Energy needed to turn 43 gallons of 2 percent sap into 1 gallon of syrup

[not_for_sale]

I want to get some feedback on this. I am trying to figure out how to most efficiently boil the sap.

1. It takes 1 btu/lb per degree to get to boiling temperature. Then it takes 970 btu to actually turn the boiling water into steam. Assuming the sap/water is 40 F that means it will take 1142 btus to evaporate one pound of water at normal pressure.

2. 42 gallons need to be evaporated, one gallon contains 8.35 lbs of water. That's roughly 351 lbs.
351x1142 btu = 400500 btu.

3. One MCF of Natural Gas costs approximately 5 bucks here in Michigan, and contains approximately 1 million btus. So 5 bucks at 100 percent efficiency should produce 2.5 gallons of syrup with Natural gas.

4. One lbs of dry wood contains approximately 8600 btu. So approximately 46 lbs of wood should produce 1 gallon of syrup at 100 % efficiency.

5. There are inefficiencies. You heat the arch, you blow btus out the chimney, and the pans aren't insulated. How much is that?

6. Using a simple RO to get to 8 % sap concentrate reduces energy requirements by 75%.

So my question is:

It obviously takes more than 46 lbs of wood to make a gallon of syrup in real life. Or if you use an RO it should take less than 12 lbs of wood. Why doesn't anybody insulate the heck out of the arch and the pans and tries to use the stack gases to heat the sap in the pans (i.e. recirculate through a heat exchanger in the stack? Long enough to have exiting flue gases at 212 degrees ?

How come no one boils in a partial vacuum? Vacuum reduces the required energy to evaporate one pound of water?

I can understand that for the hobby producers this isn't an issue, but for the larger producers it would cut energy cost dramatically if for example the efficiency of a tankless water heater at 92% could be reached?

[happy thoughts]

A lot of what you said is way over my head but you may be onto something. I'd look into what one of the new high tech operations like Crown Maple in NY are up to as they are all about increasing efficiency. You may be able to help them build a better mousetrap and profit from it yourself. That said, even on a small scale some of the simpler principles you mention are already used to some extent even by the bigger hobbyists. Arch insulation for example and even arch design and fire building to capture as much heat as possible before exiting the stack. Or sap preheating systems, even simple ones such as tubing wrapped around the stack or more complex ones like steam aways that try to capture as many btus as possible before the sap hits the pan.

[not_for_sale]

I just happened to come across maple syrup this year and I am an energy nut and a physicist.

One question that I had - I needed to do the calculations so see this. There is a discussions about preheater coils in the stack or firebox. But It takes 172 btus to get water to 212 and another 970 btus to actually evaporate it. So it takes a lot more to push the sap over the hill than up the hill.

Thus it should be utterly easy to run water/sap through a coil in the stack without it turning it to steam. However, it seems that many people talk about vapor lock and/or scorching in these types of preheaters.

I could imagine having a circulation pump, a copper or stainless coil inside the stack and pumping the sap through it, superheating it, and injecting it in the sap though injection nozzles without scorching anything.

[happy thoughts]

Sounds like a better mouse trap. Build it and they will come:) Just make sure to patent it first. I'm sure you could easily find a market for it.

And ah! A physicist! I love this site for it's interesting people with all kinds of backgrounds and experience. I also think you may be just what the doctor ordered to answer the question in this thread about what makes for better evaporation rates. I actually thought of you after reading your first post and added my own feeble answer to that one. I figured you'd probably know the real answer. Me, forget about anything involving math let alone physics, lol, I can hardly balance my checkbook:)

http://mapletrader.com/community/showthread.php?19294-What-indicates-a-better-boil-evaporation-rate-more-steam-or-more-bubbles-in-the-pan

Have a great and productive season!

[philkasza]

We also though about the same idea. The only thing that we were worried about is that sap would have to be moving through the coil inside the stack at a certain rate all the time~sometimes our float for the flue pan gets over-adjusted by some of the crew because we think the level is not right. But maybe I never understood your idea, were you think of recirculating through the stack and back into the head tank or go through the stack and then to the float box on the evap. On our 5x12 evap. we have two 601 Carlin Burners and have about 520 degree stack temp so I agree it would be nice to capture atleast a little of that heat.

Good discussion, keep it up!!!!!!!!!!!!!!!!!!!!!!

Samuel

[not_for_sale]

No - take the boiling sap and recirculate it. No float necessary. Just take the sap out of the pan, run it through a heat exchanger in the stack. Pump it in the top and take it out at the bottom. Inject it like hot air back into the pan.

If that heat exchanger is the full length of the stack it likely would result in gasses exiting at just above 212 F.

You could run it at any flow needed that would prevent scorching.

[DrTimPerkins]

How come no one boils in a partial vacuum? Vacuum reduces the required energy to evaporate one pound of water?

This is common in the cane sugar industry (as are multiple-effect evaporators). It is partly done to reduce the energy demands, but also to reduce the temperature required to boil, thus reducing color formation (which is very important in that industry). A vacuum evaporator would not be cheap or simple to make or operate, but certainly is possible.

For the largest producers, using RO (ultrafiltration) of sap to high concentration levels (20+ Brix) gets them quite good efficiences without the complications of vacuum evaporation.

[not_for_sale]

I can see a vacuum evaporator working the same way you use a Releaser to extract the maple sap. You make sure the hood over the flue pan is vacuum tight and insulated, pump out the steam, condensate it in the lines and use the energy to preheat the sap going in to the Releaser and have the same type of vacuum pump as used in the sap delivery.

Sure it would cost more but - if color depends on temperature, and you are right that temperature is lower under vacuum this would make a higher grade syrup and save energy. So there must be an economic pay back on something like this.

[jrgagne99]

The electrical energy required to maintain reduced pressure levels above the pan of evaporating sap is comparaple to the heat input required to boiling at atmospheric pressure. As Dr. Tim says, vacuum will lower the temperature and heat input, but vacuum itself is not energy-free. I suspect a detailed thermodynamics analysis will show that the energy inputs are the same. Of course the downside to vacuum is needing to make all of your equipment beefier to prevent cave-in. Heavy and not cheap.

[DrTimPerkins]

The electrical energy required to maintain reduced pressure levels above the pan of evaporating sap is comparaple to the heat input required to boiling at atmospheric pressure. As Dr. Tim says, vacuum will lower the temperature and heat input, but vacuum itself is not energy-free. I suspect a detailed thermodynamics analysis will show that the energy inputs are the same. Of course the downside to vacuum is needing to make all of your equipment beefier to prevent cave-in. Heavy and not cheap.

That is probably the case. The vacuum evaporator I saw, although quite large (20' x 40'), had walls about 1/2" thick. It was an entirely automated operation -- it could probably be made less automated, but it would still be a complicated device (much more complicated than a standard maple evaporator). It wasn't the most efficient of the processes they used.....typically the majority of the evaporation was done in multiple effect evaporators and they only ran this style of equipment when those were fully utilized or if color formation was important (and color is far more of an issue in cane sugar processing).

It would take a very long time for the small difference in profit from color to pay back the investment in complexity and robustness of the equipment involved. For example, the vacuum pump would not be a standard pump....it would have to be able to deal with high temperatures. Not saying that this couldn't be done....just that it might not be as cost-effective as other approaches, AND/OR would require a rather large operation to make it economically viable.

[Beweller]

First, the heat of vaporization increases with vacuum. (But you do gain in the heat needed to reach the boiling point.) There are two major heat losses: most importantly the heat of vaporization (or condensation) present in the steam, secondarily the heat content of the flue gases. To recover these heat values you need a heat sink that is below 212 F for the steam, and below the stack temperature for the stack gases. The possible heat sinks to absorb these heat values are the incoming sap, the combustion air, and the fuel.

You can recover a fraction of the heat content of the steam by preheating sap. You can recover more using an air-aided evaporator (piggy-back, etc), a sort of two-stage evaporator. Reducing stack temperature is easy--just provide more heat transfer surface. But this reduces the evaporation per unit surface and requires capital investment. In practice nearly everyone seems to be driven by production rate, so the stack temperature stays high or even increases.

Remember the three laws of thermodynamics:

1 You can't win
2 You can't break even
3 You can't get out of the game.

[DrTimPerkins]

Remember the three laws of thermodynamics:

1 You can't win
2 You can't break even
3 You can't get out of the game.

So true...but doesn't mean people won't keep trying. :D

[bowhunter]

The biggest problem is that you can't transfer enough heat through the pan because there isn't enough surface area. The fireside heat transfer coefficient is too low. If you made the pan big enough to reduce the stack temperature significantly it would be hugh and there wouldn't be enough sap to wet the pan. You might be able to use something called a thin film evaporator and run it under vacuum, but they are pretty expensive. It just a lot cheaper to burn a lot of wood. RO is by far the most cost effective way to remove the water.

I like your idea about stack heat recovery, but the problem is that the flow of sap is too low to recover much of the heat. One idea I was toying with is using the heat to kiln dry your wood. If you were burning kiln dried wood the process would be a lot more efficient. This is my first year and I really didn't have enough dry, dry wood so I'm experimenting with different varieties to see which one burns best when it's not fully seasoned.

[not_for_sale]

Well, I have never heard about those three laws of thermodynamics. They must not teach them in German universities. I've always asked myself why Audi can do a V8 with 50 MPG and GM and Ford can't. This then must be the reason. The engineers don't try to over here ;-p

[not_for_sale]

Bow Hunter - I think you are wrong. If you've ever taken apart a Bosch tankless water heater you'll see that they are capable of exchanging 195 kBtu over less than two feet of stack and a temperature rise of 50 F for 8 GPM of water flow. Most evaporators have 10 or more feet of stack.

This

http://www.fs.fed.us/ne/newtown_square/publications/research_papers/pdfs/scanned/OCR/ne_rp388.pdf

says that on average 16 percent of energy loss goes through the stack.

But, as Dr. Tim said, it may already be acceptable from an economic perspective to just not reduce energy consumption further.

[DrTimPerkins]

?..they are capable of exchanging 195 kBtu over less than two feet of stack and a temperature rise of 50 F for 8 GPM of water flow.

Under the proper conditions yes. Recirculating hot sap across the arch wouldn't give the same results. The temperature differential just isn't large enough.

RE: the Audi V8 and 50 mpg. What model would that be? I definitely agree with you that Audi makes very nice highly engineered automobiles. The correlary to that is that they cost three times what the Ford at 25 mpg gets. Basically you pay for it one way or the other. Same thing with hybrids...pay upfront (cost of car) or pay over time (cost of fuel).

[not_for_sale]

Audi A8 - its the Diesel TDi. I've always said it would be the perfect engine for my favorite car the Debut Conversion of the Chevy Express 4WD. The VW Passat Blue Motion, a V6 diesel also gets 70 MPG +/- .

[Beweller]

If you really want to reduce stack temperature, consider a forced circulation finned tube arrangement that operates somewhat like a long-tube vertical flash evaporator. Very compact, not too costly. Demonstrated at the gas fired domestic boiler scale, and I believe at much larger scales. Probably not practical with wood fuel. An air preheater may be more practical, but requires fan power, probably both induced and forced. If the time scale is relaxed, the idea of using stack gas to dry wood fuel is interesting, but again requires a hot gas fan.

[not_for_sale]

The maximum flame temperature of natural gas is 1500, wood is 1100. Area available in a tankless is way smaller than area available in the flue of an evaporator. Flue temperature in an evaporator is 500 to 700 degrees or more if you supply forced air and have carbon monoxide burn in the flue.

The biggest problem is not going to be the energy available. The problem for wood burners is going to be how clean they burn so that you don't get creosote condensation on the exchanger, and for gas burning its going to be what to do with the condensate on the heat exchanger while you cool the flue gasses.

So I don't think it's as simple as I'd like it to be.

However, I am sure that someone has tried this and didn't pursue it because of technical issues. It certainly isn't the heat exchanger.

My original questions where why hasn't someone looked at these efficiencies and I think the answer that I am hearing is because it doesn't make economic sense to reduce the fuel cost from 50 cents a gallon of syrup to 40 cents.

[DrTimPerkins]

Audi A8 - its the Diesel TDi.

The one referenced in this Audi press release:
http://www.audiusanews.com/newsrelease.do;jsessionid=FFECE04EE9252C8F31B43AB4 035361CB?&id=3240&mid=1

Which states:

- The Audi A8 TDI clean diesel achieves manufacturer estimated fuel efficiency of 24 city/ 36 highway

If that is the one you're talking about? It must only get 50 mpg going downhill. Of course even Audi doesn't know yet what the real mileage will be :)

It also will have an MSRP (reasonably equipped) of over $100k, which illustrates my point quite nicely. While I completely agree that you COULD engineer and build an evaporator that was more more efficient, it would probably cost a pile of money to produce. That would mean you wouldn't sell many of them, which would only exacerbate the problem of cost (you couldn't gain the manufacturing efficiencies necessary to overcome the high cost of production). Finally, it would be a fairly complex machine, requiring a fairly well trained and skilled operator to reach top efficiency, and would be costly to repair.

P.S. My wife had an Audi A6, and then a couple of Passats after that. All were great cars.....excellent engineering (mostly--except the cupholders) and lots of fun to drive, but you prayed to God that they never broke down because it would take the next two paychecks to fix it.

[Dennis H.]

Something like this
http://www.google.com/patents/US5772774?printsec=drawing&dq=maple+syrup&ei=lWQvUdrQKMy10AH5v4DoDA

[not_for_sale]

Tim,

I am not sure if you know this, but Audi and VW's sold in Europe get better gas mileage. The reason is that US emission laws prevent a couple of adjustments that are allowed in Europe that reduce fuel consumption by quite a bit. It increases emissions/gallon, but the vehicle uses less gallons per mile. So the net effect in emissions is that it's emitting less bad stuff but more per gallon used. Stupid but true.

Research the Passat Blue Motion 1.6 - it's very obvious with that vehicle in its UK vs. US version.

In addition, you know as well as I do that Audi costs more not because of he engineering but because of he name. Audi engines are the same as the engines in the VW's. Same relation between Cadillac and Chevy.

Joe

[Revi]

We don't have the most efficient set up but we get about 10 gallons per cord of wood at our sugarhouse. Of course we burn the crappiest wood we can find, which means it's a mixture of hardwood, softwood and anything I can find at the dump that doesn't have nails, paint or pressure treating.

[DrTimPerkins]

In addition, you know as well as I do that Audi costs more not because of he engineering but because of he name. Audi engines are the same as the engines in the VW's.

A little of both perhaps (engineering and name). Seems like you couldn't have one without the other.

We had an Audi A6. A friend had a Passat (forget the model, but essentially same car with fewer amenities). We each had the same silly little switch go bad. The switch was exactly the same (they were interchangeable), except for color and cost. The switch for my wife's Audi was twice the cost of the switch for his Passat.