Copper HX - series or parallel?

[gimmeWood]

I am planning on building a heat exchanger for a tank that will be storing the heat from a soon to be purchased Tarm 40. I am thinking that 100 feet of 1/2" copper will be enough. Do people agree? Also, I was wondering if that much length with just 1/2" tubing might cause a lot of resistance to water movement. Would it make sense to instead use 2 50 foot coils in parallel?

 

[WRVERMONT]

I would have to know more about your system plans to guess on the copper amount. How large will your storage be? What are your heating requirements? DHW production in the summer? That being said two 50 foot coils are going to operate with 1/2 the resistance to flow(at same flow rate) compared to one 100 ft unit. Low head pressures require smaller pumps making the system more efficient. I would say if you want the storage to charge at a pace equal to the boilers output at full bore you will need more tubing(heat exchange in the storage tank).

 

[gimmeWood]

My storage will be around 1000 to 1200 gallons. I have about 3200 square feet. I am not sure about DHW in the summer - I may try it out.

I definitely want to make sure that I can handle the Tarm 40 at full bore with 1000 gallons of storage. How much coild do you think I need to handle that?

 

[gimmeWood]

Also, is there any disadvantage to dividing up the total length into smaller sections in parallel?

 

[jebatty]

Why stay with copper coils? What are you gaining over a plate hx that offsets advantages of cost, simplicity, efficiency, and compactness of a plate hx?

 

[gimmeWood]

Here are my main reasons for using copper coils:
* I think the copper will be cheaper (using 10 ft pipes at 15 bucks each).
* Compactness doesn't matter due to the fact that the storage tank is huge anyway. In fact one might argue that a more spread out heat exchanger is better because it covers more of the tank volume.
* People say that stratification is good and large vertical coils seem to encourage that.
* It sounds like copper coils are less restrictive to flow.
* If I size them incorrectly I can easily change their size since I made them in the first place
* I enjoy making stuff (the closer to being made from scratch, the better)

 

[woodmaster]

I have 240 ft for 750 gallons of storage in conjuction with an older non gasifier similar size to the 40 . Go to the STSS website , thats where I bought the coils. Stay away from any pex coil for your DHW .

 

[jebatty]

Here are my main reasons for using copper coils:
* I think the copper will be cheaper (using 10 ft pipes at 15 bucks each).
* Compactness doesn't matter due to the fact that the storage tank is huge anyway. In fact one might argue that a more spread out heat exchanger is better because it covers more of the tank volume.
* People say that stratification is good and large vertical coils seem to encourage that.
* It sounds like copper coils are less restrictive to flow.
* If I size them incorrectly I can easily change their size since I made them in the first place
* I enjoy making stuff (the closer to being made from scratch, the better)

Not saying it won't work, and we all enjoy experimenting. Good luck, and I'm sure you will be pleased with the outcome.

Keep in mind that plate hx's are highly efficient due to turbulence introduced to maximize heat transfer from one surface to the other and the close proximity of the two transfer fluids. A coil hx doesn't do this on either side of the exchange. On the tank side there is only natural convection, works but not well, and most of the tank water is not in contact with the coil. On the tube side the water in contact with the tube surface transfers heat while the water interior to the tube does not, except to the extent of natural turbulence. A coil hx seeks to achieve with mass what a plate hx achieves through design.

 

[sparke]

gimme, Make sure you check with someone that knows the math. I don't think 100' of 1/2" copper is nearly enough for 1000 gallons.

 

[Eric Johnson]

You size the hx to the boiler output, not the size of the tank. But the point is the same--100' of 1/2-inch is probably not enough.

 

[jebatty]

I think Tarm recommends 600 gal (or more) storage for the Tarm 40. I also think the STSS tank has 360' of 3/4" copper (2 coils of 180'). You might want to do a double check on the size and length of your proposed copper coil hx.

 

[Bob Rohr]

They share some nice data at this site www.stsscoinc.com/ on the sizing method-ology.

I'd agree with Jim as you get much, much more energy transfer with a small, under $200 plate heat exchanger. The surface area and two moving flows transfers huge amounts of heat in a small block. Plus it is easily serviceable on the outside. Those copper coils will leak someday

Plenty of bargins on e-bay. A circ and HX could probably be had for under 300 bucks, very little labor involved. But it does take some of the fun out.

Stick a 100 feet of copper up high in the tank for DHW pre-heat. money well spent.

hr

 

[slowzuki]

100 feet of 1" inch is just enough, your 1/2" is not enough for an EKO 40. Two 50' loops has less than half the friction of a single 100 footer because it is half the lenght and half the flow in each. A rough way of determining the size requirement is using the surface area of the exchanger. This is not perfect but it is close. Somewhere handy to 4000 square inches is needed for an eko 40 in a tubular copper hx, so about 210 ft of 1/2" tube.

 

[Nofossil]

I'd go parallel for sure, and I'd be tempted to buy some copper flashing and form fins that could clip onto the tubing to increase the effective surface area - perhaps 6" square chunks formed around a piece of pipe so that they fit snugly on the copper OD.

 

[gimmeWood]

100 feet of 1" inch is just enough, your 1/2" is not enough for an EKO 40. Two 50' loops has less than half the friction of a single 100 footer because it is half the lenght and half the flow in each. A rough way of determining the size requirement is using the surface area of the exchanger. This is not perfect but it is close. Somewhere handy to 4000 square inches is needed for an eko 40 in a tubular copper hx, so about 210 ft of 1/2" tube.

Thanks for the numbers. I am actually looking at a Tarm 40, not EKO, but I think they are similar in output. It sounds like I need at least 200 feet of 1/2 inch, which does make the flat plate or pex sound cheaper. 200 feet of copper will be right around $300. Decisions, decisions...

I'd go parallel for sure, and I'd be tempted to buy some copper flashing and form fins that could clip onto the tubing to increase the effective surface area - perhaps 6" square chunks formed around a piece of pipe so that they fit snugly on the copper OD.

I like the copper flashing idea, but I'd have to give some thought on how to attach it to the copper pipes. Maybe I'' pick up some flashing this weekend and experiment with it. I am also wondering if it might make sense to wrap a bunch of short segments of copper electrical wire around the tubing. It would certainly be easy to twist them on very tightly with some pliers.

 

[Bob Rohr]

Cut some scrap copper tube to 2" in length. Then cut that in half, lengthwise with a hack saw. Solder these onto the tube after you roll it into shape. It makes for s lot of surface area, heavy gauge, and in-expensive. I use to make those long tube HX for water heaters that way.

I have some HX tube ordered from this company for a solar invention. Maybe they have some drops to sell for cheap. Lots more heat exchange with finned tube. Rifling or turbulators inside help, especially with low flow in large diameter tube. You need to keep the flow from turning laminar.

www.energytransferinc.com

hr

 

[leaddog]

Cut some scrap copper tube to 2" in length. Then cut that in half, lengthwise with a hack saw. Solder these onto the tube after you roll it into shape. It makes for s lot of surface area, heavy gauge, and in-expensive. I use to make those long tube HX for water heaters that way.

I have some HX tube ordered from this company for a solar invention. Maybe they have some drops to sell for cheap. Lots more heat exchange with finned tube. Rifling or turbulators inside help, especially with low flow in large diameter tube. You need to keep the flow from turning laminar.

www.energytransferinc.com

hr

hard to come up with scrap copper with the scrap price well over $3 a lb. It all gets melted. I've seen alot of NEW copper fittings sold to melt.
leaddog

 

[tom in maine]

Let me ask a question: I have been working with Dick Hill for years on wood boilers. Most of our devices were unpressurized, running directly into the tank. The tank is dosed with the appropriate amount of the right corrosion inhibitor (they are available and are cheaper than copper).
The plumbing is a lot simpler in that sense. You are doing a similar thing using a plate hx, since one side is open to the tank. The risks are the same, in the sense of the pump cavitating, but that is unlikely if the system is plumbed properly and the pump is primed. And you are there to know if it is pumping properly or not.

Of course, now, you need to remove the heat from the tank. You can pump tank water, since it is inhibited, directly into a heating loop. There will be some air noise, but I find that reassuring. The only heat exchanger is the DHW one.

The STSS hx are not very well designed, throwing a lot of copper in the place of good design. They work, but are expensive, given the current cost of copper.

I like the concept of using PEX, but would only consider AL/PEX since the straight PEX is likely to be somewhat de-rated pressure-wise
in a really hot tank. I would never consider it as a heat input exchanger. And, as mentioned before, I think you need a minimum of 4 times
the tubing to get the same performance as copper.

Tom in Maine

 

[jebatty]

You are doing a similar thing using a plate hx, since one side is open to the tank. The risks are the same, in the sense of the pump cavitating, but that is unlikely if the system is plumbed properly and the pump is primed. And you are there to know if it is pumping properly or not.

One side can, but need not, be open. Both my boiler side and my tank side are pressurized. Knowing proper pumping is easy, first, you can feel if it's hot; second, most of us use temp gauges on the in and out sides (just for fun or to know what's going on), and some have wireless temp monitors with/without alarms to provide more precise info.

Many/most home oil/gas/LP boilers are pressurized and the only way to know whether the system is pumping is whether you are getting heat. With my Tarm, if the tank side is not pumping, the Tarm will shut down on the hi-limit. It does have its own "buffer" of 50+ gallons. And it has a gravity fed and boiler pump fed over-heat loop to handle any other problem. These are needed regardless of whether an hx is used.