Balanced Draught: Induced draught and forced draught fan calculations

[Impdrew]

Hi,
Not so sure about the concept of balanced draught for forced draught in boiler systems.
I have been given an estimate by a furnace manufacturer of:
Draught (pressure) accross the grate/bed of the incinerator of 150 mmH2O supplied by primary air forced draught fan, air temperaure of 200 C.
Pressure losses accross heat recovery system and pollution control device (fabric filter) of approx:
And therefore Induced draught fan draught at the total of
500 mm H2O
(Please note, i am aware how to convert mmH2O to N/m2)
I know the temp. of flue gas entering the pollution control device to be approx. 220 C. Expect a temperature drop accross it though?

Can i just calculate the power consumption of the two fans using
V(P2-P1), where V is a volume flow rate. Then take into account of inneficiencies with a fan efficiency value?
I only know the molar flow rate of both the air and the flue gas, PV = nRT will obviously do it, but what value of P do i use in this calculation, can i use atmospheric, is this the point of balanced draught?

Thanks in advance for any help that i may receive.

 

[gunsmith]

1mmH2O=10 N/m^2 but I don´t about the rest.

Andreas

 

[Impdrew]

Apologies
noted that i left out the loss accross the heat recovery system and pollution device, it being the difference between the two fans, i.e: 350 mmH2O (i guess therefore a 'balanced draught').

 

[Ralph2]

The idea of a balanced draught is to control the pressure inside the furnace. If you just had a supply the casing would likely burst and if you just sucked the casing would implode. So the pressure is regulated to maintain XXX inches of water inside the furnace.
The supply fan must pump enough cold dense air to supply the demands of the combustion. The exhaust fan must pump all this air (gas) plus that created by the combustion and it is now much hotter and thus has expanded so there is a lot more of it.
The exhaust fans are invariably larger..... but, the height of the stack, creates a natural flow, which offsets some of the demands of the exhaust fan. Thus in some systems you may not even need an exhaust fan
-
Ralph