Page 413 - Boiler_Operators_Handbook,_Second_Edition

398 Boiler Operator’s Handbook

ing is to operate with the fan damper wide open. That the fan damper. The valve opens to a preset position
way there’s always enough air to burn the fuel, right? during the main flame trial for ignition and the linkage
Actually that’s wrong because at lower firing rates limits opening of the fan damper to another preset posi-
the high excess air quenches the fire to produce com- tion. Once a flame is established and the ignitor is shut
bustibles, primarily carbon monoxide, and sometimes down the valve opens the rest of the way and the fan
unburned fuel products that are carcinogenic. Such care- damper opens with it.
less operation is not only lacking concern for the cost For oil burners the typical setup is a small hy-
of fuel but is potentially hazardous to the health of the draulic cylinder sensing the oil pressure at the burner.
operators as well as everyone within a one or two mile Two oil shut-off solenoids are used to produce the two
radius of the boiler. Now that you’re a wiser operator different oil flows or a solenoid is powered to bypass a
you will not, I hope, poison yourself and other people by manually set throttling valve for full fire. The cylinder
failing to have adequate control of your air to fuel ratio. contains a spring and it moves the damper according to
Following are descriptions of the five most com- the burner oil pressure, low then high.
mon methods of modulating a boiler’s firing rate fol- There is little advantage to a low fire start control
lowed by four possible enhancements to some systems. system. Primarily all it does is permit the use of a cheap-
They run from the simplest to the most refined and com- er ignitor that would blow out if exposed to full load
plex. You shouldn’t be disappointed if you don’t have combustion air flow. As far as I’m concerned you should
the Cadillac nor be disgruntled because you have to deal have a high-low firing rate control if you’re considering
with a complex system. They’re selected to provide opti- low fire start; there isn’t enough difference in price that
mum performance when they’re working right. It’s your would prevent recovering the added cost of the modu-
job to ensure they’re working right, keep them working lating system in one or two heating seasons.
right as much as possible and report it when they aren’t Adjusting low fire start controls is not easy and the
doing what they’re supposed to so they can be fixed by manufacturer’s instructions should be followed to the
qualified technicians when you can’t handle it. letter. You have to establish a suitable air to fuel ratio at
There’s enough information in this book for you to the full load and ignition positions and ensure that the
make adjustments and correct problems in any of these air to fuel ratio doesn’t go too far awry as the controls
systems but that doesn’t guarantee that you can relate swing from low fire to high fire. The process requires
the indications you see to the right source of the prob- a thorough understanding of geometry to arrange the
lem. If you’re confident you can fix something let the linkage so the ratio is maintained.
chief know and get permission to fix it, otherwise let one
of the contract technicians do it. If they do something
wrong their insurance company will pay the bill, not FIRING RATE CONTROL—HIGH-LOW
your employer’s. Let’s discuss these systems and we’ll
see where you stand. High-low firing rate control is similar to the low
A simple on-off boiler doesn’t have a firing rate fire start system (described above) except the controls
control system as far as I’m concerned and the first two can switch between the low (ignition) position and high
simple systems aren’t a lot better. They do, however, firing position to vary the heat input to the boiler. An-
change fuel and air flow rates so they have to be consid- other pressure control switch is added to the boiler to
ered. control the positioning between high and low. Of course
if it’s expected to work it has to be set lower than the
setting of the on-off pressure control switch to prevent
FIRING RATE CONTROL—LOW FIRE START pressure or temperature swings above the high-low
switch settings shutting the boiler down. Setting of that
A low fire start control system only regulates the pressure switch and the on-off pressure switch can be
input of fuel and air to the furnace during the ignition varied with the season as described for the on-off pres-
period. The system limits fuel input for ignition then al- sure switch and electric positioning control.
lows it to increase to the maximum firing rate which is Maintenance of a suitable air to fuel ratio during
maintained for the rest of the burner operating time. load swings is more important with the high-low system
The controls for gas typically consist of a two position than the low fire start because the linkage has to main-
fuel safety shut-off valve with a rack and pinion on its tain the ratio as the firing rate drops to low fire as well
shaft connected to linkage that controls the position of as when it increases to high fire and the burner may be