Download - Chimney Design and Simple Flue Calculations
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Overview of RequirementsFunctions
Definitions, Classifications & Designations
System Requirements
Legal Requirements
Introduction toGeneral Principles, Flue Dynamics
Chimney Design
Chimney Construction, Materials
Draught Influence & Control Methods
Presentation Summary
Examples of Chimneys. Domestic implies 50kW appliance output ref Doc J.Definition.
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Solid Biofuel Domestic Heating
ChimneysChimneys
Examples of Chimneys. Domestic implies 50kW appliance output ref Doc J.Definition.
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Primarily
1) To discharge the products of combustion intothe enviroment without causing inconvenience
2) To safely discharge products of combustionthrough the structure of the building
3) To provide assistance or total force for
combustion air input
FUNCTIONS
References; 1) Building Regs Approved Doc J:2002, Requirements J2, J3
2) 45 kW and above - BS 5854:1980, Requirement 5.1
3) European Standard BS EN 1443:1999
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1) Products of combustion; products resultingfrom the combustion of fuel (Gaseous, liquid &solid constituents)
a) Flue gas; gaseous portion of products ofcombustion conveyed in a flue
b) Liquid; condensates, (possible rain ingress)
c) Solid ; soot, smuts, fly-ash
DEFINITIONS
Reference; European Standard BS EN 1443:1999, Chimneys- General requirements
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a) System Chimneys-installed in accordance, using a combination of compatiblecomponents obtained or specified from one manufacturingsource, with product responsibility for the whole chimney; or
b) Custom Made Chimneys-
installed as above using compatible components from morethan onesource
EN Classifications & Designations Apply Installers Product Liability Insurance ?
Designers professional indemnity insurance applies
DEFINITIONS
Ref; BS EN 1443:1999 Chimneys general requirements
BS EN 12391 Metal chimneys - Execution
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Solid Biofuel Domestic Heating
ChimneysChimneys
DEFINITIONS1 Chimney 2 Flue 3 Flue Liner 4 Thermal Insulation5 Outer wall 6 Enclosure or cladding7 Chimney section8 Multi-wall chimney (eg; Flue
Block)9 Chimney fitting10 Connecting flue pipe11 Heating appliance
Terminal
Reference; EN 1443:1999
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Solid Biofuel Domestic Heating
ChimneysChimneys
CLASSIFICATIONChimneys are classified in
accordance with the following
performance characteristics:
G
Minimum specification for wood fuel: T400 N2 D 3 Gxx
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Solid Biofuel Domestic Heating
ChimneysChimneys
1. Discharge at sufficient height for convenient dispersalof products of combustion, including any airbornesolids!
2. Discharge total volume of flue gases, p.o.c + entrained air
3. Facilitate removal of solid residues eg; fly-ash
4. Allow for dispersal of any liquid products eg; condensates
5. Structural Integrity and Stability eg; loadbearing
6. Material Durability eg; resistance to temperature & corrosion
7. Protection of Building eg; Risk of Fire
8. Protection of Occupants eg; Risk of CO poisoning
General
Requirements
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Solid Biofuel Domestic Heating
ChimneysChimneys
Building Control Approval -Prior applicationto & inspection by local authority OR installation byCompetent Person eg H.E.T.A.S followed bynotification
Planning Permission -Local Authorities,National Parks, Conservation areas, listed buildingsetc.
Clean Air Act -Smoke Control Areas Environmental Health Large non-domestic
systems Neighbour complaints
Legal Requirements
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Solid Biofuel Domestic Heating
ChimneysChimneys
Approved Document J - April 2002
England & Wales The Requirements
J1 Air Supply
J2 Discharge of products of combustion
J3 Protection of Building
J4 Provision of Information
Building Regulations
Different Regulations and guidance in SCOTLAND and NI.
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Solid Biofuel Domestic Heating
ChimneysChimneysGeneral Guidance
J1 Air Supply
Applies to all fuels
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Solid Biofuel Domestic Heating
ChimneysChimneysSpecific Guidance
J1 Air Supply
Section 2Applies to solid fuels
Closed appliance no draught stabiliser550mm of permanently open free area
Of ventilation for every kW above 5.0 kW
Closed appliance with draught stabiliser
300mm for the first 5 kW and 850mm ofpermanently open free area of ventilation
for every kW above 5.0kW
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Specific Guidance
Solid Biofuel Domestic Heating
ChimneysChimneys
J2 Discharge of theproducts of combustion
Lateral Support Stays usually required 1.5m above roof line
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Specific Guidance
Solid Biofuel Domestic Heating
ChimneysChimneysJ3 Protection ofBuilding
Ventilated Firestops sometimes required. No combustible surrounding surfaces toexceed 100C in any test conditions.
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Specific Guidance
Solid Biofuel Domestic Heating
ChimneysChimneysJ4 Provision ofinformation
Data Plate
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Natural draught
Usually gives Negativepressure
within flue
Induced draught
Usually gives Negativepressure
within flue
Forced draught
by blower fan which usually givescombustion air, can causePositive pressure within the flue
Measurement by draught gaugeto achieve manufactures target
Draught types
Definitions ref Doc J
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Flue Draught
also important in naturally
draughted appliances to control
fuel/air mix of combustion.
Influenced by
Properties & Design of chimney
& by the action of wind amongst
other factors
Flue draught
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Stack Effect , Displacement
Heated flue gas expands, becomesless dense & is pushed up thechimney by a colder, densercolumn of surrounding air of equalvolume i.e. Buoyancy
The greater the temp. differencethe greater the pressure differenceor draught
The taller the columns the greater
the pressure differential &combined with the above thegreater the natural draught
Solid Biofuel Domestic Heating
ChimneysChimneysNatural draught
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Chimney Design StrategyCombine energy conservation with low resistance
Route through structure,
Tend to the vertical
Limits to offset bends & inclined runs
Limits to horizontal runs
Cleaning Access provision (whole chimney)
To Debris collection space
For Rods, Brushes, Vacuum hoses
Terminal
Additional Accessories e.g.;
Condensate or rain drains
Draught Control devices
Flue Draught Stabilisers
Special Cowls
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Sizing of Flue Height minimum recommended 4.5m (ADJ) but must also be
Sufficient for dispersal
Be discharged at a required rate dependant on combustion of
fuel
Diameter of Connecting Flue Pipe may differ to remainder of
chimney
Remember
Adopt Guidance from Building Regs
Adhere to system manufacturers recommendations if require largerthan regulations state
Calculations can be applied to provide smaller flues if carried out in a
recognised method
Chimney Design
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Considerations include;
System & Material choice (matching appliance and
chimney designations) Lateral & Vertical support
Fire protection
Weathering details
Lightning conduction & earthing !
Access, scaffolding
Materials handling
Chimney Construction
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By design of terminal cowl
This may be possible if windstrongly influences thedraught.
Draught Control
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Additional ventilation required
Types, Sizes
Positioning considerations
Same room as combustion air
Distance from appliance
Distance from combustibles
Flue Draught
Stabilisers
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Clay Liners, Concrete
Pumice Liners &
Modular Systems
Metal;
Cast Iron 3 mm. , Mild
steel 1 mm.
Stainless steel Grade
316Vitreous enamelled low
carbon steel
Flue materials
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Sweeping
at least 2 x each heatingseason
(6 x in Austria as instructedby the sweep in Germany)
Visual Inspection
Testing; smoke, clearance
RepairsLining, re-pointing orreplacement components
Maintenance
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BS 5854 BS EN 13384
CIBSE Design Guide B (The most common method)
All requires complex calculations
Spreadsheet analysis easier
Designing a chimney to provide the correct amount of naturaldraught involves a number design factors, many of which requiretrial-and-error reiterative methods
As a "first guess" approximations can be used to estimate thenatural draught/draft flow rate by assuming that the molecularmass (i.e., molecular weight) of the flue gas and the external airare equal and that the frictional pressure and heat losses arenegligible
Sizing of Flue Methods
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Pressure difference ( P) is the driving force for thestack effect and it can be approximately calculated withthe equation below
For flue gas stacks and chimneys, where air is on theoutside and combustion flue gases are on the inside, theequations will only provide an approximation of theheight (h)of the chimney.
Where:P= available pressure difference in PaC= 0.0342a= atmospheric pressure, in Pah= height, in mTo= absolute outside temperature, in KTi= absolute flue gas temperature, in K
Simple calculation- P
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Draft or draught flow rate (Q) which is induced by thestack effect can be calculated with the equation below
For flue gas stacks or chimneys, where air is on theoutside and combustion flue gases are on the inside, theequation will only provide an approximation. Also, thecross-sectional flow area (A) and is the height (h) of theflue gas stack or chimney.
Where:Q= stack effect draft/draught flow rate, m/sA= flow area, m
C= discharge coefficient (usually taken to be from 0.65 to 0.70)g= gravitational acceleration, 9.807 m/sh= height, mTi= average inside temperature, KTTo= outside air temperature, K
Simple calculation- Q
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Appliance maximum and minimum rated outputs
Appliance efficiency (gross)
Flue outlet temperatures at both maximum and
minimum outputs
Minimum and maximum draught required at the
boiler outlet.
Full method Appliance
parameters
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Fuel type (Gas, oil type of solid fuel)
Calorific value
Required excess oxygen level
Required optimum oxygen consumption
Fuel analysis including carbon, hydrogen
and sulphur content.
Full method
Fuel parameters
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Proposed inner surface material
Proposed height
U values for heat loss
Proposed diameter
Proposed eflux velocity
Full method
Chimney parameters
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Flue terminal location
Ambient temperatures (max and min
expected)
Topography
Full method Environmental
factors
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Pressure difference
Average flue gas temperature in flue
Flue surface area
Flue cross sectional area
Volume flow rate of gases
Velocity of gases
Full method - Outputs