Air flow through small rectangular opening

Click For Summary

Discussion Overview

The discussion revolves around calculating air flow through small rectangular openings in a furnace, considering factors such as temperature differences, pressure differences, and the implications for combustion efficiency. Participants explore theoretical approaches and practical considerations related to airflow in this specific context.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant seeks to calculate airflow through a small rectangular opening with specified temperatures and pressure differences.
  • Another participant requests more details, including a diagram, to better understand the problem.
  • Some participants propose using Bernoulli's equation to estimate velocity based on pressure differences, noting that the resulting airflow will likely be low.
  • One participant highlights the need to account for both pressure and temperature differences when calculating airflow, suggesting that air density changes due to temperature must be considered.
  • A participant shares a related experience with air ingress in furnaces, suggesting that localized sealing of problem areas can mitigate issues.
  • Another participant recommends performing an order of magnitude calculation to assess the mass flow, emphasizing the importance of understanding the flow path length and temperature profile.
  • One participant mentions a practical method for assessing air leaks using a lighted candle to identify larger leaks.
  • A participant expresses the need for a new sealing concept for peep doors, indicating a desire to perform a benefit calculation based on air ingress losses.
  • Another participant suggests a bag test to measure airflow around closed peep doors, proposing a method to quantify air ingress.
  • One participant requests the equation being used for rectangular orifices to evaluate how to combine pressure and temperature effects.

Areas of Agreement / Disagreement

Participants express various viewpoints on how to approach the calculation of airflow, with no consensus on a single method or solution. Multiple competing ideas and methods are presented, indicating an unresolved discussion.

Contextual Notes

Participants note the complexity of combining pressure and temperature effects on airflow, and the need for additional information such as flow path length and temperature profiles. There are also indications that practical considerations, such as sealing methods, may impact the theoretical calculations.

marcske
Messages
3
Reaction score
0
Hello,
I have to calculate the air flow through a small rectangular opening with following data:
- temperature at one side: 1200°C
- temperature at other side: 20°C
- pressure at hotter side is 3 mm WC lower than the colder side
- the length of the opening is 240 mm
- the width of the opening is 1 mm

Does anyone know how to calculate this?
Thanks in advance for your appreciated help.

Marcske
 
Engineering news on Phys.org
Need more complete description of problem . Could you provide a clear diagram and some explanation of what is actually happening ?
 
I'd use Bernoulli's equation to calculate the velocity based on the pressure difference. The answer is going to be very low though.
 
russ_watters said:
I'd use Bernoulli's equation to calculate the velocity based on the pressure difference. The answer is going to be very low though.
And even lower than that, I think, as due to the increase in temperature, the air will expand going from the cool side to the hot.
 
The actual problem is small openings in a furnace that is under a slight vacuum. There are several openings of an average of 1 mm by 240 mm (appx. 100 - 150 openings). I would like to calculate the losses related to the excess oxygen that you need for complete combustion and the extra air that you need to heat from ambient to 1200°C in the furnace. I know that the air flow through 1 opening will be very low, but the total may be sufficient to do something about these losses. My first thought was to use an equation for rectangular orifices, but the flow is not only driven by the pressure difference, but also the difference in temperature (air density). I do not know how to combine both effects.
 
I've met this problem before in furnaces made from loose blocks with many very small gaps between blocks . Ingress of air caused localised chilling and oxidation of furnace charge . In the end we identified some areas of blocks as causing problems and some not . Problem areas were just sealed with cement and no further difficulties were experienced .

Before doing any more complicated analysis try doing an order of magnitude calculation - assume maximum density air everywhere along flow path . If mass flow calculated is very small then no further work is needed .

To work this problem out in detail the length of the flow path would be needed and some estimate would have to be made of the temperature profile along the flow path .

Bear in mind though that with such a small pressure difference convection currents inside furnace and draughts in ambient air might affect flow rates significantly .
 
Incidentally we did our first assessment of where the larger air leaks were by going around all the joints with a lighted candle - you don't always need high tech methods !
 
Thank you for your reply. The openings here are around peep doors and cannot just be covered but need a new concept of sealing. As this implies a serious cost, I would like to perform a benefit calculation based on the losses by air ingress.
 
Try doing a bag test . Large plastic bag blown up but not pressurised with mouth of bag held around closed peep door and sealed with Duck tape . Known volume of air in bag . Time to collapse .
 
  • #10
marcske said:
My first thought was to use an equation for rectangular orifices, but the flow is not only driven by the pressure difference, but also the difference in temperature (air density). I do not know how to combine both effects.
Could you provide the equation you are using and source so we can evaluate it?
 

Similar threads

  • · Replies 13 ·
Replies
13
Views
2K
  • · Replies 9 ·
Replies
9
Views
2K
Replies
1
Views
3K
Replies
24
Views
3K
  • · Replies 45 ·
2
Replies
45
Views
7K
Replies
4
Views
4K
Replies
3
Views
3K
  • · Replies 2 ·
Replies
2
Views
4K
  • · Replies 6 ·
Replies
6
Views
7K
Replies
7
Views
4K