Steam damping discussion

[HanginAround]

I don't think that slightly pressurizing the hood by dampening will slow your evaporation rate. By adding pressure it increases the temp of the steam. At 1psi steam is 215 F and at 5 psi it is 227 F. When you add pressure by dampening the steam still is coming out of the hood.
I look at it like the pressure cooker that some use for boiling food on the stove. The weight on top adds pressure so the the steam will be much hotter than 212. If you leave it the water will still boil out.

Maybe we can get some input from someone with a physics background to input.
With my hood and pre heater the first year I used it, I didn't have a damper my pipe is 10" and I couldn't find one locally The temp coming out was about 180-185. The next year I bought one on e-bay and now my sap is at 200-205 If I dampen it too much it will boil inside the preheater.
I saw an increase in evaporation rate.
Remember that 1 BTU is needed to raise 1 pound of water 1 degree so you also save on fuel by increasing the temp coming out of the preheater.
Keith

I thought I would move this so we don't highjack Steamer's thread.

I agree that you increase the temp of your steam, but disagree that it's desireable... it takes more heat to get it there and make it boil. In a pressure cooker, higher temps cook food faster, but water boils away much slower. If you could boil in a vaccum (some experimental evaps have neg pressure on the sap), you can boil at very low temps.

I also agree that not dampening the steam flow will negatively affect your preheater efficiency, just not sure at what point the balance tips when weighed against overall efficiency. I really kind of agree that some dampening is beneficial, but can also see that too much can hinder boiling. I think the differences in PSI in the hood would only be in the fractions either way.

I don't think it's physically possible for sap to boil in a steam to sap heat exchanger. The steam will always have a lower boiling point than liquid sap in a pipe with head pressure on it, and the highest theoretical temp you can get in your sap is the temp of your steam.

Russ, did you ever do any experimentation with closing down the damper in order to increase your preheater temps? And were you able to tell either way if overall throughput increased or decreased? I'm sure the differences would be small either way, so hard to judge, especially with so many other variables on any give day. You really almost need to do it in a lab to test it.

[Russell Lampron]

Hangin,

I don't notice much of a difference in the temp of the sap coming out of my preheater with the stack damped or open. I only have a 2x4 hood with 8 inch stack and it seems more beneficial to get the steam out quickly. The sap temp is usually around 185 degrees weither the damper is closed or opened. The thing that makes the most difference is the amount of sap flowing thru it. It will drop down to about 165 when I am getting near the end of a draw off but recovers quickly when the flow slows back down to normal.

Russ

[Sugarmaker]

My guess is the sap temp coming out of the preheater is affected more by the sap flow rate than steam damping. I have a very loose fitting damper, in a 10 inch stack. I have never tried keeping the damper open for a long period of time. I may try this next year as a test. I like to trap as much steam in the hood as soon as possible to get the sap in the preheater temp up. This may just be psychological? Maybe the sap would get just as hot just as fast without the damper? Since this is the first rig that I used with hoods and preheater, I took my advice from Leader as to the use of the damper. I sure can't (and do not try) to build any pressure in the hood. My home made hood tells you right away if the damper is closed , as steam is coming out every crack. So it seems to work that I keep the steam out of the room and maybe help preheat the sap a little quicker?? :?

Regards,
Chris

[Fred Henderson]

A damper in the steam hood is properly set when the steam just starts tocome from under the edges .

[Parker]

Hanging- it is definatly possibel to get the sap to boil in a steam to sap preheater,(I think),,I have to be carful with my rig,,,the first boil last year we started boiling at 8 pm,,at about 2 am steve noticed that the level in the flue pan had dropped about 1 1/2"!!!!!!! I opened the emergency dump valve and brought the level back up the looked at the float box,,there was steam comming out of the feed line from the preheater,,I held the float all the way down and scalding hot sap began spitting intermittently out,,there was a burp of steam then the sap started to flow agine as normal,,,I have a parralel flow pre heater and a peice of junk aluminum flat hood from the 70's,,the evaporator is a 5x16 with a blower,,no airtight front,the 600 gallon head tank is 14" above the preheater and was full,,,we were burning pure red oak, split fine,,the wood was standing dead when we cut it the was split in the spring and dried all summer in the wood shed,,it was quite a boil,,on a suggestion of a local sugarmaker I had my pre heater steam pipe 3/4 damped,,,I now run my steam pipe wide open,,,does not boil as fast but I dont really want to burn my flue pan,,,the preheaters like mine are know for getting air bound,,I have a vent pipe off of the preheater coil but it did not save me that time,,,wouldent you have to be boiling sap in the preheater to make it air bound??

[Sugarmaker]

Parker,
For several years I got air trap in the preheater. I have 10, 3/4 inch copper tubes coming off and ending at 1-1/4 manifold pipes. I had a vent at the upper end too. The vent was 1/4 inch and fed back into the pan.
This last spring I added two more vents, one on each end of the upper manifold. These were the type that you get to tap into a house water line for a refrigerator. I put a short drop line on each and opened them all the way. I also leveled the upper manifold so as to minimize air trap. This seemd to work very well and I did not have nearly as much problem at the float box.

Prior years I had to really watch for this problem. I dont think the sap was boiling just getting a slug of hot air that could not get out. :?

Chris

[HanginAround]

I think you could have air and vapor without actually boiling, but perhaps we're argueing semantics here.

In newer commercial preheaters, the sap enters at the low end and exits at the high end... supposed to keep them from getting airlocked because the sap flow and rising air are going in the same direction. Is your preheater set up this way or the other way around? I've never seen one vented except for the sight tube, but even then on the newest one I saw, that was on the inlet (low) end of the preheater.

Click for larger image:
http://www.ThePixPlace.com/usr/4822/i_HPIM0517.JPG (http://www.ThePixPlace.com/img82541.html)

The bottom tube come from the feed tank, the valve is to bypass the preheater, and the sight tube is above the valve. The top tube feeds the floatbox.

This is my uncle's preheater... old style, inlet and sight tube are on the high end:

http://www.ThePixPlace.com/usr/4822/i_HPIM0463.JPG (http://www.ThePixPlace.com/img82542.html)

[Parker]

I believe my pre-heater is a G.H. Grimm from the 70's,,I just ran down the the sugarhouse to make sure,,my cold sap goes in the bottom,,hot sap out the top,,1 1/4" feed line into 2" preheater parrallel coil,,1"vent out of the top,,,,,not trying to be a smart a## but if the sap is not boiling in the preheater how does the air get in the preheater to block the flow? When people say the preheater gets airbound,,where is the air comming from?

[Fred Henderson]

Parker, I think they mean a vapor lock.

[mountainvan]

On my homemade preheater I put a screw vent fitting meant for hot water heating systems, to purge the air out of the pipes. I turn it so hot sap drips a bit out of the fitting and air/steam can get out too.

[Parker]

If my preheater is filled with sap when the rig is cold where is the air,,,or vapor comming from if the sap is not boiling inside the pre heater once I get going,,,,,,,,,there was no air or vapor in the preheater when I started,,it was filled with sap,,the air or vapor has to be produced inside the preheater (by boiling?),,it is not comming from my head tank???

[HanginAround]

ARGGGGG, how can two different picture hosting sights die within a couple days?? It's enough work posting pics once, but three times is getting ridiculous. I'll leave them for a day or so, maybe the site is just down temporarily.

Parker, I would say that air dissolved in sap will start coming out of suspension before it boils, and I would think that some water vapour will be present as you get near the boiling point, but still think that it cannot boil. But the end result is kind of the same so that's why I say perhaps we are argueing semantics.

The max temp of your steam is going to be the boiling point of water at your atmos. pressure, plus maybe a slight increase due to steam pressure in your hood, whereas the boiling point of your sap in your preheater will be boiling point of water at your head pressure (higher than atmospheric pressure), plus a slight increase for your sugar content. So as near as I can figure, the temp required to make the sap boil will always be higher than can be achieved by batheing it in steam at boiling point of pure water at atmospheric pressure.

[Parker]

I see what you are saying and now that you have explained it understand,,if steam is water vapor and water boils at say 212 deg and sap boils at ___ deg above to boiling point of water then the steam will not make the sap boil cause it wont be hot enough,,,
BUT .............you can still run into trouble (with my set up) with vapor related issues if the stem pipe is closed down too much,,,there must be a sweet spot where I could avoid the vapor lock and still get a much increased temp. in the sap going into the flue pan? Seeing as most of my sugarwood this year is pine I will play with the damper a little...wonder why my preheater will lock (when damped too much) when it has a vent on it?

[Russell Lampron]

I don't have the facts to prove this or have tried to measure it but have been told that the temp of steam can be much higher than the boiling point of the liquid that it is coming out of. Sap can boil in a preheater because of this, especially when sap flow to the flue pan is slow.

Russ

[Fred Henderson]

True staem can go over the boiling point but it has to be in a pressurized container or air tight so to say. It is what is called superheated steam. The US Navy heats their steam to 1200drg to power some ships. I doubt that a hood would be considered air tight. At 1200 drg you check for leaks with a wooden 2x4 and when one is found it will cut that 2x4 in two and leaves no sawdust.

[ibby458]

A previous post is correct. All liquids can be expected to contain some dissolved gases (air). The more the sap is agitatated, and the COOLER it is, the more air it will hold. The soluability of gasses rule states that as the temperature Increases, the soluability of gasses DECREASES.

Therefore - once the sap starts warming up in the preheater, gasses (air) come OUT of solution, causing vapor lock unless vented.

I haven't built my preheater yet, and have no practical experience to back it up, but I think a vent tube at the highest point of the preheater, as large as the preheater pipes, extending to above the feed tank level, would eliminate the problem.

I hope so, anyway. THat's how I'm going to build mine.

[Fred Henderson]

Ibby
Your statement is correct. That is why hot water is uesd when makeing and ice rink because hot water contains less air and freezes faster. I never had a problem with my old set up vapor locking, at least one that I knew about. However I did run a little short on sap once but caught it in time, noo burned on syrup and no damage.

[HanginAround]

Ibby, right, sap is usually cold and very well agitated in a pipeline system with vacuum, all kinds of air dissolved. Thanks for saying what I was trying to in a better way.


True staem can go over the boiling point but it has to be in a pressurized container or air tight so to say. It is what is called superheated steam.

This is true, but if you are boiling it in a pressurized container, you aren't so much making your steam hotter than your boiling point as you are raising your boiling point and at the same time the temp of your steam (a pressure cooker, or a steam hood that is airtight and outflow is restricted). You can also make superheated steam by exposing it to an alternate source of heat afterward... in steam locomotives, the saturated steam from the boiler was passed though superheater tubes that were within the boiler flues and thus exposed to hot combustion gases, further raising the temperature and making it less likely to condense in the cylinders.

Parker, do you have your sap going in the low end of your preheater? If it goes in the low end, the sap flow and the vapour flow is in the same direction (upward) rather than opposing each other. Also, is there any chance there is a high spot in your tubes somewhere that is trapping vapour? For instance, I think if your entrance and your exit were both lower than your highest point, that would be a problem. Just a thought...

[WESTVIRGINIAMAPLER]

For what it's worth, I have no vent on my preheater and have no problem running about 50 gph and the preheater temp of 175. :?

[Parker]

As I stated the cold sap goes in the bottom of my preheater,,,yesterday I was looking at my preheater a littel closer,,,The preheater tubes are 2",, the feed line and exit line are 1 1/4",,,,,,where the hot sap comes out of the top of the preheater there is a reducing connection from 2" to 1 1/4" ,,,,the vent is 6 or so inches down the 1 1/4" line heading towards the float box,,,I am thinking gasses could be building up in the top preheater tube and not moving through the reducing connection and out the vent,,I will take a pic of this and try to post it,,,the vent does work to some degree as when the rig is really going once in a while you can hear it blow off back into the head tank

[HanginAround]

Sorry for asking again, I do remember you saying that.

Seems like your setup should work pretty good. I wouldn't think the reduction from 2" to 1 1/4" would be that much of an issue.

[WESTVIRGINIAMAPLER]

Parker,

I would think the gases would congregate at the highest point. I don't know what type of float system most of you use, but my float system on my evaporator takes care of any vapor lock. When it locks, the level will drop about 1/4" and the float system will drop open enough to allow the gases to escape and it will catch back up. It doesn't lock very much, but when it does, it takes care of itself. :)

[Parker]

I think part of the problem might be that the preheater is on the oppisit side of the evap. than the float box,,,a pipe brings the hot sap around the back of the rig,,then drops into the float box,,,I have a 2 year old leader flue pan-float box set up