In order to heat my water and still use the existing hot water heater, I was thinking about building my own liquid to liquid heat exchanger. Since there would be antifreeze in the solar collector, I need some way to make sure it will never contaminate the water in the hot water tank. So I came up with this design, tell me what you think.
Seems like it would work, though I doubt it would work as well as the commercial double-wall heat exchangers - simply difficult to home-fabricate that sort of thing with similar efficiency, and lost efficiency makes your system work less effectively. In any case, use of non-toxic antifreeze is a good idea, though some jurisdictions require both non-toxic antifreeze and a double-wall heat exchanger.
Hi, I believe that some of the commercial double wall heat exchangers use a thermally conductive paste material between the two walls (ie in the space where your copper fingers are). Apparently the paste is soft enough to wash out if there is a leak in either wall.
You might think instead about using a drainback system that does not need antifreeze in the collection loop. Your heat exchanger can just be a coil of single wall copper pipe in the drainback tank.
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I doubt if it would be practical as the air gap between the heat exchange surfaces will have so low a conductivity as to be useless. I suggest that you make a conventional tube-in -tube heat exchanger and use propylene glycol as an anti-freeze (not ethylene glycol, its poisonous). Any leak will show up as sweet taste in the heated water. Simply marking the level in the header tank will show if there is a leak anywhere in the system.
Well, you're certainly reinventing the wheel here ;-).
But, that said, my suggestion would be to mount the heat exchanger vertically, with your drain holes pointing upward. Then fill the cavity with non-toxic oil or something similar, to better transmit heat from one pipe to the outer shell. If there is a leak on either side, it will come out the top. I've been working with heat exchangers most of my professional life. You need something more than those little fingers to conduct enough heat across the gap in anything resembling an efficient fashion. It would work alot better if you could make the shell side (your outer layer; you've designed what we call a shell and tube heat exchanger, commonly used for aeseptic fermentor exhausts or in-line fluid sterilizers) extremely hot, like by putting steam through it.
You may also want to consider looking into a cross flow, countercurrent heat exchanger. I once built a heat exchanger using some copper pipe and a bucket. For my purposes I was trying to cool a water mixture down. The system used a 5 gallon bucket for its shell and copper pipe inside. The system worked quite well. If you use a single piece of pipe to go through your system, and grossly overside the pipe you will help prevent leaks due to pressure buildup. Also, if you use a large pipe and can reduce the flow rate through the heat exchanger, that will also help improve the efficiency.
The fingers are a joke and would be hugely labor intensive to install. Simple is better. Have you considered mineral oil? Anything that uses water will eventually need maintenance. Our flat plate exchangers need to be torn down once a year to keep them at optimal condition. They are aluminum alloy and are water to water medium. We spend a million a month on water treatment. Starting out with distilled water.
Way, way too elaborate, but short on surface area.
When I wanted to preheat a hot water tank, I used an extra heater tank with a heat exchanger element screwed into it. This is a commercial item, that has a spaghetti array of pipes from a round header of the size usually associated with an electric heater element. I used non poisonous antifreeze as the fluid in the solar panel loop feeding the exchanger. . I placed the extra tank adjacent to the hot water tank, as a cold water feed tank with pre-heat. I arranged valves to isolate it when necessary. It was well insulated, and worked pretty well during the relatively short two year test period. It had a pump in the circuit too, driven by a differential thermostat arrangement.
Not always, Nick. There is a "sweet spot" of residence time versus flowrate, as affected by the temperature differential. So yes, you want it put through quickly, but you also want to increase the residence time to optomize heat transfer. My experience is in ten years of heating or cooling tanks of "mostly water" fluids or exhaust "mostly air" using pretty much every type of heat exchanger made.
Nick, I'm not an engineer, obviously. I'm a millwright. Your posts indicate that you probably are an engineer. However, I've probably got ten more years working in a fermentation bio-chemical plant than you have, and pretty much every one of those days, I was running a heat exchanger of some kind attached to a feed system, fermentor, storage tank, or other process equipment. Fact is, when heating up something with a heat exchanger, there are alot of secondary concerns that effect the heating rate. And a faster flowrate isn't the best way to warm up a fluid. You need the residence time to transfer the heat across, and that residence time depends on things like steam pressure, trap size, or cooling water flowrate, or heating water flowrate, a whole handful of things like flow directions and inlet and outlet temperatures.
But whatever. You have your numbers, I have real life.
Caution: extreme thread drift - I am suddenly reminded of claims by some that if you drive fast enough on washboard roads, you'll only hit the high spots. :-) Oddly enough, the same people claim that driving fast worsens the washboard, when by their own logic it should be wearing down the high spots...
Yep, but dunno about that myself, my jury is still out ;-). I live about ten klicks "off the paved road" and we do get a fair bit of washboard seasonally. It could be that if I got going fast enough, it would become smooth, but then there's all the coffee I would have spilled by then, and then I lose the advantage of defensive driving if and when a deer/bear/wolf/porcupine suddenly appears in the road ;-).
It's important to distinguish between heat transfer RATE (which generally increases with flow speed) and the total amount of heat transfer (which is a function of both rate and residence time), which does not necessarily increase with increased flow speeds.