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Old 01-18-11, 09:33 AM   #11
Indyplumber
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Default Materials used for tube-in-tube exchanger

I saw in a few of the photos you were using a PVC adapter to connect to the tank. As a plumber I would recommend using a CPVC adapter, the difference being that CPVC is suitable for hot water piping whereas PVC is for cold water use only.

One question I have is would the efficiency be greater is the solar heated liquid circulated through the outside tube instead of the inside tube? Im getting ready to build my own system and thought it would be better that way.

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Old 01-18-11, 09:53 AM   #12
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Hello Indyplumber, welcome to ER

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Originally Posted by Indyplumber View Post
One question I have is would the efficiency be greater is the solar heated liquid circulated through the outside tube instead of the inside tube? Im getting ready to build my own system and thought it would be better that way.
No, it is more efficient when the warmer liquid is circulating inside the cooler liquid. That way the temperature differential through the outside wall is smaller, wasting less heat. In theory, if the heat exchanger was ideally insulated, then this wouldn't make any difference, but no insulation is perfect.
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Old 01-18-11, 10:44 AM   #13
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The PVC adapters were just because I couldn't find anything else that would fit the threads on the tank.

It was an off-the shelf part, at least good enough for testing.

The electric heating elements ports on the side of an electric water heater tank are straight "electric" threads. They are NOT compatible with steel or copper water pipe threads.

Later on, a friend of mine drilled out the old pair of water heater elements and tapped the inside to fit a standard pipe.

I don't know if it would make much of a difference or not if the hot fluid runs on the inside or outside layer of the heat exchanger. I kinda figured that INSIDE would be better, as the heat can ONLY go into the water. If the heated solar fluid is on the OUTSIDE, the heat can go inside to the water, OR to the outside of the heat exchanger. (Obviously, you would have insulation on the outside of the exchanger, but still, some heat would be lost.

This heat exchanger was more an experiment in soldering and design than anything.

The downside to this design is that IT JUST ISN'T THAT MUCH SURFACE AREA! A similar design, but with MULTIPLE tubes running through it would be better. For example, a 2" outside pipe with four pipes running inside it. Of course that would also be a little trickier to build, but also have 4 times the heat transfer!
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Old 01-18-11, 11:39 AM   #14
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Quote:
Originally Posted by bennelson View Post
The downside to this design is that IT JUST ISN'T THAT MUCH SURFACE AREA! A similar design, but with MULTIPLE tubes running through it would be better. For example, a 2" outside pipe with four pipes running inside it. Of course that would also be a little trickier to build, but also have 4 times the heat transfer!
Ben, what you need is an EGR cooler. The one I installed in my engine has some 30-35 small tubes inside:




With some luck you can find a truck-sized EGR cooler at a junkyard. Mine cost about 50 ($70-$80) from a crashed Peugeot 307.
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Old 01-18-11, 12:27 PM   #15
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Sure looks like a heat exchanger to me!
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Old 01-18-11, 05:34 PM   #16
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rather then trying to get multiple tubes inside 1 larger tube why not go with multiple of the original all plumbed in parallel?
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Old 01-18-11, 09:32 PM   #17
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That would work too.
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Old 01-22-11, 10:32 PM   #18
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How many would be needed?
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Old 01-23-11, 12:26 AM   #19
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the math can be done.

What's the incoming temp and desired outgoing temp of the fluid you're heating, what is the incoming temp of the fluid doing the heating, what is the flow rate of both? Somewhere I found a heat exchanger calculator that will work out the necessary sqft of surface area necessary to achieve it and it will be able to tell you the outgoing temp of the heating fluid as well.

I recently was looking for a double wall flat plate that could take 50F to 110F using 140F water on the other side both flowing at 4 gallons/min The recommended model had 12.4 sqft of surface area and weighed 16.8 pounds. Not a small or cheap thing.

in this case it looks like the inside pipe is 1/2" so unless I screwed my math up you need about 92 feet worth of inside pipe to get the same surface area. You can improve things though by increasing the flow rate of the heating fluid. As well my calc was for a double wall with airspace. Single wall should be more efficient I still would be shocked if it's less then 50' though. 60F in 1 pass is not easy to do.
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Old 01-23-11, 10:31 AM   #20
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Yeah, math is sort of the boring part of all this, but it IS much easier to figure something out on paper before cutting and soldering copper.

The cutting and soldering part is much more FUN though!!!

Another take on all this is the fewer walls, the longer length, and the more heat the fluid can hold, the more efficient the system is.

A long coil of copper pipe inside a big tub of hot water is excellent for transferring heat!

At my house, I don't have a basement, and the solar panel will be ground-mounted, so it's not appropriate for a drainback system.

For a pressurized system, you really need some kind of heat exchanger. I have finally gotten my hands on a combined heat-exchanger and solar heated water tank. It's in my garage, and the weather is below freezing for a couple of months. Once we thaw out in the spring, I can test it out and start work of hooking up my actual - real-world - solar hot water system!!!

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