Fan Applications

3.2.1 Induced Draft Fans

ID fans move the combustion flue gas through the boiler, air heater, and precipitator or the baghouse, scrubber, and chimney to the atmosphere. They are a major component of a fossil-fired plant and typically consume approximately 2% of the gross electrical output.

ID fans have the largest design margins of any major equipment in a fossil-fueled power plant.

The margins are typically 15% on flow, 30% on head, and 25°F on temperature. The large margins are intended to allow for the following:

• Uncertainty in determining system requirements

• Allowance for wear

• Operating flexibility

• Allowance for pluggage and leakage

• Air infiltration

Even with these large margins, it is not uncommon for the ID fans to be the limiting factor on the output of a coal-fired unit. ID fans are included in the top 25 causes of fossil plant outages and are responsible for approximately 2% of the total outages of fossil-fired units.

The temperature of gas to be handled by the ID fan is based on the calculated unit performance at maximum boiler load. Temperature affects fan performance, and thus, a margin on temperature should be included to allow for variations in operation.

Key Technical Point

Temperature affects fan performance, and thus, a margin on temperature should be included to allow for variations in operation.

3.2.2 Forced Draft Fans

FD fans provide combustion air for boilers. The FD fan inlet is open to the atmosphere and discharges through air preheating coils, an air heater into the boiler windbox, and finally through the burners into the furnace.

In pulverized coal-fired boilers, approximately one-third of the combustion air is PA that is used to transport the pulverized coal to the burners. Some boiler designs use PA fans, which may take suction from the atmosphere and operate in parallel with the FD fans or may take suction from the FD fan discharge and operate in series with the FD fans. The PA fan application is similar to the FD fan; therefore, the description, problem area, and maintenance requirements described for

FD fans are essentially the same for PA fans. Other boiler designs use mill exhausters that take the air and pulverized coal mixture from the mill outlet and transport the mixture to the burners.

Because of the erosive nature of the pulverizer coal-air mixture, mill exhausters have a very different application than the FD, ID, or PA fans and are not addressed in this guide.

For pressurized units without ID fans, the FD fan is sized for the entire system to the stack or to the pollution control system.

FD fans for coal-fired plants rank close behind ID fans as the cause of outages. The causes of FD fan failures are similar to those for ID fans. The FD fans for a coal-fired plant consume

approximately 0.7% of the gross electrical output.

The design margins on FD fans are typically smaller than the margins on ID fans but still larger than on other major equipment. Margins of 15% on flow and 30% on head at the maximum expected ambient temperature are common.

FD fans are normally equipped with sound trunks (inlet boxes) for noise attenuation. When specifying FD fans, pressure loss through the silencers (if they are provided) must be taken into consideration. An alternative method of noise attenuation is using a fan room. This involves the use of open inlet FD fans located in a specially designed room with acoustical baffles for air entry.

Key Technical Point

When specifying FD fans, pressure loss through the silencers (if they are provided) must be taken into consideration.

3.2.3 Balanced Draft

The balanced draft system uses both an FD fan system and an ID fan system to move air through the boiler.

FD fans on a balanced draft boiler must have the necessary volume output of air required for combustion, plus air heater losses and discharge pressure high enough to equal the total

resistance of air ducts, air heater, burners, and any other resistance between fan discharge and the furnace.

ID fans in a balanced draft boiler move the gaseous products of combustion over convection heating surfaces, pollution control system(s), plus the gas passages between the furnace and stack.

The weight of gas to be handled by the ID fan is the sum of the following:

• Theoretical air for combustion

• Excess air required at burner

• Infiltration

• Leakage air-to-gas through the air heater

The draft to be provided by the fan is determined by losses through the following boiler components:

• Furnace

• Boiler and superheater

• Economizer

• Selective catalytic reduction (SCR)

• Air heater

• Precipitator or baghouse

• Ductwork

• Flue gas desulfurization system (scrubber)

• Stack

For fan design, safety margins are added to the net weight requirement, net draft requirement, and gas temperature.

3.2.4 Cold Primary Air Fans

Cold primary air fans take ambient air and discharge it through the air heater, where the air is heated up to 650ºF (the actual temperature depends on the moisture content of the coal), and then into the pulverizers—where it is used to dry, heat, and convey the pulverized coal to the burners.

This system is used on large boilers where fans are installed in parallel in order to service a bank of pulverizers.

3.2.5 Hot Primary Air Fans

Hot primary air fans take heated air from the air heater and blow it into the pulverizers where it is used to dry, heat, and convey the pulverized coal to the burners.

3.2.6 Gas Recirculation Fans

A gas recirculation system performs either or both of the following functions:

• Controls steam temperature over a wide boiler load range. To accomplish steam temperature control, a portion of the flue gas from the economizer outlet is introduced in the lower part of the furnace by means of a suitable fan and ducts. This is known as gas recirculation.

• Controls furnace gas temperature when a portion of the flue gas from the economizer outlet is recirculated to the furnace outlet. This is called gas tempering and may be used to control NOX.

The volume requirements of the gas recirculating fan are determined by the amount of

recirculation necessary to obtain the required steam temperature. Maximum flow can occur at either full boiler load or some reduced boiler load point, depending on boiler design. The gas recirculation fan must be sized so its pressure capability will always exceed the pressure differential developed by the boiler; otherwise, backflow of high-temperature furnace gas will result through the fan, with serious consequences.

Radial tip blade fans (see Figure 3-6) can be applied for gas recirculation duty, but the straight blade fan may be needed where high concentrations of fly ash will be encountered, depending on the ash properties. Inlet dampers are the principal means of accomplishing volume control.

3.2.7 Number of Fans

One of the major design decisions for a fan system is the number of fans. The factors to be considered in selecting the number of fans are initial cost, operation and maintenance (O&M) costs, flexibility of operation, and reliability.

When evaluating initial cost, the cost of motors, ductwork, insulation, control equipment, electrical equipment, and foundations must be considered in addition to the cost of the fans. The fewest number of fans usually results in the lowest initial cost.

Key O&M Cost Point

The fewest number of fans usually results in the lowest initial cost.

Operating cost usually decreases as the number of fans increases. Fans usually have their highest efficiency near their design points. At lower loads, some of the fans can be shut off in a system with more fans. The remaining fans will then operate closer to their design points and, therefore, more efficiently. An important parameter for evaluating operating cost is the projected loading schedule for the generating unit. The variation in operating cost with the number of fans will be less for a unit that operates at or near full load most of the time than for a unit that operates at

It is difficult to assign cost values to the differences in reliability and flexibility of operation with different numbers of fans. It is also difficult to assign a cost value to plant arrangement. Plant arrangements can be improved by reducing the number of fans. For these reasons, the selection of the number of fans is not a straightforward economic evaluation.

In addition to the factors discussed above, practical aspects should also be considered. For centrifugal fans, because of fan size limitations, the maximum unit size for which two ID fans can be used is approximately 700 MW. The same number of FD and ID fans is usually selected to simplify operation of the fans.

The trend in the power industry has been to use two FD and two ID fans up to approximately 700 MW. Above this size, it is not practical to build ID fans that are large enough to use two fans. In recent years, this concept has been challenged, and units with one FD and one ID fan have been built up to a limit of 500 MW. Obviously, the cost for one fan and motor is less than for two, but the bulk of the savings comes from the reduced number of ducts as well as capital costs.

The number of ID fans can have an effect on unit availability. Many owners believe that two fans will provide better availability than one. However, with two fans, the probability of a fan failure is roughly twice that for one fan, but the impact of a failure is approximately one-half. Thus, the equivalent availability is about the same. There is not a large enough database of boilers with single FD and ID fans to verify this probability, but statistical studies (using the NERC-GADS database) of boiler feed pumps verify this theory. A boiler with one full-size boiler feed pump has more full forced outages than units with two 50% capacity pumps but fewer forced deratings.

The overall equivalent availability of one full-size feed pump is slightly higher than two half-size pumps.

On large units with four ID fans, the unit will probably be capable of operating at full load with three fans under normal operating conditions. Thus, the unit will essentially have an installed spare, which should result in improved availability.