Boiler Combustion

This application involves the use of a VFD to vary the speed of a centrifugal combustion air intake fan (50 nameplate horsepower) on a scotch marine type high pressure steam boiler. The existing system utilizes an actuator to simultaneously vary the amount of gas and air that enters the burner. The air is controlled with the use of inlet dampers (not guide vanes). As the amount of "fire" is reduced, the damper opening is reduced and visa versa. The centrifugal fan and continuous variation in fire rate make this a feasible VFD application. The load profile of the boiler and corresponding motor demand (measured with demand metering equipment) is listed in Table 1. This table also includes the corresponding VFD demand requirements (approximated from Figure 4), kW savings, and kWh savings.

The annual savings for this example totaled 88,000 kWh, which would equate to an annual savings of $4,400 (based upon a cost of energy of $0.05/kWh). Lo-bodovsky provides an average estimated installed cost of VFD's in this size range at around $350 per horsepower, or an installed cost of $17,500 (50 hp * $350/hp). This would yield a simple payback of around 4 years. This example does not take into account demand savings which may result if the demand reduction corresponds with the plant peak demand. The control of the fan VFD would be able to utilize the same output signal that the existing actuator does.

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