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**a Critical Velocity : ****a quick design calculation for Critical Air Velocities in tunnels for smoke control ** |

To determine critical air velocities in tunnel ventilation for smoke control have never been easy. **Not anymore!** With *aCriticalVel*, you can do it in just a click away at anytime, anywhere...

Aim: creating a mobile design environment for the practising engineers & designers in today's mobile world.

Results: Instant solutions at your fingertips.

**Highlights**:

- calculates critical air velocity for smoke control in tunnels based on most well-known Kennedy formula adopted in NFPA 502

- built-in customized area calculator. Shapes available are Partial circle, Segment circle, Half circle + rectangle, Half ellipse + rectangle, and Rectangle

- built-in air density & air specific heat calculator

- "save-to-file" function with built-in text file viewer

- in SI-IP Units

** a Critical Velocity****: design explained**. . . |

*aCriticalVel* program employs the most commonly used model developed by Kennedy et al. to determine the ciritcal air velocity in tunnel ventilation for smoke control.

The critical velocity, Vc, is the minimum steady-state velocity of the ventilation air moving toward a fire that is necessary to prevent backlayering as illusrated in the diagram below.

To calculate the critical velocity, one needs __to solve a coupled set of equations (as given below) by iteration__. Therefore, __it is best to let a computer program to perform this task__.

(Note: for symbol and detail, refer to NFPA 502)

*acriticalVel* is designed with a built-in calculators:

- to calculate area for various shapes: partial circle, segment circle, half circle + rectangle, half ellipse + rectangle, and rectangle.

- to determine air density and air specific heat at given conditions.

**Design Fires in Road Tunnel . . . NFPA 502** |

Typical fire heat release rates for tunnel vehicles are shown in the following table (after NFPA 502):

**Did you know. . . SES program uses Kennedy model** |

The Subway Environmental Simulation (SES) computer program for predicting ventilation flows in tunnel networks also includes a simple model to calculate critical velocity devised by Kennedy et al.

**Calculation Example 1: see to believe**. . . |

The following design calculations are done with *aCriticalVel* program.

__Worked Example (in SI units)__ Determine the critical velocity required to prevent backlayering of smoke in a cable tunnel. Assume a 5MW fire.
Critical Velocity Calculation ====================== Fire heat release reate, Qc (MW) = 5 Tunnel area, A (m^{2}) = 14 Tunnel height, H (m) = 5.2 Tunnel gradient, grade (%) = 3 Approach air temp., Ta (^{o}C) = 35 Approach air density, rho (kg/m^{3}) = 1.134 Air specific heat, Cp (J/kg.^{o}K) = 1039 **Critical velocity, Vc (m/s) = 2.15**
Airflow rate (m^{3}/s) = 30.1 Hot-gas temp., Tf (^{o}K) = 449.3 Hot-gas temp., Tf (^{o}C) = 176.1 |

**Calculation Example 2: Road Tunnel **. . . |

The following design calculations are done with *aCriticalVel* program.

__Worked Example (in IP units)__ A road tunnel has the following parameters and data:
Tunnel area = 861 ft^{2}, Tunnel height = 19.7 ft, Tunnel grade = 0 %, Fire heat = 102 MBtu/h, Air temp = 80.6 ^{o}F, Air density = 0.0687 lb/ft^{3}, Air specific heat = 0.2484 Btu/lb.^{o}F. Calculate the critical velocity required to prevent backlayering of smoke. Critical Velocity Calculation ====================== Fire heat release reate, Qc (MBtu/h) = 102 Tunnel area, A (ft^{2}) = 861 Tunnel height, H (ft) = 19.7 Tunnel gradient, grade (%) = 0 Approach air temp., Ta (^{o}F) = 80.6 Approach air density, rho (lb/ft^{3}) = 0.0687 Air specific heat, Cp (Btu/lb.^{o}R) = 0.2484 **Critical velocity, Vc (ft/m) = 415.4**
Airflow rate (ft^{3}/m) = 357650 Hot-gas temp., Tf (^{o}R) = 818.8 Hot-gas temp., Tf (^{o}F) = 359.1 |

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