FLOW BALANCING REPORT

POT LINE 7

ALCOA ROCKDALE ALUMINUM SMELTER

Rockdale, Texas

Table of Contents

1.0 Summary:................................................................................................ 1

2.0 Instrument Validation:............................................................................ 2

3.0 Flow Balance:......................................................................................... 3

List of Exhibits:

Exhibit 1 – Flow Balancing Summary

Exhibit 2 – Raw Data
Alcoa Rockdale Smelter Page 1

Flow Balancing Report – Pot Line 7 050167AJ

The information contained herein is privilege and confidential information and is intended only for the use of the addressee’s 

company. Any dissemination, distribution, or copying of this material to others, in whole or in part, is strictly prohibited without the 

written permission of the CEO/Pres. of Air Control Science Inc.

1.0 Summary:

Air Control Science , Inc. (ACS) was retained to flow balance the Line 7 pot 

fume collection system at Alcoa’s Rockdale Aluminum Smelter in Rockdale 

Texas. ACS crews completed the balancing work May 13, 2005.

The Line 7 fume control system consists of two separate scrubber systems, one 

located in the east courtyard, and one in the west courtyard. Each scrubber system 

consists of 6 reactor modules and 6 fans. The east courtyard system maintains 

draft on the east half of Line 7, pots 1 through 40 in rooms 128 and 130. The west 

courtyard system maintains draft on the west half of Line 7, pots 41 through 80 in 

rooms 128 and 130. Draft is controlled by parallel blade fan exhaust dampers, 

which are positioned based on a fan amp input. 

Alcoa conforms to a regulatory emission limit as measured at potroom building 

roof monitors, and has established an internal benchmark limit below the 

regulatory limit as a target maximum. Alcoa has experienced problems in 

previous years during summer months with measured emissions approaching 

internal target maximum limits, particularly from Line 7 and Line 8. Alcoa’s 

objective in balancing pot draft is to reduce the risk of potroom emission 

excursions as measured at the building roof monitors. 

Alcoa reports original design pot flows ranging between 3,200 ACFM and 3,500 

ACFM for Line 7 and 8 pot draft. Prior to balancing, ACS measured pot flows 

ranging from 3,000 ACFM to over 4,000 ACFM. Prior to balancing, pot draft 

was found to average 3,150 ACFM for the east half of Line 7 and 3,350 for the 

west half of Line 7. During balancing, pot draft was adjusted to approximately 

3,200 ACFM for the east half of Line 7 and 3,300 ACFM for the west half of Line 

7. Table 1 provides a summary of flow settings for each scrubber system.

Table 1 Flow Balance Summary

Scrubber System Pots Served Pre-Balance 

Average Flow

Post Balance 

Average Flow

East Scrubber 1-40, rooms 128 and 130 3,150 ACFM 3,200 ACFM

West Scrubber 41-80, rooms 128 and 130 3,350 ACFM 3,300 ACFM
Alcoa Rockdale Smelter Page 2

Flow Balancing Report – Pot Line 7 050167AJ

The information contained herein is privilege and confidential information and is intended only for the use of the addressee’s 

company. Any dissemination, distribution, or copying of this material to others, in whole or in part, is strictly prohibited without the 

written permission of the CEO/Pres. of Air Control Science Inc.

Because several pots at the east half of Line 7 required a 100% open damper 

setting to achieve target flows, it is believed that the selected target flow setting of 

3,200 ACFM for the east half of Line 7 was maximized to match the east scrubber 

system capacity. It is believed that target flow setting for the west half of Line 7 

could possibly have been increased to 3,350 ACFM to match the west scrubber 

system capacity; however additional flow testing should be completed prior to 

attempting further adjustments to verify expected capacity gains. 

Alcoa and ACS have previously recorded final damper position angle to establish 

a target setting when starting up a new pot. However, flow measurements for this 

balancing effort were successfully made using an averaging pitot tube that greatly 

simplifies the damper setting process. Further, pot flows were found to be very 

sensitive to minor adjustments in damper settings, making it difficult to measure 

and set dampers by angle. ACS recommends that Alcoa set dampers using the 

averaging pitot tube and velocity pressure setpoints identified herein when setting 

new pots in the future. The flow measurement and balancing procedure used for 

this project is described in Section 3.

2.0 Instrument Validation:

Prior to initiating flow balancing work, Alcoa requested that ACS validate 

suitability of an averaging pitot tube instrument similar to that used by the Alcoa 

Warrick facility. The averaging tube is a wand made of two hollow tubes, the 

upstream tube fitted with 4 total pressure holes on the leading edge, and the 

downstream tube fitted with a single static pressure opening in the center-back of 

the wand. Similar to a standard or “S” type pitot tube, velocity pressure is 

measured directly by an oil-filled inclined manometer as the difference between 

total pressure and static pressure. 

During validation, ACS measured flows at 13 pots using a standard pitot tube and 

a 20-point traverse at the 14” ID fiberglass duct, and using the averaging tube at a 

3⁄4” half coupling port on the grounded, 15” ID aluminum duct. The 20-point 

traverse was assumed to provide an accurate benchmark velocity pressure and 

flow, against which velocity pressure and flow as measured by the averaging tube 

was compared. Calculated flow as measured by the averaging pitot tube was 

adjusted by a constant to compensate for the following factors:

averaging tube flow coefficient (reported by the manufacturer to be 0.63), 

and
Alcoa Rockdale Smelter Page 3

Flow Balancing Report – Pot Line 7 050167AJ

The information contained herein is privilege and confidential information and is intended only for the use of the addressee’s 

company. Any dissemination, distribution, or copying of this material to others, in whole or in part, is strictly prohibited without the 

written permission of the CEO/Pres. of Air Control Science Inc.

undeveloped flow in the ductwork caused by downstream elbow adjacent 

to measurement point

Flow measured by the averaging pitot tube was found to be accurate within 10% 

of measured benchmark flow at a constant coefficient of 0.425. Table 2 provides

a summary of instrument validation results. 

Table 2 – Validation Data Summary

Measurement

Point (Temp 

deg F)

20-Point Ave. 

VP (in. wg)

20 Point Flow

(ACFM)

Averaging Tube 

VP (measured, 

in.wg)

Averaging 

Tube Flow 

(adjusted 

ACFM)

Flow 

Difference

28-80 (205) 0.54 3551 1.1 3816 266

28-79 (205) 0.61 3790 1.1 3816 27

28-70 (225) 0.47 3345 0.87 3445 100

28-64 (242) 0.41 3125 0.74 3216 91

28-65 (207) 0.67 3976 1.1 3822 -154

30-80 (214) 0.51 3493 1.1 3842 349

30-79 (244) 0.48 3415 0.83 3411 -4

30-76 (226) 0.69 4083 1.3 4216 133

30-73 (234) 0.64 3927 1.05 3811 -115

30-72 (223) 0.67 4004 1.0 3690 -314

28-70 (227) 0.45 3265 0.75 3203 -62

28-65 (227) 0.59 3766 0.98 3632 -124

30-76 (210) 0.70 4041 1.15 3917 -124

3.0 Flow Balance:

To isolate impact from pot flow adjustments to one system, ACS focused 

balancing efforts on one scrubber system at a time. To coordinate work with 

Alcoa’s carbon setting shifts, ACS started balancing work on the west scrubber 

system during the day shift while carbon crews were working on the east ends of 

the buildings. The east scrubber system was balanced during the swing shift when 

carbon crews were working on the west ends of the buildings. 

Following instrument validation, ACS used the calibrated averaging pitot tube to 

measure velocity pressure at all pots. For initial measurements, duct static 

pressures were measured at all pots, however were never found to deviate from -5 

in. wg and therefore were not measured or recorded during flow balancing work. 
Alcoa Rockdale Smelter Page 4

Flow Balancing Report – Pot Line 7 050167AJ

The information contained herein is privilege and confidential information and is intended only for the use of the addressee’s 

company. Any dissemination, distribution, or copying of this material to others, in whole or in part, is strictly prohibited without the 

written permission of the CEO/Pres. of Air Control Science Inc.

Pot draft temperatures were measured and recorded during each flow 

measurement and balancing effort except during balance iterations and final, 

same-day checking. 

Working in 10-pot manifold sections, ACS adjusted all dampers within a 

manifold to achieve a measured velocity pressure within the target range. 

Depending on initial manifold flow conditions, one to two iterations were 

required on each pot to achieve desired balance distribution. If a few pots 

exhibited significantly higher flows than others, additional iterations were likely 

required. 

ACS began by measuring flow in pots 40 through 80 in both rooms 128 and 130 –

the west scrubber system. Measured velocity pressures were tabulated and 

averaged by approximation to determine an appropriate balance set point. For the 

west scrubber system, 0.78 ±0.03 in. wg was established as a target velocity 

pressure (as measured by the averaging pitot tube). Completing instrument 

validation and balancing the west half of Line 7 required one 5-day period of 12 

hour days.

Following balancing of the west scrubber system, ACS began work on the east 

scrubber system by measuring flow in pots 1 through 40 in rooms 128 and 130. 

Measured velocity pressures were tabulated and averaged by approximation to 

determine an appropriate balance set point. For the east scrubber system, 0.73 

±0.02 in. wg was established as a target velocity pressure. Completing balancing 

of the east half of Line 7 required one 3-1/2 day period of 12 hour workdays. 

Following balancing of each scrubber system, ACS completed random flow 

checks on roughly 10 pots per system, per room. All flow checks were found to 

be within original balance targets. 

Some pots were found to have broken dampers or holes in fiberglass duct joints. 

Table 2 provides a summary of each defective or suspect pot

Flow Balancing is summarized on tables presented in Exhibit 2 of this report. 

Each exhibit contains a summary table and the raw field measurements.
Alcoa Rockdale Smelter Page 5

Flow Balancing Report – Pot Line 7 050167AJ

The information contained herein is privilege and confidential information and is intended only for the use of the addressee’s 

company. Any dissemination, distribution, or copying of this material to others, in whole or in part, is strictly prohibited without the 

written permission of the CEO/Pres. of Air Control Science Inc.

The summary table is organized with the following rows and columns:

 Branch of Main Duct Number – is the section of duct in which the airflow 

measurement was taken. The numbers correspond to the node numbers on 

the provided drawings.

 Duct Diameter – is the diameter of the duct in which the airflow was 

measured.

 Gas Temperature – is the temperature as measured in the duct, degrees F. 

 Moisture Content – not used.

 Humid Volume – assumed dry air, see note on sheet.

 Static Pressure – measured static pressure. 

 Number of points on traverse – total number of measurements made at the

location of the measurement.

 Pt – point or location of measurement on duct

 VP – measured velocity pressure

 Vel – measured velocity

 Average Velocity - is the average velocity measured in the duct.

 ACFM – is the Actual Cubic Feet per Minute of airflow measured in the 

duct.

 DSCFM – is the Dry Standard Cubic Feet per Minute of the measured 

airflow. DSCFM is the calculated airflow adjusted to standard conditions 

of air density (Dry air at 70 F and Sea Level).