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Chimneys & flues


Hitting the heights


Dennis Milligan of the British Flue & Chimney Manufacturers Association (BFCMA) explains why chimney height is a crucial factor when it comes to improving air quality


A


ir quality is currently a hot topic and there are good reasons for calling for a new and updated Clean Air Act. The Clean Air Act was introduced in the 1950s in


order to combat the pollution caused by burning coal. Our understanding of the sources of air pollution has moved on since then, but one thing that has remained constant is the need to use chimney height to disperse the products of combustion in the atmosphere. The two pieces of legislation that control the emission of waste gases in Great Britain are the Clean Air Act (1993) and the Environmental Protection Act (1990).


Chimney function


The function of a chimney or flue is to discharge the products of combustion into the atmosphere. Chimneys can act as a means of controlling local pollution by discharging the emissions high enough to ensure that the dispersion of the initial discharge plume, where pollutants are most concentrated, is sufficiently high above ground level.


The chimney/flue height should be such that at the eventual contact of the plume with the ground pollution concentrations are not harmful to health. This usually requires a distance of 50-100 times the chimney height before the plume reaches the ground.


Dispersion requirements


The Clean Air Act and the Environmental Protection Act regulate how these gases can be dispersed safely in the atmosphere. The discharge height of the chimney/flue is defined as the height at which the emissions are rendered harmless and prevented from being a nuisance or prejudicial to public health. Local authorities use the Clean Air Act


Memorandum and the Technical Guidance Note D1, (dispersion), to calculate the required height of a chimney or flue. The calculations used to determine the chimney/flue height take into account the type of fuel burned and the maximum rated output of the appliance. In the case of fuels with a sulphur content greater than 0.04% the geographical location of the plant is also considered. The Clean Air Act applies to all fuels. The discharge velocity of the emissions from a chimney/flue is another important factor in


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determining how readily emissions are dispersed. Essentially, the higher the discharge velocity the better the dispersion rate. When designing the chimney/flue system it is important to design the chimney/flue so that the required discharge velocity can be achieved. With boiler systems, it is often necessary to reduce the internal diameter of the chimney/flue top by using a tapered cone – care should, however, be taken to calculate the correct size of the tapered cone.


Consistency


The maintenance of a constant discharge velocity can be difficult when more than one appliance is serviced by one chimney/flue. In this situation, an exhaust fan may be required. It is important that the discharge velocity is sufficient to prevent the plume of gases flowing down the outside of the chimney/flue. A high discharge velocity also prevents cold air entering the top of the chimney/flue and cooling the flue gas temperature, which could lead to condensation on the chimney/flue inner wall and possible corrosion problems. Adjacent buildings can also have an impact on the


required chimney or flue height and must be taken into consideration when calculating the chimney/flue height. It should always be borne in mind that the objective is to achieve an effective and safe chimney/flue discharge height, rather than a minimum discharge height. The BFCMA recommends following a 10-point check list when considering the chimney/flue height. Start with matching the appliance and the chimney/flue, then look at the chimney/flue height and emissions. 1. Determine the necessary height to create the optimal draught to meet the appliance operating requirements. Approved Document J provides clear guidance. 2. Factor in site-specific environmental impacts on the chimney/flue draught. The local terrain (proximity of trees and higher buildings, for example) will have an impact on the performance of the chimney/flue. 3. The heat output of the appliance and the total kW/hr rating of the boiler room need to be


considered, along with the appliance efficiency. 4. The type of chimney/fuel to be used will have an impact on emissions. 5. DEFRA Smoke Control Areas require the installation of an exempted appliance. This, however, only covers the appliance and the correct chimney height has still to be calculated. 6. Building regulations requirements must always be referenced. 7. The requirements of BSEN15287-2:2008 should be followed for appliances with a kilowatt rating above 50kW. 8. When considering the requirements of the Clean Air Act for wood burning appliances with a burn rate above 45.4kg/hr the Biomass Assessment Tool (TG09) should be used. This tool takes into account the postcode of the installation. 9. A location is designated an Air Quality Management Area (AQMA) if the air quality breaches the required limits. In a AQMA it is the local authority’s responsibility to measure and approve the installation of the appliance. 10. Finally, dispersion modelling may be required for larger appliances. No one point will, on its own, determine the final required height. Usually more than one will have an actual impact and affect the final determined termination height of the chimney/flue. In a nutshell, the process begins and ends with flue sizing. The applicability of some points is dependent on the kW rating of the appliance and the boiler room.


Low level


There is a suggestion that the use of low level discharge from horizontal flues should be extended to cover Type C appliances with a net heat output between 70kW and 333kW. The BFCMA does not support this suggestion on the grounds that with these appliances chimney/flue height is still required to disperse the products of combustion. In general, low level discharge should not be


considered where the termination is adjacent to an area that is used for public access, such as a playground, school yard or any area that can be used for a public gathering. The use of low level discharge should also not be carried out if the flue termination is within a light well and must not be enclosed on more than two sides.


June 2017 37


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