Page 326 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 311

smoothly and uniformly within the furnace of the boiler. variable inlet vanes on the fan inlet and can include a
There are several components of a burner and variations VSD (variable speed drive) on the fan motor. To provide
in construction that are designed to assist in this func- stable combustion the dampers or VSD have to control
tion and I’ll try to explain most of them. First I want to the air flow without sticking or flopping around which
explain some of the important aspects of combustion produces variations in air flow.
that a burner design has to address. The dampers also have to control the flow without
The burner has to control the mixing of the fuel and producing distortions in the flow of air to the burner. If
air in a manner that ensures complete and stable combus- the dampers tend to shift air to one side of the burner
tion. Stability of combustion requires the burner produce inlet (or the fan inlet) it can shift the point of ignition to
a fuel rich mixture right at the upper explosive limit one side of the burner and that can produce instability.
where the burning begins and that mixing point has to be Sometimes obstructions around fan inlets can produce
stable as described in the chapter on combustion. If the unusual swirls that are carried through the burner.
burner fails to produce a stable ignition point the flame Installation of boilers that position building col-
front will shift around in the furnace producing pulsa- umns, pipes, racks of conduit and similar obstructions
tions that disturb the process and make it worse. within seven diameters of the fan inlet should be avoid-
The quality of the burner is indicated by noise, as ed but sometimes you’re stuck with one. There are par-
the quality of mixing gets worse the noise gets louder tial to total solutions to air distribution problems caused
and some burners are so bad that flame spurts out any by such things when it’s impossible to move the boiler.
open inspection port. Resolving that mixing problem is Of course setting portable equipment, storage, and other
not a simple matter, it’s a combination of engineering things in front of a fan inlet can also cause problems
and art with many solutions achieved solely by trial with burner operation; so don’t do it.
and error. It’s not uncommon for a service technician to The devices controlling the flow of air must pres-
try several combinations of burner tips, diffusers, and ent it at the burner throat in a manner that ensures
burner adjustments to resolve an unstable ignition prob- mixing of the air and fuel to produce a mix in the flam-
lem, sometimes taking days or weeks. mable range where the heat of the furnace will ignite it.
During startups many owners and operators get To establish that ignition point where it’s desired in the
frustrated with a new boiler because the startup takes so burner there’s always a primary air adjustment. It can
much time to resolve an unstable combustion problem be sectional dampers in a stoker, position of multiple
and, despite the fact that the problem is solved, will nev- burner registers, adjustment of cylindrical tubes in the
er trust the boiler as much as they would if the problem burner that vary air flow and, the most common, posi-
never occurred. It’s not uncommon and it’s not some- tioning of a diffuser.
thing that’s predictable so if it happens don’t blame the A diffuser (Figure 10-43) contains slots or vanes
manufacturer and take a position that the boiler will that restrict air flow. Since the flow through the diffuser
always be a lemon. Unless the owner accepts something
less than reliable operation out of a new boiler it will
always be more reliable than an older one.
It’s the nature of burners that a deviation in any
one part can produce several conditions inconsistent
with good combustion all of which can be due to several
things. Many times an operator unwittingly does some-
thing that alters a burner’s performance without being
aware of it and the owner pays the price in higher fuel
costs for long periods before the deviation is discovered
and corrected. Understanding what the many adjust-
ments on a burner do is one way of preventing such
things happening.

Air Supply and Distribution
The burner is normally fitted with some means of
controlling the amount of air supplied to the fire. The
means can vary from a simple single bladed damper to Figure 10-43. Burner diffuser