Heat loss calc for 1\" copper heat exchanger

Eric Johnson

Since I'll be working with rigid copper, I suspect all the fittings and couplings will help the cause.

Since I have 200 feet of the stuff and want to put two "coils" in the tank, are you saying that I could probably be fine with, say, a 75-foot section and a 125-foot one?

And while I've got you on the line, would it be better to make long runs with fewer connections or more compact exchangers with more fittings?

Eric Johnson

Heat loss calc for 1\" copper

Still trying to dope out my in-tank heat exchanger strategy. Somebody told me that pex loses 6 btu/hour/foot when immersed in water that is 50 degrees F. cooler than what is being pumped through the pipe.

Does anybody know offhand what the value is for 1" copper (Type M, if that matters)?

I tried various Google searches, but most of it was industrial technical stuff that's way, way over my head.

Also, would pex-al-pex transfer heat better than straight pex?

hot_rod

look up some indirect tank output data

the tank I pictured above has about 24feet of 1-1/8" smooth stainless coil. Copper is a bit better conductor.

Still it is a complicated calculation to answer your question. What will the flow be thru the coil? temperature drop? In a tank of still water? how much area around the coil? Area above the coil for stratifaction? Heat loss of the tank shell.

The folks that build that collaspable tank for Tarm and others have some fairly accurate data based mainly on real installation observations. They use a 500 gallon tank with copper coils. a header arrangement as others mentioned with 1" risers and a bunch of 1/2' copper "hoops" off the risers.

Exactly how accurate do you need to be?

If there was one simple number you would have it by now

hot rod

Eric Johnson

Just trying to size the hx

Yeah, well I appreciate that, hot rod.

Briefly, my setup involves an insulated 1,000 gallon concrete tank with an epdm liner and a one-inch line coming in from an EKO 60. The loop from the boiler to the tank is 200 feet. I'm using a Grundfos 26-96, I believe, to pump the water. Water coming into the tank should be around 180 degrees. The tank measures approx. 4'w x 7'l x 5'h.

I want to have a stratified tank with two separate heat exchange assemblies--one on the bottom for heat storage and one on the top for heat recovery. At least I think that's what I want. The house hydronics include 3 pumped zones serving 15 ci rads. So the return water from the house side should be pretty cool. No mixing valves. I should add that a separate 3/4-inch line from the EKO loops through a greenhouse. It has its own pump.

I have about 200 feet of 1-inch copper to build these heat exchangers. I'm looking for some guidance on how much to use for each one, a good design and some recommendations on placement in the tank. Not surprisingly, I want to be able to dump as much heat as possible into the tank as quickly as possible and recover it when I need it.

A poster in my other thread mentioned building a series of shell-in-tube type hx units. I've built a couple of gravity-feed sidearm DHW exchangers like that with 1.5 and 1.25-inch copper shells and 3/4-inch tubes. I could do that too, but my original thought was to try to mimic a soft copper coil with a rigid copper assembly.

Attached is a photo of the tank. It's just an old poured concrete cistern in my basement with a section blocked off. You saw a pic of it when it was full of rubble, trash and old coal ashes. It's the only remaining unfinished component of my system, and I'd like to get it done before the cold weather sets in. All I really need is the right hx.

zeke

I have found from the literature the natural heat transfer for one horizontal pipe at about 140 deg and using your delta T of 50 deg, the U coefficient is about 60 BTU/hr ft deg, so that for 1 foot of 1 inch pipe I get
3.14/12*60*50=750 BTU/hr.

which is at great odds to the 6 BTU/hr your were given.

I would however caution not to use either number in developing your design but possibly run your own test in a small test tank pumping hot water thru it.

Or/also, you might get a handle on this by going online to size a commercial shell and tube heat exchanger for your application and get the amount of tubing they required to do the job. I would be most interested to see that data as well.

Eric Johnson

I misspoke

Sorry about the bad number. I misspoke. For pex it is 6 btu per foot per degree, F., so that works out to 300 with a temp differential of 50 degrees. The amount of pex required under this scenario to transfer 150k btu/hour would therefore be 500 feet.

With your numbers for copper, it's 200 feet.

That's still a lot of copper.

I know there are other variables involved, but that gives me a rough number to work with. Thanks.

I've noticed that the commercial tank/hx places generally sell 3/4-inch coils in 120 to 160-foot lengths. I'd like to build two hx units with my 200 feet of 1-inch, so I have to do some more research to see if that will work.

hot_rod

I used a 60 foot coil of 1\"

in a 500 gallon concrete tank i had in my shop a few years back.

in all honesty the output from that wood boiler isn't a static number. i suspect that amount of coil would move the heat from that boiler just fine.

It's fairly easy to solder some fins on the coil. i used some short scraps of copper cut in half with my porta band to build my own fins. Ugly, but it did add square footage at a low cost.

hot rod

hot_rod

Seems a shame

to use all that 1" for a simple HX. If I were to do one again I would use two 1" headers with a bunch of 1/2" soft copper hoops. I would T-drill them into the 1" and silver solder that connection.

Here is a coil I built for an 800 gallon concrete in-ground solar tank. I used my Curvo to limit the fittings. I belive I used three 20 foot lengths. i tried to copy the Viessmann horitiz-cell indirect with a low and wide HX.

The DHW and radiant coils will be at the top of the tank.

I'll be back on this solar job Fri. I inheritied it from a solar guy that bailed. the tank was in the ground and 8 panels in the garage. He also had the slab tubed.

This will be my first go at an underground solar tank. We did use 4" of foam inside the tank with an EPDM liner.

hot rod

Eric Johnson

nice

That's nice. I remember seeing that when you posted it last winter when I was starting to plan my tank. It got me thinking.

It is a shame to use that pipe for an hx, but it's basically a matter of economics. I paid less than $1 a foot for the pipe when I installed my first boiler 4 years ago and if I can replace it with $1.40 a foot pex-al-pex now to avoid spending probably close to $1,000 on commercial heat exchange coils for this project, then I think I'm coming out ahead. No doubt I'll dig it out of the tank at some point and use it for something else.

One thing I like about the pex is that it's easier to insulate and since the line runs through unheated space, I'll sleep easier knowing that it has a better chance of surviving some horrible freezing mishap.

Larry C_12

Old baseboard?

How about old baseboard tubing? The copper pipe with aluminum fins? Cut off every other fin if worried about not generating enough convection flow with the density of water, versus air.

The one issue I see, might be corrosion. How does copper and aluminum react in an electrolyte? The price might be right and it easy to deal with. The copper will help with killing off biological growth.

Also add about 1/2" of clean oil on top of the water to minimize evaporation once the system is working.

Larry C

hot_rod

The old Chiles Power Systems

Mike and Dan Chiles founders of Heatway use to build solar powered radiant systems, back in the solar/ Radiant Roll days. They constructed well insulated plywood tanks with EPDM liners in the basements and copper coils. A few years back I had to replace a leaking copper coil in one of these 1980 vintage systems.

I wanted to use pex for many reasons, mostly weight and cost. One of the Watts Radiant engineers calced about 3 times as much pex as copper to get 'er done. That seems to jive with what you have calculated. I tried the pex, wasn't impressed.

The new owner scraped the whole system, as the solar had been removed years earlier after a hail storm, and had me install a boiler.

Great systems for their time. this one had a solar and a pond loop geo desuper heater feeding the tank and radiant with DHW preheat coils.

A few hundred pounds of copper when i finally dismantled the system

hot rod