FIELD OF THE INVENTION
The invention relates to a method for reducing pressure drop in the ducting
between Air Heater (AH) to Windbox, to improve the flow distribution to burners
uniformly. More particularly, the invention discloses a method for reducing pressure
drop in air ducting system of a VU60 type boiler to improve the performance of a utility
boiler and reduce auxiliary power consumption.
BACKGROUND OF THE INVENTION
This method is applicable to all industrial type boilers in general for power
generation and process industries and in particular to VU60 type boilers. A typical VU60
type utility boiler consists of aforced draft (FD) fan which supplies air to the combustion
process. After air is preheated in Air preheater (AH), it is sent to burners through a
common windbox duct. Due to space constraints, the ducting from FD Fan to Furnace
has to be routed through various equipment like SCAPH, AH, Aerofoil, Windbox and
Burners with many bends and transitions. When the flow goes through these bends and
transitions, flow separations occur, which are main source of pressure losses, leading to
more non-uniformity in the inlet or outlet profiles causingsub-optimal performance of
the upstream and downstream equipment. Normally large development lengths are
recommendedbefore or after any equipment for flow to regain to fully developed
profile, but they are seldom followed strictly in real situations due to space constraints.
The designers usually refer standard handbooks or proprietary programs for design of
ducting to calculate the flow resistances for each section and add up all the losses to
arrive at total pressure loss and accordingly will arrive at the sizing of the fan

to pump the fluids. The combined effect of these losses, will not generally be additive
and of late tools like Computational Fluid Dynamics (CFD) are in use for overall
modeling and checking the combined losses and modify suitable geometrical changes to
optimize the performance. The design changes proposed by CFD analysis for retrofit
jobs, should be easily implementable at site without major revamping of the ducting
layout and avoid major overhaul and shutdown of the boiler causing huge generation
losses.
During the process of establishing the steam generation for a peak load condition of
a 270 tph VU60 type boiler at a Site, it was noticed that after reaching steam flow for
about 250 tph, the furnace is pressurized and the FD fan was not able to supply the
rated flow, though the fan blades are in full open condition. This phenomena was
observed in Unit 1 boiler (UB1) and also corroborated with data in an identical unit UB3
with the base case geometry as shown in Fig 1. The relevant parameters of the boiler
required to study the pressure losses in the air / flue gas ducting system under base
case are studied and it is observed that the pressure drop between AH outlet to
Windbox header is on the average in the order of 123mmwc. The air ducting from AH
outlet to Windbox was having huge pressure losses, due to flow resistance and prevent
boiler reaching full capacity of steam generation.
PRIOR ART
Perforated plates are used in many practical industrial applications like when flow
is passing through a wide angled diffuser for example before Electrostatic precipitators

etc., to reduce the flow non-uniformities. Besir Sahin (1989), conducted experiments to
show how velocity profiles vary widely at the end of a diffuser, to some extent are
corrected by inclusion of perforated plates at start and end of diffuser. However the
perforated plates add additional resistance, thus increasing the pressure drop to
achieve the flow uniformity downstream after it.
There are two separate methods followed in industrial practice in a 90° bend, of
using single guide vane (Alstom standard) at roughly one-third distance of height of
duct or using three guide vanes (old Combustion Engineering standard) which divide
the duct proportionately with a standard ratio. Both methods are equivalently good for
streamlining the flow in a standard 90° bend, and reduce the velocity and pressure
gradients across the 90° bend and avoid flow separation just after the bend. In the
case of a bend with unequal inlet and outlet widths, three guide vanes design is better
compared to using a single guide vane.
No other patents were found in the prior art search with various terms like “pressure
drop reduction”, “perforated plate”, “windbox ducting”, “guide vanes or guide plates in
ducting”, “VU 60 boiler” which propose a combination of geometrical features that are
discussed in the present invention. Similarly no literature exists with the combined
features as described in the current patent to the best knowledge of the author(s).

OBJECTS OF THE INVENTION
The objects of the invention is to arrive at an easily implementable set of
modifications in AH to Windbox ducting in order to reduce the pressure losses so that
fan could pump the desired air so that the boiler could achieve the full steam
generation capacity.
SUMMARY OF THE INVENTION:
The following geometrical modifications as represented in Fig 2M, 3, 4 & 5 were
recommended based on CFD study and implemented at a power plant site:
1) Removal of partially blocked perforated plate, which is causing non-uniform flow
and increase the pressure drop due to sudden area contraction.
2) Removal of single guide vane which is not appropriately positioned and replace it
with 3 guide vane system to smoothly streamline the flow.
3) Removal of guide plates in the windbox which are kept immediately after the
perforated plate.
The above modifications are further illustrated with differences between base case
and modified cases in Fig 3 to 5. Since the duct approaching windbox geometry is
symmetrical, these changes are to be done on either side of windbox. These changes
are implemented in AH outlet to Windbox path, along with air heater maintenance.
The partially blocked perforated plate is removed on both sides of windbox entry,
and the single guide vane is replaced with three guide vanes which will better

streamline the flow passing through the 90˚ bend as cross section widths are different
at inlet and outlet side. The guide plates just after the perforated plate are also
removed, as they are no longer required once the perforated plate is removed. Other
geometric details remain same as in base case.
After collecting the performance data from the site for the base case and modified
case after incorporating geometric changes in AH out to windbox region and AH
maintenance were carried out, it is observed that the pressure drop between AH outlet
to windbox header in modified case is reduced by about 91% from base case. The
pressure drop reduction happened in this zone basically from removal of a partially
blocked perforated plate at entry to windbox duct and by providing appropriate guide
vanes to stream line the flow passing through the bends. The reduction in head greatly
enhanced the FD fan to reach its potential for desired delivery of air and also to meet
the 100% MCR steam flow generation.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING:
Figure - 1: shows the elevation view of ducting layout from AH outlet to Windbox
connecting duct to Burner outlets
Figure - 2: shows the plan view of ducting layout from AH outlet to Windbox connecting
duct to Burner outlets
Figure - 2M: shows the modified ducting layout from AH outlet to Windbox connecting
duct to Burner outlets

Figure - 3: shows the comparison of single guide vane in base case versus three guide
vanes in modified case in the 90˚bend just before windbox
Figure - 4: shows the comparison of the partially blocked perforated plate in base case
versus with plate removed in modified cases.
Figure - 5: shows the comparison of the guide plates surrounding the burners in base
case versus few guide plates removed in modified cases in the windbox zone
Figure - 6: Pressure contours (Pa) of the Base case (left) and Modified case (right)
Figure - 7: Velocity contours (m/s) of the Base case (left) and Modified case (right)
DETAILED DESCRIPTION OF A PREFFERED EMBODIMENT OF THE
INVENTION:
The ducting layout from AH outlet (1) to Windbox connecting duct (2) to Burner
outlets (3) is shown in Fig 1 & Fig 2. Hot air after heat exchange process in Air
preheater (AH) with flue gases will be entering into the AH to Windbox ducting system.
This air then goes through Aerofoil shaped flow meter (4), partially blocked perforated
plate (5), then cylindrical perforated plates splitting the flow into primary / secondary
streams (6) and then exit the burners in to the Furnace (7). The duct will branch into
two ducts after flow meter, one to the left and one to the right and then takes upturn
through three successive 90˚ bends and joins with windbox connecting duct. Since
these 90˚ bends introduce additional pressure drop due to direction change, a single
guide vane (8) is placed in each bend at roughly one-third distance from inner bend as
per industrial standard practice to reduce the pressure loss and streamline the flow.

Since the duct cross section in base case changes significantly in the last 90˚ bend
and then transits to connect to windbox connection duct with large cross section, the
single guide vane (8) is not suitably placed symmetrically and may introduce flow profile
deviation after the bend. The single guide vane in the third bend is replaced with
three guide vane system (9) in modified case, and they are extending symmetrically
proportionate to a ratio of the width of the cross section of duct equally at inlet and
outlet side of the bend as shown in Fig 3, thus streamlining the flow. The recirculation
zone due to improper location in case of single guide vane design, is avoided in three
guide vane design.
Generally for optimal combustion it is desired that all burners are supplied equal air
flow which then splits into secondary air and primary air after passing through
perforated plated cylindrical chambers and enters the furnace from the burner outlets
to mix with fuel and participate in combustion in the furnace located further
downstream. In order to supply the air equally to all burners and to nullify any
upstream effects due to bends, a perforated plate (5) is installed at entry of the
windbox on either side of the connection duct as shown in Fig 2. The top and bottom
(10) portion of the above plate is non-perforated, and only the central portion (11) is
perforated in the partially blocked perforated plate in the base case design as shown in
Fig 4. Due to sudden contraction in the area available for flow to pass through the
perforated plate which occupies only partially in the whole cross section, there is a high
pressure drop happening in the duct in air path. In the modified case ducting layout,

the partially blocked perforated plates are removed just before the entry of windbox
connection duct (5) on either side of windbox duct as shown in Fig 4. This will remove
the sudden area contraction and hence will reduce the pressure drop.
There are guide plates in the windbox connection duct that are present surrounding
burners (12) and just downstream of perforated plate (13) to direct the air flow towards
the nearest burners in the base case design. These guide plates (13) just downstream
of perforated plates are also removed in modified case, as they are no longer necessary
after removal of the perforated plate (5). However the guide plates in the interior
portion surrounding the burners (12)are retained in the windbox connection duct to
guide the flow to the burners.
Additionally Air preheater maintenance is also attempted during site modification to
arrest the seal air leakage and cleaning of AH elements.
These three modifications together implemented at a power plant site reduced the
pressure drop in air path from AH outlet to Windbox in modified case by about 91.3%
from the base case. The modifications in AH outlet to Windbox zone, caused significant
reduction of pressure losses, especially the removal of blocked plates along with the
perforated plate contributed in a major way in the total losses. Also, the FD fan
discharge pressure reduced by about 100 mmwc on average. The FD fan air discharge
increased from base case by 4% on average, and Steam flow generation increased
from base case by 10% for modified case and met the 100% MCR targeted steam flow

rate. The improvements carried out will result into an estimated savings in FD fan
power consumption up to 11.84% of reduction in auxiliary power consumption
compared to the base case. Above improvements suggested using CFD analysis have
reduced pressure drop and enabled boiler to reach full load. The pressure and velocity
contours of CFD results of base case and modified case are shown in Fig 6 and 7
respectively which show an improvement in its features in modified case compared to
base case design.
NOMENCLATURE
AH Air preheater
CFD Computational Fluid Dynamics
FD forced draft
MCR maximum continuous rating of the boiler
mmwc millimeter of water column, unit of pressure
SCAPH Steam cooled air preheater
REFERENCES
1. Idel’chik, I.E(2008), “Handbook of hydraulic resistance”, 4th edition, Research Institute
for gas purification, Moscow, Russia.
2. Besir Sahin (1989), “Pressure losses in an isolated perforated plate and jets emerging
from a perforated plate”, Int J of Mech Sci. Vol 31, No 1, pp 51-61.
3. Alstom Industrial standard practice for providing single guide vane in a 90˚ bend
(Std no 34-96-03).
4. Combustion Engineering Industrial standard practice for providing three guide vanes in a
90˚ bend (Std no 77-10).

WE CLAIM
1. A method for reducing pressure drop in an air ducting system to improve performance of a
VU60 Type Boiler comprising:
a ducting layout from air preheater AH outlet (1) to windbox connecting to windbox
connecting duct (2) for passage of hot air after heat exchange process in air preheater (AH)
with flue gases, the air passes through aerofoil shaped flow meter (4);
diverting / branching the air into two ducts after flow meter (4) and taking up turns into two
90o bends before passing the 3rd 90o bend where the single guide vane (8) replaced by 3
guide vane system (9) to smoothly streamline the flow;
the air finally entering the windbox ducting system directly without any hindrance from
partially blocked plate (5) and guide plates (13) downstream of perforated plates (5)
characterized in that the air flows directly on the surrounding burners (3) directly without
any pressure drop that enhances the boiler performance.