Open areas calculator - Smoke-free environments
This calculator is a guide to determining whether an area is 'open' or 'internal' for the purposes of the Smoke-free Environments Act 1990. It is not a substitute for independent professional advice.
Date of online publication: September 2005
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Additional notes
Please note
This calculator is a guide to determining whether an area is 'open' or 'internal' for the purposes of the Smoke-free Environments Act 1990. It is not a substitute for independent professional advice.
The New Zealand Ministry of Health accepts no liability or responsibility for any acts or omissions, or any loss or damage resulting from any acts or omissions, done or omitted in reliance, in whole or part, on this calculator or its output.
Neither the use nor the output of this calculator limits your obligations under any other enactment or bylaw, including the Health and Safety in Employment Act 1992.
The New Zealand Ministry of Health reserves the right to exercise any of the enforcement powers conferred on it under the Smoke-free Environments Act 1990, notwithstanding the use or output of this calculator.
If you are in doubt as to whether a proposed use is covered by this licence, please email webmanager@moh.govt.nz.
Sources used in the preparation of this calculator
- NZS 4304:1990,
- CIBSE Guide A, 1999
Assumptions
The foreword to NZS 4304:1990 states: "The purpose of the Standard is to specify minimum ventilation rates and indoor air quality that will be acceptable to human occupants and are intended to avoid adverse health affects. For substantive information on health effects, the Standard must rely on recognized authorities and their specific recommendations. Therefore with respect to tobacco smoke and other contaminants, this Standard does not, and cannot, ensure the avoidance of all possible adverse health effects, but it reflects recognised consensus criteria and guidance".
Minimum air supply required per person = 30 lts/s… NZS 4304:1990, Table 2, Pages 8-10.
Floor area per person = 1.4 m2/person… This figure is extrapolated from the Estimated Maximum Occupancy column… NZS 4304:1990, Table 2, Pages 8-10.
Wind speed is taken as being = 0.86 m/s (calm conditions)… For the purpose of these calculations, the driving force for ventilation is taken as being wind only.
The pressure coefficient for openings on 2 opposite sides (assumed to have 45º wind angle) is taken to be = 0.35… CIBSE Guide A, 1999, Table 4.3, page 4-6 The pressure coefficient for openings on 2 adjacent sides (assumed to have 45º wind angle) is taken to be = 0.1… CIBSE Guide A, 1999, Table 4.3, page 4-6
CIBSE Guide A states: "The theoretical value of Cd for a sharp-edged opening is 0.61 and it is common practice to refer other openings, for which the airflow is governed by the square root of the pressure difference, to this value". The discharge coefficient is thus taken as being 0.61.
Equations used
Q=0.025 AOpening Vr... for openings on single side… CIBSE Guide A, 1999, Table 4.13, page 4-12
Qw = Cd AEq vr (∆Cp)0.5 … for openings on more than one side… CIBSE Guide A, 1999, Table 4.12, page 4-11.
Page last updated: 01 July 2011
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