Acceptability of air velocity from a human thermal comfort and safety

perspective

Matthew Legg and Mark Gilbey

WSP | Parsons Brinckerhoff in the UK

Agenda

• Motivation

• Existing guidance

• Effects of air velocity

– Thermal comfort

– Mechanical comfort

– Safety

• Conclusions

Why control air velocity?

Stability and life safety

Dynamic forces on structures

Dust or litter transport

Passenger experience

0-3 m/s

4-6 m/s

6-9 m/s

9+ m/s

Normal operations

Emergency operations

Existing guidance

Comfort

• Subway Environmental Design Handbook (SEDH)

• Beaufort Scale

• ASHRAE draught index

• Project design criteria

• EN ISO 7730

• Not easily defined, most sources cite 3-6 m/s

Safety

• SEDH

• NFPA 130

• TSIs

• Project design criteria

• 11 m/s widely accepted as safety criterion

Refinement of thermal comfort guidance

• Concept of thermal neutrality

• Heat rejection dominated by convective heat transfer

• Desirable to:

• Maximize thermally pleasing air velocities (cooling breeze)

• Minimize thermally displeasing draughts (wind chill)

• Building services indices not necessarily applicable

EN ISO 7730

Refinement of thermal comfort guidance

Mechanical discomfort

Source Limit Effects

SEDH 5 m/s Disruption of clothing (Beaufourt)

Lopes et al. 6 m/s “Very windy” sensation, steady wind and acclimatised

Hunt et al.6 m/s Increase in perceived wind noise, 10% turbulent intensity

7 m/s People appear “blown about”

• Discomfort or nuisance effects due to intermittent gusts

• Less sensitive to temperature

• More sensitive to unsteadiness or turbulent intensity

Safety limits

• Buildings adopt Lawson scale - requires historical data

• SEDH and NFPA 130 cite ~11 m/s (2200 fpm)

• 11 m/s appears reasonable based on experimental data

• Corresponds to around 15% of people with difficulty walking (Lopes)

• Stability largely affected by “gustiness”

• Rapid changes in air velocity may occur spatially and temporally

• Females more susceptible than males

• Reasonable that front facing gusts are more challenging

• Hunt et al. suggested correction factor of 1+3*TI for turbulent intensity

Safety limits

• Jordan data suggested loss of stability for aerodynamic forces > 5% of body weight

• Accounts for physiological differences

Average Female 9 m/s

Average Male 10 m/s

• TSIs cite absolute limit of 15.5 m/s on platforms

Safety limits

• Thermal comfort:

• Additional guidance for selection of comfort criteria

• Mechanical comfort

• Further data around mechanical effects on people

• Safety

• Improved context on the selection of safety limits

• Considerations for unsteady air velocities

Conclusions

Questions?