Page 329 - Boiler_Operators_Handbook,_Second_Edition

314 Boiler Operator’s Handbook

the furnace tube. to the ignition temperature before they reach the proper
The actual velocity is reasonably estimated by mixture so we have stable combustion. The throat is also
multiplying the square root of the differential reading by part of the insulating portion of the burner that protects
4005. That’s done on any calculator by typing in the value the boiler front and burner housing from the heat of the
of the differential (example, 0.08 for that many inches of furnace. There is a considerable variation in temperatures
water column) pressing the square root key (√) to get the across that refractory during operation. Any large cracks,
square root then × (for multiply by), 4005 and the equals spalling, or shifting of pieces of throat tile can distort air
key to get the velocity in feet per minute (1132 in the ex- flow at the burner to produce unstable combustion.
ample). There may be some argument about how much
variation is permitted in the air flow around a burner but Gas Burners
I would try to do something to cure any deviation that A gas burner can be premix or post-mix. While
exceeded ten percent. I take the sum of the readings (add most boiler burners are post-mix, where the gas and
them up) then divide by twelve to get the average then oil mix after they enter the furnace, premix burners are
multiply that result by 0.9 and 1.1 to get upper and lower available. Many operators think of a premix burner as
limits. If any of the other readings are outside those limits hazardous, after all we make a combustible mixture
I try ways to improve the air distribution in the burner outside the furnace! Many operators that moved from
including baffles, like the angles already mentioned, then firing process equipment to the boiler plant are comfort-
proceeding to shrouds. Usually corrections made at low able with premix burners because they have a lot of ex-
fire do not alter air flow at higher firing rates so correct perience with them. As long as the mixture isn’t heated
the low fire variances first and repeat tests to determine above the ignition temperature it can’t burn and premix-
their effect at higher flow rates. ing permits a low cost arrangement of multiple burners
That’s a lot of work and is all after the fact but it which is frequently necessary for good heat distribution
doesn’t cost as much as what they do for large utility in processing equipment. There aren’t many boiler ap-
boilers. Determining the best design of air distribution plications with premix burners so I won’t spend any
is such an art that utility boiler manufacturers will make more time on them than this. Your understanding of
models of the system and test them for proper air flow. combustion and the instruction manual should be all
They’ll repeat that process to get it right before they you need to operate a premix burner.
build the boiler. It’s much easier for them to spend all Of the post-mix gas burners there are two choices
the time on a model than to try to solve distribution which are normally identified as atmospheric burners or
problems on twenty four or more burners in the field. power burners. Atmospheric burners do not normally
A large number of burners were built for staged have fans or blowers to deliver the combustion air to the
th
combustion in the last half of the 20 century. Some of burner and seldom have induced draft fans. Lacking the
those burners incorporated secondary air ports (open- power of the fan to introduce and help mix the fuel and
ings in the refractory front wall around the circumfer- air, atmospheric burners use some of the gas pressure
ence of the burner) with adjustment of the air flow to for that process.
them consisting of a piece of angle or other steel form The typical atmospheric burner has a “jet” which
surrounding the burner. I’ve noted that most of those is a nozzle the gas flows through on its way into the
provisions for adjusting that air flow are so flexible that burner and that jet acts like an inducer to draw primary
they don’t provide uniform air flow around the burner. air in with it. The gas and primary air mixture is then
Some are so limber they actually flop around in the distributed through the burner head (Figure 10-46) or
air flow. If I had to set one of those burners up today flame runners (Figure 10-47) into the furnace. Second-
I would wait until proximate requirements are estab- ary air is delivered by natural draft and mixes with
lished then measure the flows at the ports, adjust the the primary air—gas mixture as it burns. The several
flexible steel to equalize the flow through them then tack forms of flame runners shown in Figure 10-47 all seem
weld the adjustment in position at each port. to work well with no discernable difference in per-
The mixture of fuel and air has to be heated to igni- formance. Cracks between the holes and holes in the
tion temperature before it will start burning so the burner bottom, usually caused by rust, can produce distorted,
has to provide means to heat the incoming air and fuel. inefficient, and dangerous fires.
The normal and best means of heating the mix is applica- Some gas furnaces can be subjected to very corro-
tion of a refractory throat. The radiant heat of the fire is sive conditions between heating seasons so it’s always a
reflected back into the entering fuel and air to heat them good idea to check an atmospheric burner right before