Preheater size

[Father & Son]

In the process of adding hoods and a preheater to my rig, a 2x8 with a 5 ft flue pan and I have a couple of questions.

What size (W x L) should the preheater be? Info from someone that makes them says his would be 10" wide and 4' long. Is this right or should it be big enough to just fit under the hood?

How high should it be mounted above the flue pan, or does this depend on the style and shape of your hood?

Any help would be greatly appreciated.

Jim

[jrthe3]

make it as big a poss the bigger the beter it will work and the hotter it will get the sap

[Haynes Forest Products]

father and son. Im with Jrthe the bigger the better..........BUT when I built my first heater I layed it on the flue pan dividers and let the condensate drip back in (didnt know better) and it worked great. didnt have a hood. Then when I changed evaps and built a hood I took the heater and hung it and the condensate try from the top of the hood. Didnt heat up because the steam couldnt get into the coils. I would mount it low near the newly developing steam with perferated tray.

[Gary R]

Jim,

I'm with the biggest is better crowd. How about a deal with Chris? He's adding his WRU and has a 5' preheater. You would just need to narrow it.

Now, how are you going to make state champion syrup with no moths in the pan?:)

[WESTVIRGINIAMAPLER]

Jim,

I have the same size evaporator and I have pics of the one I built in my photobucket link in my signature. Without closing the damper off, I am running it around 170 to 175 degrees.

[MFarmall]

Jim, I made a close copy of Brandon's hood and pre-heater, it is shorter but 2 feet wide. The sap temp coming out averaged around 190-195 with the damper almost closed. I aded an inline thermometer to check the temp as the sap came out of the pre-heater and into the float box. Also added a vent just to make sure it didn't vapor lock. Made a BIG difference in my boiling rate. Check out Brandon's pictures, they would make it real easy to fabricate.
Mel

[SapZilla]

I've heard a hundred people talk about "vapor lock" in pre-heaters and just don't see how it could happen. If a pipe or piece of tubing is directly connected to a fluid source and terminates at a level lower then the fluid source, provided that the fluid source is not airtight, how could air ever obstruct the natural tendency of the liquid to drain????????????????? I have yet to experience such a phenomenon .

[maple flats]

Vapor lock could happen because a pre heater is oriented so the sap enters low and rises as it heats until it reaches the high point. From there, if the sap does not have a straight drop to the float box it could lock with an air bubble blocking the flow because the air would not want to flow down.

[brookledge]

I can't tell you exactually why but the major manufactuers make the vent part of their pre heaters. So there has to be a reason. And I have had my vent on mine spew hot sap out of it even though it is higher than the feed tank. Also some have had the situation accur when there is no sap coming out of the pre heater even though there is plenty of sap in the feed tank.
I don't think the sap actually boils in the pre heater but do know from watching water or sap sa it heats up and that is that it begins to release gasses prior to it boiling and if the float is closed the gasses are forced backwards pushing the sap backwards.
Also the cold sap goes in the high side and the hot sap goes out the low end and when the gasses rise it creates a void.
Keith

[Sugarmaker]

Jim,
You should stop and look at this preheater of mine. It might be able to be modified to fit your system? I am hoping that I wont have to go back to it in the near future:)
Give me a call.

Regards,
Chris

[Father & Son]

Does anyone see a problem with using 1/2" tubing for a parallel flow preheater over a 2 x 5 flue pan? Alot of preheaters are 3/4" tubing but in my feed line I'm placing a flow meter that has 1/2" fittings. If the feed line is necked down to 1/2" at the flow meter, wouldn't 3/4" tubing be overkill? It would also save alot of expence if it would work.

Jim

[Gary R]

Jim,

I'm no expert at this, but here's my thoughts. I don't think the flow meter size will restrict your flow. The sap will just go faster through the meter (up to a certain point) Other's in the past have posted that it is very difficult to get a flow meter to work. I believe it was because of the low rate of flow we are trying to measure. I have a research paper from UVM. It is on the design a construction of a parallel flow preheater. Table 4 show that if you have a 50GPH flow, you need about 20' of 3/4" copper. If you went to 1/2" you would not have as much surface area. You would have to figure out the area of the 3/4, then figure out how much 1/2" would be needed to equal that. If you PM me with your email, I will send you the paper.

[Daryl]

Jim
The flow meter will measure the flow rate the same on either but the 3/4 pipe will hold more volume to heat hotter. Your sap tempture would be higher on 3/4 pipe over the 1/2 going back the float box. Make sure to put a vent on the highest point near the exit to prevent vapor lock. Also use as many feet of pipe as possible in the hood.
Daryl

[Gary R]

OK Jim, I can't remember how to multiply decimal equivalance of fractions. Maybe someone can help us out. So I did it the easy way. I used a flexable tape and measured some copper pipe. 1/2" is about 2" circ., 3/4" is about 3 3/4" circ. So, you would need almost double the amount of 1/2" compared to 3/4".

[maple flats]

Area is Pi R squared. Line friction will reduce flow too. 3/4" = .442... sq in. 1/2"= .19625 Sq In. Not counting the increased line friction you would have 44% of the sap in 1/2" and only 67% of the surface area to exchange the heat. Surface area is Pi D or 3.14 x diameter. It is possible that adding enough lineal feet of 1/2" to equal roughly the surface area of the 3/4" you might be close.(add 50% more footage) Then a little extra for flow friction might be good) These numbers assume the nominal measurements of the copper were accurate, which they are not.

[Flat47]

Whoa...decimals and fractions and pi and...

More proof I hate math!

[Dennis H.]

I don't see any reason why steping it down to 1/2 for the flow meter if you still want it then back up to 3/4 for the heater itself wouldn't work.

With the low flows we are dealing with here you should never max it out 1/2.

[Haynes Forest Products]

Keep a few things in mind. Liquid is the train that heat rides on. With those words of wisdom................Chuck what do you mean. The heat from the steam cant transfer to the sap if its moving to fast. You need it to flow at a rate that allows the heat to transfer FULLY. As stated you are better off with 3/4 manifolds and 1/2 crossover pipes. Look at any radiator and you will see the same design. Large side tanks and many crossover lines. If you remove a thermostat from a cooling system it will over heat because it doesnt allow the liquid to slow down enough to allow the heat to hop on the train for the ride.

[brookledge]

Another thing to keep in mind is when you restric the flow into the evaporator you may run into problems with getting enough sap into the pans when the sh.. hits the fan. My thoughts are you should not restrict it any more than the original size feed pipe that it came with. When your feed tank is low and it is gravity flow, it will not flow much.
Keith

[Gary R]

How about a little info on the flow meter?:) I talked with Scott last night. He said he called the instrument manufacture that Gingrich was using. Had a custom made sap flow meter made indicating GPH:rolleyes: Some others may be interested in this.

[JFH51]

On my homemade unit I use a small gallons per hour flowmeter purchased from McMaster Carr. Cost was about 50.00.
I know that my unit when cranking does about 9 to 10 gallons per hour. I meter this amount into my preheater pan mounted behind the main pan. The preheater pan has an overflow pipe at the top which feeds into the main pan. This system works very good, by have a constant small in-flow I get maximum amount of heating in the preheater, I can boil for 10 hours and never lose the boil in the main pan. The overflow allows me to monitor the flow for possible interuptions.

[Father & Son]

Gary,
Those flow meters Scott had custom made came from King Instrument Company made to his specifications measure from 20-120 gallons per hour. Not a necessary addition but it intriged me so we'll see if it is of any use in tweeking efficiency.

Jim

[WESTVIRGINIAMAPLER]

Jim,

Go with the 3/4" pipe in the preheater. It will heat the sap a higher temp and it will hold more sap inside in the event you need to press down on the float control for a sudden burst of sap. Pics of mine on my weblink in my signature. No problem to put in a 1/2 to 3/4 fitting, but 1/2" is not a lot of flow when you get in a bind and need it badly.

[TF Maple]

The reason the sap seems to heat up hotter in 3/4 pipe is because the speed of the volume of sap moving through the pipe would be slower than through 1/2 so like Haynes says; time is needed to transfer the heat.

If the movement of the sap is the same through both sizes of pipe (this is speed not volume) the 1/2 will heat the sap hotter and transfer more heat. The 1/2 inch has more of it's volume in contact with the surface of the pipe than 3/4. So 1/2 has the potential to transfer more heat.

Bottom line is, the best pipe size for heat transfer is going to depend on the flow rate you need. So you need to know how much sap you need feeding into the evaporator.

[Haynes Forest Products]

Also keep in mind some preheaters are a single pipe zig zagging back and forth and is only a single pipe. My next one will be 1' top and bottom manifold with 1/2" cross pipes so the flow will be whatever the 1" pipe can handle and with 10 or more cross over pipes things will have plenty of time to heat up.

[TF Maple]

Also keep in mind some preheaters are a single pipe zig zagging back and forth and is only a single pipe. My next one will be 1' top and bottom manifold with 1/2" cross pipes so the flow will be whatever the 1" pipe can handle and with 10 or more cross over pipes things will have plenty of time to heat up.

I agree, big ends and as many 1/2" going across as you can make fit.

[Father & Son]

Thanks for everyone's ideas! Decided on 3/4" parallel flow but 2 layers. SDURF got the idea from GaryR and put his together the last couple of nights. His is made to fit over a 4 ft flue pan and he has about 27' of 3/4" tubing. Using the same design, over a 5 ft flue pan I figure on about 40' of 3/4" tubing. Now the planning is done and the building needs to get started.
Thanks again for all the input!
Jim

[WESTVIRGINIAMAPLER]

Jim,

40' is overkill in my opinion. With manifolds, I only have about 20' total and I run about 170 to 175 without the damper closed. 40' is going to probably cause you problems in my opinion.