Page 328 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 313

and set them in the windbox temporarily until I got
good air flow distribution.
In many single burner systems we found proper
placement of one or more 4 by 4 angles mounted near
the windbox air inlet created sufficient turbulence and
deflected the high velocity air from the fan enough to
achieve good air distribution. Cut long enough to be
a press fit they will hold position while testing for air
velocity at the burner and can be moved to find the
optimum position. One centered on the burner when
the duct entering is centered, or at the point where the
entering air velocity is highest, usually does most of the
Figure 10-45. Venturi burner with flow sensing ports work because it prevents the direct blast of the air strik-
ing the shroud or register. It has to be far enough from
normally have one air supply with the air flowing to the register so some air can eddy behind it or you’ll lose
the burner distributed within a windbox. The windbox a lot of air immediately behind it. Once you’ve got the
receives the air from the forced draft fan and provides best distribution you can manage, be certain to weld
sufficient space around the burners for the air to be them in because they’ll fall out when the windbox and
distributed evenly. At least that was supposed to be the boiler heats up in normal operation and they make a lot
idea. Several installations I’ve seen in past years didn’t of noise when they blow into the register.
really provide adequate air distribution in the windbox, To check for uniform air distribution through a
especially between burners, so the fires were not truly single burner (or each burner) measure it. First do all the
uniform. lock-out, tag-out required for access into the boiler but
A windbox has to be large enough to distribute the provide a means for operating the forced draft fan. Leave
air and that’s always larger than big enough to fit the all the normal burner components in place except for a
burner. A burner manufacturer’s dimensional tolerances center-fired oil gun. Hang a manometer against the tubes
for a burner are based solely on construction clearances in the furnace and connect flexible tubing at one end to a
so the minimum distance from the center of a burner to copper tube about three feet long that you can point at the
the inside of a windbox as listed by the manufacturer burner. Take some paper on a clipboard and pen into the
is just enough to prevent the register blades hitting the furnace with you to record your measurements.
windbox. Let’s face it, if the blades are just clearing the Be certain to wear good safety glasses because the
inside of the windbox there’s no room for the air to get breeze can do all sorts of strange things including blow
between them. If you’re stuck with one of those poorly your own hair around so roughly that it jabs you in the
designed windboxes you’ll know it because you can’t eye. With the fan in operation point the tube directly
get stack gas oxygen content down without generating a at the burner while holding it so the end is about flush
lot of carbon monoxide. with the face of the furnace wall and the tube is horizon-
If you have air distribution problems you can try tal to get each reading. Begin with air flow consistent
adding shrouds. Shrouds were developed to resolve the with low fire and record the total pressure read on the
problem with limited room for registers within a burner manometer at each point on the burner. I like to use
windbox that would fit on the front of small package clock positions (1 through 12) as a basis because every-
boilers. They consist of a cylinder of perforated plate, one understands where the measurement was taken and
around 50% open area, larger than the open register (of the twelve readings provide reasonable resolution of the
course) but weighted for each application to achieve velocities around the burner.
the most uniform distribution. The shroud is usually a Since the point flush with the furnace wall and the
couple of inches wider than the burner register. open tube on the manometer are both exposed to the
I found shrouds beneficial in knocking down the furnace the static pressure is the same at both points
concentrated discharge of windbox mounted fans. They and you’re reading velocity pressure. Take readings at
proved to be more reliable than the methods I originally increasing air flow rates in steps of about twenty percent
used, structural angles across the windbox and turned until you get to the top end or the velocities get so high
so the heel pointed at the fan discharge. In most applica- that you can’t stand up to hold the tube up to the burner
tions I started with several sizes of angle cut to length or, in the case of a fire tube boiler, you’re blown out of