Preheater design

[nymapleguy607]

Just thinking out loud but have any of you ever thought about building two of the preheaters shown in the maple producers guide and placing one on top of the other then teeing them together. Has anyone tried that or think that it might work? Thanks Jeff

[Sugarmaker]

nymapleguy607,
Sorry I did not see the stacked preheaters. Were they trying to get more feet of pipe run in a small hood area? Seems like it might work OK. I believe the goal no matter how you design it is to get the sap to near boiling without vapor locking the preheater. I used the Leader design and can get about 200 deg F when it comes out to the float box. I think it improves 5-10% efficiency?

Chris

[nymapleguy607]

Just curious.. what does your leader design look like

[Sugarmaker]

nymapleguy607,
In the leader catalog they have a picture of the Parallel flow type which has as many tubes as possible side by side and running from larger cross manifold a the bottom (entry) to the higher manifold (exit) I have 10 3/4 inch copper tubes about 4 fee long under a 3 foot wide hood and 5 feet long. I did have to add vents at the top to eliminate (knock on wood) the air trap problems we had for two years. This system works good and does not require and maintenance except for draining when freezing temps between boils.

Regards,
Chris

[nymapleguy607]

thanks for the info I was wondering how the paralell flow preheaters worked. I am just trying to figure out the best design I am building a new hood for my 2x6 and want to and a new preheater. so just looking at different designs. How do the tubes run...do thy run at a diagonal or do the go out horizonal and then U back towards the rear?
thanks Jeff

[maple flats]

On my 2x6 I made a double decker preheater. I made 2 grids of 3/4" cu about 2' long with 5 or 6 parallel tubes, soldered into tees and spaced as close as std tees allowed, with the outer one into an ell at opposite corners. I then soldered a tee at the inlet in tube and on the op. corner a tee to add the same thing again on a second level. When I mounted this preheater it was lowest in the inlet and highest on the outlet at the opposite corner. The whole thin was tilted side to side and any air migrated to the outlet. At the outlet it just dropped straight into the feed valve and trapped air was never a problem. Under the preheater I made a drip pan that drained out the side and into a bucket. The sap as it hit the pans was near but not at boiling. It varied from about 170 to 205 but was usually between 180 and 190 once the rig was at a rolling boil. If you try something similar you might want to vent the high spot unless you will be a straight drop to the float valve or you might get an air lock. All i did to get it started was hold the float open until there was a solid stream and then let the float valve do it's job. The entire preheater was in a hood that fully covered the flue pan. The heater was a parallel set up with a header on each end and on each level.

[Sugarmaker]

nymapleguy607,
The lower back manifold has all 10 tubes coming off it running towards the front of the pan at a slight up angle then all 10 connect to the front manifold. From there the sap is piped to the float box. the pressure in the feed tank over comes the slight rise in the tubes.

Regards,
Chris

[Haynes Forest Products]

Has anyone used the condensing coil out of a forced air conditioner. They are made to transfer as much heat or cold and there copper and they come with a condensate tray. Now the big question is can you acid wash out the coils to remove the oil. I believe the refridgerent would disipate on its own.

[briduhunt]

I have a HVAC background and I can say for certian that even if you try to flush out the coil there will still be some oil residue in there. That is if you are using a used coil. If you are using a brand new coil there is some lubericant oils inside the pipes but not sure what they use, I am pretty sure it is not food grade.
I personelly would stay away from A/C coils.
Just my 2 cents

[peacemaker]

brandon on your prehaeter i couldnt tell where u cought the condensate ??

[WESTVIRGINIAMAPLER]

I have some pics on the weblink in my signature if you are interested in looking at it. There is already a preheater post just below this one in this section that has some good info.

As far as condensation, I have a stainless drip pan that is probably about 10 to 12 gauge that has the brackets for the preheater bolted to it that has several bolt holes to allow adjustment up and down. It has a lip on both ends that is bent down and the lip sits into the drip channel on each end. This eliminates any brackets and catches 100% of the condensation and the drip pan is plenty strong to support itself and the preheater lifts out of the brackets as it just sits in the brackets. If you look close, you can see what I am talking about.

[Fred Henderson]

One can use all the fitttings a cooper pipe you want. I made a header using 1-1/4" cu pipe. I clamped it to a piece of angle iron for drilling puroposes. I the laid out the holes that I wanted for 3/4"cu. I used 3/4" cu fin tube for my tubing. I drill 7/8" holes in the header pipe and then I silver brazed(food grade) the fin tube into the header. The Alu fins pick up so much more heat then just a straight run the cu pipe.

[Haynes Forest Products]

What is CU pipe. I used finned base board pipe in the past it does work well. I think it drips less. Im not certain but I think the heat overpowers the cold and causes less condensation.

[Clan Delaney]

What is CU pipe. I used finned base board pipe in the past it does work well. I think it drips less. Im not certain but I think the heat overpowers the cold and causes less condensation.

Haynes-

Cu is the elemental symbol for copper. Just shorthand.

Also - how well does that finned pipe clean up? Just hose it off at the end of the season? No problems with mold or the like?

[Clan Delaney]

I have some pics on the weblink in my signature if you are interested in looking at it. There is already a preheater post just below this one in this section that has some good info.

As far as condensation, I have a stainless drip pan that is probably about 10 to 12 gauge that has the brackets for the preheater bolted to it that has several bolt holes to allow adjustment up and down. It has a lip on both ends that is bent down and the lip sits into the drip channel on each end. This eliminates any brackets and catches 100% of the condensation and the drip pan is plenty strong to support itself and the preheater lifts out of the brackets as it just sits in the brackets. If you look close, you can see what I am talking about.

I've been inspectin' yer pics, WV... Tell me if I've got this right:

http://i203.photobucket.com/albums/aa160/wvmapler/Preheater/7ffc.jpg

Unheated sap travels in to the system via the pipe in the center top of the pic, then travels down. It gets heated as it rises through the parallel pipes, then out to the pan at the end of the manifold close to where it came in? I assume that the valve on the lower manifold is for draining the system?

[sapsick]

should the outlet of the preheater in your pic go to a float or ball valve?

[WESTVIRGINIAMAPLER]

Clan,

You hit it right. The raised pipe is where the sap comes in from the feed tank and it actually enters the preheater on the lower end where the ball valve is. Then it has aprox a 3 to 4" climb back up hill thru the 5 parallel pipes to where it exits outside the hood and down to the float box.

I am not claiming mine is the best or anywhere near it, but it is simple and anyone can build one and not have much expense in it and it works good and it made so that it should last forever pending I don't forget to drain it sometime and it freezes and busts. The only change as I have mentioned before is the pics of the outside connections on the outside of the hood. They are now brass quick couplers(3/4") instead of the flexible pipe with a threaded male and female coupling and 2 crescent wrenches takes it apart in about 10 seconds.

[Haynes Forest Products]

Clan The finned pipe cleaned up fine plus I used to start every season boiling water to get thing cleaned up so thing were steam cleaned.

[partsrus1974]

WV i noticed there is no bypass in case it gets to hot..you ever had a problem with it getting to hot?Also any one have a pre heater made with the fins on it?(baseboard)i was told u have to be real careful with them cause they get the sap super hot..Steam makes me nervous LOL.But i need to figure something out cause my pre heater is to small after seeing all your guys pre heaters,mines only 30" wide and about 22" long,I don't think there is more then 10' of pipe to it.It gets it hot but don't last long once u start drawing off.

[RileySugarbush]

partrus:

Preheating using the steam and water vapor rising off of the pan can't superheat the sap inside the tubes to a temperature higher than the boiling sap. That's the Second Law of Thermodynamics, and it's what makes this method of preheating safe. There is no way it can flash to steam.

It is possible to vapor lock, but not due to real boiling, just due to some vapor forming in the tubes. You know, like how a pan of water steams a bit before it actually boils? If you have your feed tank high enough it pushes the bubbles or pockets of vapor through the float valve. If not, flow can stop. Many add a vent on the high point to let the gas bubbles vent back to the tank. In our rig the feed tank is several feet higher than the preheater and it just pushes them through, though it makes the float valve spit and hiss a bit now and then when the bubbles go through.

If you run sap or water through a tube inside the stack it will be tremendously efficient and potentially very dangerous. Not worth it in my opinion, when the hood preheater works fine and is safe.

[partsrus1974]

Thanks for the info john..My 275 gallon feed tank is about 4 feet higher then my rig,so it does have some push behind it.Is there such a thing as to big of a pre heater?problay a dumb question.

[WESTVIRGINIAMAPLER]

As long as you are not taking up too much space in your flue pan, I wouldn't think there is such a thing as too much preheater. Normally this isn't a problem due to space issues in the flue pan and hood. Don't want a drip pan so big it is restricting steam rising around it.

As far as vapor lock on my preheater, I don't worry about it. I run my pan depth around 1.5 to 2 inches and when the level drops a 1/4" or so, the arm on the float drops and the vapor comes out. I have plenty of head pressure, so I see no reason to worry about vapor lock. If I was running a 1/2" level in the pans, I might be concerned, but even when I used to run at a 1" or less level, I didn't worry about it.

I don't see any reduction in boiling rate running the higher level in the pans and there is a lot less to worry about this way too. Last year I was averaging stack temps of 1650 and pushing 760 cfm air thru it, I am not going to worry about burning the pans running low levels even if it did cost me a couple of gph which I didn't see. It was boiling so hard, the higher levels just allow more liquid to put off steam at a given time.

[Beweller]

Riley,

No, that is the zeroth law--the existence law for temperature. The second law is the existence law for entropy.

[danno]

I had a heck of a time with vapor lock last year - especially when my head tank was low. I have a pre heater by-pass and I'm glad I do, I had to use it often. I do have a pressure release on the preheater that I had plumbed back intot he feed tank - but it must have had too much rise for it to work.

Differance between a full and low head tank is huge. I'm constantly changing the float settings depending on how much sap is in the head tank.

[Haynes Forest Products]

BOOKEM DANNO what size pipe do you have from your head tank to the valve? You might try and go with a bigger pipe. I use clear plastic tubing with about a 1" inside diameter. I like to see how things are running in and with a larger size pipe you get a more constant pressure on the valve. Thats got to be a pain to keep changing. What make pans do you have. My 3x10 waterloo small has a feed box float about the size of a small shoe box.

[Beweller]

No such thing as too much preheater? Maybe if you have no concern about economics.

Very roughly, the following table represents condensation heated preheater performance.

Sap temperature * Units of surface
range, from-to * needed
40 to 50 F * 1.0
60 to 70 * 1.1
70 to 80 * 1.2
------ * ----
170 to 180 * 4.5
180 to 190 * 6.2
190 to 200 * 9.8
200 to 210 * 23.9

Would you like to guess how much surface it will take to go from 210 to 211 F?

Note that that last 10 degree increment represents about 0.88 percent of the total heat demand. How much can you afford to spend on a preheater to improve your thermal efficiency by less than an additional 0.9 percent? More than 50 percent of the cost (of surface area) to reach 10.5 percent increase in thermal efficiency?

Sorry, the table format did not reproduce. I have added an * to separate columns. Hope you can figure it out.

[tom jr.]

How much is a unit of surface???? What is the formula?

[dano2840]

my uncle built one out on 1 3/4 i think copper pip that went down 1 side of the flue pan over the divider and back up the other side and connected into the feed pipe he had a pipe that went above his feed tank to prevent vapor lock it was open so vapor could escape he had sap coming in at 115, work real well

[Beweller]

Tom,

A unit of area is the area needed to heat sap from 40 to 50degrees. Its value in square feet depends on the sap flow rate and to a lesser extent on the physical design of the heat exchanger. Using a heat quantity equal to that needed to raise the sap ten degrees and the above defined surface area, the equation is simply 167/(212-Taverage), where Taverage is the average temperature of the sap. For example, for the temperature interval 160-170 F, the average temperature is 165 F, the average temperature difference is 212-165=47 F, and the required surface area is 167/47=3.6 units.

[Dave Y]

Danno,
To help control the flow from your head tank into the per heater and float box , try installing a ball valve you can tweak instead of your float box. Once you get your level where you want it dont touch your float box. Adjust the ball valve. Another thing you can do to help with the head pressure, is run your evaporator with a smaller head tank say 100gal and pump sap in it as needed. I have mine set up with a normally closed float switch that atuomaticlly kicks on the pump when the tank is about 2/3 empty. It works well for me.

[Beweller]

Under the hood preheaters function by heating the sap with the latent heat from condensing steam. When the steam condenses the resulting water occupies far less volume than did the steam. As a result, the steam flows towards the preheater surface.

If there is any air in the steam, it is pulled along with the steam. But the air does not condense--it has nowhere to go. It ends up blanketing the surface, preventing direct access by the steam. The result can be a drastically reduced heat transfer coefficient, the need for more preheater tubing, and a reduced maximum possible sap temperature.

To avoid that situation, minimize any leakage between the hood and pan. Further, consider adding a damper to the steam stack. Adjust the damper so that a small quantity of steam escapes from every leak. That asures that no air is leaking in.

[tom jr.]

beweller,

did you factor in steam pressure inside the hood? I do believe it has a big effect on preheater efficency.

[tom jr.]

sorry you beet me to it

[RileySugarbush]

The actual pressure inside a hood is hardly different from atmospheric pressure. You can actually calculate the maximum possible pressure by dividing the weight of your hood in lbf by the area of your pan in square inches.

In my case, assuming my hood weighs about 25 lbs over my 2x3 flue pan. if the internal pressure got up to .03psi my hood would lift off and rattle like a sauce pan lid when you boil eggs! So that is less than a 0.2% change in pressure over atmospheric pressure. Insignificant.

[Haynes Forest Products]

I used 3/4 finned baseboard pipe for my first preheater and it worked fine for a 2x6 that only did about 25 GPH. Finned pipe was not designed to transfer massive amounts of heat if 190 degree water was pumped threw it and 170 degree water came out a 20 d heat transfer was good for home heating. What we want to do is put 40 degree sap into the same 3/4 pipe and get 180 out and that takes eather a long pipe or slow GPM.The longer the pipe the bigger the heat transfer the hotter the sap enters the evaporator. I would use the thin wall copper pipe that they use in heat exchangers there is no pressure so its safe.

[tom jr.]

Then what is the purpus of a damper in the stack? if you close it down you are creating pressure in the hood, thus drasticly increasing preheater efficiency, that is in my case anyway.

[RileySugarbush]

No, you are not increasing the pressure much at all. Just a tiny, tiny bit, enough to minimize air infiltration as mentioned by Beweller. The increase in heat transfer that you see is not due to increased pressure, but instead is due to the higher concentration of steam. Think of an open pan of boiling water: Would you rather put you hand over it with or without a small fan blowing air into the steam? The damper shuts of the imaginary fan.

[Beweller]

Tom,

You want the pressure inside the hood to be very slightly above atmospheric. So that any leakage is steam outward, not air inward. This small pressure, typically less than one inch of water, has itself no detectable effect on heat transfer.

If you could significantly pressurize the hood, it would increase the heat transfer

[Beweller]

Haynes,

I would expect the heat transfer rate to be dominated by the inside (sap side) coefficient. Fins on the outside where the condensing coefficient is already large (if you exclude air!) don't accomplish much.

You need the fins on the sap side.

[Beweller]

While we are at it, I notice that there seems to be a preference for parallel flow preheaters. My experience with some related types of equipment is that great care is needed to get more or less equal flow in each path.

A single 3/4 inch copper water tube flowing at one foot per second has a discharge of about 83 gal/hr The dynamic pressure is only 0.007 psi. At such low velocities the friction pressure drop in the tubes is not sufficient to control how the flow splits among parallel paths. Rather, details of the inlet and exit geometry will control, and typical preheater designs do not attempt to make the geomety for every path identical or provide some way to adjust the flow.

The sap flowing in "slow" tubes will become very hot, while the sap in "fast tubes" will have a lesser temperature. Because with a finite source temperature (the water boiling point) the heat transferred decreases as the sap temperature rises, the non-equal flow causes the heat transferred (the mixed sap temperature) to be less that if the flow in every path were the same.

[Haynes Forest Products]

belweller I would agree. When it comes to heating air with water its more about how fast the air can obsorb the heat from the finned tubes and move on out into the space to be heated. with a sap preheater we are doing the oposite....heating sap with steam that has lost alot of its energy. Now if we can play along with the idea that a evaporator with a steam hood is a very basic steam engine if you had a fan that turned from the exiting steam at the top of the stack. If it was a closed system that could built pressure the steam would carrie alot more energy so as you said if you keep outside air that is cold out of the system the preheater will get steam that carries more heat. Its the steam you cant see that hurts.

[sapsick]

what if you put valves in each of the parrell lines with temp gauges so you could adjust the rate of flow thru each line allowing for slower flow rates in the cooler lines?

[Clan Delaney]

While we are at it, I notice that there seems to be a preference for parallel flow preheaters. My experience with some related types of equipment is that great care is needed to get more or less equal flow in each path.

A single 3/4 inch copper water tube flowing at one foot per second has a discharge of about 83 gal/hr The dynamic pressure is only 0.007 psi. At such low velocities the friction pressure drop in the tubes is not sufficient to control how the flow splits among parallel paths. Rather, details of the inlet and exit geometry will control, and typical preheater designs do not attempt to make the geomety for every path identical or provide some way to adjust the flow.

The sap flowing in "slow" tubes will become very hot, while the sap in "fast tubes" will have a lesser temperature. Because with a finite source temperature (the water boiling point) the heat transferred decreases as the sap temperature rises, the non-equal flow causes the heat transferred (the mixed sap temperature) to be less that if the flow in every path were the same.

It sounds like you're making a case for a series flow pre-heater (as opposed to parallel). This is a design I was thinking of going with. Anyone else have one? Are there any downsides or is a preference thing?

[Haynes Forest Products]

Sounds good just make sure you get the greenhorn to reach into the hood to change the valve settings LOL I would opt for the keeping it simple.

[RileySugarbush]

I have a 6 pass serial flow preheater. About 22 feet of 3/4 in copper tube, normal wall thickness for water plumbing. It works fine. Others have parallel set ups of various configurations. They work fine too.

The key here is that we have about 5 times as much thermal energy in the steam as we need to heat cold sap to reasonably close to boiling. That means we don't have to be too careful about it. We are going to throw away a lot of thermal energy, and there is nothing else we can do with it anyway. Just put enough tubing with sap in it to get it warmed up. Pick your layout, put enough head on it so a little vapor doesn't stop the flow (or put in a vent) and off you go!

[WESTVIRGINIAMAPLER]

Either will work fine, just make sure you have a way to drain them at the end of the boil as the copper will freeze and bust fast. The reason parallel preheaters are more popular is because they are easier to drain every boil.

[RileySugarbush]

That makes perfect sense Brandon. Mine is a royal pain to drain, I have to blow it out.

[Uncle Tucker]

I have read somewhere, most likely here, that with series flow is that the bends in the tubing cerate a lot of friction. The way to over come that is to increase your head pressure (raise the feed tank). The other issue is on a bigger evaporator you need so much copper it won’t fit in the hood. With the parallel flow, you can stack the pipes.

And a final though about parallel flow and equal heating.
If you make the sap flow up hill, the warmer sap would rise and the cold sap would stay in the bottom until it got warm. Just my thought.

[TimJ]

Clan,

You hit it right. The raised pipe is where the sap comes in from the feed tank and it actually enters the preheater on the lower end where the ball valve is. Then it has aprox a 3 to 4" climb back up hill thru the 5 parallel pipes to where it exits outside the hood and down to the float box.



I am confused - you say it enters at the top raised pipe - then you seem to say two contradictory things - or more likely I am not understanding very well.

I think I am missing the two connections - one for the ball vale and the other for the top right of that picture.

Can you draw the flow for me - I think that would be easier for my simple mind - thanks

(I am hoping to build one of these for next season.)

Another question I have - how does one disburse the steam and condensation with a home-made steam/heat/scavenger? Specifically - would a large bathroom vent and fan work at the "top" of the hood? What sort of pan/mechanism does one use to catch the condensation (this is a drip pan?) How small/large should it be and wouldn't that impinge on the steam getting to the copper pipes?

Thanks
Tim

[WESTVIRGINIAMAPLER]

Check out the pics of the preheater and hood in my photobucket link in my signature below. Pics should do more to explain vs trying to explain it to you by typing a description.