• Volume varies with speed.
• Static pressure varies as the speed squared.
• HP varies as the speed cubed.
  

In this example, changing speeds results in saving 1200HP. If energy is evaluated at $1700/KW x 1200HP x .746KW/HP, the savings are $1,521,840.

In this example, to attain high unit availability, the fundamental requirement is to size the ID fan such that the boiler will not become ID fan limited. Yet, care is taken not to increase the potential for problems with draft controllability and implosion because of over-sizing. Sufficient margin is added to the gas flow rate estimated from the boiler manufacturer's predicted MCR heat input so that the ID fans can handle the gas flow at the maximum demonstrated heat input without difficulty.

Additional margins are provided for increased air leakages, boiler gas path fouling, air-heater fouling, fan performance deterioration, and other uncertainties.

To account for all these allowances, typically a 15% mass flow margin and a 32% static pressure margin is applied to the MCR values to obtain the Test Block conditions.

Some fans are specified to be designed for initial base load operation with provisions for future cyclical duty. Additional revised design conditions due to scrubber and balanced draft conversion affect the ID fan size.

These requirements are shown in the Pressure / HP vs. Volume curve.

Thus the fans and motors are designed for Test Block requirements but normal operation is a MCR. This sets the stage for two-speed conditions.