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low pressure monotubes

Posted by richard orr 
Re: low pressure monotubes
May 07, 2012 03:36AM
Hi Richard, here is a table over pressure drop per foot vs flow for various pipe diameters.
And here are some formulae.

Edited 1 time(s). Last edit at 05/07/2012 03:40AM by sidrug.
Re: low pressure monotubes
May 07, 2012 06:22AM
For all temperatures/pressures of fluids and high pressure steam the B&W equation is:
deltaP=f L V/d X (G/100,000) squared. f would be determined by a friction table based upon the Reynolds Number, L in feet(equivalent feet if there are bends) V is the specific volume of the fluid, d the inside tube diameter in inches, G is the mass flow in pounds per hour per square foot. Its all in the wonderful B& W book. The Reynolds Number is dG/12u where u is the absolute viscosity of the fluid which changes with temperature. I believe your equations are for basic flow of cold water. Thus if we look at a monotube/Lamont/ Ofeldt boiler each foot of tubing has a different pressure drop as the V(specific volume) and u(viscosity) are changing with each foot of tubing when heat is being added to the tube. That is why it takes so long by hand to add up all the changing conditions along an entire coil bank. Then we have to deal with the change in V(specific volume) and u(absolute viscosity) that ocure in a boiler with higher pressures. I am sure there is a handy engineering program to do this for probably $10K or more!
Best, George
Re: low pressure monotubes
May 07, 2012 10:45AM
George - I've mentioned this web based calculator before - it covers a lot of this subject and could probably do what you are asking for. You'd still need to treat each short length separately and would need to work out the heat uptake for each bit to know where to start the calculation for the next bit but there is a lot of information on the site which is useful.


Re: low pressure monotubes
May 07, 2012 09:22PM
All the formula that are easy to use are for stady state conditions. But it not exactly a steady flow now is it.

I have found quite a few web pages on calculating steady flow. Very very few that address dynamics flow.

There seams to be some that say the steam state in a flow is isentropic while othere say isenthalop. There are some
guide lines to deturmin which but the rules are different on different sites.

The general flow simulation are quite involved. I think it simplest described as a point flow simulation. There are many many points in space and the flow model is looking at all the interaction between naboring points. Very costly in turms of compute time.

I am wondering if a simpler balk flow couldn't be used for tube flow. That divide the tube into short sections and calculate the midpoint state of each section. The pressure between the points acts on the balk mass between. A pressure differential results in a flow and possably an an acceleration. I have been working on this as an idea.

The simulation is of 7 short tube sections. The plots of the steam pressure at each sections mod point. The simulation starts with the exit end closed and equal pressure through out the tube. The same steam state through the tube. At the start the exit pressure os opened to a lower pressure limitless accumulator so to speak. Basiclt the exit pressure is a constant state. The plots show the dynamic pressure fluctuation as the flow stabalizes. I attached the pressure of two different flow models. A third is the type 2 with basicly a different k factor. the k factor is basicly a flow resistance. I figure it needs to be dynamicly calculated.

The last is of two points ahead of a valve that quickly opens and closes. These are simple tests having in infinite sourse and sink. Bassicly the steam states at the tube ends and across the valve are constant.

You can see the dampaned resonation in the pressure plot. In the type 2 model the frequency matches up with the speed of sound of the steam state. The change is being calculated as an isentropic change. An isentropic flow.

These are done in VisSim using my steam properties plug-in.

The examples are not low pressure. But the principals would be the same. Any comments on the accuracy? Sorry I can not do a specific run for low pressure. Right now it would be a pain. I am in the process of converting to version 8 of VisSim. One big problem is my block conection type are getting lost. If I save and reload a sim I have to go through an disconnect all my state point outputs and reconnect them to get my conversion blocks to recognize. Same happens of I change the units on one of the blocks. It has to disconnected and reconnected to work again. My state point output is an array type. When you connect blocks the blocks event code handler gets called to return it conection type. That was happing on load in the old version. It no longer is happing for inputs at load in the new version. But it is happing on outputs when the units are changed. But the block inputs are not getting called except when a connection is done.

Feel free to point fingers and whatever.

I do wont to be able to sumulate these transiate states.

The plots look like they could be right. But the basic model is a second order dufferential equation. Kind of like basic spring weight differential problem in the first differential equation course. The spring force is the pressure differance. The mass is varing. And it a chain varing spring weights. The ocesolation frequency match up gives some fath that it is close.


Re: low pressure monotubes
May 08, 2012 01:55PM
Hi George, you are right, one should take a piece of pipe, and calculate its heat transfer and pressure drop from
pipe diameter
pipe roughness
mass flow
Delta T wall/water

then add that heat to the water, and remove the pressure drop, then go on to the next piece of pipe.
I have a free steam table formula called X-steam, for the free spreadsheet Open Office Calc.
So I went on the internet and found some formulae to put into it. Once the calculation for one segment is done it is just mark and drag, and it has suddenly done all the segments. So far I have only done pressure drop and heat transfer for turbulent flow, fixed delta T and without bubbles (supercritical). The air side is probably more difficult.
Here is D'arcy-Weisbach equation for pressure loss, there is a link to the Colebrook equation for the friction coefficient as well.
Here is a chapter from the Wolverine Data Book, which seems like a big and fine resource: [www.wlv.com]
That one has the heat transfer equations I used, and here is Databook 2 and 3 tables of contents with hotlinks to the whole works inside:

I tried to plug in the Cyclone MK2, from pictures and what specs I could find. There is something odd at around 374 'C at 220 bar, in the steam table, that looks sketchy. Anyone have an explanation for that?

EDIT: Richard, if you can provide more data, then maybe it is possible to get at least a vague idea of the pressure drop for a low pressure lamont.

Edited 1 time(s). Last edit at 05/08/2012 02:44PM by sidrug.

Re: low pressure monotubes
May 09, 2012 07:12AM
Hi Sidrug,

I can't remember the explaination, but right at about 705 deg f you will see a skyrocketing peak. Finer divisions will can show a huge spike. If interested I'll rehash the reasoning with my experts.

Re: low pressure monotubes
May 09, 2012 11:41AM
I'm very interested.
Re: low pressure monotubes
May 10, 2012 09:45AM
That is the critical point. The steam properties are not real stable around the critical point. Expecially with the industrial formulations.

Re: low pressure monotubes
May 28, 2012 02:29PM
Humble apologys to one and all. Had no idea that this thread was being added to....got side tracked by John Wetz's amazing side draft / down draft boiler. Don't have time at present to respond . More later.

And now it's later.

Hello one and all...
Once again I find myself the hobbling turtle trying to keep up with jack rabbits leaping around equations and termonologys that send me scrambling to wiki for definitions. Wiki in turn provides with snapy precision aimed at the college educated and lacking metaphore by way of examples....I so apprecaite the turn of the century Audels written in such a way as to teach farmers the wonders of steam! I do also appreciate all your efforts to provide information .
Sidrug,... difficult, I suppose to convey specific information needed without a picture of intended boiler, but I will try.
Economiser will be 240 liniar feet of 1" type k copper tubing hooked up in parallel of 120 liniar feet each and configured is serpolet fashion. ( both also having a seperate displacer pump)
Generating tubes will be 630 liniar feet of 1/2" sched. 40 seam welded pipe.
Pipe will be in 7 foot sections with 180 elbows making up 90 hair pin turns.
This 630 liniar feet will be hooked up in parallel of four sections of 170.???" each. (don't have calculater handy)
So...two economiser tubes... each one Y,d off and feeding into two generating tubes each.
Economiser sections will be inter stacked on top of each other...one and two...one and two.
Generating tubes will also be inter stacked and running in twin layout on either side of firebox......one, one....two, two....etc.
I am aware that one coil of the economiser will run slightly hotter then the other, but it will not matter so much as each twin tube circut will be governed by it's own pump and all will dump into verticle steam / water drum.

I have built a four displacer reciprocating pump (adjustable while running)
Displacers are 1" dia. with 5" stroke.
They are configured in an H pattern so each economiser tube will be under full control of pump and will be fed on forward and back stroke.
I am hoping that the generating tube system, being identical on each side of fire box and only Y,d off in two circuts each, will recieve the same amount of gas flow and therefore have the same amount of water flow.
Does this information help at all?

Edited 3 time(s). Last edit at 05/29/2012 04:12AM by richard orr.
Re: low pressure monotubes
June 01, 2012 03:26AM
Good. Pics would be cool, but your description is good. Sorry, I don't know how to calculate bends or boiling water yet. Wolverine says pressure loss in boiling water is difficult. I'm looking into it - [www.wlv.com] . The Serpollet economizer is coils with no angle bends? Then at least that seems to have imperceptible resistance. What is the steaming rate lb/h?
Re: low pressure monotubes
June 01, 2012 06:04AM
Greetings Sidrug,
Several errors made in boiler tube info. Sorry.....we are in transition...moving into the mountains and off grid... life packed into little boxes...chaos reins!
Cannot get to my Audels yet to calculate steam rate but here are some corrections pertaining to last figures sent:

Economiser....240 liniar ft. of 1 in. copper tubing = 62 sq.ft
Generating tubes...520 liniar ft. of 1/2 in.seam welded pipe =140 sq.ft.
160 (not 180) elbows.

Steaming rate....Am aiming at generating 16 hp. from boiler to have 8 b.h.p. at crankshaft
Am figuring roughly 10 sq. ft of heating surface for each h.p. needed....a concervative estament because opperating at low pressure of 80 p.s.i.,
7/8 ths. cut off and ineffecient boiler configuration due to design trade offs of gasifier.

Yes, no hard bends in serpollet configuration of economiser.

(Hopefully) can get to scanner this evening and send schematic drawing of boiler.
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