So here is the puzzle:

I have one of those mobile a/c units that come with one exhaust duct that you’re supposed to hang out the window or so. For reasons that I don’t need to explain to you, I modified the unit so that I now have two ducts hanging out the window: one sucking in outside air and one blowing it out again about 20 degrees C warmer. So far so good.

I have now designed and 3D-printed a fitting that allows me to close the window to such an extent that almost no outside air comes into the room. The basic principle is to transform the round cross section of the duct into a slim rectangular one while maintaining the area of the cross-section because my intuition told me that reducing the duct size will make my a/c less efficient (and ultimately perhaps even damage it).

So the basic puzzle is this: in order to close the window as far as possible, I need to minimize the size of the rectangular end of the fitting while not making it too small and the question is: how small can I make it?

From the above blog post, I now understand that what I’m trying to do is minimize the pressure drop of my exhaust fitting.

The first version of my design is based on the following reasoning: Although the outside diameter of the duct is 150 mm, the inside is only about 135 mm because of its accordion-type design. So the area of the cross-section is 143 cm2. The with of the rectangular end can be max 36 cm long so that it needs to be about 4 cm high in order to maintain the same area.

I have obviously ignored that the walls of the accordion duct have quite some resistance while the walls of my 3D-printed part are nice and smooth. So there might be some room for further reducing the height of the rectangle, but if I start considering turbulence, I would also have to consider that my exhaust forces the air to change direction at a 90 degrees angle and I have no idea how that weighs against the resistance of the accordion walls. Probably the walls are a lot worse.

But here’s the thing: even if I were able to calculate (or measure) the exact pressure drop of my fitting, what does it mean? I’m starting to suspect that my consumer grade mobile a/c doesn’t really care about the extra pressure drop of my exhaust, given that the designers didn’t consider the accordion walls a problem. But even if it tolerates a lot, it will definitely become less efficient, so I’m nonetheless curious to try and optimize my design. So what’s most important and ho do I know whether I can reduce the size of the exhaust?

Another great article Allison
Another great article Allison. I am looking for input on supply grill pressure drop.
Years ago wood/metal grill multipliers were topical but manufacturers didn’t pick up on this.
A good looking, free flowing wall decoration shouldn’t require a translator….

Just one question :

I have two duct going to the same cooler. One is 3 inches diameter, the other one 2 inches diameter.
Air speed is the same on both inlet, so same pressure.
What happen to the speed and pressure inside both duct and does one duct take over the other one and can return some air and/or pressure in the other one if some conter pressure is done by the cooler?

Thanks for your help!

Jacques