FIED OF INVENTION
The present invention relates to the operation of fans used for boiler to generate
steam in the field of Power and Process Industries. More particularly it relates to
a method for parallel operations of primary fan with variable frequency drive
(VFD).
BACKGROUND OF THE INVENTION
Fan is a turbo machine designed to deliver a given volume and pressure to meet
the system requirement. Fans are very essential auxiliaries to boiler for steam
generation in the field of Power and Process Industries. Fans are used in the
system of steam generation to meet specific purposes. These are used as Forced
Draft Fan for providing air for combustion, as Primary air fans for carrying
pulverized coal to combustion chamber while induced draft fan being used for
exhausting the flue gases to chimney as per the requirement of the system. The
application requirement can be catered by radial and axial flow fan. Suitable
control mechanism is used to regulate the fan performance to meet the demand
of the system and also to meet part load operating conditions.
In Conventional power boilers, forced draft fans and induced draft fans are
provided with variable frequency drive (VFD) for power saving because of line
resistance of the system being linear. But in case of primary air fans where the
system resistance line is not linear, the operation of primary air fan with VFD Is
not applicable.

The present invention relates to a method of ensuring parallel operation of
primary air fan with VFD for power saving.
In conventional design as shown in fig.1, a boiler (1) generates steam by firing
coal in furnace (2). For firing coal, a supply of secondary air (7) is made with the
help of a forced draft fan (3). After the combustion process the flue gas (9)
leaving the furnace is being evacuated by using an induced draft fan (5). Both
these forced draft fan (3) and induced draft fan (5) are driven by VFD so that it
consumes less power for operation due to the linearity of system resistance. But
in case of primary air fan (4) which is used at high pressure for carrying the coal
(8) coming from mill (6) to the furnace, the fan (4) is driven with constant speed
motors due to non linearity of system resistance. As such the power consumption
is much higher and is even more particularly at part loads due to nature of
variable resistance control envisaged.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method of a parallel
operation of primary air fan which uses a variable frequency drive.
Another object of the invention is to propose a method of a parallel operation of
primary air fan using variable frequency drive which enables power savings.
SUMMARY OF THE INVENTION
Accordingly, the present invention relates to the use of variable frequency drive
for primary air fan in spite of non linearity in system resistance line by
introducing a bypass arrangement with a pass duct and a flow regulating device

which makes the primary discharge being reused in the secondary side saving
the power of forced draft fan and the primary air fan is more stable in operation
with VFD. The bypass duct taps the flow from primary air side with flow
regulating device to tune the system for stable and parallel operation of the
primary air fan with variable frequency drive which saves over all power
consumption of the boiler from primary and secondary side. For verifying this
present novel method computational fluid dynamics has been used as supporting
tool.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Fig-01 - It shows the steam generation of prior art.
Fig-02 - It shows the present invention with bypass arrangement.
DETAILED DESCRIPTION OF THE INVENTION
As shown in fig-02, the forced draft fan (3) is used to supply the secondary air
(7) for firing coal in furnace (2) to generate steam in boiler (1). After the
combustion process, the flue gas (9) leaving the furnace (2) is being evacuated
by the induced draft fan (5). Both these forced draft fan (3) and induced draft
fan (5) are driven by VFD so that it consumes less power for operation due to
linearity of system resistance.
For primary air fan (4) variable frequency drive is used in spite of non linearity in
system resistance line by a bypass arrangement. This arrangement comprises of
a suitable by pass duct (11) which taps the flow from primary discharge so that
the primary air fan (4) is more stable in operation with VFD with proper flow

regulating device (10) for system tuning. The tapped flow from primary
discharge is reused in secondary side there by saving the power of forced draft
fan (3). The present invention comprising of suitable by pass duct (11)
connecting primary air fan outlet to secondary air system so that the primary air
fan (4) is more stable in operation with VFD.

WE CLAIM
1. A method for parallel operation of primary fan with variable frequency
drive comprising;
a bypass arrangement of primary air fan outlet (4) connected to
secondary air fan outlet (3) through a by pass duct (11) and a flow
regulating device (10);
2. A method for parallel operation of primary fan with variable frequency
drive as claimed in claim 1, wherein the primary air fan operation is VFD
controlled and stable.
3. A method for parallel operation of primary fan with variable frequency
drive as claimed in claim 1 and 2, wherein the said method enables power
saving by effectively changing non linearity of primary air fan (4)
characteristic

A primary air fan used at high pressure for carrying the coal (8) coming from mill (6) to the furnace is driven by variable frequency drive in spite of non linearity in system resistance line by making a bypass arrangement with a pass duct (11) and a flow regulating valve (10) which makes the primary discharge being reused in the secondary side saving the power of forced draft fan and saving over all power consumption of the boiler from primary and secondary side. The bypass duct (11) taps the flow form primary discharge so that primary air fan is more stable in operation with VFD with proper flow regulating device (10) for system tuning.