Chapter 7: Auxiliaries
n this chapter, we will cover a broad range of cumulation of dirt or dust can decrease the
auxiliary equipment, including fans, boiler efficiency of the fan by slowly changing its
feedwater pumps, motors and turbines, elec- configuration and adding weight to the fan.
tronic motor drives, materials handling equipment, This inspection should be done at least twice
and boiler sootblowers. We will also discuss cogen- yearly, more frequently if the fans are lo-
eration and the sale of excess electricity. cated in a high-dust environment (pulveriz-
ers, conveyor belts, etc.)
Fans 2. Fan bearings should be checked for wear and
adequate lubrication. Clearance between the
There are three types of induced draft (ID) and fan and its housing should also be checked.
forced draft (FD) fans most commonly used in
power plants; the backward-curved, also known
as the airfoil; the straight blade and the radial Boiler Feedwater Pumps
tip. The most efficient is the airfoil; it has a 90%
efficiency. Other advantages of the airfoil in- Boiler feedwater pumps function as the delivery
clude: very stable operation, low noise level and system to the boiler, providing water under pres-
the suitability for high-speed service. sure to the boiler. They do this by taking suction
Straight blade and radial tip fans are more sub- from a deaerating feedwater heater and pump-
ject to erosive conditions. As an example, straight ing the feedwater into the boilers through high
blade fans are used mainly for pulverizer exhaust pressure feedwater heaters. There are numerous
and radial tip fans for applications downstream methods available to control feedwater volume
from a low efficiency particulate removal system. and provide sufficient suction head to the
The desired air or gas flow and its pressure pumps.
in a fan are determined by the employment of One way to control volume is to use a vari-
one of the following: able speed drive motor with the boiler feed
pump. Another way is to use an electric motor
Inlet damper control running at a constant speed, coupled to a vari-
Inlet vane control able speed hydraulic drive. Where the expense of
Variable speed control a variable speed drive is not justified or desir-
able, volume can be controlled by a throttling
Dampers or vanes are used when the fan is cou- valve installed between the pump and the boiler.
pled to a single or two-speed motor. Fan manufac- A decision depends on whether the capital ex-
turers usually favor vanes over dampers as they feel pense of a variable speed drive is offset by a sav-
they are slightly more efficient due to the pre-swirl ings in electrical power. A feedwater control valve
effect that vanes impart to the air or gas flow. A requires a large drop in pressure for proper op-
variable-speed drive, however, is the most efficient eration; therefore the pump must have added
control. It provides only the power necessary to head capacity.
overcome system resistance at a given condition. It In this type of operation, the condensate in the
is particularly effective when operating conditions deaerator will be saturated, requiring the deaerator
call for frequent low load periods. The diagram be- to be elevated as far as possible above the boiler
low illustrates the amount of power necessary at feedwater pump to provide sufficient net positive
various speeds for a hypothetical centrifugal fan and suction head (NPSH) for the pump. This elevation
system resistance. may require additional structural and piping costs
which can be offset by the installation of booster
Fan Maintenance Checkpoints pumps that provide the additional suction head for
the boiler feedwater pumps.
1. Fans and their housings need to be checked A minimum of two feedwater pumps are usually
periodically for dirt and dust buildup. An ac- installed, depending on the need to maintain ca-
Energy Efficiency Handbook 35
pacity in the event of pump failure. This also allows perform. These motors represent another exces-
pump deactivation to perform normal mainte- sive energy consumption area. Some surveys
nance, such as replacing the bearing seals, packing have shown as many as 60% of all motors in in-
or repairing the electric motor driving the pump. dustrial facilities to be oversized.
Recirculation of the feedwater back to the
deaerator is necessary when a single speed motor Electronic Motor Drives
is used and flow is at or below the pump’s mini-
mum flow. Steam turbines are often used to drive A promising area for saving energy and reducing
feedwater pumps because the speed of the tur- maintenance is the marrying of energy-efficient
bine and pump can be varied like a variable electric motors to the latest generation of elec-
speed motor. An automatic recirculation control tronic motor drives. These drives can make mo-
valve or a modulated control valve controlled by tors more productive by controlling starting,
the operation’s distributed control system can stopping, speed regulation, reversing and even
perform this function. Pumps should be in a re- positioning.
circulation mode as little as possible to maximize The new drives offer protective features for
energy savings. the motor. They can control high inrush current
and its subsequent voltage sag on the power dis-
tribution grid. Other features can limit current
Motors and Turbines and shut down overloads that can allow a motor
to self-destruct. Electronic motor drives can also
Electric induction motors or steam-driven tur- minimize motor burnout, a potential fire hazard.
bines can be used to drive fans, pumps or other Most importantly, these new drives save en-
industrial process machinery. Picking one or the ergy. For example, varying a pump’s speed
other depends on the availability of steam versus rather than throttling the output flow can save
the cost of electricity. If high pressure steam is 25-40 percent of the electricity consumed. Put-
available and there is a need for low pressure ting a motor on standby rather than allowing it
steam, a turbine can serve as a pressure reduc- to continue running when it’s not needed can
tion station while driving the fan or pump. If also save energy. A typical motor can consume
there is no need for low pressure steam, a con- 10-20 times its cost per year; the payback period
densing steam turbine could be used, although for installing drives is often under two years.
turbines of this size are not efficient. If electricity It is estimated that less than 3 percent of all
is being generated on-site, it is usually more effi- installed AC motors have electronic drives. Other
cient to use electric motors. estimates indicate that at least a quarter of all AC
Motors are available today that are known as motors could benefit from the use of drives.
“premium efficiency.” They are more expensive, Other factors that make the use of electronic
but the efficiency is more than offset by the cost. drives attractive include declining prices, better
With electricity costs typically at $0.065/kwh and accuracy and reliability, the discovery of new ap-
higher, the payback analysis justifies the extra plications and increased user acceptance.
expense. Here are some typical motor efficien- Another advantage of electronic drives is
cies available today from “premium efficiency” their easy setup and adjustment. These functions
motor manufacturers: are done through an operator keypad or inter-
face device. Since the settings are entered digi-
HP Size Efficiency tally, they are very accurate and not subject to
drifting. Older analog drives were set by electro-
mechanical potentiometers, similar to older radio
15-40 90% controls. These were subject to dust and dirt con-
50-150 94% tamination and often requiring frequent calibra-
200 or greater 95% tion and readjustment.
Today’s electronic drives have a much larger
tolerance of line voltage fluctuation, thanks to
Motors should be checked on a regular basis the integration of an electronic technique known
for excessive vibration, bearing wear and ade- as pulse width modulation. The older power-
quate lubrication in accordance with manufactur- switching devices, silicone-controlled rectifiers,
er’s recommendation. had tolerances as low as 5-10 percent; with the
It is common to find motors installed that newer systems, tolerances start at 10 percent and
are oversized for the task they are required to can go up to as high as 30 percent.
36 Council of Industrial Boiler Owners
Materials Handling Equipment vacuum on the piping system, using a steam ex-
hauster or motor-driven blower. Water exhaus-
Solid fuel-fired power plants need systems to re- ters have been used for flyash conveying from
ceive, store and deliver fuel and to collect, store large pulverized coal fired units.
and remove ash. This can include furnaces de- Systems with capacities in excess of 15 or so
signed to burn coal, wood, food processing wastes tons- per-hour tend to use vacuum blowers where
and municipal and industrial processed refuse. smaller systems have historically used steam ex-
The size and complexity of these systems vary hausters. Water pollution and efficiency prob-
widely but all of them require energy to operate. lems with steam exhausters have caused many of
these systems to be converted to motor-driven
Fuel Handling Systems blowers, a move that almost always reduces
Pneumatic systems are available for certain re- If the decision is made to retain the vacuum
quirements but most fuel-handing systems are system, the maintenance of the piping is essential
mechanical, using belt or drag conveyors. They for efficient operation. An active leak detection
are very efficient on the basis of tons per kW. program is essential as leaks will cause loss of
Energy-saving options for conveyors are few; conveying capacity, increase running time and
however, some of the equipment installed as part maintenance of the pipe, fittings and air cleaning
of the conveyor system might deserve a second equipment as well as consuming more energy.
look. See Chapter 11, Compressed Air Systems and Diesel
For example, a major power consumer in a Engine Power Cogeneration, for more discussion on
coal handling system can be a crusher, used to leak detection. If steam exhausters are used, it is
size coal. Power can be saved, in some cases, by important to maintain the steam nozzles and
changing to a different crusher or installing a by- venturi sections to maintain efficiency.
pass which separates properly sized particles Pneumatic ash systems are most often run in-
from the crusher. Some installations require that termittently, pulling ash on a batch basis. Power
the crusher need only be used intermittently. A consumption can be reduced by maximizing the
change in the coal supply could cause a crusher conveying rate of the system. If the system is
to be taken out of service. Dusty areas of the sys- manually operated, the operator should feed the
tem containing electrical equipment need to be intakes at the highest obtainable rate. System
monitored and kept clean to minimize power controls should be optimized to minimize the
draw. non-conveying cycle times.
Fugitive dust collection systems on coal con- In the case of continuous pull systems, often
veyors should be maintained in the same way as found on dust hoppers in the gas cleaning train,
other dust collectors. These usually have bag fil- controls should be set to avoid pulling on empty
ters and the cleaning cycles should be adjusted to hoppers.
minimize draft loss and reduce fan power con- Larger industrial coal-burning installations
sumption. may find a conversion to a mechanical system
Pneumatic fuel-handling systems of both the practical. Power consumption is far less but this
lean and dense phase kinds are used by a num- would have to be weighed against the capital cost
ber of plants. They usually consume more power of a new system.
than mechanical types on a kW per ton basis. Ash systems serving biomass-fired boilers
Good maintenance procedures can conserve generally use mechanical systems. Finally, hy-
power in these systems. In some cases the system draulic ash systems are rarely used because of
can be redesigned to eliminate changes in direc- high power consumption and water usage. These
tion, significantly reducing line loss. Longer term systems can be converted to mechanical or pneu-
solutions might include replacing the pneumatic matic to solve probable water cleanup problems
system altogether. and potentially reduce power use depending on
the type of replacement system selected.
Ash Handling Systems
Although ash handling systems move lower ton- Boiler Sootblowers
nage of material than fuel handling systems, they
often consume far more power than necessary Efficient heat transfer is one of the major con-
because of their design and mode of operation. tributors to an energy-efficient boiler system.
The most common types move ash by inducing a One of the most important boiler auxiliary op-
Energy Efficiency Handbook 37
Generation and cogeneration operation and
Figure 7-2. Industrial steam-electric cogeneration, system
thermal efficiency 80 to 90%.
blowing frequency depends on slag buildup, but
Figure 7-1. Static pressure rise and power versus air a normal range would be 4-8 hours.
flow. Superheater, reheater and economizer sec-
tions of the boiler are cleaned with long, retract-
able lances which are most effective at cleaning
erations is the on-line, in-service fireside cleaning radiant and convective heat surfaces. Effective
of heat absorbing surfaces. This operation per- cleaning radius, using a helical blowing pattern
forms two important functions; it assures proper from two nozzles, is 4 to 9 feet.
heat transfer and also prevents sections of the
boiler from becoming plugged. Plugged sections Sootblower Operation
can restrict gas flow and cause load limitations.
Sootblowing systems are required on coal Since deposits in the radiant and convection sec-
and oil-fired furnaces. Because oil has a low ash tions of the boiler can vary from hard slag to a
content and the residue is a thin, water-soluble dry powdery coating, the blowing sequence is not
substance, it’s removal is done by water-washing set by a hard and fast rule. Sequence and fre-
the furnace walls during the annual shutdown. quency, instead, must be adjusted during initial
For this reason, furnace-wall sootblowers are not operation by starting with an assumed sequence
required on oil-fired units. and frequency of perhaps one complete cycle per
In the superheater and reheater sections of 8-hour shift. The operator should observe foul-
oil-fired units, there is ash buildup on the tubing ing patterns either through observation doors
surfaces. High-vanadium content oils, containing (during operation) or by gas-side inspection dur-
additives to combat high-temperature corrosion, ing shutdowns. Necessary adjustments can then
are especially prone to this. But when solid- be made. Once the pattern is established it can
powder additives are employed, ash deposits in be implemented and run automatically.
the high-gas-temperature areas increase mark-
edly. Fortunately, these deposits crumble and
pulverize easily and are readily removed with Cogeneration
Coal-fired units require large numbers of Cogeneration combines the production of electri-
permanently installed sootblowing equipment. cal and thermal energy for eating and process
Factors such as ash-fusion temperatures and the use from a single act of combustion. It uses less
percentage of ash in the coal determine just what total fuel than needed to produce the two forms
sootblowing coverage is required. separately. Cogeneration represents a major step
Superheated steam or compressed air is the towards maximum energy efficiency and also
medium used to remove deposits, employing a contributes to reduced pollution.
short, single-nozzle retractable blower and clean- For example, in most cases, a company gen-
ing a surface with a five-foot radius. The effective erates its own steam and purchases electric power
38 Council of Industrial Boiler Owners
Generation and cogeneration operation and
Figure 7-3. Industrial combination turbine combined cycle
cogeneration, system thermal efficiency 80 to 90%.
from the local utility. The typical industrial
steam generation steam-electric cycle has a ther-
mal efficiency of 75-85 percent, whereas the util-
ity cycle has a thermal efficiency of only about 35
percent. The net thermal efficiency of this sys-
tem, depending on the relative amount of steam
and electricity required could be in the range of
66 to 80 percent.
In a typical industrial steam turbine cogen-
eration cycle, however, high pressure steam first
produces electric power and then is used for
process needs. Because such a facility can attain
an overall efficiency of up to 88 percent the sav- Figure 7-4. Generation and cogeneration operation and
ings in fuel use can be as high as 15-20 percent. benefits comparisons. Above: Separate steam and elec-
Fuel used can be gas, oil, coal, wood, municipal tric cogeneration. Below: Combined industrial steam and
solid waste or industrial wastes. electric cogeneration.
In a gas turbine combined-cycle cogeneration
facility, after generating electricity, the hot ex-
haust from the gas combustion turbine-generator temperature processes to generate electricity by
is used to make steam. The steam can be used generating steam in a waste heat (heat recovery)
for electric power, process needs or space heat- boiler.
ing. Overall thermal efficiencies could be as high
as 90 percent. This compares to a maximum fuel
efficiency to produce the steam and power sepa- Sale of Excess Electricity
rately of 58 percent when a combustion turbine is
used. Fuel is usually limited to natural gas or oil. A further economic consideration for Cogenera-
Cogeneration plants are either topping or tion is the ability to sell excess power generated
bottoming systems. A topping system has the to the local utility. Federal regulations, intended
steam producing electric power first and all or a to encourage cogeneration, compel the utility to
part of the exhausted thermal energy is then purchase a cogenerator’s excess power at a price
used in industrial processes or for space heating determined by the utilities (called “avoided cost”)
or cooling. Bottoming systems use the waste heat or cost of power displaced by the cogenerator’s
from industrial processes or other high- output.
Energy Efficiency Handbook 39