Exhaust blower controls

We have blower motor of 75KW that controls the pressure inside the ovens. Can you suggest how VFD controls should work with pressure transmitter? We also have a modulating damper.

I would like to know start up and shutdown precautions of blower too.

You did not indicate whether the blower actually blows air into the furnace or whether it extracts the hot gases from the furnace to maintain constant furnace pressure. In addition how is the furnace fired, and is there as burner/s involved?

In addition it seems that you are contemplating replacing/adding the VFD to replace the modulating damper, to save on pumping power. If this is the case, you need to check what is the transient response you need on your fan, as varying the speed of the fan to control the air flow is more 'cumbersome' than operating a damper, due to the fan's inertia. If transients on the furnace are slow and small, then there is a high probability that you can do away with the modulating damper, however if there are large and/or fast transients then the use of the damper is a must together with the VFD to maintain good controllability of your furnace pressure.

Startup and shutdown precautions mainly depend on the furnace design, and burner controls.

Further details are required so that one can help.

You did not say if this was a gas-fired oven, or if solvents are present on the load. NFPA86 & 86A cover purging requirements of ovens.

That said, I'm assuming this is a gas-fired oven, with perhaps solvents in the load. Therefore, you MUST have a purge cycle that provides a minimum of 4 volume changes of air before ignition. Consequently, you DO NOT want to slow down the fan or modulate the dampers closed during the purge cycle. Otherwise, you may be creating a bomb.

After the purge cycle, you may modulate the damper and/or reduce the exhaust fan speed. The damper MUST be cut away so that it has a minimum opening sufficient to exhaust products of combustion. Likewise, there must be a minimum amount of exhaust to remove the products of combustion and any solvents that are being given off by the load. Typically the solvents will be reduced or eliminated within a short span at the beginning of the cycle, so the fan can be programmed to slow down (to a minimum level) after the solvents have been released. Examine the fan curves to know how much air you are moving at a given RPM of the fan.

Also, the exhaust damper (sometimes fresh air dampers and/or oven doors) must be proven open and interlocked with the purge cycle. Make sure you detect the damper position and NOT the motor position. Damper linkages can come loose or slip, allowing the motor to move while the damper is jammed shut.

These are minimum general requirements, but lacking information on your application or the product being processed, I suggest you familiarize yourself thoroughly with NFPA 86. A 75KW (100HP) exhaust suggests a very large oven and/or huge amounts of solvent. What you are proposing, if implemented incorrectly, can be very dangerous, and could result in an explosion.

Bruce A.