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flue duct Smoke / chimney (diameters and calculation software)

The Software mecaflux allows the calculation of circulation and size of flue but for those who do not have the software,

Here is the method of calculation used to evaluate the diameter of flue duct , the circulation conduit and the flow of smoke

depending on the power, the type of boiler and its performance

interface calculation flue integrated mecaflux

The sizing diameter duct Smoke / chimney must respect the rules concerning pollution fixing an exhaust velocity as a function of sulfur content.

The velocity is dependent of: friction pressure loss , elements pressure drop loss ,the flue, the chimney height and the temperature difference between entry and exit of vent pipe smoke.

What we are interested primarily in our calculation and dimensioning of flue or chimney is the speed minimum smoke emission. The minimum speed to be respected, we will check if the system generator and smoke evacuation realize this condition of thermal circulation an driving pressure
. We will touch here only the rules of minimum speeds of emissions must ensure that the leads according to the sulfur smokes. But the regulation of smoke emissions far beyond this framework.

The minimum speed of smoke outlet duct shall be rated for the smaller boiler , above the minimum velocity versus emission rate as defined below:

 Type of running generators Power to the burner [th / h] / PCI Sulfur content 'x' fuel [g / th] / PCI <0.10 0.10 8000 2 3 6 Continuous 3 3 6 Modulated P <8000 4 6 9 9 P> 8000 4 6 9 12

- Walk All or Nothing: The generator operates at its rated speed or not working

- Walk continues: The power to the burner is never less than 66% of rated output.

- Walking modulated: The power to the burner may be less than 66% of rated output.

Knowing now the minimum speed smoke exhaust we need the flow of smoke to know the proper size of conduit.

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

excess air can be estimated from the type of generator

§ Values of excess air supplied through the DTU:

. Coal: e = 70%

. Oil: e = 45%

. Gas: e = 30%

the flow of smoke will be held the following formula:

With q m Smoke: [kg / h] e:% excess air, P: Power to the burner [th / h] / PCI

As you probably noticed the smoke flow is given here in mass flow to convert the volumetric flow we need to know the average density of smoke.

A table of the estimated density of smoke in the function of temperature is provided in mecaflux. But do not let you plan if you do not have mecaflux here the density of smoke at 150 °: 0.9 kg/m3 and 50 ° 1.1 (area)

The temperature is therefore crucial to estimate the density of smoke in the duct.

This density is also necessary to calculate the actual speed of movement of smoke in the duct, because the standard minimum speed and el flow of smoke from the boiler does not tell us if the buoyancy of the duct work!

So the inlet temperature leads need to go further.

The temperature of the fluid (smoke) is given by the manufacturer or default by the following:

 Combustion efficiency Fuel Excess air% 5% 15% 30% 45% 60% 96% Gas 95 88 Oil 103 95 94% Gas 142 132 118 107 98 Oil 156 142 127 115 105 92% Gas 190 176 157 143 131 Oil 208 190 170 153 140 90% Gas 238 220 198 179 164 Oil 260 238 212 191 175 88% Gas 286 264 236 215 197 Oil 310 286 254 229 209 86% Gas 332 308 275 250 230 Oil 362 332 297 268 244

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

the temperature outside is taken as:

. 18 ° C for Use in Boilers WINTER

. 30 ° C for boilers operating all year

Example:

If h c = 94% for gas with 15% excess air is obtained for a boiler operating throughout the year:

q smoke = (132 - 30)

q smoke = 102 ° C at the entrance to the chimney.

the average temperature in the duct is slightly lower than the inlet temperature because the smoke cooled by contact with the walls of the duct.

we estimate the loss of temperature depending on the length and type of leads following the fall of temperature following approximate: (data DTU)

-- 3 ° C / ml for metal flues

-- 1.5 ° C / ml for masonry flues

-- 0.8 C / ml for insulated flue

The average temperature in the duct is approximately equal to (inlet temperature + outlet temperature) / 2. (This is only an approximation because in reality the average temperature evolves differently)

thanks to our average temperature we will select the average density in the leads, so we can estimate the approximate diameter duct that carries out the speed required by the rule of pollution by section m² = (mass flow x average density) / minimum speed exhaust fumes.

To verify that our system works we must check if the buoyancy caused by the difference in density between the air outside and the average density of smoke in the duct realize the draw.

For this we use the Bernoulli formula which gives the relationship between the energies of static pressure dynamics

it shows that the pressure difference due to difference in density between the outside and inside the duct produces a speed difference.

This pressure difference is the driving pressure

This driving pressure is reduced by pressure resistant: the losses of the pipe and the boiler and depression in the local heating (arround 2.5 pascals by lack of wind)

The actual speed in the duct can be evaluated as follows:

: Speed = square root (((driving pressure - indeed resistant) X2) / average density in the leads)

we will check that this speed is well above the rule of pollution.

With Mecaflux

the design method and Calculation of flue cited above is used in the software mecaflux menu tool / flues. This tool allows you to quickly know and estimate the diameter and length of smokes duct an chimney

The software allows the calculation mecaflux diameter flue according to various parameters such as boiler efficiency, the sulfur content, the height of the duct, the pressure loss, depression in the local heating or the temperature of into the flue ...