Does evap rate fall as sugar content rises?

[Paperman]

In a batch set-up or even a continuous flow system does the rate of evaporation fall off as sugar content rises? Seems in the begining you can push off 30 GPH but later you can only drive off 15-20. I know boiling point raises but is that the sole reason the rate falls off or dies it really keep an even rate and I am seeing things.

[WESTVIRGINIAMAPLER]

I would think if anything it would go up as the higher the sugar content in a substance, the faster and harder it boils. Maybe your are backing the fire off some if it is falling.

[Meridian Maples]

The evaporation rate of your machine will remain the same. If your machine goes through 180 gal of sap an hour it will do the same with any concentration. When i didn't have an RO we would run 180 gallons of sap through my machine an hour with 2% sap, now with the RO we run 12% sap through and its still at 180 gallons an hour. You will draw off more with higher concentration though. The evaporation rate doesn't change.

[Paperman]

That is how I thought it should be. We must have been changing something without thinking about it as the boil progressed. Thanks again gentlemen.

[red maples]

A few variables that might change the boiling level. Some might cause a big difference some only minute changes. Assuming wood fired evap.

1. Atmospheric Pressure
2. Air Temperature
3. wood temperature
4. wood moisture %
5. type of wood
6. firing times
7. loading times
8. pre boiling sap Temperature
9. Carbon build up on the bottom of the pans.
10. Niter build up inside the pan.
11. air flow

once your pans are sweet the %sugar content in each channel of each pan should remain fairly constant granted your evap is running and being run correctly.

[3rdgen.maple]

Lets think about this a little bit. Im willing to bet that if you are on a batch pan setup your rate will drop the closer you get to syrup. If you are on a continous drawoff setup I bet it doesnt.
If you batch boil then the closer you get to syrup the less sap you are adding to the pan and the less you are boiling off towards the end. Continous you are constantly adding sap as you produce syrup.

[Paperman]

I am refering to a batch type setup as that is how I am rolling right now. It appers to us that the evap rate falls off as you get closer to syrup. Not as much water available to boil off?

[DrTimPerkins]

In a batch set-up or even a continuous flow system does the rate of evaporation fall off as sugar content rises?

Yes, but it is so miniscule that you'd not really notice it. It certainly doesn't account for the difference you mention.

[TapME]

Dr. Tim, while we are on evap rates and the amount of steam coming off the pans I have a question. Are there different kinds of steam? By this I mean that sometimes you get a rolling steam nice and thick and when you get closer to the finish side of syrup the charactor or properties of the steam change. It appears to less evaporation but it is still the same. This would be on the same boil with approx. the same feed rate of sap and wood to the fire.
I don't say this much but correct me if I am wrong or tell me I'm on the right tract Dr. Tim

[DrTimPerkins]

Dr. Tim, while we are on evap rates and the amount of steam coming off the pans I have a question. Are there different kinds of steam? By this I mean that sometimes you get a rolling steam nice and thick and when you get closer to the finish side of syrup the charactor or properties of the steam change. It appears to less evaporation but it is still the same. This would be on the same boil with approx. the same feed rate of sap and wood to the fire.
I don't say this much but correct me if I am wrong or tell me I'm on the right tract Dr. Tim

Generally there isn't a great deal of difference in the physical process, but a lot of small things can affect the appearance of the steam. The only real difference that is distinguished is low pressure vs high pressure steam, but that doesn't come into play in this case.

[Beweller]

The steam leaving boiling sap is saturated steam. The steam leaving finished syrup is superheated steam with 7+ degrees of superheat.

The saturated steam quickly condenses to a fog, which is what you see. The superheated steam must be cooled to the boiling point of water before it can condense and form fog. The steam itself is invisible.

[TapME]

The steam leaving boiling sap is saturated steam. The steam leaving finished syrup is superheated steam with 7+ degrees of superheat.

The saturated steam quickly condenses to a fog, which is what you see. The superheated steam must be cooled to the boiling point of water before it can condense and form fog. The steam itself is invisible.

That is what I was thinking, the reason I say that is that it burns a lot quicker than the sap pan does. It also make sence to me

[DrTimPerkins]

That is what I was thinking, the reason I say that is that it burns a lot quicker than the sap pan does. It also make sence to me

I don't believe that is correct. It is more related to the amount of water that has to be boiled off to change the density. Hard to explain with such a small change from 60-70 Brix....so let's start back at the beginning.

In a gallon of sap, to go from 2% to 4%, you have to remove 1/2 gallon of water.

In a half gallon of 4% partially-boiled sap, you have to boil off 1 quart of water to get to 8%.

In that quart at 8%, you have to boil off 1 pint to get to 16%.

Of that 1 pint at 16%, you have to boil off 1/2 pint to get to 32%

Of that 1/2 pint at 32%, you have to boil off 1/4 pint to get to 64%.

Of that half pint at 64%, you boil off just a very small amount to get to black char.

(Changes in RED font above courtesy of Professor 3rdgen)

So basically, as you approach the density of syrup, very small removals of water make large differences in the density. Since you're always evaporating (removing) water from the sap at pretty much the same rate (unless you forget to throw wood in the arch), the increase in density accelerates as you approach syrup. To go from 32% to 64% takes a certain period of time. The change from 64% to >75% takes very little time at all. Step away for a few seconds when it is almost syrup, come back to a big mess.

The temperature of steam coming off an open boiling pot is always going to be 212 deg F unless it is under pressure (PV=nRT), which it is NOT in a standard evaporator. You only will get superheated steam when it is under pressure (a pressure cooker or autoclave). The temperature of what is left in the pot can be quite a bit different though.

[3rdgen.maple]

I don't believe that is correct. It is more related to the amount of water that has to be boiled off to change the density. Hard to explain with such a small change from 60-70 Brix....so let's start back at the beginning.

In a gallon of sap, to go from 2% to 4%, you have to remove 1/2 gallon of water.

In a half gallon of 4% partially-boiled sap, you have to boil off 1 quart of water to get to 8%.

In that quart at 8%, you have to boil off 2 pints to get to 16%.

Of that 2 pints at 16%, you have to boil off 1 pint to get to 32%

Of that pint at 32%, you have to boil off 1/2 pint to get to 64%.

Of that half pint at 64%, you boil off about a 1/4 cup and you've got black char.

So basically, as you approach the density of syrup, very small removals of water make large differences in the density. Since you're always evaporating (removing) water from the sap at pretty much the same rate (unless you forget to throw wood in the arch), the increase in density accelerates as you approach syrup. To go from 32% to 64% takes a certain period of time. The change from 64% to >75% takes very little time at all. Step away for a few seconds when it is almost syrup, come back to a big mess.

The temperature of steam coming off an open boiling pot is always going to be 212 deg F unless it is under pressure (PV=nRT), which it is NOT in a standard evaporator. You only will get superheated steam when it is under pressure (a pressure cooker or autoclave). The temperature of what is left in the pot can be quite a bit different though.

Okay Doc you lost your marbles somewhere in your math lol. You were doing good up to the point where you have to boil off 2 pints from a quart. By those calculations you just burnt up your pan before you have a chance to boil off any more water. Pint 16 ounces quart 32 ounces. 16x2=32. Not everyday I get to correct a scientist thanks for that it was fun. Just dont take the shine away from me and prove me wrong:D

[DrTimPerkins]

Okay Doc you lost your marbles somewhere in your math lol. You were doing good up to the point where you have to boil off 2 pints from a quart. By those calculations you just burnt up your pan before you have a chance to boil off any more water. Pint 16 ounces quart 32 ounces. 16x2=32. Not everyday I get to correct a scientist thanks for that it was fun. Just dont take the shine away from me and prove me wrong:D

3rdgen....it's the new math. :) No....you are absolutely right. I was thinking 4 pints to a quart. I don't deal with english units real often (I actually get to use metric....like they said back in school in the 1970s).

Actually, it makes my p(o)int event more. It requires even less water to boil off to go from 32% to 64%.

Don't gloat too much. :lol:

[TapME]

Are you saying as the water (steam) leaves the pan at different sections that the temp of the sap boiling will not produce different kinds of steam? Little confused now( doesn't take much some days). Why does it not change with the increase in temp from boiling sap? (The difference in the texture of the steam) I think.. Now I have something to think about all day instead of snow and roofs to be sholved. It should make the day go a little faster. Thanks

[DrTimPerkins]

Are you saying as the water (steam) leaves the pan at different sections that the temp of the sap boiling will not produce different kinds of steam? Little confused now( doesn't take much some days). Why does it not change with the increase in temp from boiling sap? (The difference in the texture of the steam) I think.. Now I have something to think about all day instead of snow and roofs to be sholved. It should make the day go a little faster. Thanks

OK...I see this as a great opportunity to confuse you even more. :) Physics doesn't always make intuitive sense.

Water vaporizes at 100 deg C (212 deg F). This doesn't change whether there is sugar in it, salt in it, or a Buick in it. The boiling point of the solution will change with a change in solute (sugar or salt) concentration, but not the vaporization temperature of the water in the solution. The only thing that will change the vaporization temperature is a change in pressure, a change in volume, or the addition of more heat within a confined space. If you pull a vacuum on it, water will boil at a lower temp. Put pressure on it, it will boil at a higher temp.

So, the only way (in an evaporator system) that you might possibly get steam that is somewhat hotter is under a closed hood, because you're adding more heat to it, and keeping it (somewhat) confined. But the steam coming off the sap in your backpan is normally the same temperature as the steam coming off the (near) syrup in your frontpan.

[Paperman]

Here's a question for you then, if all water boils at 212 than how can 20% or so still be in solution when I pull off near syrup at 217? The entire solution "should" be a uniform temp with all the mixing from the boil. Does the water bond to another compound or element that it needs to be striped off of before it can be evaporated?

[DrTimPerkins]

Here's a question for you then, if all water boils at 212 than how can 20% or so still be in solution when I pull off near syrup at 217? The entire solution "should" be a uniform temp with all the mixing from the boil. Does the water bond to another compound or element that it needs to be striped off of before it can be evaporated?

Because the water that is in the solution is not pure. It has dissolved sugar in it. However you're not boiling off any of the sugar (if you are....your evaporator is WAY too hot), just the water fraction. As the water vaporizes off, the boiling point of the solution increases, however the temperature of the steam (water vapor) coming off of the surface does not (unless you change the pressure or the volume...which doesn't normally occur).

Same kind of thing happens in reverse when you have the phase change from liquid to solid. If you have a sugar solution, the water starts to freeze, and the solution increases in density (sugar content). This continues until all the water has frozen (however you will end up with some sugar entrained -- captured -- in the ice, so this isn't a good method to concentrate sap). The water that is left doesn't have a lower freezing point..it just appears that way because the solute concentration is increasing.

All of these concepts are part of what is called the "Ideal Gas Law", which is mathematically summerized as PV=nRT (P=Pressure, V=Volume, n=number of molecules, R=a constant, T=Temperature).

[Paperman]

:cool: Very Nice Dr. Love it when you ask a question and get a clear answer.

[Beweller]

Water evaporates anytime the vapor pressure exceeds the partial pressure of water in the surrounding atmosphere--regardless of the temperature.

Dr Tim: Please check your steam tables. Steam--water vapor--is superheated when its temperature exceeds the saturation temperature at the existing pressure. Steam can be superheated at pressures less that one standard atmosphere--vacuum. See the tables.

Water vapor leaving a liquid at 219 F is at a temperature of 219 F. How can it be otherwise? Check the enthalpy. It must be cooled to reach 212 F. How does this cooling occur if it instantly becomes saturated vapor at 212 F.

If water vapor leaving a solution at 219 F was at 212 F, a perpetual motion machine of the second kind would be possible.

[Toblerone]

I'm probably wrong, but I believe the temperature difference between 219 liquid and 212 steam is now reflected in the latent heat of vaporization now locked up in the steam. A preheater captures this heat back when steam condenses on the pipes.

[Ausable]

Water evaporates anytime the vapor pressure exceeds the partial pressure of water in the surrounding atmosphere--regardless of the temperature.

Dr Tim: Please check your steam tables. Steam--water vapor--is superheated when its temperature exceeds the saturation temperature at the existing pressure. Steam can be superheated at pressures less that one standard atmosphere--vacuum. See the tables.

Water vapor leaving a liquid at 219 F is at a temperature of 219 F. How can it be otherwise? Check the enthalpy. It must be cooled to reach 212 F. How does this cooling occur if it instantly becomes saturated vapor at 212 F.

If water vapor leaving a solution at 219 F was at 212 F, a perpetual motion machine of the second kind would be possible.

This is a great question - We start our boil with sap and very little sugar in solution -- close to being water. But as we continue to boil there is a change going on and the almost water sap is becoming something else as the water is removed in the form of steam. The sugar in solution is becoming greater and the water less and less and it takes a greater temperature to make it boil. Is the steam being released when we are near syrup still all water or something else? Would seem like when we are near syrup the steam being released is less - even though the syrup is boiling - as most of the water has already been released. --- Mike

[Beweller]

Syrup is 0.9073 mole fraction water. If there is no interaction between the sugar and water, to boil water from the syrup at a pressure of 760 mm Hg requires a water vapor pressure of 760/0.9073 = 837.65 mm Hg. This vapor pressure occurs at a temperature of 102.7 C or 216.9 F. Consequently, we would expect this to be the boiling point for syrup.

In the actual case there is some interaction between the sugar and water that depresses the availability of water. This interaction causes the actual boiling point to be 219.3 F, and the simple calculation is off by 2.4 F. Such deviations from the simple case are common.

[OneLegJohn]

That's it! I'm gonna hook my vacuum pump to my steam hood. I bet I can get a few more GPH out of it! About 10" of Hg should allow me to finish syrup at what, 200F?! Lol!

[Beweller]

Closer to 190 F.

[OneLegJohn]

Not in my sci-fi world it doesn't. Water boils at 192F at 10" of Hg. You have to add add 7 degrees for finished syrup!

[TapME]

[QUOTE=DrTimPerkins;129400]OK...I see this as a great opportunity to confuse you even more. :) Physics doesn't always make intuitive sense.

you have no idea how this did me in. I am more confused now.


Water vaporizes at 100 deg C (212 deg F).

This makes me feel good I know this one is right at sea level.

[WESTVIRGINIAMAPLER]

I would think you wouldn't be able to get them steam hood tight enough to maintain 10" of hg inside of it.

[Beweller]

OneLeg--

Yep. Sorry, I passed over that you were talking syrup, not water.

[Ausable]

Yep --- I guess the old saying "Keep it simple - Stupid" applies once again - All I have to know is that if I boil Maple sap long enough it becomes Maple Syrup and too long -- something nasty --- lol --- Mike

[Brent]

Off topic but:

If you thought you had a handle on how water behaves, you need to dail in to the Oasis chanel when they are running the show call "the Mystery of Water"

The stuff in it boggles the mind. Not sci-fi, but stuff that can repeated over and over and nobody knows what is going on. Water has memory ????!!!! Yup.
You can burn water ????!!!! Yup.

For example they put two beakers of water side by side, filled to within about a half an inch of the top. Stick a couple of probes in the water and turn on the juice. The water climbs the wall of each beaker until it joins. Then they pull the beakers apart about and inch and this worm like string of water stays attached between the beakers, wiggling. After a rather long time, like 20 minutes, the bridge collapses.

One segement in the show talks about 'naturalizing water'. I couldn't help but wondering if we could naturalize sap and what it would do.

It's a 1 hour show. Up here they are showing it on Sunday morning at 9:00 AM. You will have to be numb not to be amazed.

[DrTimPerkins]

If water vapor leaving a solution at 219 F was at 212 F, a perpetual motion machine of the second kind would be possible.

I just so happen to have a couple of those I could sell you. Just send a brown paper bag of small bills to..... :D

In any case....sorry....I left on Thurs afternoon for a meeting, was gone to other meetings all day Friday and Sat., had some real work to do Sun-Tues. First chance I got to get back on the trader to look back at this thread.

I'm not even certain where this thread started, and I don't have time to go back and look, so I will concede the point....to some degree. Steam coming off syrup boiling at 219 deg F will be 219 deg F (at least briefly), and thus, by definition, is (slightly) superheated steam.

However....in a standard evaporator system (with a backpan and frontpan), most of the time there is VERY little syrup, and it is there only very transiently at that density before being drawn off. Further, there is relatively little "sweet" that is even close to syrup (see Fig 7.11 on page 130 of the North American Maple Producers Manual). The weighted average density of all the liquid in the pan (when boiling concentrate) is somewhere around 12-16 Brix (keep in mind that the majority of the liquid volume is in the backpan where it is fairly dilute. The boiling point of "sweet" at that temperature is about 213.2 deg F. If you're boiling sap, the weighted average Brix is closer to 5-7 Brix, resulting in about a 0.2 deg F elevation in the boiling point.

So all in all, the vast majority of the steam from an evaporator system is going to be at a temperature far closer to the boiling point of water than the boiling point of syrup.

Finally....it's getting to be that time of year when I'll be on here much more sporadically, if at all. So don't be surprised if I don't respond to posts for the next couple of months. Just in the few days I missed there were 186 threads to go over....I just don't have the time to do it right now. I may pop in here and there, but won't have time to go back through all the old posts. If you really need something, email me directly at Timothy.Perkins@uvm.edu

[TapME]

Doc. have fun playing in the bush:). have a good time with all the test and projects going on. you are living the dream that a lot of us would want( me included).