Type of running generators	Power to the burner [th / h] / PCI	Sulfur content 'x' fuel [g / th] / PCI
<0.10	0.10 <x <1	1 <x <2	2 <x
All or Nothing	P <8000	2	2	5
P> 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

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