Does a hose nozzle work as a venturi or a restriction on a chimney?

I remember somebody saying that if you close up a chimney with a baffle that is just a metal plate that closes over it, that it would be friction and slow down the fire.  Does a hose nozzle work as a venturi or a restriction?  If you put your finger over a hose, it will cause it to shoot out fast.  I think that slowly closing the chimney, but still have it open, would speed it up, but also at the same time be a restriction that would allow soot to burn up completely (actually in a restriction that speeds it up, soot particles should shrink just on that).  And as long as there is some tiny flow, you should be able to like a hose nozzle close it down completely but really burn up those soot particles and shrinking them too, but closing it down like that may require a hose nozzle.  Also, since it would speed it up even if just a tiny leak, the fire due to high speed tiny particles may burn just as well but more efficiently giving off the same heat.

I'm pretty sure that it wouldn't work anything like how you are hoping it will.

Here's a link about water pressure in hoses which might help.  Basically, as I understand it, sticking your thumb over the end increases the pressure of the water that comes out, but reduces the overall flow considerably.  Try it with a hose and bucket and time how long it takes to fill up with and without your thumb over the end. It might surprise you!

Does using a smaller pipe increase water pressure?

This looks useful too - Reducing Flow vs Reducing Pressure - which is it?

'k, there are a whole bunch of questions here - but first I want to lead off with a warning:

Don't block the flue of any kind of stove.  This is how people die from Carbon Monoxide poisoning.  You potentially get that with any kind of flame stove (gas, wood, coal, whatever) because of incomplete combustion.  CO is odorless, tasteless, heavier than air and if your stove starts making CO it will probably kill you.

Firstly, a hose nozzle increases the speed of the water exiting the hose (if there's enough water feeding in from the other end).  It does that by restricting the volume of water exiting the hose, and so making more pressure.  Thus the water comes out faster so you can jet it across your garden, but it takes longer to get the same amount of water.  You can easily try this by filling a bucket from your hose, first with and then without the nozzle.  Unless there's only a trickle of water coming out of the tap, the bucket will fill faster without the restriction.

Secondly, a stove and its flue aren't a garden hose and for it to work well you want different things.  

With the hose nozzle you want the increased pressure so you can squirt the water the other side of the veg patch without needing more hose.

With a stove you want complete combustion to make it efficient and as clean as possible.  For combustion you need oxygen, which is around 21% of the air - so, you need air.  The more volume of air you can push through the stove, the more fuel you can burn.  The whole system [from the air intake into the house to the flue] needs to maximise the volume of air coming in.  If there's not enough air, you don't burn all the fuel or it burns slow, leads to un-burnt carbon (soot) which clogs the flue, for example. [1]

You can get more air through your stove a few ways.  

The simplest is to design a stove with a nice big air intake, nice big unobstructed straight flue ('cos bends slow the gas down).  This stove will burn fast and clean but a lot of the heat shoots out the flue, meaning you have to burn more to heat the house.

The rocket stoves work by making a small fire burn really fast and clean - the hot small fire makes a fast gas flow in the stove, sucking air in faster  - but then extracting the heat from the flue, e.g. by heating a mass, so the heat isn't lost.  This is more efficient.  Compared with a regular stove, the air intake and the flue are big vs the size of the actual fire.

You can also used forced draft.  This is what many car and truck engines now do - and essentially, they're all stoves too.  Rather than just sucking air in, they blow air in with a compressor.  That means you get to burn more fuel in a small engine which is more efficient.  A blacksmith's forge uses a blower to force more air into the fire, making it burn fast and hot.

All of these stoves have an unobstructed flue.  Anything that obstructs the airflow, anywhere in the system, will make your stove burn less well. [2]

Forced draft is mostly done with a fan, but that means you rely on power for your stove to work.  Power goes out your stove burns badly if at all.

You mentioned a venturi - for anyone unsure about what that is, here's a picture:

I can see how the venturi principle could, in theory, make forced draft in a stove using wind.  The flue from the stove would enter in the middle, where it says "pressure least" in that picture.  However, it would have to be huge to work, would need to rotate like a windmill to face into the wind, and wouldn't work on days the wind didn't blow.  I doubt it'd be worse than a simple flue if the wind wasn't blowing, but I'm fairly sure the increased cost and complexity of building it wouldn't pay off.


footlingnotes!  'cos I'm a scientist by training

[1]When you burn stuff, you use up the oxygen in the air but you create more gases out of the stuff you burn.  'most everything we burn is hydrocarbons, so you make CO2 and H2O.  Can't get around that.  You can, however, burn as completely as possible, thus maximising the energy from the fuel you put in, and reducing the emissions generated by making the amount of heat you want/need.
[2] OK not the cars.  but that's 'cause people don't want them to be noisy.  They work better without a muffler

If your solid wood heater is having trouble with draft, the easiest way, and cheapest way to improve it, is to make the stovepipe taller. Adding only a few extra feet in height makes a radical difference. This is why the old brick chimneys of the old days were so tall. It was believed wrongly that it was to get the smoke up out of the city, but it was really about natural draft!

You can also get more draft by using stovepipe that is double or triple walled. The reason is, with single wall stovepipe, the cold outside air acts upon the thin steel, and cools the smoke as it rises, reducing draft. This is why masonary chimneys work better then steel ones; over time they warm up, and stay warm improving draft.

Powerdrafts really are not a real solution to the problem of a lack of draft to a stove because, when lighting the stove, the powerdraft has to be off until the stove is adequately going. Without natural draft, the smoke will just pour into the house.

Making the chimney into a restricted nozzle just slows down the flow, unless you stick a big electric fan on the inlet. A proper sized venturi on the outlet would only increase the air volume after the venturi. Wouldn't help any with the combustion inside.

The methods to improve combustion is more heat (burn chamber insulation), and sizing the system so the combustion pulls in enough air properly.  

Gasifiers do all sorts of cool stuff with restricted flows and starved combustion. But that’s not the principle with a rocket stove. I just read a great article about the thermal unit in Cycle World magazine. A car running full throttle is pumping out a million or so thermal units. A racing engine can a billion or so thermal units!. With something like 80% being unused, wasted, heat. And lots of moving parts to wear out. The beauty of rocket stove is no moving parts and heat catpture. But they all need to breath. Some tractor pull engines have turbo chargers in series feeding each other. Crazy stuff! A typical modern stick and mortar house has a 150K thermal unit heater if memory serves.
With rocket stove I’d like to try a draft inducer on the exit of the flue. Especially if there is no chimney but simply a horizontal exhaust. Is there a thread on this?? I’m sure it’s been tried. The heat shouldn’t be enough to damage a big computer fan? It would be interesting to have speed control to see what it does.
I learned CO poisoning can be cumulative. Build up a little everyday in the blood . Ever since learning this I’ve been warning people. Be careful, stay alive. With my luck I’d just get more brain damage.

Another factor is differential between the heat in the pipe verses the heat outside the pipe.      If you want more flow start with an insulated pipe, and also to get the stove started  a bypass from the thermal mass and going directly to the chimney also can help jump start the flow.


Just like a carburetor on a car there is the right air mixture for a given fire / setup,   you can go by the books out there and save much experimenting time, or you can experiment and go where no one has been before, and loose lots of sleep like I do ;-)

But  to your point it is not like a venturi, more like a vortex if you inject air with the right air diameter in the burn chamber in the right way.

I am sure that it does pull in more air but it also helps to burn the gases with more oxygen.

One thing to keep in mind too is, hot air does not rise, it never has, nor will it ever according to the laws thermal dynamics.

Warm is less dense, and so cool air rushes in to displace it.

This phenomenon is easily understood if you note how the air is under draft UNDER the coal bed; in other words, under the grates. That is air that is not being warmed, so it should not be rising, but it is because of convection, it is cool air replacing the warmed air inside the stove.

Travis Johnson wrote:One thing to keep in mind too is, hot air does not rise, it never has, nor will it ever according to the laws thermal dynamics.

Warm is less dense, and so cool air rushes in to displace it.

This phenomenon is easily understood if you note how the air is under draft UNDER the coal bed; in other words, under the grates. That is air that is not being warmed, so it should not be rising, but it is because of convection, it is cool air replacing the warmed air inside the stove.

Explain to me how a hot air balloon works then...

Travis Johnson wrote:
One thing to keep in mind too is, hot air does not rise, it never has, nor will it ever according to the laws thermal dynamics.

Warm is less dense, and so cool air rushes in to displace it.

The way I always saw it was that warm air is less dense than the cooler air around.  So it rises through the cooler air, which then rushes in to replace it.

the air is under draft UNDER the coal bed; in other words, under the grates. That is air that is not being warmed, so it should not be rising, but it is because of convection, it is cool air replacing the warmed air inside the stove.  

I think the warm air expands, becomes less dense and rises up the chimney, which reduces the pressure below it, so that air enters to replace it, cool or otherwise.

But essentially, warm air DOES rise.

Austin Shackles wrote:Anything that obstructs the airflow, anywhere in the system, will make your stove burn less well.

In absolute terms, I won't second that. In a batchrocket there is an obstruction, smaller than anywhere in the system and in spite of that these stoves are burning extremely well.

Namely the detail that is known as the port, which is 70% of the chimney's cross section area. See http://batchrocket.eu/en/. This design is using the phenomenon of "pressure drop through an orifice" (Bernoulli's theorem) to add secondary air in order to create an afterburner function. Development of this system has been done using a Testo gas analyzer throughout.

Presumably, there could be some debate whether this port is an obstruction or rather a restriction. The difference between those terms is not christal clear to me, English isn't my first language.

Well the prefix "ob" means "towards", or "Over", or "Onward" whereas the prefix Re means "back" or "Again".

In this situation though I prefer obstruction because the part allows air to move over it. The net effect is that this Venturii Effect brings in more oxygen which is what causes combustibles to readily burn.

I cannot recommend masonary stoves enough. Slow safe warmth that is very efficient and comfortable and low maintenance. Also good for cooking preserving, water etc. https://www.mha-net.org/

Mart Hale wrote:

Travis Johnson wrote:One thing to keep in mind too is, hot air does not rise, it never has, nor will it ever according to the laws thermal dynamics.

Warm is less dense, and so cool air rushes in to displace it.

This phenomenon is easily understood if you note how the air is under draft UNDER the coal bed; in other words, under the grates. That is air that is not being warmed, so it should not be rising, but it is because of convection, it is cool air replacing the warmed air inside the stove.

Explain to me how a hot air balloon works then...

The ambient air, cooler than that in the balloon, is denser and heavier, and so settles at the lowest point possible. The warm air is displaced by the cold, and the bag and basket go with it.

It's much like the phenomenon experienced when you hold your hand over cold rocks on a hot day. It feels like the cold is radiating towards your hand, but in actuality, the heat energy in your hand is travelling to the area of low-energy, trying to reach a state of equilibrium. "Cold" is simply a lower energy state, not a separate, equal force to balance "heat."

-CK

Apologies if this posted twice.

Ted:

Unfortunately there is quite a bit of misdirection in the replies below and not wanting to offend anyone I will try to give you some guidance in principle and for your application.  Please everyone, tolerate my alternative interpretations as just that.

>I remember somebody saying that if you close up a chimney with a baffle that is just a metal plate that closes over it, that it would be friction and slow down the fire.

This is correct, such a device is called a chimney damper for the reason that you "dampen the fire" in the sense of slowing down the burn rate. A damper is quite permissible provided that it is not able to close off the chimney completely. There are often regulations about this where the area can be reduced by 2/3 leaving 1/3 open when fully closed.   This is essential for late fire emissions to escape.  Most stoves (including rocket mass heaters) have poor combustion in the late fire due to high excess air (far too much air for the fire's requirement) and poor control over it.  

Mostly, it is not a good way to control a fire's burn rate. In all cases possible, the system should be constructed so as to limit the ingress of air, not the egress of gases. When the entrance is limited, and the chimney is pulling, any leaks permit air to enter the stove, whereas in the opposite case the gases inside try to leak out.  A negative draft maintained on the top of a fire should be about 20 pascals if you can arrange it.  That is very little, but it means a warm chimney pulling and air entering the stove through the controller with the entrance as the restriction on the system.

CO leaking out of the stove into the room is not heavier than air. CO2 (44 mole mass) is and sinks, but not CO. CO is almost exactly the same density as air (28 mole mass) and disperses very effectively throughout the room causing problems. The mechanism is it displaces oxygen on the red blood cells so it lowers the oxygen carrying capacity of the blood.

>Does a hose nozzle work as a venturi or a restriction?  

Anything that reduces the flow up a chimney will reduce the exhaust gas flow rate.  The temperature of the chimney above and below the restriction will change if you change the amount of that restriction.  This will affect the total draft on the stove. Generally speaking, all stoves in the USA have exhausts that are too large in diameter to work properly, the chimneys are too large in cross-section, and the exit temperature is too low to be effective.  That is a pretty broad condemnation, but it is just how it is.   Most stoves require a barometric draft regulator but do no get it. If the chimney is hot enough to work properly it tends to generate thermal runaway: more heat leads to more firepower leads to more heat in the chimney leads to more firepower.  That is not how most people want their devices to operate.

Draft in a chimney is a source of "power".  You could use a fan, but a warm chimney is capable of "doing work".  So a chimney is really a natural fan.  The power available can be directed in various ways: You could turn the energy into a "suction" on the fire and the air entrance (the preferred option). You could dissipate the energy creating a high velocity flow gases past the restrictive plate you are considering, slowing down the total flow.  Given any constant temperature in the chimney, the available energy could pull a lot more air quickly, or more slowly but having a high flow rate past the restriction point.  You may have a reason for wanting that.  You could use a small diameter chimney so the energy was dissipated along the whole pipe length limiting the flow to a certain desirable rate.  Usually that is a good idea.  The main point is there is energy available and you can choose were it is applied.  It might be at the secondary air entrance adding velocity (one of my favourite design ideas.)

Now for your main motivation:

>...also at the same time be a restriction that would allow soot to burn up completely (actually in a restriction that speeds it up, soot particles should shrink just on that).  

What you describe is a fire that is not burning cleanly, followed by some attempt to fiddle with the flow rate of the gases in order to reduce the smoke generated.  It is already too late. If the fire is not burning cleanly in the first place, it is far better to burn better than to try to clean up what should not be there. Slowing down the gas flow rate in the chimney is very unlikely to correct a bad combustor design.  That is not how things work.  Poor combustion has several classic causes so addressing them is a better approach.

>...but really burn up those soot particles and shrinking them too

Smoke particles don't "shrink".  They grow, rapidly.  You are quite correct that they are combustible. Smoke is almost entirely made up of unburned fuel and water vapour. A small amount of a typical stove's emissions is ash, not the products of incomplete combustion. Creating good operating conditions for combusting biomass requires meeting certain conditions.

One is an oxygen level of about 8% to 10% in the exhaust.  This is air which is not required to burn the fire.  Biomass is not as easy to burn as gaseous fuels which can tolerate very low excess air levels (even 1% or less O2). If you have 18% O2 in the exhaust, that is 600% excess air and it FAR too much to burn cleanly.  

The calculation is:  Measured Oxygen %/(21% minus measured Oxygen %)  The answer is the excess air factor, so multiple by 100 to get %.  
Note that it is not the total air demand, which is Excess air + 100% also called "Lambda".

All that unwanted excess air cools the combustion.  Slowing down the exhaust does not guarantee the O2 level will drop. It might, it might not.  It depends on the architecture of the stove.  But one thing for sure, it should not be more than 10% when running on high power. If you have a technically sophisticated burner, you can get it down to 6% but that is not for the ordinary hacker. Instrumentation is required.

You are correct that if you burn the smoke it will yield more heat.  However burning the CO gives far more heat than burning the smoke. C to CO yields about 8 MJ per kg of fuel. CO to CO2 gives more than 24 MJ/kg.  So if you lose a lot of energy to CO (bad combustion), you can get it back by burning the CO instead of carefully venting it outside. Emitted CO is a called a "chemical loss".

To keep CO burning needs about 820 C above the combustion zone. This is NOT provided by blowing harder on the air supply, which will just force more cold air though the chamber, cooling the fire. Blowing on a charcoal or coke fire like a blacksmith usually create a lot more heat, but huge amounts of CO.  Such a fire is not a good example of clean combustion at all.  Hot, yes, but not clean.

To get a clean, hot burn, your fuel load should match the grate area and a lot of other things.  Most stoves have:

too large a grate
too large a fire chamber for the typical fuel load
too much primary air
too large a chimney diameter
too little insulation around the combustion zone to maintain a hot fire needed to light the CO produced
too low an operating temperature
too little draft or
Too much draft without a barometric draft regulator

Putting an air control at the air entrance is how to control the fire intensity.
Secondary air (if supplied) should be approximately twice the primary air supply and where possible, preheated. If you want to burn wet wood, the primary air has to be preheated (big emissions reduction).

Most stoves have far too much primary air so any secondary air is just a waste of energy as the air goes in, is not used or needed, and carries heat up the chimney.

Very clean burns can be obtained without any grate - see Masonry Heaters available from the Masonry Heater's Association website. If the floor of the chamber is made from firebrick, effective combustion can be maintained for a long time. Recent developments in combustion include placing a layer of small stoves (20-30mm) over the whole fuel bed (still no grate).  This may not suit people burning 24/7 however, try it and see the difference.  It reduces smoke generation a lot in some cases.

Very high performance stoves (called HELE for high efficiency low emissions) have a surprisingly low chimney gas velocity of 150 to 250mm per second in a chimney that is about 1 square inch (6.5 cm^2) per kW of firepower. When you compare that with what is usually provided or "required" you may be surprised.

General rules are: burn first, clean and hot.  Then transfer heat to some surface, then vent what is needed to operate the chimney properly. Do not use a chimney as a heat exchanger save for the first few feet.  Once heat enters the chimney, keep it there to operate the draft into the stove properly. You gain nothing from a warm chimney after it departs the heated envelope, i.e. above the ceiling.

Good luck with your investigations. Modern HELE stoves can be see (drawings and all) in the Library of www.newdawnengineering.com under Library / Stoves, then by country.  One you might check out is the KG2.5 under Kyrgyzstan.  Have a look at the function of the "flame tube" which burns smoke and CO as it enters the heat exchanger. It is so effective that the top of the heat exchanger serves as a third cooking position.  The Kyrgyzstan project is quite interesting. You can read about the development of the main stoves here:

https://openknowledge.worldbank.org/handle/10986/31282
(click the download button)

A precis is available at https://openknowledge.worldbank.org/bitstream/handle/10986/31774/Beyond-the-Last-Mile-Piloting-High-Efficiency-Low-Emissions-Heating-Technologies-in-Central-Asia.pdf

And the dramatic impact it produced on people here:

https://www.nature.com/articles/s41533-019-0144-8.epdf

Stay well.
Crispin

Impressive. How is the design for burning wood using cross draft gasification advancing?

The venturi is generally misunderstood. It is a constriction used to create low pressure for side suction (like gasoline into the venturi in the air intake throat of a carburetor). Any restriction adds friction to the system and takes out energy. Nozzles and venturis increase flow speed not pressure.

Flues can be lengthened (to a point, eventually the extra cooling reduces draft) or insulated to increase draft.

The best source of information on woodstove draft is Chapter 5 "Chimneys" in The Woodburners Encyclopedia, which is available used from various sources.