Firewood Furnaces

[Willy]

Does moisture (in superheated state) improve the performace of a furnace in any way?

 

What are the parameters to be considering when evaluating this?

 

Does the fact that moist air has a higher heat capacity (Cp) have anything to do with this?

[chitrangda]

Furnace is designed depending upon its kind of use.parameters are decieded on the type of furnace.

example

An industrial furnace is an equipment used to provide heat for a process or can serve as reactor which provides heats of reaction.

Furnace designs vary as to its function, heating duty, type of fuel and method of introducing combustion air. However, most process furnaces have some common features.

[Willy]

Is it implied that there is no connection with the Cp of moist/dry air?

 

for the simplicity, let us take the same furnace recommended for fire wood.

To get the optimum output, in terms of fuel efficiency and heat transfer, what are the parameters we need to consider?

one parameter is the moisture content of fire wood which will give the actual weight of fuel and energy needed to vapourize and superheat this water.

 

what are the parameters we need to consider?

[CaptainPanic]

The parameters that are of importance are:

 

1. The heat of combustion (J/kg). This is about 17-19 MJ/kg for dry wood, and it can be as low as 8-10 MJ/kg for wet wood.

2. Cp value of the products. Air has a Cp of 1000 J/kgK, CO2 has a Cp of 0.82 J/kgK and water (gas) has a Cp of 2.0 J/kgK. So, the more water the gas contains, the lower the temperature.

 

With this you can calculate the temperature of the flame, which gives you an efficiency. Simply said: hot fires are more efficient.

 

Take for example a steam engine (considering the efficiency, it is a typical heat engine). It cannot be more efficient than defined by Carnot. It's alright if you don't understand what the formula's on that wiki-page mean... The bottom line is:

 

[math]\eta=\frac{W}{Q_H}=1-\frac{T_C}{T_H} [/math]

where

η is the efficiency

W is the work done by the system (energy exiting the system as work),

QH is the heat put into the system (heat energy entering the system),

TC is the absolute temperature of the cold reservoir, and

TH is the temperature of the hot reservoir.

 

You see: the hotter, the more efficient. The TH here is not the temperature of the fire, it's the temperature of the water in the boiler... so a very hot fire can make really hot steam (and really high pressure, and that's the main problem with high efficiency). And really hot steam will make an efficient system.

 

 

If you're just considering to heat your house: the only thing that really matters is the fact that you have to evaporate a lot of water. Because this water will not condense in your heating system, it's a pure loss. For the rest, it matters how hot the smoke is coming out of the chimney. The heat produced can only go in 2 "places". It can heat the house, or leave through the chimney. I think that wet wood combustion will result in (relatively) more heat leaving through the chimney.

 

I like the fact that you can just copy paste formula's from wikipedia. Makes life easy

Edited August 29, 2008 by CaptainPanic

[Willy]

Wow thx. that's an overload of technical stuff. will take some time to digest. dont get how that equation is related to what i'm asking.

 

but i'm clearer on the issue now from what CaptainPanic and book worm say. this is what i understand. feel free to comment/correct me.

if the furnace is not designed for dry wood, then we shouldnt use dry wood. because that is not going to give as much performace as you expect and the flue gas temperatures will be high. in other words you'll be wasting energy.

[CaptainPanic]

That's true... especially for big installations. For the small ones that you use for heating your house, it's less important if the wood isn't completely dry. If it's still fresh, I don't recommend burning it in any case.

 

You came to the scienceforum... expect an overload of technical (and scientific) stuff.

(If for some reason you think it's too much, I can recommend the happy campers' forum for cozy campfire discussions... in any case, they'll also recommend dry wood).