VFD Control and Inlet Vane Control for ID Fan

Ah! Hah! GE, Westinghouse, or B&W copied the old logic which originated during the old pneumatic control days. You got snuckered! How many neg press boiilers do either of them own and operate? None.

It can be set up either way especially if you love night mares and boiler explosions (excuse me, boiler implosions). Foregoing all the negatives about feed forward - just consider one simple question.

What is the penalty if the negative half inch slightly swings positive for a moment? ABSOLUTELY NONE! Slowly the positive will become negative and suck back in all it expelled. About once after that and the operator will adjust the set point such that never occurs again. The operator will contact the I&C person to come out at midnight and retune the control system. One midnight trip and there will be no second swing.

Now list the many unknown swinging situations that feed forward derrives. The choice belongs to the operator owner. Surely with digital controls, you can develop and tune logic for ID fan controls that maintains a negative pressure. If not, you certainly do not need to be tinkering with feed forward.

Do you have a pressure transmitter or sensor for the pressure in the furnace? If so, you may use it to form a feedforward control system based on your current VFD fan control system. And, your ultimate control objective is the pressure, not the inlet damper position, right?

In the HVAC world, many fans have had the inlet vanes removed, and are running with VFD control only. It's generally more efficient to slow the motor down than to increase the system pressure drop by reducing fan efficiency with inlet vanes. Find a good control person from the power generation industry to provide some estimates of operating costs.

We have a pressure transmitter that tell us the furnace pressure process value at all time.

And clearly our target is to maintain a negative draft not a damper position.

We gonna try to see if we can find someone to modify the control to use only the vfd.

I don't know yet if we should first lock up the damper in an opened position and then remove it completely once the vfd control modification has been proven fully reliable or remove it directly.

Thanks!

Keep in mind that local codes may prohibit modifing the original equipment specs, as the equipment is usually certified with it. Removal of any components may require recertification / field approval by the governing agency. Additions to equipment are something else. This may be why both devices are currently present. Second you should have the manufacturers spec on allowed over fire draft parameters. Typically over fire draft ( negative values ) are very small ie: 0.05 inches water column, the manufacturer's specs will give you the allowable range to be used. Has any thought been given to the possibility that the vfd may not be able to provide a fine enough adjustment to maintain such a small tolerance; I am assuming you have a modulating burner requiring the system to alter over fire draft with burner input. It may be a simple case of allowing the vfd to perform a coarse adjustment , with the oem damper providing the fine setting. For a complete answer more data will have to be supplied. Firing rate, over fire draft specs, sequence of operation, control diagram etc.

I am not sure what feedforward you speak of but traditionally air demand feedforward is used to gain quick response from the ID when the FD changes; its a good thing to have in your design.

Unless I am missing something, I do not see the advantage of using both damper and VFD together since they will both work to control air flow through the fan in series. Typically the faster responding device would work better and the VFD should be fast enough on its own with the damper set full open to reduce operating costs. I know one job that used both because the damper was faster than the hydraulic coupling ID drive. Furnace press controlled the damper and the PID output was cascaded into a VSD PID as the PV with 75% as SP. The damper would move first but if it got out of deadband 70-80% the drive would change speed and bring the damper back. But I don't think this applies to your case.

I am working on a project with both as well. I plan to control the VFD directly and use the damper during upset and MFT conditions or when VFD is malfunctioning. I plan to have one PID plus a M/A station for each allowing only one to be placed in auto. Since I have a fast damper I plan to slow it down so the characteristics of each (loop gain) is similar.