15. Preheater and Cooler Vent Fan Selection Criteria
15.1 Preheater Fan
In earlier time the preheater fan selection mode was very conventional and
accordingly the fan was designed on keeping 25 % margin on flow and accordingly
56 % on pressure above operating flow & pressure at guaranteed capacity and fan
law was applied for designing the fan which states that the pressure increases in
square with respect to flow and power increases in cube. So designing the fan in this
concept was creating huge difference in operating and design flow and pressure and
losses in PH fan power due to inefficient operation.
It is also observed that this fans were required to replace to meet the gas flow
requirement due to gradual increase in capacity after every up gradations /
optimization.
In the past the preheater cyclones were designed for 500-550 mmwg pressure drop
and preheater fans were designed at higher pressure say 800-900 mmwg, which
was leading for higher preheater specific power at around 8-10 kwh/t of clinker on
guaranteed output while operating at potential capacity it comes to 10-12 kwh/t and
efficiency of fan was poor i.e. 65-70 % on account of designing fan at higher pressure
and operating at lower pressure.. Now we decided to give the supplier the flow and
pressure what we desire by which it has reduction by 1-2 unit in power consumption
as fan law was not applied.
In case if we increase the production level then pressure drop requirement is higher
so in that condition it is better to change the impeller generally this requirement
comes after 3-4 years of plant operation hence it is economical to design the required
pressure drop initially and replace the impeller or fan later on.
Presently due to increasing trend of installing WHRS system and also the increased
usage of Alternative fuels which has lower NCV and very high moisture the designing
of fan has become complex and needs special attention on it so that better efficiency
can be achieved in all mode of operations.
Also the PH fan design pressure to be around 80 mm above to the pressure
requirement of potential output and if WHRS is being install/planned then around
further 80 -100 mm additional pressure drop can envisaged on account of boiler but
in actual operation the pressure drop across boiler remain around 60-70 mm only
and further with 150 Deg C PH fan gas temperature at boiler outlet volumes reduces
drastically which compensate the additional pressure drop as explained in the table
below so taking additional margin on pressure drop is not required.
Also the fan is equipped with VVFD hence volume can be considered higher side
which will not affect the fan efficiency and flow can be controlled as per requirement
and with extra flow any pressure requirement in the system can be fulfilled. Margin
in flow can be any time utilised for higher clinker production requirements.
For example, in a plant if the pressure requirement is 500 mmwg at fan inlet @