How to calculate head loss in coiled copper tubing...

Since a continuous coiled tube is not like a series of 90-degree bends placed one after another...

This is counterintuitive. I will ask this and then run for cover, Why it isn´t?

It would seem more like a series of 45° bends to me, but what is the change in height? Is there really going to be enough head pressure in even a 3/4 pipe to be a large factor?

Perhaps I don't understand the problem well enough.

Drew

From the practical hands on approach you could just build it and go from there!

Many times over analyzing something will just cause you unnecessary headaches and waist a good deal of time effort and money while chasing details that are not critical or of great consequence to the actual operation of something. Some times powering down the brain and powering up the hands will provide far better results!

Given the balance of pressure from gravities influence all you will have is the internal friction of the system to contend with. Given the basic specs sheet data for your circulator pump/s and its rated head pressure just by using the standard losses specs for your type of tubing you should be able to come up with a fairly forgiving ratio of pump to flow rate capacity.

In a typical in floor heat system its fairly common to run four (and sometimes more) sets of 1/2 inch by 300 feet runs in parallel or two runs of 500 feet of 3/4 inch line off of a pump with a head pressure capacity of around 20 feet and a flow rate of around 5 GPM or higher.

Also a long run of pipe will reduce flow it wont ever stop it in reality.

Assuming a conservative specific heat value of .8 for a 50/50 mix of antifreeze and water and a flow rate of around 5 GPM with a 100 degree F temperature change you would still have around a 192000+ BTU heat transfer capacity. I doubt your solar collectors and heat exchangers will actually catch and transfer more than that in reality in most typical working conditions.

Should flow rate become an issue you can always just put in a bigger pump too!

I don't know the circuit design regarding elbows versus radius bends, etc. Typical copper 90s for water systems are very short radius, and their equivalent length is on the order of 10-20 inside diameters. (This is only rough memory.) Refrigerant 90s are longer radius, and hence add considerably less equivalent length to the tubing run. Radius bends are even less yet, and probably negligible (or maybe add 5-10% to be conservative).

I don't know the insolation value in your neighborhhood (4 watts/sf maybe?), but this will set an upper limit on how much energy the tubing grid can collect. This will be higher with supplemental reflectors or glass/plastic enclosed panels, less for an open grid. Painting the tubing black will improve energy absorption.

Collectible energy (watts )= insolation value (watts) x area x efficiency.

Btu/h = watts x 3.41.

In the pump circuit, Btu/h = gpm x 8.34 lb/g x 60 m/h x Δt (dF). A suitable Δt for this would be 10-20 dF. Try some values, and equate this quantity to the previous; this will give you the total gpm needed. Divide this by the number of circuits (5 as proposed preliminarily) to get the gpm per circuit. Then select the tube size accordingly. As a double check, calculate the fluid velocity, which should be in the range of around 3 f/s (180 f/m).

This is rather rule-of-thumby, but it should get you into the right range.

Hello Tornado,

How are you today?

I have a few questions for you regarding the formulas that you provided in your post.

1). In the calculation for determining "Collectible Energy (Watts), do you know what range of efficiencies can be expected for Encapsulated solar collector tubes? Are there manufacturer's published values relative to efficiencies?

2). In regard to delta T: Am I correct to assume that this is the temperature difference between what temp the solar collector heat bulb is producing and the supply side water-glycol (or other fluid) mixture temp flowing though the manifold? dF, as in degrees Fahrenheit????

3). Am I correct to assume that the calculated value of BTU/Hr would be valid for a single solar collector tube, and if for instance, I have 5 such tubes producing similar but different BTU/Hr values (each tube is attached to the pipe manifold in a single row), then I could use summation of the heat produced for the group of collectors? It's cumulative, right?

TIA, and please have a great sunny day?

CaptMoosie,

1) I don't have a good handle on these efficiencies.

2) By Δt I mean the rise or fall in temperature in one medium as it traverses whatever heat exchanger. Sometimes this is also called "range." When comparing the temperature difference between two media, the term "TD" is commonly used. Yes, dF is degrees Fahrenheit. I sometimes forget that I can insert the ° symbol, but when it isn't available, dF works. (More technical is LMTD = logarithmic mean temperature difference, which I could address in a separate post.)

3) If the tubes are in parallel, this would add cumulatively. If they are in series, the water/glycol warms up more as it traverses more tubes. This decreases its TD with whatever is outside the tubes, and the rate of heat transfer thus also decreases as you go along. Thus they are not fully additive in series.

Many thanks Tornado for the explanations and clarifications!

I was afraid that my assumption was correct about the collector tubes being placed one after another in a "series" arrangement! You just verified my suspicions. Even though it'd be easier to construct a "series" pipe manifold, I may rethink my design and go with a parallel manifold concept, even if it involves more tubing, pipe, fittings and valves, and hence increased initial materials cost and labor. I think in the long run it'll be a much more efficient design.

Again, thanks! Have a great sunny day up there in AK!!!!

If the rise in water/glycol temperature is small compared to the initial TD, the series arrangement may still be fine; i.e., the final TD will remain fairly high. I don't know how the external temperature of your tubes is determined, so for the moment I can't guess what the TD in this system is.

Hello Tornado,

Many thanks again for the advice. I suppose I'll have to contact Owings in Ohio to find out the rated efficiencies of the collector tubes. They manufactured them for the NASA and USDOE test trials.

This past August I set up a test rig with 3 of my solar thermal collectors all put together. I ran them for a week when we had a period of hot hazy sunny weather that was in the low to upper 90's.....barely a cloud in the sky during that week. The tubes were inclined to 57 degrees above the horizontal plane which would be what I'd need for winter time ops (N 42 Deg. Latitude + 15 degrees). The measured temps taken at the heat bulbs (upper end) showed a consistent reading slightly over 350 degrees F, with a single tube running slightly over 360 degrees F (dunno why it was higher...perhaps more or lesser amount of MEK inside the heat tube???). Temps were taken with a brand new calibrated digital lab thermometer. Temps were verified with standard laboratory mercury thermometer. And I was running a constant high volume stream of cold water from the garden hose over the bulbs the entire week and still got these temps! I don't know how the collectors would perform in the dead of Winter on a cold cloudy day, but i assume they'll still generate close to 300 degrees F because of the special Gold IR film present on the interior glass tube. Good thing that I soldered all the copper fittings with silver-tin solder in light of the recorded temps!

Now if

Now, if my coolant water temp range is roughly in the 120-140 dF range, then I assume the Delta T would be pretty good so I need not worry about the series vs. parallel manifold configuration then???? Correct??? Again, I'm not a Mechanical Engineer, and at this point I'm only guessing until I run the Hydronic model(s). Even if the range was 140-180 dF, then the Delta T result should again be pretty high and could use either manifold config?

Now, if I installed the curved Linear Fresnel lenses, I can expect much more heat output from the collector tubes. The theoretic boost from the Fresnel lenses is roughly in the range of 15-20 suns. I may have to revisit the idea of brazing the copper fittings that make up the heat tube and heat bulbs!!!! LOL

I'm not too worried about heat losses along the heavily insulated copper tubing in the deeply buried carrier pipes running to and from the thermal storage tank, nor worried too much about heat losses from the tank itself, which I have already have. The specially built poly tank is meant specifically for solar applications and has an usable water storage capacity of 600 gallons. It features a factory-adhered 6-inch think layer of foam insulation all around, and top & bottom. I plan on wrapping the entire tank with a layer of ARMA Foil dual faced metallic radiant barrier film plus placing the tank on a 6-inch layer of rigid foam insulation.

Again, I thank you! May you have a great sunny day!

Your collector tubes resemble the geometry of the Apricus [spelling??] solar collector. As I recall, they are described as "heat pipes." I don't know how the temperature of their internally circulating fluid varies along the length of the tube. But if this fluid is say 330-350 dF, and the cooling water/glycol fluid is 120-140 dF, you have high TD everywhere, whether in series or parallel.

I wonder about the 2 gph figure you mention. That seems very slow, and would make boiling of the water/glycol more likely. I.e., at such a low flow rate, the water/glycol temperature could approach close to the methanol or MEK [?] temperature in the collector tubes.

I hate to make my self look dumb here, but what exactly are you using this for? Home heating, energy generation?

Drew

(if I really cared about 'looking' dumb I would post as guest!)

Hello Tornado and Drew:

I'm installing the solar thermal tubes so as to heat my house and provide DHW....it'll be the primary and my existing boiler will act as the secondary heat producer.

The sealed copper heat tube with heat bulb are sealed and have a low vacuum...there is a very small amount of MEK or Acetone present in it. They are not entirely filled with either solvent, and thus they act like an old fashioned coffee percolator....the generated heat is not constant. I should have made that clarification in my last posting...my BAD!!!! When the heat tubes and heat bulbs cool off (the MEK is cooling off too and condensation back into a liquid as it flows down the heat tube to its base), the heat bulb temps do lower into the upper 170's to mid-180's. I should have mentioned that too....I'm a bad bad boy! *LOL* I have to agree with you Tornado, the 2.0 GPH per collector may be insufficient, and I have to do more testing this winter, this time with a test manifold and flow meter, and get some realistic temp readings at the heat bulbs as well as the water temps entering and leaving the manifold.....especially if I plan on adding the Curved Linear Fresnel Lenses to boost the captured radiation from good 'ole Sol. Of course that'll mean me modifying my test rig somewhat to include some thermocouples and the lenses w/ a new framework. Right now, nothing is set in stone, especially since I haven't run any software sims yet!!!! Whatever I get in the way of new temp readings from the tests they'll be used to calibrate the model........

Hope this info helps!

Can you keep us posted here or will you create a blog? I would like to follow this story to its conclusion.

Drew

Hello Drew K!

Yes, sure I could keep all informed of my progress or even create a blog sometime early next summer. That'd be neat to share with everyone! But first, I have to erect my 2-story steel toolshed/storage building that I designed before I can mount the solar collection tubes and their mounting frames together with all the piping, fittings valves of all types, flowmeters and thermocouples w/ wiring for the SCADA system. The entire shed will be erected on a very stiff and heavily reinforced "Alaskan Slab". All the steel to be used is recycled or salvaged and mostly comprises of galvanized steel wall studs, galvanized sheet steel and steel structural plate. I'll be fabricating 7 steel trusses and their support columns, all comprised of built-up members using those steel studs.

Normally I would have mounted the solar collector tubes on the house roof, but our solar PV Contractor is slated to install the system there sometime in the next few weeks. That system will comprise of 36 - 210 Watt PV modules equating to 7.56 kW, which will provide about 80 to 85 percent of our electrical needs. I can always add more on later to bring up to the max. permissible 10 kW, plus I'll most likely be adding on a 2 or 3 kW Vertical Axis Wind Turbine (VAWT) that I'm currently working on in the preliminary design concept and research stages ....

You could try this software. I have not used it myself, but it seems to be what you are looking for. Good Luck. As for the

Hello All,

I want to thank each and every one of you for your assistance. I greatly appreciate the efforts and have found almost all comments have been very helpful.

I did forget to mention in my initial plea for help that the preliminary design of my system will include one or two copper (in parallel) tubing coils immersed in a water holding tank that's roughly 66-inches tall and has a 15-inch manway and cap. I'll probably will be using a closed system rather than an open one just to reduce the amount of glycol needed and hence keep down the costs. As it stands now, I think each of the coils will be constructed of 100 linear feet of 1/2-inch refrig. copper tubing having a 6-inch continuous radius bend. Any larger of a coil radius and I'll have too much trouble getting the coils inside the tank. Anyways, I have not had the chance to perform the heat transfer calcs yet to see if this coiled heat exchanger approach will work.....if all else fails I can always use a couple of flat parallel plate heat exchangers that I've already purchased, but that will mean installing an additional pump for it to work (for the Hydronic system). At least those exchangers have a known heat transfer based of actual lab tests. Offhand, I don't have those numbers in my head....they're downstairs in the basement buried in a stack (and what a huge stack it is!) of parts boxes, etc.

I have 50 encapsulated borosilicate glass solar thermal collector tubes, each 2 1/8" D. x 46" long. There is a vacuum present between the inner and outer glass tubes. The inner tube has a gold film. These are NASA surplus tubes. Each tube has been rated to produce at least 350 to 450 BTU/Hr and requires at least 2.0 GPH/each tube of water flow in the manifold for efficient heat transfer. Of course, if I cannot get that much heat out of them, then I will experiment with boosting the radiant capture by installing curved linear Fresnel lenses, where I could possibly expect a boost of 10 to 15 Suns. If that's too intense, I could always back off the application of the Fresnel lenses some......as I have said, I'll have to do some experimenting on a test rig set up in the backyard. And yes, there's a manufacturer of these Linear Fresnel lenses.....I can either buy a roll of the linear Fresnel film or fabricated lenses that feature the film adhered to a curved acrylic base directly from the manufacturer.

I wish I could find a decent USED copy of John Siegenthaler's "Modern Hydronic Heating for Residential and Light Commercial Buildings" (Delmar Learning publishers) that is cheap enough to afford, but the lowest cost I've found online is Amazon.com and the textbook costs a whopping $145! I'd love to own a copy of the "bible", but I've received A BIG "NoGo" from the little Mrs. about it's purchase. Oh well, so much for that!!!! ***ACKKK!!!!***

Heck, I was hoping that some library close by would have it so I could borrow it for a week or two, but even the libraries at RPI and Union College don't have it as far as I can tell. The closest library for me that has it is located at Mohawk Valley Community College in Utica, but that is a 2 hour drive one-way for me.....a definite no go there as well....especially ifI drive all the way up there and find out it was taken out by a student, etc. 15 minutes prior to my arrival!!!! That would definitely burn my Rice Krispies!!! ***ACCCKKKKK ACCCKKKK***

Does anyone out there want to sell their old used copy REALLY CHEAP or at least know a library in NE NYS that has it????? Too bad it's not on a CD....ditto with the Hydronic software package that you have to purchase to makes the book more usable!!!!

Have a great day guys and gals!

Have you checked to see if the local colleges have any kind of inter-library loan with the Mohawk Valley Community College? I know most (if not all) of the Ohio colleges are connected by OhioLINK and will send books all over the state.

Are you running copper from your collectors to your heat exchanger? I have heard of this before, but I am not sure it makes sense to me. Wouldn't cpvc provide better thermal protection? Even if you insulate it wouldn't the copper transfer more heat to the insulation?

Drew

Hello Everyone!

Everyone has been wonderful with their answers! THANK YOU FOR THE GA'S!!!!!!!!

What's the maximum working temps of the Foam-Core (Sch. 40) pipe and the CPVC pipe? I believe I'll be operating over 180 degrees F, but less then boiling. BTW, what's the ASTM designation for the Foam-Core pipe so I can hunt it down ad research it???? And IF I go with curved linear Fresnel Lenses, then my system could get up to around 200 degrees F and therefore require that I adjust the water flow rate accordingly for proper heat transfer and cooling the heat bulbs and manifolds. Also, many thanks for the software links everyone! I'll give everyone of them a workout!!! hahahaha

Sandman, in regards to the tube supplier: A fellow from Richmond VA sells them on Ebay from time to time. His Username is Fossilfreedom and he usually doesn't deliver or ship them, so you may have to go to him and pick them up. I was lucky when I bought my 52 tubes from him a year ago as he was travelling through my area on his way to Buffalo, where he has a stash of them in a warehouse I think....delivered right to my doorstep early one morning before I even got up!!!! LOL He only sells the tubes, not the guts such as the copper heat tube, stainless steel wool, cap and rubber stopper, and copper heat bulb-------you'll have to build those yourself. I was lucky and was able to buy them for roughly $22.50 apiece or so. They cost the US Government $250 apiece when they were new!!!!! After running a few very fast calcs this morning I may be buying some more collector tubes to augment my current stash of collectors in addition to buying some very expensive linear Fresnel lense film from the Brits!!!

He does have a website:

http://fossilfreedom.com

I had planned on heavily insulating all piping indoors and outdoors.....with several coats of insulating paint, then wrapping the piping with rock-wool followed with a wrap of ARMA-FOIL radiant foil, with the entire assembly placed inside a 3 or 4-inch PVC carrier pipe. The distance between the tool shed (where the collector tubes will be installed on the roof) and the house basement, the carrier pipes will be buried 4 feet deep and enveloped with pieces of rigid insulation board. All this is meant to minimize heat losses between the collectors and the heat storage tank.

I tried the MVCC library and the textbook I'm looking for is not available to borrow since its a Reference Book and cannot leave the library.....BIG BUMMER!!!! I had hoped that either the RPI or Union College libraries would have had it, but they don't list it at all....perhaps one of the Mechanical Engineering faculty profs at either school has it??? Hmmmm....

Also, many thanks for the comps regarding the correlation of flowrate, BTU's, Insolation value!!! Mucho appreciated and saved me a lot of time digging it all out!!!!!!