Siemens LMV5 Series Technical Instructions page 203

LMV Series
Method shown in Figure 6‐13:
Pressure P1 relative to pressure P2 (differential pressure across the FGR damper) is more constant with
firing rate. Differential pressure will be generated across the FGR damper at low fire by using the
suction of the blower against the air damper. The differential pressure across the FGR damper can also
be adjusted with the speed of the VFD. As firing rate increases, P2 will increase (less vacuum) and P1
will increase due to increased pressure behind the stack damper, yielding a more consistent differential
pressure. As a result, the FGR damper can be sized more appropriately and should offer better control.
Other points to consider:
1. If using O trim, the effect that the trim has on FGR flow can be adjusted. Three possibilities:
2
a. If only the air damper is trimmed (set to "air influenced"), trimming closed and reducing air flow
will increase the vacuum at P2 and increase FGR flow relative to air flow.
b. If only the VFD is trimmed (set to "air influenced"), trimming slower and reducing air rate will
reduce the vacuum at P2 and reduce the FGR flow relative to the air flow.
c. If both the air damper and VFD are trimmed (set to "air influenced"), trimming closed should
keep the vacuum at P2 relatively constant and keep the FGR flow constant relative to the air
flow.
2. Differential pressure across the FGR damper is significant even at low fire due to the vacuum
provided by the blower. Small changes in the differential pressure across the FGR damper will cause
small changes in FGR flow. FGR flow will be more repeatable as compared to the method in Figure
6‐12, especially at low to mid fire.
3. No fixed air dampers exist for the blower intake, and the fixed damper in the stack may be able to
be opened further, reducing the blower pressure requirements.
SCC Inc. Page 31
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Technical Instructions
Document No. LV5‐1000
Section 6