Hot furnace air removal via venturi

[Carlo1982]

Hi all,

We are designing a new plant in which we will operate several furnaces.

Maximum temperatures will be around 550°C which needs to be vented outside the building. Instead of using a high T ventilator for sucking the air out, we are thinking of using the venturi principle. I've added a simple sketch.

Anyone experienced with this type of setup? Main questions are:
- angle between furnace and ventilator tube (we don't want air going back to the furnace);
- flow of the ventilator;
- length of ventilator tube (laminary flow required?);
- ...

Any comments are more than welcome.
Thanks,
Carlo.

 

[LittleInch]

Well a branch as shown won't work.

At the junction you need an educator - something like this

https://commons.wikimedia.org/wiki/File:Eductor_pump.svg

Many people offer these.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Why do you need this arrangement? If exhaust flue gas is at 500degC, then install a common stack that discharges at a high enough elevation that will induce the flue gas out to atm by natural buoyancy effect. Ask a process engineer to compute the diameter and height required for this common stack to develop the necessary draft.
Its a shame these days to be disposing of a high energy stream to the atmosphere - why not try to recover some of this heat with a waste heat recovery unit so that final exhaust temp is much lower. Stack height can be made higher to compensate for the additional pressure drop incurred.

 

[TiCl4]

The only reason to use an eductor system as stated by LittleInch would be to provide some level of vacuum on the furnace, i.e. to provide additional air draw because natural ventilation isn't enough. At that point, you pretty much might as well have a forced-air furnace. I'm going to assume you have natural ventilation furnaces, as you have not indicated otherwise. Georgeverghese has provided a cheaper option for increasing natural ventilation rate, provided there aren't height restrictions on stacks in your planned installation location.

Start with your furnace air requirements and design the stack(s) to meet the required flow rate. I'm not all that familiar with furnaces; You could manifold the flues together, but you would have to put in precautions to avoid back-flow in the case that a furnace is down. Depending on your number of furnaces, arrangement, and level of automation available, it may be much cheaper to vent all furnaces to a common stack rather than have individual ones.

You will not develop much available pressure from the stack effect - maybe few hundred Pa using the calc from the link below. You'll need to be careful in not only your flue sizing but also your building design to ensure you don't restrict airflow into your building.

Stack effect:

https://en.wikipedia.org/wiki/Stack_effect

 

[Carlo1982]

Hi all,

First of all, thanks for your replies.

1/ we cannot put the exhaust through the roof, we have to go up and horizontally through the outside wall. A stack is not feasible;

2/ the furnace has a forced-air exhaust (800m³/h), but can only 'throw' the air 9m further which is not enough to reach the outside wall;

3/ to avoid forced extraction of furnace air (by the ventilator pipe), we will/can install a Chinese cap on the exit of the furnace exhaust;

Thanks.

 

[georgeverghese]

If you cannot place a free standing stack outside this wall due to some space limitation, then would suggest an induced draft fan or blower ( ID fan) rather than this concept which is a crude version of an eductor. Main reasons being (a) eductors are low efficiency devices and the associated forced draft fan ( FD fan) will consume 4-5 times more power than the ID fan concept (b) there is no guarantee this concept will work.

There are high temp ID fans operating at similar temperatures even in electric arc furnace service, which draw out high temp fumes from the furnace. Same for similar blowers in FCC regenerators in oil refineries.