DIY Solar Hot Water Heat Exchanger - Page 2

Indyplumber · 01-18-11, 10:33 AM

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.

Piwoslaw · 01-18-11, 10:53 AM

Hello Indyplumber, welcome to ER

Quote:

Originally Posted by Indyplumber

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.

bennelson · 01-18-11, 11:44 AM

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!

Piwoslaw · 01-18-11, 12:39 PM

Quote:

Originally Posted by bennelson

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.

bennelson · 01-18-11, 01:27 PM

Sure looks like a heat exchanger to me!

strider3700 · 01-18-11, 06:34 PM

rather then trying to get multiple tubes inside 1 larger tube why not go with multiple of the original all plumbed in parallel?

bennelson · 01-18-11, 10:32 PM

That would work too.

Drake · 01-22-11, 11:32 PM

How many would be needed?

strider3700 · 01-23-11, 01:26 AM

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.

bennelson · 01-23-11, 11:31 AM

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!!!

01-18-11, 10:33 AM	#11
Indyplumber Lurking Renovator Join Date: Jan 2011 Location: Indiana Posts: 18 Thanks: 3 Thanked 1 Time in 1 Post	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.

01-18-11, 11:44 AM	#13
bennelson Home-Wrecker Join Date: Sep 2008 Location: SE Wisconsin Posts: 546 Thanks: 3 Thanked 165 Times in 96 Posts	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! __________________ To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

01-18-11, 01:27 PM	#15
bennelson Home-Wrecker Join Date: Sep 2008 Location: SE Wisconsin Posts: 546 Thanks: 3 Thanked 165 Times in 96 Posts	Sure looks like a heat exchanger to me! __________________ To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

01-18-11, 10:32 PM	#17
bennelson Home-Wrecker Join Date: Sep 2008 Location: SE Wisconsin Posts: 546 Thanks: 3 Thanked 165 Times in 96 Posts	That would work too. __________________ To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

01-23-11, 11:31 AM	#20
bennelson Home-Wrecker Join Date: Sep 2008 Location: SE Wisconsin Posts: 546 Thanks: 3 Thanked 165 Times in 96 Posts	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!!! __________________ To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

01-18-11, 06:34 PM	#16
strider3700 Master EcoRenovator Join Date: Dec 2008 Location: Vancouver Island BC Posts: 745 Thanks: 23 Thanked 37 Times in 30 Posts	rather then trying to get multiple tubes inside 1 larger tube why not go with multiple of the original all plumbed in parallel?

01-22-11, 11:32 PM	#18
Drake DIY Guy Join Date: Jan 2011 Location: Mpls,MN Posts: 315 Thanks: 2 Thanked 17 Times in 17 Posts	How many would be needed?

01-23-11, 01:26 AM	#19
strider3700 Master EcoRenovator Join Date: Dec 2008 Location: Vancouver Island BC Posts: 745 Thanks: 23 Thanked 37 Times in 30 Posts	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.