Forced convection for a wood burner 3

[Stoveman]

Its my first post...

Its been over 20 years since I completed my Mechanical engineering degree..and I've forgotten many of the heat transfer calcs.

I am pondering a challenge. I have a 45ft lounge and a woodburner at one end in an alcove. It heats up part of the room, but the other end is cold. I bought a 15" fan and pointed this at the stove and it heated the room up considerably. The problem is that it is incredibly noisey.

I would be grateful if you could help me:
1. How do I work out the difference in stove efficiency, i.e. heat transfer into the room rather than combustion, for forced vs natural convection. I've googled but it looks more complicated than it probably needs to be and I don't have some of the source information for the parameters. Can anyone help or point me in the direction of existing analysis.

2. Advise on whether I would be better off with a film of air over the stove top through a long slot or just blast air everywhere via a fan (i.e. trying to make efficient use of the airflow).

3. I've looked at the eco fan type arrangement, but air volumes are very low. How much heat can I extract from the stove before I affect the combustion efficiency or condensation of flue gases = chimney fires, and how would I calculate the required airflow?

A huge thanks in advance

 

[IRstuff]

> Heat transfer into the room is the same, regardless. Your stove is putting out a certain amount of energy no matter how the heat is removed from it. To the first order, anyway.


TTFN

FAQ731-376

 

[Stoveman]

Thanks for the reply. I had assumed this too. Heat transfers into the room via radiation too. However, my experience is that with the doors closed in the room in winter I can heat the room and keep it at a constant temperature of at least 5 degrees higher with the fan running - with an ambient of 10deg C and a forced convection temp of 23 deg C (rather than 18), a rough calc would suggest that its 60% more efficient. The problem is that I can't get my head around the heat transfer calculations to explain it.

I assume that it is something to do with the temperature differential between the stove and local ambient: natural convection has a smaller temperature differential (the air immediately around the stove is warm) than forcing cooler air onto the stove, therefore the heat extracted with forced convection is greater.

If I can explain it and refine it, I see an opportunity to extract more efficiency from a wood burning stove. But I would like to understand the science, and understand whether its a volume of air issue, or whether its just as efficient to push a film of air over the surface.

Any help gratefully received.

 

[IRstuff]

I suggest you read up on heat transfer; there's a free, on-line text:

http://web.mit.edu/lienhard/

http://www/ahtt.html

However, the short answer lies in the fact that many HT texts include "Mass Transfer" in their titles. That's because the movement of the air molecules, laden with heat energy is what convection is all about.

TTFN

FAQ731-376

 

[rmw]

I assume your wood burner draws its combustion air from within the room. If so, the reason the other end is colder is that the makeup air is entering that part of the room and then traveling toward the stove end. Try finding a makeup air opening near the stove and it will reduce air leakage into the room on the other end and the convection currents will work better.

rmw

 

[Dave41A]

To answer your question about minimum temperatures, I think 250 F is considered the point at which creosote begins to condense. I suggest purchasing a magnetic wood stove thermometer (it sticks to the outside) and keeping your wood stove burning at least this hot (they are usually labelled with the minimum recommended temp). A small, hot burning stove is more efficient and safer than an oversized stove smoldering away. This is because about 50% of the heat energy in wood is in the various volitile compounds that evaporate as the wood is burned. If burned in a low-oxygen environment, these compounds escape up the chimney unburnt--and condense in your chimney as creosote--where they pose a chimney fire hazard. Note that by "efficient" I mean BTU's of heat provided per BTU of wood burned. A modern wood stove is also much cleaner burning (and hence, more efficient) than older (i.e. 1970's) models.

Note also that as the room gets warmer, the draft up the chimney will increase, which will increase the rate of combustion. The room may be warmer with the fan running, but you will also have to feed more wood into the stove...hence, BTU for BTU, "efficiency" likely hasn't changed much.

You probably don't need as large and loud a fan as you are using. I would start with the outside combustion air RMW suggests because drawing make-up combustion air into the room via cracks in windows, etc. is a sure way to cool off a room (modern woodstoves feature an outside air connection). For example, some open fireplaces draw so much make up air from the room that they actually have a net cooling effect on rooms that are also heated by central heat (I.e. the heating load on the central heat system goes up when the fireplace is used, as the heat required to warm the make-up air is greater then the heat provided by the fireplace).

Another thing to consider is an air duct running from one end of the room to the other. An small fan installed within will probably help keep the air well-mixed much better than a big fan randomly blowing air all over the place. The fact that the woodstove is in an alcove doesn't help much either. A longer stovepipe (which would allow placement out of the alcove) might also do the trick.

Good luck,
Dave

 

[Stoveman]

Thanks to everyone for the great posts. My installation is pretty fixed so I probably have to live with what I have, but its really good advice for anyone fitting a new stove.

I'm still pondering how much additional energy can be extracted from a stove through forced convection rather than natural convection. I assume I have to extract a huge amount of heat before I start to affect condensates. I've been reading up on heat sink design, which is a similar challenge. Forced convection makes a massive difference to heat extraction because of the increased molecular transfer of energy, but I can't find any like for like comparisons with natural convection. I also concluded that the greatest effect is by disturbing the boundary layer, where the temperature gradient is the greatest. Turbulent flow appears to improve performance. I assume a curtain of air blowing across the surface is more efficient than just blowing air at the face of the stove. But how do I work out the volumetric flow rate?

 

[Compositepro]

You will get more heat out of your stove with a blower or fan versus natural convection. How much improvement will be very difficult to calculate with any accuracy. An more important benefit to a fan is that natural convection alone will result in very cold floors and very warm ceilings. Placing the stove in a basement was one historical solution to this problem, before electricity. Also historically, the stove pipe would exit a room at a far wall from where the stove was located. This maximized the extraction of heat. This is not allowed by modern fire codes.

I suggest you find a quieter fan. I have a 1940's Emerson 15" fan next to my wood stove. They really just do not make fans of such high quality anymore.

 

[IRstuff]

There are several benefits of forced convection, not all of them necessarily related to efficiency.

> Right now, your convection heat output is basically localized near the stove, and along the surface of the ceiling, neither of which helps you to feel warm. Counting on diffusion or convection is slow. A fan will mix the air more thoroughly, thereby significantly speeding up the warming of the nether regions of the room.

> Thermal transfer is directly proportional to temperature difference. With natural convection, the outer surfaces of the stove at kept at a relatively higher temperature, so heat output is limited. By forcing cooler air to be nearer the stove, there should be some increase in heat output into the room.

TTFN

FAQ731-376

 

[amorrison]

Methinks your problem is not stove heat output but poor natural convection in the room.

Buy two of the 20inch square box fans and arrange them to cycle the air around the entire room to keep the temperature everywhere equal.

 

[gwolf2]

I think amorrison has hit the nail on the head. For my 10 cents, I would seal the room against drafts, put an air vent in the floor just in front of the wood burner and fit a 6 inch horizontal duct the full length of the room at the apex. Starting from directly above the burner and ending above the opposite end if the room. Fit a relatively low power fan into the duct and it will spread the heat around rather than allowing it to a) stratify vertically and b) horizontally across the room.

 

[rmw]

Stoveman,

Unless you are located in deep South America or Southern Africa (or Oz) this seems a strange topic for the weather outside my home right now. Last time I looked earlier this afternoon, the outside temperature was equal to average normal body temperature; 98.6F.

rmw

 

[Stoveman]

I'm in the UK looking our the window now at light rain and about 16 deg C.

I would have to dig up the concrete and oak floor to change the went for the fire. Stoves are rated at 85% efficiency but I'm not convinced that all of this heat is transferred into the room. The challenge remains, how do I calculate the volumetric flow rate assuming I blow air over the surface of the stove? If there are no reliable calcs, I can knock something up and see what works.

I know that inset stoves have forced convection but don't understand why freestanding stoves don't. If it increases efficiency in our green age, why not fit them? The ecofan circulates heat around the room, but it doesn't really extract more heat from the stove - as far as I can make out.

 

[IRstuff]

As an engineer you should do the research. This textbook is free for downloading:

http://web.mit.edu/lienhard/

http://www/ahtt.html

TTFN

FAQ731-376

 

[Dave41A]

Stoveman:

Modern freestanding stoves are available with circulation fans. They don't actually increase the combustion efficiency of the stove--they just increase the convection coefficient from the stove to the air and help circulate the air around the room a little...reducing the temperature variation from one side of the room to the other. You might not see them in use very often as people are apt to trip over the cord! An internet search for "wood stove blower" will return hundred of hits.

Good luck,
Dave

 

[heathexer]

The 'drum' type fans used in auto systems and some 'split A/C units, and forced air furnaces can be very quiet. The more turbulants you create on the stove surfaces the more heat you can transfer. It will not increase fuel burning efficiency. I agree with IRstuff Aug 12th.

 

[Twoballcane]

I would think that laminar would be better than turbulant? A laminar film has more surface area in contact than eddie curves. I would have to look into R, but I'm in the field right now...

Tobalcane
"If you avoid failure, you also avoid success."

 

[rmw]

I remember a line from one of my heat transfer texts while discussing laminar vs. turbulent flow, giving the Reynolds numbers for each of the respective flow regimes and it stated "with laminar flow at Re #'s below 2000, heat transfer is not deemed to occur."

rmw

 

[vpl]

It's often hard to retrofit when the engineering isn't done up front and it might be worthwhile to call in someone who specializes in heating systems (or not -- sounds like the noisy fan does the trick, it might just mean buying a more expensive but quieter fan.)

Having said that, this doesn't seem to be a work-related project, so I'm not sure if it meets the posting policies for the forum.

Patricia Lougheed

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