1
Eclipse ThermJet Design Guide No. 205, 3/5/07
Modulating gas & air
On-ratio control or excess air @ low fire
A burner system with modulating control gives an input that
is in proportion with the demands of the process. ANY input
between high and low fire is possible.
Air
The control valve
1
is in the air line. It can modulate air
flow to any position between low and high fire air.
Gas
The ratio regulator
2
allows the on-ratio amount of gas to
go to the burner. Low fire gas is limited by the ratio regula-
tor
2
. High fire gas is limited by the manual butterfly valve
3
.
Note:
The ratio regulator can be biased to give excess air at low fire
Note:
Do not use an adjustable limiting orifice (ALO) as the high
fire gas limiting valve
3
. ALO’s require too much pressure
drop for use in a proportional system.
1.
2.
Step 2:
Control Methodology
(continued)
Multiple burners
Automatic shut-off at the burner
Multiple burners
Automatic shut-off by zone
Single burner
Figure 3.1
Modulating gas & air.
On-ratio control or excess air @ low fire
NC
NC
NC
P
to other Zones
to other Burners
Main gas
shut-off
valve train
to other Burners
Automatic
shut-off at
the burner
➊
➋
➌
Main gas
shut-off
valve train
P
to other Zones
to other Burners
Automatic
shut-off by
zone
to other Burners
➌
➋
to other Zones
➊
➊
➋
➌
Main gas
shut-off
valve train
P
Optional IF flame monitoring system controls the main gas shut-off
valve train AND ignition above 40% of maximum is NOT required.
