Page 401 - Boiler_Operators_Handbook,_Second_Edition

386 Boiler Operator’s Handbook

Figure 11-19. Pressuretrol—modulating motor schematic

the operating range (the steam pressure that will center to the minimum differential for smooth operation you
the wiper in the middle of the coil) and the other is the adjust the differential of the pressure control switch and
differential which is the change in steam pressure neces- the setting of the pressuretrol to establish an operating
sary to drive the wiper from one end of the coil to the range as depicted in Figure 11-20. In many plants you
other. You tune it like any proportional controller, reduc- will find that you can allow some of the differential of
ing the differential until the operation becomes erratic the pressuretrol to fall below the minimum operating
then increase it until it operates smoothly. pressure because the boiler doesn’t have to modulate to
The setting of the center of the operating range of a high fire to handle the maximum summer load.
pressuretrol should always be such that the entire oper- Heating boiler plants may have more than one
ating range is below the start pressure of the operating boiler and a need to control operations where two or
pressure switch. How far below? Enough so the steam more boilers are required to serve the needs of the facil-
pressure after the boiler has started and purged at a load
equal to low fire is slightly higher than the top of the
operating range of the pressuretrol.
When a boiler is cycling on and off the steam re-
quirement is less than the boiler produces at low fire. At
those loads the boiler shouldn’t be modulating because
that increases the input during the firing cycle to shorten
it and increase cycles. (See cycling efficiency)
If all you have is an operating pressure switch
it’s manufactured switch differential is your operating
range. When you also have modulating controls your
operating range is from the stop setting of the pressure
control switch to the pressure that generates the maxi-
mum firing rate.
After you have tuned your modulating control Figure 11-20. Range of control, modulating and on-off