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#BoilerManual #AirAndGasFlow #Section3 #Page42

Answers for air and gas flow

1. During a fan startup, both the inlet vanes and discharge damper should be closed until the fan is up to speed. This prevents overloading the motor.

2. Secondary air is the major source of oxygen required for proper combustion in the cyclone. Primary air starts the coal in a whirling motion in the burner. Tertiary air simply cools the burner front.

3. The three types of heat transfer are:

............. 1.Radiation
............. 2. Convection
............. 3. Conduction

4. This sharp change in direction causes large ash particles to fall outof the gas stream.

5. Air flow calculations require: differential pressure, air density and temperature.

6. A lower air temperature causes an increase in density, and a corresponding increase in air flow. Therefore, you must decrease the delta-P {sic}.

7. Gas tempering will lower temperatures entering the superheater.

8. Gas recirculation forms an insulating layer around the furnace walls. This decreases heat transfer in the furnace.

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#BoilerManual #AirAndGasFlow #Section3 #Page42

Answers for air and gas flow

1. During a fan startup, both the inlet vanes and discharge damper should be closed until the fan is up to speed. This prevents overloading the motor.

2. Secondary air is the major source of oxygen required for proper combustion in the cyclone. Primary air starts the coal in a whirling motion in the burner. Tertiary air simply cools the burner front.

3. The three types of heat transfer are:

............. 1.Radiation
............. 2. Convection
............. 3. Conduction

4. This sharp change in direction causes large ash particles to fall outof the gas stream.

5. Air flow calculations require: differential pressure, air density and temperature.

6. A lower air temperature causes an increase in density, and a corresponding increase in air flow. Therefore, you must decrease the delta-P {sic}.

7. Gas tempering will lower temperatures entering the superheater.

8. Gas recirculation forms an insulating layer around the furnace walls. This decreases heat transfer in the furnace.

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#BoilerManual #AirAndGasFlow #Section3 #Page41

7. Protecting the superheater tubes from high temperatures during startup is accomplished by gas recirculation, or gas tempering?

8. Does gas recirculation in the lower furnace cause an increase or decrease in the heat transfer rate?

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#BoilerManual #AirAndGasFlow #Section3 #Page41

7. Protecting the superheater tubes from high temperatures during startup is accomplished by gas recirculation, or gas tempering?

8. Does gas recirculation in the lower furnace cause an increase or decrease in the heat transfer rate?

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#BoilerManual #AirAndGasFlow #Section3 #Page40

Questions for Air and Gas Flow

1. When starting up a fan, what position should its inlet vane and discharge damper be in?

2. What are the purposes of secondary air, primary air and tertiary air?

3. Name the three types of heat transfer that occur in the boiler.

..........1. ____________________________________________

..........2. ____________________________________________

..........3. ____________________________________________

4. To exit the boiler, flue gas must take a sharp change in direction. What does this cause?

5. To calculate air flow, what three types of data are needed?

6. To keep secondary air flow constant while the gas temperature drops, would you increase or decrease the differential pressure?

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#BoilerManual #AirAndGasFlow #Section3 #Page40

Questions for Air and Gas Flow

1. When starting up a fan, what position should its inlet vane and discharge damper be in?

2. What are the purposes of secondary air, primary air and tertiary air?

3. Name the three types of heat transfer that occur in the boiler.

..........1. ____________________________________________

..........2. ____________________________________________

..........3. ____________________________________________

4. To exit the boiler, flue gas must take a sharp change in direction. What does this cause?

5. To calculate air flow, what three types of data are needed?

6. To keep secondary air flow constant while the gas temperature drops, would you increase or decrease the differential pressure?

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#BoilerManual #AirAndGasFlow #Section3 #Page39

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Alt = Labeled Fig. 33 at the bottom; labeled at top REHEATER DESIGN CURVES. This graph is laid out similarly to Figure 32 with the x axis laid out 1.4 to 4.2 in increments of .2 but is marked as Main steam flow, 10 to the 6th power lb/hr and likely applies to the x axis in Figure 32. The y axis is in 2 segments with the lower portion marked 400 to 1000 in increments of 100 labeled Steam temperatures F. The upper portion is 0 to 10 incrementing by twos, and is labeled Pressure, 100 psig water flow 10,000 lb/hr. in the upper left corner of the graph is printed "900 psig design water pressure at spray nozzle". Two lines occupy the upper section of the graph, marked (top-down order) Steam pressure at R.H. inlet which ends at the 4.2 mark on the x axis and a dot, marked Orifice design flow 54,000 lb/hr; below that line is a straight line drawn just above the 0 mark labeled "Expected R.H. spray water flow".

The lower section has two lines marked, in top-down order, Reheat outlet and Reheat inlet. The upper line gradually curves from the 1.4 mark at about 950 F until it reaches the 1000 mark at about 2.8 then flattens out for the rest of the graph; The lower line starts at just short of the 500 F mark at 1.4 and gradually rises to just about 600 at the 4.2 mark.

#BoilerManual #AirAndGasFlow #Section3 #Page39

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Alt = Labeled Fig. 33 at the bottom; labeled at top REHEATER DESIGN CURVES. This graph is laid out similarly to Figure 32 with the x axis laid out 1.4 to 4.2 in increments of .2 but is marked as Main steam flow, 10 to the 6th power lb/hr and likely applies to the x axis in Figure 32. The y axis is in 2 segments with the lower portion marked 400 to 1000 in increments of 100 labeled Steam temperatures F. The upper portion is 0 to 10 incrementing by twos, and is labeled Pressure, 100 psig water flow 10,000 lb/hr. in the upper left corner of the graph is printed "900 psig design water pressure at spray nozzle". Two lines occupy the upper section of the graph, marked (top-down order) Steam pressure at R.H. inlet which ends at the 4.2 mark on the x axis and a dot, marked Orifice design flow 54,000 lb/hr; below that line is a straight line drawn just above the 0 mark labeled "Expected R.H. spray water flow".

The lower section has two lines marked, in top-down order, Reheat outlet and Reheat inlet. The upper line gradually curves from the 1.4 mark at about 950 F until it reaches the 1000 mark at about 2.8 then flattens out for the rest of the graph; The lower line starts at just short of the 500 F mark at 1.4 and gradually rises to just about 600 at the 4.2 mark.

#BoilerManual #AirAndGasFlow #Section3 #Page38

Reheat Spray Attemperator (1)

The reheat spray attemperator functions in the same manner as the superheater sprays. The reheat spray attemperator is located in the cold reheat line between the high pressure turbine exhaust and the reheat inlet header. Spray flow to the reheat attemperator is taken off a bleed point on the boiler feed pumps.

The design curves associated with the reheat spray attemperator are illustrated in Figure 33. Notice that throughout the load range, there is no expected spray flow. Of course, this is based upon proper gas recirculation flows and normal operating conditions.

We have now discussed how the combustion air is brought to the cyclones and how the combustion gases are made to flow through the furnace, convection pass, air heaters, fans and stacks. We have also examined methods of steam temperature control with the use of gas recirculation and spray attemperation. The more detailed effects of gas recirculation were related to heat absorption patterns throughout the unit.

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#BoilerManual #AirAndGasFlow #Section3 #Page38

Reheat Spray Attemperator (1)

The reheat spray attemperator functions in the same manner as the superheater sprays. The reheat spray attemperator is located in the cold reheat line between the high pressure turbine exhaust and the reheat inlet header. Spray flow to the reheat attemperator is taken off a bleed point on the boiler feed pumps.

The design curves associated with the reheat spray attemperator are illustrated in Figure 33. Notice that throughout the load range, there is no expected spray flow. Of course, this is based upon proper gas recirculation flows and normal operating conditions.

We have now discussed how the combustion air is brought to the cyclones and how the combustion gases are made to flow through the furnace, convection pass, air heaters, fans and stacks. We have also examined methods of steam temperature control with the use of gas recirculation and spray attemperation. The more detailed effects of gas recirculation were related to heat absorption patterns throughout the unit.

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#BoilerManual #AirAndGasFlow #Section3 #Page37

The design curves associated with the superheater spray attemperators can be seen in Figure 32.

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Alt = A rather confusing graph where the x axis isn't labeled anything but is set out from 1.4 to 4.2 marked out in .2 increments. The y axis has 3 label segments but appears to have only two. The lowest y segment is in steam temp. (F) from 700 to 1000 in increments of 100 then above the 1000 mark is the 0 line, starting the second segment marked Spray, water flow attemperator, M lb/hr; it's marked out 0 to 200 in increments of 50. Above the 200 mark begins the third segment marked Pressure - psig with a line marked 2400. It runs in increments of 200 until the 3600 mark, then an unmarked line above that. Just above the 3600 line is a solid line drawn across to the 3.4 point on the x axis, marked Design pressure of spray piping from economizer to control valve. Below those, in top down order, are lines representing Water at econ. inlet; Steam at sec. S.H. inlet; Normal spray 4% (these 3 lines are across the middle segment of the y axis); Leaving secondary superheater (a straight line across the 1000 mark in segment 1); between the 700 and 800 lines on segment 1 are marked Leaving primary superheater; Entering secondary superheater.

#BoilerManual #AirAndGasFlow #Section3 #Page37

The design curves associated with the superheater spray attemperators can be seen in Figure 32.

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Alt = A rather confusing graph where the x axis isn't labeled anything but is set out from 1.4 to 4.2 marked out in .2 increments. The y axis has 3 label segments but appears to have only two. The lowest y segment is in steam temp. (F) from 700 to 1000 in increments of 100 then above the 1000 mark is the 0 line, starting the second segment marked Spray, water flow attemperator, M lb/hr; it's marked out 0 to 200 in increments of 50. Above the 200 mark begins the third segment marked Pressure - psig with a line marked 2400. It runs in increments of 200 until the 3600 mark, then an unmarked line above that. Just above the 3600 line is a solid line drawn across to the 3.4 point on the x axis, marked Design pressure of spray piping from economizer to control valve. Below those, in top down order, are lines representing Water at econ. inlet; Steam at sec. S.H. inlet; Normal spray 4% (these 3 lines are across the middle segment of the y axis); Leaving secondary superheater (a straight line across the 1000 mark in segment 1); between the 700 and 800 lines on segment 1 are marked Leaving primary superheater; Entering secondary superheater.

#BoilerManual #AirAndGasFlow #Section3 #Page36

The spray nozzles consist of an inlet pipe leading to a whirling chamber and orifice. The orifice discharges the spray. Steam passes the spray nozzle, picks up the spray, and flows through the liner which completes the evaporation and thoroughly mixes the steam prior to entering the secondary superheater. The connection of the inlet pipe to the steam line embodies a thermal sleeve which protects the steam lines from temperature differential between the spray water and the steam.

Spray water is supplied to the SH spray attemperators from feedwater entering the economizer. It is important that thte spray water be of the highest purity, since solids entrained in the spray water enter the steam, and if excessive may cause deposits on superheater tubes, piping or turbine blades. Care should be taken not to spray the steam so much as to bring the final steam temperature below the saturation point. If the temperature falls below saturation, water droplets form, which can erode tubes and headers or damage the turbine.

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Alt = labeled Fig. 31 Spray attemporator. Image is a diagram of an attemperator sliced lengthwise, with its parts pointed out.Spray water inlet is atop a cylinder laid horizontally connected to a steam line. The cylinder part has an internal liner and the Spray water inlet has an internal tube that enters the center of the cylinder with a sprayer head a the end, and the spray direction is indicated by an arrow that runs the same direction as the arrow marked Steam flow.

#BoilerManual #AirAndGasFlow #Section3 #Page36

The spray nozzles consist of an inlet pipe leading to a whirling chamber and orifice. The orifice discharges the spray. Steam passes the spray nozzle, picks up the spray, and flows through the liner which completes the evaporation and thoroughly mixes the steam prior to entering the secondary superheater. The connection of the inlet pipe to the steam line embodies a thermal sleeve which protects the steam lines from temperature differential between the spray water and the steam.

Spray water is supplied to the SH spray attemperators from feedwater entering the economizer. It is important that thte spray water be of the highest purity, since solids entrained in the spray water enter the steam, and if excessive may cause deposits on superheater tubes, piping or turbine blades. Care should be taken not to spray the steam so much as to bring the final steam temperature below the saturation point. If the temperature falls below saturation, water droplets form, which can erode tubes and headers or damage the turbine.

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Alt = labeled Fig. 31 Spray attemporator. Image is a diagram of an attemperator sliced lengthwise, with its parts pointed out.Spray water inlet is atop a cylinder laid horizontally connected to a steam line. The cylinder part has an internal liner and the Spray water inlet has an internal tube that enters the center of the cylinder with a sprayer head a the end, and the spray direction is indicated by an arrow that runs the same direction as the arrow marked Steam flow.

#BoilerManual #AirAndGasFlow #Section3 #Page35

to zero at full load. Superheater (SH) spray flow is less at full load than at lower loads, but is still required. Reheat spray is normally not required since only enough gas recirculation flow is used to raise the reheat steam temperature to design. The exception to this is at or very near full load, when no gas recirculation is required. Even with no gas recirculation, reheat steam temperatures may be high at full load.

Some gas tempering with a slight reheat spray may be necessary. The main steam attemperator control valves are normally set at their minimum spray position. They are sized to spray 4% of full load feedwater flow at full load in this position. During upset conditions, they vary flow until stable conditions are attained.

[b]Superheat Spray Attemperators (2)

Steam temperature can be controlled to within plus or minus 10 F of design parameters by the use of spray attemperation. It provides a quick acting and sensitive means of steam temperature control. One attemperator spray nozzle (Figure 31) is located in each of the two connecting lines from the primary superheater outlet to the secondary superheater inlets.

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Alt = Labeled Fig. 30 Steam temperature with controlled G.R. flow and attemperator flow. A small simplified graph with x axis in terms of Boiler load (percent) with only the 0 and 100 points are marked. The y axis is in terms of Steam temp. with only the points of 0 to 1005 marked, and there's a vertical dashed line at some unidentified x position to the left side of the graph, and a horizontal line drawn across at the 1005 mark, and there are 3 curves depicted: Reheat, Controlled G.R. flow (this arcs from high near the Reheat curve above it, then intersects with the lower S.H. attemp. flow curve below it; the S.H. curve isn't curved but takes a sharp downward angle somewhere to the right of that dashed line). The topmost Reheat curve arches briefly from left to right where the dashed line and the 1005 mark intersect, and presumably flattens out across the 1005 line after that. To the right of the graph is marked "Gas recirculation Attemperator spray flow".

#BoilerManual #AirAndGasFlow #Section3 #Page35

to zero at full load. Superheater (SH) spray flow is less at full load than at lower loads, but is still required. Reheat spray is normally not required since only enough gas recirculation flow is used to raise the reheat steam temperature to design. The exception to this is at or very near full load, when no gas recirculation is required. Even with no gas recirculation, reheat steam temperatures may be high at full load.

Some gas tempering with a slight reheat spray may be necessary. The main steam attemperator control valves are normally set at their minimum spray position. They are sized to spray 4% of full load feedwater flow at full load in this position. During upset conditions, they vary flow until stable conditions are attained.

[b]Superheat Spray Attemperators (2)

Steam temperature can be controlled to within plus or minus 10 F of design parameters by the use of spray attemperation. It provides a quick acting and sensitive means of steam temperature control. One attemperator spray nozzle (Figure 31) is located in each of the two connecting lines from the primary superheater outlet to the secondary superheater inlets.

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Alt = Labeled Fig. 30 Steam temperature with controlled G.R. flow and attemperator flow. A small simplified graph with x axis in terms of Boiler load (percent) with only the 0 and 100 points are marked. The y axis is in terms of Steam temp. with only the points of 0 to 1005 marked, and there's a vertical dashed line at some unidentified x position to the left side of the graph, and a horizontal line drawn across at the 1005 mark, and there are 3 curves depicted: Reheat, Controlled G.R. flow (this arcs from high near the Reheat curve above it, then intersects with the lower S.H. attemp. flow curve below it; the S.H. curve isn't curved but takes a sharp downward angle somewhere to the right of that dashed line). The topmost Reheat curve arches briefly from left to right where the dashed line and the 1005 mark intersect, and presumably flattens out across the 1005 line after that. To the right of the graph is marked "Gas recirculation Attemperator spray flow".

#BoilerManual #AirAndGasFlow #Section3 #Page34

SPRAY ATTEMPERATION FOR STEAM TEMPERATURE CONTROL

Gas recirculation is used to control reheat steam temperature. Spray attemperators, located in the fluid path upstream of the secondary superheater, are used to lower main stream temperatures to design.

The normal steam temperatures with gas recirculation and attemperator flows are shown in Figure 30. Notice that gas recirculation (GR) drops

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Alt = Labeled Fig. 29 Gas recirculation requirements -- it has a top title of GAS TEMPERING TO UPPER PORTS GAS RECIRCULATION TO LOWER PORTS-- and in the upper right corner has the notation "Three fans at T-50 2,250,000 lb/hr". the x axis is in terms of Main steam flow - 10 to the 6th power lb/hr, incremented from 0 to 4.4 in .4 increments. The y axis is in terms of recirculated gas weight - M lb/hr; incremented zero to 2400 in increments of 200. It maps out what was covered in detail in the main text.

#BoilerManual #AirAndGasFlow #Section3 #Page34

SPRAY ATTEMPERATION FOR STEAM TEMPERATURE CONTROL

Gas recirculation is used to control reheat steam temperature. Spray attemperators, located in the fluid path upstream of the secondary superheater, are used to lower main stream temperatures to design.

The normal steam temperatures with gas recirculation and attemperator flows are shown in Figure 30. Notice that gas recirculation (GR) drops

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Alt = Labeled Fig. 29 Gas recirculation requirements -- it has a top title of GAS TEMPERING TO UPPER PORTS GAS RECIRCULATION TO LOWER PORTS-- and in the upper right corner has the notation "Three fans at T-50 2,250,000 lb/hr". the x axis is in terms of Main steam flow - 10 to the 6th power lb/hr, incremented from 0 to 4.4 in .4 increments. The y axis is in terms of recirculated gas weight - M lb/hr; incremented zero to 2400 in increments of 200. It maps out what was covered in detail in the main text.

#BoilerManual #AirAndGasFlow #Section3 #Page33

Flue gas tempering and recirculation are used from initial firing through initial turbine loading in order to obtain maximum furnace absorption with minimal gas temperature entering the convection pass. Flue gas temperatures leaving the furnace and entering the secondary superheater must be limited to 1000 F until 10% of full load steam flow is passing through the superheater. As stated before, this is to insure that a proper cooling flow exists in the superheater in order to prevent overheat tube failures. Gas recirculation requirements over the entire load range is illustrated in Figure 29.

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Alt = Labeled Fig. 28 Effect of gas tempering on heat-absorption pattern at a constant firing rate. Image of a sectioned-off graph where the x axis is in terms of Change in heat absorption-- % and increments every 5 but marked in only 10s. The lower section of the y axis is in terms of Change in heat absorption -- % but point of origin starts at -40. It increases incrementally every 10 but only the 20 lines are marked, and there's a miniature drawing, a rendering of a furnace with 4 arrows pointing inward toward it between the lines for +40. There's an arrow pointing away from the area marked Econ at the line for 1800 of the upper graph segment, marked to AH. superheater/reheat areas are also marked on the drawing. The upper graph segment is marked in terms of Gas Temp. - F from 1800 to 2400 at every 2000 increment although increments are every 1000. it shows a sagging curve from the 2400 point at 0y, declining to just over 1800 at the point of 60y.

The lower segment displays one curve each for, in top down order, Economizer, Primary Superheater, Reheater, Furnace and Secondary superheater.

#BoilerManual #AirAndGasFlow #Section3 #Page33

Flue gas tempering and recirculation are used from initial firing through initial turbine loading in order to obtain maximum furnace absorption with minimal gas temperature entering the convection pass. Flue gas temperatures leaving the furnace and entering the secondary superheater must be limited to 1000 F until 10% of full load steam flow is passing through the superheater. As stated before, this is to insure that a proper cooling flow exists in the superheater in order to prevent overheat tube failures. Gas recirculation requirements over the entire load range is illustrated in Figure 29.

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Alt = Labeled Fig. 28 Effect of gas tempering on heat-absorption pattern at a constant firing rate. Image of a sectioned-off graph where the x axis is in terms of Change in heat absorption-- % and increments every 5 but marked in only 10s. The lower section of the y axis is in terms of Change in heat absorption -- % but point of origin starts at -40. It increases incrementally every 10 but only the 20 lines are marked, and there's a miniature drawing, a rendering of a furnace with 4 arrows pointing inward toward it between the lines for +40. There's an arrow pointing away from the area marked Econ at the line for 1800 of the upper graph segment, marked to AH. superheater/reheat areas are also marked on the drawing. The upper graph segment is marked in terms of Gas Temp. - F from 1800 to 2400 at every 2000 increment although increments are every 1000. it shows a sagging curve from the 2400 point at 0y, declining to just over 1800 at the point of 60y.

The lower segment displays one curve each for, in top down order, Economizer, Primary Superheater, Reheater, Furnace and Secondary superheater.

#BoilerManual #AirAndGasFlow #Section3 #Page32

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#BoilerManual #AirAndGasFlow #Section3 #Page32

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#BoilerManual #AirAndGasFlow #Section3 #Page31

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Alt = The image is sideways with the bottom along the right edge of the page and the top along the left. There are 4 different graphs arrayed in this image; Fig. 23 labeled Steam temperature without G.R. flow is in the upper left; Fig. 24 labeled Steam temperature with constant G.R. flow is on the upper right. The bottom two, from left to right, are Fig. 25 labeled Steam temperature with controlled G.R. flow and Fig. 26 labeled Steam temperature without G.R. flow. All 4 graphs have y axes in terms of Steam temp. (F) from 0 to 1005, with a dotted line across the graph at the 1005 mark; x axes are all marked Boiler load (percent), 0 to 100.

Fig. 23 displays an upward arching curve marked Uncontrolled steam temp. Fig. 24 has a solid horizontal line across the graph of undetermined value marked Constant G.R. flow and similarly displays an upward arching curve marked Steam temp. which overshoots the 1005 mark. Fig. 25 shows an upward arch in a curve that flattens out at the 1005 mark, marked Stem temp. Another curve is arching but going downward near the 100 mark; the curve is marked Controlled G.R. flow. Fig. 26 shows 2 gently arching curves, both below the 1005 mark, and not quite parallel to each other, with the upper curve marked Main steam and the lower curve marked Reheat. The two curves approach convergence from left to right but don't converge, as they approach the 100 mark on the x axis and 1005 mark on the y axis.

#BoilerManual #AirAndGasFlow #Section3 #Page31

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Alt = The image is sideways with the bottom along the right edge of the page and the top along the left. There are 4 different graphs arrayed in this image; Fig. 23 labeled Steam temperature without G.R. flow is in the upper left; Fig. 24 labeled Steam temperature with constant G.R. flow is on the upper right. The bottom two, from left to right, are Fig. 25 labeled Steam temperature with controlled G.R. flow and Fig. 26 labeled Steam temperature without G.R. flow. All 4 graphs have y axes in terms of Steam temp. (F) from 0 to 1005, with a dotted line across the graph at the 1005 mark; x axes are all marked Boiler load (percent), 0 to 100.

Fig. 23 displays an upward arching curve marked Uncontrolled steam temp. Fig. 24 has a solid horizontal line across the graph of undetermined value marked Constant G.R. flow and similarly displays an upward arching curve marked Steam temp. which overshoots the 1005 mark. Fig. 25 shows an upward arch in a curve that flattens out at the 1005 mark, marked Stem temp. Another curve is arching but going downward near the 100 mark; the curve is marked Controlled G.R. flow. Fig. 26 shows 2 gently arching curves, both below the 1005 mark, and not quite parallel to each other, with the upper curve marked Main steam and the lower curve marked Reheat. The two curves approach convergence from left to right but don't converge, as they approach the 100 mark on the x axis and 1005 mark on the y axis.

#BoilerManual #AirAndGasFlow #Section3 #Page30

Normally,steam temperature is lower than design at low boiler loads due to the reduced convection heat transfer which exists with low gas flows. This is shown in Figure 23. With a constant level of gas recirculation, steam temperature would increase throughout the load range, as shown in Figure 24. By running with high levels of gas recirculation at low loads, and gradually reducing gas recirculation as load increases, a constant steam temperature can be obtained over a wide load range, as shown in Figure 25.

When both main steam and reheat steam temperatures must be controlled, additional measures are required. Normally, without gas recirculation, reheat temperatures are lower than main steam temperatures, as shown in

Figure 26. With the proper levels of gas recirculation required to bring reheat steam temperatures to design

levels, main steam temperatures would be high as shown in Figure 27.

FLUE GAS TEMPERING

Another method of steam temperature control is by use of flue gas tempering. Tempering gas is introduced into the furnace near the furnace outlet so it mixes with the gaseous products of combustion to reduce the gas temperature leaving the furnace. Generally, gas tempering has little effect on steam temperature.

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#BoilerManual #AirAndGasFlow #Section3 #Page30

Normally,steam temperature is lower than design at low boiler loads due to the reduced convection heat transfer which exists with low gas flows. This is shown in Figure 23. With a constant level of gas recirculation, steam temperature would increase throughout the load range, as shown in Figure 24. By running with high levels of gas recirculation at low loads, and gradually reducing gas recirculation as load increases, a constant steam temperature can be obtained over a wide load range, as shown in Figure 25.

When both main steam and reheat steam temperatures must be controlled, additional measures are required. Normally, without gas recirculation, reheat temperatures are lower than main steam temperatures, as shown in

Figure 26. With the proper levels of gas recirculation required to bring reheat steam temperatures to design

levels, main steam temperatures would be high as shown in Figure 27.

FLUE GAS TEMPERING

Another method of steam temperature control is by use of flue gas tempering. Tempering gas is introduced into the furnace near the furnace outlet so it mixes with the gaseous products of combustion to reduce the gas temperature leaving the furnace. Generally, gas tempering has little effect on steam temperature.

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#BoilerManual #AirAndGasFlow #Section3 #Page29

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Alt = Labeled Fig. 22 Effects of gas recirculation. Image is sideways with the bottom along the right edge and the top along the left edge. It is an image of two simlified drawings of a boiler furnace with arrows indicating direction of gas flow. the drawing on the left is marked "Gas flow without gas circulation" and the one on the right is marked "Gas flow with gas recirculation". The drawing on the right has two additional arrows drawn pointing up in the cyclone bank firebox, drawn thickly, and the arrows are marked Gas recirculation. On the penthouse at the top of the left drawing, two thin arrows are marked Convection heat. In the corresponding area of the drawing on the right, one arrow is added and the arrows are marked Convection heat increased.

#BoilerManual #AirAndGasFlow #Section3 #Page29

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Alt = Labeled Fig. 22 Effects of gas recirculation. Image is sideways with the bottom along the right edge and the top along the left edge. It is an image of two simlified drawings of a boiler furnace with arrows indicating direction of gas flow. the drawing on the left is marked "Gas flow without gas circulation" and the one on the right is marked "Gas flow with gas recirculation". The drawing on the right has two additional arrows drawn pointing up in the cyclone bank firebox, drawn thickly, and the arrows are marked Gas recirculation. On the penthouse at the top of the left drawing, two thin arrows are marked Convection heat. In the corresponding area of the drawing on the right, one arrow is added and the arrows are marked Convection heat increased.

#BoilerManual #AirAndGasFlow #Section3 #Page28

negligible effect on the total boiler heat absorption, the gas weight up the stack, and the efficiency.

Figure 21 shows the variation in heat absorption with gas recirculation introduced into the lower furnace. Introduction of gas at this location produces a reduction in furnace absorption and increases convection passs absorption. Gas recirculation introduced in the vicinity of the burning zone forms a protective envelope around much of the furnace walls and reduces the amount of radiant heat absorbed in the furnace . This results in a higher gas temperature and increased mass flow, both of which mean that more heat is available for convection heat transfer to the superheater and reheater. Steam temperature then increases as the amount of gas recirculation increases. This is illustrated in Figure 22.

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Alt = Labeled Fig 21 Effect of gas recirculation on heat-absorption pattern at a constant firing rate. Image of a sectioned-off graph where the x axis is in terms of Gas recirculation-- % and increments every 5 but marked in only 10s. The lower section of the y axis is in terms of Change in heat absorption -- % but point of origin starts at -40. It increases incrementally every 10 but only the 20 lines are marked, and there's a miniature drawing, a rendering of a furnace with 4 arrows pointing inward toward it between the lines for 50 and 60. There's an arrow pointing away from the area marked Econ at the line for 70, marked to AH. superheater/reheat areas are also marked on the drawing.

The upper section of the y axis consists of just three horizontal lines representing 1800 thru 2000 F Gas Temp. A curve is drawn across the upper section labeled Furnace Exit.

#BoilerManual #AirAndGasFlow #Section3 #Page28

negligible effect on the total boiler heat absorption, the gas weight up the stack, and the efficiency.

Figure 21 shows the variation in heat absorption with gas recirculation introduced into the lower furnace. Introduction of gas at this location produces a reduction in furnace absorption and increases convection passs absorption. Gas recirculation introduced in the vicinity of the burning zone forms a protective envelope around much of the furnace walls and reduces the amount of radiant heat absorbed in the furnace . This results in a higher gas temperature and increased mass flow, both of which mean that more heat is available for convection heat transfer to the superheater and reheater. Steam temperature then increases as the amount of gas recirculation increases. This is illustrated in Figure 22.

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Alt = Labeled Fig 21 Effect of gas recirculation on heat-absorption pattern at a constant firing rate. Image of a sectioned-off graph where the x axis is in terms of Gas recirculation-- % and increments every 5 but marked in only 10s. The lower section of the y axis is in terms of Change in heat absorption -- % but point of origin starts at -40. It increases incrementally every 10 but only the 20 lines are marked, and there's a miniature drawing, a rendering of a furnace with 4 arrows pointing inward toward it between the lines for 50 and 60. There's an arrow pointing away from the area marked Econ at the line for 70, marked to AH. superheater/reheat areas are also marked on the drawing.

The upper section of the y axis consists of just three horizontal lines representing 1800 thru 2000 F Gas Temp. A curve is drawn across the upper section labeled Furnace Exit.

#BoilerManual #AirAndGasFlow #Section3 #Page27

For clarity, recirculated gas introduced in the immediate vicinity of the initial burning zone of the furnace and used for steam temperature control will be referred to as gas recirculation. Recirculated gas introduced near the furnace outlet and used for control of gas temperature will be referred to as gas tempering. Figure 20 shows the application of recirculated gas and its two components.

An important feature of recirculated gas is that its use changes only the pattern of heat absorption through a boiler. Gas recirculation has a

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Alt = Labeled Fig. 20 Gas recirculation. Image is a simplified drawing of an entire boiler furnace focused on gas/air flow from just above the cyclone furnace firebox on the lower left, but a pair of arrows pointing in opposite directions at the point where the Gas recirculation fan ductwork enters, pointing up as well as down. The windbox ductwork around the cyclones runs to the right, to the Tubular air heater marked at the lower right. Just above that ductwork is located the Gas recirculation fan, appearing to be suspended in the middle between the air heater on the right and the windbox on the left. Following the upward pointing arrow up the furnace wall, then curving through the penthouse archway, then down the right side through the area marked Economizer, the flow continues on to both the Tubular air heater and the input to the Gas recirculation fan.

#BoilerManual #AirAndGasFlow #Section3 #Page27

For clarity, recirculated gas introduced in the immediate vicinity of the initial burning zone of the furnace and used for steam temperature control will be referred to as gas recirculation. Recirculated gas introduced near the furnace outlet and used for control of gas temperature will be referred to as gas tempering. Figure 20 shows the application of recirculated gas and its two components.

An important feature of recirculated gas is that its use changes only the pattern of heat absorption through a boiler. Gas recirculation has a

------------------------------------------------- 27 ------------------------------------------------------
Alt = Labeled Fig. 20 Gas recirculation. Image is a simplified drawing of an entire boiler furnace focused on gas/air flow from just above the cyclone furnace firebox on the lower left, but a pair of arrows pointing in opposite directions at the point where the Gas recirculation fan ductwork enters, pointing up as well as down. The windbox ductwork around the cyclones runs to the right, to the Tubular air heater marked at the lower right. Just above that ductwork is located the Gas recirculation fan, appearing to be suspended in the middle between the air heater on the right and the windbox on the left. Following the upward pointing arrow up the furnace wall, then curving through the penthouse archway, then down the right side through the area marked Economizer, the flow continues on to both the Tubular air heater and the input to the Gas recirculation fan.

#BoilerManual #AirAndGasFlow #Section3 #Page26

maintenance of safe clearances, it must be regulated within permissible limits by some means of accurate control.

Due to these turbine requirements, steam temperature must be controlled to correct fluctuations caused by operating variables. Probably the most frequent cause necessitating the need for control in regulating steam temperature is the deposit of ash in some form on the surface of saturated and steam heating elements. This condition can usually be corrected by simply changing the operating sequence and frequency of sootblowing.

The removal of feedwater heaters for servicing may be cited as another instance off variable operation. To maintain the same load, with the feedwater heaters out, requires an increase in the heat input to the boiler unit, with a corresponding increase in superheater or reheat absorption. Unless some means of control is available for regulating steam temperature to cover this condition, a drop in load might be required to protect both thte boiler tube metals and the turbine.

Flue Gas Recirculation for Steam Temperature Control

One method of steam temperature control is by the use of flue gas recirculation. Flue gas exiting the boiler is recirculated with a suitable fan and flue to the furnace. The flue gas is introduced in such a manner that furnace heat absorption is reduced by shielding a substantial part of the furnace wall area with a layer of recirculated gas. This results in making more heat available for superheater and reheater absorption.

In order to minimize fan erosion, specific requirements have been set as to location, direction and velocity of the recirculated flue gas takeoff. This will minimize the carryover of large ash particles into the recirculated gas stream and hence into the recirculated gas fan.

------------------------------------------------- 26 ------------------------------------------------------

#BoilerManual #AirAndGasFlow #Section3 #Page26

maintenance of safe clearances, it must be regulated within permissible limits by some means of accurate control.

Due to these turbine requirements, steam temperature must be controlled to correct fluctuations caused by operating variables. Probably the most frequent cause necessitating the need for control in regulating steam temperature is the deposit of ash in some form on the surface of saturated and steam heating elements. This condition can usually be corrected by simply changing the operating sequence and frequency of sootblowing.

The removal of feedwater heaters for servicing may be cited as another instance off variable operation. To maintain the same load, with the feedwater heaters out, requires an increase in the heat input to the boiler unit, with a corresponding increase in superheater or reheat absorption. Unless some means of control is available for regulating steam temperature to cover this condition, a drop in load might be required to protect both thte boiler tube metals and the turbine.

Flue Gas Recirculation for Steam Temperature Control

One method of steam temperature control is by the use of flue gas recirculation. Flue gas exiting the boiler is recirculated with a suitable fan and flue to the furnace. The flue gas is introduced in such a manner that furnace heat absorption is reduced by shielding a substantial part of the furnace wall area with a layer of recirculated gas. This results in making more heat available for superheater and reheater absorption.

In order to minimize fan erosion, specific requirements have been set as to location, direction and velocity of the recirculated flue gas takeoff. This will minimize the carryover of large ash particles into the recirculated gas stream and hence into the recirculated gas fan.

------------------------------------------------- 26 ------------------------------------------------------

#BoilerManual #AirAndGasFlow #Section3 #Page25

Why Control is Necessary

To hold the optimum heat rate of a turbine designed to utilize full throttle pressure over a given load range, it is essential to regulate steam temperature over t his range by an effective means of control.

The time in which a turbine may be brought to full load is established by the turbine manufacturer in accordance with a safe steam temperature-time curve. Since the temperature of the steam is directly related to the degree of expansion of the turbine elements, and consequently the

------------------------------------------------- 25 ------------------------------------------------------
Alt = A graph labeled Fig. 19 "Typical" calibration curve. The x axis is labeled "Primary air flow (1000 lb/hr)" and increments from 0 to 100 every 10 units.The y axis is labeled Entrance cone air flow differential (inches of water), and increments sequentially from 0 to 10. Three curves are plotted from the 0 origin curving upward, all of them stopping at the 8 mark on the y axis but on slightly different points along the x axis, from slightly below the 90 mark to not quite the 100 mark; the rightmost is marked 500 F. ; the middle curve is marked 600 F; the rightmost curve is marked 500 F.

#BoilerManual #AirAndGasFlow #Section3 #Page25

Why Control is Necessary

To hold the optimum heat rate of a turbine designed to utilize full throttle pressure over a given load range, it is essential to regulate steam temperature over t his range by an effective means of control.

The time in which a turbine may be brought to full load is established by the turbine manufacturer in accordance with a safe steam temperature-time curve. Since the temperature of the steam is directly related to the degree of expansion of the turbine elements, and consequently the

------------------------------------------------- 25 ------------------------------------------------------
Alt = A graph labeled Fig. 19 "Typical" calibration curve. The x axis is labeled "Primary air flow (1000 lb/hr)" and increments from 0 to 100 every 10 units.The y axis is labeled Entrance cone air flow differential (inches of water), and increments sequentially from 0 to 10. Three curves are plotted from the 0 origin curving upward, all of them stopping at the 8 mark on the y axis but on slightly different points along the x axis, from slightly below the 90 mark to not quite the 100 mark; the rightmost is marked 500 F. ; the middle curve is marked 600 F; the rightmost curve is marked 500 F.

#BoilerManual #AirAndGasFlow #Section3 #Page24

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Alt = Labeled Fig. 18 Entrance cone. Depiction of a barrel laying on its side and it's flared on the left side, with arrows going from the right pointing left, marked Primary air flow. On the top of the barrel is a capped pipe protruding, marked Low pressure tap (duct pressure). The left edge of the flared portion is marked Perforated screen, and the flared portion plus a portion of the barrel's left end is depicted as being inside thte wall of the windbox and is marked as such. Just above the barrel and protruding from the wall of the windbox is a capped pipe marked High pressure tap (windbox pressure).

#BoilerManual #AirAndGasFlow #Section3 #Page24

------------------------------------------------- 24 ------------------------------------------------------
Alt = Labeled Fig. 18 Entrance cone. Depiction of a barrel laying on its side and it's flared on the left side, with arrows going from the right pointing left, marked Primary air flow. On the top of the barrel is a capped pipe protruding, marked Low pressure tap (duct pressure). The left edge of the flared portion is marked Perforated screen, and the flared portion plus a portion of the barrel's left end is depicted as being inside thte wall of the windbox and is marked as such. Just above the barrel and protruding from the wall of the windbox is a capped pipe marked High pressure tap (windbox pressure).

#BoilerManual #AirAndGasFlow #Section3 #Page23

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Alt = Labeled Fig. 17 at the bottom, with a title at the top: "TYPICAL" CALIBRATION CURVE"
It is a graph with the x axis incrementing by 50 from 0 to 500 marked Secondary air flow (1000 lb/hr); y axis has standard increments from 0 to 15 and is marked Bellmouth air flow differential (inches of water). Three curves are plotted from the 0 origin curving upward, the rightmost curve not going much beyond the 400 mark along the x axis, and this curve is marked 500 F. The middle curve is similar but closer to the 350 mark on the x axis, and is marked 600 F. The leftmost curve goes closer still to the 350 mark on the x axis and is marked 700 F.

#BoilerManual #AirAndGasFlow #Section3 #Page23

------------------------------------------------- 23 ------------------------------------------------------
Alt = Labeled Fig. 17 at the bottom, with a title at the top: "TYPICAL" CALIBRATION CURVE"
It is a graph with the x axis incrementing by 50 from 0 to 500 marked Secondary air flow (1000 lb/hr); y axis has standard increments from 0 to 15 and is marked Bellmouth air flow differential (inches of water). Three curves are plotted from the 0 origin curving upward, the rightmost curve not going much beyond the 400 mark along the x axis, and this curve is marked 500 F. The middle curve is similar but closer to the 350 mark on the x axis, and is marked 600 F. The leftmost curve goes closer still to the 350 mark on the x axis and is marked 700 F.

#BoilerManual #AirAndGasFlow #Section3 #Page22

for the same pressure differential readings on all cyclones. Figure 17 shows a typical air flow calibration curve.

PRIMARY-TERTIARY AIR FLOW MEASUREMENT

Primary-tertiary air flow is measured much in the same manner. An entrance cone located inside the windbox provides the pressure differential required to calculate air flow (Figure 18). In both cases, a high pressure reading (windbox pressure) and a low pressure reading (duct pressure) are compared and create the necessary pressure differential. Primary-tertiary air flow should be calibrated so that equal primary-tertiary air flows are obtained for the same flow differential readings on all cyclones. Figure 19 shows a typical air flow calibration curve.

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Alt = Labeled Fig.16 Bellmouth for measuring secondary air flow differential. This is a depiction of the secondary air ductwork that would be attached to a cyclone; air flow is designated by two arrows on the left of the duct pointing rightward. Two dampers are in the illustration, the Shut-off damper is marked on the left and next to it on the right is marked Secondary air control damper (velocity damper). Capping the left side of the duct is a dome shape marked as Perforated screen, and the air flow arrows indicate air flowing through it. Attached to the topside of the dome is what appears to be a pocket next to but below where the Shut off damper pivots, and the pocket is marked Low pressure tap (duct pressure). On the opposite side of the dome, at the bottom, is a circular representation with a gap indicated, pointed to with the marking High pressure tap (windbox pressure).

#BoilerManual #AirAndGasFlow #Section3 #Page22

for the same pressure differential readings on all cyclones. Figure 17 shows a typical air flow calibration curve.

PRIMARY-TERTIARY AIR FLOW MEASUREMENT

Primary-tertiary air flow is measured much in the same manner. An entrance cone located inside the windbox provides the pressure differential required to calculate air flow (Figure 18). In both cases, a high pressure reading (windbox pressure) and a low pressure reading (duct pressure) are compared and create the necessary pressure differential. Primary-tertiary air flow should be calibrated so that equal primary-tertiary air flows are obtained for the same flow differential readings on all cyclones. Figure 19 shows a typical air flow calibration curve.

------------------------------------------------- 22 ------------------------------------------------------
Alt = Labeled Fig.16 Bellmouth for measuring secondary air flow differential. This is a depiction of the secondary air ductwork that would be attached to a cyclone; air flow is designated by two arrows on the left of the duct pointing rightward. Two dampers are in the illustration, the Shut-off damper is marked on the left and next to it on the right is marked Secondary air control damper (velocity damper). Capping the left side of the duct is a dome shape marked as Perforated screen, and the air flow arrows indicate air flowing through it. Attached to the topside of the dome is what appears to be a pocket next to but below where the Shut off damper pivots, and the pocket is marked Low pressure tap (duct pressure). On the opposite side of the dome, at the bottom, is a circular representation with a gap indicated, pointed to with the marking High pressure tap (windbox pressure).

#BoilerManual #AirAndGasFlow #Section3 #Page21

While others have insufficient air to burn the fuel flowing through them. The total air flow to each cyclone is the sum of the secondary, primary and tertiary air.

SECONDARY AIR FLOW MEASUREMENT

Secondary air flow is measured on an individual cyclone basis. The differential pressure required to measure air flow is created by a bellmouth arrangement located at the secondary air inlet to the cyclone, Figure 16. A perforated screen which covers the bellmouth opening functions to distribute the air flow in a more uniform manner.

Secondary air flow calibration should be done at the bellmouth at three different air flows to check that equal secondary air flows are obtained

------------------------------------------------- 21 ------------------------------------------------------
Alt = Labeled Fig. 15 Percent of O2 in flue gas vs. excess air (dry basis). Graph is laid out in terms of percentage on both axes; X axis is marked Percent of O2 in flue gas (by volume) incremented every 2%, up to 12%. Y axis is marked Percent excess air--incremented every 20% up to 100. There are 2 curves on the graph, one in solid line and the other in dashed line. They both beginat the graph's origin point, going slightly upward but diverge when 2% flue gas O2 is reached. The solid line goes higher than the dashed line only slightly and converge again at the upper right corner area. The solid line is marked Sub-bituminous while the dashed line is marked Bituminous.

#BoilerManual #AirAndGasFlow #Section3 #Page21

While others have insufficient air to burn the fuel flowing through them. The total air flow to each cyclone is the sum of the secondary, primary and tertiary air.

SECONDARY AIR FLOW MEASUREMENT

Secondary air flow is measured on an individual cyclone basis. The differential pressure required to measure air flow is created by a bellmouth arrangement located at the secondary air inlet to the cyclone, Figure 16. A perforated screen which covers the bellmouth opening functions to distribute the air flow in a more uniform manner.

Secondary air flow calibration should be done at the bellmouth at three different air flows to check that equal secondary air flows are obtained

------------------------------------------------- 21 ------------------------------------------------------
Alt = Labeled Fig. 15 Percent of O2 in flue gas vs. excess air (dry basis). Graph is laid out in terms of percentage on both axes; X axis is marked Percent of O2 in flue gas (by volume) incremented every 2%, up to 12%. Y axis is marked Percent excess air--incremented every 20% up to 100. There are 2 curves on the graph, one in solid line and the other in dashed line. They both beginat the graph's origin point, going slightly upward but diverge when 2% flue gas O2 is reached. The solid line goes higher than the dashed line only slightly and converge again at the upper right corner area. The solid line is marked Sub-bituminous while the dashed line is marked Bituminous.

#BoilerManual #AirAndGasFlow #Section3 #Page20

The amount of O2 in the flue gases is significant in defining the status of the combustion process. Its presence means that more oxygen (excess air) is being introduced than is being used. O2 is related to excess air, as shown in Figure 15. Assuming complete combustion, low values of O2 in the flue gas indicates a moderate amount of excess air and reduced heat losses up the stack. Higher values of O2 indicate that excessive heat losses may be present. The flue gas analysis which normally is done at the economizer outlet flue represents only the results of the average state of combustion in the furnace. It does not indicate that there is a correct or desirable fuel/air ratio on all of the cyclones. It is up to the operator, with the help of the fuel/air flow meters for each cyclone, to keep the fuel/air ratio the same on all the cyclones. This is often called balanced firing. If and only if the cyclones are balanced will the flue gas analysis indicate the state of combustion at the cyclones. It is quite possible to have a normal O2 reading and yet still be carrying over large amounts of unburned fuel to the precipitator. This seemingly contradictory situation is generally the result of unbalanced firing. That is, some cyclones have a lot more air going to them than is needed to burn the fuel,

------------------------------------------------- 20 ------------------------------------------------------
Alt = A graph labeled Fig. 14 Effect of boiler load on excess air requirements. Both x and y axes are in terms of percentage, marked only with 0 and 100. X axis is is marked Boiler load (percent). Y axis is marked Boiler load (percent). The curve starts on the left at a high but unmarked percentage level and diminishes as it approaches a higher boiler load percentage.

#BoilerManual #AirAndGasFlow #Section3 #Page20

The amount of O2 in the flue gases is significant in defining the status of the combustion process. Its presence means that more oxygen (excess air) is being introduced than is being used. O2 is related to excess air, as shown in Figure 15. Assuming complete combustion, low values of O2 in the flue gas indicates a moderate amount of excess air and reduced heat losses up the stack. Higher values of O2 indicate that excessive heat losses may be present. The flue gas analysis which normally is done at the economizer outlet flue represents only the results of the average state of combustion in the furnace. It does not indicate that there is a correct or desirable fuel/air ratio on all of the cyclones. It is up to the operator, with the help of the fuel/air flow meters for each cyclone, to keep the fuel/air ratio the same on all the cyclones. This is often called balanced firing. If and only if the cyclones are balanced will the flue gas analysis indicate the state of combustion at the cyclones. It is quite possible to have a normal O2 reading and yet still be carrying over large amounts of unburned fuel to the precipitator. This seemingly contradictory situation is generally the result of unbalanced firing. That is, some cyclones have a lot more air going to them than is needed to burn the fuel,

------------------------------------------------- 20 ------------------------------------------------------
Alt = A graph labeled Fig. 14 Effect of boiler load on excess air requirements. Both x and y axes are in terms of percentage, marked only with 0 and 100. X axis is is marked Boiler load (percent). Y axis is marked Boiler load (percent). The curve starts on the left at a high but unmarked percentage level and diminishes as it approaches a higher boiler load percentage.

#BoilerManual #AirAndGasFlow #Section3 #Page19

This bellmouth differential is effected by the secondary air control damper and the FD fans and will be discussed in greater detail in the Cyclone Description and Operation section.

TOTAL OR EXCESS AIR

The continuous measurement of combustion air is necessary to optimize efficiency of operation and to assure effective control of available heat losses. The amount of total air that should be used can be determined by continuously monitoring the constituents of the flue gas (CO2, O2, and unburned gases). However, unburned gases, primarily carbon monoxide (CO) should never be present with proper combustion.

Ideally, a given amount of air is required to completely burn a given amount of fuel. The exact amount of air can be calculated from an analysis of the specific fuel, and is called theoretical air. 100% total air means 100%of the air theoretically required for complete combustion of the fuel without excess. Higher percentages indicate theoretical air plus excess air. For instance, 125% total air means 100% theoretical air plus 25% excess air. Some excess air is always required to account for minor imperfections throughout the system. The cyclone is not 100% efficient, coal flows are not perfectly balanced, air flow is not distributed perfectly, etc.

The cyclone is designed to operate most efficiently at full load. At lower loads, excess air must be increased. When cyclones are removed from service, some air passes through the idle dampers. This leakage does notcontribute to combustion, but does show up as excess air to the O2 monitoring equipment. Consequently, to maintain good combustion over the entire load range, excess air requirements are higher as cyclones are removed from service (Figure 14).

------------------------------------------------- 19 ------------------------------------------------------

#BoilerManual #AirAndGasFlow #Section3 #Page19

This bellmouth differential is effected by the secondary air control damper and the FD fans and will be discussed in greater detail in the Cyclone Description and Operation section.

TOTAL OR EXCESS AIR

The continuous measurement of combustion air is necessary to optimize efficiency of operation and to assure effective control of available heat losses. The amount of total air that should be used can be determined by continuously monitoring the constituents of the flue gas (CO2, O2, and unburned gases). However, unburned gases, primarily carbon monoxide (CO) should never be present with proper combustion.

Ideally, a given amount of air is required to completely burn a given amount of fuel. The exact amount of air can be calculated from an analysis of the specific fuel, and is called theoretical air. 100% total air means 100%of the air theoretically required for complete combustion of the fuel without excess. Higher percentages indicate theoretical air plus excess air. For instance, 125% total air means 100% theoretical air plus 25% excess air. Some excess air is always required to account for minor imperfections throughout the system. The cyclone is not 100% efficient, coal flows are not perfectly balanced, air flow is not distributed perfectly, etc.

The cyclone is designed to operate most efficiently at full load. At lower loads, excess air must be increased. When cyclones are removed from service, some air passes through the idle dampers. This leakage does notcontribute to combustion, but does show up as excess air to the O2 monitoring equipment. Consequently, to maintain good combustion over the entire load range, excess air requirements are higher as cyclones are removed from service (Figure 14).

------------------------------------------------- 19 ------------------------------------------------------

#BoilerManual #AirAndGasFlow #Section3 #Page18

To illustrate this, consider the following example while referring to Figure 13. In order to maintain a secondary air flow of 300,000 lb/hr with a secondary air temperature of 500 F, the differential across the bellmouth (cyclone air flow measuring device) should be 8.4". In order to maintain the same air flow with an increase of air temperature to 700 F, it now requires an increased air flow differential of 10" at the bellmouth.

------------------------------------------------- 18 ------------------------------------------------------
Alt = A large graph labeled Fig. 13 Effect of density on air flow. The y axis is labeled "Bellmouth air-flow differential--(inches of water), and increments sequentially. The x axis is labeled "Scondary air flow (1000 lb/hr)" and increments from 0 to 500 every 50 units with only the 100s lines marked as such.
Definition of "bellmouth" can be found here: en.wikipedia.org/wiki/Bell_mou…

#BoilerManual #AirAndGasFlow #Section3 #Page18

To illustrate this, consider the following example while referring to Figure 13. In order to maintain a secondary air flow of 300,000 lb/hr with a secondary air temperature of 500 F, the differential across the bellmouth (cyclone air flow measuring device) should be 8.4". In order to maintain the same air flow with an increase of air temperature to 700 F, it now requires an increased air flow differential of 10" at the bellmouth.

------------------------------------------------- 18 ------------------------------------------------------
Alt = A large graph labeled Fig. 13 Effect of density on air flow. The y axis is labeled "Bellmouth air-flow differential--(inches of water), and increments sequentially. The x axis is labeled "Scondary air flow (1000 lb/hr)" and increments from 0 to 500 every 50 units with only the 100s lines marked as such.
Definition of "bellmouth" can be found here: en.wikipedia.org/wiki/Bell_mou…

#BoilerManual #AirAndGasFlow #Section3 #Page17

------------------------------------------------- 17 ------------------------------------------------------
Alt = Labeled DENSITY OF AIR TABLE 2 subtitled [b]Change with Temperature - Pressure Constant at 29.92 in. in Hg --that translates into 29.92 inches of Mercury column. The C & I Department did indeed have such a measuring device in its shop.
The table consists of 8 columns, actually 4 pairs with each pair of columns titled, respectively, Temp. F; Density Lb. per cu. ft.; the first pair lists Temp F from 0 to 90 in increments of 10 with corresponding Density measurements. Second pair similarly lists Temp F from 100 to 190; Third pair similarly lists Temp F from 200 to 450; finally, the fourth pair similarly lists Temp F from 500 to 1000.

#BoilerManual #AirAndGasFlow #Section3 #Page17

------------------------------------------------- 17 ------------------------------------------------------
Alt = Labeled DENSITY OF AIR TABLE 2 subtitled [b]Change with Temperature - Pressure Constant at 29.92 in. in Hg --that translates into 29.92 inches of Mercury column. The C & I Department did indeed have such a measuring device in its shop.
The table consists of 8 columns, actually 4 pairs with each pair of columns titled, respectively, Temp. F; Density Lb. per cu. ft.; the first pair lists Temp F from 0 to 90 in increments of 10 with corresponding Density measurements. Second pair similarly lists Temp F from 100 to 190; Third pair similarly lists Temp F from 200 to 450; finally, the fourth pair similarly lists Temp F from 500 to 1000.

#BoilerManual #AirAndGasFlow #Section3 #Page16

A second factor which affects air flow is density. Density is measured as a unit of mass within a given volume (lb/cubic foot). A given mass (lb) of air will occupy a certain volume (cubic foot) {Now is a good time to review Boyle's Laws} at a particular static pressure and temperature. Assuming the mass and the static pressure do not change as the air is heated, its volume will expand. It will become less dense as illustrated in Table 2.

Applying this relationship to air flow, it is found that as air is heated and density decreases, there will be a corresponding drop in air flow. Again, assuming no change in pressure.

------------------------------------------------- 16 ------------------------------------------------------
Alt = Labeled Fig. 12 Rated steam flow (percent) and is almost but not quite identical to Fig. 10. The x axis is identically labeled in terms of precentage increments but is marked Rated flow (percent); the y axis is labeled Differential pressure (inches of water) but is incremented from 0 to 2.0. The curve is nearly identical.

#BoilerManual #AirAndGasFlow #Section3 #Page16

A second factor which affects air flow is density. Density is measured as a unit of mass within a given volume (lb/cubic foot). A given mass (lb) of air will occupy a certain volume (cubic foot) {Now is a good time to review Boyle's Laws} at a particular static pressure and temperature. Assuming the mass and the static pressure do not change as the air is heated, its volume will expand. It will become less dense as illustrated in Table 2.

Applying this relationship to air flow, it is found that as air is heated and density decreases, there will be a corresponding drop in air flow. Again, assuming no change in pressure.

------------------------------------------------- 16 ------------------------------------------------------
Alt = Labeled Fig. 12 Rated steam flow (percent) and is almost but not quite identical to Fig. 10. The x axis is identically labeled in terms of precentage increments but is marked Rated flow (percent); the y axis is labeled Differential pressure (inches of water) but is incremented from 0 to 2.0. The curve is nearly identical.

#BoilerManual #AirAndGasFlow #Section3 #Page15

restriction does not matter (as long as it does not change), this relationship has several applications in boiler operation. A bank of tubes, Figure 11, is a restriction to gas flow. If the differential across a bank of tubes is 1/2 inch of water at 50% boiler load, then the differential will be about 4 times as great, or 2 inches of water at 100% load. Remember that this is all based on a constant restriction. Slagging of the tube bank will alter the relationship, depending upon the severity. Differential pressures are useful as an operating guideline when comparing normal or expected differentials to the actual readings.

Steam flowing through a tube section behaves similarly. Figure 12 helps explain the reason for limiting the gas temperature at the superheater during low load or pressure raising periods. Below 10% steam flow, the pressure drop through the superheater is extremely low. If a tube were to have a water leg, it could block steam flow through that tube. The tube could overheat and fail. To protect the superheater tubes, the gas temperature entering this region is limited to a 1000 F until steam flow is greater than 10% of full load flow. Above 10% steam flow the pressure differential becomes large enough to insure that there is flow through all superheater tubes.

------------------------------------------------- 15 ------------------------------------------------------
Alt = Figure 11 Labeled Air flow across a tube bank. It does in fact show a manometer connected between the air flow inlet across an unlabeled tube bank, at the bottom of the inlet duct. The diagram on the left shows a manometer reading marked delta-P 1/2" at air flow of 50%. The diagram on the right shows a manometer reading of delta-P 2" with air flow of 100%. Each diagram depicts the section of furnace that contains a generic tube bank and is marked as such. The left diagram is marked 50% AIR FLOW; the right diagram is marked 100% AIR FLOW.

#BoilerManual #AirAndGasFlow #Section3 #Page15

restriction does not matter (as long as it does not change), this relationship has several applications in boiler operation. A bank of tubes, Figure 11, is a restriction to gas flow. If the differential across a bank of tubes is 1/2 inch of water at 50% boiler load, then the differential will be about 4 times as great, or 2 inches of water at 100% load. Remember that this is all based on a constant restriction. Slagging of the tube bank will alter the relationship, depending upon the severity. Differential pressures are useful as an operating guideline when comparing normal or expected differentials to the actual readings.

Steam flowing through a tube section behaves similarly. Figure 12 helps explain the reason for limiting the gas temperature at the superheater during low load or pressure raising periods. Below 10% steam flow, the pressure drop through the superheater is extremely low. If a tube were to have a water leg, it could block steam flow through that tube. The tube could overheat and fail. To protect the superheater tubes, the gas temperature entering this region is limited to a 1000 F until steam flow is greater than 10% of full load flow. Above 10% steam flow the pressure differential becomes large enough to insure that there is flow through all superheater tubes.

------------------------------------------------- 15 ------------------------------------------------------
Alt = Figure 11 Labeled Air flow across a tube bank. It does in fact show a manometer connected between the air flow inlet across an unlabeled tube bank, at the bottom of the inlet duct. The diagram on the left shows a manometer reading marked delta-P 1/2" at air flow of 50%. The diagram on the right shows a manometer reading of delta-P 2" with air flow of 100%. Each diagram depicts the section of furnace that contains a generic tube bank and is marked as such. The left diagram is marked 50% AIR FLOW; the right diagram is marked 100% AIR FLOW.

#BoilerManual #AirAndGasFlow #Section3 #Page14

should be calibrated, preferably at normal operating pressure and temperature. The relationship between air flow and pressure differential is a square root curve, which means that the differential varies with the square of the flow.

To illustrate this, consider this example. Looking at Figure 10, we see that at 50% rated air flow, the differential pressure is 1 1/4 inches of water {they used a water-filled manometer for the measurement as depicted in Figure 11--the C&I Dept. did in fact have such a manometer in the shop, with red colored water in it}. At twice this air flow, or 100%, the differential pressure is four times greater at 5 inches of water. Anytime the flow is doubled, the differential pressure will be quadrupled. The inverse of this relationship is also true. If the differential pressure is known the flow can be calculated. If twice the present flow is required across a certain restriction, then the flow must be increased until the differential has quadrupled.

As long as the temperature of the fluid, its pressure and restriction do not change, this relationship olds true. Because the actual nature of the

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Alt = Labeled Fig. 10 Square root curve for air flow. X axis is marked Rated air flow (percent) and is incremented in 10s from 0 to 100 with only the 50% line and the 100% line marked as such. Y axis is marked as Differential pressure (inches of water) incremented 0 to 5 with each line marked sequentially. The curve originates at zero and increases at a low angle until around the 40% mark and increases rapidly to the 100% mark.

#BoilerManual #AirAndGasFlow #Section3 #Page14

should be calibrated, preferably at normal operating pressure and temperature. The relationship between air flow and pressure differential is a square root curve, which means that the differential varies with the square of the flow.

To illustrate this, consider this example. Looking at Figure 10, we see that at 50% rated air flow, the differential pressure is 1 1/4 inches of water {they used a water-filled manometer for the measurement as depicted in Figure 11--the C&I Dept. did in fact have such a manometer in the shop, with red colored water in it}. At twice this air flow, or 100%, the differential pressure is four times greater at 5 inches of water. Anytime the flow is doubled, the differential pressure will be quadrupled. The inverse of this relationship is also true. If the differential pressure is known the flow can be calculated. If twice the present flow is required across a certain restriction, then the flow must be increased until the differential has quadrupled.

As long as the temperature of the fluid, its pressure and restriction do not change, this relationship olds true. Because the actual nature of the

------------------------------------------------- 14 ------------------------------------------------------
Alt = Labeled Fig. 10 Square root curve for air flow. X axis is marked Rated air flow (percent) and is incremented in 10s from 0 to 100 with only the 50% line and the 100% line marked as such. Y axis is marked as Differential pressure (inches of water) incremented 0 to 5 with each line marked sequentially. The curve originates at zero and increases at a low angle until around the 40% mark and increases rapidly to the 100% mark.

#BoilerManual #AirAndGasFlow #Section3 #Page13

device is installed in the air duct and the differential pressure is measured across this device. Along with the differential pressure, the temperature of the air must also be measured so the density of the air can be calculated. From experiments with air flow devices, it is known that air flow is proportional to the square root of the sum of the density of the air times the differential across the device. This relationship can be expressed by the following equation:
{deleted image due to formatting bugs}
To find the exact relationship, the device must be calibrated. Once calibration has been done, the proportionality constant can be found. For general purposes, this constant is usually called K.
{Image deleted due to formatting bugs, please refer to the Alt description below the page number. Thanks}

Once this "K" factor is known, the absolute air flow can be found by measuring the pressure differential and the temperature of the air. This "K" factor is independent of the temperature of the air. Periodically, the air flow device's calibration should be checked for accuracy. {As well as functionality, which was the job of the Control & Instruments Dept., fondly called "Seein' Eye" for obvious reasons. Most of those measurement devices were made by Leeds & Northrup, all analog.}

Differential Pressure

The measurement of fluid flow is necessary to permit intelligent, safe, and efficient operation of steam generating equipment. This includes the measurement of water and steam as well as air and gas flow. While there are many means of measuring flow, the common denominator is that allflow measuring devices produce a pressure drop or differential pressure. Differential pressure is created by restrictions in the cross-sectional area of a fluid flow path. Differential pressures created by restrictions can be converted into a flow rate.

For a dependable determination of air flow, the primary element used

------------------------------------------------- 13 ------------------------------------------------------
Alt = Equation 1 depicts Air flow is proportional to the square root of (differential)(density).
Equation 2 depicts Air flow = K is the square root of (differential)(density).

#BoilerManual #AirAndGasFlow #Section3 #Page13

device is installed in the air duct and the differential pressure is measured across this device. Along with the differential pressure, the temperature of the air must also be measured so the density of the air can be calculated. From experiments with air flow devices, it is known that air flow is proportional to the square root of the sum of the density of the air times the differential across the device. This relationship can be expressed by the following equation:
{deleted image due to formatting bugs}
To find the exact relationship, the device must be calibrated. Once calibration has been done, the proportionality constant can be found. For general purposes, this constant is usually called K.
{Image deleted due to formatting bugs, please refer to the Alt description below the page number. Thanks}

Once this "K" factor is known, the absolute air flow can be found by measuring the pressure differential and the temperature of the air. This "K" factor is independent of the temperature of the air. Periodically, the air flow device's calibration should be checked for accuracy. {As well as functionality, which was the job of the Control & Instruments Dept., fondly called "Seein' Eye" for obvious reasons. Most of those measurement devices were made by Leeds & Northrup, all analog.}

Differential Pressure

The measurement of fluid flow is necessary to permit intelligent, safe, and efficient operation of steam generating equipment. This includes the measurement of water and steam as well as air and gas flow. While there are many means of measuring flow, the common denominator is that allflow measuring devices produce a pressure drop or differential pressure. Differential pressure is created by restrictions in the cross-sectional area of a fluid flow path. Differential pressures created by restrictions can be converted into a flow rate.

For a dependable determination of air flow, the primary element used

------------------------------------------------- 13 ------------------------------------------------------
Alt = Equation 1 depicts Air flow is proportional to the square root of (differential)(density).
Equation 2 depicts Air flow = K is the square root of (differential)(density).