Increasing Ventilation by adding a blower at 45' to a stack

It a form of a Venturi effect with a Bernoulli Equation.

To get the best efficiencies one would have to model it.

I had done something something similar in a different way.

It was for a cooling line I did at a process plant, where the main line was 18” diameter running 9,000 GPM, with 4” laterals coming off. My boss shared with me that he had done a CFD model where the laters are at a 45 degree angle with the flow, the delta P had been reduced and would like me to try that.

as far as your situation, the effects may be nominal… but without actually numbers to run it’s only speculation.

Stack is 8 Inches dia and blower fitted at 45' angle is 50 cfm capacity run by 1 HP Motor. Objective is to assist normal flow of hot gases from the furnace. May be it will give you some idea about my application

You’re looking for answers and not insights… ?

I would like to know how to get best results

for myself, I do not have CDF software to do any kind of analysis that your require I can only speculate.

PWSlack has good advice,...

Good luck

A 1 hp motor that is only producing 50 cfm is very unusual...What would be the discharge orifice size? 50 cfm would ordinarily only require a small fractional hp....an influx of air that is less than or equal to the existing stack velocity, would probably slow the stack discharge velocity due to increased turbulence...

The discharge orifice size is about 2 inches and the air from the blower is entering the stack at an angle of 45' degrees to the flue gas motion to assist and accelerate the flow further and not to impede the natural draft.

It would be simpler to cut the <...stack...> where the <...discharge orifice size is about 2 inches...> so that the stack is <...8 Inches dia ...> all the way to the top. That certainly would increase the draught without any need for <...adding a blower at 45'...>.

Try that first.

Yes. A local Process Engineer can.

https://www.britannica.com/science/Bernoullis-theorem

More supply air at the burner I would think...why do you need more air flow, not exhausting the fumes properly? This could be a cracked heat exchanger....need more information....

https://www.eng-tips.com/viewthread.cfm?qid=128511

It can increase the draft, as far as combustion air that’s dependent upon the combustion fan…

The important question here is what is the problem...You wouldn't want to increase the exhaust air flow for no reason...If the stack is overheating, or fumes not exhausting, would be 2 reasons, both could be corrected by opening up the inflow air...but there could be and unstated problem...increasing the velocity rather than the quantity of airflow, could present its own problems...

Increasing air flow in the stack will create a negative pressre in the chamber and this will cause sucking of air from outside into the chmber for which a small pipe is already provided. Introduction of fresh air or constant supply of oxygen is part of the chemical process being conducted in the furnace chamber. Hope this explains.

One doesn't need an excess of air in the combustion chamber, though. Such a thing increases fuel consumption, as more of it is needed to achieve a certain temperature at the heat exchange surfaces. Further, depending on the nature of the materials in the <...stack...>, the flue gas exit temperature ideally needs to be kept above 135^oC; as anything less may cause droplets of sulphuric acid to form by condensation. Sulphuric acid will rot the <...stack...>, starting at the tip and working its way downwards.

Balancing the combustion air so that it is just enough is a better course of action than introducing too much by increasing the draught in the <...stack...>.

It sounds as though letting a local Process Engineer take a squint at this installation would be a wise investment.

At this juncture, #11⇓ remains unanswered.

So if the aim is to just control the inflow supply air to a specific location in the chamber, a blower could be used to create a positive pressure on your inflow pipe, this would adjust the total inflow to the proper quantity makeup flue discharge...you could convert the fan motor to DC and add a speed controller to make it adjustable/tweakable...then you could add a flue gas in line exhaust fan and do the same thing...

This way you could control the temperature for the combustion chamber and stack...

Yes, you're right SE, I like to add then any changes can affect the dwell time in the combustion chamber. If that an Oxidizer, that will affect the process.

PWSlack was correct in have a Process Engineer look into it, where he can gather critical information on the process itself.

The perceived need for doing so will be of interest to the forum.

My replacement furnace in my house (5-10 years back) has a squirrel-cage blower right in the chimney pipe that starts whenever the thermostat calls for heat. After a delay, the fire starts. This I assume is to make sure there is flow up the chimney so smoke doesn't go in the house. The motor is a large shaded pole type (smaller that 1/4 HP).

Putting your fan right in the pipe will increase suction.

If your furnace is gas fired,the fan starts before ignition to purge any possible build up of gas in the combustion chamber..a safety feature.

There is a type of Dyson fan with a toroidal plenum of relatively high pressure low flow air supplied by a blower, which induces a lower pressure higher flow of air through the ring. If such a unit exists matching the size of your stack, it might be the way to go.

Here is the idea: https://en.wikipedia.org/wiki/Air_multiplier

The question "what has changed since the <...stack...> was installed?" needs to be answered.

How about a Pitot tube in reverse?

How about a ssecorP reenignE in reverse?