Dear elwathig,

Consider ...... Circuit 1 - Kiln ID fan and Primary air fan for pushing the fuel through Burner pipe and

Circuit 2 - Cooler, F.D. Fans together with cooler exhaust fan.

In the first circuit when fuel is burnt theoretical minimum combustion air is required; and for complete combustion around  2% excess air is necessary so that there is no CO formation in the circuit and proper heat exchange between kiln feed and hot gases takes place for formation of clinker.Had there been no cooler & cooler exhaust fan (e.g.planetary cooler) all the combustion air would have been drawn from the hot end or kiln hood.

The purpose of ID Fan is to vent out all the gases from the Kiln once heat exchange job is done efficiently.

In second circuit i.e.during clinker cooling process the total quantity of air (NM3/KG Clinker) required for cooling is more than the total air + gases (NM3 / KG Clinker) handled by the Kiln ID fan.Hence, to keep the total system in balance additional air that is left out after the usage of secondary and tertiary air drawn from cooler, is required to be vented out through Cooler exhaust fan.

Depending upon clinker size, its bed thickness,cooler speed,FD fans, cooler exhaust fan controls and Kiln combustion conditions, feed rushes; hood draft value would behave + - 0. This is one of the key process parameter while operating Kiln and cooler.

Thus, relation between IDfan and cooler fan suction gets clarified with the above explanation so also the meaning of zero point I suppose!