diameter pipe smoke flue calculation software

Page contents

1. How is the draught of a chimney (flue) calculated?
2. Draught and sizing formulas
3. Theoretical draught versus flue height and gas temperature
4. Detailed explanations and calculation in Mecaflux
5. Frequently asked questions

How is the draught of a chimney (flue) calculated?

The natural draught of a flue is the pressure difference created by the density difference between the hot, lighter flue gases and the colder outside air: it equals the height of the flue multiplied by g and by the density difference. A 10 m flue with gases at 200 °C produces about 45 Pa of draught. This draught must cover the pressure losses of the flue and the boiler and ensure the minimum gas exit velocity required by regulations; the diameter follows from the flue gas flow rate and this velocity.

Draught and sizing formulas

• ΔP_t: theoretical draught, in pascals (Pa); 1 mm of water column = 9.81 Pa
• H: height of the flue between the gas inlet and the outlet, in metres; g = 9.81 m/s²
• ρ_air, ρ_gas: densities of the outside air and of the flue gases at their mean temperature in the flue, in kg/m³
• ρ₀: density at 0 °C (air: 1.293 kg/m³, flue gas: 1.25 to 1.35 depending on the fuel); T: temperature in °C
• ΔP_{pressure losses}: friction losses of the flue and local losses (bends, connection tee, cap, outlet)
• D: inside diameter of the flue, in m; Q_v: volume flow of flue gas at its temperature, in m³/s; V: chosen exit velocity, in m/s

Theoretical draught versus flue height and gas temperature

Outside air at 20 °C (ρ = 1.204 kg/m³), flue gas treated as hot air (ρ₀ = 1.293 kg/m³)

Flue gas temperature	ρ gas	Draught for H = 5 m	H = 10 m	H = 20 m
100 °C	0.946 kg/m³	13 Pa	25 Pa	51 Pa
150 °C	0.834 kg/m³	18 Pa	36 Pa	73 Pa
200 °C	0.746 kg/m³	22 Pa	45 Pa	90 Pa
300 °C	0.616 kg/m³	29 Pa	58 Pa	115 Pa

Detailed explanations and calculation in Mecaflux

Mecaflux, The software allows the calculation of the thermal flow, and the size of duct flue but for those who do not have the software,

here is the calculation method used for the evaluation of the diameter of flues, heat flow duct, and flue gas flow rate, depending on power, on the type of boiler and its performance.

This method of determining the diameters of flue is based on the DTU which is downloadable from this link: Assistance Sizing Ducts Smoke :DTU 12/75 (P 51-701) valid only natural heat flow and for S P inst > 75 th/h

Objectives: Design of dimensional characteristics of Fireplaces (height, diameter) Compliance with rules on pollution and the environment (SO 2 content.)

Interface calculation flue pipe, integrated with mecaflux standard (details below)

Sizing diameter flue or chimney must comply with the rules relating to pollution, setting an exhaust velocity, depending on the sulfur content, but also the principle of circulation, computed with Bernoulli , depending on smoke pipe head loss , the chimney height and the temperature difference between the inlet and the outlet flue gas discharge duct.

What we are concerned primarily in our calculation and dimensioning of flue pipe or chimney , this is the minimum speed of smoke emission. The minimum speed to be respected, we will check if the generator and smoke evacuation system realize this circulation condition. We will discuss here the rules of minimum rates of emissions that should ensure the conduit, depending on the sulfur content in flue gas. But regulation of smoke emissions far beyond this framework.

• The minimum speed smoke exhaust duct must be for the rated power of the smallest boiler debiting alone in the duct, greater than or equal to the minimum emission rate as defined below :

• operating All or Nothing :The generator is operating at its rated pace or does not work
• operating continuous :The power burner is never less than 66% of the nominal power.
• operating modulated :Power to the burner may be less than 66% of the nominal power.

Knowing now the minimum speed smoke exhaust, we need the flow of flue, to know the right size pipe..

an estimate of the flow rate of smoke (in the absence of boiler manufacturer data) can be calculated from the power of the boiler or generator and the excess air.

the excess air can be estimated from the type of generator

Average values ??of excess air:

• Average values ??of excess air:
• Coal: e = 70%
• Oil: e = 45%
• Gas: e = 30%
• Wood logs: e = 50%
• Wood Granulated: e = 30%

the flue gas flow rate will be estimated by the formula:

With flue mass flow :[kg/h], e:% excess air, P :Power burner [th/h]_/PCI

As you have probably noticed the flue gas flow rate is given here in mass flow, to convert in volume flow, we need to know the average density of flue gas. A table of the flue gas density estimated in function of the temperature is provided in mecaflux. But not to leave you stranded if you do not have mecaflux here is the density of smoke at 150 °:0.9 kg/m3 and at 50 °1.1 . The temperature is crucial to estimate the smoke density in the conduit.

This density is also necessary to calculate the actual movement speed of the smoke in the pipe , because the minimum standard of speed and flow rate of smoke from the boiler does not tell us whether the thermal chimney flue works!! . So the temperature of duct inlet is necessary to go further.Fluid temperature (smoke) is given by the manufacturer or by default in the following table:

Depending on the performance, fuel and excess air, we obtain the temperature difference between the boiler flue gas outlet and outside temperature.

the outside temperature is taken equal to :

. 18 °C for boilers operating in WINTER only

. 30 °C for boilers operating throughout the year

Exemple :

if h_c = 94 % for gas with 15 % excess air, is obtained for a boiler running throughout the year:

Temp flues = (132 - 30)

Temp flues = 102 °C at the entrance to the chimney.

the average temperature in the conduit is slightly lower than the inlet temperature, because the smoke cooled down in contact with the walls of the duct. we estimate the loss of temperature depending on the length and type of duct according to the following approximate temperature drop :

• - 3 °C/ml for metal chimneys
• - 1,5 °C/ml for masonry chimneys
• - 0,8 °C/ml for insulated chimneys

The average temperature in the pipe will be approximately equal to (inlet temperature + temperature output)/2. (this is an approximation because in reality the average temperature evolves differently)

with the average temperature we select the average density in the duct, So we can estimate the approximate diameter pipe that carries the speed requested by the rule of pollution

section m� = (mass flow X average density) / minimum velocity of flue pipes.

To verify that our system works, we need to check if the stack effect, caused by the density difference between the outside air and the average density of the flue in the duct, providing heat circulation

For this we use the Bernoulli, which gives the relationship between the energy of static and dynamic pressure

it appears that the difference in pressure due to the density difference between the outside and inside the duct produces a difference in speed. this pressure difference is the driving pressure

the driving pressure is reduced by pressure resistant: pressure drops of the conduit and the boiler and also the pressure in the boiler room (about 2.5 Pascals by lack of wind)

The actual speed in the duct can be evaluated as :

velocity=sqrt(((driving pressure - resistant effect)X2)/average density in the duct)

so we check that speed is greater than the rule of pollution.

With Mecaflux standard:

the design method and calculation of flue pipe, mentioned above, is integrated in the software mecaflux menu tool/ flues.

This tool allows you to quickly know and estimate the duct diameter and length depending on the parameters of the heating system.

A simplified mode, gives the diameter to be applied to realize a velocity in the pipe, depending on the generator and the excess air, and a mode with more options, allows you to enter the advanced parameters ...

Simplified calculation of pipes flue:

Mecaflux The software allows the calculation of the diameter of flue pipe, depending on various parameters such as boiler efficiency, the sulfur content, the height of the duct, the pressure drop, the pressure in the boiler room, or temperature entry into the flue duct...

Frequently asked questions

What flue gas velocity must be respected at the outlet?

The French DTU 12/75 (P 51-701) requires, for boilers above 75 th/h (87 kW), a minimum emission velocity of 2 to 12 m/s depending on the boiler operating mode (on/off, continuous, modulating), its power and the sulphur content of the fuel: the detailed table is further down this page. Too low a velocity disperses pollutants poorly and encourages backdraught.

Why does a flue backdraught?

When the available draught is insufficient: a flue that is too short or too cold (cooled gases, uninsulated outside flue), excessive pressure losses (bends, undersized section, fouling), an outlet badly placed in a zone of wind overpressure, or insufficient combustion air supply into the room.

Does a bigger flue draw better?

Not necessarily. A larger diameter reduces pressure losses, but it lowers the velocity and lets the gases cool, which reduces the draught and encourages condensation (dew point of gas-fired flue gases around 55 °C). The right diameter is the one that respects the minimum exit velocity at the boiler's lowest output.

How do I size the flue of my own boiler?

The flue tool of Mecaflux Standard computes the flue gas flow and temperature from the power, fuel, efficiency and excess air of your boiler, then the draught, the pressure losses and the diameter needed to respect the emission velocity, following the DTU 12/75 method valid above 87 kW in natural draught. For a boiler room to bring into compliance or a shared flue to justify, Mecaflux carries out the study of your project, or offers training on the software applied to your own installation.