heat transfer of the condensing heat
exchanger.
l The condensing heat exchanger design
was an active draft system, imparting
no change to the boiler combustion
system. This design feature is particu-
larly important due to maintaining
the boiler air-fuel ratio at lower loads
while allowing bypass of the system
during operation.
l The counter-flow design – in
which the condensed products of
combustion flow downward opposite
the direction of the makeup water
– prevented the re-evaporation
quenching of flue gas. This maintains
the humidity ratio of the flue gas,
optimizing the heat transfer to the
makeup water.
l Flue gas condensate collected within
the dropout volume under the down-
stream stainless coil is pumped back
to the makeup water line coming out
of the upstream coil, contributing
up to 7 percent of the total makeup
water flow to the boiler.
l;Capacity for preheating up to 50 gpm
of process or makeup water was added.
110 F saturated flue gas still contains
nearly 7 percent of the total energy
input to the boiler. The additional flow
capacity of the last coil would increase
efficiency to 94.5 percent.
System Control
and Optimization
The supervisory control system
was designed based on an Allen-Bradley
ControlLogix platform. The system
allowed for combustion control with
additional control added for feed
water flow control, makeup water flow
control, SCR ammonia flow control,
condensing flow control, feed water-to-makeup water flow control, steam
superheat control and the emission-monitoring system control. A propor-tional-integrative-derivative controller
was developed for the human machine
interface, running on a PC platform and
written in Visual Basic.
The control system relied on variable-frequency drives for controlling the
various flows throughout the system.
While each of the water flow loops had
venturi-based flow measurement, the
system control using variable-frequency
drives greatly simplified the system control over that of using setpoint control.
Economic Justification
Mortensen finalized the steam
system upgrade of his facility, which
comprised the following:
The back-pressure turbine was
financed over 10 years with an upfront
down payment and a yearly cost of
$68,000. The cost of the steam plant
upgrade was estimated to be $1,665,000,
which included 14 percent additional
contingency due to the novel aspects of
the proposed system. The baseline energy
costs included natural gas at $8/MMBtu
and electricity at $0.07/k Wh. Subtracting
a $262,000 rebate from Pacific Gas &
Electric, the total upfront cost of the
upgrade was $1,403,000, resulting in
a simple payback of 3.85 years.
Verified Results
The project commenced construction
in fall 2006, and the system was commis-
sioned in June 2007. PG&E performed an
energy audit of the system that verified
a boiler fuel-to-steam efficiency of 94.5
percent, the higher efficiency resulting
from preheating additional process water
needed by the plant.
Robert P. Benz, PE, is president
of Benz Air Engineering Co.
Inc. With more than 27 years
of experience in combustion
engineering, Benz supervises the
implementation of combustion
control solutions for maximum efficiency and
lower emissions for central plants. He pioneered
the use of variable-speed drive technology of
fans for precisely metering air flow, a process
for which he was granted a U.S. patent. Benz
holds a bachelor of science degree in mechanical
engineering from Oregon State University
and is a registered professional engineer
in California, Nevada and Texas. He may be
reached at RBenz@benzaireng.com.
