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Heat Exchanger Build

Posted by kdc2 
Heat Exchanger Build
January 04, 2014 07:27PM
Here is my current proto heat exchanger fab. A lot of compromising, material on hand, existing parts etc etc., but I have managed to make it self fixturing for a single setup in the lathe for winding. What a lot of work that was.

I want to keep the fluid temp exiting the center at around 750F. Looks like it may be self correcting as it won't hang out there very long. I've been debating to make the center section out of .055 id as at 800F the speed is 30 ft/sec.

Something to chat about if interested. Happy New Year 2014, Keith



Re: Heat Exchanger Build
January 27, 2014 11:04PM
Since no replies, I thought to update. Perhaps I'll get suggestions, especially with any lathe experts.

The tooling I feel comfortable with looks like this:

Reasoning: we have a pretty large Supermax gap bed lathe, one with a pretty sturdy tool post. Since this is a double multipass helicoil I thought 2 people should be winding, one per tube. Also it should be possible to match the coil pitch with the lathe feed to avoid setting up some free following arrangement.

My next issue: the thread pitches listed on the chart aren't ideal, and I believe there are hundreds more possible combinations. I've been searching but coming up empty except for a program that calculates from actual tooth count and screw pitches, but I don't want to disassemble the damn lathe to count teeth. We also have another lathe with a lot more gears, but its tool post isn't as sturdy. (this is a balanced load so I could adapt. Any one with more lathe experience with pointers?

The 14mm (.5512" ) pitch is close to my desired .568, little too fine for all things considerable. 3 TPI (.3333) is really close to my desired .3359.

Any ideas or inputs are welcome.

Cheers, Keith

(edit: fixed gab bed lathe and inch mark to smiley conversion)



Edited 1 time(s). Last edit at 01/28/2014 12:04AM by kdc2.
Re: Heat Exchanger Build
January 27, 2014 11:29PM
Hi Keith:

My reply is hardly going to be helpful--but I can assure you that I find your effort to be fascinating and admirable, if well beyond my potential level of participation.

I'm drooling over your machinery!

Not sure--but maybe if you "talk down to the proles" a little bit more by providing more basic info you'll get some further participation. FWIW.

Best wishes, never give up.

Bill
Re: Heat Exchanger Build
January 28, 2014 12:02AM
Hi Bill, thanks for the reply. What info would you include said chats with the "proles" smiling smiley
Re: Heat Exchanger Build
January 28, 2014 12:20AM
kdc2 Wrote:
-------------------------------------------------------
> Hi Bill, thanks for the reply. What info would you
> include said chats with the "proles" smiling smiley

Keith:

I hope that wasn't too rude, we need to promote interaction with everyone interested in steam and regardless of their engineering prowess! I think a lot of folks are out there watching and fearful of asking questions or having opinions--and we need to welcome more of them instead.

Maybe some general interest info such as steam generation capacity, fuel flow, hoped-for heat exchange efficiency. And of course, intended operating temp and pressure.

Actually (and admitting to not doing the necessary research to find out, you may have posted such) I would be interested in what the ultimate application for your project might become.

Just some ideas.

Bill
Re: Heat Exchanger Build
February 01, 2014 07:40PM
Its been a while since I worked with anything like this but if I'm remembering correctly you do need to change out a few gears at times to get exactly what you want. Back in college I did that all the time and it took at most 1/2 hour to do so. But we had all the gears and charts needed.
Do you have a machinery's handbook? If not then find one as its well worth the money you spend on it. I expect that most of the folks here have one or access to one.
Nice looking setup you have going. I wish you well on this!

Steve
Re: Heat Exchanger Build
February 02, 2014 09:32AM
Yes. More info. What control, especially of temperature. If multiple passes, how is flow divided or temperature controlled in each pass?
Re: Heat Exchanger Build
February 04, 2014 05:48PM
What, 1.4mm ID pipe? How can anything flow through that? Wasn't it the British turbine car that melted down some super thin pipes? Or was it the SAAB or SES thing, and it worked, or was it both?

Anyways, very interesting - I think the gas-from-outside-in donut seems very logical, with complete combustion of the heavier gases along the hot outer wall, but on the other hand the inside-to out boiler doesn't need much insulation around it. You get a smaller diameter without squeezing the combustion space too much. I guess it may depend a bit on what sort of flame you are able to make as well? What is the fuel, and what is the burner like?

Sidrug



Edited 1 time(s). Last edit at 02/07/2014 05:16AM by sidrug.
Re: Heat Exchanger Build
February 05, 2014 01:50AM
Karl, Sidrug,

According to conversations I had with Dr. Platel, SAAB had continuous burnout problems with their tiny tubing and certain other people today who are foolishly doing the same thing do not seemed to have learned the lesson. A problem with the lack of real hands on work with steam systems and substituting PR puffery nonsense and the waving of arms for facts.
I recall the Brits burned out several of those so called steam generators in that Bonneville race car. Theoretically it is a lovely idea; but in the real world it just doesn't work. Also, consider that the pumping load goes up with the use of this hypodermic tubing.
Two or three tubes in parallel; is one thing as long as you drastically increase the water flow rate when overheat occurs AND each superheater tube is protected with it s own normalizer.
That obtains with a more or less monotube/Doble style layout. The Lamont neatly steps around this entire problem. Another lesson some need to learn it seems.

Jim
Re: Heat Exchanger Build
February 05, 2014 03:33AM
Seems I got some of it right at least. I suppose you maybe could do it in the economizer section. But the equalization of the tube flows would depend upon a higher pressure differential. Problem is, unequal heating leads to more expansion in the hotter tube, leading to increased backpressure, leading to less flow, and thus you have a positive feedback loop.

Sidrug



Edited 1 time(s). Last edit at 02/07/2014 05:15AM by sidrug.
Re: Heat Exchanger Build
February 05, 2014 01:26PM
The outside fired donut shaped combustion chamber has a high radiant heated surface area to volume ratio and as the flow area is reducing as gasses flow to the center it keeps the flow rate of combustion gasses through the tube stack high.

I tried to solve the spacing problem. But one thing seams backwards. The upper chart is T/1" (Threads per inch) The next appears to mm spacing between centers. The threads per inch and mm spacing both increase along the ten steps. Sense threads per inch is the reciprocal of inches per thread. I would expect a length per thread to progress opposite to threads per length.

As an example in the threads per inch I find the gearing between I II to be 4:1. IAFR is 16 T/1" and IIAFR is 4 T/1" Looking at the mm/T ICEU is 1 and IICEU is 4. Thats a 1:4 ratio for mm/T the reciprocal of the threads/inch. That makes sense. Look at the 1,2,3...10 setting we see threads/inch increassing lest to right 64,72,76,80... But also mm/thread increase left to right. 8,9,9 1/2, 10,11,12...

It's going to take a bit of work to figure that out. My simple MathCad gearing tables do not take into acount the reversal.Anyway it's an interesting problem.

Will get back to it. Right now I am having computer problems with it shouting down.; It's a laptop. Replaced the battery. Got a new charger on the way.
Re: Heat Exchanger Build
February 05, 2014 03:12PM
Inch threads are always described as threads per inch while metric threads are quoted as mm per thread which is how they are shown on the lathe gearing tables. I blame Napoleonsmiling smiley
Re: Heat Exchanger Build
February 06, 2014 03:38AM
Quote

The outside fired donut shaped combustion chamber has a high radiant heated surface area to volume ratio and as the flow area is reducing as gasses flow to the center it keeps the flow rate of combustion gasses through the tube stack high.

The flow area can be adjusted by tube spacing, also with finned tube in the economizer. How does your second point weigh then?

To your first point, yes donuts could have a high radiated surface to volume, although it seems normally they end up rather flat.

I took measurements of kdc2's drawing, and of a drawing of a Cyclone boiler.
With an arbitrary scale, the inside fired boiler measurements are:

Height: 35 cm
Diameter: 42.5 cm
Coilstack diameter: 39 cm
Coilstack inner diameter: 12.3 cm
Coilstack height: 31.3 cm

Scaling the outside fired boiler to have the same coilstack occupied volume, its measurements are:

Height: 24.8 cm
Diameter: 101.3 cm
Coilstack diameter: 57.1 cm
Coilstack inner diameter: 19.4 cm
Coilstack height: 14.8 cm

So the outside fired one is rather huge in comparison.
Total volumes are:

Inside fired: 50 liters
Outside fired: 200 liters

The respective radiated coil areas (projected) are:
Inside fired: 1203 cm2
Outside fired: 902 cm2

I did not include the external piping and connections in the measurements, nor the insulation.

Sidrug



Edited 1 time(s). Last edit at 02/07/2014 05:15AM by sidrug.


Re: Heat Exchanger Build
February 07, 2014 12:57AM
I came up with slightly different measurements for the inside fired boiler.

Coil stack height 11.5 inches. 29.21 CM
Coil stack circumference 14,2 inches, 36.068 CM

I then figured the tubing dimensions:

Economizer section 0.25 OD tubing.

Four layers at 12.2", 12.75", 13.35", 13.9" of 29 wraps length 395.553 feet surface area 233 in^2

The super-heater section 0.5 OD tubing

Four layers at 6.8", 8.25", 9.35" 10.5" of 21 wraps. Length 192.148 feet surface area 452.737 in^2

Evaporator section section 0.25 OD tubing

TWO layers at 4.75", 5.2" of 29 wraps Length 75.922 feet surface area 44.722

Laying out an outside fired boiler of similar heating areas as above.

Economizer section 0.25 OD tubing .. 17 layers of 19 wraps. Surface area: 245.567 in^2 length:416.887 feet
Super heating section 0.3125 OD tubing .. 13 layers of 16 wraps. Surface area: 477.204 length: 516.826 feet
Evaporating section 0.25 OD tubing .. 2 layers of 13 wraps. Surface area of 48.489 length: 82.318 feet

The innermost wrap of the Econimizer is 2.5" The outermost wrap of the evaporator section is 12.275"
The height of the boiler coils is 5.72"

The over all dimensions would be(with insulation) 7.72" x 22.85" Having 5.36 sq feet of heating
surface are. These outside fired boiler dimensions allow for a 6 foot flame before overlapping it's self.

I do not know the specifics if the operating parameters. But the section area relation do not seem optimal.

The inside fired steam generator occupies 5.0 ft^3 not including insulation.
The outside fired steam generator having more heating surface area occupies 3.8 ft^3 not including insulation.

Andy
Re: Heat Exchanger Build
February 07, 2014 03:34AM
First of all, your outside fired boiler (OFB ) coil stack occupies less than a third of the volume that the inside fired one does. This may be due to tube diameters or tube spacing, the point is that if you can do it in an OFB, you can also do it in an IFB.

Secondly, the outside fired one is going to need more insulation, so not measuring insulation gives the outside fired boiler an unrealistic advantage.

That being said, my comparison was not entirely fair either. The Cyclone boiler has a bigger inner diameter of the coil stack, which I assume has nothing to do with it being outside fired.

I went over my measurements. My scale for the inside fired was 20 pixels/cm.

Quote

Coil stack height 11.5 inches. 29.21 CM
I agree, by averaging the economizer and the rest, this is the number, not 31.3.

Quote

Coil stack circumference 14,2 inches, 36.068 CM
You mean diameter, but no, it is more like 39 cm.

Sidrug



Edited 1 time(s). Last edit at 02/07/2014 05:15AM by sidrug.
Re: Heat Exchanger Build
February 07, 2014 04:46AM
Hello All,

Nice to see some good thoughts bouncing around.

Been scrambling to get a complete system build .. a lot of work. I've redone the winder to allow a one man operation, locking in control rollers rather than relying on touchy feely by 2 people in sync. Looks more like this:

Karl, although the multiple circuits are somewhat pressure balanced, the main effect is the pump also has multiple circuits and very little can bypass a circuit. I will attempt to limit the temperature of the working fluid to 1150 F for this go. Main control algorithm is monitor output temp and should anything gets too hot it gets dumped into the condenser, bypassing expander consumption.

I was quite intrigued by the inside/outside thoughts. This build is stuck on inside due to many prefabbed systems and parts. I really like the natural flow out to in for all reasons mentioned, but bottom line is getting something to work without excess heat out the stack. These coils are very finely pitched, but not optimized for flow velocity as that would require an even finer pitch, beyond my fab capability. (I got a few no quotes on the double nested 2 pass in 1/8, so it is what it is) I will embed several thermocouples to monitor and control air flow through the coils.

I had a hard time determining which heat exchanger was up for comparison to which numbers. (dog gone system won't let me review the thread to check names vs comments.

I believe steamrandy's numbers were almost exactly double scale, very close. there is 200 ft in the econ., 100 in superheater and about 49 feet in the buffer coil. Tubes are 1/8 and 1/4 od. Target BTU is about 70,000 max btu/ hr, heating about 0.14 gpm. Pressure is 4000 psi, expecting less than 500 psi drop. The od of the can is 8". Taller coils are about 6.00" tall.

Did I mention this was a lot of work? Unbelievable details, but I need to be cautious with the $$. I'll have a couple grand in materials, fab and tooling for this guy. I can't afford to scrap much.

Appreciate all the comments, Keith
Re: Heat Exchanger Build
February 07, 2014 05:09AM
kdc2, good stuff. It's not clear how many parallel paths it is going to be. What steel is it?

Sidrug



Edited 2 time(s). Last edit at 02/07/2014 05:23AM by sidrug.
Re: Heat Exchanger Build
February 07, 2014 05:13AM
Quote

I had a hard time determining which heat exchanger was up for comparison to which numbers. (dog gone system won't let me review the thread to check names vs comments.
What, you can't see who wrote what? I'll edit my posts to add a signature.

In short, I took your drawing and scaled it 20 pixels to one cm. Then I scaled a drawing of a Cyclone boiler to match the coil stack volume to yours. Then Andy layed out another outside fired boiler to compare to yours.

Sidrug



Edited 1 time(s). Last edit at 02/07/2014 05:14AM by sidrug.
Re: Heat Exchanger Build
February 07, 2014 05:34AM
Hi Sidrug,

304 ss for the housing and 316 for the water path.

I've been told 316 and di are a must for this temp and pressure. All my initial calcs didn't take the significant density and fluid speed changes from stp, hence the jump to the larger od tube section. It's almost a crazy linear pressure/diameter/wall thickness progression. I really only have a 3x safety margin on the 1/4"....it just doesn't stop. The tube manufacture's claim you have to heat to 1900 f and quench to get any properties approaching/exceeding 1250. and even with the quench the tube needs derating to 30% at 1250 vs near 65ish% at 1150. At the end of the day I just have to build something and try it, and I won't be quenching or hip treat the damn tubes either, maybe later with a PCHE. Cheers, Keith
Re: Heat Exchanger Build
February 07, 2014 05:39AM
sidrug Wrote:
-------------------------------------------------------
Quote

What, you can't see who wrote what? I'll edit my
posts to add a signature.

No, after I previewed the attached picture, I would have to open a new browser window to see the thread. I could easily figure it out if it could be read while typing. smiling smiley
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All files from this thread

File Name File Size   Posted by Date  
HE ft_sec.PNG 9.7 KB open | download kdc2 01/04/2014 Read message
heat exchanger cross arrow.png 884.7 KB open | download kdc2 01/04/2014 Read message
HE TOOLING.PNG 246.5 KB open | download kdc2 01/27/2014 Read message
HE TOOLING 2.PNG 212.9 KB open | download kdc2 01/27/2014 Read message
IMG_20140127_185145_785a.jpg 445.7 KB open | download kdc2 01/27/2014 Read message
IMG_20140127_185301_419a.jpg 205.1 KB open | download kdc2 01/27/2014 Read message
cyclone boiler.png 276.2 KB open | download sidrug 02/06/2014 Read message
winder 2.jpg 76.8 KB open | download kdc2 02/07/2014 Read message