FAQ 111 – Chimney Height Calculation Spreadsheet

What is the Supplementary Assistance: Chimney Height Calculation Spreadsheet?

The Supplementary Assistance Chimney Height Calculation Spreadsheet has been designed to estimate the minimum permissible stack height for a small boiler emitting sulphur dioxide (SO₂), to ensure compliance against the air quality objective of 266 μg/m³ (expressed as the 99.9^th percentile of 15-minute means, i.e., allowing a maximum of 35 exceedances per year). The spreadsheet could indicate taller stacks than either the HMIP D1 calculation method, the Guidance on Stationary Sources, or the Chimney Heights Memorandum, because it includes more conservative assumptions on meteorology, and uses a more stringent assessment criterion.

Limitations of Method

This calculation method is a screening tool; therefore, it gives a conservative estimate for the minimum permissible stack height. For certain situations, a shorter stack could suffice, but would need to be justified using detailed dispersion modelling.

This spreadsheet can only be used to calculate the minimum permissible stack height for a single chimney emitting SO₂. The calculation method is not applicable to multiple stacks in close proximity, or any other pollutant. As the 15-minute mean objective is the most stringent air quality objective for SO₂ in the Air Quality Strategy, the stack height calculated by this spreadsheet would therefore also ensure compliance against the 1-hour and 24-hour mean objectives.

The spreadsheet is based on the results of a large number of runs of the ADMS dispersion model. The range of validity of the spreadsheet is as follows:

• Emissions which vary with time cannot be treated by the method; a constant SO₂ emission rate is assumed in the calculations.
• The method assumes flat terrain in the vicinity of the stack, and no coastal wind effects.
• The method can model stack volumetric flow rates in the range 2.5 – 20 m³ s^-1.
• The method can model exit diameters in the range 0.5 – 2 m.

Instructions

The screening tool is available from the Supplementary Assistance Chimney Height Calculation Spreadsheet page. The spreadsheet will calculate the minimum permissible stack height which will ensure compliance against the 15-minute mean air quality objective for SO₂, taking account of the local background SO₂ levels in year 2005.

STEP 1: Background Concentration

The annual average background concentration must be entered, in units of ‘μg m^-3‘. In the absence of local monitoring data for annual mean SO₂ concentrations, estimates for year 1996 may be obtained from the UK Air website. The figure entered onto the spreadsheet must relate to year 2005.

STEP 2: Emission Characteristics

Enter the following data in the appropriate cells:

• Stack internal diameter at exit point (m).
• Volumetric flow rate at exit temperature (m^-3 s^-1).
• SO₂ emission rate (g s^-1).

STEP 3: Results

The results table at the bottom of the spreadsheet gives three values for the minimum permissible stack height:

• The stack height which would ensure compliance with the 15-minute mean objective, if there are no significant buildings close to the stack.
• The stack height which would ensure compliance with the 15-minute mean objective, if it was centred on a cubic building, of height 3 m less than the stack height. This is effectively a ‘worst-case’ building for downwash effects; a clearance of less than 3 m between building and stack top is not advisable.
• The stack height which would ensure compliance with the 15-minute mean objective, if it was centred on a cubic building, of height 6 m less than the stack height.

The dispersion algorithms which attempt to account for building wake effects have not been as well validated as models without building downwash. Therefore, it must be appreciated that the results including building downwash effects are less robust than the calculations in the absence of a building.