MIL-HDBK-1003/6
where:
Wa
=
Weight of air to be handled by fan, lb/hr
Tac
=
Theoretical air for combustion
Ea
=
Excess air required at burner
La
=
Leakage in air heater
Tnr
=
Total net requirement
Wsm
=
proper safety margin
Wbr
=
Total test block requirement
EQUATION:
Ps = Pfr
(18)
Pfr + Psm = Pbp
where:
Ps
=
Static pressure at fan discharge, inches of water
Pfr
=
Total air-resistance, fan to furnace
Psm
=
Add proper safety margin
Pbp
=
Total test block pressure
EQUATION:
Ta + 25
degrees F (13.9 degrees C) = Ttb
(19)
where:
Ta
=
Temperature of air, degrees F
Ttb
=
Total test block temperature
8.3.6.2
Induced Draft Fan
a) Weight of gas to be handled by fan, lb/hr = Total net
requirement = Theoretical air for combustion + Excess air required at burner +
Fuel burned (assume no ash) + Infiltration + Leakage in air heater.
Add proper safety margin = Total test block requirement.
b) Draft to be provided by fan, inches of water =
Total net draft
requirement.
Add proper safety margin = Total test block pressure
c) Temperature of air, degrees F
Net temp. + 25 degrees F (13.9 degrees C) = Total test block
temperature.
Selection of fan controls is a balance of
8.3.7
Fan Control Methods.
engineering and economic factors.
The manufacturer's fan curves must be
supplied for each fan.
a) Dampers. Dampers are used on fan discharge to raise system
resistance and raise operating points higher on the fan curves. Input power
151
Previous Page
