3DP Heat Exchanger

Heat exchangers are devices that transfer heat from one flow to another. This flow can be any liquid or gas, but all exchangers described on this page exchange air to air. If there is heat involved, a heat exchanger might benefit the process. For domestic use heat exchangers are a great addition to any ventilation system in a cold environment. By adding an exchanger between the outgoing and incoming flow, the temperature can be exchanged between them. In the case of a 100% efficient heat exchanger, all of the heat will be exchanged. The dry, fresh air from the outside can be replaced with the humid, stale air from the inside while keeping the heat inside.

Heat exchangers work by passing 2 flows of air past each other separated by a (thin) wall. The heat naturally moves from the hotter side to the cooler side, through the wall. The only energy required is to get the air moving. There are several ways to make the flows of air pass each other. The flows can cross, perpendicular to each other, run parallel to each other. In all examples on the page the flows of air run opposite to each other. This setup gives the highest potential efficiency (nearing 100%).

My own house has ventilation issues. While I can just open a window in the Summer, in the winter I would waste a lot of heat that way. I always wanted to make a heat exchanger, so I used this opportunity to make one.

More information on logging and efficiencies can be found on the Instructables page: http://www.instructables.com/id/Heat-Exchangers-and-3D-Printing/

Data logger

Making something like a heat exchanger without testing it is rather pointless. Because of this I also made a data logger to measure all temperatures and get a ball park figure of how efficient all exchangers run. The logger consists of an Arduino Uno with 4 10k NTC’s. The Arduino measures the temperature of every NTC at set intervals and logs these temperatures on an SD card. This data can then be pulled from the card and imported into something like Excel to get fancy graphs.

 

Partially 3D printed version 1

The partially 3D printed version uses 3D printing to make the complicated parts. The parts that do the exchanging are fairly simple to come by. A 90mm PVC tube is used as a body, and long drinking straws are used to exchange the heat. Straws are used because they are cheap, have lots of surface area and are fairly easy to come by. The end caps cap of the tube. The straws are fed through the end caps, making lots of tiny channels through the tube. End caps separate the flow through the straws and the flow around it. One of the flows of air moves through the straws, while the other moves around the straws.

The exchanger with end caps is around 1m long, 24cm wide and 14cm deep. It is made from PVC, PLA and probably PE. The straws are 75cm long, have a diameter of 6.5mm and a wall thickness of 0.1mm. There are a total of 91 straws in this exchanger. Accounting for some losses in manufacturing, the total exchanging surface area of the exchanger is 12500cm². Over a square meter. Measure efficiency ranges from 65% to 75%.

 

HXC download button P3DP

If you downloaded the files from this site and liked it, please consider going to the Donations page. This will help the development of more free designs and plans.

 

Fully 3D printed version 1

The fully 3D printed heat exchanger is (as the name suggests), completely 3D printed. It is printed on an Ultimaker with an E3D V6 with 0.25mm nozzle. The material is PLA, the wall thickness is 0.3mm and the layer thickness is 0.16mm. The dimensions of exchanger without caps is 15x8x7cm and the internal surface area is 1000cm². The total exchanger takes around 10 hours to print. While the walls are thicker than on the partially printed exchanger, the fully printed version requires little assembly. The thicker walls make the exchange rate a bit slower, but it still performs quite good with an efficiency of 50% to 65%.

 

HXC download button F3DP

If you downloaded the files from this site and liked it, please consider going to the Donations page. This will help the development of more free designs and plans.

 

Real test

The original test was to properly ventilate my home without wasting all the heat. On the ventilation grille above the window 2 adapter were glued. Vacuum cleaner hose was attached to these adapters. The rest of the grill was taped of to prevent unwanted flow in and out of the house. Fresh cool air is drawn from the right adapter, and the exchanged stale air is exhausted to the left adapter. The exchanger sits in between, with the logger logging the temperatures.

The air quality of the room has improved quite a lot from this setup. Without the fans running the air feels used up. After 8 hours of running, the air is just fresh, without anything special to note about it. There are some issues in the implementation. The hoses prevent the curtains from being closed and fans are quite noisy. Without filters I am wondering how long it will last before clogging up, but time will tell.

 

Conclusion

The heat exchangers tested so far work better than expected. All of them preformed above 50%, and in some cases above 75% efficiencies were measured. The most surprising thing was not how good the partially 3D printed version worked, but how well the fully 3D printed version worked. For something with those thick walls, the efficiency was rather surprising.

I think that I need to know more. I want to to a bigger followup experiment, with more variables measured (pressure, flow, humidity, temperature) and more different, smaller designs. I want to see the effect of different materials, different flow paths, wall thicknesses, length and surface area. I also want to plot graphs on flow vs. efficiency and temperature difference vs. efficiency. For this I will design a better logger in the future, that will plot these graphs for me and preform all of the testing automatically.

There is a lot more to learn about these things and I want to know it.

 

License

by-sa

The project described on this page is licensed under the Creative commons - Attribution  - ShareAlike license.

5 Comments

  1. I had a chance to look at your design and I think it can be further improved if you use shell-tube design with baffles (these are complex to construct for mass production, but hey you are using 3D printing.
    My suggestion is also to make the tubes with hexagon shape and arrange them as https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcSgLBG4VJ2tLsUTfp-wuPT7HSt9MGDPj2tM7-PIbq1rvIOW7xz-, but rotate the image by 90 degrees so the flow to be from left to right, then baffle will force it from right to left and so on. Additionally you can try the tubes to change hexagon size slightly in zik-zak manner so it can force turbulent flow inside the tube (which should improve more air to get in contact with the walls).
    This design will increase the air resistance, but you should get better efficiency.

    Best of luck!

  2. Hi,
    Did you do any experiments with the 90 degree grid pattern like in the old Hitachi systems? Why did you go for this design? I’m thinking of making something like this for my camper van, and your design is very neat looking..

    • The designs here are mostly experimental and proof of concept. The counter flow is said to be the most efficient, with 95% efficiency possible. I don’t know if the designs are already mature enough to be put to use, but you can always try. The 90 degree grid to my knowledge is easiest to make and has the lowest resistance, but efficiency peaks at 70%.

      I do want to make the proper tools to measure these exchangers and do a lot more tests this winter.

  3. I absolutely love this. I live in southern california, and our problem is the opposite, we want to keep our incoming air cool. I wonder, what kind of efficiencies you would get on the fully printed heat exchanger with copper or silver filled pla?

    • This is actually a discussion filled topic that I hope to learn more about next winter.

      It is actually not completely clear whether metals such as copper improve the efficiency by that much (or at all). The gut feeling says it should, after all, better heat conduction means better efficiency, but plastics seem to work in heat exchangers way more than they should. The idea is that getting the heat from the air to the wall material is disproportionally more difficult than getting it through the wall itself. The next winter I hope to do a lot of experiments with a better test setup to determine the effects of heat transfer of the material, surface area, turbulence, thickness and general design. This will also include a showdown between copper, aluminium and plastic.

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