A fire pit or fire box is the area of the smoker where we burn wood












 








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Fire
Pits



 




A fire pit or fire box is the area of the smoker where we burn wood to
produce smoke or to generate heat to cook meat products. It can be an
integral part of the smoker or free standing separate unit connected to
a smoker by a pipe or digged out trench in the ground. It could be a
smaller barrel or a metal box. It may be made of reinforced concrete or
from firebricks. The combustion efficiency inside the fire box depends on the ways fresh
air is supplied into the burning wood. All smokers employ a very simple
system of draft control as the the main purpose of smoking is to create
smoke at low temperatures and that can be often accomplished without any
draft system.

If we want to control the burning process by using fresh air dampers,
how we place wood controls the rate of burning and temperatures in a
fire box. A baffle can be placed vertically and it will direct heat in a
different direction now. The less clearance between the top of the
baffle, the more heat will be supplied to the top of the fire box.

Note: warm air or smoke will always go up. 









Simple firebox

 Fire box with a vertical baffle



The three classical ways of placing wood in a fire box follow below:

• Directly on the ground, known as the hearth botom-grate
• Above the ground on a grate, known as a stool-grate
• Above the ground on a grate, known as a raised stool-grate

Hearth Botom Grate

The drawing below shows the most common way of positioning wood when
smoking. When wood is placed directly on the ground the whole air enters
frome above. This is the way fireplaces work and though all air seems to
bypass the wood, enough of it reacts with fire to maintain combustion.
It is obvious from the drawing that a strong draft has very little
influence on a combustion process as the air will only stream faster
over the fire.




Hearth bottom grate

The rate of combustion and temperature is
controlled by the amount of wood placed in a fire pit and as the
air is consumed, a new fresh air comes into its place. The rate
of combustion is independent of the draft and the fire burns as
if no chimney existed. To be able to control burning, the air
opening has to be greatly decreased and a damper will start
exercising control. Wood burns from front to back. When placing
wood on the ground in a firebox with fresh air flowing in
freely, the higher temperature will be in the back of a firebox
or the barrel




Stool Grate

The drawing below is an example of a closed fire box used in a heating
stove. The entering air below the wood has a greater density than the
hot air above the wood, this creates a difference of pressure (draft) and
the only possible way is to go up through the wood. The burning coals
will deprive the streaming air of its oxygen and that sets the
temperature as no more fresh air is available.




Stool grate

       Placing wood on a grate permits the introduction of primary air
below the burning wood and better combustion. Here the coal bed
physically separates fresh air below from the warm air above, a natural
draft is created and all air has to flow through burning coals or wood.
This is the most effective solution permitting a great deal of control.




Raised Stool Grate

The drawing below depicts a smaller amount of air flowing through the
burning wood and the larger portion blows over the wood increasing the
rate of burning. In this case the rate of combustion can be easily
controlled by adjusting the air supply under the fire.


 Raised stool grate
 
Summary on fire box types

The rate of combustion is easiest to control in the stool-grate design
and most difficult in the hearth bottom-grate type. Smoking can be
easily done with an open fire (hearth-bottom grate) and adding more
sawdust to kindling fire allows for some control at least in respect to
smoke. The raised stool grate type can be an effective tool for
switching over from smoking to the cooking process. When smoking, the
bottom damper has to be shut tight and the top damper open. The firebox
becomes the hearth bottom-grate type and by adjusting the top damper we
can choke down the air supply and the wood will start smoking. Needless
to say we still need to add sawdust or wood chips into the hot coals.

To control combustion and temperatures when cooking, a bottom damper can
be adjusted to provide more air which will increase the burning rate and
will create more heat and higher temperatures. By the same token the
amount of smoke created will be decreasing. A correctly designed
stool-grate fire pit will allow to switch from smoking to cooking by
opening the fresh air damper located below the grate level.

The way most smokers operate is to have an unrestricted flow of the
incoming air into the flue (pipe) and the smoke is rushing from the fire
pit into the smoking chamber with some unburned particles. The
temperature is set by the rate of burning wood and the excess fresh air
is rushing into the pipe together with the smoke. This is a welcome
scenario as this fresh air brings moisture with it into the smoker
preventing the drying out of sausage casings.

Note: in order for the dampers to perfom their designated function, the
fire box and its door must be made tight. If proper rules for efficient
combustion are observed the wood becomes a very clean fuel.

The grate significantly increases the efficiency of combustion. The
firebox design is further enhanced by sloping the floor of the firebox
from the sidewalls down to the grate.

Sloping the floor towards the grate 
Secondary combustion

Secondary combustion means adding pre-heated fresh air above the burning
wood which will add fresh oxygen into the flames and the temperature
will rise allowing cleaner and more efficient combustion . There will be
less soot and other unburned particles and less smoke as well. This is
not needed during the smoking process but may be of use when we start
cooking meats. 



Hearth bottom grate and the secondary air

It is achieved
by placing a baffle in the rear of the fire pit to prevent smoke
from rushing directly into the flue and to re-direct it into the
area where a secondary air is entering the chamber. In old times
the blacksmith was manually pumping more air into the fire to
create high temperatures that made metals softer and pliable and
that was excactly secondary combustion even though the air he
supplied was not pre-heated.
Incoming air is preheated in a confined but hot area before being
divided into two separate streams: the primary (lower) and the secondary
(higher). The smoke cannot go straight up the flue anymore and has to
return to sneak through the only available opening where it picks up
fresh and hot secondary air and burns cleaner now due to much higher
temperatures.




Note: bluish flames indicate complete combustion

The air for secondary combustion is delivered in a higher part of the
fire box, just before smoke enters the flue. The idea behind secondary
combustion is to recover some of the heat that will otherwise go up the
chimney by more efficient and cleaner burning of the wood. Secondary combustion can be employed in any of the fire pit
arrangements as long as there is a baffle and secondary air coming close
to the flue as shown in the following drawing. 



Raised stool grate stove a baffle and the secondary air
 The efficiency will further improve if
the secondary air in C is preheated.


 
Preheating secondary air
 








In the above drawing the pipe travels through the flame area
and the air inside gets heated and escapes through a series of
holes drilled in the end section of the pipe.The simplest secondary air delivery system is just a covered hole
located near the upper part of the baffle. The hole can be drilled in the required area and the pipe can be
directly inserted into the hole. It can be welded or threaded and
secured with nuts.

     
The above drawing depicts a very clever solution where the baffle
acts as a pre-heat air delivery vehicle. Depending whether a grate is
employed a pipe can be connected to the baffle on the bottom of a fire
box or can run over the grate. Baffle plates should be made of a thicker
material.



 



Grate made of metal pipe 1/2"-1" diameter. Small holes 1/8"
diameter supplying secondary hot air.




Fire pit placement in smokers

If a fire pit will occupy the bottom part of a barrel some steps must be
taken to slow down the flow of the smoke and evenly distribute it inside
of the chamber. This requires the installation of one or two baffle
plates. The heat conduction will also be slowed down and the whole
smoking and cooking process will be easier to control. The unit will
have higher efficiency and less wood will be consumed. A fire pit can be
a 2
3 ft deep hole in the ground, it can be round, square,
rectangular, whatever, but it has to be covered with a sheet of metal.
The hole should be 18
24" wide, the bigger diameter the more wood can
be burned inside and more heat will be produced, which may be needed
when cooking at low outside temperatures. The deeper the hole the better
the draft will be. Anything deeper than 3 ft will make it impractical to
clean up the ashes.

A metal barrel of smaller size can be used for a fire pit or a large 55
gal (200 l) barrel can be cut in half and imbedded in the ground. A
metal box of suitable dimensions can also be used. Using concrete blocks
or masonry bricks will only make a temporary arrangement as they will
start cracking. Firebricks and refractory fire clay is an acceptable and
commonly used solution.
 



 





 


The following information
comes from the book
"Meat Smoking and Smokehouse Design" by Adam Marianski


Many different types of smokers, some
of them very pretty, can be viewed on our site in
Photo Galleries. 

Copyright © 2006
wedlinydomowe.com. All rights reserved.

Page
added on May 7, 2006

 





 
 
 
 
 
 
 
 

 






 










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