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FORCED-DRAFT SYSTEMS

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DESIGN PRACTICES

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CONTENTS Section

Page

SCOPE ............................................................................................................................................................ 2 REFERENCES ................................................................................................................................................ 2 DESIGN SPECIFICATION REQUIREMENTS................................................................................................. 2 BASIC DESIGN CONSIDERATIONS ............................................................................................................. 2 BURNERS............................................................................................................................................... 2 DUCTING................................................................................................................................................ 2 FAN......................................................................................................................................................... 3 NOISE CONTROL .................................................................................................................................. 5 CONTROL OF COMBUSTION AIR FLOW ............................................................................................. 5 AIR FLOW MEASUREMENT .................................................................................................................. 6 CALCULATION PROCEDURES..................................................................................................................... 6 REQUIRED WEIGHT FLOW OF DRY AIR AT GIVEN FIRING RATE.................................................... 6 VOLUMETRIC AIR FLOW AT SPECIFIED ENVIRONMENTAL CONDITIONS...................................... 6 CORRECTING VOLUMETRIC FLOW RATE FOR ALTITUDE ............................................................... 6 AIR VELOCITY IN DISTRIBUTION DUCTS ........................................................................................... 7 PRESSURE LOSS IN STRAIGHT DUCTS ............................................................................................. 7 OTHER DUCTING PRESSURE LOSSES .............................................................................................. 7 ESTIMATING DRIVER POWER ............................................................................................................. 7 NOMENCLATURE .......................................................................................................................................... 8 TABLES Table 1A Table 1B FIGURES Figure 1 Figure 2 Figure 3A Figure 3B Figure 4A Figure 4B Figure 5A Figure 5B

Design Specification Information on Fan and Ducting (Customary)............................. 9 Design Specification Information on Fan and Ducting (Metric) .................................. 10

Typical Forced - Draft System.................................................................................... 11 Typical Characteristic Curves for Forced-Draft Fan With System Resistance Curve Over-Plotted .................................................................................................... 12 Psychrometric Chart (Customary).............................................................................. 13 Psychrometric Chart (Metric) ..................................................................................... 14 Air Velocity Versus Velocity Pressure (Dynamic Head) (Customary)......................... 15 Air Velocity Versus Velocity Pressure (Dynamic Head) (Metric) ................................ 16 Change of Air Density With Altitude (Customary) ...................................................... 17 Change of Air Density With Altitude (Metric) .............................................................. 18

Revision Memo 12/01

Added API Standard to reference list. Editorial revision to Fan paragraph. Added drop out door requirement to safety section. Editorial revision to Control of Combustion Air Flow paragraph. Clarification on Air Flow Measurement paragraph.

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FORCED-DRAFT SYSTEMS DESIGN PRACTICES

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SCOPE This section covers the fan, ducting and related instrumentation required for a forced-draft burner system. requirements for systems, which contain air preheaters, are covered in Section VIII-K.

Additional

REFERENCES DESIGN PRACTICES (BESIDES OTHER SECTIONS OF THIS SECTION) Section XIV

Fluid Flow

GLOBAL PRACTICES GP 2-1-1 GP 7-1-1 GP 10-13-1 GP 15-1-1

Equipment Noise Level Data Requirements Fired Heaters Special Purpose Centrifugal Fans Instrumentation for Fired Heaters

API STANDARD ➧

650

Fired Heater for General Refinery Service Including Appendices

OTHER REFERENCES R. Jorgensen, Editor, Fan Engineering, Buffalo Forge Company, Buffalo, NY (1961). Heinz P. Bloch, Process Plant Machinery, Butterworth Publishers, Stoneham, MA (1989).

DESIGN SPECIFICATION REQUIREMENTS Table 1 lists the necessary fan and ducting information in the form that it should be presented in the fired heater section of the design specification. This table lists all requirements not covered by the referenced Global Practices. The required safety and control instrumentation should be shown on the design specification flow plan. Any necessary instrument connections should be shown on the fired heater sketch, so that these connections can be properly located.

BASIC DESIGN CONSIDERATIONS BURNERS Selection and arrangement of the forced-draft burners are covered in Sections VIII-B and VIII-F. When sizing the fan and ducting, the designer must know the required air pressure at the burner inlet for the normal and maximum firing rates. This information is given in Section VIII-F.

DUCTING Air is ducted from the atmosphere to the fan inlet and from the fan exhaust to the forced-draft burners. Figure 1 shows a typical ducting system. Duct Arrangement - The ducting must be located so that: 1. Burners can be removed onstream as required in GP 7-1-1. 2. Personnel escape routes from the fired heater are provided. 3. There is adequate clearance for maintenance of equipment beneath the fired heater. Fan Inlet - To prevent potentially damaging foreign material from entering the fan, the fan inlet duct (or the fan inlet itself, if no duct is used) must be covered with a screen of 1-1/2 in. mesh (wire spacing of 38 mm), as required by API Standard 673. Finemesh screens should not be used, since they tend to ice over in the winter or to become plugged with fine material. A rain shield should be called for in the Design Specification (see Table 1A or 1B), if needed to prevent rain from entering the fan.

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BASIC DESIGN CONSIDERATIONS (Cont) Fan Discharge - Ducting from the fan discharge includes (see Figure 1): 1. Supply Duct - Connects the fan exhaust and the distribution duct. 2. Crossover Duct - Connects the ducting systems of two fired heaters. This duct permits operation of two fired heaters with a single fan. 3. Distribution Duct - A manifold which must be sized to assure equal air flow to all burners. 4. Riser Ducts - Connect burners with the distribution duct. Construction - Above-ground ducting should be constructed of carbon steel at least 3/16 in. (5 mm) thick and braced to minimize vibration. This must be noted in the Design Specification (see Table 1A or 1B). Dampers - An isolation damper must be installed in the crossover between two fired heaters (even if both fired heaters are in a common box, as POWERFORMING Unit fired heaters often are). The designer must note this in the Design Specification. As required by GP 7-1-1, the following dampers must be installed in the ducting: 1. 2. 3.

Tight shut-off dampers at each burner, upstream of the burner flange and operable from grade. Discharge isolation facilities on any fan that can be shutdown for maintenance while the heater continues to operate. If two or more fans discharge into a common duct, each fan must be provided with an automatic damper which prevents backflow through the fan when it is not in operation (similar to a check valve). This automatic damper may serve to satisfy Requirement 2. Sizing - Duct size (cross-sectional area) for a specified air flow is fixed by the design air velocity. A low velocity will result in an excessively large and costly duct. Too high a design velocity will result in large pressure drop and high power costs. A design air velocity in supply and crossover ducts of 40 ft per second (12 m/s) has been found to result in ducting of reasonable size and pressure drop. Use of higher velocities is not recommended unless justified by an economic study. Air velocity in the distribution duct - and in the riser ducts, if they feed more than one burner (see Figure 1) - must be low enough to assure equal air flow to all burners. This velocity may be below the optimum 40 ft per second (12 m/s) recommended above. Uniform air flow to the burners from the distribution duct is assured by limiting the velocity so that the dynamic head (velocity pressure) in the distribution duct is less than 5 percent of the static pressure required in the burner windbox at normal firing rate. Pressure Drop - Pressure drop in the ducting must be calculated before the final fan design requirements can be established. Since the final ductwork arrangement and sizing are generally established by the contractor, the pressure drop calculations are also normally performed by the contractor during the detailed engineering stage and are checked by the Owner's Engineer. Duct pressure drop calculation procedures are given below, under CALCULATION PROCEDURES. Calculated pressure drop in distribution and riser ducts should be based on normal design air flow rate (heater design firing rate at design excess air rate divided by total number of burners).

FAN

➧

➧

The fan is the key element in a forced-draft system. Since fans are not usually spared, failure of a fan results in fired heater shutdown in a single fan installation, or in reduction of unit throughput if more than one fan is used to supply combustion air. Manufacturers will rarely guarantee a fan/driver combination for a run length consistent with the desired fired heater run length, which may be as long as 3 to 4 years. Therefore, to provide a fan/driver combination with the desired run length, critical design features, such as bearings and lubrication, must be carefully specified. GP 10-13-1, Special Purpose Centrifugal Fans, defines these design requirements. In addition to reliability, the fan must be properly sized to provide the required air flow at the pressure required by the burners. Number of Fans - One fan per fired heater is sufficient except for critical units, such as atmospheric pipestills, where shutdown of the fired heater would necessitate shutdown of downstream units. In such critical units two fans should be provided, each sized for 50% of the normal air flow requirements. With one of the two fans in operation, the fired heater could be operated at about 85% of design capacity, rather than 50%, because of margins incorporated in the fan sizing criteria, and because the single fan will be operating at a lower head point on its characteristic curve. In some non-critical units, a spare is provided when it can serve as a common spare between two or more heaters.

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BASIC DESIGN CONSIDERATIONS (Cont) Fan Specification - The specification of a fan to meet the requirements discussed above has two parts: 1. General Requirements for Forced Draft Fans - Most of these general fan requirements are covered by GP 10-13-1 and GP 7-1-1 and should not be repeated in the Design Specification. Those requirements not covered by Global Practices are given in Table 1A or 1B. 2. Criteria for Sizing the Fan - To size a fan, its rated point must be specified. The rated point and the information needed to define it quantitatively are discussed below. Fan Curves - Figure 2 shows the static head versus flow characteristics for a typical constant-speed, centrifugal fan with backward curved blades. Backward curved blades are specified (as opposed to straight or forward curved blades), because the peak efficiency of a fan with backward curved blading occurs close to the point of maximum fan power, thus minimizing driver power. As can be seen in Figure 2, the fan has a separate characteristic curve for each position of the variable inlet guide vanes, which are used to control air flow. The following are also shown on Figure 2: 1. System Resistance Curve - This curve shows the fan head needed to overcome the system pressure drop at a given air flow rate. System pressure drop includes the pressure to force air through the burners (see Section VIII-F), the ducting pressure losses, and losses in any sound dampers or flow metering devices. Thus, the system resistance curve is the fan operating line. Any increase in back pressure, such as would result from closing a burner damper, moves the system resistance curve upward. The system resistance curve is also higher when the fired heater is operating with some burners turned off, even though the total air rate is constant. The operating line (system resistance curve) is defined as follows:  F2  H = Hn  2v  = R Fv2 F   vn  where: Fv Fvn H Hn R Note:

= = = = =

Eq. (1)

Volumetric flow, cu ft/min (dm3/s) Volumetric flow at normal firing rate, cu ft/min (dm3/s) Static head at Fv, in. of water (kPa) Static head at normal firing rate, in. of water (kPa) Hn System resistance = (Fvn ) 2

Use summer design temperature and humidity, and correct for altitude (if necessary) in defining the operating line.

2. 3.

Fan Operating Points - These are intersections of the system resistance curve with the fan characteristic curves. Fan Rated Point - This is not an actual operating point, but includes margins on both flow and head, as defined below. These margins are needed to provide a safety factor on fan/system performance and to assure that the fan is not normally operating with its inlet vanes fully open (i.e., without control). 4. Fan Stability - Fan operation is unstable near and to the left of the peaks of the characteristic curves. In this unstable region, any reduction in air flow (e.g., as would result from closing a burner damper) reduces the fan head, which then causes a further reduction in air flow, and so forth. To assure stable operation, the static head should show a continuous increase from rated air flow back to a nominal 60% of rated air flow. Sizing - To provide a fan of proper size, the fan rated point must be defined (see Figure 2). At the rated point, the fan must provide the following: 1. 115% of the air flow (weight flow) needed for fired heater design firing rate and excess air. 2. 115% of the static head required to overcome the sum of the ducting pressure drop plus the pressure at the burner needed for maximum burner firing rate. The fan must be sized based on design summer air temperature and relative humidity and at the altitude of the installation. No altitude correction is needed unless the installation is more than 1000 ft (305 m) above sea level. The calculations needed to establish the rated point are normally performed by the fired heater vendor, using information given in the Design Specification (Table 1). See CALCULATION PROCEDURES.

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BASIC DESIGN CONSIDERATIONS (Cont) NOISE CONTROL Noise emission from the fan and ductwork must meet the hearing conservation criteria defined in GP 2-1-1, Equipment Noise Level Data Requirements. To assure that these noise criteria are met, the fan/duct system should incorporate the following features (as noted in Table 1): Elevation - The fan inlet should be 15 ft (4.6 m) above the first fired heater platform. Insulation - Typically, aboveground ducting and fan casings require 2 in. (50 mm) or more of acoustic insulation (e.g., 80 to 100 lb/ft3 gunite - 1280 to 1600 kg/m3). In addition to meeting the hearing conservation noise criteria, the noise contribution of the fan/duct system must be consistent with the community noise criteria for the particular installation. Since these requirements vary from installation to installation, the Mechanical Engineering Section of EETD should be consulted regarding establishment of fan/duct noise limits and the possible need for noise suppression features, such as inlet duct sound dampers, in addition to those mentioned above.

S

➧

SAFETY Fired heaters equipped with forced-draft systems require certain safety features in addition to those for natural draft heaters. These are covered in GP 15-1-1 and are summarized below: Loss of Combustion Air - Main fuel and pilot fuel must be cut off automatically and a warning alarm must be sounded if air flow is blocked, or if the fan fails. This can be accomplished by a low combustion air flow cut-off. If the heater is equipped with drop out doors, FD fan failure shall initiate a heater shutdown with a time delay. To override the shutdown signal, opening of the bypass damper and air door must be confirmed. Time delay shall be approximately 10 seconds. Loss of Fan - The preferred measurement method is low fan speed. Fired Heater Overpressure - The maximum internal pressure that a fired heater can withstand without structural damage is about 5 in. of water gauge (1.25 kPa). Since the fan is capable of delivering higher pressures, the fired heater must be protected from the overpressure which would result if the fired heater outlet were blocked. To prevent overpressure, the following features are required: 1. A high-pressure alarm, set to sound whenever the box pressure becomes slightly positive; say 0.1 in. of water (0.025 kPa). 2. Partial dampers in the flue gas ducting, in lieu of standard dampers. These partial dampers must be sized to provide a maximum pressure drop of 5 in. of water gauge (1.25 kPa) in the fully closed position with normal air flow. If partial dampers are impractical (such as when an air preheater is used), then a pressure relieving device must be provided to assure that heater box pressure does not exceed 5 in. of water (1.25 kPa). Sizing of pressure relieving doors is discussed in Section VIII-K. Fired Heater Shutdown - Fans are to continue running when the fired heater is shut down by safety equipment, such as the pilot gas PLCO.

CONTROL OF COMBUSTION AIR FLOW ➧

To provide the proper excess air at varying heater firing rates, the air flow to the burners must be adjustable. Variable-position inlet guide vanes on the fan are the preferred means of controlling air flow. As the inlet vanes are closed, the entering air is given a spin in the direction of fan rotation. This spin results in reduced driver power required for a given air flow and static head (see Figure 2). Other methods of controlling air flow, such as varying fan speed or use of a variable damper in the fan discharge, are not generally desirable. Throttling the fan discharge raises the system resistance curve (see Figure 2), thus requiring greater driver power for the same air flow and discharge pressure (downstream of the throttling damper) than does inlet guide vane control. Varying the fan speed requires a variable-speed driver or a variable-speed drive (e.g., fluid drive), both of which are more expensive than inlet guide vane control. Even when economics dictate use of a steam turbine driver, which is capable of variable-speed operation, constant-speed operation of the turbine in combination with inlet guide vane control is preferred, because a less complicated system results. Inlet guide vane position may be controlled by any of the following: 1. A local, manual positioner, such as a hand wheel. 2. A hydraulic or pneumatic positioner controlled by a Manual Loading Station (MLS) from the control house. This approach is preferred when the indicator of the heater oxygen analyzer (if provided) is located in the control house. Using the oxygen indicator as a guide, the boardman sets the MLS at the position providing the desired excess air. Since burner windbox pressure is a measure of burner air flow (see Section VIII-F) it may be used as a check on fired heater air flow by installing a pressure indicator in the control house. 3. An automatic control system, which sets air flow as a function of firing rate to maintain a preset excess air level.

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BASIC DESIGN CONSIDERATIONS (Cont) AIR FLOW MEASUREMENT ➧

➧

Airflow measurement must be provided for all forced draft applications as well as special requirements, such as automatic airfuel ratio control, which dictate that air flow be measured. A venturi in the fan inlet is the most practical device for this purpose. The venturi may also be located in the fan discharge provided that sufficient run length between the fan exhaust and venturi inlet is available for good metering. In some special applications, particularly retrofits, there may not be overhead or plot space available to provide venturis for airflow measurement. Orifice meters require too great a pressure drop relative to venturis, and single pitot tubes require a greater velocity for proper measurement than is typically avialable in air ducts. Multiple pitot tube arrays provide a solution that results in reasonable accuracy, although they are subject to plugging in dusty environments and should be used only when a venturi is not feasible.

CALCULATION PROCEDURES REQUIRED WEIGHT FLOW OF DRY AIR AT GIVEN FIRING RATE Wa = Fg (FG – 1) where: Wa = Fg = FG =

Eq. (2a)

Air weight flow, lb/hr (kg/s) of dry air Gross fuel required, lb/hr (kg/s) Lb flue gas/lb fuel (kg flue gas/kg fuel) (Figure 12, Section VIII-M)

or  100 + EA   100 + EA  Wa = 0.0008 Qf   or Metric Wa = 0.344 Qf   100 100     where: Qf = EA =

Eq. (2b)

Heat fired, Btu/hr (MW) (LHV) Percent excess air

VOLUMETRIC AIR FLOW AT SPECIFIED ENVIRONMENTAL CONDITIONS Given: Summer temperature, relative humidity, and weight flow of dry air Volumetric flow rate, Fv Find: From psychometric chart (Figure 3A or 3B) determine v = specific volume in ft3/lb (m3/kg) dry air  v Wa  Fv =   or Metric Fv = v Wa x 1000  60  where: Fv

=

Eq. (3)

Volumetric flow rate, cu ft/min (dm3/s)

CORRECTING VOLUMETRIC FLOW RATE FOR ALTITUDE  1 .0   (Fv )corr =    Sa  where: Sa

=

(Fv )sea level Specific gravity for air at desired altitude, from Figure 5A or 5B

If the altitude is less than 1000 ft (305 m) above sea level, this correction can be neglected.

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Eq. (4)

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Section VIII-G

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CALCULATION PROCEDURES (Cont) AIR VELOCITY IN DISTRIBUTION DUCTS See Figure 1 for definition of distribution ducts. A riser duct is to be considered a distribution duct if it feeds two or more burners. Velocity head should not exceed 5% of the air pressure required at the burner inlet. Example:

Burner pressure Velocity pressure

= =

12 in. of water (3.0 kPa) 0.05 (12) = 0.6 in. of water (0.15 kPa)

From Figure 4A or 4B, air velocity = 52 ft/sec (15.8 m/s). However, duct air flow is not to exceed 40 ft/sec (12 m/s). Therefore, use 40 ft/sec (12 m/s) for pressure drop calculations. Note: Temperature and altitude effects on air density may be neglected in this calculation.

PRESSURE LOSS IN STRAIGHT DUCTS Use a design air velocity of 40 ft/sec (12 m/s) for all ducts except distribution ducts. Calculate the pressure drop using normal fluid flow correlations. See Section XIV-C. For pressure drop in a duct of rectangular cross-section, the equivalent diameter: dc =

2xy x+y

where: x and y are the dimensions of the sides of the rectangle.

OTHER DUCTING PRESSURE LOSSES Elbows, Tees and Changes in Flow Area - Use the procedure in Section VIII-C. Silencers - If silencers (mufflers) are used, obtain pressure loss data from the manufacturer.

ESTIMATING DRIVER POWER Approximate driver power can be found from Eq. (5), which is useful for estimating utility consumption:  0.000996 (Fv )corr H M   Ts + 273   0.000157 (Fv )corr H M   Ts + 460   or Metric P =   P=       N T 460 N +   w    Tw + 273   where: P (Fv)corr H M N Ts Tw

= = = = = = =

Eq. (5)

Approximate driver power, hp (kW) Air flow rate, cu ft/min (dm3/s) [from Eq. (4)] Fan static head, in. of water (kPa) [See Section XIV-C] Driver load factor, use 1.05 Fan static efficiency, use 0.70 Summer design air temperature, °F (°C) Winter design air temperature, °F (°C)

Eq. (5) can be used for estimating driver size by using the air flow and static head required at the fan rated point.

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NOMENCLATURE EA

=

Percent excess air

Fg

=

Gross fuel required, lb/hr (kg/s)

Fv

=

Volumetric air flow rate, cu ft/min (dm3/s)

Fvn

=

Volumetric air flow rate at normal firing rate, cu ft/min (dm3/s)

FG

=

Lb flue gas/lb fuel (kg flue gas/kg fuel)

H

=

Static head, in. of water (kPa)

Hn

=

Static head at normal firing rate, in. of water (kPa)

M

=

Driver load factor, dimensionless

N

=

Fan static efficiency, dimensionless

P

=

Approximate driver power, hp (kW)

Qf

=

Heat fired, Btu/hr (MW) (LHV)

Sa

=

Specific gravity of air at altitude in question

Ts

=

Summer design air temperature, °F (°C)

Tw

=

Winter design air temperature, °F (°C)

v

=

Specific volume, cu ft/lb (m3/kg) dry air

Wa

=

Air weight flow rate, lb dry air/hr (kg/s)

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TABLE 1A DESIGN SPECIFICATION INFORMATION ON FAN AND DUCTING (CUSTOMARY) FAN C - ________________________ Designation: Normal Air Flow: _____________________ lb/hr of dry air (air flow at normal heater firing rate) Air Pressure at Burner Inlet with Normal Air Flow: (burner windbox pressure at normal firing rate, data in Section VIII-F) _____________________ in. of water Design Environment: Altitude: _____________ ft above sea level (for altitudes less than 1000 ft, use sea level) Ambient Conditions: (Use G.I.I. design values) Summer

Winter

Temperature, °F

____________________

____________________

Relative Humidity, %

____________________

____________________

Air Flow Control: Variable inlet guide vanes (type of positioner must be specified) Driver Type: Constant speed (either electric motor or steam turbine) Blading: Backward curved, non-overloading Fan Seals: Manufacturer's standard; leakage is to be accounted for when the fan is sized (exception of GP 10-13-1) Rated Point of Fan: Rated Air Flow shall be 115% of normal air flow and attainable throughout the range of ambient conditions and at local altitude. Allowance for leakage from fan or ducting is to be added. Rated Static Head shall be 115% of the sum of (1) plus (2), below: 1.

Air pressure at burner inlet flange of ___________ in. water. (Value to be used here is the burner wind box pressure at maximum burner firing rate, obtained in Section VIII-F.)

2. Pressure losses in inlet and exhaust ducting calculated based on final ducting arrangement. System Resistance Curve shall be overplotted on the fan characteristic curves. System resistance is to be calculated based on normal air flow and burner inlet air pressure and ducting losses at normal firing rate. AIR DUCTING Air Velocity in ducting is not to exceed 40 ft per second. Above-ground Ducting shall be: 1. 2. 3.

Constructed of carbon steel at least 3/16 in. thick. Braced for rigidity. In compliance with noise requirements in GP 2-1-1. Typically, this requires 2 in. or more of acoustic insulation (e.g., 80 to 100 lb/cu ft gunite). Ducting Inlet shall be: 1. Elevated to 15 ft above the first fired heater platform. 2. Provided with a rain shield. Isolation Dampers are required in crossover ducts between heaters. Final Ducting Arrangement and pressure drop calculations are to be submitted for approval by the Owner's Engineer.

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TABLE 1B DESIGN SPECIFICATION INFORMATION ON FAN AND DUCTING (METRIC) FAN C - ________________________ Designation: Normal Air Flow: _____________________ kg/s of dry air (air flow at normal heater firing rate) Air Pressure at Burner Inlet with Normal Air Flow: (burner windbox pressure at normal firing rate, data in Section VIII-F) _____________________ kPa Design Environment: Altitude: _____________ m above sea level (for altitudes less than 305 m, use sea level) Ambient Conditions: (Use G.I.I. design values) Summer

Winter

Temperature, °C

____________________

____________________

Relative Humidity, %

____________________

____________________

Air Flow Control: Variable inlet guide vanes (type of positioner must be specified) Driver Type: Constant speed (either electric motor or steam turbine) Blading: Backward curved, non-overloading Fan Seals: Manufacturer's standard; leakage is to be accounted for when the fan is sized (exception of GP 10-13-1) Rated Point of Fan: Rated Air Flow shall be 115% of normal air flow and attainable throughout the range of ambient conditions and at local altitude. Allowance for leakage from fan or ducting is to be added. Rated Static Head shall be 115% of the sum of (1) plus (2), below: 1.

Air pressure at burner inlet flange of ___________ kPa. (Value to be used here is the burner wind box pressure at maximum burner firing rate, obtained in Section VIII-F.)

2. Pressure losses in inlet and exhaust ducting calculated based on final ducting arrangement. System Resistance Curve shall be overplotted on the fan characteristic curves. System resistance is to be calculated based on normal air flow and burner inlet air pressure and ducting losses at normal firing rate. AIR DUCTING Air Velocity in ducting is not to exceed 12 m/s. Above-ground Ducting shall be: 1. 2. 3.

Constructed of carbon steel at least 5 mm thick. Braced for rigidity. In compliance with noise requirements in GP 2-1-1. Typically, this requires 50 mm or more of acoustic insulation (e.g., 1280 to 1600 kg/m3 gunite). Ducting Inlet shall be: 1. Elevated to 4.6 m above the first fired heater platform. 2. Provided with a rain shield. Isolation Dampers are required in crossover ducts between heaters. Final Ducting Arrangement and pressure drop calculations are to be submitted for approval by the Owner's Engineer.

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FIGURE 1 TYPICAL FORCED - DRAFT SYSTEM SIDE ELEVATION Rain Shield Fired Heater Screen

15ft (4.6 m)

Burner

Line for Pumping Out Sump

Inlet Duct Fan

A

A Access Door

Bolted Flange Connection

Tight Shutoff Damper

Riser Duct

Supply Duct

Sump

Tight Shutoff Damper

SECTION A-A Isolation Damper Crossover Duct

Driver

Coupling with Shield Supply Duct

Pedestal-Mounted Bearings

Isolation Damper Distribution Duct

Vertical Riser Feeds Two Burners

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FIGURE 2 TYPICAL CHARACTERISTIC CURVES FOR FORCED-DRAFT FAN WITH SYSTEM RESISTANCE CURVE OVER-PLOTTED

Rated Air Flow Air Flow at Normal Firing Rate

15% Margin

60% of Rated Air Flow Region of Unstable Operation

Static Head, Inches of Water (kPa)

Rated Point

15% Margin

Duct Pressure Loss

Normal Operating Point System Resistance Curve with All Burners Firing Closing of Inlet Vanes

Burner Windbox Pressure at Maximum Firing Rate

Inlet Vanes Fully Open

Rated Static Head

Burner Windbox Pressure at Normal Firing Rate

0

Air Flow Rate, ft3/min (dm3/s)

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December, 2001

FIGURE 3A PSYCHROMETRIC CHART* (CUSTOMARY)

85

80

14.75

* From Mechanical Engineering Thermodynamics by D.A. Mooney (Prentice-Hall, 1953)

14.5 ft3 per lb. of Dry Air

70 100% 90% 80% 70%

Wet Bulb Temperature, ° F

60% 50%

60

40% Relative Humidity 14.25

30% 50

20% 40 14.0

30 20

DP8GF3a

30

13.5

12.75

12.5

20

10%

13.75

40

50

Wet Bulb Temperature

13.25

13.0 60

70

80

90

Dry Bulb Temperature, °F

ExxonMobil Research and Engineering Company – Fairfax, VA

100

110

-10

0,75

-5

-5

ExxonMobil Research and Engineering Company – Fairfax, VA

0

0

30

+ 1,0

5

5

25

+ 0,4 + 0,8 + 0,2 + 0,6

10

15

Wet Bulb or Saturation Temperature ° C

0,80

10

10

40

45

50 15

55

60

65

20

20

25

25

0,85

30

30

- 0,4

* Reproduced by Permission of Carrier Corporation

35

35

40

130 45

135

40

20%

30%

40%

50% Relative Humidity

60%

70%

80%

90%

Volume m3 /kg Dry Air

Dry Bulb Temperature ° C

15

70

75

80

85

100

105

110

125

0,90

45

10%

50

50

140

55

0,032 0,031 0,030 0,029 0,028 0,027 0,026 0,025 0,024 0,023 0,022 0,021 0,020 0,019 0,018 0,017 0,016 0,015 0,014 0,013 0,012 0,011 0,010 0,009 0,008 0,007 0,006 0,005 0,004 0,003 0,002 0,001 0,00

55 0,03390

145

0,75 0,80 0,85 0,90 0,95 1,00

0,70

0,65

0,60

0,55

0,50

0,45

0,40

0,36

December, 2001

Below 0°C Properties and Enthalpy Deviation Lines are for Ice

- 10

-5

0

5

10

20

25

35

40

Enthalpy at Saturation kJ / kg Dry Air - 0,05

95 90

- 0,2

DP8GF3b

- 0,1

14 of 18

- 0,6

120

Moisture Content kg / kg Dry Air

VIII-G

- 0,8

Page

Sensible Heat Factor

- 1,2

0,95

-1,0 Enthalpy Deviation kJ / kg Dry Air

Section

115

ExxonMobil Proprietary FIRED HEATERS

FORCED-DRAFT SYSTEMS DESIGN PRACTICES

FIGURE 3B PSYCHROMETRIC CHART* (METRIC)

ExxonMobil Proprietary FIRED HEATERS

Section VIII-G

FORCED-DRAFT SYSTEMS DESIGN PRACTICES

Page 15 of 18

December, 2001

FIGURE 4A AIR VELOCITY VERSUS VELOCITY PRESSURE (DYNAMIC HEAD)* (CUSTOMARY) 100 90 80 70 60

Air Velocity, ft/sec

50

40

30

20

10 0.07

0.1

0.2

0.3

0.4

0.6

0.8

1.0

Velocity Pressure, Inches of Water

Velocity Pressure (In. H2O) Velocity, ft/sec = 18.3

Air Density (lb/ft3)

Curve Based on ρ = 0.075 lb/ft3 DP8GF4a

* From Fan Engineering, (Buffalo Forge Co., N.Y., 1961)

ExxonMobil Research and Engineering Company – Fairfax, VA

2.0

3.0

ExxonMobil Proprietary FIRED HEATERS

Page

Section VIII-G

16 of 18

FORCED-DRAFT SYSTEMS DESIGN PRACTICES

December, 2001

FIGURE 4B AIR VELOCITY VERSUS VELOCITY PRESSURE (DYNAMIC HEAD)* (METRIC)

30.

Air Velocity, m/s

20.

10. 9. 8. 7. 6. 5. 4. 0.01

0.02

0.03

0.04 0.05 0.06

0.08

0.1

0.2

Velocity Pressure, kPa

Velocity, m/s = 44.8

Velocity Pressure (kPa) Air Density (kg/m3)

Curve based on 20°C and ambient pressure of 101.325 kPa

DP8GF4b

* From Fan Engineering, (Buffalo Forge Co., N.Y., 1961)

ExxonMobil Research and Engineering Company – Fairfax, VA

0.3

0.4

0.5

0.6 0.7

ExxonMobil Proprietary FIRED HEATERS

FORCED-DRAFT SYSTEMS DESIGN PRACTICES

Section VIII-G

1.0

0.95

0.90

Specific Gravity of Air

17 of 18

December, 2001

FIGURE 5A CHANGE OF AIR DENSITY WITH ALTITUDE* (CUSTOMARY)

0.85

0.80

0.75

0.70

0.65 0

5000 Altitude above Sea Level, Feet

DP8GF5a

Page

* From Fan Engineering, (Buffalo Forge Co., 1961)

ExxonMobil Research and Engineering Company – Fairfax, VA

10,000

ExxonMobil Proprietary Section

FIRED HEATERS

Page

VIII-G

18 of 18

FORCED-DRAFT SYSTEMS DESIGN PRACTICES

December, 2001

FIGURE 5B CHANGE OF AIR DENSITY WITH ALTITUDE* (METRIC)

1.00

0.95

Specific Gravity of Air

0.90

0.85

0.80

0.75

0.70

0.65 0

500

1000

1500

2000

Altitude Above Sea Level, m DP8GF5b

* From Fan Engineering, (Buffalo Forge Co., 1961)

ExxonMobil Research and Engineering Company – Fairfax, VA

2500

3000