Fletchers Equations – Excel Spreadsheet

Fletchers Equations are the way to go when either designing a Captor Hood or assessing the Captor Distance during a TExT.  This is the methodology expected by HSE.

Do not use the ACGIH Industrial Ventilation Manual equations as these are based on the Dallavalle Equations and (according to HSE) are too optimistic when applied to (especially) low Aspect Ratio Captor hoods.

Fletchers Equations ar a bit challenging for a simple calculator:-

But, fortunately Excel ‘crunches’ these equations quite well.  Note – if you have a ‘silly’ sized Captor Hood – such as 2mm x 2mm – the equations stop working correctly.  For all ‘normal’ sized hoods they are perfect.

The Excel spreadsheet has two ‘Tabs’ at the bottom – one for Designers who wish to calculate the required hood entry velocity (Face Velocity) and the second for TExT Engineers who, knowing the actual measured Captor Hood Face Velocity, need to calculate the “Effective Captor Distance”.

These equations are also available in a handy FREE OXYL8 App.  To download the App – visit your App Store and search using OXYL8 (there is only one App).

Download the Fletcher Excel spreadsheet here:- Fletcher 2022_v2.1