Boiler FD Fan

[Dimenssionless]

I need help on the below drawing (copy and paste the following link in your browser)

http://pulse.**********/_WT5VBCZXHFLS.../photos/412030

Right now steam is being produced from boiler and steam demand is controlled via PT-1000 by manupulating air/fuel ratio using a mechanical linkage scheme. This mechanical linkage gets OP from PIC-1000 and adjust the fuel valve and air damper opening. There are two air dampers before and after air FD fan. Fan is running on full speed where air dampers are throttled to control air flow rate. I want to install VFD on FD fan motor and open suction/discharge air dampers to 100%. In this way I will reduce the fan speed and will save energy cunsumption. Following are suggestions from myside:

1- Take out Inlet Damper. Will open it to 100 % on permanent basis. This is valid for all three below options
AND
2- Put VFD on the FD fan motor, Three options for this:
a. Option 1: To install a P.T before the discharge damper. All other things as it is. Pressure set point will control the speed of FD fan, P.T increase speed will decrease, if P.T decrease speed of fan will increase. Option dependent on space constraint to install P.T. Signal to inlet suction damper FCV 1001 will be made nonfunctional.

b. Option 2: Second option not clear, but thinking to incorporate the signal for inlet (suction) damper ( i-e signal from FCV 1001 to VFD. All other things remain as it is.

c. Option 3: Third option to disengage the mechanical link of discharge damper and fuel damper. Giving signal from PIC 1000 to FCV 1002 to both the fuel damper (as existing via modulating motor) and separately to discharge damper. In this option signal for suction damper (signal FCV 1001) will be nonfunctional.

However your expert opinion will make me understand the same better and develop a strategy to harness the potential safely, effectively and efficiently.

Regards,
Usman
Process Engineer

[riazahmadmalik]

This is not recommended unless advised by the boiler manufacturer. The whole air / fuel ratio is controlled by the governor and it affects the overall performance of the boiler. The fan has to move as per the design parameters. Fiddling with it might damage the tubes due to performance beyond limits and other stress relevant issues.
By variating the FD fan speed you will simultaneously reducing the stack pressure and it will be more detrimental if you are having an economizer on it.