Page 422 - Boiler_Operators_Handbook,_Second_Edition

Controls 407

flow through the valve (precise provided the feedwater
header and steam pressures are near design). The con-
troller will not see an unexpected response and control
operation will be smooth. The added benefit is that you
can always expect a proportional change in flow when
operating in hand.

START-UP CONTROL

The only type of start-up control that I believe I
didn’t create in a system is one that’s called “low fire
hold.” The control consists of an extra pressure or tem-
perature switch that opens contacts to prevent automatic
modulation of a burner when the pressure of tempera-
ture of the boiler is below the switch setting. Once the
temperature, and temperature can be used even on steam
boilers, or pressure rises to a level higher than the switch
setting the automatic controls can operate.
What usually follows is a modulating control run-
Figure 11-34c. Blank positioner cam

Figure 11-34b. Graphs for positioner