Sizing of the exhaust pipe of the kitchen extractor hood

  • Erstellt am 2017-05-20 21:13:33

Kaspatoo

2017-05-22 13:34:00
  • #1

If executed correctly (follow the assembly instructions), nothing can get between the plate and the facade. The pipe extends up to the flap, i.e., beyond the base plate. The gap between the pipe and the base plate opening is then sealed with silicone. However, the exhaust air does not reach this sealing area at all due to the protruding pipe. Only condensation moisture may accumulate here.
And with the others, where the mechanics are in the wind tunnel, condensate can form just the same. A look at the assembly drawings shows quite a few corners and edges where the air must flow past. Likewise, the other wall boxes also protrude beyond the pipe on the sides and thus have a contact surface with the exterior wall, where condensate could form as well.
Furthermore, for the remaining boxes, an incoming pipe must be inserted. How is it ensured here that the transition from the pipe to the box is sealed and no condensate forms? With Weibel, the pipe can run all the way through to the outside.
But as I said, Weibel is a fixed factor.

I only wanted to point out the disadvantage of heat loss. And regarding this topic about air volume in relation to pipe size etc., you gave me a very good and plausible answer. Many thanks for that. But I’m not concerned about “a few cents,” though theoretically that might add up over the year if you were to calculate it. But exhaust air is also a fixed factor.

If the difference in exhaust air volume is only slight and the exhaust stage does not change as a result, then a smaller pipe seems more sensible to me because it tends to cause less heat loss.
Smaller pipe means 125 is sufficient according to the Miele table.
Then the only "question" is whether I have chosen the right size.
 

Knallkörper

2017-05-22 14:33:36
  • #2


How do you want to seal a several-centimeter-wide gap with silicone?



The dew point will certainly not be undershot inside the pipe during operation, so no.



For the other wall boxes, a seal between the base plate and the pipe is provided. Condensation always forms, but I wouldn't want it between the base plate and the facade because it doesn't dry out there so quickly.



I absolutely don't understand the logic behind that. But I recommend you use a 50 mm pipe if the lower performance and higher noise level apparently don't bother you.



There is hardly a right or wrong to that. Over 200 mm probably brings hardly any benefit for a typical household system; below that, it's "bigger=better." Just take a look at a fan characteristic curve. The pressure loss is roughly inversely proportional to the square of the pipe diameter. Normally, you want the highest possible volume flow, so a large pipe.
 

Kaspatoo

2017-05-22 14:56:35
  • #3
If the gap is really that large, it is not necessary to seal the entire gap, but it is sufficient to seal the pipe and plate, with masonry in between. Alternatively, a 150 pipe can be installed and still reduced to 125 on the inside if desired/required.

Sealing is also provided between the base plate and masonry at the Weibel. Therefore, it is just as tight as others at this point.

The misunderstood logic is described in the following quote, that 125 is the smallest reasonable pipe size. The sentence about the 50 pipe was inappropriate.

bigger = better -> 150 pipe and not 125 after all. Okay, thanks, then it will be that way.
 

11ant

2017-05-23 23:30:51
  • #4
Summary: don’t simply work with a larger pipe diameter, but use your head.

.

Detailed version:






just ask Lesch

(Not only) he can explain physics better than I can. I’ll still try (after I had several good conversations with RobsonMKK today, I finally have to give him one last chance to shake his head at my words as usual).

Soooo, let’s imagine ourselves as totally dumb for a moment, what happens there: the fumes have to be drawn off. There must be no mess in the exhaust pipe, even if the fumes are grease-laden vapor. So a filter has to go in. Unfortunately, the filter slows down the exhaust. This braking moment has to be "overcompensated" by appropriate stimulation of airflow. We let a fan take care of that.

The fan draws the fumes through the filter. Behind the filter is initially a funnel-shaped chamber (due to the fact that the rectangular intake cross-section must be converted into a round pipe).

In this chamber there is a swirling (caused by the interaction of - ideally regular - airflow stimulation and the complex and moderately asymmetrical shape of the funnel). This swirling continues into the pipe. Thanks to its non-axial impact (and because its wavelength is never quite adequate to the resonance frequency of the pipe) it screws itself elliptically through there. In the pipe, the spring stiffness of the air column opposes it.

In the process, the fume draft compresses the air column in the pipe, its coil spring shape supports it in this drilling work. When it reaches the outlet opening, the system begins to oscillate continuously - pulsating only because of the weight of the flaps that close the outlet opening - until the fan stage is switched.

The perceived noise is not only dependent on sound pressure, but also on frequency - with a thicker pipe, longer wavelength means deeper tone.


Thicker pipe = better? - not necessarily:

With pipe diameter, the outflow speed of the coiled column of fumes increases, but also: the spring stiffness from the weight of the air column in the pipe.

Which of the two effects wins also depends on the ratio of the segments in the pipe: if a large part of the pipe length runs horizontally, the speed will increase with a thicker pipe; the greater the vertical section portion, the more the air column loads.


My suggestion would be: don’t work on the air column with thrust alone (as one does in the classic case, where the only fan sits immediately behind the filter, i.e. at the start of the tunnel), and also not with pull (i.e. a fan only at the outlet), but with both combined. Rotating at the same speed, this (as optimal as theory unfortunately only is) effectively shortens the pipe length to zero - at least as far as spring stiffness is concerned.

All clear (?)
 

Knallkörper

2017-05-23 23:46:40
  • #5


Hello 11ant,

I read your post with great pleasure, but I cannot follow you factually on the quoted passage. In general, the flow velocity decreases with increasing pipe cross-section. The "spring constant" also decreases, since a lower pressure will establish itself in the larger pipe.
 

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