Using VSD for Boiler FD Fan Application

A VSD should be ideal for this application. However, there may be an antiquated code provision that requires a damper. I don't understand why the fan starts and stops; what kind of control scheme is making this happen?

Sounds like the thermal control is tripping, due to overheating.

Thank you all. Sorry for the delay in replying, there is a timezone difference between us.

Following operational interlocks exist in control scheme.

ID Fan stops when steam pressure touches its high value, due to load variations (boiler supplies steam to a single plant and processing stops due to some equipment breakdown there will be no load on boiler).

Alongwith ID, FD & Booster Fans stop as they are interlocked ID.

As ID has a VSD, its start/stop is smooth. But FD & Booster have no VSDs.

Dear Sitaram, What I don't know is in a fluidized bed operations, what I do know is the boiler setting is not built to withstand pressurization, it is heavy refractory and a light steel skin, designed to operate at a slight negative pressure ....Always. Admission air can be added to assist in developing turbulence (the goal being complete combustion). The forced draft fan is the primary air. How do you insure enough air penetration to prevent clinkers from forming and blocking off needed air flow? Secondary Air.... ID fan is controlled by the primary element???? that seems wrong to me .... it would be vary difficult to control this operation, with out having a steam release dump system to give you the needed time to throttle down the rate of fuel feed. -hope this helps some

What is ID Fan and what is the booster fan for?

FD means Forced draft fan = OK can understand that.

Normally, the combustion air is supplied by the FD fan and control used to be with opening and closing the dampers at the inlet of the FD in synch with the fuel supply requirement to the burner.

Using a VSD + Damper control on the FD fan is not a simple matter to properly set up. Specially if you need to set the boiler to vary its load to demand requirement (against a fixed permanent load). This need some knowledge of the system and some training with checking of the combustion exhaust gases.

It is easier to have a fixed speed FD fan and adjust the damper settings for the different stages (or continuous varying) of loading.

As far as concerning the start and stopping of the FD fan: You can either be OK with it if not too frequent (don't forget that the minimum run time is at least 5 minutes between starting, running, postpurge). or keep the motor running and just bring the damper to minimum opening. The interlocks could be modified...

Anyway, the commissioning engineer is the ultimate adviser since he will be responsible to install, run, commission and report to the maker.

FD fan is a forced draft fan

ID fan Induced draft fan

Booster fan is used to fluidize coal.

FBC is fluidized combustion boiler.

FD fan handles cold air so profiled blade can be used to get good efficiny of fan at various loads.

ID fan handles flue gas so profiled blade is not preferred so variable speed drive is used for better efficiency.

variable speed FD fan can be used but cost will go up.

most probably your steam requirement is the existing boiler capacity why the commissioning enginer disregards the use of VSD and assume that when your plant operates boiler will operate at max capacity. Please recheck the settings of your control panel otherwise contact the commissioning enginer. REcheck also the electrical components for FD and FD motor.

If the FD fan is using the damper, that is it's not fully open, ii's hard to see any reason not to use a VFD.

The reduced airflow will be simulated by VFD exactly the same not changing the process - the only thing to check and be sure of is the draft when both stopping and starting to maintain the conditions inside the boiler during these phases. You already should have control in place for this, I imagine, interlocked to burner control/feed.

Thanks again for the replies. Following is the complete system what we have, for clarity.

This is a Coal fired Fluidized bed combustion boiler (6TPH 11psi saturated steam and not super heated) with both Underbed & Overbed options for fuel feeding. Complete system is controlled through a PLC. ID Fan, connected between boiler (through dust filters) & chimney, has a VSD (VFD) for maintaining the required –ve draft in boiler. Infeed air is maintained by FD with a manual damper. Part of infeed air from FD is fed to Booster Fan as its inlet. Fuel is fed in to Booster fan's outlet air stream that enters the boiler furnace. FD air gives the required fluidization to the sand bed in the furnace. Booster air along with fuel joins the fluidized sand to burn completely without forming into clinker. Feed water pumps maintain the required water level in steam drum. ID, FD & Booster Fans are sequentially interlocked in that sequence, ID being the first drive to start. Booster cannot run when FD is not running and FD cannot run when ID is not running.

Boiler pressure is maintained between 8 & 10psi and Process requirement of steam is at around 7-9psi and around 8TPH. Any breakdown in processing plant means no steam is drawn from boiler. When this continued, obviously, boiler pressure starts to build-up and once it reaches a preset max pressure of 10psi the boiler stops i.e., Fuel feed stops, ID Fan stops, FD & Booster.

Now, my thought for improvement is to reduce operational inconvenience to operator by providing VSD for FD that is presently controlled manually operated damper or by switching-on or switching-off the drive. My other concern is about possible failure of FD Fan motor due to increased start-stop operations.

Hope I made my question more clearer to the participants.

Thanks again for all the participations.

Dear Sitaram, The primary (controller) element should be on the (secondary air) ID Fan..Your commissioning engineer did every thing correctly. I suspect since the difficulty in maintaining a minimum stable firing rate is the reasoning behind his decisions not to utilize a VFD on the (primary air) FD fan. As to your original post, Straining FD fan.with stops and starts.... If starting and stopping is a problem perhaps you can balance steam load demand of the operation. Possible avenues could include increasing auxiliary users (chillers) have the current major users call in advance of bringing their equipment online. That will assist your operators in balancing (reacting time) the steam load demand on the boiler. Ray

try this, i hope the tripping of fdf is only due low steam consumption. Have you tried checking the configuration of your plc. visit the response time for fdf on/off control.

Run the fdf with seperate control but do not remove the sequence of operation interlock with idf to avoid over pressure in the furnace. It will like you start and stop the fdf manually but you cant start it if the fdf is not running. So during steam consumption is low your fdf run continously but the fuel stop. you can add also a safety, time delay to off fdf. Watch out you need to maintain the fire in the furnace also.