Heat Exchangers and 3D Printing

I live in a one bedroom apartment and while I love living there, there are some issues with ventilation. There are only windows on one side with small ventilation grilles above two windows. The air tends to get stale in the winter because there is not enough air flowing. Opening the windows every day would solve the problem, but that would waste a lot of heat.

There is a way around this, a 'heat exchanger' or 'heat recovery system'. Now normal people would just open a window, but I always wanted to try and make one. In this Instructable I will share a few designs I tried and share efficiencies and my conclusion for now.

All exchangers are at least in part 3D printed and there is even a completely 3D printed design.All files are available (and will be kept up to date) here:

http://ytec3d.com/3dp-heat-exchanger/

Simply put a heat exchanger is a device that transfers the heat from one flow of to another (in this case air to air). They do this by making 2 flows of air with different temperatures pass one another separated by a thin wall. Heat travels from the hotter flow of air, through the wall and into the cooler flow of air. The only part that requires energy is making the air move. Everything else happens by temperature difference alone.

The heat exchanger is great for ventilation systems for homes in cooler countries because homes can then be ventilated without losing the heat. The humid, stale air of inside can be replaces with the dry, fresh air from the outside without also replacing the heat.

In an ideal world, heat exchangers can transfer up to 100% of the heat from one flow to the other, but in the real world, percentages are lower. My initial goal was to make an exchanger with 80% efficiency. Efficiency in heat exchanger is how much of the heat is transferred between the flows of air. An example:

The air outside is 0°C and the inside air is 20°C. If there is a 80% efficient heat exchanger in the ventilation system, the 0°C air from the outside will be heated to 16°C when it exits the exchanger. The 20°C air will be cooled to 4°C when it exits the heat exchanger.

As seen in the image, there are a few design options. Also there are 3 distinct flow directions.

• Cross flow: The streams flow perpendicular to each other. Exchanging heat sideways (the 50 - 70% exchanger)
• Parallel flow: Both flows enter from the same side and exit the same side. This way allows for at most 50% heat transfer.
• Counter flow: The most efficient, where the flows are opposite from one another. This creates a hot side and a cool side. Depending on the design, counter flow can get close to 100% efficiency. All my tests use counter flow heat exchangers.

The partially 3D printed heat exchanger has all of the complex parts printed. These are the end caps, adapters and shrouds. The main frame of the exchanger is a PVC tube. For those who want to duplicate the design, it is a 90mm PVC tube and I dare anyone to find it in an ordinary hardware store. I messed up and designed this heat exchanger around a tube that is only available on the internet.

The exchanging is done in drinking straws. Normal straws are only around 20cm long but I managed to find straws 75cm long on the internet. The straws have a wall thickness of 0.1mm and a diameter of 6.5mm. 91 were used in the current design. Effectively each straw has a surface area of around 140cm² (it is 150, but not all is used). With a total of 91 straws, this gives a total surface area of 12500cm². Over a square meter or 13.5 square feet.

To assemble the exchanger, the tube was cut to length. The straw holes in the end caps were drilled to give a loose but clamping fit. Both end caps were glued to the tube and aligned. The straws were fed through by hand, one at a time. After all straws were in place, one side was glued using super glue. Then from the other side the straws were pulled to tighten them and the other side was glued as well.

The fans are standard 60mm fans. The fans are rated at 38m²/h, but due to the resistance of the exchanger, a fraction of that is probably achieved. The fans consume 1.75W each. The fans have adapters to mount them to things like vacuum cleaner hoses. 60mm fans are not ideal. They are noisy and not that efficient. In the future I might replace them with something quieter and more efficient like 120mm case fans.

To download the files, go here:

http://ytec3d.com/3dp-heat-exchanger/

The completely 3D printed version is, as the name suggests, completely 3D printed. To make it I modified my Ultimaker with an E3D V6 with 0.25mm nozzle.

The walls of the exchanger are 0.3mm thick. The outside dimensions of the exchanger are 15x8x7cm but it has an internal surface area of around 1000cm² (1/10th of a square meter or about a square foot). It is printed in PLA and takes around 10 hours to print at 0.16mm layer thickness. With special adapters it can fit 60mm fans and all the other adapters I have designed.

special adapters were printed to connect the 60mm fans to the 3D printed exchanger. The accessories used on the partially 3D printed heat exchanger also fit on the completely 3D printed version.

To download the files, go here:

http://ytec3d.com/3dp-heat-exchanger/

Making a heat exchanger is meaningless without determining how efficient it is. The test is fairly straight forward and gives a decent ball park efficiency figure. In the real world efficiency is also dependent on flow, but for this test the heat exchanger will be ran at one speed only.

The basics of the testing equipment is an Arduino Uno with 4 10k thermistors (NTC) and an SD card for logging. Thermistors are not known for their accuracy, but with some calibration I got them to operate within +/- 0.5°C of each other. The logger takes a sample of each airflow every few seconds and stores it on an SD card. These values can be imported into excel to make a functional graph.

The flow will not be equal on both sides. I currently have no way to measure actual flow. To still get a good efficiency reading, both in and output flows were measured. By calculating both efficiencies, I can average them out and get a decent ball park on the efficiency.

Efficiency is measured as full potential energy vs. energy actually being used. In the case of the heat exchanger it is total temperature difference vs. transferred heat. This is assuming equal flow and equal mediums. The 4 temperatures (unit does not matter):

• Hot in (the warmer air that enters the hot side of the exchanger)
• Hot out (the warmer air that exits the cool side of the exchanger)
• Cool in (the cooler air that enters the cool side of the exchanger)
• Cool out (the cooler air that exits the hot side of the exchanger)

Hot flow efficiency: (Hot in - Hot out) / (Hot in - Cool in) x 100%

Cool flow efficiency: (Cool out - Cool in) / (Hot in - Cool in) x 100%

To measure the actual efficiency a one of the inputs was suspended over a heat source (ie. a heater) and the other input was in a cooler place. Both setups have 4 fans, 2 for each flow. The heat source is approximately 35°C while the cooler side is around 19°C.

Partially 3D printed exchanger

The first graph is a short test of the partially printed exchanger inside of my house. Every bottom number is 1 second, the total test was around 18 minutes. From top to bottom, the lines are: Hot in, Cool out, Hot out, Cool in. The hot air was removed too soon, the line was not yet stable. The stables situation was: HI: 34°C, HO: 23°C, CI: 18°C, CO: 32°C. Putting these values through the efficiency formulas gives 68.75% for the hot flow and 87.5% for the cool flow. This averages to 78.125% efficiency. The difference in efficiencies is due to the different flows and measuring errors. An interesting this to see is the thermal 'lag' of the exchanger. It takes over 10 minutes for the exchanger to reach a stable temperature.

The second graph is a longer test in my local (cooler) hacker space with an electric heater for a heat source. Every bottom number is 6 seconds, the total test lasted about 2,5 hours. Again from top to bottom, the lines are: Hot in, Cool out, Hot out, Cool in. Interesting to see here is that the temperature of the room slowly rose over time, due to all the electric heaters used. The heat was periodically removed to see the effect. 2 stable lines can be found, one at 450 and one at 1000.

450 -> HI: 35°C, HO: 20°C, CI: 14.5°C, CO: 30.5°C, giving 73.2% for the hot flow and 78.0% for the cool flow

1000 -> HI: 31°C, HO: 19.5°C, CI: 15.5°C, CO: 28°C, giving 74.2% for the hot flow and 80.6% for the cool flow

The average efficiency of the partially 3D printed heat exchanger is 76.5%.

Fully 3D printed exchanger

The third and final graph is the fully 3D printed heat exchanger. The fans were run at 10V to reduce the flow. Every bottom number is 6 seconds, the total test lasted just over an hour. Again from top to bottom, the lines are: Hot in, Cool out, Hot out, Cool in. The thermal 'lag' of this exchanger is a lot lower. The 2 times used will be the peak at 120 and the valley at 210.

120 -> HI: 38°C, HO: 27°C, CI: 22.5°C, CO: 33°C, giving 70.9% for the hot flow and 67.7% for the cool flow

210 -> HI: 33°C, HO: 25.5°C, CI: 21°C, CO: 30°C, giving 62.5% for the hot flow and 75% for the cool flow

The average efficiency of the Fully 3D printed heat exchanger is 69%. An important note to make here is that in a smaller (unlogged) test the 3D printed heat exchanger ran at 12V, the efficiency was closer to 50 - 60%.

The initial goal was to make this exchanger part of an active ventilation for my house. The goal is to properly ventilate my house without pumping all the heat from my home. Because I lack a centralized ventilation system and this is a rented apartment, I decided the ventilation above my window was the best place for the ventilation. One side of the ventilation grille is used to draw new air in, the other side is used to vent old air out. The rest of the grille is taped off.

This setup has a few drawbacks. First, the fans are quite noisy. This is not a setup I can run while I am at home. The second is that the hoses leak heat. They are not insulated and will preheat and precool the air. The test was done with outside air at around 0°C, yet the incoming air was already 6°C after only 50cm of hose. The longer hose has a difference upwards of 10°C. The Third thing is that I cannot really close my curtains with the tubes in the way. I can remove the hoses to close my curtains while I am home. It will not run when I am home anyway.

With those drawbacks out of the way,

Does it work?

The answer, YES. After running for over 8 hours while I was at work, the air was a lot fresher. Usually when I come home there is a certain staleness to the air, but now I came home to nothing. Just nice air.

I had the logger running for the entire time. The test started around 8 o'clock, every number on the X is 6 seconds. There are 3 zones of interest.

0-3000: Here the air outside is slowly heating up. Temperatures around this point are: HI: 17°C, HO: 10°C, CI: 6°C, CO: 14.5°C, giving 63.6% for the hot flow and 77.3% for the cool flow, averaging 70.5%.

3000-4000: Here the sun hits the window and there is a spike in temperature. No useful data can be gathered from this time.

4000-6000: The air outside is slowly cooling. Temperatures around this point are: HI: 17°C, HO: 12°C, CI: 8°C, CO:
15°C, giving 55.6% for the hot flow and 77.8% for the cool flow, averaging 66.7%.

There are 3 conclusions I can draw from this project so far

The project as a whole was a success. I got fresh air into my home and the heat exchanger exchanged a large percentage of the heat. The execution of the ventilation could have been quieter and more compact, but as a whole, it works better than expected.

The big partially 3D printed version works remarkably well. Running at 70 - 80% efficiency, it is good for one of my first attempts. It does have some issues with unequal resistance and pressure drop. The side that goes through the straws has less resistance than the side around the straws and it takes quite some pressure to get real flow out of it. The 60mm fans can get air to move, but they are struggling hard. Whether the efficiency drops significantly if the flow increases much is not yet known.

The 3D printed heat exchanger works surprisingly well. I did not expect it to have the efficiency 69% it had. That said, from other smaller test it does seem that if the flow goes up, the efficiency goes down quickly (for the fully 3D printed version at least). The big logged test I did was at a lower flow. When I later preformed the test with a higher flow, I got 50 - 60%. Not bad, but less than initial testing.

What my tests so far have shown me is that I know too little. I need better measurements, more different measurements and a more controlled test. What I want to do is design a better test setup.

I want to manually heat the air using electrically heated wire. This way I cannot only control the exact temperature, I can also determine the flow by comparing the energy I added to the air to the temperature rise of the air. I want to use better temperature sensors that are both calibrated and more accurate. This way I can get more precise results.

For the test itself I do not only want to determine efficiency at one temperature and flow. I also want to see what the influence is on the efficiency if the temperature difference (ΔT) becomes bigger and smaller, and how the efficiency changes when the flow is changed.

In this setup I want to test small test exchangers, so I can see the effect of wall thickness, flow paths, surface area and more. If I learned one thing from the tests I have done so far, it is that there is a lot more to know here than I already do.

While I cannot promise when I will do these tests, I really want to do them.