Transcript Design Considerations for Main Exhaust Fan Systems at

The Application of Vertically-Mounted
Jet Fans in Ventilation Shafts for a Rail
Overbuild
Richard Ray, Mark Gilbey and Praveen Kumar
PB Americas, Inc.
Railroad Tunnel Ventilation
Requirements

Normal Operations:
 Removal
of Heat
 Dilution
of Combustion Products from Diesel
Locomotives

Train Fire Emergency:
 Provide
Tenable Evacuation Path for Evacuating
Passengers per NFPA 130
“Piston Effect” Longitudinal
Ventilation (Normal)
Ventilation Shafts
Portal
Portal
Direction of Train Travel
“Push-Pull” Longitudinal
Ventilation (Emergency)
Supply Fans
Exhaust Fans
Direction of Passenger Egress
Reversible Axial Fans for
“Push-Pull” System
Isometric View of Overbuild
Building “A”
Building “I”
North
Portal
Length = 914.4 m (3,000 ft)
Width = 9.75 to 15.24 m (32 to 50 ft)
Building “O”
South Portal
Height = 5.56 to 8.53 m (18.25 to 28 ft
Building “I” Overbuild
Overbuild Natural/Mechanical
Ventilation: Buildings “A” – “E”
Damper
Shaft Fan
Portal
Damper
Extraction Duct Fans
Stopped Train
Shaft Fan
Portal
Overbuild Ventilation System
Buildings “F” – “O”

Jet Fans Vertically-Mounted on Shaft
Walls Near Base of Shaft

Dampers at Top of Shaft Eliminated

Jet Fans Run for Fire Emergencies and
High NO2
Jet Fan Performance

Momentum Exchange Between Faster
Moving Jet of Air Discharged from Fan and
Surrounding Airstream

Only a Portion of the Total Flow Passes
through Jet Fan

Remainder Passes Around Fan and is
Accelerated by the Jet

Work Best with Low Resistance, Low Velocity
Tunnel or Shaft
Saccardo Nozzle
High Velocity Nozzle
Induced Airflow
Through Tunnel
Vehicular Tunnel Jet Fan
Installation
Design Considerations:
Building “I” Jet Fans

Target Airflow Total of 236 m3/s (500,000 cfm)
for the Two Shafts

Determine Required Jet Fan Thrust

Calculate Shaft and Plenum System
Resistance and Resulting Pressure Drop

Offset by Pressure Rise Due to Shaft Stack
Effect (Estimated Smoke Temperature =
107°C [225°F])
Preliminary Building “I” Shaft and
Plenum Geometry

Shaft Areas: 4.5 m (14.8 ft) by 3.05 m (10 ft)

Shaft Heights: 24.8 m (81.2 ft )

Single Approach to Shaft from Plenum
w/Turning Vanes at Bottom of Shafts

Series of 1.5 m (5.0 ft) by 1.5 m (5.0 ft )
Openings in Top of Crash Wall to Plenum

Plenum Height 1.6 m (5.1 ft) to 2.1 m (6.9 ft)
Plan View of Building “I” North
Shaft and Plenum
Calculated Overall
Pressure Drop =
0.184 kPa
(0.738 in. w.g.)
Stack Effect
DPstack = Dr g h = 0.062 kPa (0.25 in. w.g.)
Where:
DP= Stack effect pressure rise (kPa [in. w.g.])
Dr = Difference between ambient temperature and the
average smoke temperature air density (kg/m3 [lb/ft3])
g = Acceleration due to gravity (m/s2 [ft/s2])
h = Vertical height of shaft (m [ft])
 DPtunnnel / shaft  DPstack
Thrust  



 r
 Ashaft  std
 r smoke




Jet Fan Thrust
Where, Thrust in N (lb) is calculated from:
 = Jet fan effectiveness
Ashaft = Shaft cross sectional area (m2 [ft2])
rsmoke = Smoke density (kg/m3 [lb/ft3])
rstd = Air density at which fan was rated (kg/m3 [lb/ft3])
Jet Fan Effectiveness ( )


Ability of Fan to Transfer Momentum to
Surrounding Airstream
 = 1.0 for Fans Located in Center of
Shaft and Away from Shaft Walls

For Fans Close to Corners, Walls and
Other Fans,  Could Be as Low as 0.77
Correction Coefficient for Shaft
Velocity

Tunnel Air Velocity “Offloads
the Fan Compared to Still Air
Conditions” (Woods)

Jet Fan Velocity of 36.3 m/s
(7,140 fpm); Shaft Velocity of
11.6 m/s (2,280 fpm)

Coefficient of 0.68 x  of 0.77 =
Overall Correction of 0.52
Jet Fan Selection

Overall Coefficient of 0.65 Used

Total Thrust Required per Shaft =
3,514 N (790 lb)

Three 0.9-m (2.96-ft) Dia. Jet
Fans per Shaft Assumed for
Initial CFD Runs

Thrust = 3,079 N (687 lb) to
Match Fans in Other Shafts
Results of Initial CFD Analysis

Total Airflow for Two Shafts of 310.4 m3/s
(668,000 cfm)

Smoke Layers Still Unacceptably Low in
Some Segments of the Evacuation Path

Shafts Increased to 5.84 m (19.2 ft) by 3.05
m (10 ft) for Next Iteration

4th Jet Fan Added – Total Thrust of 4,095 N
(916 lb) Per Shaft
Revised Building “I” Shaft and
Plenum Configuration
South Shaft
North Shaft
Plenum
Crash
Walls
Revised Fan/Shaft Performance
Air Velocity (fpm)
South Shaft
North Shaft
184.53 m3/s
(391,000 cfm)
164.24 m3/s
(348,000 cfm)
Section View of Air Velocity
Vectors Through Shafts
Air Velocity
(fpm)
Air Velocity
(fpm)
Air Velocity Contours at Fan
Discharge and Top of Shafts
Air Velocity
(fpm)
South Shaft
North Shaft
Calculations vs. CFD Analysis

Stack Effect Less than Calculated Due to Dilution
from Make-Up Air

Calculations Repeated Using CFD Output

Shaft Smoke Temperature of 58°C (136°F)

Make-up Air Pressure Drop of 0.027 kPa (0.110 in. w.g.)

Overall Correction Coefficient of 0.625 Calculated

With Shaft Coefficient from Table of 0.71, yields  of
0.88 instead of 0.77
Conclusions

Jet Fans Can Be Used to Induce High Airflow
Quantities Through Shafts in Tunnel Ventilation
Systems

Jet Fan Thrust Estimates Should Account for
Efficiency () and Shaft Velocity Correction Factor

Jet Fan Efficiency () Not as Adversely Impacted
by Shaft Length and Proximity to Walls/Corners as
Predicted