundant fuel, who cares.

Point is just to ramble during season a bit, but also to solicit other opinions and examples where perhaps someone took a step back to go forward for what their goal was, but also see the basic evolution of the big, fancy setups available for the pros.. A few main categories keep showing up implicitly in these discussions,
1. Efficiency of the heat transfer to the sap.
This covers quite a bit regarding what type of setup, depth of boil, size of pans, gap between pan and fire, and even pre-heating sometimes. General answers without the mathematical equations point that heat transfer efficiency increases with temperature gradient (Delta T) across a unit area. So you can improve both with higher firing and larger surface area of transfer. Simple explanation of why flue type pan versus flat pan can be more efficient here. Similarly, a stock pot on open fire, filled up, with flames surrounding it, can utilize the cylindrical area, not just the bottom. It only becomes a trouble when the sides get burned as level drops (but I digress). The same principle can also hurt you however, when you also use them to describe heat losses such as the sides of your pan exposed to ambient air (difference in temp can be positive or negative). Hotter fires, and high flue stack temperature means that this efficiency higher, due to higher Delta T.
2. Efficiency of fuel quantity used to evaporate.
This area seems to get little attention for u smaller operations, as we seem to just get more wood, often from free sources. You can bet the larger operations running on gas or oil, or other purchased fuel may be paying a bit more attention. In my case, I am running low on dry firewood, have access to plenty of green big logs, and with some transportation time I can get some pallets. Next year, I will plan better. This year, I want to maximize the yield from what I have. The equations here center first around the energy available in the fuel (BTU per lb) but then get interesting. The above efficiency matters of course, but from there we need to subtract the energy that escapes.. everywhere. First is incomplete combustion which is prominent on open fire but even in a good complete combustion, a lot goes up the chimney. In this way, it is contrary to having high stack temps. There are a couple threads on gasification arches and 'rocket' stoves. The goal of a rocket stove is to use less fuel, primarily by full combustion. It seems that good, hot arch setup will do this as well, especially with forced air. For me, this year is too late for forced, but I may try and rig naturally aspirated hot air tubes somehow, time permitting.. and on that...

3. Efficiency of your labor and time in basically everything.
We maplers seem to yippie yap on everything here, and I daresay that for most posters, this efficiency wins out, but is often overlooked. the details vary in that if there is a dollar value in your time, hobby or not, we need to consider. For example, I can just order a cord of delivered, split, seasoned firewood, and save myself many days of cutting and splitting. I can save a small fee of delivery, and go pick it up, for probably two hours cost. Easy choice? Same goes for sap collection, bottling, you name it. For us small time folks, an enjoyable activity that becomes a burden needs to be modified. Herein lies our problem. We relentlessly pursue this area and get addicted to getting.. better, more efficient!

[mspina14]

This conversation is a bit above my pay grade.

But you did get my attention when you started talking about time.

I can only practice my hobby on the weekends, as I work out of town during the week and am only home where the sugar shack is on weekends.

I can make or buy a better evaporator, add RO, split more wood, add more taps, or do any one of a number of different things to make more syrup But at the end of the day, its time that I lack the most and is the most precious. I can't make more time, so in the end, its what limits me the most.

[SPILEDRIVER]

ummm no thanks....your trying to take all the fun out of it

[BSD]

i started a thread the other day about improving my boil rate on my rig and got some great feedback from members here.

efficiency is the name of my game, i try to make everything i do efficient, at work, home, play or whatever. I never have enough time to do everything i need to do, so i try to be smart about it.

as far as fuel source. i work with an arborist, and i have unlimited amounts of wood to burn. but i don't. i burn pallets. they're uniform, easy to work with, i can move a stack with my tractor right over next to the arch and cut them up with a chainsaw when i need them, not like firewood where you manhandle it about 12 times before it goes in the fire box. i can get an unlimited supply from a number of sources around here, i don't have to look at it all **** year and remember to split and stack it early enough in the year. more importantly, they burn extremely hot, albeit fast. they also provide a nice consistent burn.

[motowbrowne]

There was a conversation about the length of evaporators on the other forum. We hit on the idea that while a cooker is more efficient at extracting heat the longer it is, there's a point at which it's a more efficient use of materials to build a cooker that's faster, rather than more efficient.

Personally, I was very very pleased when I moved from flat pans to a raised flue setup. I know that home made rigs are cool, and it's awesome that people build cookers that do the job. Making syrup yourself on something you built is an awesome feat. However, the efficiency of a thin metal flue pan type setup on an ideal arch is amazing. Obviously these machines come with a hefty price tag in most cases, but I think they really pay it back in fuel and time savings. Heck, a new 2x10 from A&A is only like $3000. No way could I build something that fast and efficient for that kind of money. Not to mention the time it would take.

[Helicopter Seeds]

ummm no thanks....your trying to take all the fun out of it

Sorry if that was the message - exactly opposite intent. I intentionally avoid the hard thermodynamics that I frankly have not studied in 20 years anyway, never liked the subject then either. My point was that we all get tired, run out of time, and have the threat of something we enjoy becoming a burden because of it. We end up seeking solutions to make it a little better next year... to preserve the fun. So matter what complexity we can imagine on exactly how it boils, it usually comes down to personal time available that limits us, and of course cash.

[Big_Eddy]

Helicopter
I'll bite - another Engineer here.
1) Efficiency of heat transfer to the fluid.
I do remember my thermodynamics, but I won't try to explain them. Some simple truths though. More area = more heat transfer. Bigger temperature difference (hotter fire) = more heat transfer. For the greatest evaporation, get the hottest fire and apply it to the largest surface area possible. Hence flue pans.
A few comments though. Every once in a while, someone will come along and state "a steel pan is more efficient than a stainless pan" or "a thin pan is more efficient than a thick pan". Technically - both statements are correct. However, in practice the difference is negligible. Sure - copper conducts better than steel which conducts better than stainless, and a thick pan does have a proportionally higher "resistance" to heat transfer than a thin one - but in the case of boiling sap in a pan the overall resistance is the sum of the flame to metal, metal itself and then metal to liquid resistances. The boundary resistances (flame to pan and pan to sap) are SOOOO many times greater than the thermal resistance within the metal itself, that it's not worth worrying about the pan thickness or material.

The other discussion that comes up all the time, and is generally accepted as truth is that the shallower the sap depth, the faster the evaporation rate. I don't dispute it (and I run 1/2" to 3/4" typically) but I have never been able to figure out the science behind it. Every liquid has a "latent heat of evaporation". Transfer that much energy to it and it changes from liquid to gas. If the amount of heat being transferred is dependent on temperature difference and the surface area, then the amount of heat being transferred to the sap will be the same regardless of depth. The latent heat of evaporation is a property of the liquid and doesn't change - so X energy = Y steam, again regardless of depth. (Yes - the latent heat of evaporation of a liquid does go up with pressure, and pressure goes up with depth - but the difference in pressure for another 1" depth is trivial.) So - depth shouldn't matter - but it does. DRAT!!

2) Efficiency of Fuel
You are correct that this is not discussed anywhere near as much as 1. Efficiency of fuel is a combination of the conversion of stored energy from fuel to heat, and the percentage of the energy that's extracted from the gasses and transferred to the sap.
The longer the arch, the more opportunity there is to remove heat from the hot gasses before they leave the arch and go up the stack. That's why a long narrow arch will use less wood than a short square arch. It's also why it's best to have a short firebox at the front of the arch rather than one the full depth of the arch. To be most efficient at using your fuel, you want the hottest fire possible AND you want the coolest stack temperatures possible. i.e you want to extract as much of the energy as possible from the gasses before they exit up the stack.
This is contradictory to 1) above - where you want the highest possible temperatures across the largest possible surface area.

Add a blower -> hotter fire -> higher evaporation -> GOOD. But add AUF = higher stack temperatures = less efficient use of the available heat in the wood = more wood consumed for the same amount of syrup produced. Is that good? It is at 2am when you have 60 gallons to go before you can hit the sack, but it might not be in May when you're out cutting wood in the early summer heat. This is something that every sugar maker will need to decide for themselves. I'm happy with my 20+/- gals /hr on my 20x64 natural draft. I could probably push it to 30 with a blower, but if it took twice as much wood to do it, it's not for me.

Gasification is a different discussion. Gasification increases the amount of the wood that is actually converted to heat - the other half of the fuel efficiency equation.

3) Efficiency of time
Buy syrup in the store. $20 a litre. Comes nicely packaged in a glass bottle with a convenient screw cap.

[sappytapper]

Also an engineer here, so I am right on board with you on this thread. This is the kind of stuff that goes through my head with just about every hobby I pick up... And as some of my friends and family have commented in the past, I seem to collect hobbies like some folks collect stamps. You could say my biggest hobby IS hobbies ;-)

Regardless, this leads to mostly the focus on efficiency of time, as that's the one thing that you really just can't get more of. Thankfully, increasing the efficiency in the other areas usually yields more available time. But also usually requires some investment of time (and money... oh lord, the money...) up front, so I've had to force myself to notice sometimes that it can just cancel itself out.

I'm only on my second year into this, but jumped in to building a barrel stove with AUF for this season to try to bump my boil rate up from 1gph and change to 4+, figuring that I was planning on increasing the number of taps by 3 or so, so increasing my boil rate by more than that factor should yield more time in my pocket when the dust settled. Yeah right... Now I have spent at least a couple full days, all said and done, building a barrel evap that still needs plenty of tweaking - and the boil process is more involved, to boot, since keeping the boil rate up means firing more often with more thinly split wood! So much for being able to make dinner while the boil is on! But, I digress.

End of the day what makes some of these hobbies so appealing to me, is trying to make it as efficient as possible to minimize the parts I don't enjoy as much, but still maximize the parts I do. On my gardens I have set up automatic watering and weed block fabric, since i hate watering and weeding, next year I'll be running my 15'ish taps on tubing to a temperature-probe controlled shurflo, so I can just let the sap collect during the week instead of having to empty buckets every day during a run. Just using those two examples, though, am I REALLY saving time when I take a step back? Probably not... The up front time to set up the systems that keep me from weeding my garden or emptying my sap buckets probably negates just doing those tasks. I just figure that's the curse of the engineer in me. :-)

[Big_Eddy]

3. Efficiency of your labor and time in basically everything.
We maplers seem to yippie yap on everything here, and I daresay that for most posters, this efficiency wins out, but is often overlooked. the details vary in that if there is a dollar value in your time, hobby or not, we need to consider. For example, I can just order a cord of delivered, split, seasoned firewood, and save myself many days of cutting and splitting. I can save a small fee of delivery, and go pick it up, for probably two hours cost. Easy choice? Same goes for sap collection, bottling, you name it. For us small time folks, an enjoyable activity that becomes a burden needs to be modified. Herein lies our problem. We relentlessly pursue this area and get addicted to getting.. better, more efficient!

Not evaporator efficiency, but efficiency nevertheless. We use buckets. Tubing is more efficient than buckets (especially if it runs right into the sugar house) but where's the fun in that? By visiting 200+ trees every day and collecting the day's yield, we are in tune with the trees and out of touch with the rest of the world. I's relaxing. It's enjoyable. It's family time. And it's not efficient. So what? My kids are lined up with buckets and the trailer when I get home from work. They are raring to go. Once we get back with a tank full of sap, they disappear. Collecting sap is fun. "Watching water boil" - not so much. Everyone asks me why I still use buckets. That's what makes syrup making fun for us. If it wasn't fun I wouldn't be making it.

[Helicopter Seeds]

Glad I hit a spot and I really laughed reading the responses.
I enjoy collecting buckets too, and frankly cannot string full lines across the neighbors yard anyway. I get intrigued by the idea of vacuum just the same, but not for the laziness sake, just the efficiency of higher yield per tap. Really intrigued at the idea of sapling farms with vacuum limbs. Try to convince wife to plant 50 small ones in my back yard.. won't happen. now I digress. Don't like cleaning, which was principle in me raising pans out of flame to heat only the bottom. which reminds me of Big Eddy's question on depth.

I was touching on the heat transfer, which in a bottom only pan, goes from fire to sap by the bottom as discussed. The comment further that it hurts you also - that was in regard to the transfer of heat from sap through sidewall to air. so a six inch filled pan has 6 times the surface area than a one inch filled pan for heat energy to escape the pan and heat the air instead. even if shallow, I have noticed small condensation on the interior sides, so there is still heat loss going on, but not as much. A well insulated wall could probably help, but if boiling only an inch deep, then maybe not worth it. Admittedly I am entertaining wrapping this area, but I don't think I could really measure the result in output. It would be simple enough to measure the sidewall temperature with a laser thermometer above and below the level line, and calculate the heat loss. But as I said, although I cannot stop thinking about it, I resist the temptation to actually do the math. I tell my young engineer staff all the time when they pose an idea for testing - "what are you going to do with the information?" In this case it would only serve to guide on yes or no to bother insulating the sidewall.

[pyro]

I've been debating about the thickness of the pan since I boil with a relatively thick 14 gauge stainless pan. I am getting 3-3.5 gph on a 2x3 pan with new custom steel arch. I'm not convinced the thickness is insignificant. I blame my poor efficiency on this. The thinner the pan the better heat transferred. I don't follow some of your additional resistances such as flame to pan and pan to liquid. The bottom of the pan is at some hot fire temp, say >500 F. The sap is always 212 F or boiling point, never more no matter what (excluding sugar content, elevation, etc). The better the thermal conductivity of the pan, the better heat transfer. Metal thickness is proportional to thermal resistance. So going from 14 to 20 gauge is half the thickness and twice as much heat transfer. Now going from 20 to 22 gauge is much less significant and probably not worth the effort/cost.

Now I have to decide whether I believe this enough that its worth spending money on a 20 gauge pan for the gamble that I am actually correct and it significantly improves gph!

[Helicopter Seeds]

pyro,

my take is that it won't matter much. The thinner more conductive pan would start the boil faster, less steel mass to heat up, but the heat would also keep evaporating the sap after the fire cycles down, so that is just an energy balance equation heat in = heat out. My pan level dropped an inch overnight long after the boiling and steaming stopped. a thicker bottom would technically prolong that a smidge. The heat loss however would slightly be higher as again, you lose a little in your sidewall conductivity. I watched a video of sugar cane syrup production and they traditionally use a very large wok shaped boiler in the center of a swirling brick oven. The boiler is thick cast iron, and still works for them. No reason that concept would not work on maple, but draw off is by scupper.