Calculate Open Storage Tank HX Coil Size/Qty requirements

I posted this reply in the other thread before you moved it here. I'll post it here and add to it too, so as to help stir up some commentary.

I think you need to size your heat exchangers (coils or flat plate type) to your rate of heat transfer not to the volume of the storage. The faster you need to get the heat out of your boiler and into your loads (either storage or heated space and DHW or a combo of any of these) the larger heat exchanger you'll need to do that, regardless of the size of the storage.
If your heat exchanger only drives the storage it still needs to handle all the output of your boiler when the house and DHW are satisfied. Otherwise the boiler will shut down to idle until the HX can catch up. The larger tank means you can run it longer and store more heat but doesn't determine the rate of heat exchange. I mean like BTUs/hour.

I believe this is going to be determined mostly by the size of your boiler and how hard you are running it. And the higher the average temp of your storage the harder it is to get that heat transfer. The higher the average temperature of your storage tank the larger the heat exchanger you will need to transfer the same amount of heat. It gets tricky to calculate with coils in a tank because there are convection currents that effect transfer of heat and your best bet may be some empirical rules of thumb. STSS might be your best source for this if you really want to use tubing.

I think you might find flat plate exchangers more efficient and cheaper to use unless the price of copper takes a nosedive. The bigger the better. Pay attention to the port size as well as the number of plates. You'll need an additional circulator but if your heat exhcanger is not undersized it doesn't need to be a very big one.
You could find a lot on this forum from people with real experience (both good and disappointing) by searching for “flat plate” or such.

Though coils work I think they are an expensive excess and they work against themselves and by design are the reason it is harder to get the last few degrees without having an elongated energy curve. Without going into a lengthy explanation I think the extraction coil (I say "coil" but I mean something like a large radiator that takes a lot of soldering and you make yourself) needs to be in the uppermost layer of stratification and the heater/charging "coil" (which can be smaller than the extraction coil) needs be in the lowest layer of stratification. The above is just added for your consideration and really does not help you determine the btu output you need.

If you have had an oil or gas or electric boiler or furnace in you garage then you should have a reference for btu demand. For the sake of having enough it is usually safer to have more than you need for normal times than to have less than you need for critical times. Since your storage tank will not keep the 180 or so that you plan to charge it with for long you may need to consider the "average" temperature nearer the realistic mark. How do they mean average? 180 upper level and 140 lower level gives an average of 160 and an 160 upper with a 130 lower is a 145 average in simple terms where volume amounts are equal. There are heat consumption calculators that will give you your btu needs for daily heat loss usage.

We manufacture three coils for our tanks.
Two space heat coils are rated for 100k and 150k btu boilers. They do double duty for input and extraction.
We also make a DHW coil with an antiscald valve on it.

Our coils are significantly different than our competitors. Without disclosing our specific design information, they all hang on the side of the tanks
and are self contained (being single coils). There is no plumbing that the installer has to do inside the tank and the space heat exchangers cost a fair bit less than others.

I think our designs (which we have used for 25 years) work with and preserve tank stratification. Plate heat exchangers might be less expensive, but do require two circulators. If one is to analyze the two types of hx, you really need to figure the cost of the additional hardware and perhaps the labor? There is a reason that most commercial installers use coils, they are simpler and quicker to install, eliminate some hardware and usually work well. It then is a factor of cost of the hardware and moving parts replacement costs and service.

There is no one size fits all and we have a lot of fine choices.

Tom

Hi...I don't know zip about this yet hence a sore head every night from reading this stuff. However a friend uses HX's he gets from a company called Princess Auto....in Canada....the HX's are used for hydraulic cooling and are fairly inexpensive.. .
$180.00 for 16 gpm and $200.00 for 26.4 gpm...that's Cdn. dollars so cheaper for U.S. ....Not sure how efficient they are but they seem to do the job for him....mind you he uses an OWB with his backhoe to push mammoth piles of wood to burn in the beast..! He recently found a used HX at a salvage yard...a big one for $100.00.....again I'm just learning about all this..Maybe check out solar stuff.. my brother-in-law has just imported a load of Vacuum tube solar water heaters...one of the components is a DHW tank...electric with built in HX....I was thinking also that you could get an older style pressure tank...non-bladder...cut it open and install a coil HX...weld it back up...use it in line with your regular hot water tank... at times even just pre-heating water is a bonus...chilly billy well water here.....here's the url for Princess Auto HX's... http://www.princessauto.com/hydraulics/components/coolers/list-all ...Have fun...............

...forgot to say that if the aforementioned HX's are useable they seem to come on sale every couple of months for quite a bit less...maybe two could be used in tandem.............

Tom in Maine said:

...... Plate heat exchangers might be less expensive, but do require two circulators. If one is to analyze the two types of hx, you really need to figure the cost of the additional hardware and perhaps the labor?

Tom

Click to expand...

And the cost of running two pumps reduces the overall system efficiency. 1.2 amps at 120 volts X 12 hours per day (average Oct - Apr.) = $6.74 per month at .13 per kWH.

Tom,

I knew those were your tanks over at Tarm (as it was still called when I was last there) but I didn't know they were your coils as well. I just assumed STSS had finally hired some craftsmen that took pride in the appearance of their work! You're too modest. Which is how the webmaster likes to keep it, I guess.

Beside being more elegantly engineered looking, do you think having the coils along the walls of the tank gives you a better overall heat exchange rate by a 'turnover' convection effect than by having the coils scattered around the interior? I guess what I'm imagining is a narrower thermocline because of less eddy mixing. Just a leftover question kicking around in my skull's attic.
Before I fell onto a couple of propane tanks that would just fit through my basement doors and decided to go pressurized I was planning the pool and coil option.

Comparing the TCO of copper coils vs. flat plates is a very different thing if you have to figure in the cost of professional trade labor rates for installation and maintenance. We DIYers typically use $0 as our labor rate to justify doing what we want to do anyway. When you hire pros to do the work very different results come out of the overall cost analysis. Simpler assembly can be a whole lot cheaper.

Dave,
Thanks for the kind words.
We spent a lot of time designing and testing the coils. After we thought we were done, we wound up collaborating with Dick Hill at UMO and
further refined the design.

When we started with Tarm/Bioheat, I knew that our heat exchangers would be a bit of a shock, since they are so different than others. They are much smaller and are quite compact.
Most people just throw a lot of copper into a tank until it works. That can get expensive and is a real pain to plumb.

We brought our hx to Bioheat, asked them to try them out and just waited. They found they worked well and we have had similar feedback from the field.
I can tell you that our heat exchangers are somewhat overdesigned for the application, but that is probably a good thing.

Being adjacent to the sidewall probably helps a bit with channeling convection.


Tom