Page 420 - Boiler_Operators_Handbook,_Second_Edition

Controls 405

used to relate pressure drop and flow to get linear air the percentage scale of your air controllers output then
flow characteristics. The easiest one to use is the square all you have to do is use the scale on the bottom of the
root graph paper in Appendix H. Measuring the pres- chart, remembering that each value indicated should
sure drop between furnace or burner housing and stack have a zero after it and one is one hundred. If you have
with a manometer and while the fan is running (no fire) the typical cam with twelve positions then 100% is 12
will provide all the information necessary for establish- and 1 is zero adjustment with a span of 11. For each cam
ing linear air flow. position (1 through 12), subtract one from it then divide
Setting your manometer on a slope (Figure 2-3) by 11. Note the result on the calculator, locate it on the
will allow you to measure the pressure drop in hun- bottom of the graph, draw a vertical line on the graph
dredths of an inch. Extend tubing from the manometer, and write the cam position under it.
connecting the bottom end of it, into a hole in the stack. For each corresponding differential pressure read-
Be certain the end of your tubing is not pointing towards ing, divide the reading by the maximum measured
or away from the direction of air flow so you avoid get- differential. Locate that value on the vertical scale of
ting any velocity pressure reading. Extend another piece the graph and draw a light line horizontally until it
of tubing through the observation port of the burner intersects the corresponding cam position or control-
and connect it to the top end of the manometer. The end ler output line and make a big dot there. Once you’ve
inside the burner has to be positioned to avoid velocity applied the ten or twelve dots draw a line connecting
pressure as well; it’s best to put a 90 degree bend in the them. The line should always extend to the upper right
end so the end is perpendicular to the flow. corner where both values are 100%. Don’t be surprised
If you have air flow measurement then you could if a line from zero and zero isn’t appropriate, the lowest
use plain graph paper and simply record the air flow. position or controller output is at low fire and the air
This exercise is useful, however, when there is any rea- flow at that point should be anywhere from 10 to 25%
son to question your air flow measurement. Compare and the differential would be between 0.001 and 0.06.
the flow indicated at the differential using the graph in Now what? Hey, if the line is straight, or nearly so,
appendix G. you’re done. If, however, the line is anything but straight
To ensure the boiler will not fire while you’re (like the curves A, B, E, F or G in Figure 11-31) you had
working on this it’s best to remove the burner manage- better adjust that linkage to get a more linear system. You
ment chassis. On small boilers it may be necessary to want something that falls in that narrow gray band on
jumper the fan starter to get it running independent of Figure 11-31 for good control.
the burner management system. Once you get the fan If you’re dealing with a jackshaft you’ll have to
running locate the terminals that drive the modulating change the position of the linkage. When possible, restore
motor so you can jumper them to control the position or, the original settings by the manufacturer, they should be
with other control systems, simply put it in manual so linear. Otherwise, opt for changes that make sense to you
you can stroke the damper. Run the controls up to high then take some more readings to see how you did, repeat-
fire to get the maximum air flow and record the pressure ing the process until you get something linear.
differential on the manometer. For the best world, a damper actuator with a po-
If you’re working with a jackshaft system you sitioner, the data you just collected will allow you to
should operate the modulating motor to lower air produce a new positioner cam. Linear control should pro-
flow, stopping when you’re even with each screw and duce a straight line from low fire to 100% so simply draw
recording the air differential. With more sophisticated a straight line from the low fire point to the upper right
controls set the air flow controller output at maximum corner. Draw horizontal lines through your data points
then decrease it and read the differential at 10% inter- until they intersect that line. The height of the existing
vals (90%, 80%, 70%, etc.) Once you have differential cam at the data point is the height you need for the new
pressure readings for all the flow values you can draw cam at the position coinciding with the straight line.
up your graph. Achieving linearity with a valve positioner or an
Make a copy of the graph in Appendix H and sit actuator for dampers and other shaft operated devices
down with it and a calculator. Write “air flow—%/100” was simply a matter of plotting the control signal, pro-
on the bottom of the graph and “differential—%/100” on cess flow, and height of the positioner or actuator cam
the left side. The chart values are 0 to 1 so the “%/100” along with its rotation. Then, take the blank cam and
indicates that the range of your data is from zero to one mark off the desired height at each angle of rotation,
hundred percent. If you had ten cam positions or used the desired height being determined by graphing the