What are the good practices for the construction of a masonry chimney of a Zigzag Kiln?
A Zigzag Kiln (a Natural Draught Zigzag Kiln [NDZK] or an Induced Draught Zigzag Kiln [IDZK]) consists of:
Centrifugal fan (in an IDZK)
Flue gas ducts
Central island (Miyana)
Wicket gates (Ghaati).
The construction quality of these components has to be good for the efficient functioning of a kiln. In this Knowledge Brief, good practices for the construction of the masonry chimney of a Zigzag Kiln (NDZK or IDZK) are discussed. The good practices for the construction of the flue gas ducts and miyana, and the outer wall and wicket gates are discussed in other Knowledge Briefs.
In an NDZK, the chimney is built within the kiln in the centre of the miyana. In an IDZK, the chimney is built either within the kiln in the centre of the miyana, or outside the kiln area. The chimney can be circular or rectangular in shape. The main parts of a chimney are (a) the foundation and (b) the wall. The depth of the foundation, the thickness of the chimney wall, and the construction features depend on the shape and size of the chimney. The self-supporting structure of the chimney must be designed to withstand various loads such as wind loads, earthquake loads, and temperature loads, apart from self-weight.
What are the good practices for constructing the foundation of the chimney?
The design of the chimney should be done in consultation with a qualified structural engineer. However, some general guidelines based on the best practices observed in the field are listed below.
The depth of the foundation pit is kept at least 5 feet from the floor level of the trench (trench level).
The floor of the foundation pit is compacted and evenly levelled.
A layer of river sand is spread over the floor. Layers of plain cement concrete (PCC), reinforced cement concrete (RCC), and mixture of mortar and brick pieces are placed in sequence, one on top of the other, over the sand layer.
The base of the chimney walls is constructed (in step footing) from the top of the ‘mortar and brick pieces’ layer till the trench level. High quality (good load bearing capacity) Class-1 bricks are used for the construction of the foundation.
What are the good practices for constructing the chimney structure?
Listed below are the good practices for constructing the chimney structure.
From the trench level up to at least 50-feet height, good quality Class-1 bricks are used for constructing the chimney.
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). Only a thin layer of mud mortar is applied.
RCC bands are provided in the chimney at regular height intervals.
The inner surface of the chimney wall is plastered with mud mortar and the outer surface by cement mortar to prevent infiltration of ambient air.
To monitor air pollution, four sampling ports, which are mutually perpendicular to each other, are provided.
The sampling port is generally in the form of a pipe of about 4-inch internal diameter. The pipe extends outwards from the exterior chimney wall (approximately 6-inch projection). An easily removable cover is provided to close the port when not in use.
To access the ports, a work platform is provided all around the chimney. Minimum platform width should be 4 feet regardless of the diameter of the chimney. The height of the ports from the floor of the work platform should be between 3 and 4 feet.
For safety, a guardrail is provided around the platform. The work platform should be strong and should be able to support a weight of about 500 kg (2–4 persons + weight of the monitoring equipment).
Access to the work platform is provided through a caged ladder.
Power supply points (220 V, 15 A, single phase AC circuit with grounded outlet) are provided at the platform and chimney base.
Lightning protection system and earthing are installed on the chimney to shield it from structural damage or electric shock caused by lightning.
Holes left on the chimney walls to put the scaffoldings are properly closed with bricks and mortar after removing the scaffoldings.
How is a fan installed in an Induced Draught Zigzag Kiln?
To induce artificial draught in an IDZK, a centrifugal fan is installed between the central duct and the chimney. From the central duct, hot gases are made to flow into the fan inlet through a connecting duct. The outlet of the fan is connected to the base of the chimney through a duct, which directs the gases into the chimney. It is advisable to install a diverging duct between the fan outlet and the chimney to minimise the loss of energy due to the sudden expansion of hot gases inside the chimney.
What is the minimum stack height for Zigzag Kilns?
The minimum stack height according to the Environment (Protection) Fourth Amendment Rules, 2009 of the Ministry of Environment, Forest, and Climate Change, is given below.
Trench width (m)
Minimum stack height (m)
With fan operating with minimum draught of 50 mm WG