In a Natural Draught Zigzag Kiln (NDZK), the setting of bricks is different from that in a Fixed Chimney Bull’s Trench Kiln (FCBTK). Usually, double or triple zigzag brick settings are employed in NDZKs. Now-a-days, in some of the bigger kilns, quadruple zigzag brick setting is also being practised.
In an FCBTK, bricks are stacked in vertical columns in a line (row) across the width of the trench. The rows of brick columns are arranged one after the other in the direction of air flow.
In an NDZK, bricks are stacked in such a manner that distinct chambers of brick setting are formed in the kiln. Just like in the case of FCBTK, in NDZK also, the bricks are stacked in vertical columns in a line (row) across the width of the trench. However, unlike in FCBTK, all the brick columns are not of the same width. The rows of brick columns are arranged one ahead of the other in the forward direction of fire travel. In an NDZK, one chamber of brick setting consists of five such rows.
Because of different brick settings, the path of air flow in the two kilns is also different. In an FCBTK, the air flows in a straight-line path along the length of the trench. In an NDZK, the air flows both across the width and along the length of the trench in a zigzag path. How the air is made to follow a zigzag path is explained in detail later in this Knowledge Brief.
Chamber dimensions are fixed in the following way:
In a chamber, bricks are arranged according to the following method:
In a double zigzag brick setting, in the last row (fifth row) of a chamber, openings are left near both ends of the trench width. In the next chamber, in the last row, openings are left at the central part of the trench width. In this way, alternately, openings are left either at the centre or at the two ends of the trench width in the last row of successive chambers. This arrangement of openings in the chambers results in the zigzag flow of air, and is repeated throughout the length of the rectangular trench.
In double zigzag flow, air alternately enters the chambers either through the openings at the ends or through the openings at the centre.
When air enters a chamber from the ends (as in Chamber-1 in the image), it is made to flow inwards across the width of the chamber. It leaves the chamber through the openings at the centre and enters the next chamber.
When air enters a chamber from the centre (as in Chamber-2 in the image), it is made to flow outwards across the width of the chamber. It leaves the chamber through the openings at the two ends and enters the next chamber.
This reversal of the flow direction in alternate chambers creates a zigzag flow. This flow pattern is repeated in alternating chambers throughout the length of the rectangular trench.
The zigzag brick setting is done only in the longer lengths of the rectangular trench. In the shorter lengths of the rectangular trench, which is called gully, brick setting is done as in an FCBTK.
In the gully region (shorter length of the rectangular trench), all the rows, except the last row, are made of equally spaced, same size brick columns in exactly the same way as in an FCBTK. Air flows in a straight-line path in this region. At the entry and the exit of the gully region, openings are left only near the outer wall end of the trench. This arrangement ensures that air also flows through the extreme corner of the trench thereby exchanging heat with as many bricks as possible. Openings at only the outer wall end also prevent short-circuiting of the air flow near the miyana of the kiln.