Material Selection for KILN WALLS

Submitted to

Dr. Jayant Jain

Submitted by

Ajay kuntalM.Tech 1st Year(Design

INTRODUCTION

A kiln is a thermally insulated chamber, a type of oven, that produces temperatures sufficient to complete some process, such as hardening, drying. Various industries and trades use kilns to harden objects made from clay into pottery

HistoryThe earliest kilns were certainly no more than the hearths used by peoples for cooking, warmth, light, and protection. In fact, very simple 'pit' kilns are still in use today.

Neolithic period, when agriculture began, is generally cited as the origin of fired clay objects, approximately 10,000 years ago.

These early farmers needed containers for seeds, for harvested foods to be stored, and for water transportation and storage. Fired clay served these needs well, and was locally available and easy to form. The earliest kilns were nothing more than a shallow 'pit' dug in the ground.

Pots fired in this way were very fragile and porous due to the low temperatures possible in such a firing (1000°-1200° Fahrenheit).

http://seco.glendale.edu/~rkibler/kilns.html

History and development This kiln does have

insulation-the earth itself.

This kiln does have insulation-the earth itself.

The fuel is any flammable organic material such as wood, straw, or manure

…History and development.. The insulation, in the form

of an arch is on top, retaining the heat better. 

Burnt clay bricks are used for dome

http://www.fao.org/docrep/X5328E/x5328e08.htm#7.2. the brazilian beehive kiln

…History and development.. An interesting variation on the

beehive design was first built in China around 500 A. D.

This kiln utilized the basic format of the beehive.

Multiplied chambers.

Saggar boxes, which were used to protect the pottery from flying wood ash and were made of clay.

http://seco.glendale.edu/~rkibler/kilns.html

…History and development.. Natural gas is the fuel

(propane).

Better quality insulating brick.

It produces very low levels of pollution, and that the fuel is relatively inexpensive compared to electricity.

 NATURAL GAS KILNS

http://seco.glendale.edu/~rkibler/kilns.html

Design RequirementFunction -Thermal insulation for kiln

Constraints -Maximum operating temperature 1000C ----Possible limit on kiln-wall thickness for space reasons

Objective Minimize energy consumed in firing cycle

Free variables -Kiln wall thickness, w-Choice of material

Material Selection• When a kiln is fired, the internal temperature rises

quickly from ambient, To, to the operating temperature, Ti, where it is held for the firing time t.

• At steady state the heat loss by conduction, Q1, per unit area, is given by the first law of heat flow

Here is the thermal conductivity, dT/dx is the temperature gradient and w is the insulation wall-thickness.

…..Material Selection• The heat absorbed by the kiln wall in raising it to

Ti, and this can be considerable. Per unit area, it is

where Cp is the specific heat of the wall material and is its density. • The total energy consumed per unit area is the sum

of these two:

…..Material Selection• A wall that is too thin loses much energy by

conduction, but absorbs little energy in heating the wall itself. One that is too thick does the opposite. There is an optimum thickness, which we find by differentiating equation with respect to wall thickness w and equating the result to zero, giving:

• Q is minimized by choosing a material with a low value of the quantity (λρCp)1/2, that is, by maximizing-

Material Selection

Final Selection

Postscript• It is not generally appreciated that, in an efficiently-

designed kiln, as much energy goes in heating up the kiln itself as is lost by thermal conduction to the outside environment. It is a mistake to make kiln walls too thick; a little is saved in reduced conduction-loss, but more is lost in the greater heat capacity of the kiln itself.

• That, too is the reason that foams are good: they have a low thermal conductivity and a low heat capacity. Centrally heated houses in which the heat is turned off at night suffer a cycle like that of the kiln. Here (because Ti is lower) the best choice is a polymeric foam, cork, or fiberglass (which has thermal properties like those of foams).

References• http://seco.glendale.edu/~rkibler/kilns.html• http://www.directindustry.com/industrial• http://www.traditionaloven.com/articles/81/

insulating-fire-bricks• Materials Selection in Mechanical Design - Michael

F. Ashby• http://en.wikipedia.org/w/index.php?

search=insulating+brics&title=Special%3ASearch

Thank You

Ajay kuntal2012 AMD 3062Design Engineering