Knowledge Brief

What are the good practices for the construction of flue gas ducts and Miyana of a Zigzag Kiln?

A Zigzag Kiln (a Natural Draught Zigzag Kiln (NDZK) or an Induced Draught Zigzag Kiln (IDZK)) consists of:

1. Chimney
2. Centrifugal fan (in an IDZK)
3. Flue gas ducts
4. Central island (Miyana)
5. Outer wall
6. Wicket gates (Ghaati).

The construction quality of these components has to be good for the efficient functioning of a kiln. In this Knowledge Brief, the good practices for the construction of the flue gas ducts and miyana of a Zigzag Kiln (NDZK or IDZK) are discussed. The good practices for the construction of a masonry chimney of an NDZK or an IDZK, and the outer wall and wicket gates are discussed in other Knowledge Briefs.

Flue ducts are used to convey hot gases from the kiln to the chimney. The walls of the ducts (and miyana) must be designed and constructed to minimise air leakage, and to withstand expansion and contraction of the brick walls (due to heating or cooling of brick walls).

What are the good practices for constructing the flue gas ducts?

The good practices for constructing the flue gas ducts are as given below.

1. At least 1-foot-deep foundation pit is dug for the construction of flue gas ducts.
2. The foundation pit floor is compacted and evenly levelled.
3. Mud mortar (Gara) is used. The mortar must be aged for at least 2–3 days prior to its usage. It should be in the form of a homogeneous paste (texture similar to toothpaste).
4. Flue gas duct walls can be constructed with Class-3 bricks and mud mortar.
5. The arch of the duct is constructed with Class-1 bricks.
6. Care should be taken during construction to ensure that the brick joints are never in a straight line between the inner and outer surfaces of the walls so as to minimise the leakage of flue gases.

What are the good practices for constructing the miyana (central island)?

The good practices for constructing the miyana are as given below.

1. At least 1-foot deep foundation pit is dug for the construction of the miyana.
2. The foundation pit floor is compacted tightly and evenly.
3. Mud-mortar (gara) is used to construct the walls. The mortar must be aged for at least 2–3 days prior to its usage. It should be in the form of a homogeneous paste (texture similar to toothpaste).
4. The inner side of miyana walls is made in step footing arrangement, but the outer side is straight.
5. The top half of the outer side of miyana walls is tapered by 1-1/2” towards the inside.
6. The corners of the miyana are reinforced by L-shaped iron rods at regular intervals from the bottom.
7. The outer side of miyana walls is plastered with mud.
8. Expansion joints are made on the outer side of miyana walls to reduce the possibility of cracks developing in the walls due to thermal stress developed during firing of the kiln.
9. One expansion joint is provided between two side ducts.
10. The central duct is made at the middle of the miyana. The remaining space between the miyana walls and the central duct is filled and levelled with soil. Soil is filled in layers. Once soil is filled in a layer of about 1 foot, it should be compacted, and only after that the next layer of soil is filled. This will minimise the possibility of vacant spaces being left between the walls.
11. The arch of the central duct is covered from outside with plastic sheet before filling the soil.